Boric acid as biocatalyst for living ring-opening polymerization of ε -caprolactone

Boric acid (1, B(OH)3) was investigated as metal-free biocatalyst for its activity towards the bulk ring-opening polymerization (ROP) of ε-caprolactone (CL) initiated by benzyl alcohol (BnOH). Moreover, other boric acid derivates such as phenylboronic acid (2), 2-thienylboronic acid (3), 2-furanylboronic acid (4) and 5-pyrimidinylboronic acid (5) were also used as acidic catalysts to synthesize poly(ε-caprolactone) (PCL), poly(δ-valerolactone) (PVL) and poly(trimethylene carbonate) (PTMC). The studies on the polymerization kinetics for all catalysts confirm that the reaction rates are first-order with respect to monomer and their catalytic activities follow the order 1 > 3 > 2 ≈ 4 > 5. Furthermore, the block copolymerization of ε-caprolactone (CL) with δ-valerolactone (VL) and trimethylene carbonate (TMC) successfully proceeded by the sequential monomer addition to give PCL-b-PVL and PTMC-b-PCL copolymers, indicating the living nature of the present polymerization system. The end group analysis by 1H NMR and MALDI-TOF MS measurements suggested that the benzyl moiety of initiator inserted into the synthesized polymeric chain. The existence of the interaction between the catalyst B(OH)3 and the monomer CL supports that B(OH)3 catalyzed ROP of CL preferred an activated monomer mechanism. A possible model for the initiation and propagation procedures of ROP of cyclic esters catalyzed by boric acid in the presence of benzyl alcohol was proposed.