POLYMERS
Polymers are usually divided into two categories:
 Addition polymers
 Condensation polymers
ADDITION POLYMERS
In addition polymerisation, a large number of molecules of a monomer combine to
produce a polymer, but no other product is formed. The polymer has the same
empirical formula as the monomer but has a larger molecular mass, which could be
as high as 500,000.
Polymerisation is initiated by a radical or an ion (this depends on the catalyst system
which is used) which adds to a carbon-carbon double bond. For example:
.
X + CH2=CH2
X
.
CH2CH2
Further molecules of monomer then add sequentially to the reactive end, and the
chain grows in length.
The overall reaction for addition polymerisation can be represented as:
H
n
R
C
(
C
H
H
H
R
C
C
H
H
)n
The repeating unit is enclosed within the brackets. n is a large number, which
typically lies in the range 100 to 10,000. The end groups constitute such a small
fraction of the polymer molecule that they are usually omitted.
The backbone of a polyalkene molecule is made up of singly-bonded carbon atoms.
The molecule is saturated and is chemically unreactive. These polymers are nonbiodegradable. The attractive forces between polymer chains are weak. This means
that, on heating to relatively low temperatures, they soften and become mouldable:
these are thermosoftening plastics.
With the possible exception of p.v.c., these plastics are flammable. When heated,
p.v.c. decomposes releasing HCl; this acts as a self-extinguisher.
Examples of addition polymers are:
Monomer
ethene, CH2=CH2
propene, CH3CH=CH2
phenylethene, CH2=CHC6H5
chloroethene, CH2=CHCl
Polymer
Use
poly(ethene)
bags, buckets
poly(propene)
ropes, containers
poly(phenylethene)
yoghurt pots
poly(chloroethene)
window frames,
(PVC)
guttering, pipes
propenenitrile, CH2=CHCN
poly(propenenitrile)
acrylic fibres
tetrafluoroethene, CF2=CF2
poly(tetrafluoroethene)
non-stick surface
methyl 2-cyanopropenoate,CH2=C(CN)COOCH3 poly(methyl 2-cyanopropenoate) super glue
TOPIC 13.10: POLYMERS 1
CONDENSATION POLYMERS
Condensation polymers are formed by the reaction between molecules containing
two functional groups. The reaction is accompanied by the elimination of a small
molecule such as water.
POLYESTERS
A polyester may be formed by the reaction of a dicarboxylic acid and a diol. The
reactants are heated together at temperatures up to 180oC, often in the presence of a
titanium-containing catalyst. Water is released during the reaction.
If the methyl ester of the dicarboxylic acid is used, there is less likelihood of subliming
reactants/products blocking the condenser. In this case, methanol is released during
the reaction.
The most important polyester is formed from benzene-1,4-dicarboxylic acid and
ethane-1,2-diol; it has the trade name Terylene. It is used to make plastic bottles,
seat belts, parachute harnesses, magnetic recording tapes and fleece clothing.
O
O
C
C
HO
+
OH
HOCH2CH2OH
loss of water
HO
O
O
C
C
OCH2CH2O
H
n
repeating unit
Hydrolysis
Like simple esters, polyesters can be hydrolysed into their component monomer
units. Polyesters are, therefore, biodegradable. For example:
O
O
C
C
+
OCH2CH2O
2n H2O
n
O
O
C
n
HO
TOPIC 13.10: POLYMERS 2
C
+
OH
n
HOCH2CH2OH
POLYAMIDES
A polyamide may be formed by the reaction of a dicarboxylic acid and a diamine. The
most important polyamide is nylon-6,6; it is called nylon-6,6 because it is made from
a 6 carbon dicarboxylic acid, hexanedioic acid, and a 6 carbon diamine, 1,6diaminohexane. Nylon-6,6 is used to make fishing line, hosiery, casings for power
tools, bearings, tyre cords and ropes.
1,6-diaminohexane is a base and reacts initially with hexanedioic acid to form a salt,
known as nylon salt. When the salt is heated to about 250oC, water is liberated and
the polymer, nylon-6,6 forms.
O
O
C
C
(CH2)4
HO
OH
O
O
-O
C
C
(CH2)4
O-
+
H2N(CH2)6NH2
+
+
+
H3N(CH2)6NH3
nylon salt
loss of water
O
O
HO
C
(CH2)4
C
NH(CH2)6NH
H
nylon-6,6
n
repeating unit
The two functional groups (carboxyl group and amine) can be in the same molecule,
so when the amino acid, 6-aminohexanoic acid is heated, nylon-6 is formed.
+
H3N
O
(CH2)5
C
O-
O
loss of water
H
NH
(CH2)5
C
OH
n
repeating unit
nylon-6
TOPIC 13.10: POLYMERS 3
Hydrolysis
Like simple amides and proteins, synthetic polyamides can be hydrolysed into their
component monomer units. Polyamides are, therefore, biodegradable.
For example:
O
NH
(CH2)5
C
+ n H2O
n
TOPIC 13.10: POLYMERS 4
+
n H3N
O
(CH2)5
C
O-