AbstractA series of castor‐oil‐modified hyperbranched polyurethanes were synthesized via an A2 + B3 approach based on castor oil, macroglycol poly(ϵ‐caprolactone)diol or poly(ethylene glycol) (PEG), and diphenyl methane diisocyanate, with or without a chain extender. The yield of the product was 95%, and the polymers were soluble in common organic polar solvents. The synthesized polymers were characterized with Fourier transform infrared spectroscopy, NMR, wide‐angle X‐ray diffraction, and gel permeation chromatography measurements. The degree of branching of the polymers was calculated from their respective 1H‐NMR spectra with the help of model compounds and found to be varied from 0.7 to 0.8. The hard‐segment content, the nature of the macroglycol, and the composition of the polymers had prominent effects on the thermal degradation and crystallization of the hyperbranched polymers. The crystallization of poly(ϵ‐caprolactone) and PEG as soft segments occurred well in the hyperbranched polymers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009