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 name | Monomer structural formula | Monomer'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 |
|
|
+ |
| | | 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.
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 name | Monomer structural formula | Monomer's functional groups |
|---|
trivial: terephthalic acid
IUPAC: 1,4-benzenedicarboxylic acid |
HOOC COOH |
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 |
|---|
| 1 | 1 | 1 | 1 |
| 2 | 2 | 3 | 3 |
| 3 | 3 | 5 | 5 |
| 4 | 4 | 7 | 7 |
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 diol | → | polyester | + | (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.
Poly(butylene terephthalate)
Polybutylene terephthalate or PBT can be prepared by reacting 1,4-butanediol with terephthalic acid (1,4-bezenedicarboxylic acid).
| Monomer name | Monomer structural formula | Monomer'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 diol | → | polyester | + | (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.
