Chain transfer
From Wikipedia, the free encyclopedia
Chain transfer is a reaction in radical polymerization by which a radical center on a growing polymer chain is transferred to another molecule.[1]
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 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 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.
- 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.
- Catalytic chain transfer. A variety of cobalt and chromium coordination complexes are able to abstract a hydrogen atom from a growing free radical chain and transfer that hydrogen atom to monomer, thereby starting a new polymer radical chain. The original chain is thereby terminated with an olefinic unsaturation. The net effect is to lower the molecular weight of the resulting polymer. This is a convenient process for the generation of reactive methacrylate and styrene macromonomers for inkjet inks and automotive paint.[2]
