A method of controlling the molecular weight of poly(cyclic)olefin (norbornene-type) polymers and activating the polymerization thereof with a single material is provided. Such method include adding a chain transfer/activating agent to a mixture of monomer(s), catalyst, solvent and an optional cocatalyst and polymerizing the mixture to form a polymer. It is shown that the amount of chain transfer/activating agent in the mixture can serve to control the molecular weight of the resulting polymer, its percent conversion or both, and in some embodiments the optical density of the resulting polymer.