Dialkylmagnesium‐Promoted Deprotonation of Protic Precursors for the Activated Anionic Ring‐Opening Polymerization of Epoxides

Abstract

A family of extremely strong Brönsted bases called phosphazene superbases, [13] was also employed as effective organic catalysts or promoters for the polymerization of various types of monomers, including epoxides. [14][15][16][17][18][19][20][21][22] Due to their high basicity, non-nucleophilic nature, good solubility in a wide range of solvents, and easy handling, phosphazene bases can be readily used to turn protic moieties into nucleophilic initiating sites through deprotonation or activation of weak nucleophiles. Hydroxyl groups were the most commonly employed protic moieties due to their wide variety and high availability. Some recent reports have also demonstrated the deprotonation of other protic precursors, i.e., thiol, [23][24][25][26] amide, [27][28][29] and carboxylic acid [30] for effective polymerizations of cyclic monomers with mostly t-BuP 4 superbase. The obtention of hydroxy telechelic poly(propylene oxide) appears still limited by transfer to monomer reactions, giving chains initiated by an allyloxy group and limiting the molar masses. Their present high cost may also limit their use in industry.Besides, some studies have shown that a simple dibutylmagnesium (Bu 2 Mg) enables the synthesis of polyesters by ring-opening polymerization of lactides or lactones with residual transesterification side reactions. [31][32][33][34][35] Alcohols with different steric bulk and acidity, including iPrOH, PhCH 2 OH, Ph 2 CHOH, and Ph 3 COH, were employed with Bu 2 Mg to form magnesium alkoxides. The generated system was shown to improve the polymerizability of cyclic esters in dilute solutions by increasing the molar masses and narrowing the dispersity values where alcohols acts as control transfer agents. [36][37][38][39] We demonstrated in a previous work that propylene oxide can be polymerized with magnesium alkoxides (ROMgX) prepared from Grignard precursors in combination with triisobutylaluminum. [40] Starting from an alcohol, Grignard reagents were used as sustainable deprotonating agents for the generation of alkoxides of magnesium halide. The so-formed active species were able to initiate and poly merize propylene oxide in a controlled way. Competitive initiations were nevertheless observed. The main one was coming from the halide of the Grignard. The work presented in this article concerns a halide-free system based on dialkylmagnesium compounds used as deprotonating agents of protic