Controlled ring opening polymerizations in milliseconds by using sterically hindered strong bases

Abstract

Anionic ring-opening polymerization (ROP) of epoxides, cyclic esters, and cyclic carbonates with strong bases such as potassium tert-butoxide (KOtBu) or potassium bis(trimethylsilyl)amide (KHMDS) is typically performed as batch reaction, leading to broad dispersity and poor control over the molecular weight (Mn) mainly because of insufficient initiator efficiency and transesterification reactions. We now present that combining a sterically hindered strong base such as KOtBu or KHMDS with a primary alcohol and using a continuous-flow apparatus can afford control and reliability over the ROP and, remarkably, enable the controlled polymerization of traditionally low-activity monomers such as δ-valerolactone and ε-caprolactone in milliseconds. These reactions exhibit characteristics capable of producing narrow dispersity with predictable molecular weights and can rapidly generate well-defined block copolymers at residence times below 0.1 s. With reaction rates that are orders of magnitude higher than the fastest rates reported for urea anions, these reaction conditions offer great potential to expand the breadth of materials accessible.