Facile Synthesis of Aliphatic ω-Pentadecalactone Containing Diblock Copolyesters via Sequential ROP with l-Lactide, ε-Caprolactone, and δ-Valerolactone Catalyzed by Cyclic Trimeric Phosphazene Base with Inherent Tribasic Characteristics
Degradable long-chain aliphatic polyesters are promising alternatives to polyolefins. The ring-opening polymerization (ROP) of ω-pentadecalactone (PDL) enables the synthesis of aliphatic polyesters with melting temperature close to low-density polyethylene (LDPE). However, this ROP reaction is of great challenge because of low ring strain of the PDL monomer. The occurrence of intra- and intermolecular transesterification reactions during ROP made it difficult to prepare well-defined block copolyesters for advanced properties. In this context, cyclic trimeric phosphazene base (CTPB) in combination with benzyl alcohol (BnOH) was proved to be an efficient initiator system for ROP of PDL, showing an excellent polymerization rate (TOF up to 600 h–1) at 80 °C. Well-defined diblock copolyesters, i.e., PPDL-b-PLLA, can be easily synthesized by sequential addition of PDL and l-lactide (L-LA). In contrast, sequential addition of PDL and ε-caprolactone (CL) or δ-valerolactone (VL) only led to the random copolyesters under the same conditions. On the other hand, CTPB is an inherent tribasic superbase and can be partially neutralized to decrease its basicity for better control over active lactones such as CL and VL. Hence, we introduced 0.33–1.0 equiv of benzoic acid (PhCOOH) relative to CTPB and demonstrated that it can diminish the transesterification during ROP of CL or VL. Based on this strategy, PPDL-b-PCL or PPDL-b-PVL diblock copolyesters were successfully prepared by sequential addition of monomers of CL or VL after ROP of PDL. The block copolyester structures were confirmed by the 13C NMR spectroscopy and DSC characterizations.
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