Highly active catalyst systems for the ring-opening metathesis polymerization (ROMP) of strained (norbornene) and low-strain (cyclooctene) olefins are readily prepared from RuCl2(arene)(PR3) precursors (directly available by addition of a phosphine to the stable [(arene)RuCl2](2) dimers) after activation with (trimethylsilyl)diazomethane. Durene or p-cymene as arene ligands, together with a sterically demanding basic phosphine (typically tricyclohexylphosphine), promoted the formation of the most active polymerization catalysts. The effects of arene and phosphine ligands and of the solvent on polynorbornene and polyoctenamer molecular weight distributions and microstructures were investigated. The excellent functional group compatibility of the catalyst system was illustrated by the synthesis of a variety of polyoctenamers bearing epoxide, acid, ether, eater, acetal, and bromine functionalities. The polymers were isolated in quite good yields. The striking positional influence of the functional group on the polymerization was revealed by comparing two 4,5-substitutd cyclooctenes with the corresponding allylic derivatives. Sulfide and azide functionalities in the monomers resulted in a deactivation of the catalyst. The characterization of the polymers by IR- and NMR-spectroscopies revealed a lack of high regio- and stereospecificity in the propagation step.