Ethylene-propylene rubbers catalyst systems

Figure 2 shows the profile of the 27-29 ppm spectral region of three polymers which served as models (1J ) for ethylene propylene rubber. The better agreement between the observed spectrum and the five-parameter model strongly suggests the three-parameter model is less realistic as an explanation for the polymerization mechanism. Table VII compares the observed profiles of EPDM rubbers made with a Ziegler catalyst system. The ratio of... 

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. 

At present tubular turbulent apparatus system of divergent-convergent design is used in ethylene-propylene rubbers production at the stages of catalytic system decomposition, removing of catalyst from polymer and introduction of stabilizer-antioxidant into rubber solution [78, 79]. 

Commercial ethylene-propylene rubbers (EPR or EPM ) generally contain about 35 mole % propylene although rubbery properties are shown by copolymers with a propylene content ranging from 30—60 mole %. At the present time, these materials are prepared exclusively by Ziegler-type processes. Generally, true solution processes are preferred in which a soluble catalyst system is used and the polymer remains in solution rather than form a slurry. A common soluble catalyst system is based on vanadium oxychloride/aluminium trihexyl. Catalysts of this type favour the formation of amorphous atactic polymers and lead to narrower molecular weight distributions than solid catalysts. Typically, polymerization is carried out at about 40°C in a solvent such as chlorobenzene or pentane and the polymer is isolated by precipitation with an alcohol. 

In addition to titanium-based Ziegler-Natta catalysts, vanadium-based systems have also been developed for PE and ethylene-based co-polymers, particularly ethylene-propylene-diene rubbers (EPDM). Homogeneous (soluble) vanadium catalysts produce relatively narrow molecular mass distribution PE, whereas supported V catalysts give broad molecular mass distribution.422 Polymerization activity is strongly enhanced by the use of a halogenated hydrocarbon as promoter in combination with a vanadium catalyst and aluminum alkyl co-catalyst.422,423... 

Several polymerization processes are carri out in single liquid phase systems. The most widespread process of this type is the high pressure polymerization of ethylene (for "low-density" polyethylene). Other well Imown examples are the newest high temperature versions of processes for the polymerization of ethylene with Ziegler-type catalysts (for "high density" polyethylene), the preparation of some synthetic rubbers (polybutadiene, polyisoprene, ethylene-propylene co- and ter-polymers), the "bulk" polymerization of styrene, etc. All these processes have in common that the reaction product is a polymer melt or solution, that is relatively viscous. Since at the same time the intrinsic reaction rates are usually quite high, the conversion rates are often limited by diffusion. These processes are usually carried out in stirred reactors, for which the effects of micro-mixing have to be taken into account. 

Recently a codimerization of ethylene and butadiene using various organometallic catalyst systems to give cw-1,4 hexadiene was found, equation (7-5). Hexadiene is a potential third component (or termonomer) of ethylene-propylene-diene-rubber [sometimes called ethylene propylene-terpolymer, EPT or EPDM (according to the ASTM)]. 

The RCP part of the mixture is designed to have ethylene contents on the order of 40-65% ethylene and is termed the rubber phase. The rubber phase can be mechanically blended into the ICP by mixing rubber and HPP in an extruder or it can be polymerized in situ in a two-reactor system. The HPP is made in the first reactor and the HPP with active catalyst still in it is conveyed into a second reactor where a mixture of ethylene and propylene monomer is polymerized in the voids and interstices of the HPP polymer powder particle. The amount of rubber phase that is blended into the HPP by mechanical or reactor methods is determined by the level of impact resistance needed. The impact resistance of the ICP product is determined not only by its rubber content but also by the size, shape, and distribution of the rubber particles throughout the ICP product. Reactor products usually give better impact resistance at a given rubber level for this reason. 

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