Cationic substitutionally inert cyclometalated ruthenium (II) and osmium (II) complexes, ([Mt(o-C6H4-2-py)(LL)(2)]PF6), where LL-1,10-phenanthroline (phen) or 2,2'-bipyridine (bipy), were used for radical polymerization of styrene. Gradual modification of the complexes within the series allowed comparison of the catalytic activity and the redox properties. There was no correlation between the reducing powers of the complexes and their catalytic activities. The osmium compound of the lowest reduction potential was not active. All the ruthenium complexes catalyzed the polymerization of styrene in a controlled manner; but the level of control and the catalytic activity were different under the same polymerization conditions. [Ru(o-C6H4-2-py)(phen)(2)]PF6 demonstrated the best catalytic performance though its redox potential was the highest. It catalyzed the "living" polymerization with a reasonable rate at a catalyst-to-initiator ratio of 0.1. 1 equiv. of Al(OiPr)(3) accelerated the polymerization and improved the control, but higher amount of Al(OiPr)(3) did not speed up the polymerization and moved the process into the uncontrollable regime. Under the most optimal conditions, the controlled polymerization occurs fast without any additive and the catalyst degradation. Added free ligands inhibited the polymerization suggesting that the catalytically active ruthenium intermediates are generated via the reversible dechelation of bidentate phen or bipy ligands.(C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3814-3828, 2009