A STEREOCHEMICAL AND STATISTICAL-ANALYSIS OF METALLOCENE-PROMOTED POLYMERIZATION

The metallocene catalysts active in propylene polymerization can be di vided into five classes, depending on whether the catalytic sites are chirotopic or not, and on the symmetry relationship between the cataly tic sites bound to the same metal atom. Under the hypothesis of a pure ly alternating mechanism for propagation, class III and class IV catal ysts, respectively, give rise to prevailingly isotactic and syndiotact ic polymers. Four probabilistic schemes have been developed for interp reting the stereosequence distribution of polypropylene in terms of pe ntads. They take into account the two sources of steric control, chiro topicity of the catalyst site and configuration of the chain end, and the presence, besides the alternating mechanism, of successive additio n steps at the same catalytic site.