Polymerization reactions triggered by thermal energy require an initiator but those triggered by irradiation do not require an initiator. Why?

A polymer is a large, complex chemical unit that is composed of numerous smaller repeating units.  These smaller repeating units are called monomers.  In order to build a polymer chemically, you start with numerous equivalents of the monomer and allow it to undergo a polymerization reaction whereby the monomers react with one another in repeating fashion to build up the polymer chain.  Polymerization reactions require some kind of event to initiate the reaction.  Most commonly the initiation event is brought upon by the presence of a radical initiator chemical compound like azobisisobutyronitrile (AIBN).  The initiator compound forms a radical (unpaired electron) under thermal conditions and the presence of that radical species is what catalyzes the polymerization reaction.  Polymerization reactions that require light (irradiation) do not require a separate chemical initiator because the light energy itself is the radical initiator.  The energy from the light source is all that is required to form a radical and catalyze the reaction.