Highly stereospecific catalytic systems for the polymerization of α‐olefins to isotactic polymers

The highly stereospecific catalytic systems for the polymerization of α‐olefins have been compared. These are obtained starting either from the α, γ, or δ modifications of violet TiCl3 with or without AlCl3 present in solid solution, and from aluminum diethyl monohalides, or from metalloorganic products obtained by reacting two molecules of aluminum monoalkyl dihalide with one molecule of particular electron donor compounds. The catalytic systems considered polymerize propylene, giving rise to crude polymers containing 90 to 100% isotatic propylene (nonextractable in boiling n‐heptane) for a wide range of polymerization temperatures. By fractionation of crude polymers prepared with the use of these catalytic systems, highly isotactic polypropylenes, having a melting temperature up to 177°C., have been separated. The results, correlated with those obtained in x‐ray determinations of the structure of the modification of TiCl3 used, demonstrate that the stereospecificity of all the catalytic systems examined is independent of the particular type of violet modification (α, γ, or δ), whether or not these contain aluminum as AlCl3 in solid solution. This proves that the single active centers are substantially of the same type; a variation in their number causes only a variation of the overall polymerization rate.