A kinetic study of three ruthenium carbene catalysts, (H2IPr)(PCy3)(Cl)(2)Ru=CHPh, 3 (investigated extensively by Mol), (H(2)IMes)(Cl)(2)Ru=CH(o-iPrOC(6)H(4)), 4 (Hoveyda's catalyst), and (H2IPr)(Cl)(2)Ru=CH(o-iPrOC(6)H(4)), 5 (a new catalyst structure), was conducted under ADMET polymerization conditions. The kinetic behavior of these catalysts was compared to the classical first- and second-generation Grubbs' complexes at 30, 45, and 60 degreesC. Complex 3 exhibits the highest initial ADMET rate (80 DP s(-1)) of any phosphine complex to date and efficiently promotes metathesis even at temperatures as low as 0 degreesC. Complex 4 alone does not polymerize 1,9-decadiene in the bulk; however, addition of a polar solvent induces polymerization. Combining elements of catalysts 3 and 4 yielded the new complex 5. This complex results in higher polycondensation rates than previous Hoveyda-type structures and exhibits an increased stability over its parent phosphine complex. The new catalyst polymerizes 1,9-decadiene in the bulk to high polymer (M-n = 40 000 g/mol) using low catalyst loadings (0.1 mol %). The isomerization chemistry induced by complexes 3 and 5 was investigated using a model compound, 1-octene.