This methodology ( Figure 1) is generally referred to as iterative exponential growth (IEG) ( Barnes et al., 2015 Leibfarth et al., 2015). One of the most efficient ways to precisely control the length and sequence of synthetic polymers is by iteratively coupling ( Jones et al., 1997) polymer strands together in a convergent/divergent fashion ( Hawker et al., 1997 Read et al., 2000 Grayson and Frechet, 2001 Li et al., 2005 Liess et al., 2006 Binauld et al., 2011). However, while the precise chemical structures, lengths, and sequences of such macromolecules ( Dobscha et al., 2019 Gerthoffer et al., 2020 Zhao et al., 2020) dictate their folding, and with it their functionalities and physical properties ( Chen et al., 2015 Hanlon et al., 2017), it remains challenging to synthesize polyaromatic structures with precise lengths and/or sequences as unimolecular entities. This reduced amount of conformational freedom can help enhance the folding of aromatic polymers, to advance a variety of useful properties such as selective supramolecular recognition ( Goodman et al., 2007 Liu et al., 2015 Otte, 2016 Schneebeli et al., 2016 Adhikari et al., 2017 Xie et al., 2020), selective catalysis ( Rajappan et al., 2020 Sharafi et al., 2020), and self-assembly ( Cole et al., 2017 Greene et al., 2017 Bonduelle, 2018 Ong and Swager, 2018 Delawder et al., 2019 Zhao et al., 2019). The backbones of conjugated and heteroatom-linked aromatic polymers tend to possess fewer conformational degrees of freedom than polymers with more flexible aliphatic or partially aliphatic backbones. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules. As indicated by 1) 1H- 1H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C–H bonds of the electron-poor pyrimidine functions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (S NAr) reactions.
Clark X-Ray Facility and 3M Materials Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, United States 1Department of Chemistry, University of Vermont, Burlington, VT, United States.McKay 1, Kassondra Little 1, Reilly Osadchey Brown 1, Danielle L. Campbell 1 †, Jessica Bocanegra 1, Kyle T.
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