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

Abstract

ABSTRACT:The kinetics of the polymerization (propagation) process of protonated polybutadienyllithium (hPB) in a nonpolar solvent, deuterated benzene (dBz), was examined with 1 H 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 ¼ $ 4 through Li at their ends. The residual monomer fraction ðtÞ did not rigorously exhibit the single-exponential decay with time t expected for the conventionally considered propagation through the dissociated chains, ðtÞ ¼ expðÀt=Þ withassociation/dissociation equilibrium constant, k p = propagation rate constant, ½I 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 7 Li NMR data): This fusion-aided propagation should have been osmotically suppressed on an increase of the hPB concentration (decrease of ) 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 ðtÞ 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 2 f -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.

Macromolecular Assemblies in Solution: Characterization by Light Scattering

“…Micellization plays an important role in polymerization reaction kinetics, because only dissociated free living anions are believed to be active species for the propagation reaction. [33][34][35][36][37] Thus the precise aggregation number m, the association constant (or the free energy of micellar formation), and the structure of the reversed micelle formed by polymer living anions are prerequisite to the profound understanding of the anionic polymerization.…”

“…Recently, Oishi et al 37 investigated the kinetics of living anionic polymerization for butadiene in deuterated benzene (a poor solvent for PB) by 1 H NMR. They observed nonsingle-exponential decays of the monomer concentration [M] during the propagation reaction, and the deviation from the single exponential decay was enhanced by addition of inert deuterated PB.…”

Kinetics of Living Anionic Polymerization of Polystyrenyl Lithium in Cyclohexane

“…Furthermore, the decay was retarded in the presence of chemically inert, neutral (non-anionic) PB chains. 25 These results unequivocally indicated that the conventional molecular picture, shown in Scheme 1 and eq 3, does not accurately hold for the PBLi/Bz systems.…”

“…25 The residual monomer fraction ðtÞ was found to exhibit the single-exponential decay only in the early stage of propagation where ðtÞ > 10%, and the decay of ðtÞ became slower in the late stage. 25 (This nonsingle-exponential behavior was not observed in the previous studies [1][2][3][4][5]8,9,13,14 simply because these studies traced ðtÞ only in the early stage.) Furthermore, the decay was retarded in the presence of chemically inert, neutral (non-anionic) PB chains.…”

“…A simple analysis incorporating this osmotic effect was in harmony with the measured ðtÞ data, which lent support to this hypothesis. 25 This osmotic effect cannot be unique to the PBLi system but is expected for all living anionic polymerization systems in nonpolar solvents. Thus, we tested this effect through 1 H NMR measurements for polystyrenyl lithium (PSLi) polymerized in cyclohexane (CH) at 34.5 C (= theta temperature for neutral high-M PS chains) and other temperatures.…”