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 1 H 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.