Poly(methyl methacrylate)-block-poly(methacrylic acid) (PMMA-b-PMAA) with and without pyrene labels attached to the end of the PMAA block have been synthesized and characterized. These polymers formed micelles with PMAA as the core in ethyl acetate/methanol mixtures when the ethyl acetate volume fraction was higher than 80%. After mixing a micelle solution of PMMA-b-PMAA without pyrene labels, i.e., polymer I, with a unimer solution of the sample with pyrene, i.e., polymer II, the fluorescence intensity IPy(t) of pyrene increased with time. This was caused by the insertion of the pyrene group of polymer II chains into the rigid core of polymer I micelles. The pyrene fluorescence quantum yield is higher in a more rigid environment due to the reduced quenching of pyrene fluorescence by oxygen and the possible suppression of certain nonradiative deactivation pathways. A kinetic model has been proposed for describing the chain insertion process. Fitting the experimental fluorescence intensity data using the derived expression for IPy(t) allowed the first determination of the rate constant kn for diblock copolymer chain insertion into micelles.