Dynamics of Monofunctional Polybutadienyl Lithium Chains Aggregated in Benzene

Viscosity, 7Li-NMR, and neutron/light scattering measurements were conducted for living monoanionic polybutadienyl lithium chains of various molecular weights M and concentrations C. In benzene, the living chains aggregated with each other at their Li ends to form star-like tertramers as the major component, as confirmed from the viscosity and scattering measurements. An average time τdif required for the tetrameric aggregate to diffuse over a distance to a neighboring aggregate was estimated from the viscosity data with the aid of the bead-spring model (valid at the small C and M examined). This τdif, of the order of 10−5–10−7 s, was much smaller than the Li–Li exchange time τex determined from the 7Li-NMR measurement. The large difference between τdif and τex indicates that the dissociation of the aggregates, occurring through the Li–Li exchange, is much less frequent than the thermal collision of the aggregates. The M, C, and T dependencies of the exchange time (dissociation time) τex were satisfactorily described by an empirical equation, τex∝Qosτdifexp(ΔE/RT) where R and T were the gas constant and absolute temperature, respectively, and Qos denoted an osmotic barrier for mutual approach of the aggregates carrying the aggregated Li species at the center. The activation energy ΔE (≅88 kJ/mol) was considerably smaller than the bare energy required for breaking the Li–Li bonds and releasing isolated Li species. These results suggest that the collision of the aggregates (under the osmotic barrier) is required for the dissociation of the aggregates and the dissociation results from a cooperative exchange of Li species occurring through formation of a larger, transient aggregate.