Photoinitiation of polymerization by chloro-oxobis(2,4-pentanedionato)vanadium(V) in the presence of electron donors

Abstract

When a strong electron donor D, such as dimethyl sulphoxide (DMSO) or one of a wide range of amino-compounds, is added to VO(acac)2Cl a colour change from deep blue to pale green-yellow occurs. The change appears to be complete if [D] is slightly in excess of [VO(acac)2Cl] and all strong donors examined give effectively similar final absorption spectra. At the same time the solution acquires marked electrical conductivity. It is proposed that ion-pair complexes of the type (I) [graphic omitted] are formed between donor and chelate.The complexes are effective photoinitiators of free-radical polymerization; the reactions occurring at λ= 365 nm with D = DMSO and D = pyridine (Py) have been studied with methyl methacrylate as monomer. The polymerization is not complicated by retardation and the rate of initiation at a volume fraction of additive va= 0.1 is independent of monomer concentration (with benzene and ethyl acetate as diluents). The quantum yields of initiation are relatively high, 0.59 and 0.125 for D = DMSO and Py, respectively; these values are 29 and 6 times those obtained with VO(acac)2Cl in the absence of D. When D = DMSO both chlorine and DMSO residues are found in the polymers, but with D = Py only chlorine has been detected. Negligible quantities of acetylacetone fragments are incorporated in either case. The products are VIV derivatives, VO(acac)2D with D = Py and VO(acac)2D and VOCl2D3 with D = DMSO. In the latter case acetylacetone is also formed; both this product and VOCl2D3 are considered to arise from interaction of the chelate with HCl produced in another secondary reaction.Spectrophotometric measurements show that the rate of decomposition of (I) for a given incident intensity is independent of va; on the other hand, the rate of initiation decreases markedly with increasing va for va > 0.1, approximately.Several reaction mechanisms are discussed and it is concluded that the observations are consistent with a primary act consisting of electron transfer from D to vanadium. Secondary processes involving the resulting radical cation D+˙ and Cl– occur leading to Cl atoms and also, when D = DMSO, to ĊH2SOCH3 and HCl; both Ċl and ĊH2SOCH3 initiate polymerization. With increasing va, solvent separation of D+˙ and Cl– retards reaction between these species and D+˙ is consumed in a competitive processes; the decrease in the rate of initiation with increasing va is thus understandable. The relevant kinetic parameters for this mechanism are evaluated.During the course of this work it was found that pyridine has a greater influence on the propagation coefficient of methyl methacrylate than any other solvent so far examined.