A thermally expandable composition which includes at least one thermoplastic polymer, at least one chemical blowing agent, at least one free radical initiator, and at least one co-agent having at least two maleimide groups. The invention is also related to a baffle and/or a reinforcement element for hollow structures including the thermally expandable composition, to a process for manufacturing the baffle and/or the reinforcement element, to use of the baffle and/or the reinforcement element for sealing, baffling, or reinforcing of a cavity or a hollow structure, and to a method for sealing, baffling and/or reinforcing a cavity or hollow structure.