Provided is a process for esterifying or transesterifying side chains in polymers, which involves esterifying or transesterifying the carboxyl group having the R4 radical of a copolymer obtained by polymerizing a (meth)acrylate of formula (I), and a monomer of formula (II): wherein R1 and R3 are independently H or CH3, R2 and R4 are independently H or an alkyl radical which is optionally substituted by functional groups such as acryloyl, ether, amino, epoxy, halogen or sulfonic acid groups, and n is from 1 to 40, with an alcohol in the presence of an enzyme which catalyzes the esterifying or transesterifying. Further provided is the esterified or transesterified polymer product and its use.