Abstract: The present invention relates to expandable, polymerizable compositions comprising at least one benzoxazine and at least one cyclic carbonate, to polymerization products of these expandable, polymerizable compositions, to a process for preparing these polymerization products as well as to uses of these expandable, polymerizable compositions. The present invention is based on the surprising finding that copolymerizing benzoxazine monomers with cyclic carbonate monomers results in novel copolymers having unforeseeably high expansion rates, wherein the properties (e.g. solid/brittle, solid/soft, rubbery) of the resulting copolymers can be easily and reproducibly tuned/adjusted, depending on the ratio of the benzoxazine equivalents/cyclic carbonate equivalents present in the composition and copolymer, respectively.