Diene reactions: Part 2.—Retardation by isoprene of polymerization initiated by Ni{P(OPh)3}4+ CCl4—a novel termination reaction

Abstract

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.