the invention relates to a block copolymer containing : a ) hydrophobic biodegradable polymer ; b ) a hydrophilic polymer and c ) at least one reactive group for covalent binding of a surface - modifying substance d ) to the hydrophilic polymer b ). the invention relates to shaped bodies consisting of the block copolymer and to their utilization , particularly as carriers for tissue culture and active substances and for controlled release and targeted administration of active substances .