Chain Transfer

Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule.

P• + XR' → PX + R'•

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 addition polymerization.

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

• Transfer to chain transfer agent. Chain transfer agents have at least one weak chemical bond, which therefore facilitates the chain transfer reaction. Common chain transfer agents include thiols, especially DDM, and halocarbons such as carbon tetrachloride. Chain transfer agents are sometimes called modifiers or regulators.

• Transfer to monomer. Chain transfer to monomer may take place in which the growing polymer chain abstracts an atom from unreacted monomer existing in the reaction medium. Because, by definition, polymerization reactions only take place in the presence of monomer, chain transfer to monomer determines the theoretical maximum molecular weight that can be achieved by a given monomer. Chain transfer to monomer is especially significant in cationic addition polymerization and ring-opening polymerization.

• Transfer to polymer. Chain transfer may take place with an already existing polymer chain, especially under conditions in which much polymer is present. This often occurs at the end of a radical polymerization when almost all monomer has been consumed. Branched polymers are formed as monomer adds to the new radical site which is located along the polymer backbone. The properties of low-density polyethylene are critically determined by the amount of chain transfer to polymer that takes place.

• Transfer to solvent. In solution polymerization, the solvent can act as a chain transfer agent. Unless the solvent is chosen to be inert, very low molecular weight polymers (oligomers) can result.