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