Copolymers from promoted preformed solid catalysts

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Abstract
Promoted preformed solids catalyze polymerization of ethylene and propylene and formation of copolymers of ethylene with such olefins as propylene and butene‐1. The structure of the copolymers helps explain how they are formed and their properties. Kinetic studies alone do not distinguish between polymerization mechanisms involving either anionic centers or ion‐radicals. Structural studies of ethylene–propylene copolymers of low propylene content show that they contain isolated propylene units in accord with the ion‐radical theory of polymerization in a chemisorbed layer. Isolated short sidechains, so introduced, reduce density and increase flexibility. They give copolymers with flow properties more like those of linear polyethylenes than those of branched polyethylenes with long sidechains. Furthermore, they interfere with recrystallization and crystal growth; the resulting polymers have better stress‐crack resistance and form clearer films.