This book gives idea about how to modified reactive functional group Polymer structure using acidic substrate. Book represents the Applications of different heterogeneous catalyst with their selectivity and specificity. Synthesis of novolac phenol formaldehyde resin (NPFR) from formaldehyde and phenol is covered and Synthesis of epoxy novolac phenol formaldehyde resin (ENPFR) from NPFR by epoxidation using NaOH and epichlorohydrine and their modification with sulphanilic acid to from ENPFSA are discuss well. Characterization of NPFR, ENPFR and ENPFSA by IR spectroscopy, Functional group analysis, Thermo Gravimetric Analysis (TGA) for the decomposition temperature and degradation pattern are made well. Selectivity and specificity of catalyst is discuss well with suitable examples of organic substrate with more than one functionality. Catalyst stability comparison using thermogravimetric data is discuss well.