Tacticity and Temperature Dependence of Acrylate Radicals

Computer simulations of the fully converged, high-temperature limit spectra are shown at the bottom of each data set in Fig. 14.2. The hyperfine coupling constants used for each simulation are nearly identical and will be discussed in the next section. 

The fast motion spectrum of the /-PMMA radical consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. There is coupling to the methyl group to form a quartet (22.9 G) that is then split further into a triplet from one set of p-methylene protons (16.4 G) and another triplet from the other set (11.7G). Theoretically, this should lead to 36 lines (4 x 3 x 3), but a fortuitous degeneracy exists because one of the fast motion p-methylene couphng constants is almost exactly 

FIGURE 14.2 X-band TREPR spectra of main-chain polymer radical la produced from 248 nm laser flash photolysis of atactic, isotactic, and syndiotactic PMMA in propylene carbonate at 0.8 ps delay time. The temperature for each spectrum is shown in °C, and the magnetic held sweep width is 150 G for aU spectra, which exhibit net E CIDEP in aU cases. Simulations of each fast motion spectrum (highest temperature) are shown at the bottom of each data set. Hyperflne values for each simulation are 3 anCCHs) = 22.9 G, 2aH(CH2)= 16.4 G, 2aH(CH2) = 11.7G for isotactic PMMA 3ah(CH3) = 22.9G, 2aH(CH2 = 16.2G, 

In the meso and racemic radical structures, the hyperflne coupling constants to the [3-methylene protons are not perturbed greatly by the change in stereochemistry on 

FIGURE 14.3 Nuclear spin symmetry relationships for main-chain radical la from PMMA, as a function of polymer tacticity. The two radicals shown have mirror plane ( meso ) or C2 axis ( racemic ) symmetry elements establishing magnetic equivalence of each set of 3-methylene protons. 

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