Controlled ring-opening (co)polymerization of macrolactones: a pursuit for efficient aluminum-based catalysts†

The synthesis of high molar mass polymacrolactones and their well-defined block copolymers with small-ring lactones utilizing aluminum-based catalysts remains challenging due to the concurrent transesterifications. In this work, we report the catalytic ring-opening polymerization (ROP) of macrolactones, specifically ω-pentadecalactone (PDL), ω-hexadecalactone (HDL), and ω-6-hexadecenlactone (6HDL), and their copolymerization with small lactones, ε-caprolactone (ε-CL) and l-lactide (l-LA). Various types of aluminum-based complexes with different ancillary ligands, including phenoxy-imine (1), pyrrole-imine (2), salen (3–7), salan (8), and bis(pyrrolidene) (9, 10) ligands, were investigated as potential effective aluminum catalysts. The controlled ROPs of PDL, HDL, and 6HDL with the highest catalytic activity were achieved by bis(pyrrolidene) aluminum complex 9. For the ROP of PDL, complex 9 displayed the highest catalytic activity of k_app = 9.6 × 10⁻⁴ s⁻¹ (TOF = 360.0 h⁻¹) among the active aluminum-based catalysts reported to date and also afforded the high molar mass polypentadecalactone (PPDL) with the M_n value of up to 379 200 g mol⁻¹. The random and perfect diblock copolymers obtained from the copolymerizations of macrolactones and ε-CL initiated by complex 9 were successfully prepared via single-feed and sequential-feed polymerization strategies, respectively. Noteworthily, we achieved the efficient preparation of the perfect diblock copolymer poly(PDL-b-l-LA) with high conversion of both monomers via a sequential-feed copolymerization between PDL and l-LA using a “PDL block first route”. The microstructure of the copolymers was characterized by quantitative ¹³C NMR spectroscopy, differential scanning calorimetry, and MALDI-TOF mass spectrometry.