Photoinitiation of polymerization by chloro-oxobis(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 V^IV 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.