13C-labeled nitrile N-oxides (13CNOs) were prepared to investigate the structural characteristics of nitrile N-oxide in solution. In the 13C NMR spectra at room temperature, 13C signals of 13CNOs appeared at 35–38 ppm. The chemical shift of adjacent aromatic carbon Ca to 13CNO was remarkably upfield-shifted to indicate a positive mesomery effect of CNO. The 13C NMR signal appeared as a triplet that originated from the scalar coupling of the adjacent 14N nucleus, while it fused into a singlet as the temperature decreased. The UV spectra of 13CNOs also indicated the temperature-dependent dynamic character of CNO.

A one-pot synthesis of polymer nitrile N-oxides was achieved via the Michael addition of living polymer anions derived from vinyl monomers to commercially available trans-β-nitrostyrene and subsequent dehydration with concd H2SO4. The polymer nitrile N-oxides are effective as grafting agents in catalyst- and solvent-free 1,3-dipolar cycloadditions to unsaturated-bond-containing polymers with high conversion and exhibit higher reactivity compared to that of nitrile N-oxides prepared from 1,1-diphenylnitroethene. Application to the preparation of a functional glass surface was demonstrated using PtBMA nitrile N-oxide as a grafting agent.

This review discusses seven synthetic approaches for fabricating topologically crosslinked polymers with rotaxane structures at the crosslinking points (rotaxane-crosslinked polymers, RCPs). RCPs exhibit unique properties such as high swelling capability, high elasticity and stimuli-responsiveness attributed to the rotaxane-crosslinking points. This article primarily focuses on the four most recently developed strategic approaches for synthesizing RCPs in which various rotaxane components, such as crown ethers/ammonium salts, cyclodextrins/hydrophobic guests and metal-coordinated macrocycles/ligands, are employed. Two of these RCP synthetic methods efficiently introduce special structures and functions into the RCP crosslinking points. Using these methods, we inserted well-studied rotaxane systems that exhibit deslipping behavior into the crosslinking points of network polymers. Notably, each of the resulting RCPs exhibited chemostimuli-responsive or photoresponsive degradability due to the deslipping behavior of the rotaxane system incorporated into the RCP. The other two methods are based on the use of rotaxane crosslinkers to fabricate versatile RCPs containing vinyl trunk polymers. The efficiencies and applications of these recently developed methods are compared with those of conventional methods.

Fluorescent poly(boron enaminoketonate)s (PBEKs) were synthesized via the polycycloaddition of a homoditopic nitrile N-oxide to diynes followed by other polymer reactions. Click polycycloaddition of the nitrile N-oxide to various diynes effectively produced polyisoxazoles in high yields. The polyisoxazoles were transformed into the corresponding fluorescent PBEKs by forming the poly(β-aminoenone) intermediates and reacting these intermediates with (C6F5)2BF·OEt2. The solution and solid-state optical properties of the PBEKs were evaluated by ultraviolet–visible (UV–vis) and fluorescence spectroscopy.

A mild annulation reaction of a propargyl-terminated pseudorotaxane with a homoditopic stable nitrile N-oxide enabled the efficient synthesis of catenanes consisting of not only dibenzo-24-crown-8-ether (DB24C8) but also dibenzo-30-crown-10-ether (DB30C10) as a wheel component. A dynamic 1H NMR study showed the highly enhanced mobility of the components of the DB30C10-based [2]catenane due to the enlarged wheel cavity.

The effect of rotaxane shuttling on the fluorescence properties of a fluorophore was investigated by exploiting fluorophore-tethered [2]rotaxanes. A fluorescent boron enaminoketonate (BEK) moiety was introduced in a rotaxane via transformation of an isoxazole unit generated as a result of an end-capping reaction using a nitrile N-oxide. The rotaxane exhibited a red shift of the fluorescence maximum along with a remarkable enhancement of the fluorescence quantum yield through wheel translation to the fluorophore.

A general method for the one-pot synthesis of stable polymer nitrile N-oxides was developed by a combination of 1,1-diphenylnitroethene with a living anionic polymer. The polymer nitrile N-oxide served as a facile and effective grafting tool for use with polymers containing unsaturated bonds in a catalyst-free and solvent-free [2+3] cycloaddition.

The first synthesis of polyrotaxanes consisting of polyester axles and α-cyclodextrin (α-CD) wheels was achieved by the catalyst-free click end-capping reaction of polypseudorotaxanes using nitrile N-oxide. The polypseudorotaxanes contain acrylate-functionalized polyesters that are obtained by the living ring-opening polymerization of lactones. The yield and coverage ratio of polyrotaxanes are highly dependent on the reaction time, molecular weight of the polyester, polyester structure, and solvent used. From the thermal properties of the resulting polyrotaxanes, it was found that coverage with α-CDs efficiently suppresses the crystallization of the polyester main chain.

We developed a facile protocol for grafting-onto and cross-linking unsaturated-bond-containing common polymers via the formation of masked-ketene-functionalized polymers. The protocol utilizes a cascade functionalization agent 1 that has nitrile N-oxide and masked ketene functionalities. Through the model ligation reactions using 1, it turned out that the 1 facilitates the catalyst-free click introduction of masked ketene moieties to the unsaturated bonds such as CN and CC bonds, capable of undergoing catalyst-free ligation with not only a nucleophilic amine but also a neutral alcohol with low nucleophilicity. On the basis of these results, the catalyst-free grafting reactions of PEG onto EPDM and PAN using 1 were performed to afford the corresponding graft copolymers in excellent conversion yields. In addition, it was also revealed that heating of both PAN and NR with masked ketene moieties at 250 °C for 1 h without catalyst enables the efficient conversion to give the respective cross-linked polymers.

A general method for the one-pot synthesis of stable polymer nitrile N-oxides was developed by a combination of 1,1-diphenylnitroethene with a living anionic polymer. The polymer nitrile N-oxide served as a facile and effective grafting tool for use with polymers containing unsaturated bonds in a catalyst-free and solvent-free [2+3] cycloaddition.