Stereoregular Two-Dimensional Polymers Constructed by Topochemical Polymerization

Programmable Solid-State [2 + 2] Photocycloadditions of Dienes Directed by Structural Control and Wavelength Selection

Small differences in molecular or solid-state structure can afford significant differences in properties. Here, a diene derivative, 1,3-bis((E)-2-bromostyryl)benzene (2Brm), is synthesized and crystallized into two unique solid-state forms, each exhibiting a different π-π stacking geometry, which imparts distinct reactivity and photoresponsivity. Upon exposure of the two solids to UV-Vis light, a [2 + 2] photocycloaddition occurs to afford regioisomeric products due to the difference in the stacking geometries of the dienes. From a single molecular precursor, we further demonstrate that under different wavelengths of light, the chemical functionality can be programmed into discrete and distinct products containing one, two, or three cyclobutane rings as well as oligomeric/polymeric products. Moreover, the two initial solid forms exhibit wavelength-dependent photomechanical behaviors (i.e., photosalience). This work demonstrates a rare, template-free, self-assembly-based strategy that enables access to a suite of discrete and oligomeric/polymeric products via regiocontrolled solid-state photocycloadditions and further presents potential routes toward the design of photoactuating materials.

Truxillic calcium supramolecular skeleton fortified pH responsive and biodegradable alginate hydrogel films promoting fruit preservation

This work presented a unique alginate hydrogel film fortified by truxillic calcium skeleton with notable physicochemical and mechanical properties, as well as the promising application in fruit preservation. A series of truxillic‑calcium-alginate (CBDA-Ca/SA) films were prepared. It was found that CBDA-10-Ca/SA can block over 50 % of UV-visible light. The maximum breaking strengths of (6 % CBDA-1-Ca)/SA and (2 % CBDA-10-Ca)/SA are 82 MPa and 79 MPa, about twice that of Ca/SA. CBDA-11-Ca/SA showed the highest WVP of 2.18 × 10-10 g ∙ m-1 ∙ s-1 ∙ Pa-1 and CBDA-10-Ca showed the best performance in reducing the OTR of the films by 4.1× 10-5 cm3 ⸱cm ⸱ m-2 ⸱ 24 h-1 ⸱ Pa-1. With water absorption ranging from 3480 % in an acidic environment to 9520 % in an alkaline environment, the CBDA-10-Ca/SA film demonstrated exceptional pH responsiveness. The degradation rate of all films was around 70 % after four weeks of burial under the soil. The above studies indicate that polyphenols can not only act as "hooks" to grasp Ca2+ to form supramolecular skeleton structure improving the mechanical strength of SA-based films, but also act as active components to endure the functional properties of SA-based films with antibacterial and antioxidant properties.

Truxinates and truxillates: building blocks for architecturally complex polymers and advanced materials

Significant advancements in the syntheses of cyclobutane containing small molecules and polymers are described in the last 15 years. Small molecule cyclobutanes are under investigation for their diverse pharmacological activities, while polymers with cyclobutane backbones are emerging as novel mechanophores, stress-responsive materials, and sustainable plastics. Within these chemistries, [2 + 2] photocycloadditions to yield truxinates and truxillates are highly efficient offering a versatile strategy to access complex scaffolds. This article provides a comprehensive review on the synthetic methodologies, properties, and applications of polymer truxinates and truxillates, providing the background necessary to understand current developments and envision future applications. Additionally, we highlight the links between the development, discoveries, and synthetic methodologies of small molecules and cyclobutane polymers. We emphasize structure property relationships and discuss methods to control composition and structure for desired applications. We begin with a discussion of synthetic techniques for small molecule and polymer cyclobutanes followed by their greater applications, including pharmacological and material properties with examples including sustainable plastics and stimuli-responsive systems.

Hierarchical single-crystal-to-single-crystal transformations of a monomer to a 1D-polymer and then to a 2D-polymer

Designing and synthesizing flawless two-dimensional polymers (2D-Ps) via meticulous molecular preorganization presents an intriguing yet challenging frontier in research. We report here the single-crystal-to-single-crystal (SCSC) synthesis of a 2D-P via thermally induced topochemical azide-alkyne cycloaddition (TAAC) reaction. A designed monomer incorporating two azide and two alkyne units is synthesized. The azide and alkyne groups are preorganized in the monomer crystal in reactive geometries for polymerizations in two orthogonal directions. On heating, the polymerizations proceed in a hierarchical manner; at first, the monomer reacts regiospecifically in a SCSC fashion to form a 1,5-triazolyl-linked 1D polymer (1D-P), which upon further heating undergoes another SCSC polymerization to a 2D-P through a second regiospecific TAAC reaction forming 1,4-triazolyl-linkages. Two different linkages in orthogonal directions make this an architecturally attractive 2D-P, as determined, at atomic resolution, by single-crystal X-ray diffraction. The 2D-P reported here is thermally stable in view of the robust triazole-linkages and can be exfoliated as 2D-sheets.

A Biorenewable Cyclobutane-containing Building Block Synthesized from Sorbic Acid Using Photoenergy

A novel cyclobutane-containing diacid building block, CBDA-3, was synthesized from sorbic acid using clean, efficient [2 + 2] photocycloaddition. This photoreaction can be performed using commercially available germicidal lamps, which represent a form of ECO-UV. SC-XRD showed that the cyclobutane ring in CBDA-3 has a unique semi-rigid character, unlike more rigid aromatic rings or more flexible types of aliphatic rings. C=C bonds present in the structure of CBDA-3 provide opportunities for derivatization which could be used to alter the characteristics of polymers made from this monomer. Additionally, TGA and DSC analysis showed CBDA-3 to have excellent thermal stability. These characteristics make CBDA-3 a promising building block with the potential to be used as a sustainable alternative to traditional petroleum-derived diacids. Finally, a facile and reliable Fischer esterification of CBDA-3 was performed to tune its melting point and solubility for different applications and to demonstrate the applicability of this building block in polymer synthesis.

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A novel cyclobutane-containing diacid building block

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A potentially sustainable alternative to petroleum-derived diacids

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Photoreaction using ECO-UV (Energy-efficient, Cost-effective, and Operator-friendly)

A recyclable thermoset with built-in thermocleavable group developed from a cis-cyclobutane-1,2-dicarboxylic acid

A novel class of recyclable thermoset has been developed from cis-3,4-diphenylcyclobutane-1,2-dicarboxylic acid (CBDA-4) due to its thermocleavability at high temperature. This key CBDA-4 building block was synthesized from β-trans-cinnamic acid using a [2+2] photocycloaddition reaction. CBDA-4 was subsequently linked with glycerol via esterification to give a thermoset with T_g of 68 °C. The thermoset was heated to 300 °C to analyze its degradation. A key intermediate was successfully obtained after purification of the degraded polymer. NMR, FT-IR, HRMS, and single crystal X-ray diffraction confirmed the intermediate was glycerol cinnamate, which was the result of splitting cyclobutane in the polymer backbone at high temperature. Glycerol cinnamate was readily hydrolyzed reforming the starting materials glycerol and trans-cinnamic acid to complete the recycling loop.

Solution-processable and functionalizable ultra-high molecular weight polymers via topochemical synthesis

Topochemical polymerization reactions hold the promise of producing ultra-high molecular weight crystalline polymers. However, the totality of topochemical polymerization reactions has failed to produce ultra-high molecular weight polymers that are both soluble and display variable functionality, which are restrained by the crystal-packing and reactivity requirements on their respective monomers in the solid state. Herein, we demonstrate the topochemical polymerization reaction of a family of para-azaquinodimethane compounds that undergo facile visible light and thermally initiated polymerization in the solid state, allowing for the first determination of a topochemical polymer crystal structure resolved via the cryoelectron microscopy technique of microcrystal electron diffraction. The topochemical polymerization reaction also displays excellent functional group tolerance, accommodating both solubilizing side chains and reactive groups that allow for post-polymerization functionalization. The thus-produced soluble ultra-high molecular weight polymers display superior capacitive energy storage properties. This study overcomes several synthetic and characterization challenges amongst topochemical polymerization reactions, representing a critical step toward their broader application.

Highly Efficient Preparation of Single-Layer Two-Dimensional Polymer Obtained from Single-Crystal to Single-Crystal Synthesis

A two-dimensional polymer (2DP) single crystal (T-2DP) with submillimeter size was synthesized by single-crystal to single-crystal transformation based on photochemical [2 + 2]-cycloaddition. A successful conversion from monomer to polymer was achieved in the single-crystal state. The structure information with an atomic resolution of both the monomer and 2DP was given through single-crystal X-ray diffraction. By simply treated with trifluoroacetic acid (TFA) under mild conditions, an unprecedented efficiency of exfoliation was achieved. The triazine core in T-2DP could be protonated by TFA, which resulted in a solution-like sample with >60% of monolayers. The size of the exfoliated monolayer reaches to several hundreds of μm². This is another precious example of 2DP single crystal with nearly perfect structure and large enough size. The successful preparation of the highly desirable 2DP "solution" for a long time containing large sized and large amount of 2DP monolayers may open up new prospects for the basic properties study and the applications of 2DPs.

Polymers with advanced structural and supramolecular features synthesized through topochemical polymerization

Polymers are an integral part of our daily life. Hence, there are constant efforts towards synthesizing novel polymers with unique properties. As the composition and packing of polymer chains influence polymer's properties, sophisticated control over the molecular and supramolecular structure of the polymer helps tailor its properties as desired. However, such precise control via conventional solution-state synthesis is challenging. Topochemical polymerization (TP), a solvent- and catalyst-free reaction that occurs under the confinement of a crystal lattice, offers profound control over the molecular structure and supramolecular architecture of a polymer and usually results in ordered polymers. In particular, single-crystal-to-single-crystal (SCSC) TP is advantageous as we can correlate the structure and packing of polymer chains with their properties. By designing molecules appended with suitable reactive moieties and utilizing the principles of supramolecular chemistry to align them in a reactive orientation, the synthesis of higher-dimensional polymers and divergent topologies has been achieved via TP. Though there are a few reviews on TP in the literature, an exclusive review showcasing the topochemical synthesis of polymers with advanced structural features is not available. In this perspective, we present selected examples of the topochemical synthesis of organic polymers with sophisticated structures like ladders, tubular polymers, alternating copolymers, polymer blends, and other interesting topologies. We also detail some strategies adopted for obtaining distinct polymers from the same monomer. Finally, we highlight the main challenges and prospects for developing advanced polymers via TP and inspire future directions in this area.

Topochemical polymerizations for the solid-state synthesis of organic polymers

Topochemical polymerizations are solid-state reactions driven by the alignment of monomers in the crystalline state. The molecular confinement in the monomer crystal lattice offers precise control over the tacticity, packing and crystallinity of the polymer formed in the topochemical reaction. As topochemical reactions occur under solvent- and catalyst-free conditions, giving products in high yield and selectivity/specificity that do not require tedious chromatographic purification, topochemical polymerizations are highly attractive over traditional solution-phase polymer synthesis. By this method, polymers having sophisticated structures and desired topologies can be availed. Often, such ordered packing confers attractive properties to the topochemically-synthesized polymers. Diverse categories of topochemical polymerizations are known, such as polymerizations via [2+2], [4+4], [4+2], and [3+2] cycloadditions, and polymerization of diynes, triynes, dienes, trienes, and quinodimethanes, each of which proceed under suitable stimuli like heat, light or pressure. Each class of these reactions requires a unique packing arrangement of the corresponding monomers for the smooth reaction and produces polymers with distinct properties. This review is penned with the intent of bringing all the types of topochemical polymerizations into a single platform and communicating the versatility of these lattice-controlled polymerizations. We present a brief history of the development of each category and comprehensively review the topochemical synthesis of fully-organic polymers reported in the last twenty years, particularly in crystals. We mainly focus on the various molecular designs and crystal engineering strategies adopted to align monomers in a suitable orientation for polymerization. Finally, we analyze the current challenges and future perspectives in this research field.

The impact of solid solution composition on kinetics and mechanism of [2 + 2] photodimerization of cinnamic acid derivatives

The effect of the solid solution composition on the kinetics of solid-state [2 + 2] photocycloadditions was evaluated via a combination of single crystal XRD, FTIR, Raman spectroscopy, and principal component analysis (PCA).

Crystal Engineering Construction of Caffeic Acid Derivatives with Potential Applications in Pharmaceuticals and Degradable Polymeric Materials

Natural products are precious feedstock in drug discovery and sustainable materials. This work using crystal engineering strategy, visible light, and solvent-free cycloaddition successfully constructed two caffeic acid derivatives, rel-(1R,2R,3S,4S)-2,4-bis(3,4-dihydroxyphenyl)cyclobutane-1,3-dicarboxylate and rel-(1R,2R,3S,4S)-2,4-bis(3,4-dihydroxyphenyl)cyclobutane-1,3-dicarboxylic acid. Because of the multiple stereocenters, it is challenging to prepare those compounds using traditional organic synthesis methods. The crystal engineering Hirshfeld surface analysis and 2D intermolecular interaction fingerprints were applied to synthetic route design. The light resources used in this work was visible LED or free, clean, and renewable sunlight. The evidence suggested that pure stereoisomer was obtained demonstrating the stereospecificity and efficiency of the topochemical cycloaddition reaction. The derivatives exhibited free radical scavenging and antioxidant biological activities, as well as the potential inhibitory activity of fatty acid binding proteins. One of the derivatives is the precursor of the natural product Shimobashiric acid C which paves the way for the total synthesis and further study of Shimobashiric acid C. In addition, the derivatives possess photodegradability at a specific wavelength, which is very attractive for "green" degradable polymeric materials.

How to use X-ray diffraction to elucidate 2D polymerization propagation in single crystals

Covalent long-range ordered (crystalline) sheets called 2D polymers have recently been synthesized by irradiating single crystals of suitably packed monomers. To have such an action proceed successfully, billions of bond formation processes have to be mastered exclusively in two dimensions within 3D crystals. This raises questions as to how to elucidate the mechanism of these unusual polymerizations as well as their entire strain management. The article will show that single crystal X-ray diffraction based on both Bragg and diffuse scattering are powerful techniques to achieve such goal. The very heart of both techniques will be explained and it will be shown what can be safely concluded with their help and what not. Consequently, the reader will understand why some crystals break during polymerization, while others stay intact. This understanding will then be molded into a few guidelines that should help pave the way for future developments of 2D polymers by those interested in joining the effort with this fascinating and emerging class of 2D materials.

Synthesis and characterization of BPA-free polyesters by incorporating a semi-rigid cyclobutanediol monomer

A trans-1,3-cyclobutane-containing diol (CBDO-1) has been synthesized and introduced to materials science as a versatile monomer and a possible phenol-free BPA replacement.

A reversible photochemical solid-state transformation in an interpenetrated 3D metal-organic framework with mechanical softness

We synthesized a two-fold interpenetrated 3D MOF with two crystallographically distinct C[double bond, length as m-dash]C bonds, which undergoes [2+2] photo-cycloaddition and thermal reversible reaction, in a single-crystal-to-single-crystal (SCSC) manner. The softer nature and comparable mechanical properties of the crystals of the parent and cyclized MOFs revealed by nanoindentation allowed rationalizing their structural softness and SCSC transformation behaviour.

Scalable preparation and property investigation of a cis-cyclobutane-1,2-dicarboxylic acid from β-trans-cinnamic acid

Scalable synthesis of β-truxinic acid (CBDA-4) was accomplished by capturing and photodimerizing a metastable crystalline solid of trans-cinnamic acid. This synthetic approach builds a foundation for investigating the properties and applications of the useful diacid. The X-ray crystal structure of CBDA-4 was determined for the first time. The cyclobutane ring in CBDA-4 was cleaved upon heating, making it a promising building block for thermally recyclable/degradable materials.

Cyclobutane-1,3-Diacid (CBDA): A Semi-Rigid Building Block Prepared by [2+2] Photocyclization for Polymeric Materials

A previously overlooked building block, cyclobutane-1,3-diacid (CBDA), is introduced to materials synthesis due to its great potentials. As an example of CBDA, α-truxillic acid or 2,4-diphenylcyclobutane-1,3-dicarboxylic acid, was readily synthesized from commercially available trans-cinnamic acid. This CBDA showed outstanding stability both in sunlight and upon heating. While its two carboxylic acid groups can be readily utilized in connecting with other molecules to form new materials, the cyclobutane ring was able to tolerate acid and base treatments showing good chemical stability. A series of cyclobutane-containing polymers (CBPs), namely poly-α-truxillates, were obtained by condensation between α-truxillic acid and diols including ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-petanediol, and 1,6-hexanediol. The structures of these poly-α-truxillates were analyzed by NMR, FT-IR, and HRMS. Powder X-ray diffraction results of the poly-α-truxillates indicated that they are semi-crystalline materials. Preliminary thermal, chemical, and photochemical tests showed that the poly-α-truxillates exhibited comparable stabilities to PET.

Exfoliation of two-dimensional polymer single crystals into thin sheets and investigations of their surface structure by high-resolution atomic force microscopy

The acid-assisted wet-chemical and the adhesive-tape induced micromechanical exfoliation of differently sized single crystals of a 2D polymer (approx. 20 μm and 100 μm) is shown to result in thin sheet stacks. Tuning of the thickness is achieved via duration and frequency of the exfoliation, respectively. A color code is established that correlates interference colors of sheet stacks on SiO₂(300 nm)/Si as observed under an optical microscope with their thicknesses measured by atomic force microscopy. This facilitates reliable monitoring of the exfoliation and quick identification of sheet stacks of a desired thickness. Furthermore, high resolution atomic force microscopy is applied to investigate the surfaces of starting crystals and both wet-chemically and micromechanically exfoliated sheet stacks aiming at exploring whether exfoliation proceeds with preservation of surface periodicity and with a low frequency of sheet rupturing. These investigations also aimed at uncovering possible point defects and domain (grain) boundaries in the surfaces. It appears that all investigated objects have a high molecular scale perfection and that both exfoliation methods proceed mild enough to largely preserve the molecular structure of the 2D polymer including the not covalently bonded template molecules being part of the crystal packing.

Single crystal to single crystal [2+2] photoreactions in chloride and sulphate salts of 4-amino-cinnamic acid via solid-solution formation: a structural and kinetic study

SCSC [2+2] photodimerizationviathe formation of solid state solution is achieved in chloride and sulfate salts of 4-amino-cinnamic acid, and structural and kinetic analyses have been performed.