koinitiator

A koinitiator is a substance that, when added to a monomer system, enhances the rate or efficiency of polymerization that is initiated by another substance, the primary initiator. Koinitiators typically do not initiate polymerization on their own under the reaction conditions. Their role is to participate in a redox reaction or some other chemical process with the primary initiator or its decomposition products, generating a greater concentration of active species, such as free radicals or ions, that can then initiate polymerization.

In free radical polymerization, common koinitiators include reducing agents. For example, in redox initiation systems, a persulfate salt might act as the primary initiator, decomposing upon heating to produce sulfate radicals. A koinitiator, such as a reducing sugar or a metal ion in a lower oxidation state, can react with these radicals or with the initial persulfate to produce more initiating radicals, thereby accelerating the polymerization process. This allows polymerization to occur at lower temperatures or with shorter reaction times.

The concept of a koinitiator is also relevant in other polymerization mechanisms, including cationic and anionic polymerization, where it may assist in the generation of active propagating species. The selection of a suitable koinitiator depends on the specific monomer, the primary initiator, and the desired polymerization conditions, such as temperature and solvent. The efficiency of a koinitiator is often measured by its ability to increase the polymerization rate or to lower the required temperature for initiation.