Controlled and effective ring-opening (co)polymerization of rac-lactide, ε-caprolactone and ε-decalactone by β-pyrimidyl enolate aluminum complexes

Abstract

The use of biodegradable polyesters, such as polylactide (PLA), polycaprolactone (PCL), and poly(ε-decalactone) (PDL), is one solution to environmental problems caused by petroleum-derived polymers and is also beneficial for a sustainable future. This work describes a series of β-pyrimidyl enolate aluminum complexes with the general formula LAlMe₂ (1–6) {L = [(2-C₄H₂N₂Cl)-CHC(O)C₆H₄R], R = H (1), Me (2), OMe (3), CF₃ (4), CN (5), Ph (6)} that efficiently produce PLA, PCL, and PDL with controlled molar masses and narrow dispersity values. Under immortal polymerization conditions, all aluminum complexes also demonstrated a living polymerization with the obtained molar masses of polylactide proportional to the ratio of monomer/benzyl alcohol and relatively narrow dispersity values. While the electronic characteristics of the ancillary ligands had no substantial influence on the catalytic activity for the rac-LA polymerization, the catalytic activity of the ε-CL and ε-DL polymerizations was affected by the ligand substituent (R), with complex 4 exhibiting the highest catalytic performance in both cases. In addition, six different types of well-defined block copolymer between rac-LA, ε-CL, and ε-DL were successfully prepared via a sequential-feed approach and the second monomer could be polymerized to complete conversion regardless of the order of the first monomer. The results unveiled that the order of monomer addition was not crucial in this catalytic system, which was advantageous for synthesizing triblock copolymers. However, the overall polymerization time was the significant factor in determining the order of monomer addition. Remarkably, the perfect random copolymer between ε-CL and L-LA, poly(L-LA-r-CL), with a strict Bernoullian distribution, was successfully produced by complex 3via a single-feed polymerization without the transesterification side reactions. This is the first time that the random copolymer can be produced by utilizing sterically unhindered aluminum complexes with a strong electron-withdrawing character. The electronic influence from the pyrimidine ring of the ligand framework was believed to play an important role in the formation of the random copolymer. The success of this aluminum-based catalyst system allows for the future development of other bio-derived lactones and copolymers with varying properties.