Browsing by Author "Mullik, S. U."

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    Activities of Olefinic Derivatives as Components of Photoinitiating Systems based on Transition-Metal Carbonyls
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (09), 1977) Bamford, Clement H.; Mullik, S. U.
    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.
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    Photosensitization of Free-radical Polymerisation by Mn2(CO)10+C2F4
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (6), 1973) Bamford, C. H.; Mullik, S. U.
    A study has been made of the free-radical polymerization of methyl methacrylate at 25°C photo-initiated (λ= 435.8 nm) by Mn2(CO)10+ C2F4. The manganese and fluorine contents of the polymers are consistent with the view that initiation leads to the incorporation of one manganese atom and one C2F4 unit in each growing chain. In the absence of C2F4 no initiation and no significant decomposition of manganese carbonyl occur. With increasing [C2F4] the rate of initiation î…ƒ at first increases and then reaches a plateau value at high [C2F4]. Decomposition of Mn2(CO)10 in the presence of C2F4 follows a first-order law, the rate coefficient k being proportional to the incident intensity; k, like î…ƒ, reaches a plateau value at high [C2F4]. At low incident intensity î…ƒ=k[Mn2(CO)10], within the limits of experimental error, each manganese carbonyl molecule decomposing giving rise to one growing chain. However, at high intensity î…ƒ[double less-than, compressed]k[Mn2(CO)10] for low [C2F4]; the difference between the two quantities decreases as [C2F4] increases. At high [C2F4] the quantum yields for initiation at low intensity and carbonyl decomposition are close to unity. The rate coefficient k is independent of methyl methacrylate concentration so long as this exceeds 2 mol l.–1; it falls off at lower concentrations. Two possible types of initiation mechanism are discussed. In the first, photodissociation (assisted by the monomer M) gives rise to two fragments M ·· Mn(CO)4, Mn(CO)6 of which the former reacts with C2F4 to produce an initiating radical M ·· Mn(CO)4—CF2ĊF2 or (CO)5Mn—CF2ĊF2. The second mechanism assumes formation of an exciplex [C2F4·· Mn2(CO)10] which ultimately yields (CO)5Mn—CF2ĊF2. The low values of î…ƒ at high intensity and low [C2F4] are ascribed in both cases to recombination between the initiating radicals and other manganese species e.g. Mn(CO)6. The polymerizations of styrene and acrylonitrile may also be photoinitiated by Mn2(CO)10+ C2F4. The activity of C2F4 in these initiating systems is attributed to the relatively high energy of the Mn—CF2CF2 bond.