We synthesized bottlebrush polymers with polyaziridine brushes and a polynorbornene backbone by a grafting-through approach. Polyaziridine macromonomers were synthesized by aza-anoinic polymerization of an N-tosylaziridine, initiated with a norbornene-functionalized sulfonamide anion. These macromonomers were then polymerized by ring-opening metathesis polymerization (ROMP) in dichloromethane to produce bottlebrush polymers with molecular weights of 136–456 kDa. To investigate potential macromonomer aggregation that would hinder grafting-through polymerization, we used dynamic light scattering (DLS) to measure the change in macromonomer aggregation and the growth of bottlebrush chains during ROMP. We observed that the macromonomers aggregate in solution, but once ROMP is initiated, these aggregates disperse over the course of the polymerization. This solution behavior appears to be an example of polymerization-induced deaggregation.