Linear not cyclic – unravelling an anionic initiation pathway for Lewis pair polymerization of lactones

Abstract

Zwitterionic Lewis pair (LP) catalysis is potent towards the polymerization of lactone monomers to form cyclic polymers. In pursuit of faster polymerization kinetics, the use of weaker Lewis acids, such as diethylzinc (ZnEt₂), has hitherto been suggested. However, the strong Brønsted base character of ZnEt₂ brings the question of the actual initiation mechanism. Here, the ZnEt₂-1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) LP was studied as catalyst for the initiation and polymerization reactions of ω-pentadecalactone (PDL), ε-caprolactone, δ-valerolactone, and γ-butyrolactone. Collective MALDI-ToF MS, NMR, FT-IR, and Ubbelhode viscometry studies revealed a polymerization mechanism proceeding through deprotonation of the α-protons on the lactone and not zwitterionic ring-opening, yielding an anionic propagation mechanism and linear polymers. The polymerization kinetics display an initiation period that correlates to ethyl decomposition on ZnEt₂ and the initiation period is shortened by increasing the reaction temperature, Lewis base equivalents, and the lactones, e.g. ε-caprolactone, δ-valerolactone, and γ-butyrolactone in the system.