Anilidopyridyl-Pyrrolide and Anilidopyridyl-Indolide Group 3 Metal Complexes: Highly Active Initiators for the Ring-Opening Polymerization of <i>rac</i>-Lactide

Three new group 3 metal complexes, bearing anilidopyridyl-pyrrolide (<b>L</b>^<b>1</b>) and anilidopyridyl-indolide (<b>L</b>^<b>2</b>) as dianionic tridentate ligands, with the general formula LMN­(SiHMe₂)₂ were synthesized (complex <b>1</b>, M = Y, L = <b>L</b>^<b>1</b>; complex <b>2</b>, M = Sc, L = <b>L</b>^<b>1</b>; complex <b>3</b>, M = Y, L = <b>L</b>^<b>2</b>). All complexes were fully characterized and tested as initiators for the ROPs of <i>rac</i>-lactide. The yttrium complexes <b>1</b> and <b>3</b> resulted in highly active catalysts (TOF up to 10⁴ mol_lactide mol_Y^–1 h^–1), whereas the scandium complex showed moderate activities. This class of catalysts allowed a good control of the macromolecular architecture of the polymer, namely, the nature of end groups, the molecular weights, and their distribution. Moreover, the obtained PLAs showed <i>P</i>_r values in the range of 0.57–0.84, depending on the nature of the initiator and solvent. Well-controlled and rapid ROPs of <i>rac</i>-lactide were obtained in the solvent-free polymerizations at 130 °C as well, suggesting that complexes <b>1</b>–<b>3</b> are stable at high temperature. Finally, in the presence of 2-propanol, complex <b>1</b> promoted the <i>immortal</i> ROP of <i>rac</i>-lactide, showing a remarkable TOF of 3.5 × 10⁴ mol_lactide mol_Y^–1h<sup>–1