13C-NMR spectroscopic analysis of vinyl groups in styrene–divinylbenzene networks

p-Divinylbenzene (DVB) ¹³C-labeled at the methine carbon of the vinyl group was copolymerized in suspension with styrene at 70, 70–95, and 135–155°C using 2,2′-azobisisobutyronitrile (AIBN) as the initiator. The number of unreacted vinyl groups in each copolymer was determined by ¹³C CP–MAS NMR analysis of solid samples, direct polarization ¹³C-NMR analysis of CDCl₃-swollen gels, and bromination. Results from the three methods agree methods agree qualitatively. Even the 1% DVB-crosslinked networks contained 40% unreacted DVB-vinyl groups when prepared by high conversion polymerization at 70°C and 16% unreacted DVB-vinyl groups when polymerization was finished at 95°C. The analyses were also applied to some commercial crosslinked polystyrenes. Every sample examined contained pendent vinyl groups