An efficient approach for the synthesis of block copolymers of poly(acrylonitrile-co-butadiene) (NBR) and poly(styrene-co-acrylonitrile) (SAN) is described. Conjugation of preformed polymer building blocks is achieved via a hetero-Diels-Alder (HDA) mechanism employing cyclopentadiene-capped NBRs with dienophile SAN copolymers, both synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The protocol is further extended toward the synthesis of 4-miktoarm star polymers, consisting of two NBR and two SAN arms. Molar masses of the obtained complex macromolecular architectures range from below 10 000 g.mol(-1) up to 110 000 g.mol(-1) with dispersities below 1.5. Molecular verification of the coupling moieties is provided via NMR spectroscopy as well as ESI mass spectrometry. Size exclusion chromatography (SEC) traces of the obtained block copolymers and miktoarm star polymers were analyzed via deconvolution techniques, revealing the presence of 9.9-12.6 wt % (block copolymers) and 20 wt % (stars) of polymer chains not participating in the HDA conjugation, respectively. The residual polymers were analyzed toward their origin from either the loss of functionality during RAFT polymerization or incomplete conversion during the conjugation process. The comprehensive analysis of the macromolecular material was underpinned by kinetic simulations to estimate the fractions of nonfunctional polymer chains generated during the NBR and SAN polymerizations. The simulations evidenced that NBR-b-SAN samples cannot contain more than 94.4 wt % (M-n 13 000 g.mol(-1)), 93.6 wt % (M-n 57 000 g.mol(-1)), or 93.9 wt % (M-n 110 000 g.mol(-1)) of polymer chains actually possessing the targeted block copolymer structures when assuming an ideal RAFT process. These results unambiguously reveal that nonfunctionalized polymer chains formed during RAFT polymerization cause the incomplete conjugation of polymer building blocks, evidencing the limitations of end-group control in controlled/living radical polymerizations.