Dynamics of Monofunctional Polybutadienyl Lithium Chains Aggregated in Benzene

Abstract

ABSTRACT:Viscosity, 7 Li-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 ( dif 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 ( dif , of the order of 10 À5 -10 À7 s, was much smaller than the Li-Li exchange time ( ex determined from the 7 Li-NMR measurement. The large difference between ( dif and ( ex 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) ( ex were satisfactorily described by an empirical equation, ( ex / Q os ( dif expðÁE=RTÞ where R and T were the gas constant and absolute temperature, respectively, and Q os denoted an osmotic barrier for mutual approach of the aggregates carrying the aggregated Li species at the center. The activation energy ÁE (¼ $ 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.

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

“…Thus, as the propagating species sensitive to the osmotic effect, we first consider the fused 2 f -mer aggregate (Scheme 2) that appears to play an important role in the Li-Li exchange process. 16 The 2 f -mer aggregate is less stable/ more reactive than the f -mer aggregates (as suggested from the previous 7 Li NMR study 16 ) and is the simplest candidate of this propagating species. For completeness, we will also discuss the contribution of the other species (such as n f -mer and f =2-mer aggregates) to the polymerization.…”

“…For the average aggregation number estimated previously, 15,16 f ¼ 4, the data points are mostly collapsed onto a universal curve within small experimental scatters. Furthermore, in the top panels of Figures 4-7, the data points in a range of ðtÞ !…”

“…4,9,10 On the other hand, for the propagation process of the anionic polymerization after full consumption of the RLi molecules, a simple kinetics shown in Scheme 1 seems to have been accepted. In this kinetics, the anionic polymer chains, PLi, are in chemical equilibrium with their star-like f -mer aggregates (PLi) f bound at the Li ends (as detected from scattering experiments [10][11][12][13][14][15][16] ), and the propagation occurs only through the dissociated PLi chains because the aggregates are considered to be well stabilized and thus inert for the propagation. The time (t) evolution of the molar concentration of the monomer, [M], is then described by…”

“…A recent experiment 16 for living polybutadienyllithium (PBLi) chains poses a delicate question related to this chemical inertness of the aggregates. For monodisperse PBLi chains of the molecular weights M ¼ 2:6 Â 10 3 -9:6 Â 10 3 and the mass concentrations C ¼ 0:016{0:096 g cm À3 fully polymerized in benzene (i.e., after completion of the propagation process), light/neutron scattering experiments 15,16 indicated that the PBLi chains predominantly form the (PBLi) f aggregates with the average aggregation number f ¼ $ 4 (and a very minor fraction of huge aggregates with f ) 4), and a 7 Li NMR experiment 16 revealed that several different Li species coexist in the system and a thermal exchange among these species has a Cand M-dependent characteristic time ex (¼ 0:1{10 s at room temperature).…”

“…For monodisperse PBLi chains of the molecular weights M ¼ 2:6 Â 10 3 -9:6 Â 10 3 and the mass concentrations C ¼ 0:016{0:096 g cm À3 fully polymerized in benzene (i.e., after completion of the propagation process), light/neutron scattering experiments 15,16 indicated that the PBLi chains predominantly form the (PBLi) f aggregates with the average aggregation number f ¼ $ 4 (and a very minor fraction of huge aggregates with f ) 4), and a 7 Li NMR experiment 16 revealed that several different Li species coexist in the system and a thermal exchange among these species has a Cand M-dependent characteristic time ex (¼ 0:1{10 s at room temperature). Furthermore, this ex was associated with an activation energy, ÁE ¼ 88 kJ mol À1 , that was considerably lower than the energy ÁE C-Li required for cleaving a C-Li bond 17 (> 150 kJ mol À1 ).…”