Two new dimethylvinyl carbene complexes, RuCl2(SIMes)(PPh3)CHCHC(CH3)(2) and RuCl2(SIMes)(3BP)(2)CHCHC(CH3)(2), were synthesized from RuCl2(PCP3)(2)CHCHC(CH3)(2). Complex RuCl2(SIMes)(3BP)(2)CHCHC(CH3)(2) does not suffer from the problem of incomplete initiation that has been observed for the other dimethylvinyl carbene complexes, as witnessed by complete and rapid reaction with ethyl vinyl ether. Acyclic diene metathesis (ADMET) polymerization of 1,9-decadiene with these complexes was found to give polymers with chemical and thermal properties similar to those obtained with Schrock's molybdenum catalyst. These complexes are also catalysts for ring-opening metathesis polymerization. The parent complex RuCl2(SIMeS)(PCP3) CHCHC(CH3)2 was found to give polyoctenamer with high initial heats of fusion, suggesting a dependence of the "as formed" crystallinity of the polymer on the rate of the ROMP reaction. (c) 2005 Wiley Periodicals, Inc.