The synthesis of flawless two-dimensional polymers (2D-Ps) is a challenging area of research due to the difficulty in controlling polymerization in two orthogonal directions. Current methods, such as on-surface/interfacial polymerizations, face limitations in scalability and structural elucidation. Covalent organic frameworks (COFs), while promising, often suffer from lability due to their dynamic covalent bonds. Topochemical polymerization, where pre-organized monomer molecules react in the crystal lattice upon stimulation, offers an advantage in producing perfectly structured 2D-Ps. Previous work has successfully used light-induced cycloaddition reactions for SCSC synthesis of 2D-Ps, but these polymers often undergo thermal depolymerization, limiting their high-temperature applications. This study aims to overcome these limitations by exploring thermally induced topochemical azide-alkyne cycloaddition (TAAC) as a route to synthesizing robust, thermally stable 2D-Ps with diverse architectures. The authors hypothesize that a carefully designed monomer can undergo sequential, hierarchical, and regiospecific SCSC transformations to achieve this goal, expanding the scope of topochemical 2D-polymerization beyond light-induced reactions.

Significant progress has been made in designing and synthesizing linear polymers with complex topologies. However, directing polymerization in two orthogonal dimensions to create 2D-Ps remains a significant challenge, primarily due to the entropic cost associated with increasing dimensionality. On-surface and interfacial polymerization methods offer potential, but issues with scalability, isolation of polymer sheets, and structure elucidation persist. While covalent organic frameworks (COFs) have gained attention as organic 2D materials, their dynamic covalent bonds often lead to topological imperfections and lability. Topochemical polymerization has emerged as an elegant approach to overcome these challenges, allowing for the synthesis of crystalline 2D-Ps with precise layered structures. Pioneering work by King, Sakamoto, and Schluter demonstrated the topochemical synthesis of 2D polymers using photo-induced cycloaddition reactions, achieving SCSC synthesis and structure determination via single-crystal X-ray diffraction (SCXRD). However, these photo-induced reactions often result in polymers susceptible to thermal depolymerization, limiting their applicability in high-temperature environments. Therefore, exploring alternative topochemical reactions, such as the thermally induced TAAC reaction, is crucial for broadening the scope of 2D-P synthesis.

The researchers designed and synthesized an X-shaped monomer (M) with a benzene core, incorporating two azide and two alkyne groups. The monomer's crystal structure was determined using SCXRD, revealing a packing arrangement conducive to TAAC polymerization along specific crystallographic axes. The study employed differential scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) spectroscopy to monitor the thermal events associated with the polymerization process. DSC thermograms showed two distinct exothermic peaks, indicative of a step-wise polymerization. Crystals were heated to temperatures above each exothermic peak, followed by cooling, and then re-analyzed by DSC and FT-IR to confirm reaction completion at each step. SCXRD analysis was performed on crystals heated to specific temperatures to determine the structures of the intermediate 1D-P and final 2D-P products. The authors also investigated the temperature dependence of the polymerization reactions using time-dependent DSC and powder X-ray diffraction (PXRD) analyses. To further elucidate the polymerization process, variable-temperature Raman spectroscopy was employed to track the consumption of alkyne groups during the reaction. Finally, the exfoliation of the 2D-P was explored using different solvents. Optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) were utilized to characterize the exfoliated sheets.

The designed monomer successfully underwent two sequential, regiospecific TAAC polymerizations in a hierarchical manner. The first polymerization, occurring at a lower temperature, yielded a 1D-P with 1,5-triazolyl linkages. Subsequent heating induced a second polymerization, forming a 2D-P with both 1,5- and 1,4-triazolyl linkages in orthogonal directions. The SCSC nature of both transformations was confirmed by SCXRD. The 2D-P crystal structure revealed a unique architectural feature: the incorporation of two different types of triazole linkages in orthogonal directions, leading to a distinct pore structure compared to previously reported 2D-Ps with hexagonal cavities. The 2D-P exhibited excellent thermal stability due to the robust triazole linkages. The study also demonstrated the successful exfoliation of the 2D-P crystals into thin, crystalline sheets using trifluoroacetic acid (TFA), likely facilitated by protonation of the triazole nitrogen atoms and subsequent electrostatic repulsion. AFM analysis indicated that the exfoliated sheets ranged in thickness from approximately 1 nm to 4 nm, consistent with single to few-layer structures. Time-dependent DSC and PXRD studies confirmed that the polymerizations were complete at the targeted temperatures. Variable-temperature Raman spectroscopy provided further evidence of the consumption of alkyne groups during the reaction. The mechanism of the hierarchical polymerization was found to differ from previously proposed models for 2D polymerization, resembling cooperative 1D polymerization mechanisms in which each step of polymerization only proceeds in a particular direction.

This study successfully demonstrates a thermally induced SCSC topochemical approach for synthesizing robust and crystalline 2D-Ps. The use of TAAC reaction to produce thermally stable triazole linkages addresses a key limitation of previously reported light-induced methods. The hierarchical, step-wise polymerization is mechanistically unique, offering new insights into topochemical 2D-polymerization. The resulting 2D-P exhibits a novel architecture, featuring different linkages in orthogonal directions, allowing for the tunability of pore size and shape by using different monomers. The successful exfoliation of the 2D-P into thin sheets expands its potential for practical applications. The findings suggest that exploring different topochemical reactions and monomer designs can lead to new families of 2D-Ps with tailored properties. This work significantly advances the field of 2D-polymer synthesis by expanding the range of available reactions and providing new strategies for controlling the architecture and properties of these materials.

This research successfully synthesized a thermally stable and exfoliable 2D-polymer using a thermally induced, hierarchical, and regiospecific SCSC topochemical polymerization via TAAC reaction. The resulting 2D-P exhibits a unique architecture with two different triazole linkages, offering a new avenue for tuning the properties of 2D-polymers. The successful exfoliation of the material into thin sheets opens up potential applications. Future research could explore other thermally induced topochemical reactions and different monomer designs to further expand the diversity and functionalities of 2D-Ps.

The insolubility of the reaction intermediates (oligomers/1D-P) prevented kinetic studies to further elucidate the mechanism of the polymerization. The exfoliation method using TFA is limited to acids, and the search for alternative, scalable exfoliation methods should be pursued for broader applications. The relatively small scale of the synthesis needs to be improved for mass production purposes. Further investigations could focus on exploring the applications of these exfoliated 2D-P sheets in various fields.