A process for preparing graft copolymers of polyvinyl esters by polymerization of a) at least one vinyl ester of aliphatic C.sub.1-C.sub.24-carboxylic acids in the presence of b) polyethers which are solid at room temperature and have the general formula I, c) and, where appropriate, at least one other monomer using a free-radical organic initiator system, wherein liquid polyalkylene glycol is used as solvent for the free-radical initiator system (feed).