Monodisperse polymer particles and dispersions thereof

A monodisperse dispersion of polymer particles having an average particle diameter of >5 .mu.m produced by a process which comprises:

• (a) preparing an emulsion or dispersion of monodisperse particles comprising:
• (i) polymer and
• (ii) Substance I which is one or more materials having a molecular weight of <5000 and a water solubility of <10.sup.-2 g/l,
• by adding said Substance I to an aqueous dispersion of monodisperse polymer particles, whereby Substance I is transported through the aqueous phase to said particles which are swelled therewith, under conditions which cause the rate of transport of Substance I through the aqueous phase to be relatively high and then,
• (b) under conditions which are such that the rate of transport of Substance I through the aqueous phase is relatively low, adding to said emulsion or dispersion, Substance II which is one or more partly water-soluble materials, at least one of which is a polymerizable monomer, having a water solubility at least 10 times higher than that of Substance I, whereby Substance II diffuses into the polymer particles containing Substance I at a rate substantially in excess of the rate of departure of Substance I from said polymer particles, the amount of said Substance II diffusing into said particles containing Substance I being at least 20 times that of the original polymer particles used in (a) based on volume and then causing polymerization to take place.