Chemistry Reference
In-Depth Information
product effectively equal to zero. With more polar copolymerizing pairs, the use
of a solvent with a higher dielectric constant increases the alternation tendency
and reduces the
r
1
r
2
product to a small, but significant, extent. Similarly, com-
plexing of one of the monomers increases alternation (
Section 9.12.4
).
The reactivity of a polar monomer can be considerably enhanced in copoly-
merization with a species of the opposite polarity. Maleic anhydride does not
homopolymerize under normal free-radical reaction conditions but it forms 1:1
copolymers with styrene under the same conditions and even reacts with stilbene
(9-6), which itself will not homopolymerize.
H
H
9-11-1
C
C
9-6
9.10.3
Steric Effects
Steric influences may retard some radical polymerizations and copolymerizations.
Double bonds between substituted carbon atoms are relatively inert (unless the
substituents are F atoms) and 1,2-substituted ethylenes do not homopolymerize in
normal radical reactions. Where there is some tendency of such monomers to
enter into polymers, the
trans
isomer is more reactive. When consideration is
restricted to monomers that are doubly substituted on one carbon atom, it is usu-
ally assumed that steric effects can be neglected and that the influence of the two
substituents is additive. Thus, vinylidene chloride is generally more reactive in
copolymerizations than is vinyl chloride.
9.11
Analysis of Reactivity Data
Several attempts have been made to codify the relations between monomer reac-
tivity ratios and structures. These approaches are essentially empirical but they
are useful for predictions of reactivity ratios.
The
Qe
scheme
[25]
assumes that each radical or monomer can be classified
according to its “general reactivity” and its polarity. The general reactivity of rad-
ical
M
i
is represented by
B
i
while the corresponding factor for monomer M
j
is
Q
j
. Polarity is denoted by
e
i
and
e
j
, with the
e
values for a monomer and its
resulting radical being assumed to be equal. The rate constant for reaction (9-3)
would then be expressed as
k
12
5
B
e
1
e
2
). From analogous expressions
for the three other propagation reactions, one obtains the following expressions
for the reactivity ratios:
B
1
Q
2
exp(
2
r
1
5
ð
Q
1
=
Q
2
Þ
exp
½
2
e
1
ð
e
1
2
e
2
Þ
(9-69)
