A Mechanochemically Triggered “Click” Catalyst

“Click” chemistry represents one of the most powerful approaches for linking molecules in chemistry and materials science. Triggering this reaction by mechanical force would enable site‐ and stress‐specific “click” reactions—a hitherto unreported observation. We introduce the design and realization of a homogeneous Cu catalyst able to activate through mechanical force when attached to suitable polymer chains, acting as a lever to transmit the force to the central catalytic system. Activation of the subsequent copper‐catalyzed “click” reaction (CuAAC) is achieved either by ultrasonication or mechanical pressing of a polymeric material, using a fluorogenic dye to detect the activation of the catalyst. Based on an N‐heterocyclic copper(I) carbene with attached polymeric chains of different flexibility, the force is transmitted to the central catalyst, thereby activating a CuAAC in solution and in the solid state.     

“Click” coupling between alkyne‐decorated multiwalled carbon nanotubes and reactive PDMA‐PNIPAM micelles

Covalent functionalization of alkyne‐decorated multiwalled carbon nanotubes (MWNTs) with a well‐defined, azide‐derivatized, thermoresponsive diblock copolymer, poly(N,N‐dimethylacrylamide)‐poly(N‐isopropylacrylamide) (PDMA‐PNIPAM) was accomplished by the Cu(I)‐catalyzed [3 + 2] Huisgen cycloaddition. It was found that this reaction could simultaneously increase the molecular size and bonding density of grafted polymers when PDMA‐PNIPAM micelles were employed in the coupling system. On the other hand, attachment of molecularly dissolved unimers of high‐molecular weight onto the nanotube resulted in low‐graft density. The block copolymer bearing azide groups at the PDMA end was prepared by reversible addition–fragmentation transfer polymerization, which formed micelles with a diameter of ～40 nm at temperatures above its critical micelle temperature. Scanning electron microscopy was utilized to demonstrate that the coupling reaction was successfully carried out between copolymer micelles and alkyne‐bearing MWNTs. FTIR spectroscopy was utilized to follow the introduction and consumption of alkyne groups on the MWNTs. Thermogravimetric analysis indicated that the functionalized MWNTs consisted of about 45% polymer. Transmission electron microscopy was utilized to image polymer‐functionalized MWNTs, showing relatively uniform polymer coatings present on the surface of nanotubes. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7187–7199, 2008     

“Click”‐Triggered Self‐Healing Graphene Nanocomposites

Strategies to compensate material fatigue are among the most challenging issues, being most prominently addressed by the use of nano‐ and microscaled fillers, or via new chemical concepts such as self‐healing materials. A capsule‐based self‐healing material is reported, where the adverse effect of reduced tensile strength due to the embedded capsules is counterbalanced by a graphene‐based filler, the latter additionally acting as a catalyst for the self‐healing reaction. The concept is based on “click”‐based chemistry, a universal methodology to efficiently link components at ambient reaction conditions, thus generating a “reactive glue” at the cracked site. A capsule‐based healing system via a graphene‐based Cu₂O (TRGO‐Cu₂O‐filler) is used, acting as both the catalytic species for crosslinking and the required reinforcement agent within the material, in turn compensating the reduction in tensile strength exerted by the embedded capsules. Room‐temperature self‐healing within 48 h is achieved, with the investigated specimen containing TRGO‐Cu₂O demonstrating significantly faster self‐healing compared to homogeneous (Cu(PPh₃)₃F, Cu(PPh₃)₃Br), and heterogeneous (Cu/C) copper(I) catalysts.

     

Synthesis of dendrimers through divergent iterative thio‐bromo “Click” chemistry

The development of a novel nucleophilic thio‐bromo “Click” reaction, specifically base‐mediated thioetherification of thioglycerol with α‐bromoesters, is reported. Combination of this thio‐bromo click reaction with subsequent acylation with 2‐bromopropionyl bromide provides an iterative two‐step divergent growth approach to the synthesis of a new class of poly(thioglycerol‐2‐propionate) (PTP) dendrimers. This approach is demonstrated in the rapid preparation of four generation (G1–G4) of PTP dendrimers with high‐structural fidelity. The isolated G1–G4 bromide‐terminated dendrimers can be used directly as dendritic macroinitiators for the synthesis of star‐polymers via SET‐LRP. Additionally, the intermediate hydroxy‐terminated dendrimers are analogs of other water‐soluble polyester and polyether dendrimers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3931–3939, 2009     

Thio‐Bromo “Click” Reaction Derived Polymer–Peptide Conjugates for Their Self‐Assembled Fibrillar Nanostructures

The synthesis and self‐assembly of peptide–polymer conjugates into fibrillar nanostructures are reported, based on the amyloidogenic peptide KLVFF. A strategy for rational synthesis of polymer–peptide conjugates is documented via tethering of the amyloidogenic peptide segment LVFF (Aβ_17‐20) and its modified derivative FFFF to the hydrophilic poly(ethylene glycol) monomethyl ether (mPEG) polymer via thio‐bromo based “click” chemistry. The resultant conjugates mPEG‐LVFF‐OMe and mPEG‐FFFF‐OMe are purified via preparative gel permeation chromatography technique (with a yield of 61% and 64%, respectively), and are successfully characterized via combination of spectroscopic and chromatographic methods, including electrospray ionization time‐of‐flight mass spectrometry. The peptide‐guided self‐assembling behavior of the as‐constructed amphiphilic supramolecular materials is further investigated via transmission electron microscopic and circular dichroism spectroscopic analysis, exhibiting fibrillar nanostructure formation in binary aqueous solution mixture.     

“Click” generation of a conjugated polymer library for SWNT dispersion

Noncovalent functionalization of single‐walled carbon nanotubes (SWNTs) with conjugated polymers enhances SWNT processability and allows for selective dispersion of various SWNT species. Selective dispersions can be obtained by tuning the nature of the polymer, which can involve using various polymer backbones or side‐chains. However, a clear understanding of selectivity determinants is elusive, as the degree of polymerization (DP) has a large effect on SWNT selectivity. Additionally, preparing libraries of conjugated polymers with varying functionality while keeping DP consistent is difficult. Here, we report the utilization of a strained cyclooctyne‐containing conjugated polymer that serves as a versatile scaffold, enabling systematic preparation of a small library of conjugated polymers with different side‐chain functionality, while maintaining a consistent DP. The resulting polymers were used as dispersants for SWNTs, forming supramolecular polymer‐SWNT complexes that were characterized by UV‐Vis‐NIR absorption and Raman spectroscopy. In the series of polymers, we were able to probe the effect of small changes within the side chains, such as the incorporation of a carbonyl group or an aromatic unit, on the quality of the polymer‐SWNT dispersion. The results of these studies provide new insight into the factors that dictate the ability of a polymer to form strong interactions with SWNTs. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2053–2058     

Degradable star polymers with high “click” functionality

Degradable polyester‐based star polymers with a high level of functionality in the arms were synthesized via the “arms first” approach using an acetylene‐functional block copolymer macroinitiator. This was achieved by using 2‐hydroxyethyl 2′‐methyl‐2′‐bromopropionate to initiate the ring‐opening polymerization (ROP) of caprolactone monomer followed by an atom transfer radical polymerization (ATRP) of a protected acetylene monomer, (trimethylsilyl)propargyl methacrylate. The hydroxyl end‐group of the resulting block copolymer macroinitiator was subsequently crosslinked under ROP conditions using a bislactone monomer, 4,4′‐bioxepanyl‐7,7′‐dione, to generate a degradable core crosslinked star (CCS) polymer with protected acetylene groups in the corona. The trimethylsilyl‐protecting groups were removed to generate a CCS polymer with an average of 1850 pendent acetylene groups located in the outer block segment of the arms. The increased functionality of this CCS polymer was demonstrated by attaching azide‐functionalized linear polystyrene via a copper (I)‐catalyzed cycloaddition reaction between the azide and acetylene groups. This resulted in a CCS polymer with “brush‐like” arm structures, the grafted segment of which could be liberated via hydrolysis of the polyester star structure to generate molecular brushes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1485–1498, 2009     

Thio‐bromo “Click,” post‐polymerization strategy for functionalizing ring opening metathesis polymerization (ROMP)‐derived materials

The use of a thio‐bromo click strategy as an efficient postpolymerization tool is described. Norbornene derivatives bearing an α‐bromo ester could be polymerized using Grubbs 2^nd generation initiator to provide α‐bromo ester‐containing homo‐and block copolymers that could be efficiently functionalized through reactions with various thiols. A one‐pot strategy was also used, in which up to four different thiols were reacted simultaneously. This chemistry could also be used as an efficient cross‐linking strategy to form ROMP‐based gels as well as a tool for terminal functionalization of polypropylene‐based oligomers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 179–185     

Diels–Alder “Click” Chemistry in Designing Dendritic Macromolecules

Simple, versatile and green : Diels–Alder “click” chemistry is a simple, versatile and “greener” approach in the design of a diverse range of dendritic macromolecules (see scheme).

     

Polysiloxane‐Based Luminescent Elastomers Prepared by Thiol–ene “Click” Chemistry

Side‐chain vinyl poly(dimethylsiloxane) has been modified with mercaptopropionic acid, methyl 3‐mercaptopropionate, and mercaptosuccinic acid. Coordinative bonding of Eu^III to the functionalized polysiloxanes was then carried out and crosslinked silicone elastomers were prepared by thiol–ene curing reactions of these composites. All these europium complexes could be cast to form transparent, uniform, thin elastomers with good flexibility and thermal stability. The networks were characterized by FTIR, NMR, UV/Vis, and luminescence spectroscopy as well as by scanning electron microscopy, thermogravimetric analysis, and X‐ray photoelectron spectroscopy. The europium elastomer luminophores exhibited intense red light at 617 nm under UV excitation at room temperature due to the ⁵D₀→⁷F₂ transition in Eu^III ions. The newly synthesized luminescent materials offer many advantages, including the desired mechanical flexibility. They cannot be dissolved or fused, and so they have potential for use in optical and electronic applications.     

Cyclometalated Mono‐ and Dinuclear IrIII Complexes with “Click”‐Derived Triazoles and Mesoionic Carbenes

Orthometalation at Ir^III centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C?H activation sites, we present here mono‐ and dinuclear Ir^III complexes with “click”‐derived 1,2,3‐triazole and 1,2,3‐triazol‐5‐ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir^III centers. Structural characterization of the complexes reveal a piano stool‐type of coordination around the metal centers with the “click”‐derived ligands bound either with C^N or C^C donor sets to the Ir^III centers. Furthermore, whereas bond localization is observed within the 1,2,3‐triazole ligands, a more delocalized situation is found in their 1,2,3‐triazol‐5‐ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer‐pure, dinuclear cyclometalated Ir^III complexes with poly‐mesoionic‐carbene ligands.     

“Click” Chemistry Mildly Stabilizes Bifunctional Gold Nanoparticles for Sensing and Catalysis

A large family of bifunctional 1,2,3‐triazole derivatives that contain both a polyethylene glycol (PEG) chain and another functional fragment (e.g., a polymer, dendron, alcohol, carboxylic acid, allyl, fluorescence dye, redox‐robust metal complex, or a β‐cyclodextrin unit) has been synthesized by facile “click” chemistry and mildly coordinated to nanogold particles, thus providing stable water‐soluble gold nanoparticles (AuNPs) in the size range 3.0–11.2 nm with various properties and applications. In particular, the sensing properties of these AuNPs are illustrated through the detection of an analogue of a warfare agent (i.e., sulfur mustard) by means of a fluorescence “turn‐on” assay, and the catalytic activity of the smallest triazole–AuNPs (core of 3.0 nm) is excellent for the reduction of 4‐nitrophenol in water.     

Discrete macromolecular constructs via the Diels–Alder “Click” reaction

Functional polymeric materials with desired properties can be designed by precise control of macromolecular architectures. Over the recent years, click reactions have enabled efficient synthesis of a variety of polymers with different topologies via efficient polymer–polymer conjugations. While the copper catalyzed Huisgen type (3+2) dipolar cycloaddition between azide and alkyne has been widely used toward this goal, the Diels–Alder (DA) reaction offers an alternative click reaction that allow efficient macromolecular conjugations, oftentimes without the need of any additional reagent or catalyst. This article highlights, with illustrative examples, the power of the DA “click” reaction to efficiently synthesize a variety of different well‐defined macromolecular constructs in a modular fashion. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Thiol‐epoxy “click” chemistry: Application in preparation and postpolymerization modification of polymers

Base‐catalyzed reaction between a thiol and an epoxide group is a simple fusion process that leads to the formation of a β‐hydroxythio‐ether linkage. This reaction is efficient, regio‐selective, and fast. In addition, it produces a reactive hydroxyl group upon completion. Therefore, it is of considerable potential in synthesis of reactive and functional soft materials. Here, we discuss the fundamental aspects of this process, the so‐called thiol‐epoxy “click” reaction, and its utility in the preparation and post‐polymerization functionalization of polymers and crosslinked networks. Furthermore, its application in surface modification of solid substrates is also considered. Finally, utility of multifunctional materials created using the thiol‐epoxy reaction is discussed in the biomedical arena. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3057–3070     

“Clickable” polymers via a combination of RAFT polymerization and isocyanate chemistry

We describe a facile, one‐pot, two‐step polymerization towards synthesizing block co‐polymers bearing reactive isocyanate functional groups. Reversible addition fragmentation chain transfer (RAFT) polymerization is used to mediate the co‐polymerization of isocyanate‐bearing monomers dimethyl meta‐isopropenyl benzyl isocyanate (TMI) and 2‐isocyanatoethyl methacrylate (ICEMA) with styrene and methyl methacrylate (MMA), respectively. ICEMA was incorporated into the polymer at a faster rate than TMI and its unhindered isocyanate group was found to be more reactive than the hindered isocyanate group of TMI. Both the TMI/styrene and the MMA/ICEMA systems maintain the reactivity of the isocyanate functionality, which was exploited by attaching representative hydroxyl‐bearing small and large molecules as well as solid substrates to the block co‐polymers. Thus, we demonstrate the versatility of the block co‐polymer system as a basis for forming branched polymers or as grafts for a solid substrate. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Thiol–Ene “Click” Reaction Triggered by Neutral Ionic Liquid: The “Ambiphilic” Character of [hmim]Br in the Regioselective Nucleophilic Hydrothiolation

Thiol–ene “click” chemistry has emerged as a powerful strategy to construct carbon–heteroatom (C? S) bonds, which generally results in the formation of two regioisomers. To this end, the neutral ionic liquid [hmim]Br has been explored as a solvent cum catalyst for the synthesis of linear thioethers from activated and inactivated styrene derivatives or secondary benzyl alcohols and thiols without the requirement of using a metal complex, base, or free radical initiator. Furthermore, detailed mechanistic investigations using ¹H NMR spectroscopy and quadrupole time‐of‐flight electrospray ionization mass spectrometry (Q‐TOF ESI‐MS) revealed that the “ambiphilic” character of the ionic liquid promotes the nucleophilic addition of thiol to styrene through an anti‐Markovnikov pathway. The catalyst recyclability and the extension of the methodology for thiol–yne click chemistry are additional benefits. A competitive study among thiophenol, styrene, and phenyl acetylene revealed that the rate of reaction is in the order of thiol–yne>thiol–ene>dimerization of thiol in [hmim]Br.