Chapter 3 Free-Radical Photoinitiators and Initiation Mechanism

Abstract

RADIATION POLYMERIZATION follows the general scheme for any polymerization. Suitable reactive monomers must be first initiated to form a species that is capable of being polymerized, then be propagated so as to form a polymeric chain, and finally, the growing polymer chains must be terminated in some manner. These radiation polymerizations are effected or initiated through one or the other of two general mechanisms: free radical initiation or cationic initiation. Free-radical initiation is achieved either by use of an electron beam or other suitable means that generates ionizing radiation capable of generating free radicals from suitable ethylenically unsaturated monomers, or by use of ultraviolet radiation and a photoinitiator that will photolyze (degrade) in the presence of such radiation and produce free radicals. Cationic initiation is achieved by photochemical means and requires the use of a photoinitiator that will photolyze to form Lewis or Bronsted acids. Species that generate Bronsted or protonic acids are currently in commercial use. In addition to generating an acid, the photolysis of cationic photoinitiators also results in the generation of free radicals. This allows dual or hybrid cures of cycloaliphatic epoxides and acrylate esters, as well as other combinations.