Polymers

Novel Polymers

Polymers (macromolecules) are long chain organic molecules created by manyrepeating units called monomers. E.g., necklace made from many small beads (monomers). Many synthetic polymers - called plastic originate from Greek word plastikos means suitable for molding, shaping or malleable. Molecular weight is important in determining many physical properties. For examples, transitions temperatures for liquids -- waxes -- rubbers -- solids and mechanical properties as stiffness, strength, viscoelasticity, toughness, and viscosity.

Applications: common house hold like utensils to replacement of human body parts, packing, wrapping, building materials , textiles, TV's, CD's,automobiles, coat metal wires and many others.Natural polymers: Proteins - silk, collagen, keratin. Carbohydrates - cellulose, starch, glycogen DNA - RNARubber (hydrocarbon base) and silicones (alternating silicon and oxygen). Hideki Shirakawa said that polymer (polyacetylene) can be made conductive almost like a metal.

What is Polymerization? Monomers------- polymers------

Common Polyolefins

Polyesters, Amides, and Urethanes

Natural Polymers

Classification of Polymers based on monomer units(i) Homopolymers - consist of chains with identical bonding linkages to each monomer unit. This usually implies that the polymer is made from all identical monomer molecules. E.g., -[A-A-A-A-A-A]- (ii) Copolymers - consist of chains with two or more linkages usually implying two or more different types of monomer units. a. Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-Bb. Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-Bc. Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-

(i) Thermoplastics - plastics that soften when heated and become firm again when cooled. This is the more popular type of plastic because the heating and cooling may be repeated.(ii) Thermosets - plastics that soften when heated and can be molded, but harden permanently. They will decompose when reheated. For example, Bakelite used in toasters, handles for pots and pans, dishes, electrical outlets and billiard balls. Classification based on the physical property & heating:

Classified based on the reaction mode(i) Addition (chain-growth) Polymers - the monomer molecules bond to each other without the loss of any other atoms. Alkene/alkyne monomers are the biggest groups of polymers in this class. Reactive intermediates formation take place. M.Wt.: quite high (e.g., 46 x 106 for polystyrene)(ii) Condensation (step-growth) Polymers - two different monomer combine with the loss of a small molecule, usually water. Monomers contain functional groups capable of reacting with each other. No reactive intermediates formation take place. For example, polyesters, polyamides (nylons), polyurethanes, etc. M.Wt.: usually < 50,000 g/mol

Addition: one monomer at a timeAlso called chain growth.Condensation: anything goes! Also called step growth.

Two general ways of preparing polymers1. Addition Polymerization:2. Condensation PolymerizationFor example, Polyethylene Glycol:

Addition PolymerizationBased on reaction mechanism in addition polymerization: Radical reaction path; Anionic reaction path and(iii) Cationic reaction path.

Three consecutive reaction steps takes place for the polymerization:Initiation step; Propagation step and Termination step.

1. Free Radical Polymerization:Initiation step:Propagation step:Termination step:

Typical Radical Initiators:

For examples:

Copolymerization:

2. Cationic Polymerization: Initiated using a proton (H+), a Lewis acid or a carbocation (R+, generated from R-X + Lewis acid, AlCl3, BF3, etc). For example,

3. Anionic Polymerization: Initiator is an anion (often a carbanion). For example:Block Copolymerization

What are Ziegler-Natta Catalysts? (Nobel Prize in Chemistry in 1966) K. Ziegler (1953) found that Ni-Et3Al dimerized olefins. This prompted a survey for other transition metals (TMs). Group IV metals (Ti & V)- effective catalysts for ethylene polymerization. Ziegler-Natta catalyst is composed of at least two parts: TM part - the catalyst and a main group metal alkyl compound (usually Al-alkyl) - the co-catalyst. A 3rd component is used to control stereo regularity either be incorporated into the catalyst as internal donor (aromatic esters) or external donor (alkoxysilanes & hindered amines). This catalyst system demonstrates that the stereo-regular polymerization of polypropylene could be achieved.

Coordination Polymerization: Ziegler-Natta Catalyst

Ziegler-Natta catalysts are used to produce the following classes of polymers from alpha olefins:1. High density polyethylene (HDPE) 2. Linear low density polyethylene (LLDPE) 3. Ultra-high molecular weight polyethylene (UHMWPE) 4. Polypropylene (PP)--homopolymer, random copolymer and high impact copolymers 5. Thermoplastic polyolefins (TPOs) 6. Ethylene propylene diene monomer polymers (EPDM) 7. Polybutene (PB)

Condensation Polymerization Making ester linkages Polyesters Making amide linkages PolyamidesFor example, Polyesters

Analogously:

Ring-Opening Polymerization

Polyamides (Nylons):Ring Opening of Lactams:

Polyureas:Analogs Polyurethanes

Stereochemical control of Polymer MicrostructureWith so many stereocenters, the stereochemistry can be complex. There are three main stereochemical classifications for polymers.

Ease of crystallizationTacticity (order of placement of side groups)

Tacticity affects the physical properties-Atactic polymers will generally be amorphous, soft, flexible materials-Isotactic and syndiotactic polymers will be more crystalline, thus harder and less flexiblePolypropylene (PP) is a good example -Atactic PP is a low melting, gooey material - Isoatactic PP is high melting (176), crystalline, tough material that is industrially useful -Syndiotactic PP has similar properties, but is very clear. It is harder to synthesize

Polyurethanes (PU) Generalized polyurethane reactionPU reaction mechanism catalyzed by a tertiary amine

PU reaction mechanism:

carbon dioxide gas formed when reacting with water

Summary PU reaction:

STRETCHABLE Fibres

Commodity PolyolefinsPolyethyleneLow Density (1939-1945) LDPEPackaging Film, wire and cable coating, toys, flexible bottles, house wares, coatingsHigh Density (1954) HDPEBottles, drums, pipe, conduit, sheet, filmLinear Low Density (1975) Shirt bags, high strength films LLDE

Commodity PolyolefinsPolypropylene (1954) PP

dishwasher safe plastic ware, carpet yarn, fibers and ropes, webbing, auto partsPolyisobutylene (1940)PIBinner tubes, flexible adhesives, raincoats

Commodity Vinyl PolymersPolystyrene (1920)

PSStyrofoam, clear plastic cups envelop windows, toysPoly(vinyl chloride) (1927)PVCgarden hose, pipe, car trim, seat covers, records, floor tiles

Semi-Commodity PolymersPoly(methyl methacrylate) (1931) PMMAplexiglass, embedding resin, resist for X-ray applications

Polytetrafluoroethylene. (1943)teflon, non stick cookware, no grease bearings, pipe-seal tape

Commodity Condensation PolymersNylon 6 bearings, molded partscarpet yarnmarine ropecooking/boiling bagsNylon 66 (1939)bullet-proof vests, clothes, Fibers, tire cord, fishing line, canoes, kayaks, sailboats, fishing poles and tennis racquets.

Commodity Condensation PolymersPolyester (1941

PET, dacron, mylar, kodel fibers, film-backing, magnetic tapes, soft drink bottles, tire cord, moldings

Polycarbonate (1957) PC, Lexan

shatter proof glass, CD-disks, car doors and roofs, appliance housings

Conducting polymers or Intrinsically conducting polymers (ICPs) (polymers plastics being somehow the opposite of metals)Organic electrically conductive polymers: poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(aniline)s, poly(fluorene)s, poly(p-phenylene sulfide). The melanin pigment in animals is a mixed copolymer of polyacetylene, polypyrrole, and polyaniline. Some fungal melanins are pure polyacetylene. (A. J. Heeger, A. G. MacDiarmid, H. Shirakawa 2000 (Nobel Prize).Oxidation with Cl2, Br2 or I2 vapor made polyacetylene films >109 times conductive than original called doping. The doped form of polyacetylene had a conductivity of 105 Siemens/meter, higher than any known polymer like teflon -1016S m1 and silver and copper 108 S m1.

The halogen doping that transforms polyacetylene to a good conductor of electricity is by oxidation (p-doping). Reductive doping (n-doping) is also possible using, e.g., an alkali metal.[CH]n + 3x/2 I 2 .......> [CH]n x+ +xI3 oxidative doping[CH] n + xNa > [CH]n x + xNa + reductive dopingThe doped polymer is thus a salt. However, it is not the counter ions, I3 or Na+, but the charges on the polymer that are the mobile charge carriers.By applying an electric field perpendicular to the film, the counter ions can be made to diffuse from or into the structure, causing the doping reaction to proceed backwards or forwards. In this way the conductivity can be switched off or on.

Radical cation (polaron) formed by removal of one electron on the 5th carbon atom of a undecahexaene chain (a b). Polaron migration shown in c e.Types of Conducting Polymers: a. Electron- Conducting Polymersb. Proton- Conducting Polymers c. Ion- Conducting Polymers

What makes a material conductive?1D forms of carbon materials3D2DIn many materials- crystals, stretched polymers or liquid crystals, macroscopic properties -Strength/optical/ electrical properties generally depend on direction said anisotropic.

108 -107S m1103 -102S m1Doping cis-polyacetylene with AsF5 resulted in an increase of conductivity by a factor of 1011. The high conductivity found opened up the field of plastic electronics.Applications: Doped polyaniline - conductor and for electromagnetic shielding of electronic circuits. Polyaniline -a corrosion inhibitor.Poly(ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonic acid as an antistatic coating material to prevent electrical discharge exposure on photographic emulsions and serves as a hole injecting electrode material in polymer light-emitting devices.Poly(phenylene vinylidene): have been major candidates for the active layer in pilot production of electroluminescent displays (mobile telephone displays).Poly(dialkylfluorene): as the emissive layer in full-color video matrix displays.Poly(thiophene): field-effect transistors: for supermarket checkouts.Poly(pyrrole): tested as microwave-absorbing stealth (radar-invisible) screen coatings and active thin layer of various sensing devices.

Electroluminescent polymers second-generation conductive polymers

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