Cycloaddition Polymerization Research Articles

Unclicking the Click: A Depolymerizable Clicked Polymer via Two Consecutive Single-Crystal-to-Single-Crystal Reactions.

We report here a clicked polymer that can be depolymerized by a declicking reaction. A designed dipeptide monomer, upon heating its crystals, underwent a single-crystal-to-single-crystal topochemical ene-azide cycloaddition polymerization to form a triazoline-linked polymer, which upon further heating, underwent a remarkable SCSC denitrogenation, resulting in an imine-linked polymer quantitatively. As both the TEAC polymerization and the denitrogenation occurred in SCSC fashion, the structures of the triazoline-linked polymer and the imine-linked polymer could be determined at atomic resolution by SCXRD. Acid hydrolysis of the imine-linked polymer leads to quantitative depolymerization, yielding a dipeptide, showcasing the degradability and depolymerizability of such polymers. This solid-state click polymerization and denitrogenation yielding depolymerizable polymer is attractive over the usual click polymers that cannot be unclicked.

Unclicking the Click: A Depolymerizable Clicked Polymer via Two Consecutive Single‐Crystal‐to‐Single‐Crystal Reactions

AbstractWe report here a clicked polymer that can be depolymerized by a declicking reaction. A designed dipeptide monomer, upon heating its crystals, underwent a single‐crystal‐to‐single‐crystal topochemical ene‐azide cycloaddition polymerization to form a triazoline‐linked polymer, which upon further heating, underwent a remarkable SCSC denitrogenation, resulting in an imine‐linked polymer quantitatively. As both the TEAC polymerization and the denitrogenation occurred in SCSC fashion, the structures of the triazoline‐linked polymer and the imine‐linked polymer could be determined at atomic resolution by SCXRD. Acid hydrolysis of the imine‐linked polymer leads to quantitative depolymerization, yielding a dipeptide, showcasing the degradability and depolymerizability of such polymers. This solid‐state click polymerization and denitrogenation yielding depolymerizable polymer is attractive over the usual click polymers that cannot be unclicked.

Large Molecular Rotation in Crystal Changes the Course of a Topochemical Diels-Alder Reaction from a Predicted Polymerization to an Unexpected Intramolecular Cyclization.

A designed anthracene-based monomer for topochemical Diels-Alder cycloaddition polymerization crystallized with head-to-tail arrangement of molecules, as revealed by single-crystal X-ray diffraction (SCXRD) analysis. The diene and dienophile units of adjacent monomer molecules are aligned at an average distance of 4.6 Å, suggesting a favorable crystalline arrangement for their intermolecular Diels-Alder cycloaddition reaction to form a linear polymer. Surprisingly, heating the monomer crystals at a temperature above 125 °C resulted in the formation of intramolecular Diels-Alder cycloadduct, which could be characterized by various spectroscopy and SCXRD analysis. Various time-dependent studies such as NMR, PXRD, and DSC, studies established that the reaction followed topochemical pathway. Schmidt's topochemical postulates are generally used to predict the topochemical reactivity and product, by analyzing the crystal structure of the reactant. Though the crystal arrangement predicted polymerization, upon heating, the molecule avoided this pathway by undergoing a large rotation to form an intramolecular cycloadduct. Theoretical calculations supported the feasibility of the rotation, exploiting the flexibility of the molecule and voids present. These findings caution that the reliance on Schmidt's criteria for topochemical reactions may sometimes be misleading, especially in heat-induced reactions.

Large Molecular Rotation in Crystal Changes the Course of a Topochemical Diels–Alder Reaction from a Predicted Polymerization to an Unexpected Intramolecular Cyclization

AbstractA designed anthracene‐based monomer for topochemical Diels–Alder cycloaddition polymerization crystallized with head‐to‐tail arrangement of molecules, as revealed by single‐crystal X‐ray diffraction (SCXRD) analysis. The diene and dienophile units of adjacent monomer molecules are aligned at an average distance of 4.6 Å, suggesting a favorable crystalline arrangement for their intermolecular Diels–Alder cycloaddition reaction to form a linear polymer. Surprisingly, heating the monomer crystals at a temperature above 125 °C resulted in the formation of intramolecular Diels–Alder cycloadduct, which could be characterized by various spectroscopy and SCXRD analysis. Various time‐dependent studies such as NMR, PXRD, and DSC, studies established that the reaction followed topochemical pathway. Schmidt's topochemical postulates are generally used to predict the topochemical reactivity and product, by analyzing the crystal structure of the reactant. Though the crystal arrangement predicted polymerization, upon heating, the molecule avoided this pathway by undergoing a large rotation to form an intramolecular cycloadduct. Theoretical calculations supported the feasibility of the rotation, exploiting the flexibility of the molecule and voids present. These findings caution that the reliance on Schmidt's criteria for topochemical reactions may sometimes be misleading, especially in heat‐induced reactions.

Simultaneous and in situ syntheses of an enantiomeric pair of homochiral polymers as their perfect stereocomplex in a crystal

Circumventing the issues of conventional stereocomplexation of preformed polymers, herein, we synthesize two enantiopure polymers of opposite chirality simultaneously and in situ as their 1:1 stereocomplex via topochemical polymerization. We design and synthesize an inositol-based achiral monomer for topochemical ene-azide cycloaddition (TEAC) polymerization. In the crystal, the monomer exhibits conformational enantiomerism, and its conformational enantiomers are self-sorted in an arrangement for TEAC polymerization to yield two enantiopure polymers of opposite chirality. Upon heating the monomer crystals, each self-sorted set of conformational enantiomers undergoes regio- and stereospecific polymerization in a single-crystal-to-single-crystal fashion, generating two 1, 4-triazolinyl-linked polymers of opposite chirality simultaneously. The new chiral carbons in all the triazoline rings of a particular polymer chain have the same absolute configuration. These homochiral polymer strands align parallelly, forming a layer, and such enantiopure layers of opposite chirality stack alternately, forming a perfect 1:1 stereocomplex, which we confirmed using single-crystal XRD analysis.

Interaction of Cyclodextrins with Amphiphilic Alternating Cooligomers Possessing the Dense Triazole Backbone.

The interaction of cyclodextrins (CDs) with structure-controlled polymers is expected to provide significant insights into macromolecular recognition. However, the interaction of CDs with structure-controlled polymers has been an underexamined issue of investigation. Herein, alternating amphiphilic cooligomers (oligoCnAH, where n denotes the carbon number of alkyl groups; n = 4, 8, and 12) were synthesized by copper(I)-catalyzed azide-alkyne cycloaddition polymerization of heterodimers of 4-azido-5-hexynoic acid (AH) derivatives carrying N-alkylamide and t-butyl (tBu) ester side chains, followed by hydrolysis of the tBu ester, to study the interaction of CDs with oligoCnAH by 1H NMR, nuclear Overhauser effect spectroscopy, and pulse-field-gradient spin-echo NMR. These NMR studies indicated that αCD interacted with oligoC4AH, αCD and βCD interacted with oligoC8AH, and all CDs interacted with oligoC12AH. Based on the equilibrium models proposed, the binding constants were evaluated for the binary mixtures, which showed interaction. Comparing the interactions of the CDs/oligoC12AH binary mixtures with those of the binary mixtures of CDs and alternating copolymers of sodium maleate and dodecyl vinyl ether (polyC12M), it is concluded that oligoC12AH forms less stable micelles than does polyC12M presumably because of the lower molecular weight, the hydrophilic amide groups in the side chain, and the longer interval between neighboring C12 groups in oligoC12AH.

Massive Molecular Motion in Crystal Leads to an Unexpected Helical Covalent Polymer in a Solid-state Polymerization.

We designed a proline-derived monomer with azide and alkene functional groups to enable topochemical ene-azide cycloaddition (TEAC) polymerization. In its crystal, the monomer forms supramolecular helices along the 'a' axis through various non-covalent interactions. Along the 'c' axis, the molecules arrange themselves head-to-tail in a wave-like pattern, positioning the azide and alkene groups of adjacent molecules in close proximity and anti-parallel orientation, complying with Schmidt's criteria for topochemical reaction. This prearranged configuration was expected to facilitate smooth topochemical polymerization, resulting in a 1,4-triazoline-linked polymer. Upon heating, the monomer underwent TEAC polymerization in a remarkable single-crystal-to-single-crystal fashion, but, to our surprise, it yielded an unexpected covalent helical polymer linked by 1,5-disubstituted triazoline units. Remarkably, the crystal avoids the ready-to-react arrangement for polymerization, but connects monomer molecules within the supramolecular helix through the cycloaddition of azide and alkene groups, even though they are not in close proximity nor in the expected orientation. This unexpected path, involving a substantial 134° rotation of the alkene group, yields hitherto unknown 1,5-disubstituted triazoline product regiospecifically. This study serves as a cautionary reminder that relying solely on topochemical postulates for predicting reactivity can sometimes be misleading.

Massive Molecular Motion in Crystal Leads to an Unexpected Helical Covalent Polymer in a Solid‐state Polymerization

AbstractWe designed a proline‐derived monomer with azide and alkene functional groups to enable topochemical ene‐azide cycloaddition (TEAC) polymerization. In its crystal, the monomer forms supramolecular helices along the ‘a’ axis through various non‐covalent interactions. Along the ‘c’ axis, the molecules arrange themselves head‐to‐tail in a wave‐like pattern, positioning the azide and alkene groups of adjacent molecules in close proximity and anti‐parallel orientation, complying with Schmidt's criteria for topochemical reaction. This prearranged configuration was expected to facilitate smooth topochemical polymerization, resulting in a 1,4‐triazoline‐linked polymer. Upon heating, the monomer underwent TEAC polymerization in a remarkable single‐crystal‐to‐single‐crystal fashion, but, to our surprise, it yielded an unexpected covalent helical polymer linked by 1,5‐disubstituted triazoline units. Remarkably, the crystal avoids the ready‐to‐react arrangement for polymerization, but connects monomer molecules within the supramolecular helix through the cycloaddition of azide and alkene groups, even though they are not in close proximity nor in the expected orientation. This unexpected path, involving a substantial 134° rotation of the alkene group, yields hitherto unknown 1,5‐disubstituted triazoline product regiospecifically. This study serves as a cautionary reminder that relying solely on topochemical postulates for predicting reactivity can sometimes be misleading.

Topochemistry for Difficult Peptide-Polymer Synthesis: Single-Crystal-to-Single-Crystal Synthesis of an Isoleucine-Based Polymer, a Hydrophobic Coating Material.

Polymers of hydrophobic amino acids are predicted to be potential coating materials for the creation of hydrophobic surfaces. The oligopeptides of hydrophobic amino acids are called "difficult peptides"; as the name suggests, it is difficult to synthesize them by conventional methods. We circumvented this synthetic challenge by adopting topochemical azide-alkyne cycloaddition (TAAC) polymerization of a hydrophobic dipeptide monomer. We designed an Ile-based dipeptide, decorated with azide and alkyne, which arrange in the crystal in a head-to-tail fashion with the azide and alkyne of the adjacent molecules in a ready-to-react orientation. The monomer, on mild heating of its crystals, undergoes regiospecific TAAC polymerization to yield a 1,4-disubstituted-triazole-linked polymer in a single-crystal-to-single-crystal fashion. The solid obtained after evaporation of the monomer solution also maintained crystallinity and underwent regiospecific topochemical polymerization as in the case of crystals. This topochemical polymerization could be studied using different techniques such as FTIR, NMR, DSC, GPC, MALDI, PXRD, and SCXRD. Since the polymer is insoluble in common solvents and hence difficult to coat surfaces, the monomer was first sprayed and evaporated on various surfaces and polymerized on the surface. Such polymer-coated surfaces exhibited water contact angles of up to 134°, showing that this Ile-derived polymer is very hydrophobic and can potentially be used as a coating material for various applications.

Synthesis of an Alternating Polycation with the Dense 1,2,3-Triazole Backbone

AbstractPolycations are an important class of water-soluble polymers because they form polyion complexes with DNA. Thus, the synthesis of polycations with controlled monomer sequences will be of increasing importance for the formation of well-defined polyion complexes. In this study, cationic homopolymer and alternating copolymer with the dense triazole backbone were synthesized by copper(I)-catalyzed azide–alkyne cycloaddition polymerization. The polycations obtained were characterized by potentiometric and turbidimetric titrations, and by complex formation with poly(acrylic acid).

Two-dimensional fullerene-based monolayer materials assembled by C80 and Sc3N@C80.

Construction of two-dimensional (2D) materials using fullerenes as building blocks has attracted particular attention, primarily due to their ability to integrate desired functionalities into devices. However, realization of stable 2D phases of polymerized fullerenes remains a big challenge. Here, we propose two stable 2D monolayer phases with covalently bridged C80 cages, namely α-C80-2D and β-C80-2D, which are semiconductors with strong absorption in the long wave range and appreciable carrier mobility, respectively. The high stability originates from the bond energy released by the [2+2] cycloaddition polymerization of C80 is greater than the deformation energy of a cage. Starting from α-C80-2D, endohedral incorporation of the Sc3N molecule into each C80 cage leads to 2D semiconductors of α-Sc3N@C80-2D and α'-Sc3N@C80-2D, which possess exceptional stability and diverse physical properties, including unique electronic band structures, strong optical absorption in the visible (VIS) to near-infrared (NIR) regime, and anisotropic optical characteristics. Remarkably, a temperature-induced order-disorder transition in the α-Sc3N@C80-2D phase has been observed at elevated temperatures above 600 K. These findings expand the family of 2D carbon materials and provide useful clue for the potential applications of fullerene-assembled monolayer networks.

Diels-Alder cycloaddition polymerization for porous poly-phenylenes with exceptional gas uptake properties.

We report the synthesis of two-dimensional and three-dimensional porous polyphenylenes (2D/3D-pPPs) via the Diels-Alder cycloaddition polymerization reaction. The resulting 2D and 3D-pPPs showed surface areas up to 1553 m2 g-1, pore volumes of 1.45 cm3 g-1 and very high H2 uptake capacities of 7.4 and 7.1 wt% at 77 K, respectively, along with a competitive high-pressure CO2 and CH4 uptake performance.

Synthesis of an alternating copolymer of the dense 1,2,3‐triazole backbone carrying t‐butyl ester and nitrile side chains

AbstractIn these decades, synthesis of polymers with controlled monomer sequences has been of increasing importance in polymer chemistry. When we tested synthesis of a heterodimer of 4‐azido‐5‐hexynoic acid derivatives carrying t‐butyl (tBu) ester and tBu amide side chains for preparation of an alternating copolymer, we unexpectedly obtained a heterodimer precursor carrying tBu ester and nitrile side chains, in a high yield. Since dense 1,2,3‐triazole polymers carrying nitrile side chains are expected as a new functional polymer, the heterodimer is polymerized by copper(I)‐catalyzed azide–alkyne cycloaddition polymerization to obtain an alternating copolymer carrying tBu ester and nitrile side chains in this study. Since 1,2,3‐triazole and nitrile moieties are known to be ligands for metal ions, the complex formation of the alternating copolymer with PdCl2 is also investigated by NMR. In the presence of PdCl2, 1H NMR spectra show broad and separated signals due to the free and complexed copolymers, indicative of slow exchange. Pulse‐field‐gradient spin‐echo NMR data indicated that the coordination of alternating copolymer to PdCl2 made the copolymer chains take a more compact conformation under dilute conditions. The density functional theory calculations indicated that the alternating copolymer coordinated to PdCl2 preferentially through the 1,2,3‐triazole nitrogen and nitrile nitrogen atoms.

Single‐Crystal‐to‐Single‐Crystal Topochemical Synthesis of a Collagen‐inspired Covalent Helical Polymer

AbstractThere is much demand for crystalline covalent helical polymers. Inspired by the helical structure of collagen, we synthesized a covalent helical polymer wherein the repeating dipeptide Gly‐Pro units are connected by triazole linkages. We synthesized an azide and alkyne‐modified dipeptide monomer made up of the repeating amino acid sequence of collagen. In its crystals, the monomer molecules aligned in head‐to‐tail fashion with proximally placed azide and alkyne forming supramolecular helices. At 60 °C, the monomer underwent single‐crystal‐to‐single‐crystal (SCSC) topochemical azide‐alkyne cycloaddition polymerization, yielding a covalent helical polymer as confirmed by single‐crystal X‐ray diffraction (SCXRD) analysis. Compared to the monomer crystals, the polymer single‐crystals were very strong and showed three‐fold increase in Young's modulus, which is higher than collagen, many synthetic polymers and other materials. The crystals of this covalent helical polymer could bear loads as high as 1.5 million times of their own weight without deformation. These crystals could also withstand high compression force and did not disintegrate even at an applied force of 98 kN. Such light‐weight strong materials are in demand for various technological applications.

Single-Crystal-to-Single-Crystal Topochemical Synthesis of a Collagen-inspired Covalent Helical Polymer.

There is much demand for crystalline covalent helical polymers. Inspired by the helical structure of collagen, we synthesized a covalent helical polymer wherein the repeating dipeptide Gly-Pro units are connected by triazole linkages. We synthesized an azide and alkyne-modified dipeptide monomer made up of the repeating amino acid sequence of collagen. In its crystals, the monomer molecules aligned in head-to-tail fashion with proximally placed azide and alkyne forming supramolecular helices. At 60 °C, the monomer underwent single-crystal-to-single-crystal (SCSC) topochemical azide-alkyne cycloaddition polymerization, yielding a covalent helical polymer as confirmed by single-crystal X-ray diffraction (SCXRD) analysis. Compared to the monomer crystals, the polymer single-crystals were very strong and showed three-fold increase in Young's modulus, which is higher than collagen, many synthetic polymers and other materials. The crystals of this covalent helical polymer could bear loads as high as 1.5 million times of their own weight without deformation. These crystals could also withstand high compression force and did not disintegrate even at an applied force of 98 kN. Such light-weight strong materials are in demand for various technological applications.

Iodine and Nickel Ions Adsorption by Conjugated Copolymers Bearing Repeating Units of Dicyclopentapyrenyl and Various Thiophene Derivatives.

The synthesis of three conjugated copolymers TPP1-3 was carried out using a palladium-catalyzed [3+2] cycloaddition polymerization of 1,6-dibromopyrene with various dialkynyl thiophene derivatives 3a-c. The target copolymers were obtained in excellent yields and high purity, as confirmed by instrumental analyses. TPP1-3 were found to divulge a conspicuous iodine adsorption capacity up to 3900 mg g-1, whereas the adsorption mechanism studies revealed a pseudo-second-order kinetic model. Furthermore, recyclability tests of TPP3, the copolymer which revealed the maximum iodine uptake, disclosed its efficient regeneration even after numerous adsorption-desorption cycles. Interestingly, the target copolymers proved promising nickel ions capture efficiencies from water with a maximum equilibrium adsorption capacity (qe) of 48.5 mg g-1.

Two Structurally Different Polymers from a Single Monomer.

We designed and synthesized a malonamide-derived monomer, containing azide and alkyne units, for topochemical polymerization to yield nylon (n,3). This monomer on crystallization gave two concomitant polymorphs M1 and M2. Both the polymorphs show crystal packings that are suitable for topochemical azide-alkyne cycloaddition polymerization. On heating, polymorph M1 reacts regiospecifically to give 1,4-disubstituted-1,2,3-triazolyl-linked polymer, whereas polymorph M2 yields 1,5-disubstituted-1,2,3-triazolyl-linked polymer regiospecifically. In the case of polymorph M1, polymerization proceeds perpendicular to the hydrogen bonding direction, whereas in M2, the reaction occurs along the hydrogen bonding direction. This results in the two structurally different polymers having distinct topologies. These single-crystal-to-single-crystal polymerizations allowed us to study their structure at atomic resolution by single-crystal X-ray diffraction. This is the first report on the topochemical synthesis of two structurally isomeric polymers from a single monomer.

Doubly Threaded Slide-Ring Polycatenane Networks.

Crosslinking in polymer networks leads to intrinsic structural inhomogeneities that result in brittle materials. Replacing fixed covalent crosslinks with mobile ones in mechanically interlocked polymers (MIPs), such as in slide-ring networks (SRNs) in which interlocked crosslinks are formed when polymer chains are threaded through crosslinked rings, can lead to tougher, more robust networks. An alternative class of MIPs is the polycatenane network (PCN), in which the covalent crosslinks are replaced with interlocked rings that introduce the unusual catenane's mobility elements (elongation, rotation, and twisting) as connections between polymer chains. A slide-ring polycatenane network (SR-PCN), with doubly threaded rings embedded as crosslinks in a covalent network, combines the mobility features of both the SRNs and PCNs, where the catenated ring crosslinks can slide along the polymer backbone between the two limits of network bonding (covalent and interlocked). This work explores using a metal ion-templated doubly threaded pseudo[3]rotaxane (P3R) crosslinker, combined with a covalent crosslinker and a chain extender, to access such networks. A catalyst-free nitrile-oxide/alkyne cycloaddition polymerization was used to vary the ratio of P3R and covalent crosslinker to yield a series of SR-PCNs that vary in the amount of interlocked crosslinking units. Studies on their mechanical properties show that metal ions fix the rings in the network, leading to similar behavior as the covalent PEG gels. Removal of the metal ion frees the rings resulting in a high-frequency transition attributed to the additional relaxation of polymer chains through the catenated rings while also increasing the rate of poroelastic draining at longer timescales.

Monodisperse Hyperbranched Polytriazoles as Unimolecular Nanocontainers for Encapsulation of Functional Payloads.

In this work, a series of polytriazole-based unimolecular nanocontainers (UNs) with good water solubility, uniformity, and colloidal stability via a bottom-up chain-growth copper-catalyzed azide-alkyne cycloaddition (co)polymerization that features tunable size, degree of branching (DB), and functionality of the UNs is developed. A broad selection of hydrophobic payload molecules, including Nile red (NR), camptothecin, pyrene, 1-pyrenemethanol, and IR676, are successfully encapsulated to demonstrate the high versatility of these polymers as UNs. Using NR as a probe guest, the relationship between the encapsulation performance and the structural properties of UNs, including size and DB, is investigated. Furthermore, the localization and dispersity of encapsulated NR are explored and the dependence of payload's dispersity on the DB of UNs is revealed. The payload encapsulated in UNs exhibits tunable release kinetics, determined by either adjusting release conditions or including pH-responsive structural units in the UNs. Meanwhile, the dyes encapsulated in UNs exhibit improved photostability and stronger resistance to photobleaching. It is expected that these explorations address the size and stability issues widely encounter in current drug/dye nanocarriers and provide a versatile platform of polytriazole-based UNs for suitable payloads in various applications, including drug delivery and bio-imaging.

Synthesis of Dense 1,2,3-Triazole Oligomers Consisting Preferentially of 1,5-Disubstituted Units via Ruthenium(II)-Catalyzed Azide-Alkyne Cycloaddition.

Ruthenium(II)-catalyzed azide-alkyne cycloaddition (RuAAC) polymerization of t-butyl 4-azido-5-hexynoate (tBuAH), i.e., a heterobifunctional monomer carrying azide and alkyne moieties, was investigated in this study. RuAAC of the monofunctional precursors of tBuAH yielded a dimer possessing a 1,5-disubstituted 1,2,3-triazole moiety. 1H NMR data showed that the dimer was a mixture of diastereomers. Polymerization of tBuAH using ruthenium(II) (Ru(II)) catalysts produced oligomers of Mw ≈ (2.7-3.6) × 103 consisting of 1,5-disubstituted 1,2,3-triazole units (1,5-units) as well as 1,4-disubstituted 1,2,3-triazole units (1,4-units). The fractions of 1,5-unit (f1,5) were roughly estimated to be ca. 0.8 by comparison of signals of the methine and triazole protons in 1H NMR spectra, indicating that RuAAC proceeded preferentially and thermal Huisgen cycloaddition (HC) somehow took place during the polymerization. The oligomer samples obtained were also characterized by solubility test, size exclusion chromatography (SEC), ultraviolet-visible (UV-Vis) absorption spectroscopy, and thermogravimetric analysis (TGA). The UV-Vis and TGA data indicated that the oligomer samples contained a substantial amount of Ru(II) catalysts. To the best of our knowledge, this is the first report on dense 1,2,3-triazole oligomers consisting of 1,5-units linked via a carbon atom.