Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of -caprolactone: aluminum thioamidate and thioureidate systems

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for -caprolactone (CL) polymerization was evaluated. S Pr -Al exhibited a higher catalytic activity than O Pr -Al (3.2 times as high for CL polymerization; [CL] : [ S Pr -Al ] : [BnOH] = 100 : 0.5 : 2; [ S Pr -Al ] = 10 mM, conv. = 93% after 14 min at 25 °C), and US Cl -Al exhibited a higher catalytic activity than U Cl -Al (4.6 times as high for CL polymerization; [CL] : [ US Cl -Al ] : [BnOH] = 100 : 0.5 : 2; [ US Cl -Al ] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [S Pr -AlOMe 2 ] 2 decomposed into the four-membered ring S Pr -AlOMe 2 . However, [O Pr -AlOMe 2 ] 2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [S Pr -AlOMe 2 ] 2 (18.1 kcal mol −1 ) as a catalyst than when using [O Pr -AlOMe 2 ] 2 (23.9 kcal mol −1 ). This is because the TS2a transition state of S Pr -AlOMe 2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [O Pr -AlOMe 2 ] 2 , the crowded highest-energy transition state of [O Pr -AlOMe 2 ] 2 with a larger N-Al-O angle (99.63°). S-Al system exhibited a higher level of catalytic activity (1.8-5.5 times) than did the O-Al system. Furthermore, the US-Al system exhibited a higher level of catalytic activity (1.7 - 4.6 times) than did the U-Al system.