Cyclodepolymerization as a method for the synthesis of macrocyclic oligomers

Macrocyclic oligomers (MCOs) are important as starting materials for Entropically-driven Ring-opening Polymerizations (ED-ROPs). This article reviews the preparation of MCOs by the cyclodepolymerization (CDP) of condensation polymers. Many MCOs have been prepared successfully this way and in numerous cases individual macrocycles have been isolated. This approach can provide one-step syntheses of many macrocycles. The combination of CDP plus ED-ROP is an attractive potential method for recycling (‘Ring-chain Recycling’) many condensation polymers.     

Investigation nonlinear optical property of novel para-phenylenealkyne macrocycles

A series of novel molecular macrocycles, [n]para-phenylacetylene ([n]CPPA), were constructed with phenylene and acetylene. We have employed quantum chemical ab initio and semiempirical molecular orbital methods to study geometry character and the nonlinear optical property. The results show that the transition wavelength λ_max was estimated to be shorter than 400 nm even in a very large molecular cycle. While the average second-order hyperpolarizability per unit γ/n was extrapolated to be 3.93 × 10⁵ a.u. for the limiting value, showing their potential applications in the future new materials.     

Polymer-supported syntheses of oxo-crown ethers and derivatives containing α-amino-acid residues

A simple polymer-supported cyclization–cleavage method is described for the small-scale synthesis of macrocyclic oligomers, with from 12 to 18 ring atoms per repeat unit, which are either 2-oxo-crown ethers, or derivatives containing, except in one case, chiral α-amino-acid residues. In each case the major cyclic product was isolated and characterized. In all except one case this was the cyclic monomer. If the macrocyclic oligomers are needed either for the preparation of static or dynamic combinatorial libraries or for entropically-driven ring-opening polymerizations, all members of the family are of interest as they all react through equilibration to give the same final products.     

Pillar[n]arenes at the Chemistry-Biology Interface

Macrocyclic chemistry has provided chemists with a wealth of molecular ‘hosts’. Ever since resurgence in the field during the 1970s and 1980s these hosts’ similarities to natural structures, such the active sites of enzymes, have been noted. Latterly there has been great interest in the recently reported pillar[n]arenes. As if to underline the importance of these compounds, exciting applications are starting to emerge, from electrochemical sensors to antimicrobial agents. Novel uses appear destined to have an impact on clinical conditions from Alzheimer's disease to cancer treatment. In order to demonstrate the impact of pillar[n]arenes across the chemistry-biology interface this review will cover the current state of the art from biomimicry and analyte-specific detection to emerging clinical applications. Examples include pillar[n]arene-based ion channels, enzyme-responsive compounds, imaging agents, biofilm inhibiting derivatives, drug complexing and drug releasing systems.     

Superstructures of Self‐Organizing Thiophenes

The self‐assembling properties of π‐conjugated oligo‐, poly‐, and cyclothiophenes (see Figure) have been studied with different techniques. Scanning tunneling microscopy on perfectly ordered two‐dimensional monolayers at the solution–HOPG (highly oriented pyrolytic graphite) interface has successfully been applied. The submolecular resolution achieved in the STM images provides valuable lattice and molecular information. Supported by X‐ray structure analyses of the 3D bulk material and by theoretical calculations, molecular conformations, molecule–molecule, and molecule–substrate interactions of the oligo‐, poly‐, and cyclomers have been analyzed and discussed.     

A comparison of poly(ether imide)s from diamines with 2-aminophenoxy or 4-aminophenoxy units: Synthesis, characterization and properties

A series of new 3- and 4-ring bis(2-aminophenoxy) aromatic diamines were prepared. These, and corresponding, conventional bis(4-aminophenoxy) diamines were reacted with several aromatic bis(ether anhydride)s to form poly(ether imide)s. The diamines with 4-aminophenoxy groups gave high-molecular-weight polymers that were cast into films with good mechanical properties. In contrast, in almost all cases, diamines with 2-aminophenoxy groups only gave low-molecular-weight powdery products that could not be cast into coherent films. The low-molecular-weight products, prepared from stoichiometrically equal amounts of monomers, were examined by mass spectrometry and shown, in most cases, to consist primarily of cyclic oligomers; traces of linear oligomers were identified in some samples. Apart from a polyimide prepared from pyromellitic dianhydride and 4,4′-bis(2″-aminophenoxy)biphenyl, the only products found to contain significant proportions of linear oligomers were those prepared with a stoichiometric imbalance of monomers. End groups of the various linear oligomers were identified. The 2-aminophenoxy groups predispose the oligomers to cyclize as amic acids, and to remain as cyclics on imidization. In some cases [1+1] cyclic oligomers were observed although the most common species were the [2+2] cyclic dimers.     

High-surface-area CoTMPP/C synthesized by ultrasonic spray pyrolysis for PEM fuel cell electrocatalysts

Ultrasonic spray pyrolysis (USP) was used to synthesize a high-surface-area CoTMPP/C catalyst for oxygen reduction reaction (ORR). SEM micrographs showed that the USP-derived CoTMPP/C consists of spherical, porous and uniform particles with a diameter of 2-5 μm, which is superior to that with a random morphology and large particle sizes (up to 100 μm) synthesized by the conventional heat-treatment method. BET results revealed that the USP-derived catalyst had a higher specific surface area (834 m² g⁻¹) than the conventional one. Cyclic voltammetric, rotating ring-disk electrode (RRDE) and H₂-air PEM fuel cell testing were employed to evaluate the USP-derived CoTMPP/C. The kinetic current density of the USP-derived catalyst at 0.7 V versus NHE was two times higher than that of the conventional catalyst. Compared to Pt/C catalyst, the USP-derived CoTMPP/C catalyst showed a strong methanol tolerance and a higher ORR activity in the presence of methanol. In a H₂-air PEM fuel cell with USP-derived CoTMPP/C as the cathode catalyst, the cell performance was much higher than that with conventional heat-treated CoTMMP/C as the catalyst.     

Cyclic polyphosphoesters synthesized by acyclic diene metathesis polymerization and ring closing metathesis

This article describes the synthesis of cyclic polyphosphoester (PPE) by the ring-closing metathesis (RCM) of different difunctional linear PPEs. Linear PPE precursors were prepared through a selective head-to-tail acyclic diene metathesis polymerization of phenyl dienephosphate monomer using 2-hydroxyethyl acrylate as a selective chain terminator, followed by the transformation of the terminal acrylate functional group into a hydroxyl group utilizing a thiol-Michael addition click reaction. These products were then reacted with the corresponding acyl chloride containing a vinyl end group. The subsequent end-to-end intramolecular coupling reaction was performed under highly dilute conditions. The successful transformation of the linear PPE precursors to cyclic PPE was confirmed by NMR spectroscopy and gel permeation chromatography. The thermal and flame retardant properties of linear and cyclic PPEs were investigated, and their thermal degradation and flame retardance were evaluated, as these are important features for future applications.     

Supramolecular Chemistry in Solid State Materials such as Metal-Organic Frameworks

Supramolecular chemistry has enriched the scientific research for more than fifty years reaching one of its summits in 2016, when the Chemistry Nobel Prize was awarded for the design and synthesis of molecular machines, in which host-guest chemistry plays a fundamental role. Recently, the groups of Omar Yaghi and Fraser Stoddart, among others, have demonstrated that this chemistry can be extended to the pores of metal-organic frameworks (MOFs). This heterogenization of supramolecular chemistry can be achieved through the incorporation of macrocycles to the organic struts of these highly porous and crystalline materials. Throughout this short review we summarize interesting examples of selective recognition by naturally occurring and synthetic macrocycles in solution and solid state; and later we survey important milestones to achieve specific recognition sites and develop host-guest chemistry at the pores of MOFs. This summary contains examples of different synthetic strategies to incorporate macrocycles to solid state materials, and in particular, to prepare supramolecular MOFs with particular properties and related applications. Specifically, the revised research includes the incorporation of both naturally occurring and synthetic macrocycles to solid state materials such as polymers, metal nanoparticles, etc., as prelude of the solid phase recognition studied in MOFs. An important number of the contributions presented here feature porous solids with smooth access to the host's cavity incorporated in the pores, allowing specific recognition of guest molecules. This smooth access to those active recognition sites in materials with extremely high surface area such as MOFs, open the possibility to develop the next generation of frontier materials with application in fields such as selective capture of water toxins and heterogeneous catalysis, among others.     

Syntheses of Octa(2‐heteroaryl) Azaphthalocyanines

Magnesium, copper(II) and nickel(II) complexes of octasubstituted azaphthalocyanines 3 — 5 have been prepared from di‐fur‐2‐yl, di‐thien‐2‐yl and di‐pyrid‐2‐yl pyrazine‐2,3‐dicarbonitriles 2 . Compounds 2 were prepared in good yields from condensations of diaminomaleonitrile and the diketones 2,2′‐furil, 2,2′‐thenil and 2,2′‐pyridil. AzaPcs 3—5 give green pyridine solutions with Q‐bands at 650—670 nm and ε‐values of 60 000—190 000.