Chemistry Reference
In-Depth Information
In an ideal copolymerization reaction,
k
11
=k
12
¼ k
22
=k
21
and
r
1
r
2
¼
1
r
2
values are equal to or approach zero, each polymer radical reacts preferentially with the
other monomer. This results in an alternating copolymer, regardless of the composition of themonomer
mixture. That is, however, a limiting case. In the majority of instances,
If
r
1
and
r
1
r
2
is greater than zero and
less than one. When the polymer radicals react preferentially with their own monomer, and
r
1
r
2
>
1, then mainly a mixture of homopolymers forms and only some copolymerization takes place.
Reactivity of vinyl monomers is very often determined experimentally by studying copolymer-
izations. Values of many free-radical reactivity ratios have been tabulated for many different
monomer pairs [
126
]. Also, the qualitative correlations between copolymerization data and molecular
orbital calculations can be found in the literature [
136
].
Some general conclusion about monomer reactivity toward attacking radicals was drawn by Mayo
and Walling [
130
]:
1. The alpha substituents on a monomer have the effect of increasing reactivity in the following order.
O
O
>
>
>
N
>
>
OR
O
>
Cl
>
CH
2
X
>
>
OR
O
2. The effect of a second alpha substituent is roughly additive.
Giese and coworkers [
131
-
136
] developed a special technique for studying the effects of the
substituents upon the relative reactivity of vinyl monomers toward free radicals. Briefly it is as
follows. Free radicals are produced by reducing organomercury halides with sodium borohydride.
The radicals undergo competitive additions to pairs of various substituted olefins. The adducts are in
turn trapped by hydrogen transfers from the formed organomercury hydrides. Relative quantities of
each product are then determined
X
RHgH
R
R
X
k
x
X
Y
RMgX
NaBH
4
R
+
RHgH
R
R
Y
k
y
Y
This method was applied in copolymerization of acrylonitrile and methyl acrylate [
129
]. It showed
that the ratios of the rate constants for each of the two monomers are independent of their concentrations.
Copolymerization reactions are affected by solvents. One example that can be cited is an effect of
addition of water or glacial acetic acid to a copolymerization mixture of methyl methacrylate with
acrylamide in dimethyl sulfoxide or in chloroform. This causes changes in reactivity ratios [
131
].
Changes in
r
values that result from changes in solvents in copolymerizations of styrene with methyl
methacrylate are another example [
133
,
134
]. The same is true for styrene acrylonitrile copolymeri-
zation [
132
]. There are also some indications that the temperature may have some effect on the
reactivity ratios [
135
], at least in some cases.
