Browsing by Author "Ashworth, J."

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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.
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    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.