Reactor blend formation of soluble highly isotactic polystyrene (iPS) enables tailoring of bimodal iPS molar mass distributions containing variable amounts of ultrahigh molar mass iPS (UHMWiPS). A key feature is the facile iPS molar mass control, achieved by homogeneous catalytic styrene polymerization on a MAO-activated titanium bisphenolate catalyst, using 1,9-decadiene as chain transfer agent. Whereas UHMWiPS (M-w of 947 000 g mol(-1)) is formed in the absence of the diene, the molar mass M-w increases from 191 000 to 482 000 g mol(-1) with decreasing diene/styrene molar ratio. In a cascade of two parallel reactors, polymerizing styrene in the presence and the absence of diene, the mixing ratio of the resulting two iPS solutions governs the UHMWiPS content of the reactor blends (RB-2). Hence, the contents of iPS and UHMWiPS are varied without affecting the average molar mass of both blend components. In reactor blends (RB-1), produced in a single reactor with delayed diene injection, molar mass and polydispersity of iPS/UHMWiPS as well as molar mass of the iPS fraction and UHMWiPS depend on the diene/styrene molar ratio and the delay time of the diene injection. In this study, we investigate the influence of both iPS molar mass and iPS molar mass distributions on crystallization behavior and viscoelastic properties. The correlation of zero shear viscosity with iPS molar mass exhibits scaling of 3.4, typical for linear polymer chains. Below 10 wt % UHMWiPS content, bimodal iPS molar mass distribution enhances processability by shear thinning. the