Catalysts, copolymerization ethylene-propylene rubbers

When ethylene is copolymerized with substantial amounts (>25%) of propylene an elastomeric copolymer is produced, commonly known as ethylene-propylene rubber (EPR) or ethylene-propylene monomer (EPM) rubber. When a diene, such as dicyclopentadiene, is also included, a terpolymer known as ethylene-propylene-diene monomer (EPDM) rubber is obtained. EPR and EPDM are produced with single site and Ziegler-Natta catalysts and are important in the automotive and construction industries. However, EPR and EPDM are produced in much smaller quantities relative to polyethylene. Elastomers display vastly different properties than other versions of industrial polyethylene and are considered outside the purview of this text. EPR and EPDM will not be discussed further. 

As opposed to the problems associated with the formation of sequential block copolymers, the preparation of relatively random copolymers is much easier and the provision of polyethylenes having a controlled degree of branching by copolymerization with propylene and butene is now a well-established commercial operation. When ethylene and propylene are employed in approximately equal proportions the ethylene-propylene rubbers are obtained. For this purpose strictly random copolymers are desirable, for which soluble vanadium catalysts are often preferred (20). With TiCls-based catalyst the propylene monomer molecule prefers to add to a propylene end unit rather than to an ethylene end unit (and vice versa). This tends to produce nonrandom blocky copolymers. Thus a recent paper by Coover ef al. (21) selects as catalysts formulations which maximize this tendency and achieve the preparation of block copolymers in a TiCl3/AlEt2Cl catalyst system in the presence of butene and propylene together. 

Ethylene-propylene rubbers are suitable additives for isotactic polypropylene. They can be added to the matrix by melt-mixing or by block copolymerization, using Ziegler-Natta catalysts (the copolymer is made by adding ethylene, as a second monomer, during the final stages of PP polymerization). 

Impact-modified polypropylene is produced by using two reactors in series [7]. The first produces polypropylene particles, which are fed to the second reactor. A mixture of gaseous ethylene and propylene monomers (but no additional catalyst) is fed to the second reactor. The monomers diffuse to the still-active catalyst within the polypropylene particles from the first reactor, where they copolymerize to form a core of ethylene-propylene rubber within a polypropylene shell. This morphology greatly enhances the impact strength over that of polypropylene alone. 

Various monomers eontaining norbornene and nor-bomadiene structure have been employed in copolymerization reactions to obtain new products with improved physico-ehemieal and meehanieal properties [30]. Synthetic rubber with unsaturated units (33) is manufactured by copolymerizing ethylene and propylene with dicyclopentadiene in inert hydrocarbon solvents using VCI4 or VOCI3 associated with R2AICI or R A1C12 as the catalysts [30a,b]. 

Unconjugated dienes can produce an even more complicated range of macro-molecular structures. Homopolymers of such monomers are not of current commercial importance but small proportions of monomers like 1,5-cyclooctadicne are copolymerized with ethylene and propylene to produce so-called EPDM rubbers. Only one of the diene double bonds is enchained when this terpolymeriza-tion is carried out with Ziegler-Natta catalysts (Section 9.5). The resulting small amount of unsaturation permits the use of sulfur vulcanization, as described in Section 1.3.3. 

---