Shaping the Gradient: Ether-Type Polar Modifiers for the Statistical Anionic Copolymerization of Styrene and Isoprene

Living anionic copolymerization of isoprene and styrene, initiated by alkyllithium compounds in nonpolar solvent, typically yields commercially relevant tapered copolymers, widely used as thermoplastic elastomers. Lewis base modifiers enable both gradient and microstructure control. Ether-type ligands with varying coordination number were systematically investigated in styrene/isoprene copolymerization in cyclohexane using in situ near-infrared spectroscopy, focusing on their electronic and steric effects and coordination behavior. Pronounced changes in reactivity ratios (r_I ≫ r_S to r_I ≪ r_S) enabled access to tapered, gradient, inverse-gradient, and inverse-tapered architectures with increasing modifier strength. NMR analysis revealed systematic polyisoprene (PI) microstructural variations, leading to increased vinyl (1,2- and 3,4-PI) content. In summary, the most promising bidentate modifier, 2,2-di(2-tetrahydrofuryl)propane (DTHFP), provides efficient architectural tuning by changing the modifier/lithium ratio, yielding random copolymers at concentrations as low as 0.25 equiv with respect to the sec-BuLi-initiator employed.