Chemistry Reference
In-Depth Information
Table 6.9
Structures of polychloroprenes formed by free-radical polymerization
% 1,4
Polymerization temperature (
C)
Cis
Trans
% 1,2
% 3,4
References
40
5
94
0.9
0.3
[
112
]
20
6
91.5
0.7
0.5
[
113
]
10
7
-
-
-
[
112
]
10
9
84
1.1
1.0
[
112
]
40
10
86, 81
1.6
1.0
[
112
]
40
13
88.9
0.9
0.3
[
113
]
100
13
71
2.4
2.4
[
112
]
a
From refs. [
127
-
132
]
6.9 Special Polymers from Dienes
There are many reports in the literature of preparations of polymers from various other substituted
dienes. Most have no commercial significance. Some are, however, interesting materials. An example
is a polymer of 2-
-butyl-1,3-butadiene formed with TiCl
4
and either alkylaluminum or aluminum
hydride catalysts [
132
]. The polymer is crystalline and melts at 106
C. It can be dissolved in common
solvents. Based on X-ray data, the monomer placement is high
t
cis
-1,4.
Poly(carboxybutadiene)s also forms with coordination catalysts [
133
-
135
]:
R
H
H
COOR'
R
COOR'
n
R
H
CH
3
;R
0
¼
where R
CH
3
,C
2
H
5
,C
4
H
9
,orC
6
H
5
.
X-ray crystallography [
133
-
135
] showed that the placement is
¼
-isotactic.
Based on the mode of packing of the chains in the crystalline regions and from the encumbrance
of the side groups in relationship to the main chain, an
trans
erythro
configuration can be assigned [
134
].
The polymers, therefore, are
-isotactic.
Polymerization of 1,3-pentadiene can potentially result
trans-erythro
in five different
insertions of the
monomers. These are 1,4-
cis
, 1,4-
trans
, 1,2-
cis
, 1,2-
trans
, and 3,4. In addition, there are potentially
3-
cis-
1,4 and 3
trans
-1,4 structures (isotactic, syndiotactic, and atactic). Formations of
trans
-1,4
isotactic,
-1,4 syndiotactic polymers are possible with Ziegler-Natta catalysts
[
136
-
138
]. Amorphous polymers also form that are predominantly
cis
-1,4 isotactic, and
cis
-1,4, but lack tactic
order. Stereospecificity in poly(1,3-pentadiene) is strongly dependent upon the solvent used during
the polymerization. Thus,
cis
-1,4 or
trans
-1,2 in
aliphatic ones. The preparations require cobalt halide/aluminum alkyl dichloride(or dialkyl chloride)
catalysts in combinations with Lewis bases. To form a
cis
-1,4 syndiotactic polymers form in aromatic solvents and
trans
trans
-1,4 structure, a catalyst containing
aluminum to titanium ratio close to 5 must be used [
139
].
6.10 Cyclopolymerization of Conjugated Dienes
Conjugated dienes like isoprene, butadiene, and chloroprene cyclopolymerize with catalysts
consisting of aluminum alkyls, like ethylaluminum dichloride, and titanium tetrachloride [
141
].
The ladder polymers that form contain fused cyclic structures. The products prepared in hexane are
generally insoluble powders, while those prepared in aromatic solvents are soluble even when the
