Executive Summary : | The aliphatic polyester namely poly(lactic acid) (PLA) is an extensively researched biodegradable polymer of modern times. It is proposed to be an alternative to conventional polyolefin plastics since such material pose severe environmental concerns including contributing factors for global warming. The conventional route to prepare high molecular weight PLA is through the ring-opening polymerization (ROP) of lactide (cyclic dimer of lactic acid). A typical single-site catalyst contains a metal center surrounded by ligand(s) and initiating group(s) and the polymerization pathway follows the coordination insertion mechanism in the absence of external initiators like alcohols. Interactions between lactide monomer and the stereogenic metal center determines the type of stereocontrol in ROP. A favorable interaction between a medial stereogenic center of the incoming lactide and the stereogenic metal site is termed “enantiomorphic-site control” (ESC). This results in preferential homochiral enchainment leading to isotactic PLA from racemic lactide (rac-LA) and syndiotactic PLA from meso lactide (meso-LA) . A favorable interaction between the medial center of the last inserted lactidyl unit and the medial stereogenic center of the incoming lactide is termed “chain-end control” (CEC). This results either in a homochiral or a heterochiral enchainment preference for rac-LA producing an isotactic PLA or a heterotactic PLA, respectively. This eventually controls the physical properties of the PLA crucial for desired applications as a substitute to polyolefins. Literature reveals that chiral and achiral catalysts obey ESC and CEC mechanisms in rac-LA polymerization. Typically, isoselective polymerization of rac-LA gives a stereo diblock polymer rather than a mixture of two homochiral polymers. Literature observations and our work clearly suggests that ESC mechanism using chiral ligands give a much higher degree of stereoregularity in PLA synthesis in comparison to CEC mechanism. In this project, we intend to use chiral ligands based on amino acids and tartrate which are relatively inexpensive as compared to other pure chiral synthons. The ligands chosen for our studies remain unexplored in synthesizing catalysts useful for PLA synthesis. Most of the scanty literature in this direction includes the use of expensive chiral ligands and explored mostly using Mg(II) and Al(III). We would like to test our results with the suitability of other catalysts containing Ga(III), In(III), Ti(IV), Zr(IV), Fe(III) and Co(II) in addition to the ones mentioned. This is based upon our experience since almost two decades wherein bulk and denticity of the ligand in the catalyst, Lewis acidity of the metal center present in the catalyst, symmetry and nuclearity of the metal catalyst and the type of stereoregulation during the ROP are key factors that decide the eventual properties of the PLA. The copolymerization of CO₂ with epoxides will also be studied. |