Chain-growth polymerization

Lore & Background

Chain-growth polymerization involves four types of reactions: initiation, propagation, termination, and chain transfer. Initiation forms an active species I* from an initiator molecule I. Propagation occurs when the initiator fragment reacts with a monomer M, retaining the center of activity; monomers continue to add until polymers Pi* are formed with degree of polymerization i. Termination deactivates the growth center, generally via reaction involving two polymers containing active centers, resulting in dead polymer. Chain transfer transfers the active species to another molecule, such as solvent, polymeric chain, or monomer, via transfer of a group or an atom; this may or may not occur depending on the nature of the reactive species.

Reader's Guide

Chain-growth polymerization is significant because it produces many common polymers, including polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), and polyvinyl acetate (PVA). The reaction mechanism is a chemical chain reaction with an initiation step forming an active center, followed by a rapid sequence of chain propagation steps where the polymer molecule grows by addition of one monomer molecule per step. The word 'chain' refers to the reaction mechanism, not the fact that polymer molecules form long chains. Classes include radical polymerization, where kinetic-chain carriers are radicals, and ionic polymerization, where carriers are ions or ion pairs. Most polymers in daily life are synthesized by free radical polymerization, including polyethylene, polystyrene, polyvinyl chloride, and others.

Did You Know?

• Chain-growth polymerization was first classified by Paul Flory in 1953.
• IUPAC recommends simplifying 'chain-growth polymerization' to 'chain polymerization'.
• For most chain-growth polymerizations, no by-product L is formed during chain propagation.
• Chain transfer to a second polymer molecule can result in a branched structure.