Yury V. Kissin and Laura A. Rishina
A new mechanism for the hydrogen activation effect in propylene polymerization reactions with Ti-based Ziegler–Natta catalysts is proposed. A propylene polymerization reaction occasionally leads to the formation of a dormant center incapable of propylene insertion, the TiCH(CH3)2 species:
Hydrogen reacts with the TiCH(CH3)2 species and generates the TiH bond. The latter predominantly inserts propylene in the primary orientation and so reactivates the center:
Ethylene can insert into the TiCH(CH3)2 species, although relatively slowly (this reaction accounts for the ethylene activation effect in propylene polymerization reactions):
Polymer molecules generated in a series of steps starting with the latter reaction were observed in low molecular weight fractions of ethylene/α-olefin copolymers produced with Ti-based catalysts.