Particle in the shape of an encapsulated droplet comprising a core material and a shell material surrounding the core material, the shell material containing maleimide groups, preferably a copolymer of maleimide groups. The shell material contains a copolymer of styrene and maleic anhydride derivatives, of which more than 75 mole %, preferably more than 90 mole % is maleimide, and the average particle size is smaller than 300 nm. The particles are obtained by a process comprising by heating between 80.degree. C. and 195.degree. C., more preferably between 120.degree. C. and 190.degree. C. an ammonium salt of a maleic anhydride containing polymer in the presence of the core material.