Copolymerization of cycloalkenes as a probe of the propagation steps in olefin metathesis

A range of ring-opened metathesis copolymers of norbornene and cyclopentene have been prepared and C-13 NMR spectroscopy used to analyse in detail the nature of the homo and heterodyad units. This has provided significant new information on the sensitivity of the [2 + 2] cycloaddition step in metathesis to the steric and electronic factors associated with the [Mt]=C and C=C moieties involved. Novel very high cis directing catalyst systems, have also been developed for the homopolymerization of norbornene using various ethers with Mo and W-based catalyst systems or chelating dienes and phenylacetylene with RuCl3 and OsCl3 systems. The general feature of cis/trans blockiness at high cis content is investigated in much greater detail. A clearer description of the associated relaxation processes of the propagating species emerges from this work. The metathesis polymerization of the matched pair, benzonorbornadiene and 7-oxa-benzonorbornadiene has been investigated using Ru and Os-based initiators. The results illustrate the subtle way whereby the presence of the proximate 7-O-atom may facilitate [2 + 2] cycloadditions. This constitutes a valuable novel method of studying key electronic factors in the mechanism of the metathesis reaction.