Abstract: The present invention relates to polymers which have pH-depending solubility characteristics in aqueous solutions, whereby the polymers are copolymers between (a) methylmethacrylate (b) at least a monomer of the formula III ##STR1## wherein R.sup.3 is an alkyl group having 4 to 18 carbon atoms and R.sup.4 is hydrogen or methyl; and (c) a monomer of the formula I ##STR2## wherein R is hydrogen or methyl, n is an integer of from 1 to 4, and X is .dbd.CH.sub.2 or .dbd.O; and/or a monomer of the formula II ##STR3## wherein R is hydrogen or methyl, R.sup.1 and R.sup.2 are each hydrogen, methyl, ethyl, propyl, isopropyl, isobutyl, t-butyl or n-butyl, and m is an integer of from 1 to 4, as well as a process for their preparation, and a product in the form of particles which have been coated with such a polymer, whereby the product is protected against dissolution at pH above 5, but shall be dissolved at pH below 4, the product preferably being one which is administered to ruminants in order to obtain rumen-by-pass.