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SYNOPSISWell-defined sequence, wholly aromatic copolyester/esteramides have been synthesized by an interfacial polycondensation of a preformed triad diacid chloride monomer, di (parachlorocarbonylphenyl) 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 -0xybenzoate-co-p -phenylene isophthalate ) . 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.