Synthesis of aliphatic polyesters has been studied intensively due to their biocompatible and biodegradable properties and their potential applications in medical and agricultural fields. There has been particular emphasis over the past decade on the synthesis of discrete, well-characterized complexes that are active polymerization initiators for the ring-opening polymerization (ROP) of lactide (LA) and β-butyrolactone (BBL) to give, respectively, poly(lactide) (PLA) and poly(3-hydroxybutyrate) (PHB). These recent advances in catalyst design have led to a variety of polyester microstructures. This tutorial review focuses on the use of metal-based complexes for the stereoselective ROP of rac-LA and rac-BBL.

Stereocontrolled ring-opening polymerization of cyclic esters: synthesis of new polyester microstructures.

The important advances and current trends in the stereocontrolled ring-opening polymerisation of lactide are discussed in this tutorial review. Microstructures, structural characterisation methods and the properties of stereoregular poly(lactide)s are examined. The application of metal-based catalysts dominates this area although simple anionic polymerisation and organocatalytic routes that demonstrate control of the polymer tacticity are discussed.

Stereocontrolled ring-opening polymerisation of lactide

We report the preparation, structural characterization, and detailed lactide polymerization behavior of a new Zn(II) alkoxide complex, (L(1)ZnOEt)(2) (L(1) = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]methyl}phenolate). While an X-ray crystal structure revealed the complex to be dimeric in the solid state, nuclear magnetic resonance and mass spectrometric analyses showed that the monomeric form L(1)ZnOEt predominates in solution. The polymerization of lactide using this complex proceeded with good molecular weight control and gave relatively narrow molecular weight distribution polylactide, even at catalyst loadings of <0.1% that yielded M(n) as high as 130 kg mol(-)(1). The effect of impurities on the molecular weight of the product polymers was accounted for using a simple model. Detailed kinetic studies of the polymerization reaction enabled integral and nonintegral orders in L(1)ZnOEt to be distinguished and the empirical rate law to be elucidated, -d[LA]/dt = k(p)[L(1)ZnOEt][LA]. These studies also showed that L(1)ZnOEt polymerizes lactide at a rate faster than any other Zn-containing system reported previously. This work provides important mechanistic information pertaining to the polymerization of lactide and other cyclic esters by discrete metal alkoxide complexes.

Highly active zinc catalyst for the controlled polymerization of lactide.

Synthesis of biodegradable polymers from renewable resources

Single site catalysts for stereoselective ring-opening polymerization of lactides

Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization of rac-Lactide

Methoxy-modified beta-diimines HL1 and HL2 reacted with Y(CH2SiMe3)(3)(THF)(2) to afford the corresponding bis(alkyl)s [(LY)-Y-1(CH2SiMe3)(2)] (1) and [(LY)-Y-2(CH2SiMe3)(2)] (2), respectively. Amination of 1 with 2,6-diisopropyl aniline gave the bis(amido) counterpart [(LY)-Y-1{N(H)(2,6-iPr(2)-C6H3)}(2)] (3), selectively. Treatment of Y(CH2SiMe3)(3)(THF)(2) with methoxy-modified anilido imine HL3 yielded bis(alkyl) complex [(LY)-Y-3(CH2SiMe3)(2)(THF)] (4) that sequentially reacted with 2,6-diisopropyl aniline to give the bis(amido) analogue [(LY)-Y-3{N(H)(2,6-iPr(2)-C6H3)}(2)] (5). Complex 2 was "base-free" monomer, in which the tetradentate beta-diiminato ligand was meridional with the two alkyl species locating above and below it, generating tetragonal bipyramidal core about the metal center. Complex 3 was asymmetric monomer containing trigonal bipyramidal core with trans-arrangement of the amido ligands. In contrast, the two cis-located alkyl species in complex 4 were endo and exo towards the 0,N,N tridentate anilido-imido moiety. The bis(amido) complex 5 was confirmed to be structural analogue to 4 albeit without THF coordination. All these yttrium complexes are highly active initiators for the ring-opening polymerization Of L-LA at room temperature.

Yttrium bis(alkyl) and bis(amido) complexes bearing N,O multidentate ligands. Synthesis and catalytic activity towards ring-opening polymerization of L-lactide

Xiaomin Shang

Papers

Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization of rac-Lactide

Yttrium bis(alkyl) and bis(amido) complexes bearing N,O multidentate ligands. Synthesis and catalytic activity towards ring-opening polymerization of L-lactide