Remarkable alternating effect in metathesis copolymerization of norbornene and cyclopentene using modified Grubbs ruthenium initiators

High trans and mainly cis directing Grubbs Ru initiators, I-III, and IV, respectively have been investigated as catalysts for metathesis copolymerization of norbornene and cyclopentene. When the Lewis acid, MoCl5 or WCl6, is added to these Lewis bases initiators, I-III, a cage effect develops such that the polymers become alternating caused by the severe or almost total denial of access of norbornene to the propagating metallacarbenes. Addition of ethers destroys the cages and removes the effect. In marked contrast the N-heterocyclic ligand Grubbs initiator, IV, does not show the effect, as addition of MoCl5 only seems to retard the overall reaction, but does not change the copolymer composition. (C) 2003 Elsevier B.V. All rights reserved.