Chemistry Reference
In-Depth Information
Should branching become excessive, infinite networks can form. The products become
cross-linked, insoluble, and infusible. Such materials are called
polymers. This phenomenon
is more common in bulk polymerizations. The cross-linked polymers form nodules that occupy much
more volume than the monomers from which they formed and often clog up the polymerization
equipment, sometimes even rupturing it.
High molecular weight homopolymers of 1,3-butadiene formed by free-radical mechanism lack
the type of elastomeric properties that are needed from commercial rubbers. Copolymers of butadi-
ene, however, with styrene or acrylonitrile are more useful and are prepared on a large scale. This is
discussed in another section.
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6.6.1.1 Liquid Polybutadiene
Low molecular weight liquid homopolymers of 1,3 butadiene, also some liquid copolymers, find
industrial uses in many applications. These materials can range in molecular weights from 500 to
5,000 depending upon the mode of polymerization. Liquid polybutadienes formed by cationic
polymerizations are high
1,4 content. Such materials find applications in industrial coatings.
They are usually prepared with Lewis acids in chlorinated solvents. When the reactions are catalyzed
by AlCl
3
at
trans-
78
C, two types of polymers form [
108
]. One is soluble and the other is insoluble,
depending upon the extent of conversion. AlCl
3
, AlBr
3
, and BF
3
-Et
2
O produce polymers with the
same ratios of
1,4 to 1,2 adducts. These range from 4 to 5. Polymerizations carried out in
ethylene chloride [
108
] catalyzed by TiCl
4
yield products with lower ratios of
trans-
1,4 to 1,2
adducts. The ratios of the two placements are affected by the solvents. They are also affected by
additions of complexing agents, such as nitroethane and nitrobenzene [
108
]. The changes, however,
are small.
Hydroxyl-terminated liquid polybutadienes are prepared for reactions with diisocyanates to form
elastomeric polyurethanes (see Chap.
6
). Such materials can be prepared by anionic polymerizations
as “living” polymers and then quenched at the appropriate molecular weight. These polybutadienes
can also be formed by free-radical mechanism. The microstructures of the two products differ,
however, and this may affect the properties of the finished products. To form hydroxyl-terminated
polymers by free-radical mechanism, the polymerization reactions may be initiated by hydroxyl
radicals from hydrogen peroxide.
A new approach to preparation of hydroxyl-terminated liquid polybutadiene is to use a cyclic
section on metathesis ring opening polymerization) and an acetate functionalized chain transfer agent
[
109
,
110
]. The acetate-functionalized chain transfer agent is
trans-
-2-butene-1,4-diacetate. The reaction
can be carried out without a solvent and proceeds at 50
C over 6 h under an inert gas purge [
111
].
The acetate protecting groups provide compatibility with the ruthenium catalyst. Subsequent to
polymerization the acetate groups can be converted to hydroxyl end groups with the aid of a base,
like sodium methoxide.
Liquid polybutadienes that are high in 1,2 placement are also available commercially. These range
from reactive polymers containing approximately 70% of vinyl groups to very reactive ones
containing more than 90% of 1,2 units. The materials are formed by anionic polymerization with
either sodium naphthalene, or with sodium dispersions, or with organolithium initiators in polar
solvents. Carboxyl group terminated liquid polybutadienes are predominantly used as modifiers for
catalysts like diphenylethanedilithium, butanedilithium, isoprenelithium, or lithium methylnaphtha-
lene complexes. Cyclohexane is the choice solvent. The reaction is quenched with carbon dioxide to
introduce the terminal carboxyl groups.
cis
