Short chain branching (SCB) and methylene sequence length (MSL) distributions were measured by TREF and DSC coupled with successive nucleation/annealing (SNA) for a Ziegler-Natta acid a metallocene ethylene-butene copolymer. TREF analysis indicated that the copolymer made with Ziegler-Natta catalyst exhibited a broad bimodal SCB distribution, while the polymer made with the metallocene catalysts had a narrow SCB distribution. SNA-DSC analysis showed that the Ziegler-Natta copolymer had a broad MSL distribution with significant amount of long methylene sequences; the metallocene copolymer had a much narrower MSL distribution and contained a large amount of polymer with short methylene sequences. The melting and crystallization measurements on PTREF fractions of the two polymers showed that the melting temperature, crystallization temperature and enthalpy of fusion of the PTREF fractions for the Ziegler-Natta polymer decreased substantially with increasing SCB content, while these properties varied only slightly for the PTREF fractions of the metallocene polymer. This indicates that the SCB distribution has a more significant effect on melting and crystallization behaviors of polyethylene copolymers than the average SCB content. (C) 2001 Elsevier Science Ltd. All rights reserved.