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 Ni^II 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 Ni^I species, generating a terminal π-allylic derivative in the polymer chain. This is the major termination reaction for [is] 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.