Iterative Exponential Growth of Oxygen-Linked Aromatic Polymers Driven by Nucleophilic Aromatic Substitution Reactions.

Tyler J Jaynes,Jessica Bocanegra,Jianing Li,Mona Sharafi,Joseph P Campbell,Severin T Schneebeli,Danielle L Gray,Toby J Woods,Kyle T Mckay,Kassondra Little,Reilly Osadchey Brown

doi:10.3389/fchem.2021.620017

Abstract

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 (SNAr) reactions. 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. 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 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.

Highlights

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
We discovered that simple methyl sulfones (Figure 1) are best suited for this purpose; as with sulfone substituents larger than methyl, we tended to get lower iterative exponential growth (IEG) coupling yields
We decided to protect the phenols with methyl groups

Summary

Introduction

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. 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) This methodology (Figure 1) is generally referred to as iterative exponential growth (IEG) (Barnes et al, 2015; Leibfarth et al, 2015). The IEG coupling reactions proceeded under mild conditions (mild heating to ∼50°C), which represents an advantage over transition metal–catalyzed alternatives [which tend to require stronger heating (Sadighi et al, 1998)]

Methods

Results

Conclusion