Semiaromatic Polyamides with Re-Entrant Chain Folding Templated by “U-Turn” Repeat Units

Biopolymers often show deterministic chain folding templated by repeat unit sequences whose conformations are locked in by intramolecular interactions. In this work, we have used repeat units based on rigid anthracene or acridine scaffolds, which we refer to as “U-turn” repeat units, to template chain folding reminiscent of β-serpentine folds in bulk synthetic polyamides. Unlike the rigid kinked repeat units present in certain commercial semiaromatic polyamides, the acridine-based U-turn repeat units did not impede crystallization in the polyamides, whose crystalline phase was shown to be stabilized by infinite arrays of intermolecular hydrogen bonds and π–π interactions between the acridine units. This led to a unique layered crystalline structure characterized by a well-defined lamellar thickness that depended on the length of the aliphatic spacers between the U-turn repeat units and regular adjacent re-entrant chain folding, which is not usually expected for semiaromatic polyamides crystallized at high supercooling. Our work hence provides the first example of the introduction of deterministic chain folding into a nonpeptidic polyamide in order to tailor its crystalline morphology and may consequently open up new perspectives for the molecular design of structural materials.

生物聚合物通常呈现由重复单元序列模板引导的确定性链折叠，其构象通过分子内相互作用得以锁定。本研究采用基于刚性蒽(anthracene)或吖啶(acridine)骨架的重复单元（我们将其称为"U型转角"重复单元(U-turn repeat units)），用于模板引导本体合成聚酰胺中类似β-蛇形折叠(β-serpentine folds)的链折叠行为。与某些商用半芳香聚酰胺(semiaromatic polyamides)中存在的刚性弯折重复单元不同，基于吖啶的"U型转角"重复单元并未阻碍聚酰胺的结晶过程；研究证实，该聚酰胺的晶相可通过吖啶单元间的无限分子间氢键(intermolecular hydrogen bonds)阵列与π-π堆积作用(π–π interactions)实现稳定。这一设计催生了一种独特的层状晶体结构，其具有明确的层状厚度(lamellar thickness)，该厚度取决于"U型转角"重复单元与规则相邻折返链折叠(re-entrant chain folding)之间的脂肪族间隔基(aliphatic spacers)长度；而在高过冷度(supercooling)下结晶的半芳香聚酰胺中，通常不会出现此类折叠模式。因此，本研究首次实现了将确定性链折叠引入非肽聚酰胺(nonpeptidic polyamide)体系，以精准调控其晶体形貌，有望为结构材料的分子设计开辟全新研究视角。