Ring-Opening Polymerization (ROP) of cyclic esters by a versatile aluminum Diphenoxyimine Complex: From polylactide to random copolymers

Aluminum complex [AlMeL2] (1), (L = N-(2,6-diisopropylphenyl)-phenoxyimine), promotes the Ring-Opening Polymerization (ROP) of L- and rac-lactide, and the copolymerization of L-lactide with rac-beta-butyrolactone and epsilon-caprolactone. This complex exhibits good control over the ROP of rac-lactide producing heterorich-PLA (P-r = 0.69) with narrow dispersity (D-M < 1.10). When the ratio of the co-initiator (BnOH) is increased the polymerization is immortal in nature. Furthermore, a series of copolymers PLLA-co-PHB and PLLA-co-PCL with wide ranging monomer compositions were synthesized and characterized by NMR spectroscopy and differential scanning calorimetry (DSC). The microstructure disclosed by C-13 NMR analysis suggested a random distribution of the monomers along the polymer chain for both the PLLA-co-PHB and PLLA-co-PCL copolymers. In all cases, the copolymers displayed a single glass transition temperature with values intermediate between those of the corresponding homopolymers and in perfect agreement with the theoretical values calculated by the Fox equation. These data further support the random structure of the PLLA-co-PHB and PLLA-co-PCL copolymers obtained by 1/BnOH. DFT calculations have been carried out to justify the unusual but similar reactivity observed for LA and BBL with the catalyst.