THE RADIATION-INDUCED POLYMERIZATION OF DIMETHYLCYCLOSILOXANES IN THELIQUID-STATE .3. COPOLYMERIZATION OF D(3) WITH D(4) AND D(4) WITH D(5), REACTIVITIES AND INTERPRETATION

The radiation-induced copolymerization of octamethylcyclotetrasiloxane (D4) with hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasil oxane with decamethylcyclopentasiloxane (D5) was used to examine the r elative reactivities between the three cyclic monomers, under super-dr y conditions. The copolymerization results have shown very similar rea ctivities of the three monomers. These results were in agreement with previously reported monomer reactivities and homopolymerization activa tion energies, as well as with the production of similar quantities of cyclic products. A single reaction mechanism for the polymerization o f D3, D4 and D5 is proposed for both the propagation and the backbitin g reactions. This mechanism assumes that backbiting (giving D6, D5 and D4 but not D3) involves, as intermediates, silicenium ions solvated b y nearby oxygens of their own chain, and propagation by a much more re active unsolvated (unpaired) silicenium ion in lower concentration.