The morphology development of ethylene copolymers was modeled with the modified phase-field theory. The metastability of polymer crystallization was also considered in the modeling. Modeling and experimental work were simultaneously undertaken to compare the crystallization kinetics of single-site-catalyzed and Ziegler-catalyzed resins and their influence on morphology development in the rotational-molding process. With a more uniform short-chain branch distribution, the single-site copolymers developed well-defined spherulitic structures. The Ziegler-Natta catalyst resins were characterized by a higher nuclei density and a faster crystallization rate and produced finer structures in the molded parts. The modeling approach proposed in our work allowed an evaluation of the processing and the material effects on the development of morphological features during melt solidification. (C) 2007 Wiley Periodicals, Inc.