Kinetics of Living Anionic Polymerization of Polystyrenyl Lithium in Cyclohexane

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 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 10 to 276 10, 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 M 1⁄4 11{73, although deviations were noted for the polymerization of lowerand higher-M hPS.