Evaluation of Reactions of a Dilute Cross-Linker Using Solution Nuclear Magnetic Resonance Spectroscopy with Isotopically Labeled Cross-Linkers
- Citation data:
Macromolecules, ISSN: 0024-9297, Vol: 37, Issue: 10, Page: 3699-3706
- Publication Year:
- Materials Science; Chemistry; Physical Sciences and Mathematics
Cross-linked poly(methyl methacrylate) (PMMA) was synthesized to high conversion with 0.02-0.5 wt % C- or H-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 NMR spectroscopy was not applicable because poor peak resolution masked the cross-linker architecture peaks. Solid-state H 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 NMR analysis of solvent swollen polymers (gel-state C NMR spectroscopy) produced well-resolved spectra of copolymers containing less than 0.5 wt % C-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-labeled pendant EGDMA methacrylates units (incorporated in a postpolymerization reaction) were used to identify the C 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 NMR analysis on these MMA/EGDMA copolymers was as low as 0.02 wt % C-labeled EGDMA, which demonstrates the utility of this approach for characterizing lightly cross-linked polymers.