Long-Spaced Polyamides: Elucidating the Gap between Polyethylene Crystallinity and Hydrogen Bonding

Aliphatic polyamides with so far inaccessibly low amide contents were prepared by acyclic diene metathesis (ADMET) copolymerization of N-(undec-10-en-1-yl)undec-10-enamide (1) and undeca-1,10-diene (2) applying different Grubbs and Hoveyda–Grubbs type olefin metathesis catalyst precursors, followed by exhaustive postpolymerization hydrogenation to yield saturated copolymers. These polyamides, containing between 1.0 and ca. 50.5 amide groups per 1000 methylene units, fill the gap between polyamides from classical polymerization approaches, like polycondensation of diamines with diacids, and linear polyethylene. With reduced amide concentrations the melting points of polyamides converge toward polyethylene, passing through a distinct melting point minimum observed around 110 °C for polyamides with ca. 35 amide groups per 1000 methylene units. The minimum goes in hand with a change in the crystal structure related to the different ratios of intersegment interactions from hydrogen bonding and nonpolar van der Waals forces depending on the amide group content in the crystalline state. Furthermore, the influence of hydrogen bonds between amide and ester groups has been quantified for polyesteramides with various amide/ester ratios, prepared by ADMET copolymerization of N-(undec-10-en-1-yl)undec-10-enamide (1) with undec-10-en-1-yl undec-10-enoate (3) and postpolymerization hydrogenation.