Stereocontrolled ROP

Lactide can be found as L-lactide, D-lactide, the racemic mixture thereof (namely raolactide), and meso-lactide. Polylactides can thus exhibit different microstruc- 

The stereosequence distribution in PLA samples is usually determined by NMR spectroscopy through inspection of the methine region in homonuclear decoupled H NMR or the carbonyl region in proton-decoupled C NMR. The stereoselectivities are classically quantified by Pm and P, values associated with the probabiUties of meso and racemic Unkage between monomer units, respectively. 

4) For instance, pure isotactic poly-(L-lactide) is a highly crystalline material (T - 180°C), whereas related random copolymers featuring 15% of meso-lactide are amorphous with a T of only 130°C (see Ref. [1]). 

The various approaches to stereoregular PLAs will be illustrated below. Representative Pm and Pj selectivities will be provided along with polymerization conditions and, when available, Tm values for isotactic PLAs. 

Alternatively, chain-end ontroUed ROP of rac-lactide leading to isotactic PLA stereocomplexes has been demonstrated with achiral SALEN-, SALAN- or homo-SALEN-based aluminum complexes. An exploration of various ligands has begun 

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C, Pm = 0.80). In 2007, Nomura et al. reported the synthesis of Schiff base aluminum complex 4 [88] with flexible but bulky BuMe2Si substituents, which exhibited the highest isoselectivity in the ROP of rac-LA to form isotactic stereoblock PLA materials with a Pm value of 0.98 and a of 210°C. More recently, highly active yttrium phosphasalen initiators were reported for the stereocontrolled ROP of rac-lactide [89]. Changing the phosphasalen structure enables access to isoselectivities (Pm = 0.84) or hetero-selectivities (P, = 0.87) (Fig. 1). 

In this chapter we present an overview of this increasingly active research field. The first section focuses on coordination polymerization with metal complexes, classified by the nature of their ancillary ligands. The spectacular achievements reported recently in organocatalyzed and stereocontrolled ROP are then presented. The third section concerns the macromolecular engineering of poly(a-hydroxyac-ids) by varying both their substitution pattern (with alternative monomers to lactide and glycolide) and their architecture (via block, star and dendritic copolymers). The well-established and rapidly emerging applications of these synthetic polyesters are discussed briefly in the last section. 

In order to effect stereocontrol over the ROP of lactones, several research groups have examined monoalkoxide aluminum initiators of the general formula LnM(OR). Preliminary studies indicated that dialkylaluminum alkoxides, e.g., Et2Al(OR), initiate the controlled ROP of lactones.772-775... 

The magnesium initiators (273) and (274) do not display the same stereocontrol as their zinc analogs over the ROP of rac-LA in either CH2C12 or benzene. However, in THF highly heterotactic PLA is produced.826 In non-coordinating solvents the Mg and Zn initiators are believed to adopt dimeric and monomeric resting states respectively, whereas both exist as monomeric forms in THF and hence give rise to similar stereoselectivity. 

Tetradentate N,0-donor ligands have also been investigated. The yttrium complex (—)-(313) does not effect stereocontrol over the ROP of rac- or meso-LA, in contrast to related A1 initiators (262) and (263).803 Polymerization is also slower than for most lanthanide initiators with 100 equivalents meso-LA requiring 14 h at 70 °C to attain near-quantitative conversion (97%). 

A completely new type of ROP catalyst was recently reported by Rieger and coworkers. Chromium salphen complexes (Fig. 32) convert racemic (3-BL to slightly isotactic-enriched PHB (0.60 < Pm < 0. 70) with a molecular weight of up to 800,000 g/mol (PD up to 8.5). These catalysts combine high activity and high molecular weight products, featuring the desired stereocontrol at moderate reaction conditions [13]. 

Spassky and coworkers discovered a remarkable stereocontrol of an enantiomerically pure A1 complex (7 )-161a for the ROP of rac-lactide resulting in a tapered stereoblock PLA microstructure with high melting point =187 °C) (Fig. 26) [160]. Structurally analogous, racemic salen-Al complex 162 resulted in highly isotactic PLA [161]. Feijen s enantiopure chiral complex (RJ )-163 (Fig. 26) exhibited an excellent reverse stereocontrol by preferential polymerization of L-lactide over D-lactide monomer (Kss/Krr = 14) that resulted in PLA with... 

Metal complexes which initiate rac-LA ROP with a high degree of stereocontrol are currently an area of major research interest and have the potential to produce a spectrum of different materials [19, 21], Much attention focuses on iso-selectivity as this can enable production of PLA of good thermal resistance (isotactic, stereoblock or even stereocomplex PLA). There are two mechanisms by which an initiator can exert iso-selectivity in rac-LA ROP (1) an enantiomorphic site control mechanism or (2) a chain end control mechanism. Enantiomorphic site control occurs using chiral initiators (Fig. 6) it is the chirality of the metal complex which... 

Stereochemically asymmetric ROP polymerizations of cyclic esters (hereafter stereocontrolled processes) involve chiral monomers. There are two major cyclic esters that bear centers of chirality p-BL and LAs (see structures 4). In this section, a notion of the absolute configuration will be used thus, the relative configurations d and l correspond to the absolute configurations R and S, respectively. 

Figure 20 Classification of the stereocontrol led polymerization processes studied in the ROP of racemic cyclic esters and leading to isotactic (homochiral) macromolecules starting from racemic monomer.

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