Polymers radicals, TREPR spectra

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

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,... 

All these spectra were acquired at elevated temperatures ( 100°C), that is, where the observation of fast motion spectra is expected. In Fig. 14.4A, the TREPR spectrum of the main-chain polymer radical from photolysis of /-PMMA is repeated from the bottom left side of Fig. 14.2, as it is the starting point for comparisons of spectral features such as hnewidths and hyperfine coupling constants. The nomenclature used throughout this section is derived using the notations indicated in Scheme 14.1 and Chart 14.1. For example, a main-chain radical from PMMA will be denoted la, whereas the oxo-acyl radical from PFOMA will be designated as radical 6b, and so on. For all radicals simulated, the parameters used are listed in Table 14.1. 

The polymer then undergoes slower internal rotations, and the TREPR spectrum of the main-chain radical is broadened. The similarity in the spectra from PFOMA and PAMA suggests that the conformational mobility of the polymeric radical in solution plays a major role in the intensity and spectral shape of the TREPR signal from these polymeric radicals and that side chain size and structure can completely prevent access to the fast motion limit, at least at temperatures below 135°C. Higher temperatures are not currently available to us because our high temperature flow system in limited to a maximum reservoir temperature of 150°C, for safety reasons. 

If the motion of an acrylic polymer radical about the Cp bond is hindered, changing the temperature should lead to changes in the TREPR spectrum. This is indeed observed for all acrylic polymers we have examined to date. Simulation of the complete temperature dependence of TREPR spectra of acrylic polymer main-chain radicals should allow information regarding the conformational motion of the polymer in solution to be extracted, such as rotational correlation times, spin-lattice relaxation times (Ti), and activation energies for conformational transitions. 

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