Main observation and conclusionRing‐opening metathesis polymerization (ROMP) is a powerful toolbox in preparation of bottlebrush polymers for its high activity. However, the ROMP of macromonomers with repeating tertiary amine groups, for example, poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA), is inhibited due to the coordination of nitrogen atom with ruthenium center ([Ru]) in the third generation Grubbs catalyst (G3). In this work, norbornenyl functionalized polystyrene‐block‐PDMAEMA (NB‐PS‐b‐PDMAEMA) macromonomers with different length of polystyrene (PS) spacers are prepared. The PS spacers provide non‐coordinating local environment to protect [Ru] in G3 from coordinating with tertiary amine groups. Kinetic studies show that the PS spacer with polymerization degree (DP) of 15 is enough to protect G3, and the polymerization is controlled. However, the PS spacer with DP of 5 is inhibited within 10 min. In situ proton nuclear magnetic resonance studies show that the [Ru] in G3 is active during the ROMP of NB‐PS15‐b‐PDMAEMA39, which is inactive gradually for NB‐PS5‐b‐PDMAEMA42. In comparison, the ROMP of NB‐PDMAEMA macromonomer without spacer chain is inhibited and the G3 catalyst is inactive from the beginning. By introducing non‐coordinating spacer chain with enough length, the ROMP of macromonomers with tertiary amine groups is controlled, which provides a new strategy for the preparation of bottlebrush polymers with functional amine groups.