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Macromolecular scaffolds for drug delivery, self-assembly, and imaging applications have attracted significant attention over the last several decades. As polymerization techniques become more sophisticated, it becomes possible to create polymeric architectures with increasing control over structure, molecular weight, and mass dispersity. Herein, ring-opening metathesis polymerization (ROMP) is paired with highly efficient synthetic methods to create functional bottle-brush polymers for MRI imaging and self-assembly applications. In this "graft-through " approach, bivalent macromonomers bearing a terminal exo-norbornene group were synthesized and polymerized to yield bottle-brush polymers with controlled molar masses and low dispersities. This approach is first utilized in the development of organic radical contrast agents (ORCAs) for magnetic resonance imaging (MRI). These ORCAs are composed of macromonomers bearing long polyethylene glycol chains and sterically hindered bis(spirocyclohexyl) nitroxide free radicals. This approach enables facile tuning of nitroxide loading percentages and molecular size. Bottle-brush ORCAs displayed high ri and r2 relaxivities suggesting that they have potential for furthe