An aqueous polymer dispersion is composed of (i) an emulsion polymer having a weight average molecular weight of from 2,000 to 30,000 daltons, wherein the emulsion polymer comprises as polymerized units, based on the dry weight of the emulsion polymer, (a) from 10% to 90% by weight of an ethylenically unsaturated ionic monomer, (b) from 5% to 90% by weight of a photosensitive monomer, and (c) from 0 to 85% by weight of an ethylenically unsaturated nonionic monomer; and (ii) from 3.5% to 50% by weight of TEMPO and/or a derivative thereof, based on the total dry weight of the aqueous polymer dispersion. An aqueous coating composition comprising the aqueous polymer dispersion and a binder provides coatings with improved durability.