The radical homopolymerization of styrene or copolymerization of styrene (S) with N-butyl maleimide (I) initiated by tetraethylthiuram disulfide was used to prepare macroinitiators having thiyl end groups. The S-I copolymers from the feeds containing 30-70 mol % I showed approximately alternating composition. The rate of copolymerization and molecular weights decreased with increasing maleimide derivative concentration in the feed; homopolymerization of I alone did not proceed. The macroinitiators served for synthesis of further S-I copolymers. Using polystyrene macroinitiator and the S-I copolymer with thiyl end groups in the polymerization of S-I mixture and styrene, respectively, the copolymers containing blocks of both polystyrene and alternating S-I copolymer were obtained. The copolymerization of S-I mixture initiated with the S-I copolymer bearing thiyl end groups led to the extension of macroinitiator chains by the blocks of alternating copolymer. The presence of the blocks in the polymer products was corroborated using elemental analysis, size exclusion chromatography, and differential scanning calorimetry.