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

The effect of lithium, sodium, and potassium <i>tert</i>-amylates on the kinetics of the statistical anionic copolymerization of styrene and isoprene in cyclohexane was investigated using <i>in situ</i> 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 <i>tert</i>-amylate retards the polymerization at overstoichiometric concentrations. In contrast, even at low concentrations, sodium and potassium <i>tert</i>-amylate increase the rate of styrene polymerization due to a counterion exchange. Only 1/30 equiv of potassium <i>tert</i>-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<i>-co-</i>isoprene). Thus, in contrast to Lewis base modifiers, the diene microstructure can be decoupled from reaction kinetics, when potassium alkoxides are used.