Abstract:
The derivatives studied include carboxyl- and nitrile-substituted ethylenes, perfluoro-propylene, -butadiene, -benzene and -styrene, 1,1-difluoro-2,2-dichloroethylene and several chloroethylenes. The activity of each compound as a component of a photoinitiating system containing rhenium carbonyl (Re2(CO)10) or manganese carbonyl (Mn2(CO)10) was investigated. Incident wavelengths were 365 and 435.8 nm for Re2(CO)10 and Mn2(CO)10, respectively, with methyl methacrylate as monomer. All reactions were carried out at 25°C.
The results lead to the following conclusions: (1) photoinitiation of polymerization involves two stages, (i) complex formation between a fragment of the metal carbonyl produced by photolysis and the olefin and (ii) rearrangement of the complex into an initiating radical of typical structure [graphic omitted] (2) the presence of electron-attracting groups such as COOH, CN, F, enhances reactivity, but (3) steric hindrance arising from bulky substituents plays a dominating role. Quantitative conclusions about the rates of processes (1.i) and (1.ii) are drawn from the observed dependence of rates of polymerization on the concentrations of ethylenic derivatives.
Of the carboxyl derivatives, both trans CH(COOC2H5): CH(COOC2H5) and CH(COOCH3): C(COOCH3)2 are highly active with Re2(CO)10, with quantum yields of initiation 0.7–0.8; trans CH(CN): CH(CN) has similar activity. Perfluoro-propylene, -butadiene and -styrene initiate with quantum yields of unity while CF2: CCl2 has a quantum yield of 0.65 under similar conditions. All these systems photoinitiate by the “non-halide” mechanism. Perfluorobenzene is inactive.
Photoinitiation in systems containing Re2(CO)10 together with CH2: CCl2, CHCl: CCl2 or CCl2: CCl2 occurs by the “halide-abstraction” mechanism. The latter derivative is highly active, giving systems with unit quantum yield and uncomplicated kinetics; both CH2: CCl2 and CHCl: CCl2 initiate with quantum yields of 0.6, approximately, at low intensities, but complicating features appear at high intensities.
The higher activity of Re2(CO)10 compared to Mn2(CO)10 in “non-halide” systems is attributed to the higher metal–carbon bond energy with rhenium.
