The reactions of urea with 1,6-hexanediamine at 150 degrees C in a polyol continuous phase in the presence of a stabilizer for the resultant particles, produce low-molecular-weight oligomers of urea-terminated poly(1,6-hexamethyleneurea) having a M(n) in the range of 500-700 g mol(-1). This is thought to involve a polymerization/precipitation mechanism in which the molecular units are held together by hydrogen bonding into a macrostructure which separates as a stable dispersed phase. This macrostructure has a spiral fibre bundle morphology, a particle size distribution of about 1 to 10 mu m, and aspect ratios from about 6 to 20. The urea-terminated poly(1,6-hexamethyleneurea) particles are highly crystalline thermoplastics with a melting point of similar to 270 degrees C and are only soluble in strong acids, where the macroparticles dissociate into their molecular units. High-molecular-weight poly(1,6-hexamethyleneurea) has been synthesized in bulk by other methods and is reported to have a melting point in the range 270-300 degrees C. Urea-terminated poly(1,6-hexamethyleneurea) oligomers having a spiral fibre bundle morphology have not been previously reported. These materials are useful as filled polyols for polyurethanes.