Helices represent a sophisticated structural motif that has been pursued for decades through the development of foldamers (oligomers), small molecule assembly, and large polymers. Recently, we reported an unexpected helical covalent polymer (HCP), which formed mechanically entwined chiral 1D polymers from achiral building blocks through the dynamic spiroborate linkage. However, many questions remained about the evolution of the chirality, the driving force, and whether it would be possible to achieve an open framework. In this work, we explored the mechanism of the HCP polymerization and found a metastable 3D covalent organic framework with srs topology (srs-COF) as an intermediate only present in the early stage of the polymerization.
By varying the reaction parameters, HCP was determined the more thermodynamically favorable topology, and the srs-COF can be isolated as kinetically trapped species. The possible srs topologies were explored by considering the chirality of the framework and the interpenetration to understand the evolution of the chirality observed in the HCP crystal structure. We found the diffraction pattern of the srs-COF matches a racemic two-fold interpenetrated framework. The proposed mechanism for the transformation involves the srs-COF pre-organizing the helices and supplying helical oligomers that will self-sort into the HCP crystal structure. This work provides an intriguing example where the COF species is not the most thermodynamically favorable species and illuminates how incorporating weak, non-covalent bonds can be more favorable. The strategy of incorporating weak non-covalent bonds in the formation of polymeric structures is relatively unexplored and will undoubtedly lead to the formation of intricate architectures with novel functionality.

Illustration of <b>srs</b>-COF and <b>HCP</b> formation utilizing the dynamic spiroborate linkage. <b>srs</b>-COF is a racemic framework connected with purely covalent bonds, whereas <b>HCP</b> is composed of 1D helical polymer chains with single-handedness (in each crystalline domain) that are assembled through hydrogen bonding interactions. The red and blue/light blue indicates opposite handedness and guest molecules are omitted from the structures for clarity.

Illustration of srs-COF and HCP formation utilizing the dynamic spiroborate linkage. srs-COF is a racemic framework connected with purely covalent bonds, whereas HCP is composed of 1D helical polymer chains with single-handedness (in each crystalline domain) that are assembled through hydrogen bonding interactions. The red and blue/light blue indicates opposite handedness and guest molecules are omitted from the structures for clarity.

Yiming Hu

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