Five different dBCPs with a varying P4VP content (SV-15 to SV-61) have been prepared. The TEM and cryo-TEM images of the neat dBCPs showed spherical micelles with core diameters Dcore of 15 nm ± 2 nm (SV-15) to 73 nm ± 9 nm (SV-61) and Dcryo of 26 nm ± 2 nm (SV-15) to 71 nm ± 4 nm (SV-61). These polymers were utilized as nanoreactors in the synthesis of iron(II)-based CP NPs with well-defined size, shape and SCO properties. The nanocomposite particles based on the dBCPs SV-15 to SV-42 were also spherical and the size was similar to the neat dBCP micelles, underlining the templating effect of the dBCPs. Interestingly, the morphology of the nanocomposite particles changed to worm-like structures for the nanocomposite particles synthesized with SV-61 and the particle core widths were determined by TEM (Dcore = 60 nm ± 8 nm) and cryo-TEM (Dcryo = 84 nm ± 10 nm), respectively. These results indicate that the introduction of the CP induced a morphological change by increasing the fraction of the insoluble parts (P4VP cores with incorporated CP) of the nanocomposite particle. Additionally, it was shown that the magnetic properties of the SCO CP-dBCP nanocomposites can be altered by several factors to approximate a bulk-like behavior: (1) The reduction of the amount of the bridging ligand bipy from 2.5 eq to 1.5 eq. This resulted in a lower residual HS fraction γHS in all samples. (2) The enlargement of the spherical particle cores from 14 nm to 58 nm also resulted in a decrease of γHS from 30 % (1e) to 21 % (4e). ( )