Polystyrene (PS) and poly(methyl methacrylate) (PMMA) having a single di-tert-butyl phthalate (DTBP) group either at the chain end or in the middle of the chain were synthesized by Cu(I) ion mediated atom transfer radical polymerization (ATRP). The di-tert-butyl phthalate initiators 7 and 8 (for end-functional polymers) and 11 (for mid-functional polymers) were prepared from commercially available di-tert-butyl acetylene dicarboxylate (1) and myrcene (2) in four and six steps, respectively, with high overall yields. The DTBP functionalized polymers could be clearly converted to the corresponding phthalic anhydride (PA) functional polymers by pyrolysis. The pyrolysis process could be easily monitored using conventional 1H NMR spectroscopy, by observing the significant chemical shift change of the alkyl linker existing in the initiators. Kinetic study of the pyrolysis revealed that the mechanism of the DTBP group pyrolysis to phthalic anhydride (PA) group follows two first-order consecutive reactions having a phthalic diacid (DA) as an observable intermediate. When the PA-functionalized PMMA was subjected to reactive blending at 180°C with an amine-functionalized PS, the conversion reached a maximum (>90%) in less than 2 min, which is considerably faster than the corresponding reaction of an aliphatic anhydride (e.g., succinic anhydride)-functionalized PMMA. A competition experiment with small molecules showed that phthalic anhydride reacts ∼5 times faster than succinic anhydride with PS-NH2.