Stereoselective ROP of rac-LA with Zr iv

This complex was highly active for the ROP of rac-LA in the melt (130 °C) with a conversion of 78% after only 10 min, with a M = 32 300 g/mol and PDI = 1.22. Significantly, PLA with a remarkable heterotacticity was isolated P,. = 0.96 cf. 0.98 in solution at room temperature). This was the first case 

Okuda Macromoiecuies 2012 inorg. Chem. 2014 Polymer Chem, 2011 

In this work the three chiral forms of the ligand have been prepared R,Rl S,Slmeso) and complexed to zirconium(iv). The chiral ligands bind to the metal centre to afford a-cis complexes. The chirality of the metal (either A or A) is predetermined by the chirality of the ligand - R,R affords the A and S,S the A isomer - and these are locked in solution as single stereoisomers. However, the meso ligand binds in a fac-fac (p-c/s) fashion to the zirconium(iv) centre and this is fluxional in solution - which has been assigned to the 

A self-correcting mechanism has been proposed, in which the A-isomer is active for the ROP of d-LA and A l-LA. If we imagine we start with the A-isomer this will polymerise d-LA, until a misinsertion of an l-LA occurs. The complex can then invert to form the A-isomer and polymerisation can continue with l-LA. However, for the chiral complexes no such selfcorrection mechanism exists and the polymerisation halts when a misinsertion event occurs forming a dormant species that needs to be awoken with the same isomer. 

In conclusion, the utility of zirconium(iv) complexes for the stereoselective ROP of rac-LA is a buoyant and vibrant area. It is possible to prepare PLA with a high isotactic and heterotactic enchainment with judicious choice of the ligand. However, it is fair to say that it is difficult to predict what the tacticity will be and to rationalise the observed tacticity. This is perhaps the biggest area that future work will need to focus on. One imagines that computational chemical approaches will have a lot to offer this area in not only rationalising the selectivity but in the determination of structure-activity relationships that will allow the synthetic chemist to predict selectivity before the experiment. 

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