Massive Molecular Motion in Crystal Lattice Leads to an Unexpected Product in a Topochemical Polymerization

We synthesized a designed proline-derived monomer decorated with azide and alkene for topochemical ene-azide cycloaddition (TEAC) polymerization. The monomer, in its crystal, assembles as supramolecular helices along the ‘a’ axis by exploiting various non-covalent interactions. Along the ‘c’ axis, the molecules are head-to-tail arranged in a wave-like topology such that the azide and alkene of adjacent molecules are proximally and anti-parallelly organized, obeying Schmidt’s criteria for topochemical reaction. This ready-to-react arrangement, resembling the transition-state arrangement for their cycloaddition, is expected to facilitate a smooth topochemical polymerization forming 1,4-triazoline-linked polymer along the ‘c’ direction. Upon heating, the monomer underwent regio- and stereospecific TEAC polymerization in a single-crystal-to-single-crystal fashion, as evidenced by SCXRD analysis. Surprisingly, it produced an unexpected 1,5-disubstituted-triazoline-linked covalent helical polymer along the ‘a’ axis rather than the expected 1,4-disubstituted-triazoline-linked wave-like polymer along ‘c’ axis. The polymerization linked the monomer molecules within the supramolecular helix via the cycloaddition between azide and alkene groups that are neither proximal nor in a suitable orientation. Interestingly, the crystal avoided a ready-to-react arrangement and chose an unexpected path involving a massive rotation of 134o of the alkene group, leading to a transient but new reactive arrangement followed by TEAC polymerization. This is the first regiospecific ene-azide cycloaddition reaction that yielded the 1,5-disubstituted product. This study cautions that the use of topochemical postulates for the prediction of reactivity can sometimes be misleading.