Biodegradable PHB from rac-β-Butyrolactone: Highly Controlled ROP Mediated by a Pentacoordinated Aluminum Complex

The diphenoxyimine five-coordinated aluminum complex [AlMeL2] (1), (L = N-(2,6-diisopropylphenyl)phenoxyimine), has been synthesized and fully characterized. The mononuclear structure observed in the solid state by X-ray diffraction analysis is maintained in solution, as shown by diffusion-ordered NMR spectroscopy (DOSY). Complex 1 is an active ROP catalyst able to provide well-controlled living ring-opening polymerization of rac-BBL at 100 °C, with PDI = 1.03, in the presence of BnOH to give the biodegrable polyester poly(hydroxybutyrate) (PHB). The suggested active species of this catalytic process, the aluminum complex [Al(OBn)L2] (2), has been synthesized and fully characterized. DFT calculations have been carried out to study the influence of the catalytic active species pocket over the polymer molecular weight control. Moreover, 1 is also active in the copolymerization of rac-β-butyrolactone and l-lactide to provide random copolymers.