OCR Chemistry A H432 Condensation Polymers

Condensation polymerization
We have previously seen addition polymerization, where a double bond in a monomer is broken and used to
form connect one monomer to another, progressively forming very long chain molecules.

Definition: Condensation is a reaction in which two small molecules react together to form a larger
molecule, with the elimination of a small molecule such as water.

Condensation polymerisation is therefore a reaction in which many monomers join together to form a
polymer, with the elimination of a smaller molecule such as water each time a monomer joins the chain.

Polyesters
Recall that an ester can be made from the reaction of a carboxylic acid (or an acyl chloride) with an alcohol.
An ester linkage (-COO-) is formed and water (or hydrogen chloride) is eliminated.

i) using one monomer

The monomer must have both a carboxylic acid or an acyl chloride functional group, and an alcohol
functional group. The monomers can then react with themselves, forming a chain.

e.g. poly(lactic acid), PLA, formed from the monomer lactic acid (2-hydroxypropanoic acid).

CH3 O CH3 O CH3 O CH3 O CH3 O CH3 O

O C C O C C O C C O C C O C C O C C
H H OH H H OH H H OH H H H

repeat unit
H 2O H 2O

Preventing the monomers from reacting until they are required to can be problematic with monomers of
this kind, so the alternative is to form the polyester using two monomers which are combined to form the
polymer.

ii) using a pair of monomers

One monomer has two alcohol functional groups - a diol. The other monomer has two carboxylic acid or acyl
chloride groups. The monomers the alternate forming the polymer chain.

e.g. Terylene (when a fibre), also known as PET (when solid e.g. in bottles)
- made from the monomers ethane-1,2-diol and benzene-1,4-dicarboxylic acid

ester linkage
H H O O H H O O
n HO C C O +n C C O C C O C C
H H H HO OH
H H
repeat unit n

+ (2n-1) H2O

Page. 1
OCR Chemistry A H432 Condensation Polymers

Breaking down polyesters

Alkaline hydrolysis
Polyesters, like esters, are hydrolysed by hot aqueous alkalis such as sodium hydroxide solution. Each ester
linkage is hydrolysed to:
- the sodium salt of a carboxylic acid, -COO-Na+
- a hydroxyl group, -OH

H H
e.g. when terylene is hydrolyzed with an alkali:
n HO C C OH
H H O O alkaline hydrolysis H H
O C C O C C NaOH/H2O +
H H O
n O
n C C
+ - - +
Na O O Na
e.g. when PLA is hydrolysed with an alkali:

CH3 O CH3 O CH3 O CH3 O

O C C O C C O C C HO C C
H H H H O- Na+
repeat unit

Acid hydrolysis
Polyesters can also be hydrolysed with hot aqueous acid, such as hydrochloric acid. The monomer units of
the polyester are produced.
H H
H H O O
n HO C C OH
O C C O C C acid hydrolysis H H
H H +
n H /H2O
O
+
O
n C C
HO OH

Polyamides
Polyamides are condensation polymers, with the amide linkage being formed by reaction of an –NH2 group
with a –COOH (or –COCl group) and the elimination of water (or hydrogen chloride).

i) using one monomer

The monomer must have an amine group and a carboxylic acid (i.e. an amino acid) or acyl chloride group.
The polymer formed in this way is called a polyamide or polypeptide. Proteins are very long polypeptides
containing many different amino acids polymerised into a chain and linked by amide bonds.

Page. 2
OCR Chemistry A H432 Condensation Polymers

H O H H O H H O
H

N C C N C C N C C

H OH H H OH H CH2 OH
CH3
HO

H O H O H O

N C C N C C N C C

H H H CH2
H CH3
amide link HO

ii) using a pair of monomers

The monomers are a diamine with two –NH2 groups, and a dicarboxylic acid or acyl chloride with two –
COOH groups or –COCl groups.

e.g. Nylon-6,6 can be made from 1,6-diaminohexane and hexanedioyl chloride.

H H H H H H O
O O
N (CH 2)6 N Cl C C C
n +n C C C Cl ! N (CH2)6 N C (CH2)4 C
H H H H
H H H H
O n
+ (2n-1) HCl
The ‘-6,6’ comes from the number of carbon atoms between the NH2 groups of the diamine,
and in the dicarboxylic acid (including those of the COOH). Other nylons can be made by varying the
numbers of carbons in either monomer.

e.g. Kevlar is formed from benzene-1,4-diamine and benzene-1,4-dioic acid.

H H O O O O
n N N +n C C N N C C
H H HO OH H H n

+ (2n-1) H2O
Kevlar has some remarkable properties – fire resistant, and a higher strength than steel. It is used to make
protective clothing e.g. for firefighters and in bullet-proof vests.

Why is Kevlar so strong ?

In part because the polar C=O groups along one chain has a δ- oxygen atom and will align exactly with the
polar N-H groups in another chain which has a δ+ H atom. Extensive hydrogen-bond cross linking of chains is
therefore possible.

Page. 3
OCR Chemistry A H432 Condensation Polymers

Hydrolysis of polyamides
Like polyesters, polyamides can be hydrolysed by hot aqueous alkali, e.g NaOH(aq). Each amide linkage is
hydrolysed producing an –NH2 group and the sodium salt of the acid, -COO-Na+
H H
e.g.
O O n N (CH 2)6 N
NaOH/H2O
N (CH2)6 N C (CH2)4 C H H

H H
n O O

+ n C (CH 2)4 C
+ - - +
Na O O Na

Alternatively polyamides can be hydrolysed using hot aqueous acid, such as HCl(aq). The dicarboxylic acid is
produced, along with the ammonium salt of the diamine.

e.g.
H H
O O H+/H2O n +
H N (CH2)6 N H
+

N (CH2)6 N C (CH2)4 C H H
H H O O
+n C (CH 2)4 C

HO OH

Comparing addition and condensation polymerisation

Addition polymerisation Condensation polymerisation

One monomer One or two monomers
Monomer functional group: C=C Monomer functional groups: -COOH and –OH
-COCl and –OH
-COOH and –NH2
-COCl and –NH2
No additional product other than polymer H2O or HCl eliminated
Polymer chain continuous –C-C-C-C-C– Polymer chain contains –COO- or –CONH- links
Polymer chain not easily broken down Polymer can be hydrolysed

Identifying types of polymerisation

For addition polymerisation, look out for:
" monomer has a double bond
" polymer backbone is continuous chain of C atoms

For condensation polymerisation, look out for

" two monomers, each with two functional groups
" one monomer with two different reacting functional groups
" polymer contains ester or amide linkages

Page. 4
OCR Chemistry A H432 Condensation Polymers

Practice:
Nomex is a synthetic heat resistant and fire resistant polyamide O O

with the structure shown:

N N

i) identify the monomer(s) from which it is formed H H

ii) write the structures of the products of acid hydrolysis of n
nomex using hydrochloric acid
iii) write the structures of the products of alkaline hydrolysis of nomex using potassium hydroxide solution

Page. 5