Aromatic donor-acceptor charge-transfer and metal-ion-complexation-assisted folding of a synthetic polymer.

Synthetic polymers typically adopt a random-coil conformation in solution primarily driven by the tendency to maximize entropy. Generation of well-defined secondary structures in synthetic polymers, therefore, will need to include specific intrachain intersegment interactions that will provide adequate enthalpic contribution to overcome this entropic penalty associated with the formation of well-ordered conformations. The motivation to generate synthetic polymers with a precisely controlled conformation stems from the desire to emulate the elegance of nature on the one hand, while at the same time from the expectation that this will enable one to bridge the gap between the molecular and the macroscopic scales in terms of the evolution of structural order. The term “foldamer” is used to describe synthetic polymers that can be made to adopt well-defined conformations in solution, such as a helix, by inclusion of specific intrachain interactions, such as hydrogen bonding, steric and/ or bond-angle constraints along the backbone, solvophobic interactions, etc. Although several examples of synthetic polymers that adopt helical conformations in solutions are known, for example, poly(trityl methacrylate)s, polyisocyanates, cis-poly(acetylene)s, polyisocyanides, and poly(1,3-phenylene ethynylene)s, most of these deal with relatively stiff chains that have very limited conformational degrees of freedom as a result of steric and/or bond-angle constraints. There are, however, only very few examples of relatively flexible systems that have been made to adopt a well-defined conformation in solution. Iverson and co-workers described one such system in a series of elegant papers in which they demonstrated the possibility of forming folded structures in specifically designed oligomers by virtue of specific intrachain aromatic donor–acceptor charge-transfer interactions aided by solvophobic effects. Thus far, most studies on flexible foldamers have dealt only with precisely defined oligomers, and to the best our knowledge there are no reports on analogous polymeric systems. Thus, in an effort to explore directed folding in truly flexible synthetic polymers, we designed and synthesized a segmented donor– acceptor polyimide 3, which contains three design elements