a continuous process for producing polymethyl methacrylate or its copolymer comprises precooling a monomeric feedstock consisting of methyl methacrylate or a mixture of mma and up to 10 mol % of a highly thermal radical - forming monomer , forcing the feedstock into a reactor wherein a preceding charge is being circulated under pressure , instantaneously mixing the whole charge and cooling it by dint of the sensible heat of the monomeric feedstock , while thoroughly mixing the hot radicals , thus effecting the polymerization within a short time period , continuously taking out from the reactor the polymerization product and unreacted monomer or monomers being circulatingly mixed , at a polymerization product proportion of 50 % by weight or less , conducting the mixture , while preheating it , into a vacuum vessel , removing therein the remaining monomer or monomers by evaporation , and then recovering the polymerization product .