Steric Control of the Polymerization of α-Olefins in the Presence of Achiral Titanocenes-Methylalumoxane Homogeneous Catalysts

Abstract

Ewen reported stereospecific polymerization of propene in the presence of homogeneous catalysts consisting of dicyclopentadienyldiphenyltitanium(IV) (CPT) and methylalumoxane (MAO) (1). Partially isotactic polypropylene is obtained in the presence of the above catalyst at low temperature (e.g. −40°C) and the stereochemical sequence of the configurations of the substituted carbons is in agreement with the Bernoullian statistical model of the stereospecific propagation proposed by Bovey (2). This fact is peculiar since the stereochemical sequence of the configurations of the substituted carbons of isotactic polypropylene obtained in the presence of either heterogeneous catalysts or homogeneous catalysts based on chiral stereorigid titanocenes or zirconocenes (1,3) is in agreement with the statistical model of the enantiomorphic sites proposed by Shelden (1,4,5). As suggested by Ewen (1) the Bernoullian model can be accounted for by assuming that the stereochemistry of the isotactic polyinsertion is controlled by the chiral chain end (1–3 like asymmetric induction (6)).