The synthesis of nanomaterials with well-defined size and shape is one of the cornerstones of nanotechnology. The step from the bulk material to nanoscale is important for a wide range of applications, which is either due to size-dependent properties, large surface areas or a better processablity. 1-3 A well-known example for size and shape dependent properties is the plasmon resonance of nanoparticles of materials with delocalized electronic states. In case of catalytically active particles, size reduction ensures large surface areas and by this high catalytic activity. Besides the established effects of size, examples for a shape dependent activity are also known, e.g. due to different number of under-coordinated atoms depending on the shape of Pt nanoparticles or quantum confinement effects in heterostructures. Another notable example for shape-dependent properties are calciumalginate hydrogels containing polylactide nanoparticles. Here the shape of the nanoparticles has a significant impact on the final adhesive and mechanical properties of the composite material. For functional materials, nanosized particles are often necessary for an implementation of the material into devices. With regard to application, spin crossover (SCO) complexes are a highly promising family of functional materials because of their unique switching properties by different external stimuli (e.g. temperature, pressure or light irradiation) between a high-spin (HS) and a low-spin (LS) state. Switching between these two states changes physical properties like magnetism, structure, or color. This offers interesting applications, e.g. as sensors, display devices and data storage, contrast agents or molecular actuators. Here, it is a major obstacle of current research to maintain bulk properties down to very small particle sizes. Scheme 1. Top: Structures obtained by self-assembly of diblock copolymers as function of the volume fraction of the two polymer blocks in a selective solvent for the green block. Bottom: Desired nanocomposite particle structures produced by self-assembly of PS-b-P4VP dBCPs in the presence of SCO complexes in THF as selective solvent, including the used abbreviations As the SCO properties strongly depend on the crystal packing, typically an increasing residual HS fraction, a decreasing hysteresis width, and a shift of the transition temperature to lower temperatures is observed for decreasing particle sizes. However, next to size also the shape of particles can be important either for the resulting spin crossover properties, or for the implementation into devices. For applications as molecular actuators, for example, nanosized rod-like structures are desirable to realize an anisotropic volume expansion. To date, it is still challenging to synthesize nanoparticles or nanocomposites with shapes different from those observed for the bulk material or spherical ones. This applies in particular for nanoparticles of coordination polymers or networks. Here, the inverse micelle technique or fast precipitation methods are the most established methods that usually yield spherical nanoparticles or nano-crystallites with shapes close to those observed for the bulk material. The aim of the work presented here is to employ diblock copolymer (dBCP) micelles, whose shape and size can be easily tailored by composition and overall molecular weight, as nanoreactors for the synthesis of spin crossover coordination polymer nanocomposites. As illustrated in the top of Scheme1, the volume fraction of the two polymer blocks of dBCPs defines the micelle morphology obtained by self-assembly in a selective solvent for one of the blocks. This enables their use as micro-or nanoreactors for the synthesis of nanoparticles. Moreover, the formation of polymer-encapsulated composite materials promises simple processing by established methods like electrospinning for the formation of fibers and fiber mats or spin-coating and drop-casting for surface coating. In this context, recent studies of SCO nanomaterials by us and others have shown that the introduction of coating and interfaces influences the SCO behavior and conversely, how this interaction can be exploited to trigger new functionalities. Recently we showed the potential of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles as nanoreactors for the synthesis of spherical coordination polymer or networks nanoparticles. Here we address, if a systematic increase in the P4VP fraction can be used to control not only the size, but also the morphology of the obtained nanocomposites. Furthermore, the encapsulation of the coordination polymer could further influence the self-assembly properties of the dBCP in solution (Scheme 1, bottom). As a result of this approach, we herein report for the first time not only a very precise size control, but also shape control for such CP-dBCP nanocomposites based on spin crossover complexes.