Dioxaheteroatom Cyclics as Initiators for Radical Polymerization

An attractive way to incorporate flame-retarding moieties into oligomers or polymers is to use strained heterocyclic compounds which contain a thermally-labile carbon-carbon bond which will undergo thermolysis at modest temperature to generate a diradical capable of initiating vinyl polymerization. Suitably substituted, they may contain either bromine or phosphorus-containing moieties, or both. The diol precursors to these heterocycles are generally available from reductive coupling of the corresponding ketones. Conversion of the diols to the heterocycles can be accomplished using standard reagents. Phospholanes generated in this way tend to be quite thermally unstable and often degrade within hours to generate a variety of products. On the other hand, siloles derived from appropriate diols display much greater thermal stability. They can be isolated as crystalline solids and may be used to initiate polymerization at 100-200 degrees C. Incorporation of heterocycle-derived units into the mainchain can impart polymer flame retardancy directly. Alternately, oligomers may be generated for use as flame retardant additives for polymeric materials.