Aluminum Salen and Salan Polymerization Catalysts: From Monomer Scope to Macrostructure Control. In: Cheng HN, Gross RA, Smith PB, editors. Green Polymer Chemistry: Biobased Materials and Biocatalysis. Washington: American Chemical Society; 2015. pp. 147-67. [DOI: 10.1021/bk-2015-1192.ch010]

Pairing-Enhanced Regioselectivity: Synthesis of Alternating Poly(lactic-co-glycolic acid) from Racemic Methyl-Glycolide

Poly(lactic-co-glycolic acid) (PLGA) is used in vivo for various biomedical applications. Due to its biodegradability and biocompatibility, PLGA is uniquely suited for controlled drug delivery with parenteral administration. Previously, we established the synthesis of isotactic, alternating PLGA from enantiopure starting materials. Here, to fill in the gap of the current field, we have developed the synthesis of syndioenriched, alternating PLGA from racemic methyl-glycolide (rac-MeG). The synthesis of alternating PLGA is accomplished by a highly regioselective ring-opening polymerization of rac-MeG with an optimized racemic aluminum catalyst. Mechanistic studies are carried out to elucidate the pairing-enhanced catalyst regio- and stereocontrol. Polymer sequence fidelity has been established by NMR investigations, confirming a high degree of alternation of the comonomer sequence and moderate syndiotacticity within the backbone stereoconfiguration. The resulting syndioenriched material is amorphous, which will facilitate the drug complexation behavior.

Lactide polymerization using a sterically encumbered, flexible zinc complex

4-(tert-Butyl)-2-trityl-6-(di-(2-picolyl)amine)phenol, LH, was prepared from paraformaldehyde, 4-(tert-butyl)-2-tritylphenol and di-(2-picolyl)amine. Reaction with Zn(N(SiMe3)2)2 gave LZnN(SiMe3)2. The complex was shown by X-ray diffraction study, variable temperature NMR and DFT calculations to coordinate only one pyridine ligand, which allows for fast and facile complex isomerisation. LZnN(SiMe3)2 was active in rac-lactide polymerization, but in contrast to previous complexes of this type did not show any evidence for isotactic monomer enchainment via a catalytic-site mediated chain-end control mechanism. Addition of alcohol led to increased activity, but the complex was unstable in the presence of free alcohol.

Replacing sulfonate by carboxylate: application of pyridyliminocarboxylato copper(II) complexes in rac-lactide polymerization and Chan–Evans–Lam coupling

Copper(II) complexes carrying pyridylmethyleneaminobenzoate or –propanoate ligands, LCuX, were prepared in one-pot reactions from pyridinecarboxaldehyde, aminobenzoic acid or β-alanine, and CuX2 (X = Cl, NO3, OAc, or OTf). All complexes were characterized by single-crystal X-ray diffraction studies and formed either dimers, tetramers, or coordination polymers. Attempted preparation of the respective alkoxide complexes, LCu(OR), was unsuccessful, but use of LCuX/NaOMe mixtures in rac-lactide polymerization indicated under some conditions coordination–insertion polymerization via a copper alkoxide as the mechanism. The complexes performed poorly in rac-lactide polymerization, showing low activities (12 h to completion at 140 °C), low to moderate heterotacticity (Pr = 0.6–0.8), and poor polymer molecular weight control (intramolecular transesterification). They were competent catalysts for Chan–Evans–Lam couplings with phenylboronic acid, without any indication of side reactions such as deboration or aryl homocoupling. The complexes were active in undried methanol, without addition of base, ligand, or molecular sieves. Aniline, n-octylamine, and cyclohexylamine were coupled quantitatively under identical reaction conditions. There is only little influence of the anion on activities (less than a factor of 2) but a strong influence on induction periods. The complexes were not active in CEL coupling with alcohols, phenols, or alkylboronic acids.

Tetradentate iminophenolate copper complexes in rac-lactide polymerization

Copper(II) nitrate complexes of 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)phenol, 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)-4,6-dichlorophenol, 2-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol and (2,4-di-tert-butyl-6-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol, as well as a copper(II) acetate complex of 2-(((2-(piperidin-1-yl)ethyl)imino)methyl)phenol, have been prepared and characterized by X-ray diffraction studies. In combination with benzyl alcohol, all complexes are active in rac-lactide polymerization at 140 °C in molten monomer to provide moderately heterotactic polylactic acid. Most complexes showed complicated reaction kinetics, indicative of two interconverting active species. Molecular weight control was poor and a strong tendency toward intramolecular transesterification led to oligomeric products. There was no indication that the basic site of the ligand is participating in the polymerization reaction by deprotonation of the alcohol nucleophile.

IronIII Half Salen Catalysts for Atom Transfer Radical and Ring-Opening Polymerizations

A series of monometallic pentacoordinate Fe^III chloride complexes have been prepared and characterized by high-resolution mass spectrometry and elemental analysis. X-ray diffraction analysis showed that the bis-chelated Fe^III complexes bear distorted trigonal bipyramidal geometries. The air- and moisture-stable Fe^III complexes were screened as mediators in the reverse atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate. Moderate to excellent control was achieved with dispersities as low as 1.1 for both poly(methyl methacrylate) and polystyrene. Kinetic studies showed living characteristics, and end group analysis revealed the presence of olefin-terminated polymer chains, suggesting catalytic chain transfer as a competing polymerization mechanism. Although the catalysts are not the fastest Fe ATRP mediators, they are robust and flexible. Using propylene oxide as an initiator, the complexes were active catalysts for the ring-opening polymerization of rac-lactide with moderate control. While the addition of propylene oxide has been reported as an efficient method of converting a metal-halide bond to a metal-alkoxide bond in situ, we show herein that this initiation mechanism can limit polymerization reproducibility and introduce an induction period.

Zirconium and hafnium complexes bearing pyrrolidine derived salalen-type {ONNO} ligands and their application for ring-opening polymerization of lactides

The reactions of pyrrolidine derived salalen-type {ONNO} ligands (S)-L^1-3-H₂ with 1 equiv. M(OⁱPr)₄(HOⁱPr) (M = Zr or Hf) in diethyl ether yielded complexes [L^1-3M(OⁱPr)₂] (L = L¹, M = Zr (1); L = L², M = Zr (2), Hf (3); L = L³, M = Zr (4), Hf (5)). All of these complexes were well characterized by NMR spectroscopy, elemental analyses and single-crystal X-ray analysis in the case of complexes 1 and 3-5. X-ray structural determination revealed that these complexes were analogous mononuclear species and had a similar structure in which the metal centers were six-coordinated to two oxygen atoms and two nitrogen atoms of one ligand and two oxygen atoms of two isopropoxy groups. All of these complexes efficiently initialized the ring-opening polymerization of lactides to afford polymers with controlled molecular weight and narrow polydispersity. Furthermore, the ring-opening polymerization of rac-lactide catalyzed by complexes 1-5 afforded isotactic-enriched polymers in solution (P_m = 0.74-0.80) and under melt conditions (P_m = 0.63-0.72).

A broad scope of aliphatic polyesters prepared by elimination of small molecules from sustainable 1,3-dioxolan-4-ones

We report a family of monomers that are built from renewable resources and use the elimination of small molecules to access aliphatic polyesters, circumventing challenging monomer syntheses to make these functionalism polymers.

Aluminum methyl, alkoxide and α-alkoxy ester complexes supported by 6,6'-dimethylbiphenyl-bridged salen ligands: synthesis, characterization and catalysis for rac-lactide polymerization

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6'-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands (6,6'-[(6,6'-dimethyl-[1,1'-biphenyl]-2,2'-diyl)bis(nitrylomethilidyne)]-bis(2-R(1)-4-R(2)-phenol): , R(1) = R(2) = Me; , R(1) = (t)Bu, R(2) = Me; , R(1) = R(2) = cumyl; , R(1) = Br, R(2) = (t)Bu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [AlMe (), Al2Me4 ()]. Aluminum alkoxide complexes AlO(i)Pr (), AlOBn (), and α-alkoxy ester complexes Al(OCMe2CO2Me) (), Al[(S)-OCHMeCO2Me] () were prepared via in situ alcoholysis of the parent aluminum methyl complex with the corresponding alcohols. The molecular structures of mononuclear complexes , dinuclear complex , alkoxide complexes and α-alkoxy ester complexes were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69-70 ppm and 25-41 ppm were observed in the (27)Al NMR spectra of complexes and , indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex is also a rare example of an O-lactate model complex that mimics the first insertion of l-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes in the presence of (i)PrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03-1.05) and 65-73% isotacticities. Particularly, complex showed an "immortal" behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.