Chemical synthesis of biodegradable aliphatic polyesters and polycarbonates catalyzed by novel versatile aluminum metal complexes bearing salen ligands

Abstract

AbstractTwo novel aluminum metal complexes (2 and 3) bearing salen ligands were in situ prepared from trimethyl aluminum (AlMe3), methanol, and (R,R)‐N,N′‐bis(salicylidene)‐1,2‐diaminocyclohexane with original synthetic strategies, and a preliminarily resoluted (R,R)‐1,2‐diaminocyclohexane was applied as a synthetic precursor. By means of Fourier transform infrared spectrometry, NMR spectrometry, mass spectrometry, and single‐crystal X‐ray diffractometry, 2 and 3 were revealed to be distinct molecular structures with corresponding yields of 85 and 10%, respectively. Further studies via 27Al NMR techniques and single‐crystal X‐ray diffraction indicated that dimeric metal complex 3 appeared in the six‐coordinated state, whereas there was a dynamic equilibrium transition between the five‐ and six‐coordinated states for metal complex 2 in a CDCl3 solution. The more stable dimeric metal complex (3) exhibited two inequivalent aluminum metal centers coordinated to nitrogen atoms attributed to two different salen ligands, and this was different from the reported salen aluminum complex structures. Furthermore, 2 and 3 were employed as candidate catalysts for the ring‐opening polymerization (ROP) of some important biodegradable aliphatic polyesters and polycarbonates, including poly(ϵ‐caprolactone) (PCL), poly(δ‐valerolactone), poly(trimethylene carbonate), and poly(2,2‐dimethyl trimethylene carbonate). The synthetic results indicated that both metal complexes efficiently catalyzed ROP at 100 °C in an anisole solution, and 3 showed much better controlled characteristics of ROP than 2. Very narrow molecular weight distributions close to 1.21 for PCL were detected with 3 as the ROP catalyst. In addition, a catalytic mechanism study confirmed that ROP catalyzed by these metal complexes was in good agreement with the commonly accepted coordination polymerization reported for aluminum triiso [Al(OiPr)3] and stannous octanoate [Sn(Oct)2]. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 373–384, 2005