a source of any chemical species that reacts with a monomer (single molecule that can form chemical bonds) to form an intermediate compound capable of linking successively with a large number of other monomers into a polymeric compound.

The most widely used initiators produce free radicals (reactive atoms or groups of atoms that contain odd numbers of electrons); examples include peroxides and aliphatic azo compounds used to polymerize vinyl chloride, methyl methacrylate, and other monomers.

Acid-forming systems such as boron trifluoride with traces of water react with a monomer to produce a positively charged (cationic) intermediate. Such initiation is used in the conversion of isobutylene to butyl rubber.

Reaction of metallic sodium and biphenyl produces an anionic initiator that causes formation of polymer chains with reactive sites at both ends; these may be further treated with a different monomer to yield block copolymers.

Polypropylene and high-density polyethylene are prepared by use of Ziegler catalysts, which are initiators composed of organometallic compounds and metallic halides, such as triethylaluminum and titanium tetrachloride.