Hexamethylcyclotrisiloxane, solid state polymerization

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

The solid state polymerization of hexamethylcyclotrisiloxane has been investigated over the temperature range —196° to 60°C. The rates of polymerization have been related to the presence of ion scavengers, H20, NH3, in the monomer and to the size of the crystals. Using large crystals dried over sodium, G values of polymerization of 11 X 103 were obtained at 50°C. This is five times larger than previously reported values. The reaction is concluded to be surface initiated and to be terminated at a crystal face or at a defect. 

A unique feature of high-energy-radiation apphcations relates to solid-state polymerizations [27-31]. In this case, a catalyst is not required and the irradiations can be performed at deliberately chosen temperatures, because the absorption of radiation is temperature-independent. Of particular interest in this context are compounds such as trioxane, hexamethylcyclotrisiloxane, P-propiolactone and diketene, all of which polymerize readily in the solid state but slowly, or not at ah, in the hquid state. Other compounds such as styrene, acrylonitrile and formaldehyde polymerize faster in the crystalline state than in the liquid state just above the melting point Compounds that undergo solid-state polymerization upon y-hradiation are listed in Table 5.1. In contrast, vinyl chloride, vinyl acetate and methyl methacrylate do not polymerize in the solid state. 

In this chapter we report some results from an intensive investigation of the radiation-induced polymerization of a single monomer—hexamethylcyclotrisiloxane—which by virtue of its ease of polymerization and very high solid state vapor pressure affords an opportunity for comment on two of the points mentioned above. 

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