Dual catalysis system for ring-opening polymerization of lactones and 2,2-dimethyltrimethylene carbonate

Abstract

In this study, three isothioureas (ITUs), 2,3,5,6-tetrahydroimidazo[2,1-b]thiazole (ITU 1), 2,3,6,7-tetrahydro-5H-thiazolo-[3,2-a]pyrimidine (ITU 2) and 3,4,7,8-tetrahydro-2H,6H-pyrimido[2,1-b][1,3]thiazine (ITU 3) were prepared. These ITUs coupled with magnesium halides (MgX₂) as the cocatalysts, cooperatively promoted the ring-opening polymerization (ROP) of several common lactones (including δ-valerolactone, ε-caprolactone, ε-decalactone and even of macrolactone ω-pentadecalactone) and 2,2-dimethyltrimethylene carbonate. These cocatalysts were found to exhibit suitable activities and ITU 2 emerged as the most active organic component. The order of activities for the magnesium halides was found to be lying in the following order: MgI₂ > MgBr₂ > MgCl₂. The polymerizations attained high conversions (>90%) under optimal conditions, and produced linear polyesters having predictable molecular weights, narrow polydispersity indices (PDIs < 1.20) and defined end groups, which were derived from the benzyl alcohol initiator. The resulting polymers were characterized using GPC, IR, DSC, NMR and MALDI-ToF mass. The kinetic and chain extension experiments showed that the ITUs/MgX₂-cocatalyzed ROPs of ε-caprolactone proceeded a living polymerization characteristics. Furthermore, the proposed polymerization mechanism was supposed to be a dual catalytic mechanism, and involved the activation of monomers through coordination to Lewis acids, while the initiator was activated by bases.