Polymers of the above types include polyvinyl butyral, copolymers of methacrylic acid with methymethacrylate, with acrylonitrile or with styrene, copolymer of vinyl acetate with maleic anhydride, copolymer of nitrostyrene with diethylmaleate, copolymers of styrene with acrylic and methacrylic acids and esters, etc.