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Item 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.Item Redox Reactions between Halides and Reducing Metal Complexes in Non-aqueous Solution: Part 2.—Derivatives of Molybdenum Carbonyl(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Bamford, Clement H.; Sakamoto, IzumiMolybdenum carbonyl undergoes ligand exchange with vinyl monomers such as methyl metha crylate (mma) to produce complexes of the type Mo(CO)5fmma) (I) which readily enter into redox processes with organic halides to generate free radicals. The absolute rate coefficients (k3) for the redox reactions of Mo(CO)s(mma) with CCL, CCl3COOEt and CBr4 have been determined and the radical yield (J) for the first of these reactions. The rate coefficient (k)) for Ute spontaneous decay of (I) has also been measured. Activation entropies of the redox reactions are more positive than expected for a bimolecular reaction with a polar transition state in a polar medium and it is suggested that weakening of Mo—CO bonds in the transition state is responsible for this. Values of the ratios of rate coefficients are compared with those derived from polymerization kinetics. As found in Part 1 for reactions of Ni(tp)4(mma) the halide activity increases in the order CCL < CCl3COOEt < CBr4, but the range of reactivities with Mo(CO)5(mma) is much narrower. For the primary oxidation with CCl4. f->l at low [Mo(CO)6], consistent with the formation of Mo1 (although formation of Mom cannot be ex cluded). At higher [Mo(CO)fi],/assumes lower values and it is suggested that this effect is a reflection of the self-inhibition observed in these systems.Item Redox Reactions between Halides and Reducing Metal Complexes in Non-aqueous Solution: Part 1.—Derivatives of Tetrakis(triphenyl phosphite)nickel(O)(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Bamford, Clement H.; Sakamoto, IzumiTetrakisftriphcnyl phosphite)nickel(O) (Ni(tp)4) undergoes ligand exchange with electron donors such as methyl methacrylate (mma) to form complexes of the type Ni(tp)3(mma) which enter into redox reactions with organic halides to generate free radicals. Three techniques are described for determining the absolute rate coefficients of these latter processes. In two methods, concentrations of Ni(tp)3(mma) are monitored spectrophotometrically, while in the third the rate of radical generation is deduced from observations of the rate of the ensuing free-radical polymerization of (mma). Reactions of CHBr3, CBr4, CH2Br2, CCl3COOEt, CHCBCOOEt, CHClj, CH2C12 are reported in this paper. In general, bromides are much more reactive than the corresponding chlor ides, and the rate of reaction increases with the number of halogen atoms attached to a single carbon. Further, introduction of an electronegative group into the halide molecule greatly increases the rate of reaction. It is concluded that the electron-affinity of the halide and solvation of the transition com plex play important roles in determining the rate coefficients and activation parameters. The reactivities of the halides in these redox processes and in chain transfer reactions arc compared.Item Thermal reactions of perfluoromethyl and perfluoroacetyl manganese pentacarbonyls. Part 2.—Initiation of free-radical polymerization and formation of methyl(2-methyl 4-oxo-5,5,5-trifluoropentanoate)(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (07), 1978) Bamford, Clement H.; Mullik, Sanam U.This paper continues the study of the thermal reactions of low-molecular-weight analogues of macroinitiators carrying terminal Mn(CO)5 groups e.g. [graphic omitted]CF2 CF2 Mn(CO)5. The reactions of CF3Mn(CO)5 and CF3COMn(CO)5 in methylmethacrylate solution at 100°C have been examined. Both derivatives initiate free-radical polymerization; retardation is less marked than with the unfluorinated analogues, especially in the case of CF3Mn(CO)5. Rates of polymerization ω are increased by the presence of “halide” and “non-halide” type additives; with CF3Mn(CO)5 the increase corresponds to a factor of 2 in the rate of initiation. The dependence of ω on [CCl4] is very sharp and a plateau value is reached for [CCl4] > 5 × 10–3 mol dm–3. The reaction of CF3COMn(CO)5 yields methyl(2-methyl-4-oxo-5,5,5-trifluoropentanoate), CF3COCH2CH(CH3)COOCH3. Reaction in the presence of D2O yields CF3COCH2CD(CH3)COOCH3, indicating that the mechanism is of a polar type involving intramolecular attack by a CF3CO anion on a coordinated monomer molecule, with subsequent acquisition of a proton from water present in trace quantities. No corresponding reaction with CF3Mn(CO)5 has been observed under similar conditions. On heating these derivatives in methyl methacrylate at 100°C a peak with λmax= 342 nm slowly develops in the u.v.-visible spectrum and represents the formation of Mn2(CO)10. In the presence of added water a peak near 385 nm appears, attributable to Mn(CO)5OH. The bearing of these results on the use of macroinitiators for block-copolymer synthesis is discussed.Item Thermal reactions of methyl and acetyl manganese pentacarbonyls. Part 1.—Initiation of free-radical polymerization and formation of methyl(2-methyl 4-oxopentanoate)(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (07), 1978) Bamford, Clement H.; Mullik, Sanam U.The manganese pentacarbonyl derivatives CH3Mn(CO)5 and CH3COMn(CO)5 are low-molecular-weight analogues of macroinitiators which carry terminal Mn(CO)5 groups and their thermal reactions in methyl methacrylate at 100°C have been studied from this aspect. Under these conditions isomerization reactions are rapid and the two compounds behave similarly. The derivatives initiate free-radical polymerization above 60°C but give rise to marked retardation except at very low concentrations. The rate of polymerization ω is increased by the presence of “halide” or “non-halide” additives and at the same time retardation is greatly reduced. The dependence of ω upon additive concentration is of the familiar form showing a plateau value at high concentrations. The rate of polymerization is depressed by the presence of carbon monoxide. However, initiation of polymerization is only a minor component of the overall reaction at 100°C, the major process (∼90%) being formation of methyl(2-methyl 4-oxopentanoate), CH3COCH2CH (CH3)COOCH3. This reaction follows a non-radical route. A mechanism is suggested involving intramolecular interaction between an acetyl anion and the coordinated monomer (M) in CH3COMn(CO)4M; the process is completed by proton transfer from water present in the system in trace quantities. This mechanism is strongly supported by investigations with D2O. Spectral (u.v.-visible) changes accompanying these transformations are recorded. Thermal decomposition of CH3Mn(CO)5 and CH3COMn(CO)5 in benzene yields Mn2(CO)10; in methyl methacrylate an additional peak develops at 385 nm which we attribute to Mn(CO)5OH. This peak is not much affected by the presence of CCl4 or CO.Item Photolysis of alkyloxy vanadium(V) chelates. A possible general route to alkyloxy radicals(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (05), 1977) Aliwi, Salah M.; Bamford, Clement H.We have studied the photochemical reactions (λ= 365 nm) of nineteen vanadium(V) chelates of structure VOQ2OR where Q is the 8-quinolyloxo ligand and R is an alkyl group. In all cases photolysis proceeds in a first-order fashion to yield the vanadium(IV) chelate VOQ2 identified by u.v.-visible and e.s.r. spectroscopy. The VV chelates photoinitiate free-radical polymerization and the quantum yields of photoinitiation with methyl methacrylate as monomer have been determined and compared with the quantum yields of chelate photodecomposition. Each molecule of chelate decomposing gives rise to one initiating radical. The dependence of the quantum yield of photodecomposition ϕd on the nature of R is discussed. The inductive effect of R, which influences the natures of the V[double bond, length as m-dash]O and V—OR bonds, also affects ϕd and a correlation between the stretching frequency of V[double bond, length as m-dash]O and ϕd has been found. The photolytic products in benzene solution have been examined by spin-trapping and g.l.c. techniques; results are consistent with the primary formation, isomerization and β-scission of alkyloxy radicals.Item Photoinitiation of Polymerization by Chloro-o.xobis(2,4- pentanedionato)vanadium(v)(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (11), 1974) Aliwi, S. M.; Bamford, Clement H.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 VIV 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.