Studies on the Ring Opening Polymerisation of Cyclopentene

Abstract

This work describes investigations on the initiation and the propagation of the polymerisation of cyclic alkenes catalysed by tungsten based systems. Cyclopentene was polymerised using the bimetallic WCl₆/Al(iBu)₃ catalyst and infra-red spectroscopy showed that trans-polypentenamer was formed. A kinetic chain mechanism was invoked to describe the propagation step in the polymerisation and to account for the bimodal molecular weight distribution of the polymer it was proposed that two active propagating species were present in the polymerisation process. Oxidation state studies showed that during the polymerisation tungsten existed in at least two oxidation states. Subsequently a monometallic catalyst system - WC16/O2 - capable of polymerising cyclopentene, was developed. Infra-red studies showed that the product was again a trans-polypentenamer and oxidation state studies on an analogous system suggested similarities between the mechanism of the polymerisation initiated by the monometallic catalyst and that initiated by the bimetallic system. Ultra-violet spectroscopic investigations on the WCl6/Cp/O2 reaction further clarified the nature and mode of formation of the species active in the polymerisation. It was finally concluded that a kinetic chain mechanism could be invoked to describe the propagation steps in both polymerisation systems wherein the initiation step was the formation of a tungsten carbene and the polymerisation proceeded by coordination of monomer to the tungsten and insertion into the growing kinetic chain. The role of the aluminium and that of the oxygen in their respective systems were thought similar and to be the production of the metal carbene from a tungsten metallocyclic which arose from a tungsten/cyclopentene reaction. A molecular orbital treatment of the mode of formation of the various species in the polymerisation concludes the report.