Dramatic Effect of Alkali Metal Alkoxides on the Anionic Copolymerization of Styrene and Isoprene

The effect of lithium, sodium, and potassium tert-amylates on the kinetics of the statistical anionic copolymerization of styrene and isoprene in cyclohexane was investigated using in situ near-infrared (NIR) spectroscopy. The reactivity ratios and the related comonomer gradients can be adjusted over the entire range resulting in both random and inverted gradient copolymers. Lithium tert-amylate retards the polymerization at overstoichiometric concentrations. In contrast, even at low concentrations, sodium and potassium tert-amylate increase the rate of styrene polymerization due to a counterion exchange. Only 1/30 equiv of potassium tert-amylate relative to butyllithium is necessary to obtain random copolymers, which unexpectedly consist of short blocks. Remarkably, a high content of isoprene 1,4-units is maintained, leading to a low glass transition temperature of -55 °C of random or inversely tapered poly-(styrene-co-isoprene). Thus, in contrast to Lewis base modifiers, the diene microstructure can be decoupled from reaction kinetics, when potassium alkoxides are used.