Chemistry Reference
In-Depth Information
7.6
Step-Growth Copolymerizations
Step-growth copolymerization involves the use of three or more monomers which do
not ordinarily all react with each other. Examples include mixtures of acids and poly-
ols in the synthesis of alkyds, as illustrated in the recipes in
Table 7.1
. Such polymers
will contain a random distribution of monomer residues if they are synthesized under
conditions in which the polymerization is reversible and the molecular weight distri-
bution is random. Polymers like alkyds are intended to be homogeneous products
with properties which represent an average of those of all the component monomers.
The copolymerization of linoleic acid in the recipe in
Table 7.1
would confer air-
drying properties on all the macromolecules in which it is incorporated.
Copolymerization can be employed in a similar fashion to modify the properties of
the homopolymer of p-hydroxybenzoic acid (7-6). Poly(p-hydroxybenzoic acid) is an
infusible polymer which can be shaped only by compression sintering. A melt process-
able variation of this high modulus, thermally stable material can be made, however, by
copolymerizing an ester of 7-6 with equimolar quantities of terephthalic acid (7-7) and
biphenol (7-8) to produce an aromatic polyester which can be fabricated at temperatures
near 400
C but still retain many useful properties at 300
C.
HO
C
O
OH
7-6
HO
C
O
C
O
OH
7-7
HO
OH
7-8
Not all copolymers that are produced by step-growth processes are random in nature.
Block copolymers are also of major interest. An example of the synthesis of elastic poly-
urethane fibers was given in Section 1.5.4. Block and graft copolymers of polysiloxane-
poly(alkylene ethers) with segments like 7-9 are used as surfactants in the production of
polyurethane foams with uniform cell sizes.
CH
3
CH
3
H
Si
O
Si
CH
2
OCH
2
C
OCH
3
3
x
CH
3
CH
3
CH
3
7-9
