Evolution of aluminum aminophenolate complexes in the ring-opening polymerization of ε-caprolactone: electronic and amino-chelating effects

A series of aluminum complexes bearing phenolate (O-Al and O2-Al), biphenolate (OO-Al type), aminophenolate (ON-Al), aminobiphenolate (ONO-Al), bis(phenolato)bis(amine) (NNOO-Al), and Salan (ONNO-Al) type ligands were synthesized. ε-Caprolactone (CL) polymerization using these aluminum complexes as catalysts was investigated. The overall polymerization rates of Al catalysts with different ligands were found to be in the following order (kobs values): ONBr-Al (0.124 min-1) ≥ OBr2-Al (0.121 min-1) > ONNOBr-Al (0.054 min-1) > NNOBr-Al (0.044 min-1) ≥ ONOBr-Al (0.043 min-1) > OBr-Al (0.033 min-1) > NNOOBr-Al (0.015 min-1) ≥ BuONNOBu-Al (0.001 min-1) = OOBr-Al (0.001 min-1). In addition, Al complexes with electron-donating substituents on ligands exhibited higher catalytic activity than those with bromo substituents. Density functional theory (DFT) calculations revealed that a dinuclear Al complex with two bridging methoxides had to rearrange to a phenolate bridged dinuclear Al complex with terminal methoxides. This is due to the low initiating ability of two bridging benzyl alkoxides. Combining the polymerization data and DFT results, it was concluded that the electron-donating substituents on the phenolate ring and chelating amino group enhance the electron density of the Al center. This may prevent the formation of a less active dinuclear Al complex with two bridging alkoxides (initiators) or facilitate its structural rearrangement. OOMe-Al has been established as a powerful candidate with a high polymerization rate and it exhibits well-controlled polymerization for synthesizing the mPEG-b-PCL copolymer.

Hydrogen Bond Donors in the Catalytic Pocket: The Case of the Ring-Opening Polymerization of Cyclic Esters Catalyzed by an Amino-Propoxide Aluminum Complex

A new aluminum complex (NSO)AlMe2 featuring a hydrogen bond donor on the ligand backbone has been synthesized via the reaction of AlMe3 with 1-((2-(isopropylamino)phenyl)thio)propan-2-ol (NSO-H) and spectroscopically characterized. In the complex, the aluminum atom is in a distorted tetrahedral coordination sphere determined by the anionic oxygen and neutral nitrogen atoms of the ligand and by the two carbon atoms of the alkyl groups. After proper activation, the complex (NSO)AlMe2 was able to promote the ring-opening polymerization of L-, rac-lactide, ε-caprolactone and rac-β-butyrolactone. The polymerization of rac-lactide was faster than that of L-lactide: in a toluene solution at 80 °C, the high monomer conversion of 100 equivalents was achieved in 1.5 h, reaching a turnover frequency of 63 molLA·molAl-1·h-1. The experimental molecular weights of the obtained polymers were close to those calculated, assuming the growth of one polymer chain for one added alcohol equivalent and the polydispersity indexes were monomodal and narrow. The kinetic investigation of the polymerization led to the determination of the apparent propagation constants and the Gibbs free energies of activation for the reaction; the terminal groups of the polymers were also identified. The complex (NSO)AlMe2 was active in harsh conditions such as at a very low concentration or in the melt using technical-grade rac-lactide. A relatively high level of activity was observed in the ring-opening polymerization of ε-caprolactone and rac-β-butyrolactone. DFT calculations were performed and revealed the central role of the NH function of the coordinated ligand. Acting as a hydrogen bond donor, it docks the monomer in the proximity of the metal center and activates it toward the nucleophilic attack of the growing polymer chain.

A bis-chelate o-vanillin-2-ethano-lamine copper(II) complex bearing both imine and amine forms of the ligand

The mol-ecular bis-chelate complex (2-{[(2-hy-droxy-ethyl-κO)amino-κN]meth-yl}-6-meth-oxy-phenolato-κO)(2-{[(2-hy-droxy-eth-yl)imino-κN]meth-yl}-6-meth-oxy-phenolato-κO)copper(II), [Cu(C10H14NO3)(C10H12NO3)] or [Cu(HL im)(HL am); HL im = C10H14NO3; HL am = C10H12NO3, represents the first compound containing a salicyl-idene-2-ethano-lamine type ligand in both imino HL im (Schiff base) and amino HL am (reduced Schiff base) forms that has been structurally characterized on the basis of X-ray data. Two mol-ecules of the monodeprotonated ligands coordinate the CuII ion in an (N,O phen)-bidentate and an (N,O phen,O alc)-tridentate fashion in the case of the imino and amino forms, respectively. The shape of the CuN2O3 coordination polyhedron is a distorted square-pyramid (geometry index τ5 = 0.26). Inter-molecular N-H⋯O and O-H⋯O hydrogen bonds, involving H atoms of the amino and hy-droxy-ethyl groups, create a two-dimensional supra-molecular array extending parallel to (010).

Syndiotactic PLA from meso-LA polymerization at the Al-chiral complex: a probe of DFT mechanistic insights

The mechanism(s) for the formation of syndiotactic PLA by the ROP of meso-LA by a chiral-Al-complex are disclosed by DFT calculations. The contributions toward stereoselectivity have been analyzed confirming the peculiar chiral recognition for stereocontrolled ROP polymerization.

Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

A series of Mg(ii) and Zn(ii) catalen complexes have been prepared for PLA formation and recycling.

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

This review describes the recent advances (from 2008 onwards) in the use of Schiff-base metal complexes as catalysts for the ring opening polymerization (ROP) of cyclic esters. The synthesis and structure of the metal complexes, as well as all aspects concerning the polymerization process and the characteristics of the polymers formed, will be discussed.