Carbon nanotubes (CNTs) were produced by pyrolyzing polypropylene (PP) in the presence of iron nanoparticles in a vacuum. Competitive processes of the growth of the CNTs and the coalescence of the iron catalysts governed morphologies of the pyrolyzed products. Multi-walled carbon nanotubes (MWCNTs) of about 16.5-40 nm in diameter were produced at 700 degrees C. Growth of the CNT was hindered at 900 degrees C when the iron particulates became larger than 100 nm. (c) 2005 Elsevier B.V. All rights reserved.