Browsing by Author "Bamford, C. H."

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Diene reactions: Part 1.—Ligand exchange and π-allylic complex formation associated with the system Ni{P(OPh)3}4+ isoprene + CCl4
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (2), 1973) Ashworth, J.; Bamford, C. H.
We have shown that reaction between tetrakis(triphenyl phosphite)nickel(0)(Ni(tp)4), isoprene (is) and carbon tetrachloride leads to the formation of an orange-coloured π-allylic NiII derivative, without free-radical generation. The first stage, ligand exchange between Ni(tp)4 and (is) leading to a weak Ni0 complex (I), is analogous to the corresponding reactions with methyl methacrylate and styrene, but the bonding is weaker in the case of isoprene. Complex (I) then reacts with CCl4 in two distinct stages, the first, an addition reaction occurring effectively instantaneously, yielding a species which subsequently rearranges intramolecularly and relatively slowly into the π-allylic complex. Detailed kinetic analyses of the ligand exchange and the overall reaction have been carried out, good agreement between results calculated from the proposed mechanism and observation being obtained.
Diene reactions: Part 2.—Retardation by isoprene of polymerization initiated by Ni{P(OPh)3}4+ CCl4—a novel termination reaction
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (2), 1973) Ashworth, J.; Bamford, C. H.
We have found that the polymerization of methyl methacrylate initiated by tetrakis(triphenyl phosphite)nickel(0)(Ni(tp)4) and carbon tetrachloride is strongly retarded by dienes. During reaction the solutions develop an absorption spectrum very similar to that of the π-allylic complex of NiII described in the preceding paper. In systems containing isoprene (is) the rate of increase of absorbance becomes independent of [is] and the rate of polymerization becomes insignificant when [is] > 0.1 mol dm–3. At lower [is] the molecular weight of the polymer falls steadily with increasing [is]. The retardation reaction is shown to involve the attack of a propagating polymer radical on an unstable NiI species, generating a terminal π-allylic derivative in the polymer chain. This is the major termination reaction for [is][gt-or-equal] 0.02 mol dm–3, so that under these conditions nearly all polymer chains carry a terminal π-allylic unit, and the molecular weight may be estimated from the absorbance. An approximate kinetic treatment relating rates of polymerization and development of absorbance and polymer molecular weights is presented.
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.