The degree of chain-end functionality in a densely grafted, concentrated polymer brush is a critical parameter. In this study, p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) technique was applied to monitor the emergence and disappearance of substituted azide (N-3) at the chain ends of polymer brushes, allowing the accurate determination of the azide density and the click reaction efficacy. The peak separation of the N-3 absorption bands in the pMAIRS spectra showed that azide substitution occurred not only at the brush ends but also at the underlying initiators that did not react during surface-initiated atom transfer radical polymerization. Hence, the density of each type of azide group and the efficiencies of the azidation and click reaction could successfully be estimated. Furthermore, a chain-end-selective click reaction was demonstrated for the azide- functionalized polymethacrylate-type polymer brushes based on size exclusion effects and polarity differences. Thus, the pMAIRS method provides important quantitative information that can expand the applications of chain-end functionalized polymer brushes.