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 v_a= 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 V^IV 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 v_a; on the other hand, the rate of initiation decreases markedly with increasing v_a for v_a > 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 v_a, 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 v_a 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.