In the recent years, substantial amount of research has resulted in the genesis of new sustainable technologies
towards the manufacture of polymers derived from renewable sources. The major reason for such a heuristic
approach stems from the pollution problems associated with conventional plastics.1 One such biodegradable
material that has attracted increasing attention is poly(lactic acid) (PLA) which is commercially produced using
the ring-opening polymerization (ROP) of lactide (LA).This methodology allows better control over molecular
parameters (polydispersity index, molecular weight) under mild reaction conditions.1 A study by Tolman et al.
found that the stereoselectivity present in PLA is a result of thermodynamic preference of the initiating
monomer molecule from a racemic mixture of LA enantiomers.2It is now beginning to unravel thatan
unsymmetrical transition state produced from LA and the catalyst during the initiation step plays a key role in
deciding the stereospecificity of the propagation step, ultimately governing the final stereochemistry of the
PLA, in addition to the thermodynamic preference for its molecular parameters.Jones et al. reported a Zrmesosalan
complex, which is capable of producing an isotactic enriched PLA.3Recently,Kol et al. prepared an
enantiopure salan magnesium complex which produces isotactic enriched PLA.4Earlier, Chakraborty et al.
reported benzotriazole phenoxide group (IV) complexes which are capable of producing highly heterotactic
PLA.5 However, the existing literature pays little attention towards the study of the symmetry of the transition
state during initiation in LA polymerization. Thus, the objective of this problem is an understanding towards
thecontrolof the molecular parameter as well as the tacticity of the PLA which is anticipated to depend on
symmetry of the transition state.This is surmised to be solved either by using non-centrosymmetric ligand
frameworks in the catalyst, or by using an enantiopure ligand. The firstmethodology is however far more
challenging and economical in most commercial costing requirements.
References:
(1) Mathew, J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486−494.(2)Luke, M. A.; Peterson, A.; Chiniforoush,
S.; Mandal, M.; Popowski, Y.; Sajjad, H.; Bouchey, J. C.; Shopov, Y. D.; Graziano, J. B.; Yao, J. L.; Cramer,
J. C.; Reineke, M. T.; Tolman, B. W. Macromolecules 2020, 53, 1809-1818.(3) Jones, D. M.; Hancock, L. S.;
Mckeown, P.; Schӓfer, M. P.; Buchard, A.; Thomas, H. L.; Mahon, F.M.; Lowe, P. J. Chem. Commun. 2014,
50, 15967-15970.(4) Rosen, T.; Goldberg, I.;Venditto, V.;Kol, M. J. Am. Chem. Soc. 2016, 138, 12041-
12044.(5)Pappuru, S.; Chokkapu, R. E.; Chakraborty, D.; Ramkumar, V. Dalton Trans. 2013, 42, 16412-
16427.