A stable free radical process for preparing a polymer having acid anhydride functionality includes heating a mixture comprised of a stable free radical agent, maleic anhydride or derivative thereof, and at least one polymerizable electron donor monomer to initiate exothermic reaction and during exotherm of the reaction, adding additional amounts of stable free radical agent to control the reaction temperature. The mixture may also include a stable free radical initiator. In the mixture, the maleic anhydride or derivative thereof preferably comprises from about 10% to about 70%, molar basis, of a total amount of the maleic anhydride or derivative thereof and the at least one polymerizable electron donor monomer. Typically, the heating is to a temperature of from about 100° C. to about 160° C., and the rate of addition of the additional amounts of stable free radical agent is adjusted to control the reaction temperature to be about 160° C. or less.