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PLA can be synthesized using three ways: (a) The first pathway is condensation polymerization of the L(+) and D(−) isomers of lactic acid which produce low molecular weight PLA, (b) the second route involves ring-opening polymerization of the lactide ring (Figure 4.5). Cargill Dow LLC developed an alternative pathway of melt polycondensation and the use of a tin catalyst to obtain commodity PLA applicable for packaging industries. This pathway involves the pre-polymer formation of aqueous lactic acid and converting into lactide stereoisomer through intramolecular cyclization. In the next step, lactide ring-opening produces high M_w PLA [21] using stannous octoate Sn(Oct)₂ as a catalyst [22].

Figure 4.5 (a) Conventional polymerization of polylactic acid, (b) High Mw PLA preparation through catalytic lactide formation method (c) The third route involves azeotropic dehydrated condensation of lactic acid to obtain high Mw PLA without the chain extenders’ addition.