Molecular weight distribution and stereoregularity of polypropylenes produced with VCl4–AlEt2Cl catalyst

Abstract

AbstractDependences of the molecular weight distribution and stereochemical regulation of the polypropylenes produced with VCl4–AlEt2Cl catalyst on the polymerization temperature were examined. The molecular weight distributions of the polymers obtained at temperatures below −40°C were unimodal and narrow (Mw/Mn ≤ 2). The molecular weight distributions obtained at higher temperatures (above −21°C) were bimodal with one narrow distribution and one wide one (Mw/Mn > 2), and the polymer fraction of the wide distribution increased with the polymerization temperature. The fractional amount of (CH2)2 groups in the polymers, which corresponds to tail‐to‐tail linkage of two propylene units, increased to a maximum at −21°C followed by a gradual decrease with the polymerization temperature. The production of isotactic polymers was confirmed at temperatures above −21°C. From these data, it is concluded that only the homogeneous form of the catalyst system is responsible for the polymerization at temperatures below about −21°C while the heterogeneous form appears and catalyzes the polymerization together with the homogeneous one at temperatures above −21°C.