An Air Stable Cationic Indium Catalyst for Formation of High-Molecular-Weight Cyclic Poly(lactic acid)

Abstract

Overcoming entropic constraints to selectively produce high-molecular-weight cyclic poly­(lactide) (c-PLA) is a challenge. In this work, we use an air-stable cationic alkyl indium complex featuring a hemilabile pyridine pedant arm to reproducibly synthesize c-PLA exclusively with low dispersity (Đ ∼ 1.30) and high molecular weights (up to 416,000 g mol–1). The complex remains active after extended periods of exposure to high-humidity air. The process allows the formation of high-molecular-weight c-PLLA, c-PDLA, and their stereocomplex with no epimerization. We propose that polymerization occurs through a cooperative Lewis-pair-based coordination–insertion mechanism where the monomer coordination to the cationic indium center and ring-opening of lactide by the hemilabile pyridine donor group are essential steps.

Toward the Synthesis of Heteroleptic Zinc ROP Initiators Based on Pyridine-Containing Monoalcohols by Tuning Ligand Substituents

“…Yield 1.83 g (65%). 1 H NMR (C 6 D 6 ): δ (ppm) = 2.27 (s, 3H, PyCH 3 ), 2.67−2.71, 2.71−2.76 (2m, CH 2 , 4H), 6.41 (dd, 2H, J = 7.7, 2.9 Hz), 6.89 (t, 1H, J = 7.7 Hz), 7.03−7.09 (m, 2H), 7.17−7.23 (m, 4H), 7.71−7.79 (m, 4H) (13H, Py and Ph), 7.29 (s, 1H, OH). 13 Synthesis of Complex 1a.…”

“…1 H NMR (C 6 D 6 ): δ (ppm) = 0.38 (q, 2H, J = 8.1 Hz, ZnCH 2 CH 3 ), 1.25 (t, 3H, J = 8.1 Hz, ZnCH 2 CH 3 ), 2.11 (s, 3H, PyCH 3 ), 3.01 (t, 2H, J = 6.3 Hz, CH 2 ), 3.39−3.42 (m, 2H, CH 2 ), 6.22 (d, 1H, J = 7.7 Hz, C 6 H 3 N,), 6.44 (d, 1H, J = 7.7 Hz, C 6 H 3 N), 6.80 (dd appears as t, 1H, J = 7.7 Hz, C 6 H 3 N); 6.89−6.96 (m, 2H), 7.01−7.09 (m, 4H), 7.50−7.59 (m, 4H) (Ph). 1 H NMR (CDCl 3 ): δ (ppm) = −0.20 (q, 2H, J = 8.1 Hz, ZnCH 2 CH 3 ), 0.68 (t, 3H, J = 8.0 Hz, ZnCH 2 CH 3 ), 2.16 (s, 3H, PyCH 3 ), 2.95 (t, 2H, J = 6.4 Hz, CH 2 ), 3.31 (m, 2H, CH 2 ), 6.78 (d, 1H, J = 7.7 Hz), 6.98−7.03 (m, 3H), 7.08 (t, 4H, J = 7.5 Hz), 7.35 (d, 4H, J = 7.7 Hz), 7.45 (t, J = 7.7 Hz, 1H) (Ar). 13 ).…”

“…Compound was obtained as a white powder (0.24 g, 81%). 1 H NMR (C 6 D 6 ): δ (ppm) = 0.41 (q, 2H, CH 2 Zn, J = 8.1 Hz), 1.26−1.42, 1.57−1.82 (2m, 10H, cyclohexyl protons), 1.50 (t, 3H, CH 3 CH 2 Zn, J = 8.1 Hz), 2.71 (s, 3H, CH 3 Py), 3.02 (s, 2H, CH 2 Py), 6.47, 6.50 (2d, 2H, J = 7.7 Hz, Py), 6.91 (dd appears as t, 1H, J = 7.7 Hz, Py). 13 C NMR (C 6 D 6 ): δ (ppm) = 2.9 (CH 3 CH 2 Zn), 13 Synthesis of Complex 3a.…”

“…To a rapidly stirring solution of complex 2a (0.0210 g, 0.0702 mmol) were added L-lactide (1.0118 g, 7.02 mmol) and then benzyl alcohol (BnOH, 8 μL, 0.0702 mmol) in toluene (10.3 mL). The reaction mixture was stirred at 80 °C for 15 h. The conversion yield of PLLA was analyzed by 1 H NMR spectroscopic studies. Methanol was added to the system to terminate the reaction.…”

About the Influence of (Non‐)Solvents on the Ring Expansion Polymerization of l‐Lactide and the Formation of Extended Ring Crystals

“…Synthesis and characterization of cPLA have rapidly increased over the past 15 years. [ 7–40 ] The polymerization methods may be subdivided into two categories, first, polymerizations obeying the ROPPOC definition, meaning ring‐opening polymerization with simultaneous polycondensation (including cyclization), [ 41 ] and second, ring‐expansion polymerization (REP) using covalent cyclic catalysts. [ 42 ] The present work is based on REPs catalyzed by two cyclic tin compounds, 2,2‐dibutyl‐2‐stanna‐1,3‐dithiolane (DSTL) and 2‐stanna‐1.3‐dioxa‐4,5,6,7‐dibenzazepine (SnBiph, see Scheme ), because these catalysts have proven in recent REPs conducted in bulk, that they may yield medium and high molecular weight cPLA with average number molecular weights ( M n ) up to 200 000 and average weight molecular weights ( Mw ) up to 400 000) along with extraordinarily high melting temperatures ( T m ) and melting enthalpies (Δ H m ).…”