The present invention relates to the polymer particles in the form of filaments composed of block copolymers, as well as their preparation process for controlled radical polymerization in emulsion in the presence of a live macroinitiator specific derivative of a nitroxide. In characteristic manner, said particles filamentous are obtained in aqueous medium, dispersed by the synthesis of said block copolymers, carried out in situ by heating the reaction medium at a temperature of 60 to 120°C, the said macroinitiator being soluble in water, with a percentage of the molar mass of the water-soluble macroinitiator in the final block copolymer comprised between 10 and 50%, the degree of conversion of the hydrophobic monomer being at least 50%. The particles filamentous according to the invention have a weight proportion of hydrophilic defraction less than 25% by weight.
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