Lactides stereo-controlled

Since the first report of the use of 4-dimethylaminopyridine (DMAP) for the ROP of lactide in 2001 [90], the field of organocatalysts for ROP has received much more attention since it provides an alternative to the metal-based catalysts stereo-controlled ROP of lactides. In 2007 Wade et al. found that a dimeric phosphazene base stereoselectively produced isotactic PLA from rac-LA (Pm = 0.95 at —75°C) [91]. Recently, Chen reported that -isocupreidine (ICD) was applied in stereoselective ROP of rac-lactide to form an isotactic-enriched PLA (Pm — 0.74) at room temperature [92]. 

Lactic acid is currently produced by fermentation of carbohydrates and is rme of the high potential and versatile biomass-derived platform chemicals, leading to various useful polymer products. PLA is produced by ROP of lactide (derived from lactic acid) and exhibits mechanical properties similar to poly(ethylene terephthal-ate) and polypropylene. Representative examples discussed herein included the synthesis of highly stereo-controlled PLAs, such as isotactic, heterotactic, and syndiotactic PLA materials, rendered by different catalyst/initiator systems. 

In principle, rac-lactide, a racemic mixmre of d- and L-lactide, may be polymerized in a stereoselective fashion. Depending on the stereoselection as the ROP proceeds, the resulting polymer may thus exhibit different stereoregularities these directly influence the thermal and mechanical properties of the produced PLAs. In this regard, isotactic PLA stereoblocks and PLA stereocomplexes, which are of interest for their thermal and mechanical properties, may be produced via the ROP of rac-lactide initiated by an achiral derivative, provided the polymerization proceeds via a chain-end stereocontrolled mechanism i.e., the last inserted lactide unit stereo-controls the insertion of the incoming monomer. This strategy has been first validated using salen-based aluminum complexes such as 16 (Scheme 16, top) to produce PLLA-PDLA isotactic stereoblocks [95, 96]. Alternatively, the chiral racemic salen aluminum complex 17 was found to be suitable for the parallel stereoselective synthesis of isotactic poly(D-lactide) and poly(L-lactide) from rac-... 

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