Chain-growth polymerization or chain polymerization is a polymerization technique in which unsaturated monomer molecules add onto the active site of a growing polymer chain one at a time. Growth of the polymer occurs only at an end (or possibly ends) of the chain. Addition of each monomer unit regenerates the active site. In chain growth polymerization, an activated species (initiator or active center) adds one monomer molecule to create a new active center (propagation step), which again adds another monomer molecule to create another active center and so on, so that the chain growth proceeds as a chemical chain reaction.
Polyethylene, polypropylene, and polyvinyl chloride (PVC) are common types of plastics made by chain-growth polymerization. They are the primary component of four of the plastics specifically labeled with recycling codes and are used extensively in packaging.
Chain-growth polymerization can be understood with the chemical equation:
This type of polymerization result in high molecular weight polymer being formed at low conversion. This final weight is determined by the rate of propagation compared to the rate of individual chain termination, which includes both chain transfer and chain termination steps. Above a certain ceiling temperature, no polymerization occurs.
Chain-growth polymerization usually has the following steps:
- chain initiation, usually by means of an initiator which generates active centers and starts the chemical process. Typical initiators include organic compounds with a labile group: e.g. azo (-N=N-), disulfide (-S-S-), or peroxide (-O-O-). Two examples are benzoyl peroxide and AIBN.
- chain propagation by an active center on the growing polymer molecule, which adds one monomer molecule to form a new polymer molecule which is one repeat unit longer with a new active center.
- chain transfer terminates the chain, but the active site is transferred to a new chain. This can occur by reaction with the solvent, monomer, or another polymer molecule. Reaction with another polymer molecule results in the formation of a branched polymer.
- chain termination, which occurs either by combination or disproportionation. Termination, in radical polymerization, is when the free radicals combine and is the end of the polymerization process.
The active center can be one of a number of different types:
- free radical in radical polymerization, for example, polystyrene, sometimes seen as packing peanuts, is produced by polymerizing styrene with Benzoyl peroxide as its radical initiator
- carbocation in cationic polymerization, an example is Isobutyl synthetic rubber, initiated by Aluminium chloride ionizing isobutylene
- carbanion in anionic polymerization
- organometallic complex in coordination polymerization.
Under the necessary reaction conditions, an addition polymerization can be considered a living polymerization. This is most often seen with anionic polymerization as it can be easy to perform without termination steps.
Comparison with other polymerization methods
The distinction between step-growth polymerization and chain-growth polymerization was introduced by Paul Flory in 1953, and refers to the difference in reaction mechanisms with step-growth using the functional groups of the monomer compared to the free-radical or ion groups used in chain-growth polymerization.
Chain growth polymerization and addition polymerization (also called polyaddition) are two different concepts. In fact polyurethane polymerizes with addition polymerization (because its polymerization does not produce any small molecules, called "condensate"), but its reaction mechanism is a step-growth polymerization.
The distinction between "addition polymerization" and "condensation polymerization" was introduced by Wallace Hume Carothers in 1929, and refers to the type of product produced. Addition polymerization produces only a polymer molecule, while condensation polymerization produces a polymer as well as a molecule with a low molecular weight, usually water.
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