Morpholine-Stabilized Cationic Aluminum Complexes and Their Reactivity in Ring-Opening Polymerization of ε-Caprolactone.

The first structures exhibiting bidentate (N, O) chelation of a morpholine group to a p-block element (aluminum) have been prepared and characterized by X-ray diffraction methods: Al[L]+ [WCA]-, where [L] = 4-(2-aminoethyl)morpholinylamino- N, N-bis(2-methylene-4,6- tert-butylphenolate) and [WCA]- is a weakly coordinating anion. These compounds are easily synthesized by reacting Al[L]Cl with an equimolar amount of anhydrous Lewis acid and were characterized by elemental analyses, ESI-MS, MALDI-TOF MS, 1H, 13C{1H}, and multinuclear NMR spectroscopy. DFT calculations showed that Al[L]+ cations containing bidentate NO coordination of the morpholine group are at least 21.1 kJ/mol more stable when compared to hypothetical monodentate (N bound) structures. When combined with protic co-initiators (EtOH, glycerol carbonate), the cationic complexes, where [WCA]- = [GaCl4]- or [InCl4]-, are living catalyst systems for the polymerization of ε-caprolactone, producing polycaprolactone with narrow dispersity ( D̵ = 1.00-1.05). Employing glycidol as a co-initiator furnished polymers with narrow dispersity ( D̵ = 1.01-1.07) but experimental molecular weights diverged considerably from the calculated values. Similar reactivity toward ROP was observed for all complexes containing a stable [WCA]- but where [WCA]- = [AlCl4]-, upon combination with alcohols, alcoholysis was observed. Kinetic studies (Eyring analyses) allowed the determination of activation parameters, which were consistent with a coordination-insertion mechanism for the catalysts containing [WCA]- = [GaCl4]- or [InCl4]-. End group analyses using MALDI-TOF mass spectrometry and 1H NMR spectroscopy showed hydroxyl and ester end groups within the polymer, corroborating the proposed mechanism. Stoichiometric reactions of EtOH, glycidol or tert-butyl alcohol with the complex, where [WCA]- = [GaCl4]-, showed protonation of the ligand at the N-morpholine site, which leads to dissociation of this pendent group.

Copolymerization of CHO/CO2 catalyzed by a series of aluminum amino-phenolate complexes and insights into structure-activity relationships.

Two series of monometallic aluminum complexes were prepared and characterized by elemental analyses, 1H and 13C{1H} NMR spectroscopy, and X-ray crystallography: Al[L]X, where [L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate) and X = Cl, OEt, and Al[L]2Cl, where [L] = 6-{[(2R,6R)-2,6-dimethyl-4-morpholino]methylene}-2,4-bis(tert-butyl)phenolate or 6-(piperidinomethylene)-2-(tert-butyl)-4-(methyl)phenolate. All the complexes, including the previously reported morpholinyl complex Al[L]Cl, where [L] = 4-(2-aminoethyl)morpholinylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate), were tested as catalysts for copolymerization of cyclohexene oxide and CO2 in the presence and absence of PPNCl. When coupled with 1 equiv. PPNCl, the complexes exhibit similar activities and the best selectivity for poly(cyclohexenecarbonate) vs. the cyclic product, cyclohexene carbonate, was obtained with the morpholinyl complex (ca. 90%) whereas significantly lower selectivities (<1-63%) were obtained with the other complexes. Preliminary DFT calculations investigating this difference in selectivity were carried out by analyzing the aluminum partial atomic charges in the Al-carbonate intermediates.

Ring-opening polymerization of cyclohexene oxide using aluminum amine-phenolate complexes.

Remarkably active (down to 0.001% Al) catalysts for ring-opening polymerization of cyclohexene oxide under neat reaction conditions are reported. High molecular weight polymers with uniform dispersity are produced. Kinetic data from NMR studies and MALDI-TOF MS data of the polymers provide some mechanistic insight.