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Soluble catalyst systems derived from alkyl aluminum halides in combination with vanadium oxytrichloride or tetrachloride produce highly random ethylene-propylene copolymers with high catalyst efficiency. By the choice of the alkyl aluminum halide and molar ratio of aluminum to vanadium, variations in polymerization efficiency and molecular weight are possible. The copolymers prepared with these soluble catalysts show advantages in both processing and physical properties over those prepared with heterogeneous catalysts. Vulcanizates of these copolymers show somewhat different accelerated aging properties depending on the additive used along with the peroxide. Low temperature properties of copolymers containing less than 70 weight % propylene show a tendency for crystallization which is not shown by X-ray diffraction. Overall physical properties and tire tests show some preference for the 65 weight % propylene copolymer over the 50% material.

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