New polyvinylamide and/or acryl acid-based copolymers. The new structured block copolymers are based on the block copolymerization of N-vinyl formamide (NVF) with other monomers by using a bifunctional initiator. Further, a method of preparing the structured block copolymer is provided, and especially a method wherein the NVF-based block is at least partially hydrolyzed to obtain PVAm. Also the use of the new structured block copolymers in sludge dewatering and in papermaking processes is disclosed.