To understand the kinetic of synthesis of polyamide-imide (PAI) via p-chlorophenol-(PCP) blocked 4,4’-diphenyl methane diisocyanate (MDI) with trimellitic anhydride (TMA), a series of reactions of blocked MDI with excess phthalic anhydride (PA) and benzoic acid are designed. PCP-blocked phenyl isocyanate (BPl) which also released isocyanate at higher temperatures was used as a model compound for BMDI. The dissociation constants of BPI and BMDI in the presence of excess PA or BA was measured by collecting the evolved CO2. The effect of the catalyst concentration and temperatures were combined by a Hostettler equation.