Acid catalyzed poly(vinyl alcohol) (PVA) acetalization was investigated in aqueous medium at 80 degrees C for a PVA concentration of 8 wt%. The reactant, 10-undecenal, was composed of a long alkyl chain with a vinyl end group, and the functionalization reaction was studied in heterogeneous media for low reactant concentrations (from 0.33 to 2.0 mol% compared with PVA hydroxyl groups concentration). First, the reaction was scrutinized with pentane-2,4-diol, as a model compound of PVA. Besides the expected reaction, the oxidation of the aldehyde into 10-undecenoic acid in the presence of water was evidenced. This carboxylic acid appeared unreactive toward esterification of pentane-2,4-diol and PVA in water. Characterization of acetal stereochemical structure formed on the PVA backbone was performed by NMR spectroscopy in accordance to the model approach. A protocol based on H-1 NMR analysis was developed to quantify grafted aldehyde, residual aldehyde, and created carboxylic acid through direct sampling of the reaction medium. Conversions and reaction rate constants were calculated for pH ranging from 1 to 3. Finally, the acetalization yield was found to be enhanced at low pH and, in such conditions, the oxidation reaction contribution was limited. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 661-671