Chain-growth polymerization

Chain-growth polymerization, also called chain polymerization, is a method in which monomers attach to an active site on a growing polymer chain, one molecule at a time. A key feature is that only a small number of these active sites exist at any given point during the reaction. This approach was first distinguished from step-growth polymerization by Paul Flory in 1953; the International Union of Pure and Applied Chemistry (IUPAC) now recommends using the simpler term "chain polymerization" instead.

The process involves four types of reactions. Initiation occurs when an initiator molecule breaks down to form an active species. During propagation, this active fragment reacts with a monomer, starting the conversion into a polymer; the active center stays on the new molecule, and monomers continue adding in the same way until a polymer with a certain degree of polymerization is formed. Termination happens when two polymers with active centers react, deactivating the growth center and producing a dead polymer. Chain transfer involves moving the active species to another molecule—such as a solvent, polymer chain, or monomer—by transferring an atom or group (like a hydrogen atom); whether this occurs depends on the nature of the reactive species.

In chain-growth polymerization, an active center (a free radical or ion) is created, allowing many monomers to join together quickly and form a large macromolecule. The polymer grows only at one (or sometimes more) endpoint, and each monomer unit regenerates the active site. Many common plastics are made this way, including polyethylene, polypropylene, polyvinyl chloride, poly(methyl methacrylate), polyacrylonitrile, and polyvinyl acetate.

The general chemical equation is Pₓ* + M → Pₓ₊₁* + L, where P is the polymer, x is the degree of polymerization, * marks the active center, M is the monomer, and L is a low-molar-mass by-product. For most chain-growth polymerizations, no by-product L forms, but there are exceptions—for example, the polymerization of amino acid N-carboxyanhydrides to oxazolidine-2,5-diones. The term "chain" refers to the chemical chain reaction mechanism: an initiation step creates an active center, followed by a rapid sequence of propagation steps where each monomer adds to that center. It does not refer to the long chains of the polymer molecules themselves; some polymers form through a different mechanism called step-growth polymerization, which lacks these rapid propagation steps.

All chain-growth polymerizations must include chain initiation and chain propagation. Chain transfer and chain termination occur in many, but not all, cases. Initiation generates a chain carrier—an intermediate like a radical or ion—that can continue the reaction. Initiation can be thermal (using molecular motion to break a molecule), high-energy (using radiation), or chemical (using a chemical initiator). In radical polymerization, for example, a radical initiator molecule dissociates into two free radicals, and each radical adds a monomer to start a chain: I → 2 R°, then R° + M → RM₁°.

Propagation, as defined by IUPAC, is the reaction where the active center on the growing polymer adds one monomer molecule, making the polymer one repeat unit longer. For radical polymerization, the active center remains an unpaired electron. The addition of the second monomer and later steps are: RM₁° + M → RM₂°, and so on up to RMₙ° + M → RMₙ₊₁°. Chains of over 1000 monomer units can form in milliseconds.

Termination occurs when the active center disappears, stopping chain growth—unlike chain transfer, where the active center simply moves to another molecule. In radical polymerization, termination involves two growing chains reacting to eliminate their unpaired electrons. This can happen in two ways. Recombination joins the unpaired electrons of two chains into a covalent bond, producing a single polymer molecule with the combined length: RMₙ° + RMₘ° → Pₙ₊ₘ. Disproportionation transfers a hydrogen atom from one chain to the other, leaving the two chains as separate dead polymers.