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In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-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 [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al–O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol−1) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol−1). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N–Al–S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N–Al–O angle (99.63°). [ABSTRACT FROM AUTHOR]

Understanding the water state in Nafion is not only crucial for operating a proton-exchange membrane (PEM)-based fuel cell, but also intimately related to the elucidation of the proton transport mechanism in a PEM. Although many studies have been published on this subject, some controversies and ambiguities remain unresolved. In this work, we design three different types of Nafion samples by substituting protons with lithium or sodium cations. We also pay special attention to the preparation of samples for carrying out broad-range variable temperature solid state NMR experiments so that no membrane dehydration occurs during the long experimental time at low temperatures. With these precautions and improvements, clear and largely straightforward information could be obtained to ensure minimal ambiguity and complexity in the interpretation of the experimental data. Our results show that about 40–60% of water remains unfrozen at −70 °C, depending on the type of the substituting cation. Both the 1H and 2H spectral and relaxation results indicate that water freezing starts from the center of the nanopores inside Nafion and increases gradually as the temperature decreases. The protons remain dissociated with sulfonate groups even at the lowest temperature we reached (−70 °C), whereas both lithium and sodium are associated with sulfonate groups at most temperatures below 0 °C. The experimental data also suggest that besides frozen and unfrozen water, there is broad distribution of water state and dynamics in Nafion as the temperature is lowered from above zero down to −70 °C. The effect of the size of the substituting cation significantly affects the properties of supercooled water by modifying the cation–water interaction and impeding the rotation of sulfonate groups. These novel results not only help us in establishing a better understanding of the water state in Nafion and its performance as a proton exchange mebrane, but also provide insights into water freezing, antifreeze and supercooling in other nanoscopic environments. [ABSTRACT FROM AUTHOR]

A series of five-membered ring aluminum complexes bearing thiol-Schiff base ligands were synthesized, and their application in the ring-opening polymerization of ε-caprolactone (CL) was investigated. The complexes exhibited dramatically higher catalytic activity than the six-membered ring S2AlMe2 complex (approximately 4- to 10-fold higher) and the five-membered ring L5-PhAlMe2 complex (approximately 7- to 19-fold higher). Moreover, a shorter induction period was observed when the five-membered ring aluminum complexes bearing thiol-Schiff base ligands were used compared with the other types of aluminum complexes bearing Schiff base ligands. The electron-withdrawing groups enhanced the catalytic activity of the Al complexes compared with the electron donating groups. The thiol-Schiff base ligand and the five-membered ring aluminum catalysis had a synergistic effect that was stronger than the combination of their individual effects. [ABSTRACT FROM AUTHOR]

Previous studies on the ring-opening polymerization of ε-caprolactone using structurally related aluminum complexes as pre-catalysts showed inconsistent trends in the total conversion time. We propose that an induction period for Al complexes for conversion to real catalytic species, Al alkoxide, should be considered because the total conversion time consists of both an induction period and polymer propagation time. Herein, the polymerization rate of a series of Al complexes bearing ketimine ligands was investigated. The catalytic results indicated complexes with more steric hindrance with an electron-withdrawing group on the ligands, or the fact that less chelating ligands demonstrated greater propagation activity. The opposite trend for these structural effects was observed on the measurement of induction periods. These features on ligands of aluminum complexes are responsible for facilitating the conversion process to Al alkoxides. The overall catalytic performance should consider both the induction period and the propagation time. [ABSTRACT FROM AUTHOR]

A series of novel magnesium and zinc aminophenoxide complexes were successfully synthesized and one zinc complex was characterized by X-ray crystallography. They were also investigated as initiators for the ring opening polymerization of l-lactide. The complexes are effective in forming polylactides with good conversions. The nature and steric bulk of the ligands coordinated to the central metal ions enormously influenced the polymer properties. Among all the complexes, the zinc aminophenoxide complexes as initiators produced polymers with good molecular weight control and relatively narrow PDIs. [ABSTRACT FROM AUTHOR]

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-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 [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol -1 ) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol -1 ). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N-Al-O angle (99.63°).

A series of five-membered ring aluminum complexes bearing thiol-Schiff base ligands were synthesized, and their application in the ring-opening polymerization of ε-caprolactone (CL) was investigated. The complexes exhibited dramatically higher catalytic activity than the six-membered ring S 2 AlMe 2 complex (approximately 4- to 10-fold higher) and the five-membered ring L 5-Ph AlMe 2 complex (approximately 7- to 19-fold higher). Moreover, a shorter induction period was observed when the five-membered ring aluminum complexes bearing thiol-Schiff base ligands were used compared with the other types of aluminum complexes bearing Schiff base ligands. The electron-withdrawing groups enhanced the catalytic activity of the Al complexes compared with the electron donating groups. The thiol-Schiff base ligand and the five-membered ring aluminum catalysis had a synergistic effect that was stronger than the combination of their individual effects.

Previous studies on the ring-opening polymerization of ε-caprolactone using structurally related aluminum complexes as pre-catalysts showed inconsistent trends in the total conversion time. We propose that an induction period for Al complexes for conversion to real catalytic species, Al alkoxide, should be considered because the total conversion time consists of both an induction period and polymer propagation time. Herein, the polymerization rate of a series of Al complexes bearing ketimine ligands was investigated. The catalytic results indicated complexes with more steric hindrance with an electron-withdrawing group on the ligands, or the fact that less chelating ligands demonstrated greater propagation activity. The opposite trend for these structural effects was observed on the measurement of induction periods. These features on ligands of aluminum complexes are responsible for facilitating the conversion process to Al alkoxides. The overall catalytic performance should consider both the induction period and the propagation time.

A series of lithium and sodium iminophenoxide complexes have been successfully synthesized and characterized by X-ray crystallography and investigated as catalysts for the ring opening polymerization of L-lactide. The nature and steric bulk of the ligands coordinated to the central metal ions greatly influence the catalytic properties. Complexes with bidenate ligands exhibit higher catalytic activity than tridentate counterparts because the third coordination atom contends with L-lactide, which decreases activity. Oxygen is the third atom in the tridentate ligand, providing stronger chelation ability with Li and Na than nitrogen or sulfur and occupies the space with which L-lactide is coordinated.

A series of novel magnesium and zinc aminophenoxide complexes were successfully synthesized and one zinc complex was characterized by X-ray crystallography. They were also investigated as initiators for the ring opening polymerization of L-lactide. The complexes are effective in forming polylactides with good conversions. The nature and steric bulk of the ligands coordinated to the central metal ions enormously influenced the polymer properties. Among all the complexes, the zinc aminophenoxide complexes as initiators produced polymers with good molecular weight control and relatively narrow PDIs.