Evaluation of reactions of a dilute cross-linker using solution nuclear magnetic resonance spectroscopy with isotopically labeled cross-linkers

Cross-linked poly(methyl methacrylate) (PMMA) was synthesized to high conversion with 0.02-0.5 wt % C-13- or H-2-labeled ethylene glycol dimethacrylate (EGDMA). Samples were analyzed with a variety of NMR techniques to examine different cross-linker reaction products (or architectures), i.e., pendant EGDMA methacrylates, cyclized EGDMA, and cross-linked EGDMA. Solid-state C-13 NMR spectroscopy was not applicable because poor peak resolution masked the cross-linker architecture peaks. Solid-state H-2 NMR spectroscopy of deuterium-labeled EGDMA was also not useful because the deuterium line shape was convoluted by signals from natural abundance deuterium in the polymer. Solution C-13 NMR analysis of solvent swollen polymers (gel-state C-13 NMR spectroscopy) produced well-resolved spectra of copolymers containing less than 0.5 wt % C-13-labeled EGDMA. These spectra confirm that significant amounts of EGDMA were singly reacted, resulting in large numbers of pendant methacrylate units. Non-cross-linked and completely soluble model PMMA copolymers containing 0.1 wt % C-13-labeled pendant EGDMA methacrylates units (incorporated in a postpolymerization reaction) were used to identify the C-13 NMR signals characteristic of EGDMA pendant units. Signals from the stereochemical triads (syndio-and heterotactic) were identified, but specific peaks for nine-membered cyclic EGDMA units were not observed. The detection limit of gel-state C-13 NMR analysis on these MMA/EGDMA copolymers was as low as 0.02 wt % C-13-labeled EGDMA, which demonstrates the utility of this approach for characterizing lightly cross-linked polymers.