Examples of such polymer particles are grains of copolymers obtained by cross linking substances containing a. plurality of hydroxyl groups, such as carbohydrates and sugar alcohols, such as dextran, starch, dextrins and other polysaccharides, and polyvinyl alcohol with a bi-functional substance, e.g. bi-functional substances of the type XRZ, wherein, for instance, X and Z are each halogen or an e