This article describes the synthesis and molecular characterization of thermal polymeric iniferters, based on hydroxy-terminated poly(tetrahydrofuran) (PTHF), bearing thiuram disulfide groups along the chain. Thermal polymerization after the addition of styrene (S) and N-methylmaleimide (MI) to these PTHF-based polymeric iniferters yielded segmented PTHF (SMI-block-PTHF)(n) block copolymers that proved to have a single T-g. The multiblock copolymers were molecularly characterized by elemental analysis, IR, and NMR. The thermal stability, as checked by thermogravimetric analysis, proved to be good up to about 350 degrees C. A size exclusion chromatography/differential viscosity (DV) analysis showed that the molecular weights of the synthesized single-phase multiblock copolymers were sufficiently high (several times the estimated molecular weight between two adjacent entanglements) to determine the entanglement density from the rubbery plateau modulus, for which the method developed by S. Wu (J Polym Sci Part B: Polym Phys 1989, 27, 723-741) was applied. The entanglement density of flexibilized SMI proved to be about 20-25% higher than that of the nonflexibilized SMI. This increase is disappointing, and more work, based on the described concept, is required to achieve the desired enhancement of the toughness.