Synthesis of polymers with a controlled molecular weight via ring-opening metathesis polymerization (ROMP) with a ruthenium-based catalyst

Various bisallylic ruthenium(IV) complexes in combination with certain diazoalkanes show high catalytic activities in the ring-opening metathesis polymerization (ROMP) of norbornene and its derivatives. In particular, dicationic complexes in the presence of ethyl diazoacetate were found to be extraordinarily efficient. The ionic character of the catalyst determines the use of polar solvents and therefore the precipitation polymerization of monomers like norbornene. The ratio of methanol to dichloromethane in the solvent mixture could be used to adjust the number-average molecular weight of polynorbornene in the range of 3 × 10⁵ g/mol up to 1 × 10⁶ g/mol at a constant monomer to catalyst ratio of 1 × 10³/1. By polymerizing norbornene in neat methanol, the number-average molecular weights could be varied between 2 × 10³ and 3 × 10 g/mol by the addition of certain quantities of 1-octene as a chain-transfer agent. According to the proposed mechanism, these polymers contain equivalent amounts of heptylidene and methylidene end-groups. Furthermore, ethyl diazoacetate was found to act not only as a carbene-generating co-catalyst, but also as a terminating agent. Consequently, an excess of ethyl diazoacetate could be used for the preparation of telechelic polymers with two ester end-groups and a reduced molecular weight.