Chain transfer

In polymer chemistry, chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule:

$${\displaystyle {\ce {P}}^{\bullet }+{\ce {XR -> PX + R}}^{\bullet }}$$ where • is the active center, P is the initial polymer chain, X is the end group, and R is the substituent to which the active center is transferred.

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.

Chain transfer (in a chain polymerization): Chemical reaction occurring during a chain polymerization in which an active center is transferred from a growing macromolecule or oligomer molecule to another molecule or to another site on the same molecule.

Chain-transfer agent: Substance able to react with a chain carrier by a reaction in which the original chain carrier is deactivated and a new chain carrier is generated.

Chain transfer reactions are usually categorized by the nature of the molecule that reacts with the growing chain.

Chain transfer was first proposed by Hugh Stott Taylor and William H. Jones in 1930. They were studying the production of polyethylene [(C
₂H
₄)_n] from ethylene [C
₂H
₄] and hydrogen [H
₂] in the presence of ethyl radicals that had been generated by the thermal decomposition of (Et)₂Hg and (Et)₄Pb. The observed product mixture could be best explained by postulating "transfer" of radical character from one reactant to another.

Flory incorporated the radical transfer concept in his mathematical treatment of vinyl polymerization in 1937. He coined the term "chain transfer" to explain observations that, during polymerization, average polymer chain lengths were usually lower than predicted by rate considerations alone.