Well-defined sequence, wholly aromatic copolyester/esteramides have been synthesized by an interfacial polycondensation of a preformed triad diacid chloride monomer, di(para-chlorocarbonyl)isophthalate, with the disodium salt of hydroquinone or of deuterated hydroquinone, or with meta-phenylene diamine. Carbon-13 nuclear magnetic resonance (NMR) spectroscopy provided unequivocal evidence for regularly alternating polymer structures, and coupled with statistical methods, for randomization on heating in the case of regularly alternating poly(p-oxybenzoate-co-p-phenyleneisophthalate). Polymers synthesized using standard high-temperature melt-polymerization techniques were shown by statistical/carbon-13 NMR methods to have essentially random structures. Thermal properties of copolymer powders and films were found to vary with monomer sequence distribution as well as with composition.