Chain initiation

Chain-growth polymerization or chain polymerization is a polymerization technique where unsaturated monomer molecules add onto the active site of a growing polymer chain one at a time. Growth of the polymer occurs only at one (or possibly more) ends. Addition of each monomer unit regenerates the active site.

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:

where n is the degree of polymerization and M is some form of unsaturated compound: an alkene (vinyl polymers) or alicyclic compound (ring-opening polymerization) containing molecule.

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:

The active center can be one of a number of different types:

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.

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