Page 260 - Carrahers_Polymer_Chemistry,_Eighth_Edition

7 Copolymerization

While the mechanism of copolymerization is similar to that discussed for the polymerization of
one reactant (homopolymerization), the reactivities of monomers may differ when more than one is
present in the feed, that is, reaction mixture. Copolymers may be produced by step reaction or by
chain-reaction polymerization. It is important to note that if the reactant species are M and M then
2
1
the composition of the copolymer is not a physical mixture or blend, though the topic of blends will
be dealt within this chapter.
Many naturally occurring polymers are largely homopolymers, but proteins and nucleic acids are
copolymers composed of a number of different mers. While many synthetic polymers are homopo-
lymers, the most widely used synthetic rubber (SBR) is a copolymer of styrene (S) and butadiene
(B), with the “R” representing “rubber.” There are many other important copolymers. Here we will
restrict ourselves to vinyl-derived copolymers.
Copolymers may be random in the placement of units,

–M M M M M M M M M M M M M M M M M M M M M M M M – (7.1)
2
2
2
2
1
1
2
2
2
1
1
1
2
2
1
1
1
2
1
1
1
2
1
1
alternating in the placement of units,
–M M M M M M M M M M M M M M M M M M M M M M – (7.2)
2
2
1
2
1
1
2
2
1
1
2
1
1
2
2
2
1
1
1
2
1
2
block in either or both of the different units in which there are long sequences of the same repeating
unit in the chain,
–M M M M M M M M M M M M M M M M M M M M M M M M M M M M – (7.3)
1
1
2
2
1
1
2
1
1
1
2
1
1
2
2
1
1
2
2
1
1
2
1
2
1
2
1
2
or graft copolymers in which the chain extension of the second monomer is present as branches.
–M M M M M M M M M M M M M M M M M M M – (7.4)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
M 2 M 2 M 2
M 2 M 2 M 2
M 2 M 2 M 2
M 2 M 2 M 2
M 2 M 2
M
2
Each of these types of copolymers offers different physical properties for a particular copolymer
combination. It is interesting to note that block copolymers may be produced from one monomer
only if the arrangement around the chiral carbon atom changes sequentially. These copolymers are
called stereoblock copolymers.
7.1 KINETICS OF COPOLYMERIZATION
Because of a difference in the reactivity of monomers, expressed as reactivity ratios (r), the compo-
sition of the copolymer (n) may be different from that of the reactant mixture or feed (x). When x
equals n, the product is said to be an azeotropic copolymer.
223
9/14/2010 3:39:49 PM
K10478.indb 223
K10478.indb 223 9/14/2010 3:39:49 PM