Time-Resolved EPR Studies of Main Chain Radicals from Acrylic Polymers. Structural Characterization at High Temperatures

Time-resolved EPR investigation of potential model systems for acrylate polymer main chain radicals based on esters of Kemp's tri-acid

Methyl esters of Kemp's tri-acid and cyclohexanetricarboxylic acid are structurally similar to acrylate polymers, having the same functionalities and stereoregularities as poly(methylmethacrylate) and poly(methylacrylate), respectively. The photochemistry and free radicals from these model systems have been studied using time-resolved electron paramagnetic resonance spectroscopy with laser flash photolysis at 248 nm. Chemically induced electron spin polarization from the triplet mechanism (net emission) is observed. Well-resolved spectra are obtained at all temperatures for the model system radicals, which are determined to be in the slow motion condition, that is, there is no interconversion of chair conformations. The temperature dependence of the spectra is minimal; some hyperfine lines shift as the temperature increases, but without much broadening. Density functional theory calculations are presented and discussed in support of the experimental data.

Model systems for poly(acrylic acid) main-chain radicals based on the Kemp's triacid framework

Kemp's triacid (KTA) and cyclohexane tricarboxylic acid (CTA) are small-molecule model systems for acrylic acid polymers, having the same functionalities and stereoregularities as isotactic poly(methacrylic acid) (PMAA) and poly(acrylic acid) (PAA), respectively. As part of an ongoing investigation of radicals produced by photolysis of acrylic polymers, the photochemistry and free radicals from the model systems have been studied using time-resolved EPR spectroscopy as a function of temperature and pH. Radicals are created by direct photolysis of the acids at 248 nm or by sensitized photo-oxidation using quinone triplet states at 308 nm. The two methods of radical production lead to different chemically induced electron spin polarization (CIDEP) patterns in the ensuing radicals, which are simulated and discussed. Well-resolved spectra are obtained at all temperatures for the model system radicals, which are determined to be in the slow motion condition. DFT calculations of the model system radicals are presented and discussed in support of the experimental data.