Quantification of Intramolecular Cyclization in Branched Copolymers by 1H NMR Spectroscopy

Statistical copolymerization of a monovinyl with a divinyl monomer leads to macroscopic gels, microgels, or soluble branched copolymers, depending on the precise reaction conditions. This approach is widely used for the manufacture of soft contact lenses and also to prepare various biomedical hydrogels for the separation and purification of proteins, DNA, etc. According to Flory-Stockmayer theory, gelation should occur in such copolymerizations if there is more than 0.50 fully reacted divinyl comonomer per primary chain. However, many experimental studies indicate significant deviations from this classical theory, which are generally believed to be due to wastage of the divinyl comonomer via intramolecular cyclization. Unfortunately, experimental verification of this side reaction has proven elusive for the past seven decades. In the present study, we use a disulfide-based cleavable bifunctional comonomer to undertake the first experimental quantification of the extent of intramolecular cyclization in nonlinear methacrylic copolymers using H-1 NMR spectroscopy.