While electron-deficient metal catalysts like Ziegler-Natta and metallocenes are able to polymerize olefins producing linear polyolefins, Ni-diimine catalysts are able to produce polymers of varying structures, ranging from irregular and amorphous to highly linear and crystalline, due to the chain walking mechanism. Our present goal is to study the effect of polymerization temperature on the structure and properties of poly(1-hexene)s and poly(1-decene)s prepared with the catalyst (iPr(2)Ph)(2)BIAN center dot Ni(NCS)(2), where BIAN is the bis(imino)acenaphthene core. The polymers were studied by H-1 and C-13 NMR, SEC and DSC. The results show off important crystallinity increase for samples prepared at high temperature, above 40 degrees C. These results are consistent with a 1,omega insertion mechanism. The latter is also corroborated by NMR study, which shows polymers with a low branch number, and isolated methyl branches predominating for high-temperature samples. Polymerization kinetic studies performed by dilatometry suggest a living character for reactions performed close to 0 degrees C and quick catalyst deactivation at 50 degrees C and above. (c) 2007 Elsevier B.V. All rights reserved.