Some aspects of stereoselectivities and kinetics in the ring-opening polymerization of norbornenes using metathesis catalysts. The nature of the metallacarbene intermediates

Abstract

Stereoselectivities in the ring-opening polymerization of bicyclo[2. 2. 1]hept-2-ene and several 5,5-disubstituted derivatives have been extensively investigated using numerous metathesis catalysts at different temperatures and concentrations of monomer. Some studies of the kinetics of polymerization are also briefly reported. For many Group VI catalysts at moderate to high concentrations of monomer a limiting value of ca. 50% for the cis content of the polymers is observed. This feature is related to the observation that under these conditions the kinetics of polymerization are zero order with respect to monomer, and is explained by the postulate that propagation is due solely to two mirror-image forms of metallacarbene which may coordinate monomer with equal facility in either a cis or trans orientation. However, with Group VI catalysts at high dilution of monomer, or with noble-metal catalysts at moderate concentrations, polynorbornenes having a high trans content are obtained. A head–tail structural bias, which increases with dilution, also becomes evident in polymers of the 5,5-disubstituted derivatives, especially when the substituents are polar and the catalysts are IrCl₃ or OsCl₃. These results are discussed in terms of Michaelis–Menten kinetic theory with the postulate that the chiral metallacarbenes relax to a more stable symmetrical form. The intermediates described and the kinetic theory developed are used to explain several other important aspects of stereospecificities and selectivities in olefin metathesis and ring-opening polymerization.