Factors influencing chain transfer with monomer and with hydrogen in propene polymerization using MgCl2-supported Ziegler-Natta catalysts

Abstract

13C and 1H NMR analysis of polypropylene prepared using various MgCl2-supported catalysts at different polymerization temperatures at different hydrogen concentrations has revealed that chain transfer with monomer, yielding vinylidene-terminated chains, is essentially unaffected by hydrogen but is dependent on catalyst selectivity. Chain transfer with hydrogen and with monomer are therefore processes which operate independently of each other, but in both cases, the incidence of chain transfer increases with decreasing catalyst (site) selectivity. Chain transfer with monomer takes place after primary insertion, while chain transfer with hydrogen takes place after either primary or secondary insertion. Chain initiation via a secondary insertion into a Ti-H species leads either to a 2,3-dimethylbutyl terminal unit, as has recently been observed in metallocene-catalyzed polymerizations, or to a further transfer reaction with hydrogen, generating traces of propane.