Chemistry Reference
In-Depth Information
The most important current graft copolymers include impact-resistant poly-
styrenes, in which a rubber, like polybutadiene (1-28) is dissolved in styrene:
HH
CH
2
CC
2
x
1-28
When the styrene is polymerized by free-radical initiation, it reacts by adding
across the double bonds of other styrene and rubber units, and the resulting prod-
uct contains polystyrene grafts on the rubber as well as ungrafted rubber and
polystyrene molecules. This mixture has better impact resistance than unmodified
polystyrene.
A related graft polymerization is one of the preferred processes for manufac-
ture of ABS (acrylonitrile-butadiene-styrene) polymers, which are generally
superior to high-impact polystyrene in oil and grease resistance, impact strength,
and maximum usage temperature. In this case, the rubber in a polybutadiene-in-
water emulsion is swollen with a mixture of styrene and acrylonitrile monomers,
which are then copolymerized in situ under the influence of a water-soluble free-
radical initiator. The dried product is a blend of polybutadiene, styrene-
acrylonitrile (usually called SAN) copolymer, and grafts of SAN on the rubber.
The graft itself is a random copolymer.
1.5.4
Block Copolymer
Block copolymers have backbones consisting of fairly long sequences of different
repeating units.
Elastic, so-called Spandex fibers, for example, are composed of long mole-
cules in which alternating stiff and soft segments are joined by urethane
H
OCN
O
1-29
and sometimes also by urea:
H
H
N
C
N
O
1-30
