Polymer

List Learning Objective

To discuss issues relating to the disposal and recycling of polymers and the conservation of resources used in polymer manufacture

To relate the properties of polymers to their molecular structure, and make predictions about the properties of a polymer

To explain the properties of condensation polymers in terms of intermolecular attractions eg H bonding in nylon or permanent dipole - permanent dipole interactions in PET

To show awareness of the ways that chemists can modify the properties of a polymer by physical and chemical means.

To distinguish between addition and condensation polymerization.

List Learning Outcomes

Understand what polymers are made up of, their properties, and basic polymer nomenclature

Identify the different polymerisation and polymer analytical techniques Differentiate between commodity and engineering thermoplastics Appreciate how polymers are processed for specific uses Suitable polymeric material for a daily use product

Keywords in Thermochesmistry

Polymerisation

DNA Polymerase

Propagation

Terminal pathway

Linear Polymer

Ester linkages

Quasi-thermoplastic

Cross link process

Polyisoprne

Delocalisation

Subtopics

Synthetic Polymers

Addition Polymerization Mechanisms

Coordination Polymerization

Condensation Polymerization

Thermoplastic and Thermosetting Materials

Natural Rubber

Introduction

Prior to the early1920's, chemists doubted the existence of molecules having

molecular weights greater than a few thousand. This limiting view was

challenged by Hermann Staudinger, a German chemist with experience in

studying natural compounds such as rubber and cellulose. In contrast to the

prevailing rationalization of these substances as aggregates of small molecules,

Staudinger proposed they were made up of macromolecules composed of

10,000 or more atoms. He formulated a polymeric structure for rubber, based on

a repeating isoprene unit (referred to as a monomer). For his contributions to

chemistry, Staudinger received the 1953 Nobel Prize. The terms polymer and

monomer were derived from the Greek roots poly (many), mono (one) and meros

(part).

Recognition that polymeric macromolecules make up many important natural

materials was followed by the creation of synthetic analogs having a variety of

properties. Indeed, applications of these materials as fibers, flexible films,

adhesives, resistant paints and tough but light solids have transformed modern

society. Some important examples of these substances are discussed in the

following sections.

Historical development

Starting in 1811, Henri Braconnot did pioneering work in derivative cellulose compounds, perhaps the earliest important work in polymer science. The development of vulcanization later in the nineteenth century improved the durability of the natural polymer rubber, signifying the first popularized semi-synthetic polymer. In 1907, Leo Baekeland created the first completely synthetic polymer, Bakelite, by reacting phenol and formaldehyde at precisely controlled temperature and pressure. Bakelite was then publicly introduced in 1909.

Despite significant advances in synthesis and characterization of polymers, a correct understanding of polymer molecular structure did not emerge until the 1920s. Before then, scientists believed that polymers were clusters of small molecules (called colloids), without definite molecular weights, held together by an unknown force, a concept known as association theory. In 1922, Hermann Staudinger proposed that polymers consisted of long chains of atoms held together by covalent bonds, an idea which did not gain wide acceptance for over a decade and for which Staudinger was ultimately awarded the Nobel Prize. Work by Wallace Carothers in the 1920s also demonstrated that polymers could be synthesized rationally from their constituent monomers. An important contribution to synthetic polymer science was made by the Italian chemist Giulio Natta and the German chemist Karl Ziegler, who won the Nobel Prize in Chemistry in 1963 for the development of the Ziegler-Natta catalyst. Further recognition of the importance of polymers came with the award of the Nobel Prize in Chemistry in 1974 to Paul Flory, whose extensive work on polymers included the kinetics of step-growth polymerization and of addition polymerization, chain transfer, excluded volume, the Flory-Huggins solution theory, and the Flory convention.

Synthetic polymer materials such as nylon, polyethylene, Teflon, and silicone have formed the basis for a burgeoning polymer industry. These years have also shown significant developments in rational polymer synthesis. Most commercially important polymers today are entirely synthetic and produced in high volume on appropriately scaled organic synthetic techniques. Synthetic polymers today find application in nearly every industry and area of life. Polymers are widely used as adhesives and lubricants, as well as structural components for products ranging from children's toys to aircraft. They have been employed in a variety of biomedical applications ranging from implantable devices to controlled drug delivery. Polymers such as poly(methyl methacrylate) find application as photoresist materials used in semiconductor manufacturing and low-k dielectrics for use in high-performance microprocessors. Recently, polymers have also been employed as flexible substrates in the development of organic light-emitting diodes for electronic displays.

Synthetic polymers are often referred to as "plastics", such as the well-known polyethylene and nylon. However, most of them can be classified in at least three main categories: thermoplastics, thermosets and elastomers. Formed from hydrocarbons, hydrocarbon derivatives, or sometimes from silicon, polymers are the basis not only for numerous natural materials, but also for most of the synthetic plastics that one encounters every day. Polymers consist of extremely large, chain-like molecules that are, in turn, made up of numerous smaller, repeating units called monomers. Chains of polymers can be compared to paper clips linked together in long strands, and sometimes cross-linked to form even more durable chains. Polymers can be composed of more than one type of monomer, and they can be altered in other ways. Likewise they are created by two different chemical processes, and thus are divided into addition and condensation polymers. Among the natural polymers are wool, hair, silk, rubber, and sand, while the many synthetic polymers include nylon, synthetic rubber, Teflon, Formica, Dacron, and so forth. It is very difficult to spend a day without encountering a natural polymer—even if hair is removed from the list—but in the twenty-first century, it is probably even harder to avoid synthetic polymers, which have collectively revolutionized human existence.

They are not limited to having carbon backbones, elements such as silicon form familiar materials such as silicones. Coordination polymers may contain a range of metals in the backbone, with non-covalent bonding present.

Man-made polymers are used in a wide array of applications: food packaging, films, fibers, tubing, pipes, etc. The personal care industry also uses polymers to aid in texture of products, binding, and moisture retention (e.g. in hair gel and conditioners).

Laboratory synthesis

Laboratory synthetic methods are generally divided into two categories, step-growth polymerization and chain-growth polymerization[4]. The essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only[5], whereas in step-growth polymerization chains of monomers may combine with one another directly[6]. However, some newer methods such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without a catalyst. Efforts towards rational synthesis of biopolymers via laboratory synthetic methods, especially artificial synthesis of proteins, is an area of intense research.

Biological synthesis

There are three main classes of biopolymers: polysaccharides, polypeptides, and polynucleotides. In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids. The protein may be modified further following translation in order to provide appropriate structure and functioning.

Modification of natural polymers

Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of sulphur.

Organic examples

As explained in the essay on Organic Chemistry, chemists once defined the term "organic" as relating only to living organisms; the materials that make them up; materials derived from them; and substances that come from formerly living organisms. This definition, which more or less represents the everyday meaning of "organic," includes a huge array of life forms and materials: humans, all other animals, insects, plants, microorganisms, and viruses; all substances that make up their structures (for example, blood, DNA, and proteins); all products that come from them (a list diverse enough to encompass everything from urine to honey); and all materials derived from the bodies of organisms that were once alive (paper, for instance, or fossil fuels).

As broad as this definition is, it is not broad enough to represent all the substances addressed by organic chemistry—the study of carbon, its compounds, and their properties. All living or once-living things do contain carbon; however, organic chemistry is also concerned with carbon-containing materials—for instance, the synthetic plastics we will discuss in this essay—that have never been part of a living organism.

It should be noted that while organic chemistry involves only materials that contain carbon, carbon itself is found in other compounds not considered organic: oxides such as carbon dioxide and monoxide, as well as carbonates, most notably calcium carbonate or limestone. In other words, as broad as the meaning of "organic" is, it still does not encompass all substances containing carbon.

Some of the most important polymers, according to monomer composition, are:

Polyacrylates Polyamides Polyesters Polycarbonates Polyimides Polystyrenes

However, a polymer need not be wholly made from one class of monomer, in which case it is classified as a copolymer.

A non-exhaustive list of ubiquitous materials includes: acrylonitrile butadiene styrene (ABS) polyacrylonitrile (PAN) or Acrylic polybutadiene poly(butylene terephthalate) (PBT) poly(ether sulfone) (PES, PES/PEES) poly(ether ether ketone)s (PEEK, PES/PEEK) polyethylene (PE) poly(ethylene glycol) (PEG) poly(ethylene terephthalate) (PET) polypropylene (PP) polytetrafluoroethylene (PTFE) styrene-acrylonitrile resin (SAN) poly(trimethylene terephthalate) (PTT) polyurethane (PU) polyvinyl butyral (PVB) polyvinylchloride (PVC) polyvinylidenedifluoride (PVDF) poly(vinyl pyrrolidone) (PVP)

Inorganic examples Polysiloxanes Polyphosphazenes

Brand names

These polymers are often better known through their brand names, for instance:

Bakelite, i.e. phenol-formaldehyde resin Kevlar, Twaron, i.e. para-aramid Kynar, i.e. PVDF Mylar, i.e. polyethylene terephthalate film Neoprene i.e. Polychloroprene Nylon, i.e. polyamide 6,6 Orlon, i.e. polyacrylonitrile Rilsan, i.e. polyamide 11 & 12 Technora, i.e. copolyamid Teflon, i.e. PTFE Ultem, i.e. polyimide Vectran, i.e. aromatic polyamide Viton, i.e. poly-tetrafluoroethylene Zylon, i.e. poly-p-phenylene-2,6-benzobisoxazole (PBO)

Synthetic Biodegradable Polymers as Medical Devices

In the first half of this century, research into materials synthesized from glycolic acid and other -hydroxy acids was abandoned for further development because the resulting polymers were too unstable for long-term industrial uses. However, this very instability—leading to biodegradation—has proven to be immensely important in medical applications over the last three decades. Polymers prepared from glycolic acid and lactic acid have found a multitude of uses in the medical industry, beginning with the biodegradable sutures first approved in the 1960s. Since that time, diverse products based on lactic and glycolic acid—and on other materials, including poly(dioxanone), poly(trimethylene carbonate) copolymers, and poly ( -caprolactone) homopolymers and copolymers—have been accepted for use as medical devices. In addition to these

approved devices, a great deal of research continues on polyanhydrides, polyorthoesters, polyphosphazenes, and other biodegradable polymers.

A biodegradable intravascular stent prototype is molded from a blend of polylactide and trimethylene carbonate. Photo: Cordis Corp. Prototype Molded by Tesco Associates, Inc.

Why would a medical practitioner want a material to degrade? There may be a variety of reasons, but the most basic begins with the physician's simple desire to have a device that

can be used as an implant and will not require a second surgical intervention for removal. Besides eliminating the need for a second surgery, the biodegradation may offer other advantages. For example, a fractured bone that has been fixated with a rigid, nonbiodegradable stainless implant has a tendency for refracture upon removal of the implant. Because the stress is borne by the rigid stainless steel, the bone has not been able to carry sufficient load during the healing process. However, an implant prepared from biodegradable polymer can be engineered to degrade at a rate that will slowly transfer load to the healing bone. Another exciting use for which biodegradable polymers offer tremendous potential is as the basis for drug delivery, either as a drug delivery system alone or in conjunction to functioning as a medical device.

Polymer scientists, working closely with those in the device and medical fields, have made tremendous advances over the last 30 years. This article will focus on a number of these developments. We will also review the chemistry of the polymers, including synthesis and degradation, describe how properties can be controlled by proper synthetic controls such as copolymer composition, highlight special requirements for processing and handling, and discuss some of the commercial devices based on these materials.

POLYMER CHEMISTRY

Biodegradable polymers can be either natural or synthetic. In general, synthetic polymers offer greater advantages than natural materials in that they can be tailored to give a wider range of properties and more predictable lot-to-lot uniformity than can materials from natural sources. Synthetic polymers also represent a more reliable source of raw materials, one free from concerns of immunogenicity.

Table I. Properties of common biodegradable polymers.

The general criteria for selecting a polymer for use as a biomaterial is to match the mechanical properties and the time of degradation to the needs of the application (see Table I). The ideal polymer for a particular application would be configured so that it:

Has mechanical properties that match the application, remaining sufficiently strong until the surrounding tissue has healed.

Does not invoke an inflammatory or toxic response. Is metabolized in the body after fulfilling its purpose, leaving no trace. Is easily processable into the final product form. Demonstrates acceptable shelf life. Is easily sterilized.

Polymer Melting Point (°C)

Glass-Transition Temp (°C)

Modulus (Gpa)a

Degradation Time (months)b

PGA 225—230 35—40 7.0 6 to 12

LPLA 173—178 60—65 2.7 >24

DLPLA Amorphous 55—60 1.9 12 to 16

PCL 58—63 (—65)— (—60)

0.4 >24

PDO N/A (—10)— 0 1.5 6 to 12

PGA-TMC

N/A N/A 2.4 6 to 12

85/15 DLPLG

Amorphous 50—55 2.0 5 to 6

75/25 DLPLG

Amorphous 50—55 2.0 4 to 5

65/35 DLPLG

Amorphous 45—50 2.0 3 to 4

50/50 DLPLG

Amorphous 45—50 2.0 1 to 2

a Tensile or flexural modulus.

b Time to complete mass loss. Rate also depends on part geometry.

The factors affecting the mechanical performance of biodegradable polymers are those that are well known to the polymer scientist, and include monomer selection, initiator selection, process conditions, and the presence of additives. These factors in turn influence the polymer's hydrophilicity, crystallinity, melt and glass-transition temperatures, molecular weight, molecular-weight distribution, end groups, sequence distribution (random versus blocky), and presence of residual monomer or additives. In addition, the polymer scientist working with biodegradable materials must evaluate each of these variables for its effect on biodegradation.1

Biodegradation has been accomplished by synthesizing polymers that have hydrolytically unstable linkages in the backbone. The most common chemical functional groups with this characteristic are esters, anhydrides, orthoesters, and amides. We will discuss the importance of the properties affecting biodegradation later in the article.

The following section presents an overview of the synthetic biodegradable polymers that are currently being used or investigated for use in wound closure (sutures, staples); orthopedic fixation devices (pins, rods, screws, tacks, ligaments); dental applications (guided tissue regeneration); cardiovascular applications (stents, grafts); and intestinal applications (anastomosis rings). Most of the commercially available biodegradable devices are polyesters composed of homopolymers or copolymers of glycolide and lactide. There are also devices made from copolymers of trimethylene carbonate and -caprolactone, and a suture product made from polydioxanone.

Polyglycolide (PGA). Polyglycolide is the simplest linear aliphatic polyester. PGA was used to develop the first totally synthetic absorbable suture, marketed as Dexon in the 1960s by Davis and Geck, Inc. (Danbury, CT). Glycolide monomer is synthesized from the dimerization of glycolic acid. Ring-opening polymerization yields high-molecular-weight materials, with approximately 1—3% residual monomer present (see Figure 1). PGA is highly crystalline (45—55%), with a high melting point (220—225°C) and a glass-transition temperature of 35—40°C. Because of its high degree of crystallization, it is not soluble in most organic solvents; the exceptions are highly fluorinated organics such as hexafluoroisopropanol. Fibers from PGA exhibit high strength and modulus and are too stiff to be used as sutures except in the form of braided material. Sutures of PGA lose about 50% of their strength after 2 weeks and 100% at 4 weeks, and are completely absorbed in 4—6 months. Glycolide has been copolymerized with other monomers to reduce the stiffness of the resulting fibers.

Figure 1. Synthesis of

Polylactide (PLA). Lactide is the cyclic dimer of lactic acid that exists as two optical isomers, d and l. l-lactide is the naturally occurring isomer, and dl-lactide is the synthetic blend of d-lactide and l-lactide. The homopolymer of l-lactide (LPLA) is a semicrystalline polymer. These types of materials exhibit high tensile strength and low elongation, and consequently have a high modulus that makes them more suitable for load-bearing applications such as in orthopedic fixation and sutures. Poly(dl-lactide) (DLPLA) is an amorphous polymer exhibiting a random distribution of both isomeric forms of lactic acid, and accordingly is unable to arrange into an organized crystalline structure. This material has lower tensile strength, higher elongation, and a much more rapid degradation time, making it more attractive as a drug delivery system. Poly(l-lactide) is about 37% crystalline, with a melting point of 175—178°C and a glass-transition temperature of 60—65°C. The degradation time of LPLA is much slower than that of DLPLA, requiring more than 2 years to be completely absorbed. Copolymers of l-lactide and dl-lactide have been prepared to disrupt the crystallinity of l-lactide and accelerate the degradation process.

Poly( -caprolactone). The ring-opening polymerization of -caprolactone yields a semicrystalline polymer with a melting point of 59—64°C and a glass-transition temperature of —60°C (see Figure 2). The polymer has been regarded as tissue compatible and used as a biodegradable suture in Europe. Because the homopolymer has a degradation time on the order of 2 years, copolymers have been synthesized to accelerate the rate of bioabsorption. For example, copolymers of -caprolactone with dl-lactide have yielded materials with more-rapid degradation rates. A block copolymer of -caprolactone with glycolide, offering reduced stiffness compared with pure PGA, is being sold as a monofilament suture by Ethicon, Inc. (Somerville, NJ), under the trade name Monacryl.

Figure 2. Synthesis of poly( -

Poly(dioxanone) (a polyether-ester). The ring-opening polymerization of p-dioxanone (see Figure 3) resulted in the first clinically tested monofilament synthetic suture, known as PDS (marketed by Ethicon). This material has approximately 55% crystallinity, with a glass-transition temperature of —10 to 0°C. The polymer should be processed at the lowest possible temperature to prevent depolymerization back to monomer. Poly(dioxanone) has demonstrated no acute or toxic effects on implantation. The monofilament loses 50% of its initial breaking strength after 3 weeks and is absorbed within 6 months, providing an advantage over Dexon or other products for slow-healing wounds.

Figure 3. Synthesis of poly(dioxanone). -caprolactone).

Poly(lactide-co-glycolide). Using the polyglycolide and poly(l-lactide) properties as a starting point, it is possible to copolymerize the two monomers to extend the range of homopolymer properties (see Figure 4). Copolymers of glycolide with both l-lactide and dl-lactide have been developed for both device and drug delivery applications. It is important to note that there is not a linear relationship between the copolymer composition and the mechanical and degradation properties of the materials. For example, a copolymer of 50% glycolide and 50% dl-lactide degrades faster than either homopolymer (see Figure 5). Copolymers of l-lactide with 25—70% glycolide are amorphous due to the disruption of the regularity of the polymer chain by the other monomer. A copolymer of 90% glycolide and 10% l-lactide was developed by Ethicon as

an absorbable suture material under the trade name Vicryl. It absorbs within 3—4 months but has a slightly longer strength-retention time.

Figure 4. Synthesis of

Figure 5. Half-life of PLA and PGA homopolymers and copolymers implanted in rat tissue. (Figure reproduced courtesy of Journal of Biomedical Materials Research, 11:711,

1977.)

Copolymers of glycolide with trimethylene carbonate (TMC), called polyglyconate (see Figure 6), have been prepared as both sutures (Maxon, by Davis and Geck) and as tacks and screws (Acufex Microsurgical, Inc., Mansfield, MA). Typically, these are prepared as A-B-A block copolymers in a 2:1 glycolide:TMC ratio, with a glycolide-TMC center block (B) and pure glycolide end blocks (A). These materials have better flexibility than pure PGA and are absorbed in approximately 7 months. Glycolide has also been polymerized with TMC and p-dioxanone (Biosyn, by United States Surgical Corp., Norwalk, CT) to form a terpolymer suture that absorbs within 3—4 months and offers reduced stiffness compared with pure PGA fibers.

Figure 6.

Other Polymers under Development. Currently, only devices made from homopolymers or copolymers of glycolide, lactide, caprolactone, p-dioxanone, and trimethylene carbonate have been cleared for marketing by FDA. A number of other polymers, however, are being investigated for use as materials for biodegradable devices.

In addition to their suitability for medical uses, biodegradable polymers make excellent candidates for packaging and other consumer applications. A number of companies are evaluating ways to make low-cost biodegradable polymers. One method is to bioengineer the synthesis of the polymers, using microorganisms to produce energy-storing polyesters. Two examples of these materials—polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV)—are commercially available as copolymers under the trade name Biopol (Monsanto Co., St. Louis) and have been studied for use in medical devices (see Figure 7). The PHB homopolymer is crystalline and brittle, whereas the copolymers of PHB with PHV are less crystalline, more flexible, and easier to process. These polymers typically require the presence of enzymes for biodegradation but can degrade in a range of environments and are under consideration for several biomedical applications.

Figure 7. Molecular structure of two bioengineered polyesters that require specific enzymes for biodegradation.

The use of synthetic poly(amino acids) as polymers for biomedical devices would seem a logical choice, given their wide occurrence in nature. In practice, however, pure insoluble poly(amino acids) have found little utility because of their high crystallinity, which makes them difficult to process and results in relatively slow degradation. The antigenicity of polymers with more than three amino acids in the chain also makes them inappropriate for use in vivo. To circumvent these problems, modified "pseudo" poly(amino acids) have been synthesized by using a tyrosine derivative. Tyrosine-derived polycarbonates, for example, are high-strength materials that may be useful as orthopedic implants. It is also possible to copolymerize poly(amino acids) to modify their properties. The group that has been researched most extensively is the polyesteramides.

A Note on Nomenclature

A polymer is generally named based on the monomer it is synthesized from. For example, ethylene is used to produce poly(ethylene). For both glycolic acid and lactic acid, an intermediate cyclic dimer is prepared and purified, prior to polymerization. These dimers are called glycolide and lactide, respectively. Although most references in the literature refer to polyglycolide or poly(lactide), you will also find references to poly(glycolic acid) and poly(lactic acid). Poly(lactide) exists in two stereo forms, signified by d or l for dexorotary or levorotary, or by dl for the racemic mix.

The search for new candidate polymers for drug delivery may offer potential for medical device applications as well. In drug delivery, the formulation scientist is concerned not only with shelf-life stability of the drug but also with stability after implantation, when the drug may reside in the implant for 1—6 months or more. For drugs that are hydrolytically unstable, a polymer that absorbs water may be contraindicated, and researchers have begun evaluating more hydrophobic polymers that degrade by surface erosion rather than by bulk hydrolytic degradation. Two classes of these polymers are the polyanhydrides and the polyorthoesters.

Figure 8. Molecular structure of poly(SA-HDA

Polyanhydrides have been synthesized via the dehydration of diacid molecules by melt polycondensation (see Figure 8). Degradation times can be adjusted from days to years according to the degree of hydrophobicity of the monomer selected. The materials degrade primarily by surface erosion and possess excellent in vivo compatibility. So far, they have only been approved for sale as a drug delivery system. The Gliadel product, designed for delivery of the chemotherapeutic agent BCNU in the brain, received regulatory clearance from FDA in 1996 and is being produced by Guilford Pharmaceuticals, Inc. (Baltimore).

Figure 9. Molecular structure of poly(orthoester).

Polyorthoesters were first investigated in the 1970s by Alza Corp. (Palo Alto, CA) and SRI International (Menlo Park, CA) in a search for new synthetic biodegradable polymers for drug delivery applications (see Figure 9). These materials have gone through several generations of improvements in synthesis, and can now be polymerized at room temperature without forming condensation by-products. Polyorthoesters are hydrophobic, with hydrolytic linkages that are acid-sensitive but stable to base. They degrade by surface erosion, and degradation rates can be controlled by incorporation of acidic or basic excipients.

PACKAGING AND STERILIZATION

Because biodegradable polymers are hydrolytically unstable, the presence of moisture can degrade them in storage, during processing, and after device fabrication. In theory, the solution for hydrolysis instability is simple: eliminate the moisture and thus eliminate the degradation. However, because the materials are naturally hygroscopic, eliminating water and then keeping the polymer free of water are difficult to accomplish. The as-synthesized polymers have relatively low water contents, since any residual water in the monomer is used up in the polymerization reaction. The polymers are quickly packaged after manufacture—generally double-bagged under an inert atmosphere or vacuum. The bag material may be polymeric or foil, but it must be highly resistant to water permeability. To minimize the effects of any moisture present, the polymers are typically stored in a freezer. Packaged polymers should always be at room temperature when opened to minimize condensation, and should be handled as little as possible at ambient atmospheric conditions. As expected, there is a relationship among biodegradation rate, shelf stability, and polymer properties. For instance, the more hydrophilic glycolide polymers are much more sensitive to hydrolytic degradation than are polymers prepared from the more hydrophobic lactide.

Final packaging consists of placing the suture or device in an airtight, moistureproof container. A desiccant can be added to further reduce the effects of moisture. Sutures, for example, are wrapped around a specially dried paper holder that acts as a desiccant. In some cases, the finished device may be stored at subambient temperature as an added precaution against degradation.

Devices incorporating biodegradable polymers cannot be subjected to autoclaving, and must be sterilized by gamma or E-beam irradiation or by exposure to ethylene oxide (EtO) gas. There are certain disadvantages, however, to both irradiation and EtO sterilization. Irradiation, particularly at doses above 2 Mrd, can induce significant degradation of the polymer chain, resulting in reduced molecular weight as well as influencing final mechanical properties and degradation times. Polyglycolide, poly(lactide), and poly(dioxanone) are especially sensitive to ionizing radiation, and these materials are usually sterilized by EtO for device applications. Because the highly toxic

EtO can present a safety hazard, great care must be taken to ensure that all the gas is removed from the device before final packaging. The temperature and humidity conditions should also be considered when submitting devices for sterilization. Temperatures must be kept below the glass-transition temperature of the polymer to prevent the part geometry from changing during sterilization. If necessary, parts can be kept at 0°C or lower during the irradiation process.

PROCESSING

All commercially available biodegradable polymers can be melt processed by conventional means such as injection molding, compression molding, and extrusion. As with packaging, special consideration needs to be given to the exclusion of moisture from the material before melt processing to prevent hydrolytic degradation. Special care must be taken to dry the polymers before processing and to rigorously exclude humidity during processing.

Because most biodegradable polymers have been synthesized by ring-opening polymerization, a thermodynamic equilibrium exists between the forward or polymerization reaction and the reverse reaction that results in monomer formation. Excessively high processing temperatures may result in monomer formation during the molding or extrusion process. The presence of excess monomer can act as a plasticizer, changing the material's mechanical properties, and can catalyze the hydrolysis of the device, thus altering degradation kinetics. Therefore, these materials should be processed at the lowest temperatures possible.

Factors That Accelerate Polymer Degradation More hydrophilic backbone. More hydrophilic endgroups. More reactive hydrolytic groups in the backbone. Less crystallinity. More porosity. Smaller device size.

DEGRADATION

Once implanted, a biodegradable device should maintain its mechanical properties until it is no longer needed and then be absorbed and excreted by the body, leaving no trace.

Simple chemical hydrolysis of the hydrolytically unstable backbone is the prevailing mechanism for the polymer's degradation. This occurs in two phases. In the first phase, water penetrates the bulk of the device, preferentially attacking the chemical bonds in the amorphous phase and converting long polymer chains into shorter water-soluble fragments. Because this occurs in the amorphous phase initially, there is a reduction in molecular weight without a loss in physical properties, since the device matrix is still held together by the crystalline regions. The reduction in molecular weight is soon followed by a reduction in physical properties, as water begins to fragment the device (see Figure 10). In the second phase, enzymatic attack and metabolization of the fragments occurs, resulting in a rapid loss of polymer mass. This type of degradation—when the rate at which water penetrates the device exceeds that at which the polymer is converted into water-soluble materials (resulting in erosion throughout the device)—is called bulk erosion. All of the commercially available synthetic devices and sutures degrade by bulk erosion.

Figure 10. Generic absorption curves showing the sequence of polymer molecular weight, strength, and mass reduction. (Figure reproduced courtesy of Journal of Craniofacial Surgery, (8)2:89, 1997.)

A second type of biodegradation, known as surface erosion, occurs when the rate at which the polymer penetrates the device is slower than the rate of conversion of the polymer into water-soluble materials. Surface erosion results in the device thinning over time while maintaining its bulk integrity. Polyanhydrides and polyorthoesters are examples of materials that undergo this type of erosion—when the polymer is hydrophobic, but the chemical bonds are highly susceptible to hydrolysis. In general, this process is referred to in the literature as bioerosion rather than biodegradation.

The degradation-absorption mechanism is the result of many interrelated factors, including:

The chemical stability of the polymer backbone. The presence of catalysts, additives, impurities, or plasticizers. The geometry of the device.

Balancing these factors by tailoring an implant to slowly degrade and transfer stress at the appropriate rate to surrounding tissues as they heal is one of the major challenges facing researchers today.

COMMERCIAL BIODEGRADABLE DEVICES

The total U.S. revenues from commercial products developed from absorbable polymers in 1995 was estimated to be over $300 million, with more than 95% of revenues generated from the sale of bioabsorbable sutures. The other 5% is attributed to orthopedic fixation devices in the forms of pins, rods, and tacks; staples for wound closure; and dental applications.2 Research into biodegradable systems continues to increase, from the 60 to 70 papers published each year in the late 1970s to the more than 400 each year in the early 1990s. The rate at which bioabsorbable fixation devices are cleared through the FDA 510(k) regulatory process is also increasing, with seven devices cleared for sale in 1995.

What follows is a brief overview of some of the significant commercial applications of biodegradable polymers.

Sutures. While comprising the lion's share of the total medical biodegradables market in 1995, this is a mature area not expected to grow rapidly in the future. About 125 million synthetic bioabsorbable sutures are sold each year in the United States. They are divided into braided and monofilament categories. Braided sutures are typically more pliable than monofilament and exhibit better knot security when the same type of knot is used. Monofilament sutures are more wiry and may require a more secure knot. Their major advantage is that they exhibit less tissue drag, a characteristic that is especially important for cardiovascular, ophthalmic, and neurological surgeries. A recent source in the literature lists eight objective and three subjective parameters for suture selection based on criteria such as tensile strength, strength retention, knot security, tissue drag, infection potential, and ease of tying.3

Dental Devices. Biodegradable polymers have found use in two dental applications. Employed as a void filler following tooth extraction, porous polymer particles can be packed into the cavity to aid in quicker healing. As a guided-tissue-regeneration (GTR) membrane, films of biodegradable polymer can be positioned to exclude epithelial migration following periodontal surgery. The exclusion of epithelial cells allows the supporting, slower-growing tissue—including connective and ligament cells—to proliferate. Three examples of these GTR materials are Resolut from W.L. Gore (Flagstaff, AZ), Atrisorb from Atrix Laboratories (Fort Collins, CO), and Vicryl mesh from Ethicon.

Orthopedic Fixation Devices. Orthopedic fixation devices made from synthetic biodegradable polymers have advantages over metal implants in that they transfer stress over time to the damaged area, allowing healing of the tissues, and eliminate the need for a subsequent operation for implant removal. The currently available materials have not exhibited sufficient stiffness to be used as bone plates for support of long bones, such as the femur. Rather, they have found applications where lower-strength materials are sufficient: for example, as interference screws in the ankle, knee, and hand areas; as tacks and pins for ligament attachment and meniscal repair; as suture anchors; and as rods and

pins for fracture fixation. Screws and plates of poly(l-lactide-co-glycolide) for craniomaxillofacial repair have recently been cleared for marketing in the United States under the trade name LactoSorb Craniomaxillofacial Fixation System (Biomet, Inc., Warsaw, IN).

Other Applications. Biodegradable polymers have found other applications that have been commercialized or are under investigation. Anastomosis rings have been developed as an alternative to suturing for intestinal resection. Tissue staples have also replaced sutures in certain procedures. Other applications currently under scrutiny include ligating clips, vascular grafts, stents, and tissue-engineering scaffolds. A list of commercial synthetic biodegradable polymer devices by category is given in Table II.

Application Trade Name Composition a Manufacturer

  Dexon PGA Davis and Geck

  Maxon PGA-TMC Davis and Geck

  Vicryl PGA-LPLA Ethicon

Sutures Monocryl PGA-PCL Ethicon

  PDS PDO Ethicon

  Polysorb PGA-LPLA U.S. Surgical

  Biosyn PDO-PGA-TMC U.S. Surgical

  PGA Suture PGA Lukens

  Sysorb DLPLA Synos

  Endofix PGA-TMC or LPLA

Acufex

  Arthrex LPLA Arthrex

Interference screws

Bioscrew LPLA Linvatec

  Phusiline LPLA-DLPLA Phusis

  Biologically Quiet

PGA-DLPLA Instrument Makar

Suture anchor Bio-Statak LPLA Zimmer

  Suretac PGA-TMC Acufex

Anastomosis clip

Lactasorb LPLA Davis and Geck

Anastomosis ring

Valtrac PGA Davis and Geck

Dental Drilac DLPLA THM Biomedical

Angioplastic plug

Angioseal PGA-DLPLA AHP

Screw SmartScrew LPLA Bionx

Pins and rods Biofix LPLA or PGA Bionx

  Resor-Pin LPLA-DLPLA Geistlich

Tack SmartTack LPLA Bionx

Plates, mesh, screws

LactoSorb PGA-LPLA Lorenz

Table II. Some commercial biodegradable medical products.

Biodegradable Polymers in Tissue Engineering

One of the exciting current areas for applications of biodegradable polymers is in tissue engineering. Several companies are investigating using these materials as a matrix for living cells. Important properties in this regard include porosity for cell in-growth, a surface that balances hydrophilicity and hydrophobicity for cellular attachment, mechanical properties that are compatible with those of the tissue, and degradation rate and by-products. The polymer matrix may represent the device itself, or can be a scaffold for cell growth in vitro that is degraded by the growing cells prior to implantation. The device can also be formulated to contain additives or active agents for more rapid tissue growth or compatibility. For example, a bone implant may contain a form of calcium phosphate or a growth factor such as one of the bone morphogenetic proteins.

There are a number of ways of making the three-dimensional matrices required for tissue engineering. These methods include woven or nonwoven preparations from spun fibers, blown films using solvents or propellants, or sintered polymer particles. One of the newest methods is being developed by Therics (Princeton, NJ), which has licensed a system for building three-dimensional devices for use as scaffolds and in drug delivery products. In this system, small spheres of polymer are laid out in thin films. Using technology similar to that found in ink-jet printers, small amounts of solvent are used to fuse particles together. The particles not fused are removed and another layer of particles laid out. This particle placement and fusing is continued for many layers, until the exact three-dimensional structure is obtained. Because each polymer layer is applied in a separate step, different polymers can be used to obtain different properties in the interior and exterior of the device.

Polymerization is the formation of long, repeating organic polymer chains.

Polymerization dates back to the beginning of DNA based life, as both DNA and proteins can be considered polymers. The first 'synthetic' polymers of the 19th century were actually formed by modifying natural polymers. For example nitrocellulose was manufactured by reacting cellulose with nitric acid. The first genuinely man-made polymer, bakelite, was synthesized in 1872, however research into polymers and polymerization really accelerated in the 1930s after the serendipitous discovery of polyethene by the chemical company ICI.

There are many forms of polymerization, and different systems exist to categorize them. Categorizations include the addition-condensation system and the chain growth-step growth system. Another form of polymerization is ring-opening polymerization, which is similar to chain polymerization.

Addition polymerization involves the linking together of molecules incorporating double or triple chemical bonds. These unsaturated monomers (the identical molecules which make up the polymers) have extra, internal, bonds which are able to break and link up with other monomers to form the repeating chain. Addition polymerization is involved in

the manufacture of polymers such as polyethene, polypropylene and polyvinylchloride (PVC.) A special case of addition polymerization leads to living polymerization.

Condensation polymerization occurs when monomers bond together through condensation reactions. Typically these reactions can be achieved through reacting molecules incorporating alcohol, amine or carboxylic acid (or other carboxyl derivative) functional groups. When an amine reacts with a carboxylic acid an amide or peptide bond is formed, with the release of water (hence condensation polymerization.) This is the process through which amino acids link up to form proteins, as well as how kevlar is formed.

The chain growth-step growth system categorizes polymers based on their mechanism. While most polymers will fall into their similar category from the addition-condensation method of categorization, their are a fwe exceptions.

Chain growth polymers are defined as polymers formed by the reaction of monomer with a reactive center. These polymers grow to high molecular weight at a very fast rate. It is important to note that the overall conversion rates between chain and step growth polymers are similar, but that high molecular weight polymers are formed in addition reactions much more quickly than with step polymerizations.

Step growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomer. Most step growth polymers are also classified as condensation polymers, but not all step growth polymers (like polyurethanes formed from isocyanate and alcohol bifunctional monomers) release condensates. Step growth polymers increase in molecular weight at a very slow rate at lower conversions and only reach moderately high molecular weights at very high conversion (i.e. >95%).

To alleviate inconsistencies in these naming methods, adjusted definitions for condensation and addition polymers have been developed. A condensation polymer is defined as a polymer that involves elimination of small molecules during its synthesis, or contains functional groups as part of its backbone chain, or it repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded.

Addition polymerization involves the breaking of double or triple bonds, which are used to link monomers in to chains. In the polymerization of ethene (fig. 1), its double bond is broken and it is used to bond to another poly(ethene) monomer. There are several mechanisms through which this can be initiated. The free radical mechanism was one of the first methods to be used. Free radicals are very reactive atoms or molecules which have unpaired electrons. Taking the polymerization of ethene as an example, the free radical mechanism can be divided in to three stages: initiation, propagation and termination.

Ethene polymerization.png

Initiation is the creation of free radicals necessary for propagation. The radicals can be created from organic peroxide molecules, molecules containing an O-O single bond, by reacting oxygen with ethene. The products formed are unstable and easily break down into two radicals. In an ethene monomer, one electron pair is held securely between the

two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule in to another radical.

Propagation is the rapid reaction of this radicalised ethene molecule with another ethene monomer, and the subsequent repetition to create the repeating chain.

Termination only occurs when two radical chains collide. The lone electrons pair up and a stable molecule is formed, the product being a sum of the individual polymer chains.

Free radical addition polymerization must take place at high temperatures and pressures, approximately 300°C and 2000 At. There are problems with the lack of control in the reaction, specifically with the creation of variably branched chains. Also, as termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains. Finally, reactions involving larger molecules such as polypropene are difficult. For this reason new mechanisms for addition polymerization were developed. An early replacement was the Ziegler-Natta catalyst.

The problem of branching occurs during propagation, when a chain curls back on itself and breaks - leaving irregular chains sprouting from the main carbon backbone. Branching makes the polymers less dense and results in low tensile strength and melting points. Developed by Karl Ziegler and Giulio Natta in the 1950s, Ziegler-Natta catalysts (triethylaluminium in the presence of a metal (IV) chloride) largely solved this problem. Instead of a free radical reaction, the initial ethene monomer inserts between the aluminium atom and one of the ethyl groups in the catalyst. The polymer is then able to grow out from the aluminium atom and results in almost totally unbranched chains. With the new catalysts, the tacticity of the polypropene chain, the alignment of alkyl groups, was also able to be controlled. Different metal chlorides allowed the selective production of each form i.e., syndiotactic, isotactic and atactic polymer chains could be selectively created.

However there were further complications to be solved. If the Ziegler-Natta catalyst was poisoned or damaged then the chain stopped growing. Also, Ziegler-Natta monomers could be only small, and it was impossible to control the molecular mass of the polymer chains. Again new catalysts, the metallocenes, were developed to tackle these problems. Due to their structure they have less premature chain termination and branching.

Monomers: The Double Bond

In order for polymerization to occur with vinyl monomers, the substituents on the double bond must be able to stabilize a negative charge. Stabilization occurs through delocalization of the negative charge. Because of the nature of the carbanion propagating

center, substituents that react with bases or nucleophiles either must not be present or be protected.

Vinyl monomers with substituents that stabilize the negative charge through charge delocalization, undergo polymerization without termination or chain transfer. These monomers include styrene, dienes, methacrylate, vinyl pyridine,aldehydes, epoxide, episulfide, cyclic siloxane, and lactones. Polar monomers, using controlled conditions and low temperatures, can undergo anionic polymerization. However, at higher temperatures they do not produce living stable, carbanionic chain ends because their polar substituents can undergo side reactions with both initiators and propagating chain centers. The effects of counterion, solvent, temperature, Lew base additives, and inorganic solvents have been investigated to increase the potential of anionic polymerizations of polar monomers. Polar monomers include acrylonitrile, cyanoacrylate, propylene oxide, vinyl ketone, acrolein, vinyl sulfone, vinyl sulfoxide, vinyl silane and isocyanate.

You know from your studies that a double bond is two pairs of electrons being shared by two atoms. This arrangement is fairly strong, but when other molecules - molecules that like to react with electrons - are near, one of these pairs of electrons is vulnerable to attack. One such attacking species is the free radical.

Below is an example of how a free radical forms (this one starts with peroxide, HOOH):The peroxide molecule has an easy-to-break O-O single bond. Heat or light energy can break this O-O single bond ().

The single dot represents one electron from the O-O peroxide bond. The HO· fragments are the free radicals, and they are very unstable and reactive. Free radicals are very reactive. When a free radical gets close to a double bond, one of the bonds is disrupted. One of the electrons in the double bond is attracted to the free radical. The double bond breaks, and a new single bond is formed ( ).

Notice that in forming this bond, one electron from the double bond is left alone. Thus, another (larger) free radical has been formed.

By the way, we say that the free radical adds to the double bond. Get it? Addition polimarization.

Let's look at the steps involved in a typical addition polymerization.

The Mechanism of Addition Polymerization

The formation of a polymer by addition polymerization is an example of a chain reaction. Once a chain reaction gets started, it is able to keep itself going. The three steps of this reaction to focus on are

how the reaction gets started (INITIATION)how the reaction keeps going (PROPAGATION)how the reaction stops (TERMINATION)

A Note About This Example:There are various methods used to carry out addition polymerization chain reactions. The details vary according to the method used.

We will focus on a commonly used mechanism involving a free radical. Our example polymerization will combine ethylene (ethene) monomers (CH2=CH2), so our product will be polyethylene. (Polyethylene is used to make food wrap, milk jugs, garbage bags, and many other plastic products.)

I. INITIATION

The reactivity of initiators used in anionic polymerization should be similar to that of the monomer that is the propagating species. The pKa values for the conjugate acids of the carbanions formed from monomers can be used to deduce the reactivity of the monomer. The least reactive monomers have the largest pKa values for their corresponding conjugate acid and thus, require the most reactive initiator. Two main initiation pathways involve electron transfer (through alkali metals) and strong anions.

If you looked at Part 3 of this tutorial, you have already seen the first part of the initiation step of addition polimarization chain reaction. A peroxide molecule breaks up into two reactive free radicals. Light or heat can provide the energy needed for this process.

We can write an equation for this process:

The second part of initiation occurs when the free radical initiator attacks and attaches to a monomer molecule. This forms a new free radical, which is called the activated monomer.

We can write an equation for this process, too:

I nitiation by Electron Transfer

Szwarc and coworkers studied the initiation of polymerization through the use of aromatic radical-anions such as sodium naphthenate. In this reaction, an electron is transferred from the alkali metal to naphthalene. Polar solvents are necessary for this type of initiation both for stability of the anion-radical and to solvate the cation species formed.The anion-radical can then transfer an electron to the monomer.

Initation through electron transfer.

Initiation can also involve the transfer of an electron from the alkali metal to the monomer to form an anion-radical. Initiation occurs on the surface of the metal, with the reversible transfer of an electron to the adsorbed monomer.

Initiation by Strong Anions

Nucleophilic initiators include covalent or ionic metal amides, alkoxides, hydroxides, cyanides, phosphines, amines and organometallic compounds (alkyllithium compounds and Grignard reagents). The initiation process involves the addition of a neutral (B:) or negative (B:-) nucleophile to the monomer

Initiation through strong anion.

The most commercially useful of these initiators has been the alkyllithium initiators. They are primarily used for the polymerization of styrenes and dienes

II. PROPAGATION

Propagation in anionic addition polymerization results in the complete consumption of monomer. It is very fast and occurs at low temperatures. This is due to the anion not being very stable, the speed of the reaction as well as that heat is released during the reaction. The stability can be greatly enhanced by reducing the temperatures to near 0˚C. The propagation rates are generally fairly high compared to the decay reaction, so the overall polymerization rates is generally not affected.

Propagation of an anionic addition polymerization

During a chain reaction, most of the time is spent in the propagation phase as the polymer chain grows. In the propagation phase, the newly-formed activated monomer attacks and attaches to the double bond of another monomer molecule. This addition occurs again and again to make the long polymer chain.

Once again, we can write an equation for this reaction:

The "n" stands for any number of monomer molecules, typically in the thousands.

III. TERMINATION

Anionic addition polymerizations have no formal termination pathways because proton transfer from solvent or other positive species does not occur. However, termination can occur through unintentional quenching due to trace impurities. This includes trace amounts of oxygen, carbon dioxide or water. Intentional termination can occur through the addition of water or alcohol. Another method of termination, chain transfer, can occur when an agent can act as a Bronsted acid .In this case, the pKa value of the agent is similar to the conjugate acid of the propagating carbanionic chain end. Spontaneous termination occurs because the concentration of carbanion centers decay over time and eventually results in hydride elimination. Polar monomers are more reactive because they are stabilized by their polar substituents. These polar substituents can react with nucleophiles which results in termination as well as side reactions that compete with both initation and propagation.

This chain reaction cannot go on forever. The reaction must terminate, but how? A growing polymer chain joins with another free radical. We watched a peroxide break up to form two radicals. It makes sense that two free radicals could join to make a stable bond.

The equation representing this step of the chain reaction can be written simply as:

Remember: The R and R' groups here can be the original free radicals, the growing polymer chains, or even one of each. Termination reactions can, however, be more complicated looking.

An Important Note:Chemists can control the way a polymer does each of these steps by varying the reactants, the reaction times, and the reaction conditions.

The physical properties of a polymer chain depend on the polymer's average length, the amount of branching, and the constituent monomers.

This is an exciting and useful field of chemistry!

A Simulation of Addition Polymerization

In Part 4 of this tutorial, you saw that there are three steps in an addition polymerization chain reaction. You also saw that there are only two kinds of molecules in the chain reaction: the initiator molecule and the monomers. Polymerization begins at the initiator, and reaction continues until there are no more monomers to add to the growing polymer chain. The chain grows only at the reactive end, the end with the unpaired electron.

The simulation you will see displays this process graphically. Click on the button below to view the Simulation window. (If nothing happens, click here.)

The larger box in this window is the area where you'll see the polymerization of monomers, represented by black balls. The initiator molecules are in red. You input the number of initiators, press START, and the monomers will add onto the initiators linearly. As the average chain length increases, you'll see it displayed graphically in the smaller box. The graph is a bar graph of the average size of the polymer chain versus the reciprocal of the number of initiators (a red bar represents the active or most recent polymerization; a blue bar, the past polymerizations).

Note:You may not see the full length of the chain, but the numbers you see in and below the graph are correct for the given number of initiators.

Some Assumptions:First, we assume that the red initiator molecule is the activated monomer that you saw in Part 4. Second, we assume that there are only 200 monomers in the polymerization. In real life, the number of monomers are on the order of 1023. Despite the low number of monomers in the simulation, it does show the correct, real-life trend of how the number of initiators affects the average chain length. Third, polymerization is terminated when

the monomers run out. There is no visual coupling of free radicals; there are as many polymers as there are initiator molecules.

View the simulation several times with different numbers of initiators to see a trend in the bar graph. The more initiators, the shorter the chains (if there is a constant number of monomers).

Addition polymerization is not the only mechanism by which polymerization can occur.

Coordination polymerization is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center. The development of this polymerization technique started in the 1950s with heterogeneous Ziegler-Natta catalysts based on titanium tetrachloride and an aluminium co-catalyst such as methylaluminoxane. Coordination polymerization has a great impact on the physical properties of vinyl polymers such as polyethylene and polypropylene compared to the same polymers prepared by other techniques such as free radical polymerization. The polymers tend to be linear and not branched and have much higher molar mass. Coordination type polymers are also stereoregular and can be isotactic or syndiotactic instead of just atactic. This tacticity introduces crystallinity in otherwise amorphous polymers. From these differences in polymerization type the distinction originates between low density polyethylene (LDPE), high density polyethylene (HDPE) or even ultra high molecular weight polyethylene (UHMWPE). Polymerizations catalysed by metallocenes occur via the Cossee-Arlman mechanism.

In many applications Ziegler-Natta polymerization is succeeded by metallocene catalysis polymerization. This method is based on homogeneous metallocene catalysts such as the Kaminsky catalyst discovered in the 1970s. The 1990s brought forward a new range of post-metallocene catalysts.

Insertion of aluminum alkyls into olefins was studied by Ziegler:

Important discovery: R3Al + Lewis acids:

Another important discovery: tacticity control:

Results:

Nobel Prize in Chemistry for Zeigler and Natta (1963) Multibillion $ industry

A typical Ziegler-Natta catalyst can be produced by mixing solutions of titanium(IV) chloride (TiCl4) and triethylaluminum [Al(CH2CH3)3] dissolved in a hydrocarbon solvent from which both oxygen and water have been rigorously excluded. The product of this reaction is an insoluble olive-colored complex in which the titanium has been reduced to the Ti(III) oxidation state.

The catalyst formed in this reaction can be described as coordinately unsaturated because there is an open coordination site on the titanium atom. This allows an alkene to act as a Lewis base toward the titanium atom, donating a pair of electrons to form a transition-metal complex.

The alkene is then inserted into a Ti-CH2CH3 bond to form a growing polymer chain and a site at which another alkene can bond.

Thus, the titanium atom provides a template on which a linear polymer with carefully controlled stereochemistry can grow

Overall Scheme of Coordination Polymerization

Limited to ethylene and other a-olefins like propylene. (Actually, it is the only good way to polymerize these monomers.)

Produces linear polymer, with very few branches (e.g., high density polyethylene, HDPE).

Capable of producing homo-tactic polymers. Most commercial initiators are insoluble complexes or supported on insoluble

carriers. Very complex mechanism, still poorly understood for the heterogeneous systems.

Termination is almost exclusively by chain transfer. Modern "high mileage" initiators produce up to 1000's of kg per g initiator. Initiators are often called "catalysts" even though they are consumed by the

process. Many chains are started per molecule of initiator.

Mechanism of Coordination Polymerization

The mechanism is poorly understood because it takes place on the surface of an insoluble particle, a difficult situation to probe experimentally. The mechanism shown below is one of several models proposed to at least partially explain the action of the Ziegler-Natta systems, but it is only an approximation of the more complex process that actually occurs.

Hyperbranched polymers by coordination polymerization

Hyperbranched copolymers comprising at least one C2 -C20 α-monoolefin monomers and 0.2 to 20 mole % of at least one α,ω-non-conjugated diene monomers having 5 to 18 carbon atoms are prepared by coordination (metallocene) copolymerization of the monomers and quenching the reaction prior to the formation of a gelled product. The building blocks of the products are characterized by a number average molecular weight less than 5 times the entanglement molecular weight of a homopolymer prepared using the same catalyst but in the absence of the diene component.

Condensation polymers are any kind of polymers formed through a condensation reaction, releasing small molecules as by-products such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymers include polyamides, polyacetals and polyesters.

Condensation polymerization, a form of step-growth polymerization, is a process by which two molecules join together, resulting loss of small molecules which is often water. The type of end product resulting from a condensation polymerization is dependent on the number of functional end groups of the monomer which can react.

Monomers with only one reactive group terminate a growing chain, and thus give end products with a lower molecular weight. Linear polymers are created using monomers with two reactive end groups and monomers with more than two end groups give three dimensional polymers which are crosslinked.

Dehydration synthesis often involves joining monomers with an -OH (hydroxyl) group and a freely ionized -H on either end (such as a hydrogen from the -NH2 in nylon or proteins). Normally, two or more different monomers are used in the reaction. The bonds between the hydroxyl group, the hydrogen atom and their respective atoms break forming water from the hydroxyl and hydrogen, and the polymer.

Polyester is created through ester linkages between monomers, which involve the functional groups carboxyl and hydroxyl (an organic acid and an alcohol monomer).

Nylon is another common condensation polymer. It can be manufactured by reacting di-amines with carboxyl derivatives. In this example the derivative is a di-carboxylic acid, but di-acyl chlorides are also used. Another approach used is the reaction of di-functional monomers, with one amine and one carboxylic acid group on the same molecule:

The carboxylic acids and amines link to form peptide bonds, also known as amide groups. Proteins are condensation polymers made from amino acid monomers. Carbohydrates are also condensation polymers made from sugar monomers such as glucose and galactose.

Condensation polymerization is occasionally used to form simple hydrocarbons. This method, however, is expensive and inefficient, so the addition polymer of ethene (polyethylene) is generally used.

Condensation polymers, unlike addition polymers, may be biodegradable. The peptide or ester bonds between monomers can be hydrolysed by acid catalysts or bacterial enzymes breaking the polymer chain into smaller pieces.The most commonly known condensation polymers are proteins, fabrics such as nylon, silk, or polyester.

CondensationCondensation is an organic reaction when two molecules combine, usually in the presence of a catalyst, with the elimination of water or some other simple molecule. Catalysts commonly used in condensation reactions include acids and bases. The combination of two identical molecules is known as self-condensation. This process forms larger molecules, many of which are useful in organic synthesis. Aldehydes, ketones, esters, alkynes, and alcohols are among several organic compounds that combine with each other to form larger molecules.

Example:

CH3OH + CH3OH => CH3OCH3 + HOH

methanol + methanol => methoxymethane + water

For a carboxylic acid undergoing condensation reaction, it combines with another reactant, forming two products - an organic compound and the byproduct of water. In an event when a carboxylic acid reacts with an alochol to produce an ester and water, this process is called esterification.

Example:

CH3COOH + HOCH3 => CH3COOCH3 + HOH

ethanoic acid + methanol => methyl ethanoate + water

Polymerization is a process in which very small molecules, called monomers, combine chemically with each other to produce a very large chainlike molecule, called a polymer. The monomer molecules may be all alike, or they may represent two, three, or more different compounds. The monomers react to form a polymer without the formation of by-products such as water. The structure has one structural unit, or monomer, that occurs repeatedly. Through polymerization of ethylene (ethene), CH2CH2, the structure of the polymer can therefore be represented by -(CH2CH2)n- Where n can be several thousand. Because of this, polymers have incredible molar masses up to millions of grams per mole.

Example:

H H H H H H H H H H H H | | | | | | | | | | | | C=C + C=C + C=C => :C-C:C-C:C-C: OR -(CH2CH2)n- | | | | | | | | | | | | H H H H H H H H H H H H

ethylene part of polyethylene

In condensation polymerization, two functional groups of two different monomer molecules are joined together which produces a small molecule such as water. The monomers bond at where the hydrogen atoms were taken out to produce water. In order to become a condensed polymer, the monomer molecules must have at least two functional groups. The combination of two identical molecules is known as self-

condensation. The reaction between a carboxylic acid and an alcohol creates an ester. If the carboxylic acid and the alcohol were the monomers of the polymer, during polymerization, they would create polyester, and produce water. The polymerization of a carboxylic acid and an amine similarly creates polyamides.

Example:

COOHC6H6COOH + HOCH2CH2OH => COOHC6H6OCH2CH2OH + HOH

1,4-benzenedioic acid + 1,2-ethandiol => polyester + water

Condensation polymerization (part 1 of 2)

The bonds between monomer molecules are formed with the elimination of a small molecule, such as water. This is due to one of the monomers losing a hydrogen atom, and another losing a hydroxyl group. Each monomer usually has two functional groups of condensation polymers, the polyesters, and the polyamides. The condensation reaction: (diag.) Now consider some examples of polyesters; Polyethylene terephthalate (PET) is produced by the step-growth polymerization of ethylene glycol and terephthalic acid.

The presence of the large benzene rings in the repeating units, gives the polymer notable stiffness and strength, especially when the polymer chains are aligned with one another in an orderly arrangement by stretching. In this semi-crystalline form, PET is made into a high-strength textile fiber, where its stiffness makes them highly resistant to deformation, so that they have excellent resistance to wrinkling in fabrics.

Monomers: Functional Groups

The monomers that are involved in condensation polymerization are not the same as those in addition polymerization. The monomers for condensation polymerization have two main characteristics:.

Instead of double bonds, these monomers have functional groups (like alcohol, amine, or carboxylic acid groups).

Each monomer has at least two reactive sites, which usually means two functional groups.

Some monomers have more than two reactive sites, allowing for branching between chains, as well as increasing the molecular mass of the polymer. Four examples of these difunctional monomers were introduced in Part 2 of this tutorial. Here they are again:

Guess the names of each of these monomers. Give the letter that corresponds to the correct name of the structure (use each letter only once). Hints: Glycol means that a molecule has more than one alcohol (-OH) group. Amine means that a molecule has an amino (-NH2) group. Diamine (or diamino) means that a molecule contains two amino groups. Acid means that a molecule contains a carboxylic acid group (-COOH). Click the button when done.

Let's look again at the functional groups on these monomers. We've seen three:

The carboxylic acid group

The amino group

The alcohol group

You might have learned in chemistry or biology class that these groups can combine in such a way that a small molecule (often H2O) is given off.

The Amide Linkage:When a carboxylic acid and an amine react, a water molecule is removed, and an amide molecule is formed.

Because of this amide formation, this bond is known as an amide linkage.

The Ester Linkage:When a carboxylic acid and an alcohol react, a water molecule is removed, and an ester molecule is formed.

Because of this ester formation, this bond is known as an ester linkage.

In Summary:Monomers involved in condensation polymerization have functional groups. These functional groups combine to form amide and ester linkages. When this occurs, a water molecule in removed. Since water is removed, we call these reactions condensation reactions (water condenses out.). When a condensation reaction involves polymerization, we call it condensation polimarization.

Let's look at a few common examples of condensation polymers.

The Mechanism of Condensation Polymerization

You know that monomers that are joined by condensation polymerization have two functional groups. You also know (from Part 6) that a carboxylic acid and an amine can form an amide linkage, jand a carboxylic acid and an alcohol can form an ester linkage. Since each monomer has two reactive sites, they can form long-chain polymers by making many amide or ester links. Let's look at two examples of common polymers made from the monomers we have studied.

Example 1:A carboxylic acid monomer and an amine monomer can join in an amide linkage.

As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer.

Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide. The polymer made from these two six-carbon monomers is known as nylon-6,6. (Nylon products include hosiery, parachutes, and ropes.)

Example 2:A carboxylic acid monomer and an alcohol monomer can join in an ester linkage.

A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding. ( )

Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)

Let's summarize:As difunctional monomers join with amide and ester linkages, polyamides and polyesters are formed, respectively. We have seen the formation of the polyamide nylon-6,6 and the polyester PET. There are numerous other examples.

Remember: The above process is called condensation polymerization because a molecule is removed during the joining of the monomers. This molecule is frequently water.

A Simulation of Condensation Polymerization

You learned in Part 7 that condensation polymers are made from monomers that have at least two functional groups. Because of this, the polymers can grow at either end of the chain.

During the polymerization process, the monomers tend to form dimers (two linked monomers) and trimers (three linked monomers) first. Then, these very short chains react with each other and with monomers. The overall result is that, at the beginning of polymerization, there are many relatively short chains. It is only near the end of polymerization that very long chains are formed.

The simulation you will see displays this process graphically. Click on the button below to view the Simulation window. (If nothing happens, click here.)

The larger box in this window is the area where you'll see the polymerization of monomers, represented by gray balls. When you click START, you'll see a lattice of

gray, or unreacted, monomers. Once they polymerize into dimers, trimers, and so on, the monomers will turn black. Polymerization will continue for a few seconds. Then the display will change into a bar graph entitled "Distribution" and show the progression of the polymerization over time. The x-axis is the number of units in the polymer (the "n" in the formula of a polymer). This is suggested graphically with the series of polymers projected into the screen. As you move to the left, the polymers are longer. The y-axis is the number of polymers. The higher the bar, the more numerous are the polymers. The graph shows dynamically the distribution of polymers in the polymerization as the reaction progresses. Notice that at the beginning of the polymerization, the distribution lies farther to the right, meaning that there are a lot of monomers, dimers, trimers, and other short chains but few long chains. As the polymerization progresses, the distribution shifts to the left, indicating that there are fewer short chains and more of the longer ones.

The smaller box is a graph that displays the average size of the polymers versus the time of the polymerization. Again, notice that at early times, there are mostly short chains, and that near the end, there are more long.

Some Assumptions:First, we assume that there is only one type of difunctional monomer, as opposed to two types, as you saw in the two examples in Part 7. If you imagine that the polymers in the simulation are polyamides (like nylon-6,6), then the monomer has one carboxylic acid group and one alcohol group (picture the dimer you saw in Example 1 in the previous section). Second, we assume that there are only 90,000 monomers in the polymerization. In real life, the number of monomers is on the order of 1023. Despite the low number of monomers in the simulation, it does show the correct, real-life distribution of polymer chains over time

Step-growth polymerizationStep growth polymerization, also called condensation polymerization, refers to polymerizations in which bi-functional or multifunctional monomers react to form dimers, trimers, longer oligomers and eventually long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth polymerization. It requires a high extent of reaction to achieve high molecular weight, thus only a few kinds of commercial polymers can be synthesized this way, like polyester, polyamide, polyurethane,etc. The easiest way to visualize a step growth polymerization is a group of people holding hands to form a human chain: each person has two hands (=reactive sites).

A monomer with functionality 3 will introduce branching in a polymer and will ultimately form a cross-linked macrostructure or network even at low fractional conversion. The point at which this three-dimensional structure is formed is known as the gel point because it is signalled by an abrupt change in viscosity. One of the earliest so-

called thermosets is known as bakelite. It is not always water that is released in step-growth polymerization: in acyclic diene metathesis or ADMET dienes polymerize with loss of ethylene.

A generic representation of a step-growth polymerization. (Single white dots represent monomers and black chains represent oligomers and polymers)

Comparison of Molecular weight vs conversion plot between step-growth and chain-growth polymerization

This technique is usually compared with chain-growth polymerization to show its characteristics.

Step-growth polymerization Chain-growth polymerization

Growth throughout matrixGrowth by addition of monomer only at one end of

chain

Rapid loss of monomer early in the reaction Some monomer remains even at long reaction times

Same mechanism throughoutDifferent mechanisms operate at different stages of

reaction (i.e. Initiation, propagation and termination)

Average molecular weight increases slowly at low conversion and high extents of reaction are required

to obtain high chain length

Molar mass of backbone chain increases rapidly at early stage and remains approximately the same

throughout the polymerization

Ends remain active (no termination) Chains not active after termination

No initiator necessary Initiator required

Classes of step-growth polymers

Polyester has high Tg, high Tm, good mechanical properties to about 175°C, good resistance to solvent and chemicals. It can exist as fibers and films. The former is used in garments, felts, tire cord, etc. The latter appears in magnetic recording tape and high grade films.

Polyamide (nylon) has good balance of properties: high strength, good elasticity and abrasion resistance, good toughness, favorable solvent resistance. The applications of polyamide include: rope, belting, fiber cloths, thread, substitute for metal in bearings, jackets on electrical wire.

Polyurethane can exist as elastomers with good abrasion resistance, hardness, good resistance to grease and good elasticity, as fibers with excellent rebound, as coatings with

good resistance to solvent attack and abrasion and as foams with good strength, good rebound and high impact strength.

Polyurea shows high Tg, fair resistance to greases, oils and solvents. It can be used in truck bed liners, bridge coating, caulk and decorative designs.

Polysiloxane are available in a wide range of physical states-from liquids to greases, waxes, resins and rubbers. Uses of this material are as antifoam and release agents, gaskets, seals, cable and wire insulation, hot liquids and gas conduits, etc.

Polycarbonates are transparent, self-extinguishing materials. They possess properties like crystalline thermoplasticity, high impact strength, good thermal and oxidative stability. They can be used in machinery, auto-industry and medical applications. For example, the cockpit canopy of F-22 Raptor is made of high optical quality polycarbonate.

Polysulfides have outstanding oil and solvent resistance, good gas impermeability, good resistance to aging and ozone. However, it smells bad and it shows low tensile strength as well as poor heat resistance. It can be used in gasoline hoses, gaskets and places that require solvent resistance and gas resistance.

Polyether shows good thermoplastic behavior, water solubility, generally good mechanical properties, moderate strength and stiffness. It is applied in sizing for cotton and synthetic fibers, stabilizers for adhesives, binders, and film formers in pharmaceuticals.

Phenol formaldehyde resin (Bakelite) have good heat resistance, dimensional stability as well as good resistance to most solvents. It also shows good dielectric properties. This material is typically used in molding applications, electrical, radio, televisions and automotive parts where their good dielectric properties are of use. Some other uses include: impregnating paper, varnishes, decorative laminates for wall coverings.

Advances in step-growth Polymers The driving force in designing new polymers is the prospect of replacing other materials of construction especially metals using lightweight and heat-resistant polymers. The advantages of lightweight polymers include: high strength, solvent and chemical resistance, contributing to a variety of potential uses, such as electrical and engine parts on automotive and aircraft components, coatings on cookware, coating and circuit boards for electronic and microelectronic devices, etc. Polymer chains based on aromatic rings are desirable due to high bond strengths and rigid polymer chains. High molecular weight and crosslinking are desirable for the same reason. Strong dipole-dipole, hydrogen bond interactions and crystallinity also improve heat resistance. To obtain desired mechanical strength, sufficiently high molecular weights are necessary, however, decreased solubility

is a problem. One approach to solve this problem is to introduce of some flexibilizing linkages such as isopropylidene, C=O, and SO2 into the rigid polymer chain by using an appropriate monomer or comonomer. Another approach involves the synthesis of reactive telechelic oligomers containing functional end groups capable of reacting with each other, polymerization of the oligomer gives higher molecular weight, referred to as chain extension.

Aromatic polyether

The oxidative coupling polymerization of many 2,6-disubstituted phenols using a catalytic complex of a cuprous salt and amine form aromatic polyethers, commercially referred to as poly(p-phenylene oxide) or PPO. Neat PPO has little commercial uses due to its high melt viscosity. Its available products are blends of PPO with high-impact polystyrene (HIPS).

Polyethersulfone

Polyethersulfone (PES) is also referred to as polyetherketone, polysulfone. It is synthesized by nucleophilic aromatic substitution between aromatic dihalides and bisphenolate salts. Polyethersulfones are partially crystalline, highly resistant to a wide range of aqueous and organic environment. They are rated for continuous service at

temperatures of 240-280 oC. The polyketones are finding applications in areas like automotive, aerospace, electrical-electronic cable insulation.

Aromatic polysulfides

Poly(p-phenylene sulfide) (PPS) is synthesized by the reaction of sodium sulfide with [[p-dichlorobenzene]] in a polar solvent such as 1-methyl-2-pyrrolidinone (NMP). It is inherently flame resistant and stable toward organic and aqueous conditions, however, it is somewhat susceptible to oxidants. Applications of PPS include automotive, microwave oven component, coating for cookware when blend with fluorocarbon polymers and protective coatings for valves, pipe, electromotive cells etc.

Aromatic polyimide

Aromatic polyimide are synthesized by the reaction of dianhydrides with diamines, for example, pyromellitic anhydride with [[p-phenylenediamine]]. It can also be accomplished using diisocyanates in place of diamines. Solubility considerations sometimes result in using the half acid-half ester of the dianhydride instead of the dianhydride. Polymerization is accomplished by a two-stage process due to insolubility of polyimdes. The first stage forms a soluble and fusible high-molecular-weight poly(amic acid) in a polar aprotic solvent such as NMP or N,N-dimethylacetamide. The poly(amic aicd) can then be processed into the desired physical form of the final plymer product (e.g., film, fiber, laminate, coating) which is insoluble and infusible.

Telechelic oligomer approach

Telechelic oligomer approach applies the usual polymerization manner except that one includes a monofunctional reactant to stop reaction at the oligomer stage, generally in the 50-3000 molecular weight. The monofunctional reactant not only limits polymerization but end-caps the oligomer with functional groups capable of subsequent reaction to achieve curing of the oligomer. Functional groups like alkyne, norbornene, maleimide, nitrite, and cyanate have been used for this purpose. Maleimide and norbornene end-capped oligomers can be cured by heating. Alkyne, nitrile and cyanate end-capped oligomers can undergo cyclotrimerization yielding aromatic structures.

Thermoplastic

A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers

(Bakelite; vulcanized rubber) as they can, unlike thermosetting polymers, be remelted and remoulded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene.

Stress strain graph of thermoplastic material.

Thermoplastics are elastic and flexible above a glass transition temperature Tg, specific for each one — the midpoint of a temperature range in contrast to the sharp melting point and melting point of a pure crystalline substance like water. Below a second, higher melting temperature, Tm, also the midpoint of a range, most thermoplastics have crystalline regions alternating with amorphous regions in which the chains approximate random coils. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity, as is also the case for non-thermoplastic fibrous proteins such as silk. (Elasticity does not mean they are particularly stretchy; e.g., nylon rope and fishing line.) Above Tm all crystalline structure disappears and the chains become randomly inter dispersed. As the temperature increases above Tm, viscosity gradually decreases without any distinct phase change.

Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name. This quality makes thermoplastics recyclable. The processes required for recycling vary with the thermoplastic. The plastics used for soda bottles are a common example of thermoplastics that can be and are widely recycled. Animal horn, made of the protein α-keratin, softens on heating, is somewhat reshapable, and may be regarded as a natural, quasi-thermoplastic material.

Some thermoplastics normally do not crystallize: they are termed "amorphous" plastics and are useful at temperatures below the Tg. They are frequently used in applications where clarity is important. Some typical examples of amorphous thermoplastics are PMMA, PS and PC. Generally, amorphous thermoplastics are less chemically resistant and can be subject to stress cracking. Thermoplastics will crystallize to a certain extent and are called "semi-crystalline" for this reason. Typical semi-crystalline thermoplastics are PE, PP, PBT and PET. The speed and extent to which crystallization can occur depends in part on the flexibility of the polymer chain. Semi-crystalline thermoplastics are more resistant to solvents and other chemicals. If the crystallites are larger than the wavelength of light, the thermoplastic is hazy or opaque. Semi-crystalline thermoplastics become less brittle above Tg. If a plastic with otherwise desirable properties has too high a Tg, it can often be lowered by adding a low-molecular-weight plasticizer to the melt before forming (Plastics extrusion; molding) and cooling. A similar result can sometimes

be achieved by adding non-reactive side chains to the monomers before polymerization. Both methods make the polymer chains stand off a bit from one another. Before the introduction of plasticizers, plastic automobile parts often cracked in cold winter weather. Another method of lowering Tg (or raising Tm) is to incorporate the original plastic into a copolymer, as with graft copolymers of polystyrene, or into a composite material. Lowering Tg is not the only way to reduce brittleness. Drawing (and similar processes that stretch or orient the molecules) or increasing the length of the polymer chains also decrease brittleness.

Although modestly vulcanized natural and synthetic rubbers are stretchy, they are elastomeric thermosets, not thermoplastics. Each has its own Tg, and will crack and shatter when cold enough so that the crosslinked polymer chains can no longer move relative to one another. But they have no Tm and will decompose at high temperatures rather than melt. Recently, thermoplastic elastomers have become available.

Terminology

The literature on thermoplastics is huge, and can be quite confusing, as the same chemical can be available in many different forms (for example, at different molecular weights), which might have quite different physical properties. The same chemical can be referred to by many different tradenames, by different abbreviations; two chemical compounds can share the same name; a good example of the latter is the word "Teflon" which is used to refer to a specific polymer (PTFE); to related polymers such as PFA, and generically to fluoropolymers.

Furthermore, over the last 30 years, there has been tremendous change in the plastics industry, with many companies going out of business or merging into other companies. Many production plants frequently changed hands or have been relocated to emerging countries in Eastern Europe or Asia, with different trademarks.

Testing

Testing of thermoplastics can take various forms.

Tensile tests — ISO 527 -1/-2 and ASTM D 638 set out the standardized test methods. These standards are technically equivalent. However they are not fully comparable because of the difference in testing speeds. The modulus determination requires a high accuracy of ± 1 micrometer for the dilatometer.

Flexural tests — 3-points flexural tests are among the most common and classic methods for semi rigid and rigid plastics.

Pendulum impact tests — impact tests are used to measure the behavior of materials at higher deformation speeds. Pendulum impact testers are used to determine the energy

required to break a standardized specimen by measuring the height to which the pendulum hammer rises after impacting the test piece.

Table of thermoplastics

Polymer Melting point

Acrylonitrile butadiene styrene (ABS)  -

Acrylic (PMMA) 130–140 °C

Celluloid  -

Polycaprolactone (PCL) 62 °C

Polyethylene (PE) 105-130 °C

Polyphenylene oxide (PPO)  -

Polyimide (PI)  -

Polymethylpentene (PMP)  -

Polylactic acid (PLA) 50-80 °C

Polyphenylene sulfide (PPS)  -

Polyphthalamide (PPA)  -

Polypropylene (PP)  -

Polystyrene (PS) 240 °C

Polyurethane (PU)  -

Polyvinyl acetate (PVA)  -

Polyvinyl chloride (PVC) 80 °C

Polylactic acid

Polylactic acid or polylactide (PLA) is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources, such as corn starch (in the U.S.) or sugarcanes (rest of world). Although PLA has been known for more than a century, it has only been of commercial interest in recent years, in light of its biodegradability.

Chemical and physical properties

Due to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known

as L-lactide). PLLA has a crystallinity of around 37%, a glass transition temperature between 50-80 °C and a melting temperature between 173-178 °C.

Polylactic acid can be processed like most thermoplastics into fiber (for example using conventional melt spinning processes) and film. The melting temperature of PLLA can be increased 40-50 °C and its heat deflection temperature can be increased from approximately 60°C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular stereocomplex with increased crystallinity. The temperature stability is maximised when a 50:50 blend is used, but even at lower concentrations of 3-10% of PDLA, there is still a substantial improvement. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA. PDLA has the useful property of being optically transparent.

Applications

Biodegradable plastic cups in use at an eatery.

Stereocomplex blends of PDLA and PLLA have a wide range of applications, such as woven shirts (ironability), microwavable trays, hot-fill applications and even engineering plastics (in this case, the stereocomplex is blended with a rubber-like polymer such as ABS). Such blends also have good form-stability and visual transparency, making them useful for low-end packaging applications. Progress in bio-technology has resulted in the development of commercial production of the D(-) form, something that was not possible until recently.

PLA is currently used in a number of biomedical applications, such as sutures, stents, dialysis media and drug delivery devices. It is also being evaluated as a material for tissue engineering. Because it is biodegradable, it can also be employed in the preparation of bioplastic, useful for producing loose-fill packaging, compost bags, food packaging, and disposable tableware. In the form of fibers and non-woven textiles, PLA also has many potential uses, for example as upholstery, disposable garments, awnings, feminine hygiene products, and nappies.

PLA has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersomes.

PLA is a sustainable alternative to petrochemical-derived products, since the lactides from which it is ultimately produced can be derived from the fermentation of agricultural by-products such as corn starch[1] or other carbohydrate-rich substances like maize, sugar or wheat.

PLA is more expensive than many petroleum-derived commodity plastics, but its price has been falling as production increases. The demand for corn is growing, both due to the use of corn for bioethanol and for corn-dependent commodities, including PLA.

PLA has also been developed in the United Kingdom to serve as sandwich packaging

Thermosetting polymer

Thermosetting plastics (thermosets) are polymer materials that irreversibly cure. The cure may be done through heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.

Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC's).

Process

The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.

Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.[1]

Statistics

Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature.

Examples

Some examples of thermosets are:

Polyester fiberglass systems: (SMC Sheet molding compounds and BMC Bulk molding compounds)

Vulcanized rubber Bakelite , a phenol-formaldehyde resin (used in electrical insulators and

plasticware) Duroplast , similar to Bakelite Urea-formaldehyde foam (used in plywood, particleboard and medium-density

fibreboard) Melamine resin (used on worktop surfaces) Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass

reinforced plastic and graphite-reinforced plastic) Polyimides (used in printed circuit boards and in body parts of modern airplanes) Mold or Mold Runners (the black plastic part in Integrated Circuits (IC) or

semiconductors)

Some methods of molding thermosets are:

Reactive injection molding (used for objects like milk bottle crates) Extrusion molding (used for making pipes, threads of fabric and insulation for

electrical cables) Compression molding (used to shape most thermosetting plastics) Spin casting (used for producing fishing lures and jigs, gaming miniatures,

figurines, emblems as well as production and replacement parts

Thermoplastics vs Thermosetting

Thermoplastics and thermosetting plastics are terms that describe how a polymer reacts to heat. All plastics, whether made by addition or condensation polymerization, can be divided into two groups: thermoplastics and thermosetting plastics. Thermoplastics can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics, on the other hand, harden permanently after being heated once.

The Difference - Weak Van Der Waal ForcesThe reason for the difference in response to heat between thermoplastics and

thermosetting plastics lies in the chemical structures of the plastics. Thermoplastic molecules, which are linear or slightly branched, do not chemically bond with each other when heated. Instead, thermoplastic chains are held together by weak van der Waal forces (weak attractions between the molecules) that cause the long molecular chains to clump together like piles of entangled spaghetti. Thermoplastics can be heated and cooled, and consequently softened and hardened, repeatedly, like candle wax. For this reason, thermoplastics can be remolded and reused almost indefinitely.

Thermosetting Plastics Thermosetting plastics consist of chain molecules that chemically bond, or cross-link, with each other when heated. When thermosetting plastics cross-link, the molecules create a permanent, three-dimensional network that can be considered one giant molecule. Once cured, thermosetting plastics cannot be remelted, in the same way that cured concrete cannot be reset. Consequently, thermosetting plastics are often used to make heat-resistant products, because these plastics can be heated to temperatures of 260° C (500° F) without melting.

Thermoplastics The different molecular structures of thermoplastics and thermosetting plastics allow manufacturers to customize the properties of commercial plastics for specific applications. Because thermoplastic materials consist of individual molecules, properties of thermoplastics are largely influenced by molecular weight. For instance, increasing the molecular weight of a thermoplastic material increases its tensile strength, impact strength, and fatigue strength (ability of a material to withstand constant stress). Conversely, because thermosetting plastics consist of a single molecular network, molecular weight does not significantly influence the properties of these plastics. Instead, many properties of thermosetting plastics are determined by adding different types and amounts of fillers and reinforcements, such as glass fibers.

The Processes of Making PlasticsThe process of forming plastic resins into plastic products is the basis of the plastics industry. Many different processes are used to make plastic products, and in each process, the plastic resin must be softened or sufficiently liquefied to be shaped. Although some processes are used to manufacture both thermoplastics and thermosetting plastics, certain processes are specific to forming thermoplastics.

Injection Molding Injection molding uses a piston or screw to force plastic resin through a heated tube into a mold, where the plastic cools and hardens to the shape of the mold. The mold is then opened and the plastic cast removed. Thermoplastic items made by injection molding include toys, combs, car grills, and various containers.

Extrusion Extrusion is a continuous process, as opposed to all other plastic production processes, which start over at the beginning of the process after each new part is removed from the mold. In the extrusion process, plastic pellets are first heated in a long barrel. In a manner similar to that of a pasta-making or sausage-stuffing machine, a rotating screw then forces the heated plastic through a die (device used for forming material) opening of the desired shape.

As the continuous plastic form emerges from the die opening, it is cooled and solidified, and the continuous plastic form is then cut to the desired length. Plastic products made by extrusion include garden hoses, drinking straws, pipes, and ropes. Melted thermoplastic forced through extremely fine die holes can be cooled and woven into fabrics for clothes, curtains, and carpets.

Blow Molding Blow molding is used to form bottles and other containers from soft, hollow thermoplastic tubes. First a mold is fitted around the outside of the softened thermoplastic tube, and then the tube is heated. Next, air is blown into the softened tube (similar to inflating a balloon), which forces the outside of the softened tube to conform to the inside walls of the mold. Once the plastic cools, the mold is opened and the newly molded container is removed. Blow molding is used to make many plastic containers, including soft-drink bottles, jars, detergent bottles, and storage drums.

Natural rubberNatural rubber is an elastomer (an elastic hydrocarbon polymer) that was originally derived from a milky colloidal suspension, or latex, found in the sap of some plants. The purified form of natural rubber is the chemical polyisoprene which can also be produced synthetically. Natural rubber is used extensively in many applications and products as is synthetic rubber. The entropy model of rubber was developed in 1934 by Werner Kuhn.

Latex being collected from a tapped rubber tree

Varieties

The major commercial source of natural rubber latex is the Para rubber tree (Hevea brasiliensis), a member of the spurge family, Euphorbiaceae. This is largely because it responds to wounding by producing more latex.

Other plants containing latex include Gutta-Percha (Palaquium gutta),[1] rubber fig (Ficus elastica), Panama rubber tree (Castilla elastica), spurges (Euphorbia spp.), lettuce, common dandelion (Taraxacum officinale), Russian dandelion (Taraxacum kok-saghyz), Scorzonera tau-saghyz, and Guayule (Parthenium argentatum). Although these have not been major sources of rubber, Germany attempted to use some of these during World War II when it was cut off from rubber supplies[citation needed]. These attempts were later supplanted by the development of synthetic rubbers. To distinguish the tree-obtained version of natural rubber from the synthetic version, the term gum rubber is sometimes used.

Discovery of commercial potential

Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736.[2] In 1751, he presented a paper by François Fresneau to the Académie (eventually published in 1755) which described many of the properties of rubber. This has been referred to as the first scientific paper on rubber.[2]

The para rubber tree initially grew in South America, and the first European to return to Portugal from Brazil with samples of water-repellent rubberized cloth so shocked people that he was brought to court on the charge of witchcraft. When samples of rubber first arrived in England, it was observed by Joseph Priestley, in 1770, that a piece of the

material was extremely good for rubbing out pencil marks on paper, hence the name rubber.

South America remained the main source of what limited amount of latex rubber was consumed during much of the 19th century. However in 1876, Henry Wickham gathered thousands of seeds from Brazil, and these were germinated in Kew Gardens, UK. The seedlings were then sent to Ceylon (Sri Lanka), Indonesia, Singapore and British Malaya. Malaya (now Malaysia) was later to become the biggest producer of rubber. About 100 years ago, the Congo Free State in Africa was also a significant source of natural rubber latex, mostly gathered by forced labor. Liberia and Nigeria also started production of rubber.

In India, commercial cultivation of natural rubber was introduced by the British Planters, although the experimental efforts to grow rubber on a commercial scale in India were initiated as early as 1873 at the Botanical Gardens, Kolkata. The first commercial Hevea plantations in India were established at Thattekadu in Kerala in 1902.

PropertiesRubber latex.

Rubber exhibits unique physical and chemical properties. Rubber's stress-strain behavior exhibits the Mullins effect, the Payne effect and is often modeled as hyperelastic. Rubber strain crystallizes.

Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking.

Solvents

There are two main solvents for rubber: turpentine and naphtha (petroleum). The former has been in use since 1763 when Francois Fresnau made the discovery. Giovanni Fabronni is credited with the discovery of naphtha as a rubber solvent in 1779. Because rubber does not dissolve easily, the material is finely divided by shredding prior to its immersion.An ammonia solution can be used to prevent the coagulation of raw latex while it is being transported from its collection site.

Chemical makeup

Natural rubber is a polymer of isoprene - most often cis-1,4-polyisoprene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials are found in natural rubber.

Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber".

Some natural rubber sources called gutta percha are composed of trans-1,4-polyisoprene, a structural isomer which has similar, but not identical properties.

Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both; i.e., if it is heated and cooled, it is degraded but not destroyed.

Elasticity

In most elastic materials, such as metals used in springs, the elastic behavior is caused by bond distortions. When force is applied, bond lengths deviate from the (minimum energy) equilibrium and strain energy is stored electrostatically. Rubber is often assumed to behave in the same way, but it turns out this is a poor description. Rubber is a curious material because, unlike metals, strain energy is stored thermally.

In its relaxed state rubber consists of long, coiled-up polymer chains that are interlinked at a few points. Between a pair of links each monomer can rotate freely about its neighbour. This gives each section of chain leeway to assume a large number of geometries, like a very loose rope attached to a pair of fixed points. At room temperature rubber stores enough kinetic energy so that each section of chain oscillates chaotically, like the above piece of rope being shaken violently.

When rubber is stretched the "loose pieces of rope" are taut and thus no longer able to oscillate. Their kinetic energy is given off as excess heat. Therefore, the entropy decreases when going from the relaxed to the stretched state, and it increases during relaxation. This change in entropy can also be explained by the fact that a tight section of chain can fold in fewer ways (W) than a loose section of chain, at a given temperature (nb. entropy is defined as S=k*ln(W)). Relaxation of a stretched rubber band is thus driven by an increase in entropy, and the force experienced is not electrostatic, rather it is a result of the thermal energy of the material being converted to kinetic energy. Rubber relaxation is endothermic, and for this reason the force exerted by a stretched piece of rubber increases with temperature (metals, for example, become softer as temperature increases). The material undergoes adiabatic cooling during contraction. This property of rubber can easily be verified by holding a stretched rubber band to your lips and relaxing it.

Stretching of a rubber band is in some ways equivalent to the compression of an ideal gas, and relaxation is equivalent to its expansion. Note that a compressed gas also exhibits "elastic" properties, for instance inside an inflated car tire. The fact that stretching is equivalent to compression may seem somewhat counter-intuitive, but it makes sense if rubber is viewed as a one-dimensional gas. Stretching reduces the "space" available to each section of chain.

Vulcanization of rubber creates more disulfide bonds between chains so it makes each free section of chain shorter. The result is that the chains tighten more quickly for a given length of strain. This increases the elastic force constant and makes rubber harder and less extendable.

When cooled below the glass transition temperature, the quasi-fluid chain segments "freeze" into fixed geometries and the rubber abruptly loses its elastic properties, though the process is reversible. This is a property it shares with most elastomers. At very cold temperatures rubber is actually rather brittle; it will break into shards when struck or stretched. This critical temperature is the reason that winter tires use a softer version of rubber than normal tires. The failing rubber o-ring seals that contributed to the cause of the Challenger disaster were thought to have cooled below their critical temperature. The disaster happened on an unusually cold day.

Current sources

Close to 21 million tons of rubber were produced in 2005 of which around 42% was natural. Since the bulk of the rubber produced is the synthetic variety which is derived from petroleum, the price of even natural rubber is determined to a very large extent by the prevailing global price of crude oil[citation needed]. Today Asia is the main source of natural rubber, accounting for around 94% of output in 2005. The three largest producing countries (Indonesia, Malaysia and Thailand) together account for around 72% of all natural rubber production.

Cultivation

Rubber is generally cultivated in large plantations. See the coconut shell used in collecting latex, in plantations in Kerala, India

Rubber latex is extracted from Rubber trees. The economic life period of rubber trees in plantations is around 32 years – up to 7 years of immature phase and about 25 years of productive phase.

The soil requirement of the plant is generally well-drained weathered soil consisting of laterite, lateritic types, sedimentary types, nonlateritic red or alluvial soils.

The climatic conditions for optimum growth of Rubber trees consist of (a) Rainfall of around 250 cm evenly distributed without any marked dry season and with at least 100 rainy days per annum (b) Temperature range of about 20°C to 34°C with a monthly mean of 25°C to 28°C (c) High atmospheric humidity of around 80% (d) Bright sunshine amounting to about 2000 hours per annum at the rate of 6 hours per day throughout the year and (e) Absence of strong winds.

Many high-yielding clones have been developed for commercial planting. These clones yield more than 2,000 kilograms of dry Rubber per hectare per annum, when grown under ideal conditions.

Collection

In places like Kerala, where coconuts are in abundance, the shell of half a coconut is used as the collection container for the latex but glazed pottery or aluminium cups are more common elsewhere. The cups are supported by a wire that encircles the tree.This wire incorporates a spring so that it can stretch as the tree grows. The latex is led into the cup by a galvanised "spout" that has been knocked into the bark. Tapping normally takes place early in the morning when the internal pressure of the tree is highest. A good tapper can tap a tree every 20 seconds on a standard half-spiral system and a common daily "task" size is between 450 and 650 trees. Trees are usually tapped alternate or third daily although there are many variations in timing, length and number of cuts. The latex, which contains 25 - 40% dry rubber, is in the bark so the tapper must avoid cutting right through to the wood or the growing cambial layer will be damaged and the renewing bark will be badly deformed making later tapping difficult. It is usual to tap a pannel at least twice, sometimes three times, during the trees' life. The economic life of the tree depends on how well the tapping is carried out as the critical factor is bark consumption. A standard in Malaysia for alternate daily tapping is 25 cm (vertical) bark consumption per annum. The latex tubes in the bark ascend in a spiral to the right. For this reason, tapping cuts usually ascend to the left to cut more tubes.

A tree woman in Sri Lanka in the process of harvesting rubber

The trees will drip latex for about four hours, stopping as latex coagulates naturally on the tapping cut thus blocking the latex tubes in the bark. Tappers usually rest and have a meal after finishing their tapping work then start collecting the latex at about midday. Some trees will continue to drip after the collection and this leads to a small amount of cup lump which is collected at the next tapping. The latex that coagulates on the cut is also collected as tree lace. Tree lace and cup lump together account for 10 - 20% of the dry rubber produced.

The latex can be collected in its liquid state. It is sometimes necessary to add a few drops of ammonia solution to the cup, or to the transport tank, to prevent precoagulation of the latex before it reaches the factory. It can also be left in the cup to coagulate naturally into cup lump for collection before the next tapping, although this will produce a lower grade of product.

Latex is generally processed into either latex concentrate for manufacture of dipped goods or it can be coagulated under controlled, clean conditions using formic acid. The coagulated latex can then be processed into the higher grade technically specified block rubbers such as TSR3L or TSRCV or used to produce Ribbed Smoke Sheet grades.

Naturally coagulated rubber (cup lump) is used in the manufacture of TSR10 and TSR20 grade rubbers. The processing of the rubber for these grades is basically a size reduction and cleaning process in order to remove contamination and prepare the material for the final stage drying.

The dried material is then baled and palletized for shipment.

Uses

Compression molded (cured) rubber boots before the flashes are removed.

The use of rubber is widespread, ranging from household to industrial products, entering the production stream at the intermediate stage or as final products. Tires and tubes are the largest consumers of rubber, accounting for around 56% total consumption in 2005. The remaining 44% are taken up by the general rubber goods (GRG) sector, which includes all products except tires and tubes.

Pre-historical uses

The first use of rubber was natural latex from the Hevea Tree in 1600 BC by the Ancient Mayans[citation needed]. They boiled the harvested latex to make a ball for sport

Manufacturing

Other significant uses of rubber are door and window profiles, hoses, belts, matting, flooring and dampeners (anti-vibration mounts) for the automotive industry in what is known as the "under the bonnet" products. Gloves (medical, household and industrial) are also large consumers of toy balloons and rubber, although the type of rubber used is that of the concentrated latex. Significant tonnage of rubber is used as adhesives in many manufacturing industries and products, although the two most noticeable are the paper and the carpet industry. Rubber is also commonly used to make rubber bands and pencil erasers.

Textile applications

Additionally, rubber produced as a fiber sometimes called elastic, has significant value for use in the textile industry because of its excellent elongation and recovery properties. For these purposes, manufactured rubber fiber is made as either an extruded round fiber or rectangular fibers that are cut into strips from extruded film. Because of its low dye acceptance, feel and appearance, the rubber fiber is either covered by yarn of another

fiber or directly woven with other yarns into the fabric. In the early 1900’s, for example, rubber yarns were used in foundation garments. While rubber is still used in textile manufacturing, its low tenacity limits its use in lightweight garments because latex lacks resistance to oxidizing agents and is damaged by aging, sunlight, oil, and perspiration. Seeking a way to address these shortcomings, the textile industry has turned to Neoprene (polymer form of Chloroprene), a type of synthetic rubber as well as another more

commonly used elastomer fiber, spandex (also known as elastane), because of their superiority to rubber in both strength and durability.

VulcanizationMain article: Vulcanization

Natural rubber is often vulcanized, a process by which the rubber is heated and sulfur, peroxide or bisphenol are added to improve resilience and elasticity, and to prevent it from perishing. Vulcanization greatly improved the durability and utility of rubber from the 1830s on[citation needed]. The development of vulcanization is most closely associated with Charles Goodyear[3]. Carbon black is often used as an additive to rubber to improve its strength, especially in vehicle tires.

Allergic reactionsMain article: Latex allergy

Some people have a serious latex allergy, and exposure to certain natural rubber latex products such as latex gloves can cause anaphylactic shock. Guayule latex is hypoallergenic and is being researched as a substitute to the allergy-inducing Hevea latexes. Unlike the sappable Hevea tree, these relatively small shrubs must be harvested whole and latex extracted from each cell.

Some allergic reactions are not from the latex but from residues of other ingredients used to process the latex into clothing, gloves, foam, etc. These allergies are usually referred to as multiple chemical sensitivity (MCS).

Objective Questions

1. A copolymer has the following repeat unit: -{CH2-CHCI- CH2-CH=CH- CH2}- Which pair of monomers could be used to make this polymer?

A. CH2 = CHCI and CH2 = CH2

B. CH2 = CHCI and CH2 = CH-CH= CH2

C. CH3 - CH2CI and CH3 – CH= CH- CH3

D. CH2 =CCI-CH= CH2 and CH2 = CH2

Answer: B

2 Natural rubber is made up of isoprene monomer units with the structural formula: CH2 = CH(CH3) CH= CH2

Which of the following about rubber and its monomer is not true?

A The IUPAC name for isoprene is 2- methylbuta- 1,3- diene

B Raw rubber is soft and sticky when warmed

C Bulk rubber is a mixture of cis- and trans- polyisoprene

D Bulk rubber can be strenghten by vulcanization

Answer: C (Bulk rubber is raw natural rubber.Natural rubber consists only of cis- polyisoprene

molecular chains)

3. The sequences of reactions below shows how benzene is used to manufacture poly (phenylethene)

benzene phenylethane phenylethene poly(phenylethene) Which of the following statements can be used to explain the above process? 1. Using a suitable catalyst, ethene can be substituted into benzene to form phenylbenzene

2. Phenylethene is obtained from phenylethene by dehydrogenatio

3. The repeat unit in poly(phenylethene) is

Answer: B [ only1 and 2 are correct] ( Another name for poly(phenylethene) is polystrene)

4. Clerfilm is manufactured from a polymer made by copolymerizing CH2 = CHCI with. CH2 =CCI2 in a regular ‘ head to tall’ linkage, where CH2 is taken as the ‘ head’ of the monomer. Which of the following could represent parts of the polymer chain in ‘clearfilm’?

1. -CHCI-CH2-CCI2-

2. - CCI2- CCI2- CH2- CHCI

3. - CH2- CHCI- CCI2

Answer: A [ only 1 is correct](Only one arrangement for the length of the polymer is possible, - CH2- CHCI- CH2- CCI2- CH2- CHCI)

5. A part of a certain polymeric chain is shown below.

Which of the following statements about this polyner are correct?

1. Its monomer is

2. It’s empirical formula is C2H2CI

3. It has geometrical isomers

Answer: Conly 2 and 3 are correct] (Its monomer is )

6. PET or polyethreneterephthalate is the most widely used packing polymer. PET can be categorized as a

1. polyethene type polymer

2. condensation polymer

3. polyster

Answer: D [1,2 and 3 are correct]

Subjective Questions

1. The following scheme shows how nylon 6,6 is manufactured starting from phenol H2

OH OH O Catalyst J

K L

Oxidation

CO2H (CH2)4 + M Nylon 6,6

CO2H

a.) Write a balanced equation for the reaction between phenol and hydrogen.

OH OH

+3H2

b.) Name or write the formula for the i.) Catalyst J Answer: Nickel / Platinum

ii.) Compond M Answer: Hexane- 1,6 – diamine

c.) Draw the structural formula of the repeat unit in nylon 6,6

d.) Describe a simple chemical test to distingiuish between the compounds K and L Answer: Add phosphorus pentachloride. K produces dense white fumes.

OH CI

+ 5PCI5 + POCI3 + HCI White fumes

No changes are seen with LComment:Tests on the carbohynl group in L can also be accepted

e.) Which of the 2 componds K and L will have a higher boiling point? Explain your answer?

Answer: K. The alcohol K is capable of self hydrogen bonding which raises its boiling point. L is not capable of hydrogen bonding.

2.) a) Polymers can be divided into thermosets, thermoplastics and elastomers. i) What is a thermoplastic polymer? Answer: Thermoplastic polymers soften on warning and can be remoulded.

Structurally, thermoplastic polymers do not have cross-link between polymeric chains.

ii.) Give an example of a thermoplastic polymer? Answer: PE / PS / PVS / PET / NYLON

b.) Natural rubber is a elastomer. i.) What is meant by the term elastomer? Answer: An elastomer is a polymer that can be streched to at least twice its

original length and rapidly contracts to its original length when released .

ii.) What is the monomer unit in natural rubber? Answer: The monomer unit is natural rubber is 2- methylbutadiene or more

commonly known as isoprene.

c.) Poly( buta-1,3 – diene) is a synthetic rubber. It exsists as a mixture of geometrical isomers, is sticky and is not useful material. What is added to synthetic rubber to give it more resilient properties?

Answer: Sulphur

d.) SBR is a synthetic rubber used in the automobile industry. Name the monomers that are used to make SBR.

Answer: Phenylethene / Styrene and buta – 1,3 – diene

3.) a.) Explain the following terms, using suitable examples . i.) Addition Polymerization Answer: Addition Polymerization is the reaction of alkenes or functionally –

substituted alkene monomers by addition to form polymers. It proceeds by way of chain-growth mechanisms involving reactive intermediates which my be a free radical, an anion or a cation.

ii.) Condensation Polymerization Answer: Condensation Polymerization is the formation of a polymer by

functional group reaction which often occur with loss of a small by- product such as water. Reaction between the functional groups usually gives it an alternate structure.

b.) How do the properties of a polymer change under the follwing conditions? i.) Lenghtening the polymer

Answer: The softening temperature of the polymer can be raised by lengthening. Soft and sticky polymers can be made more rigid.

ii.) Forming cross-links Answer: Moderate cross-linking results in a more elastic and resilient polymer.

Generally, a cross-linked polymer is thermoset and cannot be reshapad

iii.) Adding a polymer Answer: Adding a copolymer blends the properties of polymers which are

often better than either of the orginal polymers.

iv.) Adding a plasticizer Answer: A plastizer is added to incrase the flexbility and durability of a

polymer.

CONCLUSION

We have attempted to provide an overview of the medical device uses of biodegradable polymers. While sutures were the first commercial product and still account for the vast majority of all sales, a variety of products are now on the market for an expanding range of applications, with others certain to appear in the next decade.

What is it about these materials that makes them so attractive to the device industry? First, in this conservative field, where devices serve critical, perhaps life-and-death, functions, the industry is slow to accept new materials or new designs. The polymers prepared from these materials, particularly lactide and glycolide, have a long history of safe and effective use. Building on this solid foundation, researchers will continue to evaluate these materials, taking advantage of the wide range of properties that can be obtained in polymers built with relatively few monomer units. We expect that, in the future even more than today, device designers and physicians will have available a wealth

of products using biodegradable polymers that will help speed patient recovery and eliminate follow-up surgeries.

"Polymers." University of Illinois, Urbana/Champaign,Materials Science and Technology Department (Web site). <http://matse1.mse.uiuc.edu/~tw/polymers/polymers.html> (June 5, 2001)

"Polymers and Liquid Crystals." Case Western ReserveUniversity (Web site). <http://abalone.cwru.edu/> (June 5, 2001)

Plastics.com (Web site). <http://www.plastics.com> (June5, 2001)

Plastics 101." Plastics Resource (Web site). <http://www2.plasticsresource.com/plastics_101/> (June 5, 2001

http://www.smithsonianmag.com/science-nature/plastic.html

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