after the material has reacted for about 30 minutes the mixture is poured into a citrate bottle and capped the material is maintained at about 50 u00b0 c in the citrate bath for about 24 hours and agitated throughout this time at the end of 24 hours the reaction bottle is cooled and the material is removed washed and dried a portion of the microspheres are set aside and the remainder are heated in an air oven for a period of about 30 minutes at about 150 u00b0 c a sample of the dry unexpanded and dry expanded microspheres are then studied by a coulter counter the dry unexpanded microspheres have a size of about 2 to 12 microns about half of the microspheres exposed to the heating process show expansion the procedures of example 1 are substantially repeated with the exception that 1 part by weight of methanol is added to the reaction mixture the dry unexpanded and dry heat expanded microspheres are then studied by coulter counter the dry unexpanded microspheres measure about 8 to 10 microns in size essentially all the microspheres exposed to heat expand the procedures of example 2 are substantially repeated except that after synthesis of the microspheres a slurry of the microspheres is added to an aqueous solution containing 35 weight percent hydrogen peroxide this slurry is hated to a temperature of about 50 u00b0 c for about 3 5 hours and subsequently cooled and air.