The present invention relates to use of certain chain transfer agents to control molecular weight of addition mass polymerization of certain polycycloolefinic monomers. More specifically, the present invention relates to use of a series of substituted bicycloalkenes as chain transfer agents in the addition mass polymerization of a series of functionalized norbornene-type monomers. This invention also relates to compositions containing bicycloalkenes as chain transfer agents in forming “in mold” polycycloolefinic polymers by addition mass polymerization.