Various macromonomers (MMs) were synthesized using controlled radical polymerization (CRP) by either directly growing from a norborene-functionalized initiator or chain transfer agent ("direct-growth" or DG method) or coupling a norbornenyl group to preformed polymers ("growth-then-coupling" or GC method). The degree of control for the ring-opening metathesis polymerization (ROMP) of these MMs was found to be dependent on which synthetic method was used for the MMs. Narrowly dispersed brush polymers were consistently obtained from the GC-MMs. In contrast, the DG-MMs resulted in brush polymers with a small high-molecular weight (MW) shoulder or broader molecular weight distribution (MWD). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of DG-MMs showed the presence of a small amount of alpha,omega-dinorbornenyl telechelic species resulting from the biradical combination during polymerization. A control study further revealed that even the presence of 1 mol % alpha,omega-norbornenyl telechelic polymer in the MM resulted in broadening of MWD). Our surprising findings suggest the importance of MM quality and the absence of dinorbonenyl telechelic polymers in achieving the best control for high-MW brush polymers via efficient ROMP of MMs.