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Synthetic Polyesters Tutorial

Key Concepts

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Poly(ethylene adipate)

In the 1920s, W.H. Carothers, working for Du Pont, discovered that diols (alcohols with 2 OH functional groups) and dicarboxylic acids (carboxylic acids with 2 COOH functional groups) could be mixed to make polyester polymers#.
He produced polyethylene adipate by heating ethylene glycol (1,2-ethanediol) with adipic acid (hexanedioc acid).

Monomer nameMonomer structural formulaMonomer's functional groups
trivial: adipic acid
IUPAC: hexanedioc acid
HOOC-CH2-CH2-CH2-CH2-COOH two carboxylic acid, COOH, functional groups
trivial: ethylene glycol
IUPAC: 1,2-ethanediol
HO-CH2-CH2-OH two hydroxyl, OH, functional groups

Each time an OH functional group reacts with a COOH functional group, an ester link, -O-CO- , is formed between the two molecules and a molecule of water is eliminated.

This is an example of a condensation polymerisation reaction, the monomeric units join together by eliminating the small water molecule.

ethylene glycol
(1,2-ethanediol)
+ adipic acid
(hexanedioc acid)
polyethylene adipate + water
    H
|
  H
|
   
HO-C-C-OH
    |
H
  |
H
   
+
    O
||
  H
|
  H
|
  H
|
  H
|
  O
||
   
HO-C-C-C-C-C-C-OH
        |
H
  |
H
  |
H
  |
H
       
      H
|
  H
|
      O
||
  H
|
  H
|
  H
|
  H
|
  O
||
   
-(-O-C-C-O-C-C-C-C-C-C-)n-
      |
H
  |
H
          |
H
  |
H
  |
H
  |
H
       
+ H-O-H

In order to drive the reaction forward to maximise the production of the polyethylene adipate polymer, Carothers's team developed a "molecular still" in which water was removed as a vapour and then condensed on a "cold finger". By Le Chatelier's principle, removing water shifts the equilibrium position to the right to compensate for the loss of the water so that more polymer and more water is produced.

Unfortunately, polyethylene adipate has poor resistance to heat, melting during the hot ironing process so when Carothers discovered nylon, Du Pont devoted itself to developing nylon rather than polyesters.

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Poly(ethylene terephthalate)


British Chemists Whinfield and Dickson finally patented PET or PETE (polyethylene terephthalate) in 1941.
Polyethylene terephthalate (PET) forms the basis of synthetic fibres such as dacron® (manufactured by Du Pont), terylene® (manufactured by ICI##) and 'polyester'.
These polyester fibres were very popular in the 1970s, but were declining in popularity until 1989 with the development of microfibres which are polyester fibres with the look and feel of silk.
Because polyethylene terephthalate is strong and has good resistance to impact, it is used to make containers like drink bottles.
When formed into films such as biaxially-oriented polyethylene terephthalate (BoPET), polyethylene terephthalate is the basis for Mylar® which is used in colourful helium balloons and space blankets.
Polyethylene terephthalate films can also be shaped into packaging materials like trays and blister packs.

Polyethylene terephthalate can be prepared in the laboratory by heating terephthalic acid (1,4-benzenedicarboxylic acid) with ethylene glycol (1,2-ethanediol) in the presence of an acid catalyst.*

Monomer nameMonomer structural formulaMonomer's functional groups
trivial: terephthalic acid
IUPAC: 1,4-benzenedicarboxylic acid
HOOCCOOH two carboxylic acid, COOH, functional groups
trivial: ethylene glycol
IUPAC: 1,2-ethanediol
HO-CH2-CH2-OH two hydroxyl, OH, functional groups

The reaction is a condensation polymerisation reaction in which water, H2O, is eliminated.

Each carboxyl, COOH, functional group can react with a hydroxyl, OH, functional group to produce water and an ester, -COO-, link between the two molecules.

To start the polymerisation, 1 molecule of terephthalic acid reacts with 1 molecule of ethylene glycol to produce 1 molecule of the ester and 1 molecule of water:

1HOOCCOOH + 1HO-CH2-CH2-OH HOOCCOO-CH2-CH2-OH + 1H2O

The ester molecule has a carboxylic acid, COOH, functional group at one end and a hydroxyl, OH, functional group at the other end so it can react with both the ethylene glycol monomer and the terephthalic acid monomer.
If the ester molecule reacts with the 1 terephthalic acid monomer, a new ester link is formed and a molecule of water is eliminated:

1HOOCCOO-CH2-CH2-OH + 1HOOCCOOH HOOCCOO-CH2-CH2-OOCCOOH
+ 1H2O

The new ester molecule has carboxyl, COOH, functional groups at both ends, so it can react with ethylene glycol to produce another ester link and molecule of water:

1HOOCCOO-CH2-CH2-OOCCOOH + 1HO-CH2-CH2-OH HOOCCOO-CH2-CH2-OOCCOO-CH2-CH2-OH
+ 1H2O

This new ester has a carboxyl, COOH, functional group at one end and a hydroxyl, OH, functional group at the other end so it can react with both the ethylene glycol and the terephthalic acid to produce a new ester link and water.

In this way long chains of alternating ethylene glycol and terephthalic acid monomeric units are built up to form the polyester polymer.

Each time a molecule of dicarboxylic acid reacts with a molecule of diol, 1 water molecule is eliminated:

Number of
dicarboxylic acid monomers
Number of
diol monomers
Number of
ester links formed
Number of
water molecules eliminated
1111
2233
3355
4477

If n is the number of dicarboxylic acid monomers and also the number the diol monomers, then

number of ester links formed = (2 x n) -1

Since 1 water molecule is elimated every time 1 ester link is formed, the number of water molecules eliminated during the polymerisation reaction is equal to the number of ester links formed:

number of water molecules formed = (2 x n) -1

Therefore we can write a general equation for the polymerisation of polyethylene terephthalate as:

n terephthalic acid + n ethylene glycol polyethylene terephthalate (PET) + (2n-1) water
nHOOCCOOH + nHO-CH2-CH2-OH -(-OCCOO-CH2-CH2-O-)n- + (2n-1)H2O
n dicarboxylic acid + n diolpolyester + (2n-1) water

Oxygen is more electronegative than carbon. So the oxygen atom in each ester link is slightly negatively charged, Oδ-, while the carbon atom of the ester link is slightly positively charged, Cδ+. The Cδ+ on one polymer chain is attracted to the Oδ- on another polymer chain, so the chains are held together by dipole-dipole interactions.

one polymer chain
      H
|
  H
|
      Oδ-
||
 Oδ-
||
     
-(-O-C-C-O-Cδ+Cδ+-)n-  
      |
H
  |
H
      .
.
  .
.
  dipole-dipole interactions
one polymer chain
      H
|
  H
|
      Oδ-
||
 Oδ-
||
   
-(-O-C-C-O-Cδ+Cδ+-)n-
      |
H
  |
H
             

The dipole-dipole interactions and the regularity of the stacking of the benzene rings results in a highly crystalline polymer with a melting point of about 240oC.

Polyethylene terephthalate (PET) is a thermoplastic material which means that when it is heated it becomes soft and can be molded into a new shape, making it suitable for recycling.
After materials have been collected for recycling, the polyethylene terephthalate (PET) materials must be separated from the other materials.
The polyethylene terephthalate (PET) materials are then ground into smaller particles known as flakes.
The flakes undergo further purification to remove any remaining foreign material. This can be achieved by washing and air classification, as well as by water baths which can separate particles that float from particles that sink.
The purified polyethylene terephthalate (PET) is then rinsed to remove any foreign materials including detergent and disinfectant residues, and then dried.
The polyethylene terephthalate (PET) is heated to melt it, and the molten PET material passes through a series of screens to form pellets (any non-molten material, non-PET material, will not pass through the screens).
After cooling, the recycled polyethylene terephthalate (PET) pellets are ready to be used to make new polyethylene terephthalate (PET) objects.
Recycled polyethylene terephthalate (PET) can used to make polyester carpet fibres, polyester fabric for T-shirts and underwear, shoes, luggage, upholstery, automotive parts, sheets, films and new PET containers.

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Poly(trimethylene terephthalate)

® and by Shell as Corterra®.

Poly(trimethylene terephthalate) is produced by reacting terephthalic acid with 1,3-propanediol.

Monomer nameMonomer structural formulaMonomer's functional groups
trivial: terephthalic acid
IUPAC: 1,4-benzenedicarboxylic acid
HOOCCOOH two carboxylic acid, COOH, functional groups
IUPAC: 1,3-propanediol HO-CH2-CH2-CH2-OH two hydroxyl, OH, functional groups

The reaction is a condensation polymerisation reaction in which a molecule of water is eliminated each time a hydroxyl, OH, group reacts with a carboxyl, COOH, group to form an ester, O-CO-, link:

n terephthalic acid + n 1,3-propanediol polytrimethylene terephthalate (PTT) + (2n-1) water
nHOOCCOOH + nHO-CH2CH2CH2-OH -(-OCCOO-CH2CH2CH2-O-)n- + (2n-1)H2O
n dicarboxylic acid + n diolpolyester + (2n-1) water

The carbon of the ester link has a slightly positive charge, Cδ+, and the oxygen atom has a slightly negative charge, Oδ-, because oxygen is more electronegative than carbon. Therefore dipole-dipole interactions between the polymer chains hold them together.
Polytrimethylene terephthalate has 1 extra non-polar methylene (CH2) group in the chain than polyethylene terephthalate, and this makes polytrimethylene terephathalate material softer, easier to stretch and more easily coloured.
Polytrimethylene terephthalate can be used as a carpet fibre because it has good wear and stain resistance.

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Poly(butylene terephthalate)

Polybutylene terephthalate or PBT can be prepared by reacting 1,4-butanediol with terephthalic acid (1,4-bezenedicarboxylic acid).

Monomer nameMonomer structural formulaMonomer's functional groups
trivial: terephthalic acid
IUPAC: 1,4-benzenedicarboxylic acid
HOOCCOOH two carboxylic acid, COOH, functional groups
IUPAC: 1,4-butanediol HO-CH2-CH2-CH2-CH2-OH two hydroxyl, OH, functional groups

The reaction is a condensation polymerisation reaction in which a molecule of water is eliminated each time a hydroxyl, OH, group reacts with a carboxyl, COOH, group to form an ester, O-CO-, link:

n terephthalic acid + n 1,4-butanediol polybutylene terephthalate (PBT) + (2n-1) water
nHOOCCOOH + nHO-CH2CH2CH2CH2-OH -(-OCCOO-CH2CH2CH2CH2-O-)n- + (2n-1)H2O
n dicarboxylic acid + n diolpolyester + (2n-1) water

Like polyethylene terephthalate (PET), polybutylene terephthalate (PBT) has polar C=O bonds with the carbon being slightly positive, Cδ+, and the oxygen atom being slightly negative, Oδ-, so that dipole-dipole interactions between the polybutylene terephthalate chains hold them together.
The melting point of polybutylene terephthalate, 170oC, is less than that of polyethylene terephthalate, 240oC.
The additional non-polar CH2 groups in the polybutylene terephthalate chains reduces the melting point.

Polybutylene terephthalate has low moisture absorption, good fatigue resistance, good solvent resistance, good self-lubrication, and maintains its physical properties well when heated.

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Hydrolysis of Polyesters

Simple esters can be hydrolysed by reaction with dilute acids or alkalis.

Polyesters react with water so slowly that this reaction can be ignored. For this reason polyesters can be used to make water bottles.

Polyesters react only very slowly with dilute acids. For this reason polyesters can be used to make soft drink bottles since soft drinks are acidic.

Polyesters react readily with strong alkalis. The ester links are broken and the products of the reaction are the diol and the salt of the carboxylic acid.

-(-OCCOO-CH2-CH2-OOCCOO-CH2-CH2-O-)n-
NaOH
Na+-OOCCOO-Na+ + HO-CH2-CH2-OH + Na+-OOCCOO-Na+ + HO-CH2-CH2-OH
(salt of carboxylic acid)   (diol)   (salt of carboxylic acid)   (diol)

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#Perhaps we should say rediscovered since it is believed that polyesters had been made earlier by Gay-Lussac and Théophile-Jules Pelouze in 1833 and Jöns Jakob Berzelius in 1847.

##Imperial Chemical Industries

*Polyethylene terephthalate is prepared commercially by converting terephthalic acid to the dimethyl ester first. The dimethyl ester is much more easily purified by distillation or crystallisation than the acid. The dimethyl ester reacts with the ethylene glycol in an ester interchange reaction to form polyethylene terephthalate.