Chemistry Reference
In-Depth Information
particularly the case with crystallizable polymers, such as polyethylene (1-3), where
branching impedes crystallization and affects the stiffness and impact resistance of
the final articles. Further mention of branching distributions is made in Section 3.4.4.
1.5
Copolymers
A homopolymer is a macromolecule derived from a single monomer, whereas a
copolymer contains structural units of two or more different precursors.
This distinction is primarily useful when the main chain of the macromolecule
consists of carbon
carbon bonds. There is little point in labeling poly(ethylene
terephthalate), 1-5, a copolymer, since this repeating unit obviously contains the
residues of two monomers and the polymer is made commercially only by reac-
tion (1-1) or (1-7). Some polyesters are, however, made by substituting about
2 mol % of sodium-2,5-di(carboxymethyl) sulfonate (1-24) for the 3,5 isomer for
the dimethyl terephthalate in reaction (1-7):
CH
3
OO
C
OC
OCH
3
O
O
SO
3
Na
1-24
The eventual product, which is a modified polyester fiber with superior affin-
ity for basic dyes, is often called a copolymer to distinguish it from conventional
poly(ethylene terephthalate). This is a rather specialized use of the term, however,
and we shall confine the following discussion to copolymers of monomers with
olefinic functional groups.
The most important classes of copolymers are discussed next.
1.5.1
Random Copolymer
A random copolymer is one in which the monomer residues are located randomly
in the polymer molecule. An example is the copolymer of vinyl chloride and
vinyl acetate, made by free-radical copolymerization (Chapter 9):
H
H
H
H
H
H
CH
2
C
CH
2
C
CH
2
C
CH
2
C
CH
2
C
CH
2
C
Cl
Cl
O
C=O
CH
3
Cl
O
C=O
CH
3
Cl
1-25
