THE ROLE OF SPECIFIC SIDE-GROUPS AND PH IN MAGNETIZATION-TRANSFER IN POLYMERS

The nature of water-macromolecule interactions in aqueous model polyme rs has been investigated using quantitative measurements of magnetizat ion transfer. Cross-linked polymer gels composed of 94% water, 3% N,N' -methylene-bis-acrylamide, and 3% functional monomer (acrylamide, meth acrylamide, acrylic acid, methacrylic acid, 2-hydroxyethyl-acrylate, o r 2-hydroxyethyl-methacrylate) were studied. Water-macromolecule inter actions were modified by varying the pH and specific functional group on the monomer. The magnitudes of the interactions were quantified by measuring the rate of proton nuclear spin magnetization exchange betwe en the polymer matrix and the water. This rate was highly sensitive to the presence of carboxyl side groups on the macromolecule. However, t he dependence of the rate on pH was not consistent with simple acid/ba se-catalyzed chemical exchange, and instead, the data suggest that mul tiequilibria proton exchange, a wide distribution in surface group pK values, and/or a macromolecular structural dependence on pH may play a significant role in magnetization transfer in polymer systems. These model polymer gels afford useful insights into the relevance of chemic al composition and chemical dynamics on relaxation in tissues, (C) 199 8 Academic Press.