A new route for the preparation of enriched iso-polylactide from rac-lactide via a Lewis acid catalyzed ring-opening of an epoxide

Abstract

Tetraphenylporphyrin aluminum(III) salts, TPPAlX, where X = Me, OEt, OiPr, OCHMeCH₂Cl, and Cl, and bis(triphenylphosphine)imminium chloride, PPN⁺Cl⁻ (1 : 1) react with rac-lactide, rac-LA, in neat propylene oxide, PO, to yield chains of enriched isotactic polylactide, PLA, with end groups of PO–Cl and with time these yield cyclic polymers (PO)_n(PLA) where n = 2 or 3 and even higher. There is no reaction between TPPAlOR (R = Et or ⁱPr), PPN⁺Cl⁻, and rac-LA in neat THF at 25 °C even though TPPAlOR (R = Et or ⁱPr) and PPN⁺Cl⁻ in neat PO yields polypropylene oxide with a terminal OR group, H(PO)_nOR. Taken together, Al(III) acts as a Lewis acid in the ring-opening of PO, in which PPN⁺Cl⁻ is present and the incipient ClCH₂CHMeO⁻ initiates the ROP of LA to yield anion chains of [(PLA)-OCHMeCH₂Cl]⁻, and then the ring-opening of PO yields cycles, (PO)_n(PLA), with the liberation of Cl⁻. The polymer was isolated by the addition of MeOH/HCl and end group analysis by mass spectrometry.