Photoinitiation of free-radical polymerization by vanadium chelates. Part 3.—Methoxo-oxobis(8-quinolyloxo)vanadium(V)

Abstract

The chelate complex VOQ₂OCH₃, where Q represents the 8-quinolyloxo ligand, is a photoinitiator for the polymerization of methyl methacrylate at λ= 365 nm. No photoinitiation occurs at λ= 436 or 570 nm, and the chelate does not initiate thermally up to 80°C. The photoinitiated reaction is an uncomplicated free-radical process and the rate of initiation is independent of monomer concentration (in benzene solution). The quantum yield of photoinitiation is low, 2.26 × 10^–3.

Spectral observations (infra-red, e.s.r. and ultraviolet–visible) indicate that irradiation brings about scission of the methoxy ligand with formation of the V^IV derivative VOQ₂. Comparison of the rates of photoinitiation and chelate decay on irradiation shows that one molecule of the initiating species arises from each molecule of chelate decomposed. In this respect VOQ₂OCH₃ differs from Mn(acac)₃ and VO(acac)₂Cl in which photoreaction with monomer without production of radicals accompanies photoinitiation.

The nature of the initiating species was examined by experiments with VOQ₂O¹⁴CH₃ and spin-trapping investigations with PhCHN(Bu^t)→ O as radical scavenger. At low concentrations of scavenger the trapped radical was exclusively ĊH₂OH, but at higher concentrations both ĊH₂OH and CH₃O were trapped. Reasons are given for believing that isomerization of CH₃O may be catalysed by a vanadium species. These radicals are responsible for initiation of polymerization; no indication of initiation by a species containing Q was obtained.

A mechanism is proposed for the photolysis.

Chelates of the type considered may be incorporated into polymers by two methods to yield photosensitive products which undergo grafting and cross-linking reactions on irradiation.