Polyaramides from Oxacalix[2]arene[2]hetarene monomers

Three dicarboxylic acid-functional oxacalix[2]arene[2]hetarenes were synthesized from 3,5-dihydroxybenzoic acid and heteroaromatic dihalides for use as large, difunctional monomers. The oxacalixarene cyclic monomers were prepared by nucleophilic aromatic substitution and purified by recrystallization. The monomers were polymerized with oxydianiline using Higashi-Yamazaki phosphorylation to produce oxacalixarene polyaramides. The molecular weights were characterized by gel permeation chromatography to show large polydispersities that were attributed to substantial cyclic oligomer content. The low molecular weight cyclic byproducts were removed by Soxhlet extraction and their molecular weights and cyclic nature characterized by MALDI-TOF mass spectrometry. Experimental and computational evidence suggests a strong relationship between the shape of the macrocycle and intramolecular cyclization. Copolymerization with terephthalic acid resulted in decreased cyclic oligomer production. Mechanical properties of the polymers revealed Young's moduli in the range of 1.7–3.8 GPa, elongations from 0.6 to 16.2 %, and breaking strengths of 18.2–45.9 MPa. Homopolymers tended to display brittle fracture properties while copolymers exhibited mostly ductile fracture. The work is a rare example where macrocycles have been incorporated into linear polymer structures and the first known report of oxacalixarenes used in step-growth polycondensations.