Metal(iii) complexes supported by porphyrin and salen ligands were highly efficient for rac-lactide polymerization at room temperature giving isotactic-enriched PLA.

Tetraphenylporphyrin aluminum(iii) salts, TPPAlX, where X = Me, OEt, OiPr, OCHMeCHCl, and Cl, and bis(triphenylphosphine)imminium chloride, PPNCl (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)(PLA) where n = 2 or 3 and even higher. There is no reaction between TPPAlOR (R = Et or Pr), PPNCl, and rac-LA in neat THF at 25 °C even though TPPAlOR (R = Et or Pr) and PPNCl in neat PO yields polypropylene oxide with a terminal OR group, H(PO)OR. Taken together, Al(iii) acts as a Lewis acid in the ring-opening of PO, in which PPNCl is present and the incipient ClCHCHMeO initiates the ROP of LA to yield anion chains of [(PLA)-OCHMeCHCl], and then the ring-opening of PO yields cycles, (PO)(PLA), with the liberation of Cl. The polymer was isolated by the addition of MeOH/HCl and end group analysis by mass spectrometry.

A series of tin(II) complexes supported by N 2 O 2 bis(phenol)-amine ligands were prepared from the reactions of the corresponding ligands with Sn[N(SiMe 3 ) 2 ] 2 in benzene at room temperature. The ligands were designed to have different substituted group at the ortho-position on the aryl rings (R = t Bu, CH 3 ) and N-containing side arm (E = CH 2 NEt 2 and pyridine) giving a variation of tin(II) complexes (R = t Bu, E = CH 2 NEt 2 , 2a; R = t Bu, E = py, 2b; R = CH 3 , E = CH 2 NEt 2 , 2c; R = CH 3 , E = py, 2d). All complexes were characterized by NMR spectroscopy and single-crystal X-ray analysis. The single-crystal X-ray crystallography revealed that all complexes have a monomeric four-coordinate tin center with a distorted seesaw structure. All complexes are active for solvent-free polymerization of L-lactide at 120 C giving poly(L-lactide) with narrow to moderate dispersity (Ð = 1.12-1.56). In the presence of benzyl alcohol during the polymerization, the resulting polymer was found to be linear having benzyl alcohol as the end group while, in the absence of benzyl alcohol, the polymer was cyclic. The large t Bu group at the ortho-position was found to decrease polymerization activity while the more basic CH 2 NEt 2 group was found to increase the polymerization activity. The polymerization of rac-lactide under a similar condition gave PLA having a slight heterotactic bias for all catalysts.