A compatible polymer mixture, comprising:(I) 0.1-99.9 wt. % of a polymer P1, comprised of monomer units of formula I ##STR1## wherein R.sub.1 is hydrogen or a methyl group; and (II) 99.9-9-0.1 wt. % of a copolymer P2, comprised of:30-90 parts by wt. of a methacrylic acid ester monomer of formula II ##STR2## wherein R.sub.2 is methyl or ethyl; 70-10 parts by wt. of a methacrylic acid ester monomer of formula III ##STR3## wherein R.sub.3 is a hydrocarbon group with 3-24 carbon atoms; and 0-10 parts by wt. of a third monomer M which is copolymerizable with but different from the monomers of formulas II and III.