DESC:Field of the invention: The present invention relates to a method of synthesis of Acrylamido-2-tertiary butyl sulphonic acid (also referred as ATBS) using acrylonitrile (also referred as ACRN), Isobutylene and a sulphuric acid mixture to get 80% to 82% yield and more than 99% purity with operationally simple process, and more particularly the present invention relates to the method for synthesis of ATBS using an ageing reactor that offers better quality of ATBS monomer. Background of the invention: Acrylamido-2-tertiary butyl sulphonic acid has a wide range of applications like water treatment, additives in acrylic fibers, in oilfield, latex and adhesive etc. Also, as a raw material for obtaining the polymers which is used as dispersant, hydrogel and thickening agent in diversified industries like oil industry, construction, desalination of water, mineral industry and cosmetics etc. The method for preparation of ATBS involves mixing excess of ACRN and a sulphonating agent selected from oleum, sulphuric acid, and a mixture of oleum and sulphuric acid, to get the ACRN-sulphate salt, followed by purging of isobutylene as gas to get ATBS as a white solid powder. Scheme: Preparation of ATBS ATBS is formed as a slurry during the reaction. ATBS being insoluble in ACRN, the solid content of ATBS is separated by filtration. This is followed by purification using various washing solvent to get the highly pure ATBS product. Herein, the purification step is more important to get the low level of impurities in desired ATBS, because the impurities present in ATBS strongly affect the self-polymerization, more specifically the molecular weight and water insolubility from the resulting polymers and copolymers. US10669231B2 discloses the method for producing acrylamide 2-tertiary butyl sulphonic acid using fuming sulphuric acid to quench the residual water content in ACRN. The purification is performed by recrystallization in aqueous medium to get the pure ATBS. This purification step is commercially non-feasible and also, the use of fuming sulphuric acid affects the yield and purity of ATBS. Use of aqueous medium for recrystallization generates hazardous effluents which leads to environmental concerns. US8247601B2 discloses a method of synthesis of ATBS by mixing ACRN, sulphuric acid and isobutylene in continuous manner, wherein the impurity profile is controlled by adjusting the concentration of sulphur trioxide in reaction system. However, maintaining the content of sulphur trioxide in reaction mixture is difficult at commercial scale and the unused sulphur trioxide needs to be quenched at the end of reaction. Although, the reported processes in the prior art offers ATBS monomer, unit operations are more such as recrystallization makes these protocols industrially nonviable. Reported process also not provides whole process starting from preparation of sulphuric acid to drying of ATBS monomer. Moreover, these prior arts offer the ATBS with complex unit operations and lower purity ATBS which needs additional purification operations in the manufacturing process. Some prior art methods use mixture of sulphuric acid and oleum as a sulphonating agent. Oleum is a fuming sulphuric acid and the fumes are toxic and hazardous to handle. A scrubber is required to neutralize the toxic fumes. The intensity of fumes increases (23%<30%<65%23 to 30%). The purpose behind selection of 23% to 30% oleum is to minimize the toxic fumes which are coming out while handling. On increase in concentration of oleum, intensity of fumes increases (23%<30%<65%