Facile Synthesis of Hydroxyl-Ended, Highly Stereoregular, Star-Shaped Poly(lactide) from Immortal ROP of rac-Lactide and Kinetics Study

Triethanolamine (TEA) reacted with 3 equiv of yttrium alkyl complex 1 bearing O,N,N,O-tetradentate Satan ligand to give a trinuclear yttrium alkoxide counterpart 2 via metathesis reaction. Complex 2 initiated the ring-opening polymerization of rac-lactide in a livingness mode under mild conditions. Remarkably, 2 was so tolerant of the protic TEA that under various TEA-to-yttrium molar ratios ranging from 1:3 up to 81:1, the polymerization performed smoothly, suggesting an immortal characteristic. The resultant polylactides had variable molecular weights (M-n = 1.36 x 10(4) to 32.54 x 10(4)) but narrow molecular weight distributions (PDI = 1.02-1.09), and, especially, pure heterotactic (P-r = 0.99), hydroxyl-functionalized and star-shaped architecture, which were fully characterized by NMR spectroscopy and MAL-DI-TOF mass spectrum analyses and confirmed further by calculating the contraction factor value g' (g' = 0.96) via Mark-Houwink equations. Detailed investigation of the immortal polymerization gave a unique kinetics law of -d[LA]/dt = k(p)[LA][TEA](-0.352).