Nitroxide-controlled free-radical copolymerization of vinyl and divinyl monomers. 2. Gelation

Some important features of the gel prepared by the nitroxide-mediated free-radical copolymerization of styrene with a small amount of 4,4'-divinylbiphenyl were studied. Owing to the "living'' character of this system, the cross-linking reaction in this system proceeds highly homogeneously (randomly) without forming any microgels. The gel prepared by this method showed remarkable differences from the one prepared in the conventional system, in regard to the relative fraction and swelling ratio of the gel studied as a function of monomer conversion. It was also found that the critical number density of cross-links at the gel point agreed with the mean-field theoretical value of Flory and Stockmayer (FS) within a factor 2. This value was exceptionally small compared with those previously reported for the conventional free-radical systems, which are typically 1 or 2 orders of magnitude larger than the FS value. These results, along with the previous study on the reactivity of the pendant vinyl of this system (Ide, N.; Fukuda, T. Macromolecules 1997, 30, 4268), show that the gels prepared by this method are exceptionally homogeneous compared with those prepared by the conventional free-radical method.