Chemistry Reference
In-Depth Information
The tendency to alternation increases as the
r
1
r
2
product nears zero, as long as
both
r
1
and
r
2
are less than unity. Such copolymerizations occur in free-radical
systems when the two monomers have opposite polarities (
Section 9.10.2
). The
styrene maleic anhydride copolymers mentioned in Chapter 1 are an example of a
purely alternating system (
r
1
5
acrylonitrile (M
2
)
copolymers have a pronounced tendency to alternate monomer residues (
r
1
5
r
2
5
0), while styrene (M
1
)
0.4,
r
2
5
0).
9.3.3
Long Sequences of One Monomer
When one reactivity ratio is greater than one and the other is less than one, either
radical will prefer to add monomers of the first type. Relatively long sequences of
this monomer will be formed if the reactivity ratios differ sufficiently. In that
case the product composition will tend toward that of the homopolymer of the
more reactive monomer. Such reactivity ratios reflect the existence of an imprac-
tical copolymerization. Styrene (
r
1
C
vinyl acetate (
r
2
5
0) is such a system
and these copolymers cannot be made by free radical initiation.
50)
9.3.4
Block Copolymers
If both reactivity ratios are greater than one, there will be a tendency for forma-
tion of sequences of uniform composition in the copolymers. Such reactivity ratio
combinations are not known in free-radical copolymerizations when both mono-
mers are present simultaneously in the reaction vessel, but they can be made in
other systems.
9.4
Azeotropic Compositions
Binary copolymerization resembles distillation of a bicomponent liquid mixture,
with a reactivity ratio corresponding to the ratio of vapor pressures of the pure
components in the latter case. The vapor-liquid composition curves of ideal
binary mixtures have no inflection points and neither do the polymer-composition
curves for random copolymerizations, in which
r
1
r
2
C
1(
Fig. 9.1
). For this reason,
such comonomer systems are sometimes called
ideal
.
Distillation terminology is also borrowed for instances when the copolymer
composition is the same as that of the comonomer feed from which it is derived.
Such a feed composition is called an azeotrope by analogy with distillation. Under
these conditions
d
[M
1
]
/d
[M
2
]
[M
1
]/[M
2
], and hence
Eq. (9-13)
reduces to
r
1
½
5
M
1
1
½
M
2
1
(9-21)
M
2
5
½
M
1
1
r
2
½
From
Eq. (9-21)
, an azeotropic feed composition is such that
