Polymer Synthesis Now that you have learned something about polymer microstructure, the multifarious ways monomer units can be arranged in chains, it is time to move on and look at some of the details of how these units are linked together in the first place - the science ( and sometimes art) of polymer synthesis. "I am inclined to think that the development of polymerization is perhaps the biggest thing that chemistry has done, where it has had the biggest effect on everyday life ” —Lord Todd,1980 Ethylene Polyethylene CH 2 = CH 2 [-CH 2 -CH 2 -] n Magic ?
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Polymer SynthesisNow that you have learned somethingabout polymer microstructure, themultifarious ways monomer units canbe arranged in chains, it is time tomove on and look at some of thedetails of how these units are linkedtogether in the first place - thescience ( and sometimes art) ofpolymer synthesis.
"I am inclined to think that thedevelopment of polymerizationis perhaps the biggest thingthat chemistry has done,where it has had the biggesteffect on everyday life ”
—Lord Todd,1980
Ethylene PolyethyleneCH2= CH2 [-CH2- CH2-]n
Magic ?
Polymer Synthesis - Classification
Polymerization reactions can be classifiedinto two or three basic types. Carothers, agreat and tragic figure in the history ofpolymer science, suggested that mostpolymers could be classified into two broadcategories, condensation or addition. Forreasons that will become obvious, the termsstep-growth and chain polymerization'sprovide a more accurate and completedescription.
TYPES OF REACTIONS
CONDENSATION
ADDITION
RING OPENING
CH2 = CH2
N
CO
H
H2N - (CH2)6 - NH2 + HO - C - (CH2)4 - C - OH
OO
Nylon 6,6
Polyethylene
Nylon 6
HISTORICAL CONTEXT
1920 - STAUDINGER; The macromolecular hypothesis
1926 - CHARLES STINE; Initiates a program of fundamental research at du Pont
LATE 1920's - CAROTHERS; Set out to prove the existence of macromolecules by systematically building them from small molecules using well known chemistry
MOLECULAR WEIGHT
"Drop the idea of large molecules. Organic molecules with a molecular weight higher than 5000 do not exist.”
—Advice given to Hermann Staudinger*
Condensation Reactions
Acetic Acid Ethyl Alcohol or Ethanol
ReversibleReaction!
Ethyl AcetateIf you don’tknow what thisis you shouldbe castigatedunmercifully
Ester linkage
CH3 - C - OH + CH3 - CH2 - OH CH3 - C - O - CH2 - CH3 + H2O
O O
Why do Molecules React ?
Reactants
Products
ENERGY
CH3 - C - OH
O
HO - CH2 - CH3 Acetic Acid
kersplat! Ethyl Alcohol or Ethanol
Why do Molecules React ?
H:O - CH2 - CH3 CH3 - C:OH
O
HO. + .H HO:H ~ H2O
CH3 - C. + .O - CH2 - CH3 CH3 - C:O - CH2 - CH3
OO O
Making a Polymer
Acetic Acid Ethyl Alcohol or Ethanol
ReversibleReaction!
Ethyl Acetate
Ester linkage
CH3 - C - OH + CH3 - CH2 - OH CH3 - C - O - CH2 - CH3 + H2O
O O
Having given you a very crude picture of what happens in a condensationreaction, let us now turn our attention to making a polymer, a polyester.But we want you to do some of the work, instead of just sitting there!The question is this; if we heat acetic acid and ethanol up to a just over1000C, to get the reaction going and drive off water, why don’t we formpolymer?
Making a PolymerOf course; under these conditions once you react each functionalgroup, the acid and the alcohol, that’s the end of it. Let’s look at thisschematically;
Once we form the yellow blob, in this case the ester, there is no way tomake a bigger molecule. Obviously, to make linear chains we needbifunctional molecules
Except the reaction doesn’t happen all in one go, like this, but in a step-growth fashion. Let’s look at a specific example.
+
O O
HO - C - (CH2)n - C - O - (CH2)m - OH + H2O
O O
Making a Polyester
If we take a bifunctional acid and a bifunctional alcohol, then the firststep would be simple reactions between pairs of monomers to formdimers (and water, which must be driven off in such equilibriumreactions if you want to get high molecular weight polymer):
HO - C - (CH2)n - C - OH + HO - (CH2)m - OH
Making a PolyesterThe dimers can now react with monomers to make trimers;
HO - C - (CH2)n - C - OH
O O
HO - C - (CH2)n - C - O - (CH2)m - OH +
O O
HO - C - (CH2)n - C - O - (CH2)m - O - C - (CH2)n - C - OH
O OO O- H2O
And so on M1 + M1 M2
M2 + M1 M3
M2 + M2 M4
M3 + M1 M4
M4 + M1 M5
M3 + M2 M5
M5 + M1 M6
Etc.
Nylons
H2N - (CH2)6 - NH2 + HO - C - (CH2)4 - C - OH
O
H2N - (CH2)6 - N - C - (CH2)4 - C - OH + H2O
OO
O
Adipic AcidHexamethylene Diamine
Amide Group
H
In the same step wise fashion as before, dimers react with monomers togive trimers, these in turn react, slowly building chains. The polymer,whose repeat unit is shown below, is called nylon 6,6 because of thenumber of carbon atoms in each of its constituent units;
Nylon 6,6
- N - (CH2)6 - N - C - (CH2)4 - C -
O O
n6 6
H H
Nylon 6,10
- N - (CH2)6 - N - C - (CH2)10 - C -
O O
n
What would nylon 6,10 look like?
A
- N - (CH2)10 - N - C - (CH2)6 - C -
O O
n
- N - (CH2)6 - N - C - (CH2)8 - C -
O O
n
B
C
H H
H H
H H
More on Nylons
Another aspect of step-growth polymerizations is that in addition to drivingthe reaction to high degree of conversions, it is also necessary to haveprecisely equivalent amounts of the monomers (you will see why if you study thestatistics of these reactions). This is not always easily accomplished on anindustrial scale. One trick that is possible with nylons is to convert the acid andamine (which is a base) to salts, which will precipitate out of solution in theform of 1:1 complexes;
- OOC - (CH2)4 - COO -
+ NH3 - (CH2)6 - NH3 +
Nylon Salt
Back to Condensation
O
H2N - (CH2)6 - N - C - (CH2)4 - C - OH + HCl
OO
O
Adipoyl ChlorideHexamethylene Diamine
Amide Group
Is a molecule of water always split out?
H2N - (CH2)6 - NH2 + Cl - C - (CH2)4 - C - Cl
H
Hexamethylene diamine in water
Adipoyl chloride in chloroform
Nylon 6,6 formed at the interface
Nylon "skin" carefully drawn off to form fiber or rope
The Nylon Rope Trick
Polyurethanes
An example of a reaction between bifunctional molecules that does notinvolve the splitting out of a small molecule at all can be found in thesynthesis of polyurethanes.
O = C = N - (CH2)6 - N = C = O + HO - (CH2)2 - OH
O = C = N - (CH2)6 - N - C - O - (CH2)2 - OH
O
Hexamethylene Diisocyanate Ethylene Glycol
Urethane Linkage
H
Back to Polyesters
+ 2CH3OH
CH3 - O - C - - C - O - CH3 + 2(HO - (CH2)2 - OH)
=
O
=
O
- C - O - CH2CH2OH HOCH2CH2 - O - C -
=
O
=
O
Dimethyl Terephthalate Ethylene Glycol
+ HOCH2CH2OH
- C - O - CH2CH2 - O - C -HOCH2CH2 - O - C -
=
O=
O
- C - O - CH2CH2OH
=
O
=
O
- C - O - CH2CH2OH HOCH2CH2 - O - C -
=
O
=
O
- C - O - CH2CH2OH HOCH2CH2 - O - C -
=
O
=
O
Transesterification
Transesterification
And so on
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