Chemistry Reference
In-Depth Information
CHAPTER
9
Copolymerization
It is a good morning exercise for a research scientist to discard a pet
hypothesis every day before breakfast. It keeps him young.
—Konrad Lorenz, The So-Called Evil, Chapter 2 (1966)
9.1
Chain-Growth Copolymerization
Copolymerization of two or more monomers during chain-growth polymerization
is an effective way of altering the balance of properties of commercial polymers.
Free-radical copolymerization of 20
35% of the relatively polar monomer acry-
lonitrile with the hydrocarbon styrene produces a transparent copolymer with bet-
ter oil and grease resistance, higher softening point, and better impact resistance
than polystyrene. The copolymer can be used for applications in which these
properties of the styrene homopolymer are marginally inferior. Similarly,
although polyisobutene is elastomeric, the macromolecule is resistant to the cross-
linking reactions involved in sulfur vulcanization. Cationic copolymerization of
isobutene with 1
3 mol% isoprene at very low temperatures yields a polymer
with sufficient unsaturation to permit vulcanization by modified sulfur systems.
Reactivity of a monomer in chain-growth copolymerization cannot be pre-
dicted from its behavior in homopolymerization. Thus, vinyl acetate polymerizes
about twenty times as fast as styrene in a free radical reaction, but the product is
almost pure polystyrene if an attempt is made to copolymerize the two monomers
under the same conditions. Similarly, the addition of a few percent of styrene to a
polymerizing vinyl acetate mixture will stop the reaction of the latter monomer.
By contrast, maleic anhydride will normally not homopolymerize in a free-radical
system under conditions where it forms one-to-one copolymers with styrene.
These examples do not mean that copolymerization reactions defy understand-
ing. The simple copolymer model described here accounts for the behavior of
many important systems and the entire process is amenable to statistical calcula-
tions that provide a great deal of useful information from little data. Thus, the
composition of a copolymer of three or more monomers can be estimated reliably
from a knowledge of the corresponding binary copolymerization reactions, and it
