A dispersion of polymer microparticles in organic liquid, the polymer comprising the reaction product of i) from 2 to 40 parts by weight of a first polymer being polypropylene polymer having sufficient carboxylic acid and/or carboxylic acid anhydride groups equivalent to an acid value of from 2 to 50 mg KOH/g nv polymer and ii) from 60 to 98 parts by weight of a second polymer having a molar excess of functional groups capable of reacting with the carboxylic acid and/or carboxylic acid anhydride groups of the polypropylene polymer and wherein the organic liquid is chosen to be a good solvent for the second polymer and a poor solvent for the polypropylene polymer.