Mechanism of ring‐opening polymerization of 1‐methylbicyclo[2.2.1]hept‐2‐ene using metathesis catalysts

Abstract

AbstractPolymers of the title monomer have been made using 24 different metathesis catalysts, based on Nb, Mo, W, Re, Ru, Os and Ir compounds. Their structure, determined by 13C NMR spectroscopy, varies from all‐cis (ReCl5) to all‐trans (RuCl3, OsCl3) and from fully biased (head‐tail) to unbiased. The bias stems partly from an inability to from head‐head dyads containing cis double bonds and partly from the dipolar interaction between the propagating metal‐carbene complex and the monomer in forming the [2 + 2] transition state. Arguments are presented to show that the formation of biased polymers proceeds largely through a metal carbene species in which the last‐added monomeric unit has its methyl group adjacent to the metal centre (PH). In high‐cis polymers the sequences of three double bonds are such as to indicate that a kinetic distinction must be made between species in which the previous double bond formed was cis or trans (PHc or PHt), consistent with the observation that high‐cis polymers of related monomers have a blocky cis/trans distribution.