Highly stereoregulated syndiotactic-polystyrene/polyethylene (s-PS/PE) block copolymers with variable ethylene block lengths are synthesized using the sequential monomer feed scheme with C5H5TiCl3 catalyst. The copolymer, after removing the atactic portion, is separated into two fractions by tetrahydrofuran (THF). The insoluble fraction contains a highly stereoregulated s-PS block linked with PE blocks, while the soluble fraction also contains a highly stereoregulated s-PS block but separated randomly by a shorter or isolated ethylene unit. For the I-fraction, r(E) = 25 ± 5 and r(S)= 1.9 ± 0.3, and for the S-fraction r(E) = 0.4 ± 0.1 and r(E) = 0.34 ± 0.15. These results are compared with prior studies, where mainly random type PS/PE copolymers with low styrene stereoregularity and rigio-irregular styrene and polyethylene are produced. Apparently, both the sequential feed scheme coupled with the use of a lower reactivity metallocene catalyst facilitated the switching of catalytical sites with the majority product being of block nature. (C) 2000 Elsevier Science Ltd.