Quantitative Solid State 13C NMR Studies of Highly Cross-Linked Poly(divinylbenzene) Resins

Three highly cross-linked poly(divinylbenzene) resins have been prepared from p-divinylbenzene and two grades of "divinylbenzene" from commercial sources using suspension polymerization methodology and employing toluene as a porogen. These materials along with XAD-2 and XAD-4 from the Rohm and Haas Co. have been examined by solid state 13C nuclear magnetic resonance (NMR) techniques. In particular, quantitative single pulse excitation (SPE) experiments have been carried out to analyze the molecular structure of each resin in terms of quantifying the various types of carbon atom present. This has allowed the level of residual unreacted vinyl groups to be determined and hence the effective cross-link ratio of each resin. Whereas the monomer feed composition is known reasonably accurately for the in-house resins, no such information is available for XAD-2 and XAD-4. However, the SPE NMR spectra also allow the levels of ethylstyrene and divinylbenzene residues to be assessed as well, and so a full analysis of residual double bond content and cross-link ratio is also possible for the commercially sourced resins. In general real cross-link ratios are significantly lower than the feed ratio of divinylbenzene isomers. Thus ∼100% divinylbenzene yields ∼55% cross-linking, ∼80% divinylbenzene yields ∼40−45% cross-linking, and 50% divinylbenzene yields ∼30−35% cross-linking. There is therefore some uniformity in the percentage efficiency in the consumption of divinylbenzene in cross-links, with perhaps a marginal lowering of efficiency at very high absolute levels.