Chemistry Reference
In-Depth Information
to obtain 1,2 placement of the diene. Dienes in common use are ethylidine norbornene, methylene
norbornene, 1,4-hexadiene, dicyclopentadiene, and cycloocatadiene:
ethylidine
norbornene
methyle
norbornene
1,4-hexadiene
dicyclopentadiene
cyclooctadiene
In addition to the above, the patent literature describes many other dienes. An idealized picture of a
segment of an uncross-linked gum stock might be shown as having the following structure:
6.5.2 Copolymers of Ethylene with
-Olefins and Ethylene
a
with Carbon Monoxide
Many copolymers of ethylene with
a
-olefins are prepared commercially. Thus ethylene is
copolymerized with butene-1, where a comonomer is included to lower the regularity and the density
of the polymer. Many copolymers are prepared with transition metal oxide catalysts on support.
The comonomer is usually present in approximately 5% quantities. This is sufficient to lower the
crystallinity and to markedly improve the impact strength and resistance to environmental stress-
cracking. Copolymers of ethylene with hexene-1, where the hexene-1 content is less than 5%, are also
produced for the same reason.
In most cases, the monomers that homopolymerize by Ziegler-Natta coordination catalysts also
copolymerize by them [
94
]. In addition, some monomers that do not homopolymerize may still
copolymerize to form alternating copolymers. Because the lifetime of a growing polymer molecule is
relatively long (can be as long as several minutes), block copolymerization is possible through
changes in the monomer feeds. Also, the nature of the transition metal compound influences
the reactivity ratios of the monomers in copolymerizations. On the other hand, the nature of the
organometallic compound has no such effect [
95
]. It also appears that changes in the reaction
temperature between 0 and 75
C have no effect on the
values. Copolymers can be formed using
either soluble or heterogeneous Ziegler-Natta. One problem encountered with the heterogeneous
catalysts is the tendency by the formed polymers to coat the active sites. This forces the monomers to
diffuse to the sites and may cause starvation of the more active monomer if both diffuse at equal rates.
Many different block copolymers of olefins,
r
like ethylene with propylene and ethylene
with butene-1, are manufactured. Use of the anionic coordination catalysts enables variations in
