Transformations of 2-trimethylsilylnorbornadiene-2,5 (SNBD) and 2,3-bis(trimethylsilyl)norbornadiene-2,5 (DSNBD) under metathesis and addition polymerization conditions were studied. In the presence of the WCl(6)/1,1,3,3-tetramethyldisilacyclobutane-1,3 and Cl(2)(PCy(3))(2)Ru=CHPh catalysts, high-molecular-mass silicon-substituted poly(cyclopentylidene vinylene)s were obtained with yields up to 98%. Under addition polymerization conditions, SNBD and DSNBD dimerized on the nickel(II) naphthenate-methyl alumoxane catalyst to give pentacyclic dimers. Three dimers were detected in the case of SNBD, whereas the exo-trans-exo-cyclodimer alone was exclusively produced from DSNBD. Being inactive in addition homopolymerization, both SNBD and DSBND take part in addition copolymerization with norbornene.