Chongqing Yang1,Rebecca Khoo1,2,He Li1,Christopher Anderson1,2,Christopher Jones3,Carolin Sutter-Fella1,Jian Zhang1,Hosea Nelson3,Yi Liu1
Lawrence Berkeley National Laboratory1,University of California, Berkeley2,California Institute of Technology3
Chongqing Yang1,Rebecca Khoo1,2,He Li1,Christopher Anderson1,2,Christopher Jones3,Carolin Sutter-Fella1,Jian Zhang1,Hosea Nelson3,Yi Liu1
Lawrence Berkeley National Laboratory1,University of California, Berkeley2,California Institute of Technology3
To address the negative environmental influence associated with the fabrication of synthetic polymers, the development of eco-friendly and sustainable chemistries has been crucial but challenging. Distinct from the conventional solution-based polymer growth, topochemical polymerization (TCP) offers a ‘green’ way for scalable fabrication of high fidelity polymers without the need of catalysts, auxiliaries, or solvents. With crystallinity embedded during the synthesis, it is also one of the few ways to produce single crystalline polymers suitable for atomic structure determination. However, only a handful of TCP reactions have been reported to date, and it is still challenging to expand new chemistries in this area, due to the difficulty in controlling the reactivity in crystalline solids. Moreover, most of the polymers from TCP reactions have low solubility which renders their characterization and application challenging.<br/>In this work, we present that an azaquinodimethane (AQM) family has shown great topochemical reactivity in the forms of single crystals, polycrystalline powders, and thin films under visible light irradiation or heat. The resultant crystalline polymer structures were successfully resolved by single crystal X-ray diffraction (for large crystals), or cryo-EM based microelectron diffraction with 1 Å resolution (for microcrystals). The high efficiency of solid-state polymerization was evidenced by the ultrahigh molecular weight (<i>M<sub>w</sub></i>>10<sup>6</sup> g/mol) of a soluble TCP polymer bearing pendent 3,5-dihexyloxyphenyl groups. Moreover, the solid-state reactivity of AQMs also shows remarkable functional group tolerance, allowing the introduction of different side groups without decreasing the polymerization efficiency. Notably, the intensity of the absorption peaks of monomers in the range of 400-500 nm undergoes a characteristic decrease upon polymerization. This spectroscopic change provides a distinctive optical feature to track the polymerization process via in-situ UV/vis characterization with a time resolution of 0.1 s. Kinetic studies of AQM polymerization showed a relatively small activation energy of ~19.7 kcal K<sup>-1</sup> mol<sup>-1</sup>,providing key quantitative information of the easily accessible and fast polymerization process. Further kinetic details were obtained from the temperature-dependent polymerization of AQMs bearing different conjugated aromatic units or chiral sidechains, allowing fundamental understanding of a structure-reactivity relationship of such AQMs. Such insight paves the way for the future design and synthesis of functional polymers in an environmentally friendly way.<br/><b>References</b><br/>Dou, L.; Zheng, Y.; Shen, X.; Wu, G.; Fields, K.; Hsu, W.-C.; Zhou, H.; Yang, Y.; Wudl, F., <i>Science</i> <b>2014</b>, 343, 272.<br/>Hema, K.; Ravi, A.; Raju, C.; Pathan, J. R.; Rai, R.; Sureshan, K. M., <i>Chem. Soc. Rev. </i><b>2021,</b> <i>50</i>, 4062.<br/>Anderson, C. L.; Li, H.; Jones, C. G.; Teat, S. J.; Settineri, N. S.; Dailing, E. A.; Liang, J.; Mao, H.; Yang, C.; Klivansky, L. M.; Li, X.; Reimer, J. A.; Nelson, H. M.; Liu, Y.,<i> Nat. Commun.</i> <b>2021</b>, 12, 6818.