A process for modifying polymers containing carboxyl, hydroxyl and/or phenolic hydroxyl groups. In the process, the polymers are reacted with a delta.sup.2 mono- or bis-oxazoline, each substituted in the 2-position by an optionally substituted alkyl or alkenyl group containing from 7 to 23 carbon atoms. The modified polymers show reduced melt viscosities, lower adhesion and better slip characteristics than unmodified polymers.