(A) 1 to 50% by weight of at least one end carboxyl bearing reactive vinyl monomer of the formula: ##STR1## in which R.sub.1 is hydrogen or methyl group; R.sub.2 is a substituted or unsubstituted aliphatic hydrocarbon having 2 to 10 carbon atoms, alicyclic hydrocarbon having 6 to 7 carbon atoms or aromatic hydrocarbon having 6 carbon atoms; A is a repeating unit of ##STR2## R.sub.3 is ethylene or propylene; R.sub.4 is a substituted or unsubstituted alkylene having 2 to 7 carbon atoms; R.sub.5 is a substituted or unsubstituted alkylene having 2 to 5 carbon atoms; m is an integer of 1 to 10; n is an integer of 2 to 50; X is an inorganic or organic base, and when .alpha., .beta.-ethylenically unsaturated compound(s) (B) hereinunder referred to includes a basic relative monomer, it may be hydrogen atom, and(B) 99 to 50% by weight of at least one other .alpha., .beta.-ethylenically unsaturated compounds, in an aqueous medium through an emulsion polymerization and removing the said medium therefrom.The invention also concerns an aqueous emulsion containing said microparticles in a considerable quantity, which is specifically useful in an aqueous paint.