The radical homo- and copolymerization of styrene (1) and diethyl fumarate (DEF, 2) in the presence of methylated beta-cyclodextrin (beta-CD) in water is described. It has been shown for the first time that homopolymerization of CD-complexed DEF and its copolymerization with CD-complexed styrene occur readily in aqueous solution. In the absence of CD, or in organic solvents, the homopolymerization of DEF, is strongly retarded.