Photolysis of alkyloxy vanadium(V) chelates. A possible general route to alkyloxy radicals

Abstract

We have studied the photochemical reactions (λ= 365 nm) of nineteen vanadium(V) chelates of structure VOQ₂OR where Q is the 8-quinolyloxo ligand and R is an alkyl group. In all cases photolysis proceeds in a first-order fashion to yield the vanadium(IV) chelate VOQ₂ identified by u.v.-visible and e.s.r. spectroscopy. The V^V chelates photoinitiate free-radical polymerization and the quantum yields of photoinitiation with methyl methacrylate as monomer have been determined and compared with the quantum yields of chelate photodecomposition. Each molecule of chelate decomposing gives rise to one initiating radical.

The dependence of the quantum yield of photodecomposition ϕ_d on the nature of R is discussed. The inductive effect of R, which influences the natures of the VO and V—OR bonds, also affects ϕ_d and a correlation between the stretching frequency of VO and ϕ_d has been found.

The photolytic products in benzene solution have been examined by spin-trapping and g.l.c. techniques; results are consistent with the primary formation, isomerization and β-scission of alkyloxy radicals.