How does the chemical reaction of a single polymer chain progress? This question is not proven, as long as it is studied the data of the ensemble average from the large number of molecules In this study, we succeeded for the first time in the direct measurement of when, where, and how the chemical reaction of a polymer chain proceeds on a nanometer scale That is, single-molecule imaging of the photodegradation reaction of a chiral helical it-conjugated polymer following laser irradiation of 405 nm was conducted Analysis of the chemical kinetics showed that the photodegradation of the single polymer chain proceeded stepwise as a quantum phenomenon When the motility of the chain-end increased, reactivity of the photodegradation increased It was also discovered that the photodegradation of the polymer chain proceeded continuously in one direction, like the "domino effect" Our method allowed dynamic imaging of single polymer chains at a solid/liquid interface, and hence can be applied to the study of many types of polymers. (C) 2010 Wiley Periodicals, Inc J Polym Sci Part A. Polym Chem 48 4103-4107, 2010