Isolated Organosodium and -Potassium Intermediates: Understanding Metal Effects on Anionic Styrene-Isoprene Copolymerization.

The isolation and characterization of reactive organoalkali-metal intermediates remain a major challenge. In particular, organosodium and organopotassium compounds show different reactivities from organolithium compounds, albeit the underlying molecular reasons are largely unexplored due to the absence of relevant intermediates. We present the first-step organometallic intermediates of an anionic styrene polymerization with sodium and potassium as counterions, and the first intermediate of an isoprene polymerization with potassium. The structures were crystallized and characterized by X-ray diffraction, providing insights into structural characteristics of heavier alkali-metal organyls. As the compounds were synthetically accessible, they do not only represent organometallic model compounds of the propagating chain ends but also new monometallic alkoxide-free initiators. This enables, for the first time, a fundamental understanding of structure-reactivity relationships in anionic polymerizations using organosodium or organopotassium reagents. The newly synthesized initiators were applied for a real-time in situ near-infrared spectroscopy kinetic investigation of the styrene/isoprene copolymerization with heavier alkali metals in cyclohexane. Combined with DFT studies, these results elucidate structural effects of the isolated intermediates, showing that charge stabilization in the transition state mainly influences the reactivity. With this combination of molecular methods for anionic polymerizations, molecular insights into the behavior of heavier alkali-metal organyls are obtained.