Analysis of the polyesterification in bulk without any external catalyst at 200 degrees C of o-phthalic anhydride with neopentyl glycol (2,2-dimethyl-1,3-propanediol) with a mole ratio ([-COOH]/[-OH]) = 0.7 has been carried out by high resolution C-13 nuclear magnetic resonance (C-13-NMR). Polyesters can be analyzed by C-13-NMR spectra because of the fact that both o-phthalic acid (o-phthalic anhydride) and neopentyl glycol carbons are sensitive to sequence effects. Spin-lattice relaxation times T-1, of quaternary, tertiary and secondary carbons in different structures are in the 0.1-6.5 s range depending on the neighboring residue effects in the polymer chain.