An apparatus for halogenation of a polymer is disclosed. The apparatus includes a reactor, at least one light source, a stirrer and a heater. The reactor contains a slurry of the polymer. The light source is disposed outside of the reactor at a distance ranging from 0.5 centimeter to 2 centimeters for facilitating irradiance of the slurry. The light source radiates a light of wavelength in the range of 250 nm to 355 nm. The stirrer is adapted to agitate the slurry. The heater is adapted to heat the slurry of the polymer. The blades on the stirrer and light source are arranged in such a way that slurry is maintained in uniform motion and reacted homogeneously to achieve desired conversion efficiently.