Theoretical investigation of the stereochemistry of the polymerization of α-olefins and dienes with the participation of ziegler-natta catalysts

Abstract

The processes involved in the formation of the active polymerization sites in heterogeneous Ziegler-Natta catalysts have been investigated with consideration of the real structure of the components by the atom-atom potential method, the Monte-Carlo method, a modified diatomics-in-molecules method, and the CNDO/2 method with the aid of the available experimental facts. It has been shown that three types of bimetallic active sites (AS), which differ with respect to the spatial configuration of the coordination sphere, viz., AS-1, AS-2, and AS-3, form, depending on the electronic structure of the homogeneous component R_nM, the ionic radius of M (M is a metal from groups I–III), and the unit-cell parameters of the heterogeneous component MeX_m (Me is a transition metal from groups IV–VIII). Only the AS-1 sites are stereospecific in the polymerization of α-olefins and 1,3-dienes (isotactic polyolefins and 1,4-trans-polydienes form); the AS-2 sites are nonstereospecific in the polymerization of α-olefins, but they form stereoregular 1,4-cis-polydienes; the AS-3 sites are nonstereospecific in the polymerization of both α-olefins and 1,3-dienes. The phenomenon of stereoregularization in the polymerization of α-olefins and 1,3-dienes is determined by the steric and electrostatic factors.