The mechanism of stereoselective ring-opening metathesis polymerization of norbornene and several derivatives

Abstract

The properties of several noble metal halide catalysts for ring-opening metathesis polymerization of norbornene, endo-dicyclopentadiene and norbomadiene have been investigated as a function of prior or in situ treatment by the latter pair of diolefins, which may also be monomers as stated. Microstructures of the various polymers and sometimes copolymers were examined in detail using 13C NMR, 1H NMR and IR spectroscopic techniques. In the case of Ru and Os chlorides, there is a dramatic swing from high- trans to high- cis polymer in all cases where chelating diolefin is present, whereas IrCl 3 is unaffected in this respect. High- cis polymers of norbornene, made using RhCl 3 and endo-dicyclopentadiene-modified RuCl 3, show a strong tendency for the small number of trans units that are present to occur in triplets and larger blocks, an unusual blockiness feature previously noted only when the OsCl 3 catalyst was used. The usual tendency for high- cis-poly(norbornene) made using numerous Mo, W and Re based catalysts is to have the trans units in pairs. In all cases, therefore, there is a virtual absence of ctc triads, in contradiction to expectation for random high- cis polymers. This general feature is discussed in terms of a new mechanism based on two or more kinetically distinct metallacarbenes and various conformational forms of metallacyclobutanes undergoing disrotatory [2 + 2] cycloreversion reactions.