Copolymerization of styrene (S) and diethyl fumarate (DEF) at 125degreesC in the presence of 2,2,6,6-tetramethylpiperidin-1-yloxyl radical (TEMPO) and initiated with a thermal initiator, 2,2'-azobisisobutyronitrile (AIBN), was studied. The molar fraction of DEF in the feed, F-DEF varied within 0.1-0.9. An azeotropic composition, (F-DEF)(A) = 0.38, was found for the copolymerization under study. At F-DEF = 0.1-0.4, a quasi-living process was observed, transforming to a retarded conventional radical copolymerization at a higher content of DEF in the initial mixtures. The obtained TEMPO-terminated S-DEF copolymers were used to initiate polymerization of styrene. Poly(styrene-co-diethyl fumarate)-block-polystyrene copolymers were prepared with molecular weight distributions depending on the amount of inactive polymer chains in macroinitiators, as indicated by size-exclusion chromatography. A limited miscibility of the blocks in the synthesized block copolymers was revealed by using differential scanning calorimetry.