Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-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 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°).

Controlled and effective ring-opening (co)polymerization of rac-lactide, ε-caprolactone and ε-decalactone by β-pyrimidyl enolate aluminum complexes

A series of β-pyrimidyl enolate aluminum complexes (1–6) were found to promote controlled and living ROP of rac-LA, ε-CL, and ε-DL. Six well-defined diblock copolymers and the perfect random copolymer poly(l-LA-r-CL) were successfully synthesized.

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. D^Bu₂Al₃Me₅ exhibited higher catalytic activity than did the dinuclear aluminum complex L^Bu₂Al₂Me₄ (16 times as high for CL polymerization; [CL]:[D^Bu₂Al₃Me₅]:[BnOH] = 100:0.5:5, [D^Bu₂Al₃Me₅] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert-butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.

Bimetallic aluminum complexes bearing novel spiro-phenanthrene-monoketone/OH derivatives: synthesis, characterization and the ring-opening polymerization of ε-caprolactone

A series of spiro-phenanthrene-monoketone/OH derivatives (L1–L6) were synthesized and fully characterized with ¹H/¹³C NMR spectroscopy and elemental analyses. By treating ligands with AlMe₃, oxygen-bridged binuclear aluminum complexes (Al1–Al6) were isolated and characterized by ¹H/¹³C NMR spectroscopy. The molecular structures of ligands (L2, L4 and L5) and complex Al1 were determined by single crystal X-ray diffraction. In the presence of benzyl alcohol (BnOH), these aluminum complexes demonstrated high efficiency towards the ring-opening polymerization of ε-caprolactone (ε-CL), resulting in PCL in a linear manner with the BnO-end group. In addition, complexes Al1 and Al5 exhibited good catalytic activities even without BnOH. Moreover, complexes Al3 and Al6 with the bulkier substituent of ⁱPr at the ortho-position of the arylamines demonstrated better catalytic activities than the analogs. Moreover, substituents on the backbone also affected catalytic behaviors.

Bimetallic aluminum complexes bearing novel spiro-phenanthrene-monoketone/OH derivatives were synthesized, and displayed good activity toward the ring-opening polymerization of ε-caprolactone.

Dialkylaluminum 2-substituted 6,6-dimethylcyclopentylpyridin-7-oxylates toward structural-differentiation of the ring-opening polymerization of ε-caprolactone and l-lactides

Herein, a series of dialkylaluminum 2-substituted 6,6-dimethylcyclopentyl pyridin-7-oxylates Al1-Al7 were synthesized and characterized by 1H- and 13C-NMR spectroscopy and elemental analysis. The molecular structure of Al3 was proven to be a dimer of an aluminum complex. These aluminum complexes could efficiently initiate the ring-opening polymerization (ROP) of ε-caprolactone (CL), and the structural differentiations of the resultant PCL were strongly dependent on the amount of BnOH (PhCH2OH) used. In the absence of BnOH, the resultant PCL showed a cyclic structure, whereas BnO-capped linear PCL was obtained in the presence of >2.0 equivalents of BnOH; the resultant PCL was a mixture of linear and cyclic PCLs in the presence of 1.0 equivalent of BnOH. Moreover, these aluminum complexes exhibited high efficiency towards the ROP of l-lactide (LLA); however, the activities were lower than those for the ROP of ε-CL. Without BnOH, the resultant PLLA showed a highly linear structure with the alkyl-end group from aluminum complexes; on the other hand, PLLA displayed a major cyclic structure and minor BnO-capped linear PLLA if 1.0 equivalent of BnOH was employed, and the percentage of BnO-capped linear PLLA was increased by increasing the amount of BnOH.

Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters

This review focuses on recent developments concerned with the use of well-defined main group complexes as (pre-)catalysts for the ROP of cyclic esters to give aliphatic polyesters; factors influencing catalytic activity, selectivity and polymer properties are all discussed.

Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

Two series of alkyl aluminum complexes LAlMe2 and L2AlMe supported by N,O-bidentate β-pyrazylenolate ligands have been synthesized and applied to the ROP of ε-CL and rac-LA.

Reactions of an Aluminium/Phosphorus Frustrated Lewis Pair (FLP) with α,β-Unsaturated Carbonyl Compounds: FLPs as Efficient Two-Electron Reductants with the Formation of Enolates, a cis-Enediolate, and an Allene

The Al/P-based frustrated Lewis pair (FLP) Mes₂ P-C(AltBu)₂ =C(H)Ph (1; Mes=mesityl) reacted as an efficient two-electron reductant with benzil to afford a cis-enediolate that was coordinated to the FLP through P-O and Al-O bonds and the formation of a seven-membered heterocycle (2). The phosphorus atom is oxidised from +III to +V. Similar heterocycles (3 a to 3 f) were formed if 1 was treated with various enones (acrolein, acrylate, acrylamide). The resulting enolates are bound to the FLP through P-C and Al-O bonds. Cyclopropenone gave an adduct (4) with the C=O bond coordinated by P and Al. Ynones gave a fascinating variety of different structures. 1,3-Diphenylprop-2-yn-1-one afforded a remarkable allene-type moiety with two cumulated C=C bonds (5); 3-hexyn-2-one yielded a ligand with two conjugated C=C bonds by C-H bond activation at the carbonyl methyl group (7); and 4-(trimethylsilyl)-3-butyn-2-one reacted by C-H bond cleavage, formation of an enolate group with a terminal C=C bond, and shift of the proton to the P atom (8). The C≡C bond was not affected. Allene compound 5 rearranged at elevated temperature and in daylight through the formation of a tricyclic compound by C-H bond activation and C-C bond formation. DFT calculations on this unusual rearrangement suggest insertion of the central allene C atom into the C-H bond of a methyl group and the intermediate formation of a C₃ ring.

Living ring-opening polymerization of ε-caprolactone catalyzed by β-quinolyl-enamino aluminium complexes: ligand electronic effects

Seven aluminium β-quinolyl-enaminates with varying substituents have been synthesized and applied to the ROP of ε-CL. A comparative study on catalytic activities and reaction kinetics was conducted.

β-Pyridylenolate zinc catalysts for the ring-opening homo- and copolymerization of ε-caprolactone and lactides

Eleven zinc β-quinolylenolates with varying substituents have been synthesized, and some of these complexes are excellent catalysts for the ring-opening homo- and copolymerization of ε-CL and LAs.

Synthesis of aluminum complexes supported by 2-(1,10-phenanthrolin-2-yl)phenolate ligands and their catalysis in the ring-opening polymerization of cyclic esters

Phenanthroline-phenolate based N,N,O-chelate aluminum complexes were demonstrated to catalyze the ROP of ε-caprolactone, rac-lactide, and rac-β-butyrolactone, as well as their block copolymerization.