Chemistry Reference
In-Depth Information
Table 6.8
Polymerization products of isoprene
Approximate
%
Mode of polymerization
Solvent
Cis
-1,4
%
Trans
-1,4
% 1,2
% 3,4
Free radical
Emulsion in water
32
65
6
7
Cationic
-
37
51
4
9
Chloroform (30
C)
-
90
4
6
Anionic
Lithium
Pentane
94
0
6
Ethyllithium
Pentene
94
0
6
Butyllithium
Pentene
93
0
7
Sodium
Pentene
0
43
6
51
Ethylsodium
Pentene
6
42
7
45
Butylsodium
Pentane
4
35
7
54
Potassium
Pentane
0
52
8
40
Ethylpotassium
Pentane
24
39
6
31
Butylpotassium
Pentane
20
41
6
34
Rubidium
Pentane
5
47
8
39
Cesium
Pentene
4
51
8
37
Ethyllithium
Ethyl ether
6
29
5
60
Ethylsodium
Ethyl ether
0
14
10
76
Lithium
Ethyl ether
4
27
5-7
63-65
Alfin
Pentane
27
52
5
16
Coordination catalysis
a
-TiCl
3
/AlR
3
91
VCl
3
/Al(C
2
H
5
)
3
99
TiCl
4
/Al(C
2
H
5
)
3
95-96
Ti l
4
/AlR
3
+ amine
100
-
-
CoCl
2
/AlR
3
+ pyridine
96
V(acetylacetonate)
3
/AlR
3
90
Ti(OR)
4
/Al(C
2
H
5
)
3
95
a
From various sources in the literature
Cationic polymerizations of isoprene proceed more readily than those of butadiene, though both
yield low molecular weight liquid polymers. AlCl
3
and stannic chloride can be used in chlorinated
solvents at temperatures below 0
C. Without chlorinated solvents, however, polymerizations of
isoprene require temperatures above 0
C. At high conversions, cationic polymerizations of isoprene
result in formations of some cross-linked material [
120
]. The soluble portions of the polymers are
high in
trans-
1,4 structures. Alfin catalysts yield polymers that are higher in
trans-
1,4 structures than
free-radical emulsion polymerizations [
121
].
Chromium oxide catalysts on support polymerize isoprene-like butadiene to solid polymers. Here
too, however, during the polymerization process, polymer particles cover the catalyst completely
within a few hours from the start of the reaction and retard or stop further polymer formation.
The polymerization conditions are the same as those used for butadiene. The reactions can be carried
out over fixed bed catalysts containing 3% chromium oxide on SiO
2
-Al
2
O
3
. Conditions are 88
C and
42 kg/cm
2
pressure with the charge containing 20% of isoprene and 80% isobutane [
122
]. The mixed
molybdenum-alumina catalyst with calcium hydride also yields polyisoprene.
Lithium metal dispersions form polymers of isoprene that are high in
1,4 contents as shown in
Table
6.8
. These polymers form in hydrocarbon solvents. This is done industrially and the products
are called Coral rubbers. They contain only a small percentage of 3,4-structures and no
cis-
trans
-1,4 or
1,2 units. The materials strongly resemble
Hevea
rubber.
