Polymerization

Polymerization is a chemical process in which small molecules called monomers react to form a large molecule called a polymer, made of repeating units. Polymers may be linear, branched, or crosslinked, and their properties are influenced by monomer choice, stoichiometry, and processing conditions. Monomers may be identical or different and often contain multiple reactive sites.

There are two broad categories of polymerization: addition (chain-growth) polymerization and condensation (step-growth) polymerization. In addition polymerization, an initiator generates an active center that adds monomer units one by one, rapidly building a growing chain. Propagation continues until termination or transfer occurs. This category includes free-radical polymerization (used for polyethylene, polystyrene), anionic and cationic polymerization (used for various polymers including polyacrylates), and coordination polymerization (used for many polyolefins under Ziegler–Natta or metallocene catalysts). In condensation polymerization, monomers react to form covalent bonds with the loss of a small molecule such as water or methanol; high conversion and functionality are needed to reach high molecular weight. Common condensation polymers include polyesters (e.g., PET) and polyamides (e.g., nylon).

Key concepts include the degree of polymerization, molecular weight distribution, and kinetics. Living (or controlled) polymerization techniques, such as RAFT or ATRP, enable precise control of molecular weight and architecture with reduced dispersity.

Historically, the idea of macromolecules emerged in the early 20th century, with later theoretical work by Hermann Staudinger establishing the macromolecular nature of polymers. Today, polymerization underpins plastics, fibers, coatings, adhesives, and many biomedical and electronic materials, emphasizing efficiency, selectivity, and environmental considerations.