The ring-opening metathesis polymerization (ROMP) of a variety of 5-substituted cyclooctenes by the well-defined metathesis catalyst (PCy3)2Cl2RuCHCHCPh2 (1) was accomplished. Direct polymerization of functionalized monomers allowed the incorporation of alcohol, ketone, ester, and bromine functionality along the polymer backbone. The polymers were obtained in moderate to good yields. The attempted polymerization of epoxy- and cyano-substituted cyclooctenes by 1 failed. Structures of the polymers were confirmed by IR, 1H NMR, and 13C NMR spectroscopies. The molecular weight of acetate-containing polymer was controlled by varying the monomer to catalyst ratio and by the addition of a chain transfer agent to the polymerization solution. Hydrogenation of the acetate derivative gave the corresponding ethylene/vinyl copolymer. The thermal properties of the polymers are reported.