Kinetics of Anionic Polymerization of Polybutadienyl Lithium in Benzene: An Osmotic Effect on Propagation Process

Abstract

The kinetics of the polymerization (propagation) process of protonated polybutadienyllithium (hPB) in a nonpolar solvent, deuterated benzene (dBz), was examined with 1H NMR. An oligomeric, deuterated butadienyllithium (oBLi) was utilized as an initiator in order to avoid a contamination of the initiation process to the NMR data. The hPBLi chains mostly formed aggregate with an average aggregation number f ≍ 4 through Li at their ends. The residual monomer fraction φ(t) did not rigorously exhibit the single-exponential decay with time t expected for the conventionally considered propagation through the dissociated chains, φ(t)=exp(−t/τ) with τ={f/Kd[I]0}1/f/kp (Kd = association/dissociation equilibrium constant, kp = propagation rate constant, [I]0 = molar concentration of initiator at time 0). This deviation from the conventional behavior appeared to be due to competing propagation mechanism through the transiently fused aggregates (suggested from the 7Li NMR data): This fusion-aided propagation should have been osmotically suppressed on an increase of the hPB concentration (decrease of φ) to give the deviation. The propagation was found to be also retarded in the presence of chemically inert deuterated polybutadiene (dPB) chains that just tuned the osmotic environment for the aggregates, lending support to the molecular idea of the osmotic effect on the propagation. Furthermore, the φ(t) data in the absence/presence of the dPB chains were semi-quantitatively described by a simple model considering the competition of the propagation mechanisms through the fused 2f-mer aggregates and dissociated chains, with the former mechanism vanishing in the late stage of propagation. These results suggested a non-negligible contribution of the fused aggregates to the polymerization kinetics in particular in the early stage.