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

The study addresses the challenge of synthesizing flawless 2D polymers (2D-Ps) with controlled growth in two orthogonal directions. Existing approaches, especially on-surface/interfacial and photochemical topochemical polymerizations, face hurdles including scalability, isolation, and structural elucidation; photogenerated linkages can depolymerize upon heating, limiting high-temperature applications. The authors aim to demonstrate a thermally driven topochemical route—azide–alkyne cycloaddition (TAAC)—that forms robust, thermally stable triazole linkages, enabling hierarchical single-crystal-to-single-crystal (SCSC) transformations from a monomer to a 1D polymer and then to a 2D polymer, with regiospecific linkages along orthogonal axes. They also seek architectural diversity beyond common tripodal C3-symmetric monomers by employing an X-shaped benzene-core monomer bearing two azides and two alkynes.

Prior work has shown that topochemical polymerization can yield highly ordered 2D-Ps resolvable by single-crystal X-ray diffraction (SCXRD). Pioneering studies by King, Sakamoto, and Schluter used photo-induced [4+4] anthracene and [2+2] cycloadditions to achieve SCSC 2D-Ps, enabling mechanistic insights into 2D growth. However, photochemical approaches require stringent ready-to-react packing and often produce thermally labile cycloadducts prone to retro-cycloaddition, hindering high-temperature utility. Covalent organic frameworks (COFs) have advanced 2D materials but dynamic covalent chemistry can introduce topological defects. Heat-induced TAAC has been successful for 1D polymers with less stringent geometric requirements due to thermal motion and often proceeds SCSC. The authors build on this by exploring TAAC for 2D polymerization, aiming for robust, thermally stable linkages and alternative monomer architectures to access non-hexagonal pores.

- Monomer design and crystallization: An X-shaped benzene-core monomer (M) with two azide and two alkyne groups was synthesized and crystallized from methanol–isopropanol (10:1 v/v) or chloroform–isopropanol (2:1 v/v) by slow evaporation to yield plate-like single crystals. - Structural characterization of M: SCXRD revealed triclinic P-1 with two half-molecules in the asymmetric unit. Intramolecular N–H...O and C–H...O H-bonds lock arm conformations. Columns along a are formed by π...π interactions and H-bonding, arranging azide/alkyne pairs in reactive geometries along a for TAAC to give 1,5-triazolyl-linked 1D chains; along b and c, packing is less reactive, but thermal motion could enable reaction along b (or c in the 1D-P) to form 1,4-triazoles. - Thermal analysis: Differential scanning calorimetry (DSC) identified exotherms with onsets near 110 °C and 190 °C attributed to sequential TAAC reactions, and a high-temperature exotherm (~350 °C) due to decomposition. Thermogravimetric analysis (TGA) corroborated decomposition. FT-IR tracked azide stretching reduction upon partial (160 °C) and full (210 °C) conversion. Variable-temperature Raman (rt to 200 °C, 5 °C/min) monitored alkyne stretching at 2130 cm⁻1. - Controlled SCSC polymerizations: Crystals were heated at 2 °C/min to 160 °C (hold 10 min) to obtain 1D-P, then to 210 °C (hold 10 min) to obtain 2D-P, with cooling to room temperature after each. Polarized optical microscopy checked birefringence for crystallinity retention. - Structural characterization of 1D-P and 2D-P: SCXRD solved structures after each step, confirming SCSC transformations and regiospecific linkages (1,5 along a in 1D-P; additional 1,4 linkages along c in 2D-P). Unit cell parameter changes were quantified. Void analysis assessed rotational freedom enabling the second TAAC. - Kinetic profiling: Time-dependent DSC and PXRD at fixed temperatures quantified completion times: M→1D-P at 90–100 °C; 1D-P→2D-P at 140–150 °C. PXRD patterns were compared to simulated patterns from SCXRD models to confirm phase evolution. - Exfoliation: Crystal face indexing and BFDH morphology correlated sheet orientation to (010)/(0-10) faces (sheets parallel to ac). Solvent exfoliation was tested: NMP (stirring/sonication), and common solvents (DMSO, DMF, chloroform, acetone, iPrOH, EtOH, MeOH) showed no delamination by PXRD. Strong acids (TFA, 1 N HCl) induced rapid swelling and exfoliation into thin sheets, attributed to triazole protonation and electrostatic repulsion. Exfoliates were analyzed by optical microscopy, SEM, AFM (thickness ~1–4 nm), and TEM. - Methods specifics: Polymerization protocols as above; exfoliation involved sonication of 2D-P in TFA (5 min for SEM; 1 h for AFM/TEM), centrifugation, dilution, and drop-casting on Si wafers or TEM grids.

- Hierarchical SCSC polymerization: The designed monomer (M) undergoes two regiospecific, orthogonal TAAC reactions in crystals: • First step (M→1D-P): SCSC TAAC along a forms 1,5-triazolyl-linked 1D polymer chains. Evidence: disappearance of first DSC exotherm after heating to 160 °C; FT-IR shows reduced azide stretch; SCXRD confirms 1D-P retaining P-1 and zigzag ladder-like chains stabilized by intramolecular N–H...O H-bonds. Unit cell: a contracts by 1.6%, c expands by 0.8%, b ~unchanged. • Second step (1D-P→2D-P): Further SCSC TAAC links chains along c via 1,4-triazoles to yield 2D-P. Evidence: second DSC exotherm (~190 °C onset); SCXRD confirms 2D sheets with P-1 symmetry; unit cell parameters change significantly (c −11.8%, b −10.7%, a +8.6%). - Regiospecific orthogonal linkages: 1,5-triazoles propagate along a; 1,4-triazoles along c, producing an architecturally distinct 2D network compared to hexagonal, tripodal systems. - Thermal behavior: Monomer crystals do not melt up to 300 °C; decomposition occurs near 350 °C (exotherm). The TAAC-formed triazole linkages are thermally robust (no retro-cycloaddition), unlike many photopolymers. - Kinetics (time-dependent DSC/PXRD): M→1D-P completes in ~5 h at 90 °C and ~2 h at 100 °C. 1D-P→2D-P completes in ~4 h at 140 °C and ~2 h at 150 °C. Variable-T Raman shows the alkyne stretch at 2130 cm⁻1 vanishes by 200 °C, indicating complete conversion. - Mechanistic insight: Unlike previously proposed simultaneous 2D growth mechanisms (chain-growth, step-growth, or random), this system exhibits discrete, sequential 1D-like cooperative growth along one axis at a time, with no coexistence of monomer and 2D-P. - Exfoliation: Strong acids (TFA, 1 N HCl) rapidly exfoliate 3D crystals into thin 2D sheets from the (010)/(0-10) faces; typical AFM thicknesses ~1–4 nm (single layer ~1 nm from crystal structure), preserving birefringence. Common solvents and NMP fail to exfoliate, even with prolonged soaking/sonication. - Structural features: In 1D-P, voids near unreacted azide/alkyne enable rotation (azide ~162°, alkyne ~96°) to reach antiparallel, reactive geometries (end-to-end ~3.8–3.9 Å) for the second TAAC. - Crystallinity retained: Both polymerization steps proceed SCSC, preserving single-crystal integrity, enabling atomic-resolution structures of 1D-P and 2D-P.

The findings establish a thermally driven, hierarchical route to crystalline 2D polymers via sequential SCSC TAAC reactions. This approach overcomes key limitations of photochemical topochemical 2D polymerizations—namely stringent packing requirements and thermal lability—by leveraging thermal motion and forming robust triazole linkages. The regiospecific formation of 1,5- and 1,4-triazoles along orthogonal axes demonstrates precise control over network connectivity, delivering an architecturally distinct 2D-P beyond the conventional tripodal/hexagonal motif. Mechanistically, the two-step sequence deviates from established simultaneous 2D growth models, resembling cooperative 1D topochemical polymerization along one axis at a time and avoiding coexistence of monomer with 2D-P. The ability to exfoliate the 3D crystals into crystalline 2D sheets using acid-mediated delamination broadens potential applications by providing accessible 2D materials while maintaining structural order. The work highlights the broader scope of topochemical reactions for 2D polymer synthesis and suggests a pathway to tune pore size and topology via monomer architecture and linkage regiospecificity.

This work demonstrates a thermally induced, sequential SCSC topochemical polymerization that converts a purpose-designed X-shaped azide–alkyne monomer into a 1D polymer and subsequently into a 2D polymer with orthogonal, regiospecific triazole linkages (1,5 along a and 1,4 along c). The polymers are structurally elucidated at atomic resolution due to preserved crystallinity. The resulting 2D-P is thermally robust and can be exfoliated into thin, crystalline sheets using strong acids such as TFA and HCl. The study expands the toolbox for 2D polymer synthesis beyond photochemical cycloadditions and common tripodal architectures, enabling control over topology and pore size via monomer design and linkage chemistry. Future research can explore diverse monomer geometries, alternative thermally driven topochemical reactions, structure–property relationships of the resulting sheets, and application-driven functionalization, as well as refined mechanistic/kinetic studies where solubility permits.

- The reaction mixtures (oligomers/1D-P/2D-P) are insoluble, precluding detailed kinetic studies in solution and limiting direct mechanistic quantification. - Exfoliation required strong acids (TFA or HCl); common solvents and NMP were ineffective even with prolonged treatment, which may constrain processing conditions or substrate compatibility. - While thermal motion facilitates the second TAAC step, the necessity for specific rotational freedom and packing could limit generality across other monomer/crystal systems.