Mechanism of initiation of the ring-opening polymerization and addition oligomerization of norbornene using unicomponent metathesis catalysts

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Abstract
The identities and yields of the low-molecular-weight products formed during ring-opening polymerization of norbornene in benzene solvent using several unicomponent metal halide catalysts (W, Mo, Re, Ru, Os and Ir) were extensively investigated. Dimers and trimers were observed in all cases but were only substantial relative to polymer yields for WCl6 and ReCl5. The rate of formation of the dimers exactly paralleled that of the ring-opened polymer, in marked contrast to the rate of growth of the Friedel–Crafts alkylation product, phenylnorbornene. Cyclo-octa-1,5-diene complexes of Ru, Os and Ir halides were also prepared and found to contain hydride ligands whose concentration seemed to be important in controlling catalytic activity. A general mechanism for both the ring-opening polymerization and the addition oligomerization is developed based on a sequence of steps consisting of reaction of metal hydride with monomer to give the corresponding alkyl-metal complex, which in turn forms a hydridometallacarbene intermediate. While metallacarbenes then propagate ring-opening polymerization, the detailed structures of the dimers and trimers are best explained by a mechanism involving reaction of monomer with the hydridometallacarbene species rather than by the classical alkyl/olefin insertion mechanism for Ziegler–Natta polymerization of alk-l-enes.

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