Evaluation of Reactions of a Dilute Cross-Linker Using Solution Nuclear Magnetic Resonance Spectroscopy with Isotopically Labeled Cross-Linkers

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Cross-linked poly(methyl methacrylate) (PMMA) was synthesized to high conversion with 0.02−0.5 wt % 13C- or 2H-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 13C NMR spectroscopy was not applicable because poor peak resolution masked the cross-linker architecture peaks. Solid-state 2H 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 13C NMR analysis of solvent swollen polymers (gel-state 13C NMR spectroscopy) produced well-resolved spectra of copolymers containing less than 0.5 wt % 13C-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 % 13C-labeled pendant EGDMA methacrylates units (incorporated in a postpolymerization reaction) were used to identify the 13C 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 13C NMR analysis on these MMA/EGDMA copolymers was as low as 0.02 wt % 13C-labeled EGDMA, which demonstrates the utility of this approach for characterizing lightly cross-linked polymers.