Structure of Trapped Acrylate Radicals

Conflicting views about the structure of the trapped polyacrylate radicals have been presented the three-line spectrum generally observed with polyacrylates is 

For both it is assumed that only one of the a-protons located on a CH2-group contributes to the hyperfine splitting. 

Selective deuteration of the monomer at its a-positions made it possible to determine which of the structures is the correct one. Fig. 18 shows that similar spectra were obtained with normal and deuterated samples. This proves that structure II must be the correct one in case I a two-line spectrum would be observed. Therefore, between the end of the irradiation and the time of measurement (about 5 minutes) virtually all of the propagating chain-end radicals have been terminated by combination or hydrogen abstraction. 

A similar situation exists with polystyrene but not with polymethacrylates 

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Other researchers have experimentally observed heterogeneity in crosslinked polymers by studying radical concentrations and environment with ESR [101-106], Knowledge of the structure and reactivity of trapped radicals is especially important when considering the long term physical and mechanical properties of dental polymers. Kloosterboer et al. [106] has studied the structure of trapped acrylate radicals while Hamielec and coworkers [102-105] have... 

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