Model for the aggregation state of living anionic polymers

Kanaya

et al. 2006

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.

mentioning

confidence: 99%

Dynamics of Monofunctional Polybutadienyl Lithium Chains Aggregated in Benzene

Oishi

Kanaya

et al. 2006

“…22 These fused aggregates should be less stable and thus in a higher energy state compared to the tetrameric aggregates (detected in scattering experiments 21,22 as the main component of the aggregates), suggesting a possibility that the fused aggregates, once formed transiently, contribute to the propagation. This possibility is in harmony with quantum chemistry calculations 23,24 that suggest a low probability of the propagation only through the dissociated unimers.…”

mentioning

confidence: 49%

“…For PBLi mainly forming tetrameric aggregates in benzene, 21,22 the NMR data indicated that a thermal exchange of several different Li species occurs in the system and its characteristic time ex depends on the concentration (C) and molecular weight (M) of PBLi. 22 This ex was associated with an activation energy (ÁE ¼ 88 kJ mol À1 ) considerably lower than the energy ÁE C-Li for a C-Li bond dissociation 23 (> 150 kJ mol À1 ). The C-and M-dependencies of ex as well as this ÁE value suggested that the Li-Li exchange occurs mainly through a thermal fusion of the tetrameric aggregates into larger aggregates without forming the dissociated unimers.…”

mentioning

confidence: 99%

Kinetics of Living Anionic Polymerization of Polystyrenyl Lithium in Cyclohexane

Mishima

Yamago

2008

Living anionic polymer chains polymerized with organolithium initiators usually form aggregates through their Li ends in nonpolar solvents. This aggregation structure strongly affects the anionic polymerization (propagation) kinetics. In the conventional molecular picture, the aggregates are assumed to be chemically inert and the propagation occurs only though the dissociated unimers, which leads to single-exponential decay of the residual monomer fraction ðtÞ with time t. This picture was tested by 1 H NMR measurements for protonated polystyrenyl lithium (hPSLi) polymerized in a nonpolar solvent, deuterated cyclohexane (dCH). The measurements were made mostly at 34.5 C, the theta condition for neutral (non-anionic) high-M hPS. An oligomeric, deuterated styrenyl lithium (oSLi) was utilized as an initiator so that the NMR data exclusively detected the propagation kinetics (no contamination of the initiation process). For hPSLi with the molecular weight ranging from 4:2 Â 10 3 to 276 Â 10 3 , ðtÞ was found to exhibit almost single-exponential decay at short t (where ðtÞ > 10{20%) but the decay slowed at longer t (for smaller ðtÞ). Furthermore, ðtÞ decayed more slowly when the polymerization batch contained neutral, deuterated dPS (not affecting the hPSLi chemistry) at a concentration in the semi-dilute regime. These results indicated the failure of the conventional molecular picture assuming the chemical inertness of the aggregates. Consequently, some (unstable) aggregates appeared to contribute to the propagation, and the osmotic interaction among the aggregates as well as with the coexisting dPS seemed to reduce this contribution at long t thereby giving the non-singleexponential decay of ðtÞ. A simple kinetic model considering this osmotic effect consistently described the behavior of ðtÞ in the absence/presence of dPS for the polymerization of hPS in a range of 10 À3 M ¼ 11{73, although deviations were noted for the polymerization of lower-and higher-M hPS.

“…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 ). The C-and M-dependencies of ex as well as this ÁE value suggest that the Li-Li exchange occurs mainly through a thermal fusion of the f -mer aggregates into a larger aggregate, not through an independent dissociation of respective PBLi chains from the f -mer aggregates.…”

mentioning

confidence: 99%

“…In relation to this point, quantum calculations suggested that the propagation through the independently dissociated single chain is hardly probable. 17,18 The simplest propagation route through the transient 2 f -mer aggregates is shown in Scheme 2.…”

mentioning

confidence: 99%

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

Oishi

2007

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.