Abstract: A process for preparing high molecular weight aryl alkyl polymers is disclosed, wherein oxalic acid and paraphenylene diisocyanate are reacted in substantially stoichiometric amounts in a polar, aprotic anhydrous organic solvent at a temperature which is raised progressively at a rate of about 1 to about 5.degree.C per minute from a lower temperature of about 0.degree. to 30.degree. C to a higher temperature of about 80.degree. to about 150.degree.C in the presence of, at least when the viscosity of the reaction mixture exceeds 6,000 poises, a viscosity reducing amount of a hydrogen bond breaking agent, such as lithium chloride.The resulting polymers can be formed into filaments, fibers and films, and the filaments exhibit improved tensile strengths and modulus of elasticity as compared to similar filaments of other arylalkyl polymers based on oxalic acid and other aromatic diisocyanates.