Organic solvents effect initiator decomposition rate

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

Effect of some organic solvents [4-6]. The inQience of acetonitrile (ACN) and dimethyl formamide (DMFA) on the radieal polymerization of styrene and (meth) acryhc esters initiated with azoisobutyronitrile (AIBN) or BP was studied. Basic characteristics of the gross kinetios of polymerization in solution have been foimd The reaction order with respeet to initiator eoneentration is always 0.5, which points to bimolecular chain termination. The reaetion order with respect to monomer varies within rather wide limits (above or below unity) depending on the chemical nature of the solvent, monomer, and initiator. The lowest (0.83) and highest (1.6) values were foimd for BP initiated polymerization of MMA in DMFA solution and for AIBN initiated polymerization of styrene in ACN solution, respectively. Sueh a high value of the order with respect to monomer is eaused by the abnormally low rate of AIBN decomposition in ACN, which, in turn, is explained by the donor-aeceptor interaction... 

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