Investigation of the Initiating and Catalytic Behavior of Tri-n-butyltin(IV) n-Butoxide in Ring-Opening Polymerization of ε-Caprolactone and Transesterification of Poly(L-lactic acid)

Abstract

Tri-n-butyltin(IV) n-butoxide (nBu3SnOnBu) was successfully synthesized and utilized as a stable and homogeneous initiator for the small-scale synthesis of biodegradable polymers in our laboratory. From our previous works, the initiating performance of nBu3SnOnBu in the ring-opening polymerization (ROP) of cyclic ester monomers was completely investigated by differential scanning calorimetry (DSC). It was found that nBu3SnOnBu acted as the slow initiator in the ROP of ε-caprolactone (ε-CL) due to its high steric hindrance around the active center. Moreover, it could be used in the stereoselective ROP of racemic-lactide (rac-LA). The synthesized nBu3SnOnBu showed higher stereoselectivity than the conventional tin(II) octoate (Sn(Oct)2) system. In this present work, the effectiveness of nBu3SnOnBu in scaled-up (250 g) of the synthesis of poly(ε-caprolactone) (PCL) was further investigated comparing to small-scale polymerization (4 g). Furthermore, the catalytic performance of this nBu3SnOnBu in the transesterification of high molecular weight poly(L-lactic acid) (PLA) was also investigated for the first time. The transesterification of PLA with 0.05 M and 0.10 M of soluble nBu3SnOnBu catalyst in CHCl3 was successfully investigated by the gel permeation chromatography (GPC) technique. It was found that the molecular weight of PLA dramatically decreased with increasing time and nBu3SnOnBu concentration. From the kinetics study, nBu3SnOnBu acted as a highly active transesterification catalyst for PLA. From a mechanistic study, the transesterification mechanism of PLA with nBu3SnOnBu catalyst was proposed through intramolecular transesterification.