Stereoselective polymerization of rac‐lactide with a bulky aluminum/Schiff base complex

Abstract

AbstractAn aluminum/Schiff base complex {[2,2‐dimethyl‐1,3‐propylenebis(3,5‐di‐tert‐butylsalicylideneiminato)](isopropanolato)aluminum(III) (2)} based on a bulky ligand and aluminum isopropoxide was prepared and employed for the stereoselective ring‐opening polymerization (ROP) of rac‐lactide (rac‐LA). The initiator was characterized with nuclear magnetic resonance (NMR), crystal structure measurements, and elemental analysis. It contained a five‐coordinate aluminum atom that was trigonal bipyramidal in the solid state according to the crystal structure measurements. The two conformational stereoisomers of 2 exchanged quickly on the NMR scale. Compound 2 polymerized rac‐LA into a crystalline polymer that was characterized with 1H NMR, wide‐angle X‐ray diffraction, electrospray ionization mass spectrometry, and gel permeation chromatography. The kinetics of the polymerization were first‐order in both the monomer and initiator, and there was a linear relationship between the rac‐LA conversion and the number‐average molecular weight of poly(rac‐LA) with a narrow molecular distribution (1.04–1.08). These features showed that the polymerization was well controlled. The high melting temperature (196–201 °C) and isotacticity of poly(rac‐LA) indicated that complex 2 was a highly stereoselective initiator for the ROP of rac‐LA. The stereoselectivity was as high as 90%, and the stereoblocks of poly(rac‐LA) by complex 2 contained an average of 20 units (average block length = 20) of enantiomerically pure lactic acid. The activation energy (23.6 kJ mol−1) was obtained according to an Arrhenius equation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5974–5982, 2004