Exploiting controlled transesterification as a “top down” approach to tailor poly(ε-caprolactone)-poly(lactic acid) copolymer structures with bis-Zn catalysts

Abstract

Poly(ε-caprolactone)-poly(lactic acid) (PCL-PLA) copolymers have wide-ranging applications, where the polymer properties are controlled by the chain structure (i.e. block or random). Yet the synthesis of well-defined higher order multiblock PCL-PLA copolymers remains challenging, as competitive transesterification processes can occur that disrupt the polymer structure. Herein, we demonstrate that controlled transesterification can be harnessed as a “top down” method of tailoring PCL-PLA copolymer structures, instigated by the addition of ε-caprolactone to a living PCL-PLA-Zn copolymer chain. The extent of transesterification can be enhanced by increasing the ε-CL stoichiometry and decreasing the chain length of the diblock PCL-PLA precursors. While transesterification decreases the average length of the PCL and PLA blocks, the polymers retained their “blocky” nature as evidenced by DSC analysis. Novel computer simulations on simplified oligomeric systems show that transesterification occurs in both the PCL and PLA blocks of the original copolymer. This methodology also successfully transesterified an isolated PCL-block-PLA copolymer, suggesting that this zinc-catalysed approach may be a versatile post-polymerisation method for diversifying copolymer structures.