Photoinitiation of Polymerization by Chloro-o.xobis(2,4- pentanedionato)vanadium(v)

Abstract

Chloro-oxobis(2,4-pentanedionato)vanadium(V)[VO(acac)2Cl] sensitizes the polymerization of methyl methacrylate when irradiated by wavelengths in the near ultra-violet. Under the experimental conditions employed ([VO(acac)2Cl]<10–4 mol dm–3) no retardation occurs and the rate of initiation is independent of monomer concentration (with benzene as diluent). The quantum yield for initiation at λ= 365 nm is 2.07 × 10–2. Analysis of the polymers formed shows that initiation occurs predominantly through scission of chlorine atoms from VO(acac)2Cl. Spectral changes accompanying irradiation are consistent with the formation of VO(acac)2 as the final product. Rates of photodecomposition of VO(acac)2Cl have been measured spectrophoto-metrically over a range of monomer concentrations [M] with benzene as diluent and found to increase linearly with [M]. At [M]= 0, the rate of decomposition is equal to the (constant) rate of initiation at finite [M], but for [M]>0, the rate of photodecomposition exceeds the rate of initiation. A reaction mechanism is proposed based on an intramolecular photo-oxidation-reduction process which leads to the primary formation of a VIV chelate and a chlorine atom. In addition, monomer is considered to undergo an insertion reaction into the V—Cl bond of photo-excited VO(acac)2Cl, resulting in decomposition of the chelate by a non-radical route. Relevant kinetic parameters are evaluated.