Abstract
The reaction of MgBu2, ZnEt2 or Ca(O(i)Pr)2 with 2 eq. of three-coordinating N-[methyl(2-hydroxy-3,5-dimethylphenyl)]-N-methyl-N-methyl-1,3-oxolaneamine (mpoa-H) or N-[methyl(2-hydroxy-3,5-di-tert-butylphenyl)]-N-methyl-N-methyl-1,3-oxolaneamine (tbpoa-H) gave neutral, monomeric [Mg(mpoa)2], [Zn(mpoa)2], [Zn(tbpoa)2], and [Ca(tbpoa)2] as white powders in 58-90% yields. The resulting aminophenolates were characterized in solution by NMR showing, in the case of [Zn(tbpoa)2], interesting dynamics. [Zn(tbpoa)2] and [Ca(tbpoa)2] were characterized by X-ray crystallography to show the Zn atom to be pseudo-octahedrally coordinated and the Ca atom in six-coordination mode. The new homoleptic complexes were tested in the polymerization of lactide with an external alcohol to reveal stable behaviour (during the polymerization process) only in the case of [Zn(tbpoa)2]. The high activity of the catalyst was correlated with a ligand flexibility that was further supported by theoretical studies.
Highlights
Over the past two decades biodegradable polymers have attracted increasing attention as the subject of fundamental research and as products of the chemical industry.[1]
Excellent reviews have recently appeared, describing the most effective method for the synthesis of PLAs, i.e. ringopening polymerization (ROP) of lactides catalyzed by metal alkoxides.[4]
A wide variety of different kinds of complexes have been used for this purpose, comprising compounds of biologically benign metals like lithium, sodium, magnesium, zinc, calcium, and iron, and more or even highly toxic ones like aluminum, tin, lead, bismuth, and lanthanides
Summary
Over the past two decades biodegradable polymers have attracted increasing attention as the subject of fundamental research and as products of the chemical industry.[1] One of the most prominent examples of such molecules is polylactide (PLA), which is presently developed as a commodity polymer for packaging (bottles and thin films), fibres (tissue and clothes), as well as for biomedical applications as bioresorbable sutures, screws, orthopedic implants, drug delivery agents or scaffolds for tissue engineering.[2] Due to its favorable material properties and the fact that it can be produced from inexpensive renewable sources, PLA is qualified to be a viable alternative to petrochemical-based plastics.[3]. Since it is practically unviable to completely remove catalyst residues from the polymer, which is important for biomedical applications and green packages, the most interesting remain environmentally friendly non-toxic catalysts A wide variety of different kinds of complexes have been used for this purpose, comprising compounds of biologically benign metals like lithium, sodium, magnesium, zinc, calcium, and iron, and more or even highly toxic ones like aluminum, tin, lead, bismuth, and lanthanides.
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