Synthesis of Poly(<i>n</i>-butyl acrylate)-<i>b</i>-poly(ε-caprolactone) through Combination of SG1 Nitroxide-Mediated Polymerization and Sn(Oct)₂-Catalyzed Ring-Opening Polymerization: Study of Sequential and One-Step Approaches from a Dual Initiator

Abstract

A hydroxyl-functionalized alkoxyamine derived from the SG1 nitroxide was used as a dual initiator for ring-opening polymerization (ROP) of ε-caprolactone (CL) and nitroxide-mediated polymerization (NMP) of n-butyl acrylate (BA) to obtain the corresponding block copolymer. Both sequential and one-step strategies were investigated, using the tin(II) 2-ethylhexanoate (Sn(Oct)2) ROP catalyst. The NMP first sequential approach (consisting of performing the NMP before the ROP) successfully provided the diblock copolymer through well-controlled NMP and ROP processes. This copolymer was fully characterized by size exclusion chromatography (SEC), liquid chromatography at the critical conditions (LC-CC) of PBA and PCL, and gradient polymer elution chromatography (GPEC). Conversely, the ROP first strategy led to badly defined PCL (bimodal distribution) in the first ROP step. This was attributed to a side reaction of Sn(Oct)2 with the SG1 nitroxide arising from the alkoxyamine dissociation at the ROP temperature. This strategy was consequently not suitable for obtaining the diblock copolymers. Finally, PBA-b-PCL copolymers were successfully prepared from the dual initiator through a NMP/ROP one-step process in toluene, as shown by 1H NMR spectroscopy, SEC, and GPEC. The success of this one-step approach was explained by a much faster consumption of the BA by NMP than that of the CL by ROP, making the process looking like a NMP first in a one pot experiment.