Chemistry Reference
In-Depth Information
the heat. In one of them, called “flash polymerization process” the catalyst (a Lewis acid, like BF
3
or
AlCl
3
, for instance) is added in solution to the cooled isobutylene solution. The polymerization takes
place very rapidly and is complete in a few seconds with the heat of the reaction being removed by
vaporization of the diluent. Such reactions, however, are very difficult to carry out in conventional
batch reactors. Two types of procedures were, therefore, adopted [
75
]. The first one is built around a
moving stainless steel belt contained inside a gas-tight reactor housing. Isobutylene and liquid
ethylene from one source and a Lewis acid in ethylene solution (0.1-0.3% based on monomer)
from another source are fed continuously onto the moving belt where they are mixed and moved.
The movement of the belt is adjusted at such a speed that the polymerization is complete before the
polymer arrives at the end of its travel, where it is removed with a scraper and further processed.
In the second process, the polymerization is carried out in multiple kneaders or mixers. These are
arranged in a series of descending steps. Here the reaction mixture is carried from one kneader to
another with the temperature being raised at each station and completed at the last one.
All commercially important polyisobutylenes are linear, head to tail polymers, with tertiary butyl
groups at one end of the chains and vinylidene groups at the other:
n
The differences lie in molecular weights. They range from 2,000 to 20,000 for viscous liquids to
between 100,000 and 400,000 for high molecular weight elastomers that resemble unmilled crepe
rubber. The polymers degrade readily from thermal abuse. They can be stabilized effectively,
however, by adding small quantities (0.1-1.0%) of such stabilizers as aromatic amines, phenols, or
sulfur compounds. Polyisobutylenes are soluble in many hydrocarbons and are resistant to attacks by
many chemicals.
Coordination polymerizations with Ziegler-Natta catalysts yield similar polymers that range from
viscous liquids to rubbery solids. At 0
C, a catalyst with a 1:16 Ti to Al molar ratio yields a polymer
with a molecular weight of 5,000-6,000 [
76
]. The molecular weight, however, is dependent upon the
reaction time. This contrasts with polymerizations of ethylene, propylene, and 1-butene by such
catalysts, where the molecular weights of the products are independent of the reaction time.
In addition, there are some questions about the exact molecular structures of the products [
76
].
Bochmann and coworkers [
77
] carried out polymerizations of isobutylene and copolymerizations
with isoprene using cationic zirconocene hydride complexes. The combination of [Cp
2
ZrH] with
various trityl salts of weakly coordinating anions gives binuclear cationic hydrides [Cp
0
4
Zr
2
H
(
-H)
2
]
+
X
which are powerful initiators for the polymerization of isobutene and its copolymeriza-
tion with isoprene. The temperature dependence of M is indicative of a cationic mechanism. The
highest molecular weights are obtained only under scrupulously dry conditions.
High molecular weight polyisobutylene has fair tensile strength but suffers from the disadvantage
of considerable cold flow. A copolymer of isobutylene with some isoprene for cross-linking is,
therefore, used as a commercial elastomer and called “butyl rubber.” The isoprene is present in the
copolymer in only minor proportions (1.4-4.5%). The uncross-linked material is very similar to
polyisobutylene. Copolymers of isobutylene with other dienes are also called butyl rubbers. They can
also be terpolymers, where the third component may be cyclopentadiene for improved ozone
resistance.
The molecular weights of the copolymers vary inversely with the quantities of isoprene
incorporated, the polymerization temperatures, and amount of impurities present during polymeriza-
tion. Impurities like
m
butene or water act as chain transferring agents [
79
].
To maintain uniform molecular weights, the conversions are usually kept from exceeding 60%.
n-
