Synthesis of biodegradable chiral polyesters by asymmetric enzymatic polymerization and their formulation into microspheres

Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ε-Caprolactone and Substituted ε-Caprolactones and Their Copolymerizations

Two series of titanium complexes, including salicylbenzoxazole titanium complexes (1-4) and salicylbenzothiazole titanium complexes (5-8), were successfully synthesized and characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction crystallography (for 2 and 5). The ¹H NMR spectra of complexes 7 and 8 reveal fluxional behavior in solution at room temperature, and the activation parameters were determined by lineshape analysis of variable-temperature (VT) NMR spectra in toluene-d₈: for 7, ΔH^⧧ = 73.0 ± 1.8 kJ mol^-1, ΔS^⧧ = 22.1 ± 5.5 J mol^-1 K^-1; for 8, ΔH^⧧ = 73.7 ± 1.2 kJ mol^-1, ΔS^⧧ = 20.3 ± 3.8 J mol^-1 K^-1. The positive values of ΔS^⧧ suggested that the isomerization occurred via a dissociative mechanism. All complexes were active initiators for the ring-opening polymerization of ε-caprolactone (ε-CL) and three substituted ε-CLs: γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL). Of all complexes, complex 5 was found to be the most active initiator in this study. The copolymerizations between ε-CL and three substituted ε-CLs produced completely random copolymers. The polymerization was proposed to proceed via a dissociative coordination-insertion mechanism. The catalytic activity of the salicylbenzoxazole titanium complex was lower than that of its closely related salicylbenzothiazole titanium congener. Additionally, DFT calculations unveiled that the ligand decoordination step and the less steric congestion at the titanium center in the salicylbenzothiazole titanium complexes were the key factors in enhancing the catalytic rate.

Titanium complexes of pyrrolylaldiminate ligands and their exploitation for the ring-opening polymerization of cyclic esters

A series of six-coordinate titanium complexes 1-6 supported by pyrrolylaldiminate ligands were prepared via the reaction of 2 equivalents of ligands and Ti(OⁱPr)₄ in toluene at 70 °C. The X-ray structure of 2 revealed that the two ligands were κ²-coordinated to the titanium center with the two pyrrole nitrogen atoms in trans positions and the two imine nitrogen atoms in cis positions. All complexes were active initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), ε-caprolactone (ε-CL), and three substituted ε-caprolactones (γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL)). Polymerizations of all monomers were well controlled, affording predetermined molar masses and narrow dispersity values. Complex 5 exhibited the highest polymerization activities with rac-LA and ε-CL and its performance was comparable to other highly active six-coordinate titanium complexes reported thus far. Kinetic results revealed a first-order dependency on the monomer concentration, and the rate of polymerization was greatly influenced by the substituent on the imine nitrogen. End-group analysis of the isolated PLA and PCL suggested a coordination-insertion mechanism.

Substrate Engineering in Lipase-Catalyzed Selective Polymerization of d-/l-Aspartates and Diols to Prepare Helical Chiral Polyester

The synthesis of optically pure polymers is one of the most challenging tasks in polymer chemistry. Herein, Novozym 435 (Lipase B from Candida antarctica, immobilized on Lewatit VP OC 1600)-catalyzed polycondensation between d-/l-aspartic acid (Asp) diester and diols for the preparation of helical chiral polyesters was reported. Compared with d-Asp diesters, the fast-reacting l-Asp diesters easily reacted with diols to provide a series of chiral polyesters containing N-substitutional l-Asp repeating units. Besides amino acid configuration, N-substituent side chains and the chain length of diols were also investigated and optimized. It was found that bulky acyl N-substitutional groups like N-Boc and N-Cbz were more favorable for this polymerization than small ones probably due to competitively binding of these small acyl groups into the active site of Novozym 435. The highest molecular weight can reach up to 39.5 × 10³ g/mol (M_w, Đ = 1.64). Moreover, the slow-reacting d-Asp diesters were also successfully polymerized by modifying the substrate structure to create a "nonchiral" condensation environment artificially. These enantiocomplementary chiral polyesters are thermally stable and have specific helical structures, which was confirmed by circular dichroism (CD) spectra, scanning electron microscope (SEM), and molecular calculation.

The construction of photoresponsive polymer particles with supramolecular helicity from achiral monomers by helix-sense-selective polymerization

Optically active azobenzene-containing polymer particles successfully prepared from achiral monomers for the first time by helix-sense-selective dispersion polymerization, also known as asymmetric helix-chirogenic polymerization.

Organocatalyzed ring opening polymerization of regio-isomeric lactones: reactivity and thermodynamics considerations

Study of the kinetics and thermodynamics of the organocatalyzed ring opening polymerization of a regio-isomeric mixture of β,δ-trimethyl-ε-caprolactones (TMCL).

Application of a thermostable Baeyer-Villiger monooxygenase for the synthesis of branched polyester precursors

BACKGROUND

It is widely accepted that the poor thermostability of Baeyer-Villiger monooxygenases limits their use as biocatalysts for applied biocatalysis in industrial applications. The goal of this study was to investigate the biocatalytic oxidation of 3,3,5-trimethylcyclohexanone using a thermostable cyclohexanone monooxygenase from Thermocrispum municipale (TmCHMO) for the synthesis of branched ϵ-caprolactone derivatives as building blocks for tuned polymeric backbones. In this multi-enzymatic reaction, the thermostable cyclohexanone monooxygenase was fused to a phosphite dehydrogenase (PTDH) in order to ensure co-factor regeneration.

RESULTS

Using reaction engineering, the reaction rate and product formation of the regio-isomeric branched lactones were improved and the use of co-solvents and the initial substrate load were investigated. Substrate inhibition and poor product solubility were overcome using continuous substrate feeding regimes, as well as a biphasic reaction system with toluene as water-immiscible organic solvent. A maximum volumetric productivity, or space-time-yield, of 1.20 g L-1 h-1 was achieved with continuous feeding of substrate using methanol as co-solvent, while a maximum product concentration of 11.6 g L-1 was achieved with toluene acting as a second phase and substrate reservoir.

CONCLUSION

These improvements in key process metrics therefore demonstrate progress towards the up-scaled Baeyer-Villiger monooxygenase-biocatalyzed synthesis of the target building blocks for polymer application. © 2018 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Enantio-, Regio-, and Chemoselective Lipase-Catalyzed Polymer Synthesis

In contrast to chemical routes, enzymatic polymerization possesses favorable characteristics of mild reaction conditions, few by-products, and high activity toward cyclic lactones which make it a promising technique for constructing polymeric materials. Meanwhile, it can avoid the trace residue of metallic catalysts and potential toxicity, and thus exhibits great potential in the biomedical fields. More importantly, lipase-catalyzed polymer synthesis usually shows favorable enantio-, regio-, and chemoselectivity. Here, the history and recent developments in lipase-catalyzed selective polymerization for constructing polymers with unique structures and properties are highlighted. In particular, the synthesis of polymeric materials which are difficult to prepare in a chemical route and the construction of polymers through the combination of selective enzymatic and chemical methods are focused. In addition, the future direction is proposed especially based on the rapid developments in computational chemistry and protein engineering techniques.

Strong, Resilient, and Sustainable Aliphatic Polyester Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) composed of ABA block polymers exhibit a wide variety of properties and are easily processable as they contain physical, rather than chemical, cross-links. Poly(γ-methyl-ε-caprolactone) (PγMCL) is an amorphous polymer with a low entanglement molar mass (M_e = 2.9 kg mol^-1), making it a suitable choice for tough elastomers. Incorporating PγMCL as the midblock with polylactide (PLA) end blocks (f_LA = 0.17) results in TPEs with high stresses and elongations at break (σ_B = 24 ± 2 MPa and ε_B = 1029 ± 20%, respectively) and low levels of hysteresis. The use of isotactic PLA as the end blocks (f_LLA = 0.17) increases the strength and toughness of the material (σ_B = 30 ± 4 MPa, ε_B = 988 ± 30%) due to its semicrystalline nature. This study aims to demonstrate how the outstanding properties in these sustainable materials are a result of the entanglements, glass transition temperature, segment-segment interaction parameter, and crystallinity, resulting in comparable properties to the commercially relevant styrene-based TPEs.

A highly stretchable bioelastomer prepared by UV curing of liquid-like poly(4-methyl-ε-caprolactone) precursors

UV curing of PMCL precursors in the absence of any solvent or heating led to highly stretchable bioelastomers.

Lipases in polymer chemistry

Lipases are highly active in the polymerization of a range of monomers. Both ring-opening polymerization of cyclic monomers such as lactones and carbonates as well as polycondensation reactions have been investigated in great detail. Moreover, in combination with other (chemical) polymerization techniques, lipase-catalyzed polymerization has been employed to synthesize a variety of polymer materials. Major advantages of enzymatic catalysts are the often-observed excellent regio-, chemo- and enantioselectivity that allows for the direct preparation of functional materials. In particular, the application of techniques such as Dynamic Kinetic Resolution (DKR) in the lipase-catalyzed polymerization of racemic monomers is a new development in enzymatic polymerization. This paper reviews selected examples of the application of lipases in polymer chemistry covering the synthesis of linear polymers, chemoenzymatic polymerization and applications of enantioselective techniques for the synthesis and modification of polymers.

Surface-functionalizable membranes of polycaprolactone-click-hyperbranched polyglycerol copolymers from combined atom transfer radical polymerization, ring-opening polymerization and click chemistry

Hyperbranched polyglycerols containing terminal alkyne and alkyl bromide groups (CH[triple bond, length as m-dash]C-HPG-Br) were first synthesized via propargyl alcohol-initiated ring-opening polymerization (ROP) of glycidol, followed by reaction of 2-bromoisobutyryl bromide (BIBB) with the hydroxyl groups to introduce the atom transfer radical polymerization (ATRP) initiators on HPG. Hydrophobic azido-terminated poly(ε-caprolactone) (PCL-N₃), prepared a priori via 2-azidoethanol-initiated ROP of ε-caprolactone, was then coupled to the CH[triple bond, length as m-dash]C-HPG-Br polymer through a Cu(i)-catalyzed alkyne-azide click reaction. The resultant linear-hyperbranched PCL-click-HPG copolymers were cast by phase inversion in an aqueous medium into microporous membranes of well-defined and uniform pores. Not only could the HPG contents in the PCL-click-HPG copolymers be used to control the pore size and porosity of the resulting membranes, but also the alkyl halide chain-ends of HPG allowed the subsequent functionalization of membrane and pore surfaces. The PCL-click-HPG-b-PMPC membrane was prepared via surface-initiated ATRP of zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) from the PCL-click-HPG membrane and pore surfaces. The PCL-click-HPG-b-PMPC membranes exhibit good antifouling and antibacterial adhesion properties with negligible cytotoxicity effects, making the membranes potentially useful for biomaterials and biomedical applications.