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

Abstract

Triethanolamine (TEA) reacted with 3 equiv of yttrium alkyl complex 1 bearing O,N,N,O-tetradentate Salan 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 (Mn = 1.36 × 104to 32.54 × 104) but narrow molecular weight distributions (PDI = 1.02−1.09), and, especially, pure heterotactic (Pr = 0.99), hydroxyl-functionalized and star-shaped architecture, which were fully characterized by NMR spectroscopy and MALDI−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 = kp[LA][TEA]−0.352.