On the degradation of polysilanes in solution. Flash photolysis and pulse radiolysis studies

Polysilanes (catena-polysilicons) of the structure—SiR¹R²—(R¹, methyl; R², phenyl, n-propyl, n-hexyl, β-phenethyl) and a copolymer (repeating units:—SiMe₂—and—SiPhe₂—) were irradiated with UV light (347 nm) or 15 MeV electrons. The dominant feature in both photolysis and radiolysis is main-chain scission. Typical quantum yields of main-chain scission determined by light scattering (LS) measurements are: φ(S) = 0.2 [poly(methyl-n-hexyl silane)] and φ(S) = 0.5 [poly(methyl-β-phenethyl silane)]. The radius of gyration r_G depends on M_W as r_G ∝ M^v_W; v = 0.5. Flash photolysis and pulse radiolysis studies in conjunction with optical absorption and LS measurements revealed that main-chain cleavage occurs very rapidly (τ<20 ns). The resulting polymer fragments exhibit transient optical absorption probably originating from free radicals. The rate of decrease in LS intensity after the pulse corresponds to the rate of separation of fragments generated by main-chain cleavage. Regarding solutions of polysilane in liquids of bad solvent quality the rate of fragment separation is determined by the lifetime of contact pairs corresponding to physical interactions between polymer segments [at 23°C ca 100 μs in a 66/34 (v/v) mixture of 1,4-dioxane/1-propanol]. Moreover, it turned out that in benzene solution main-chain scission is sensitized by energy transfer from solvent to polysilane.