Chemistry Reference
In-Depth Information
6.5 Copolymers of Ethylene and Propylene
Many monomers have been copolymerized with ethylene by a variety of polymerization methods.
When ethylene is copolymerized with other olefins, the resultant hydrocarbon polymers have reduced
regularity and lower density, lower softening point, and lower brittle point.
Copolymers of ethylene and propylene are a commercially important family of materials.
They vary from elastomers that can contain 80% ethylene and 20% propylene to polypropylene
that is modified with small amounts of ethylene to improve impact resistance.
Metallocene catalysts can produce both random and alternating copolymers of ethylene and
propylene [
85
]. At present there does not appear to be any commercial utilization of alternating
copolymers. They were reported to form in polymerizations catalyzed by bridged fluorenyl
catalysts [
85
].
6.5.1 Ethylene and Propylene Elastomers
ethylene-propylene rubbers
The commercial
typically range in propylene content from 30 to 60%,
depending upon intended use. Such copolymers are prepared with Ziegler-Natta type catalysts.
Soluble catalysts and true solution processes are preferred. The common catalyst systems are
based on VCl
4
, VOCl
3
, V(Acac)
3
, VO(OR)
3
, VOCl(OR)
2
, VOCl
2
(OR), etc. with various
organoaluminum derivatives. The products are predominantly amorphous. Polymerization reactions
are usually carried out at 40
C in solvents like chlorobenzene or pentane. The resultant random
copolymers are recovered by alcohol precipitation. Because these elastomers are almost completely
saturated, cross-linking is difficult. A third monomer, a diene is, therefore, included in the
preparation of these rubbers that carry the trade names EPTR or EPDM. Inclusion of third monomers
presents some problems in copolymerization reactions. For instance, it is important to maintain
constant feed mixtures of monomers to obtain constant compositions. Yet, two of the three
monomers are gaseous and the third one is a liquid. Natta [
86
] developed a technique that depends
upon maintaining violent agitation of the solvent while gaseous monomers were bubbled through
the liquid phase. This was referred to as “semi-flow technique.” The process allows the
compositions of gaseous and liquid phases to be in equilibrium with each other and to be more or
less constant [
87
]. Other techniques evolved since. All are designed to maintain constant polymeri-
zation mixtures.
The vanadium-based catalyst systems deteriorate with time and decrease in the number of catalytic
centers as the polymerizations progress. The rate of decay is affected by conditions used for catalyst
preparation, compositions of the catalysts, temperature, solvents, and Lewis bases. It is also
affected by the type and concentration of the third monomer [
88
-
90
]. Additions of chlorinated
compounds to the deactivated catalysts, however, help restore activity [
91
,
92
]. Catalyst decay
can also be overcome by continually feeding catalyst components into the polymerization
medium [
93
].
While third monomer can be a common diene, like isoprene, more often it is a bridged ring
structure with at least one double bond in the ring. In typical terpolymer rubbers with 60-40 ratios of
ethylene to propylene the diene components usually comprise about 3% of the total. Some specialty
rubbers, however, may contain 10% of the diene or even more. Reaction conditions are always chosen
