Polycondensation of H3PO4 with glycerol: From branched structures to hydrolytically reversible gels

Direct polycondensation of glycerol (GL) and phosphoric acid (PA) has been studied for the first time. Reactions were performed at 100 °C–120 °C, and in spite of the formally A3 + B3 process soluble poly(glycerol phosphates) could be obtained up to the high conversions. This behavior stems from the side dealkylation reaction, that is, nucleophilic attack of the hydroxyl groups on the carbon atom in the already formed macromolecules, in place of the “normal” attack on the phosphorous atom. Besides, formation of cyclic structures frustrated both high polymers formation as well as early gelation. At certain, however, ratios of reactants and at the high enough conversions highly hygroscopic gels are formed. Acidic gels were swelling in water (over 1000%) and then were converted into relatively stable, soluble products, resulting from hydrolysis of the triester knots, and containing more hydrolytically stable diesters in the chains. Thus formed highly branched macromolecules could be converted back into a gel by condensation. This process could be repeated several times. The mechanism of polycondensation is proposed: like in the previously studied polycondensation of ethylene glycol with PA, formation of pyrophosphoric acid is preceding the actual polycondensation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3533–3542