Telechelic copolymers of styrene and different N-substituted-maleimides (SMIs) with a molecular weight of 2000-8000 g/mol were synthesized using the starved-feed-reactor technique and were nearly bifunctional when the monomer feed had a high styrene concentration. The COOH-terminated rigid SMI blocks were poly-condensated with OH-terminated poly(tetrahydrofuran) (PTHF) blocks, with a molecular weight of 250-1000 g/mol,which are the flexible parts in the generated homogeneous multiblock copolymer. The entanglement density, which is closely related to the toughness of materials, increased in these flexible SMI copolymers (nu(e) = 5.2 . 10(25) m(-3)) compared to the unflexibilized ones (nu(e) = 2.4 . 10(25) m(-3)). The glass transition temperature of these flexibilized, single-phase multiblock copolymers was still high enough to qualify them as engineering plastics.