Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule. Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule. Chain transfer reactions reduce the average molecular weight of the final polymer. Chain transfer can be either introduced deliberately into a polymerization (by use of a chain transfer agent) or it may be an unavoidable side-reaction with various components of the polymerization. Chain transfer reactions occur in most forms of addition polymerization including radical polymerization, ring-opening polymerization, coordination polymerization, and cationic polymerization, as well as anionic polymerization.