Chemistry Reference
In-Depth Information
Table 4.2
Conditions for preparations of some living polymers
a,b
Monomer
Temp. (
C)
Initiating system
Solvent
Vinyl ethers
HI + ZnI
2
Toluene
40 to
25
Protonic acids
b
+ ZnCl
2
Isobutyl vinyl ether
Toluene
40 to
0
p
-Methyl styrene
CH
3
COClO
4
CH
2
Cl
2
/toluene (1:4)
78
Isobutylene
BCl
3
+ cumyl acetate
CH
2
Cl
2
30
N
-vinylcarbazole
HI
Toluene
40
Styrene
CH
3
CH(C
6
H
5
)Cl + SnCl
4
+
n
-C
4
H
9
NCl
CH
2
Cl
2
15
Indene
Cumyl methyl ether + TiCl
4
CH
2
Cl
2
40;
75
Indene
2-Chloro-2,4,4-trimethylpentane + TiCl
4
CH
3
Cl/CH
3
C
6
H
11
80
b
-Pinene
CH
3
CH(OCH
2
-CH
2
Cl)Cl + TiCl
2
(O-
i
-Pr) +
n
-Bu
4
NCl CH
2
Cl
2
78 to 40
a
From various sources in the literature
b
Protonic acids used were CH
3
SO
3
H, R
2
P(O)OH, and R
0
CO
2
H, where R
-C
4
H
9
;R
0
¼
¼
OC
6
H
5
,C
6
H
5
,
n
CF
3
, CCl
3
,
CHCl
2
,CH
2
Cl
As shown above, the carbon-iodine bond is stretched, with or without the help of a Lewis acid. The
Lewis acid assists in further stretching the carbon-iodine bond. Whether it is needed, depends upon
the strength of that bond. This strength, in turn, varies with the ability of the solvent, the temperature,
and the substituent R to stabilize the
+
center. Depending upon conditions, a Lewis acid can convert
the counterion, like I
to a more stable, less nucleophilic species. Lewis bases, like dioxane or ethyl
acetate, may function by reacting directly with the propagating centers.
Living cationic polymerizations were also carried out with heterogeneous catalysts. Thus,
Aoshima and coworkers [
139
] reported a heterogeneous living cationic polymerization of isobutyl
vinyl ether, using Fe
2
O
3
in conjunction with the isobutyl vinyl ether-HCl adduct in toluene. This was
done in the presence of an added base at 0
C. Such bases are ethyl acetate and 1,4-dioxane. These
bases are effective in homogeneous living cationic polymerization of vinyl ethers in the presence
various metal halides. The living cationic polymerization of isobutyl vinyl ether produced polymers
with very narrow molecular weight distribution of the product [
139
]. The number average molecular
weight increased in direct proportion to the monomer conversion, and they reported that the
molecular weight distributions were very narrow throughout the polymerization [
d
1.1].
Stereoselectivity of the products was similar to polymers obtained by soluble catalysts. Controlled
polymerization also occurred even at higher temperature (30
C). What is particularly interesting is
their report that they separated the catalyst from the mixture by centrifugation, and then used this
catalyst to catalyze a second living polymerization under the same conditions, yielding a polymer
with narrow molecular weight distribution. The ease of the catalyst separation permitted repeated
reuse of the catalyst. Up to the fifth use, the catalyst maintained its activity to give well-defined
polymers with very narrow MWD [
139
].
Heterogeneous conditions, due to poor solubility of heteropoly acid, in polymerization of isobutyl
vinyl ether with H
3
PW
12
O
40
in CH
2
Cl
2
were also studied. When bases like 1,4-dioxane or tetrahy-
drofuran were present the molecular weight distributions were very broad. By contrast,
polymerizations in the presence of dimethyl sulfide at
M
w
/
M
n
¼
30
C yielded living polymerizations of the
ether. Here too, the product had very narrow molecular weight distribution [
139
]. In summary, some
typical features of living cationic polymerizations are:
1. The quasiliving or living/controlled carbocationic polymerizations are characterized by an ioniza-
tion equilibrium between active and reversibly deactivated chains that are dormant.
2. The number average molecular weight of the polymers that form is proportional to the amount of
monomer introduced into the reaction mixture. In most cases, the reactions are rapid, often to a
point that it is impossible to stop them before all the monomer is consumed.
3. The concentration of the polymers formed is constant and independent of conversion. This
concentration is often equal to the concentration of the initiator.
