High molecular weight polybenzoxazinones have been prepared by cyclo-polycondensation reaction of 4,4′-diamino-3,3′-biphenyldicarboxylic acid with a variety of aromatic carbonyl compounds using a solution polymerization technique in polyphosphoric acid. From the model reactions of anthranilic acid, and 4,4′-diamino-3,3′-biphenyldicarboxylic acid with benzoyl chloride in polyphosphoric acid, it is established that the cyclopolycondensation proceeds through the formation of an open-chain tractable precursor, polyamic acid of high molecular weight (ninh = 2.66) in the first step, which subsequently undergoes thermal or chemical cyclodehydration along the polymer chain, to yield, in the second step, a fully aromatic polybenz-oxazinone. Polybenzoxazinones thus obtained have excellent thermal stability both in nitrogen and in air.