Chemistry Reference
In-Depth Information
monomers, using the same temperature, solvent, and counterion. One such study was carried out on
cationic copolymerizations of styrene with two substituted styrenes. These were
-methylstyrene, and
with chlorostyrene [
370
-
372
]. The relative reactivity ratios of these substituted styrenes were
correlated with Hammett
a
rs
values. The effect of the substituents on reactivity of styrene falls in
the following order:
p
- OCH
3
>p
-CH
3
>p
-H
>p
-Cl
>m
-Cl
>m
-OH
This information, however, is useful only for copolymerizations of substituted styrene monomers.
Copolymerization studies demonstrated that steric factors are very important
in cationic
copolymerizations [
373
-
375
]. For instance,
-methyl styrene is less reactive than styrene. Also,
trans
-
b
-methylstyrene is more reactive than the
cis
isomer.
The effect of solvents on the reactivity ratios in cationic copolymerizations can be seen from
copolymerizations of isobutylene with
b
p
-chlorostyrene, using aluminum bromide as the initiator
[
376
]. The
r
2
values in hexane for isobutylene and chlorostyrene copolymerization are both
equal to 1.0. In nitrobenzene, however,
r
1
and
r
1
is equal to 14.7 and
r
2
is equal to 0.15.
-methylstyrene catalyzed by SbCl
5
, AlX
3
, TiCl
4
, SnCl
4
,
BF
3
OEt
2
, SbCl
3
,Cl
3
COOH, and iodine, the copolymer composition depends upon the solvent and
on the acid strength of the catalysts. There is no difference in copolymer composition in highly polar
solvents, except when SbCl
5
is used [
377
]. Generally, the amount of styrene in the copolymer
decreases as the solvent polarity increases. In solvents of low polarity, on the other hand, the amount
of styrene in the copolymer decreases with a decrease in the strength of the Lewis acids. This depends
upon the amount of solvation of the ion pair, and on the complexation of the solvent and monomer
with the ion pair. With an increase in the tightness of the ion pairs in less polar solvents, selectivity
increases. Because
In copolymerization of styrene with
p
-methylstyrene is more polar it complexes to a greater extent than does styrene.
Polymerization temperature in cationic polymerizations affects monomer reactivity strongly. This
effect is considerably greater in cationic copolymerizations than in free-radical ones. No general
trend, however, appears to have been established, so far.
Electron withdrawing substituents in anionic polymerizations enhance electron density at the double
bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave
similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to alternate
[
378
]. The reactivities of the monomers in copolymerizations and the compositions of the resultant
copolymers are subject to solvent polarity and to the effects of the counterions. The two, just like in
cationic polymerizations, cannot be considered independently from each other. This, again, is due to the
tightness of the ion pairs and to the amount of solvation. Furthermore, onlymonomers that possess similar
polarity can be copolymerized by anionic mechanism. Thus, for instance, styrene derivatives copolymer-
ize with each other. Styrene, however, is unable to add to a methyl methacrylate anion [
379
-
381
], though
it copolymerizes with butadiene and isoprene. In copolymerizations initiated by
p
n
-butyllithium in toluene
78
C, the following order of reactivity with methyl methacrylate anions was
observed [
382
]. In toluene the order is: diphenylmethyl methacrylate
and in tetrahydrofuran at
>
benzyl methacrylate
>
methyl
methacrylate
>
ethyl methacrylate
> a
-methylbenzyl methacrylate
>
isopropyl methacrylate
> t
-
a
0
-di-methylbenzyl methacrylate. In tetrahydrofuran the
butyl methacrylate
>
trityl methacrylate
> a
,
order changes to: trityl methacrylate
>
benzyl methacrylate
>
methyl methacrylate
>
diphenylmethyl
a
0
-
methacrylate
>
ethyl methacrylate
> a
-methylbenzylmethacrylate
>
isopropyl methacrylate
> a
,
dimethylbenzyl methacrylate
-butyl methacrylate.
Copolymerizations of styrene with butadiene in hydrocarbon solvents, using lithium alkyls
initiators initially yield copolymers containing mainly butadiene. The amount of styrene in the
copolymer increases considerably, however, in tetrahydrofuran solvent.
Hashimoto and coworkers studied anionic living copolymerization by ionic mechanism of two
monomers, styrene and isoprene in a dilute benzene solution with the aid of combined time-resolved
> t
