Anionic synthesis of well-defined, poly[(styrene)-block-(propylene oxide)] block copolymers

Abstract

Well-defined, narrow molecular weight distribution (Mw/Mn ≤ 1.1) poly[(styrene)-block-(propylene oxide)] block copolymers with relatively high molecular weight poly(propylene oxide) blocks [e. g. Mn (PPO) = 10 000–12 000 g/mol] have been prepared by anionic polymerization. The polystyrene block (Mn = 5 000; Mw/Mn = 1.1) was prepared by alkyllithium-initiated polymerization of styrene followed by chain-end functionalization with ethylene oxide and protonation with acidic methanol. The resulting ω-hydroxyl-functionalized polystyrene was converted to the corresponding alkali metal salts with alkali metals (Na/K alloy, Rb, Cs) and then used to initiate block polymerization of propylene oxide in tetrahydrofuran. The effects of crown ethers (18-crown-6 and dicyclohexano-24-crown-8) and added dimethylsulfoxide were investigated. Chain transfer to the monomer resulted in significant amounts of poly(propylene oxide) formation (50%); however, the diblock molecular weight distributions were narrow. The highest molecular weight poly(propylene oxide) blocks (12 200 g/mol) were obtained in tetrahydrofuran with cesium as counterion without additives.