Water-Soluble Soft Nylons: Novel Class of Soft Matter Exhibiting LCST-type Thermo-Responsiveness

High crystallinity and low solubility are inherent characteristics of polyamides. Research efforts have primarily focused on exploring their bulk properties as fibers, with less emphasis on their solution properties. Herein, we report on the synthesis and properties of soft and soluble N-methylated nylons as a novel class of soft polymer matter. Surprisingly, N-methylated nylons have scarcely been reported and are almost not registered with CAS SciFindern, while their unit structure is very simple. The N-methylation of nylons resulted in softness and high solubility by eliminating hydrogen bonds between polymer backbones and generating two conformers (cis and trans) of the tertiary amide group. Remarkably, some of them exhibit lower critical solution temperature (LCST)-type phase separation in water. By manipulating the carbon number within polymer backbones, we have found that the quantitative hydrophilic/hydrophobic balance for LCST is its unit formula per amide group equal to C6H11NO, which aligns with typical LCST polymers like poly(N-isopropylacrylamide) (PNIPAM). Despite the structural similarity to PNIPAM, polymer scientists have never found LCST-type behavior of N-methylated nylons during over 50 years of soft polymer matter research. The long-standing assumption that polyamides are inherently rigid and cannot exhibit softness and solubility has impeded their application to soft polymer matter.