Copolymers and Hydrogels Based on Vinylphosphonic Acid

Polyelectrolyte hydrogels have the advantage of a high swelling ratio and to respond to changes in pH and ionic strength which gives them useful characteristics for biomedical applications. To investigate the possibility to synthesize hydrogels from vinylphosphonic acid (VPA), first the free radical copolymerization of VPA and methyl acrylate (MA) was investigated at temperatures between 60 and 80 °C. Second free radical copolymerization of VPA with ethylene glycol diacrylate (EGDA) at 80 °C was studied to obtain a hydrogel. These copolymerizations were successful. For poly(VPA-co-MA) the solubility behavior, NMR spectra, and titration curves of the resulting products proved the existence of copolymers, and for poly(VPA−EGDA) gels a high mass swelling ratio in water was observed, showing that they behave as hydrogels. At pH values between 3 and 9 the degree of swelling displayed low dependence on pH, but deswelling upon addition of salt was observed. A total conversion between 15 and 79% was obtained for poly(VPA-co-MA) depending on the percentage of VPA in the feed and on the temperature. A higher conversion was achieved when the percentage of VPA in the feed was lower, and the polymerization temperature was higher. A nonideal behavior of the copolymerization in terms of VPA content in the resulting polymers as a function of VPA in the feed was observed. This was attributed to the participation of VPA−anhydride in the copolymerization process as an intermediate giving rise to cyclopolymerization.