A process for incorporating carboxy and/or carboxyl derivative functionality into a hydrocarbon polymer which is substantially free of ethylenic unsaturation. The functionalization is accomplished by contacting the functionalizing agent or agents with the polymer in the presence of a free radical initiator in a blending apparatus capable of imparting high mechanical energy into the reaction medium. The blending apparatus is preferably an extruder and most preferably a non-intermeshing twin screw extruder. The amount of degradation resulting from the presence of a free radical initiator and the high mechanical shear at the elevated temperatures employed during the contacting is reduced by effecting the contacting in a suitable diluent. Similarly, the amount of polymer crosslinking and/or coupling is reduced through the use of a suitable diluent. The polymer is maintained in the molten phase during the reaction.