Suitable monomeric unsaturated compounds which can be copolymerized with the unsaturated polyesters are, for example, vinyl compounds, such as styrene, vinyl-toluene and divinyl-benzene; vinyl esters, such as vinyl acetate; unsaturated carboxylic acids and their derivatives, such as acrylic acid, acrylic ester and acrylonitrile; methacrylic acid and its corresponding derivatives; allyl esters, suc