Discussion on the mechanism of polymerization of α‐olefins with Ziegler‐Natta catalysts

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Abstract
A monometallic mechanism of monomer complexation and addition to the growing chain for the isotactic polymerization of an α‐olefin in the presence of α‐, γ‐ or δ‐TiCl3 is proposed, which avoids the back‐jump of the Ti‐R bond required by the COSSEE‐ARLMAN mechanism. The overall activation energy for propagation involved in the latter mechanism should be higher than that allowed by the mechanism proposed here, which may be more easily reconciled with the figure found for polypropylene (∼ 5 kcal/mole). The steric requirement for a Ti atom on the side surface of a crystal to be catalytically active is that monomer complexation must be possible on both the Ti valencies pointing outside: this requires the active site to be slightly protruding above the two neighbouring layers. Each active site is intrinsically chiral; the cis‐opening of the double bond is in agreement with the mechanism.