The ring-opening metathesis polymerization (ROMP) of cis-cyanocyclooct-4-ene initiated by ruthenium-based catalysts of the first, second, and third generation was studied. For the polymerization with the second generation Grubbs catalyst [RuCl2(=CHPh)(H(2)IMes)(PCy3)] (H(2)IMes = N,N'-bis(mesityl)-4,5-dihydroimidazol-2-ylidene), the critical monomer concentration at which polymerization occurs was determined, and variation of monomer to catalyst ratios was performed. For this catalyst, ROMP of cis-cyanocyclooct-4-ene did not show the features of a living polymerization as M-n did not linearly increase with increasing monomer conversion. As a consequence of slow initiation rates and intramolecular polymer degradation, molar masses passed through a maximum during the course of the polymerization. With third generation ruthenium catalysts (which contain 3-bromo or 2-methylpyridine ligands), polymerization proceeded rapidly, and degradation reactions could not be observed. Contrary to ruthenium-based catalysts of the second and third generation, a catalyst of the first generation was not able to polymerize cis-cyanocyclooct-4-ene. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 879-885, 2011