The molecular weight distribution of polymer, termed dispersity (Đ), is a fundamental parameter that determines polymer properties. Sodium azide (NaN3) functions as a catalyst in organocatalyzed living radical polymerization when the reaction medium is nonpolar. In contrast, NaN3 can act as a nucleophile when the reaction medium is polar. In this paper, we report an efficient approach to dispersity control by exploiting the dual functions of NaN3 under the varied solvent polarity. Simultaneous polymerization and chain-end substitution allowed us to tune the Đ values of various polymethacrylates and poly(butyl acrylate). Notably, the Đ value could be tuned to a wide range approximately from 1.2 to 2.0 for polymethacrylates and to 3.8 for poly(butyl acrylate). This approach afforded polymer brushes on surfaces with tailored Đ values. An interesting finding was that the polymer brushes exhibited a unique interaction with external molecules, depending on the Đ value.