Aminoxyl Macroradicals

It has also been demonstrated [52] that interaction of NO with radiation-produced macroradicals XFV at 150-200 Cleads to their decay without the production of ARs. The absence of ARs in irradiated PTFE was explained by the hindrance of spatial migration of the free valence within the rigid matrix of the fluorinated polymer in which the nitroso compounds formed by recombination of XIV with NO cannot serve as spin traps. It has been noted [52] that PTFE samples irradiated in air and exposed to NO at room temperature exhibit an ESR spectrum tentatively assigned to aminoxyl macroradicals, but the conditions under which these radicals are formed have not been clearly defined. Furthermore, to reliably identify the radicals by their solid-phase spectra, one should assess isotropic values with the appropriate parameters of low-molecular perfluoroalkylaminoxyl radicals studied thoroughly in the liquid phase [53, 54]. 

A similar procedure was used to obtain spin-labelled TEE-HEP [49]. The presence of hexafluoropropylene (HFP) groups in this polymer leads to disturbance of the structural ordering typical of PTFE to more complex dynamics of their motion. After y-irradiation of powders and films of TFE-HFP copolymer in air, there are three types of stable peroxy macroradicals in the samples end radicals CF -CF O, secondary middle-chain radicals CF -CF(00 )-CF2, and tertiary middle-chain radicals CF2-C(CF3)(00 )-CF2. In contrast to PTFE, prolonged exposure (>100 hours) of these samples in a NO atmosphere at room temperature does not lead to the formation of aminoxyl macroradicals. However, two types of macroradicals are formed if TFE-HFP is heated with evacuation after the decay of radicals in a NO atmosphere. At 90 °C, the ESR spectrum demonstrates the presence of tertiary alkyl macroradicals CF2-C (CF3)-CF2 formed upon decay of the tertiary nitroso compounds [57]. On further increasing of the temperature up to 180 "C, the tertiary alkyl macroradicals... 

In the presence of NO formed upon decomposition of the tertiary nitroso compounds, the terminal radicals can be converted into terminal nitroso compounds reacting with the adjacent double bond and forming aminoxyl macroradicals ... 

This sequence of reactions explains why CF3-NO - CF,-CF, radicals are not formed during the photolysis of polymer samples irradiated in a vacuum and then exposed to NO. Under these conditions, there is no formation of the nitrates whose photolysis produces the NO, necessary for the conversion of tertiary alkyl radicals to the corresponding alkoxide macroradicals and then to CFj. Thus, aminoxyl macroradicals obtained in the TFE-HFP copolymer are optimal spin labels. Behaviour of CF,-C(CF3)(X)-CF,-NO -CF, radicals can reflect the motion of the segments located in the middle of polymer chain, whereas the ESR spectra of CF3-NO -CF,—CF, radicals contain information about the movements of terminal groups of macromolecules. 

These parameters coincide with those of ARs obtained by interaction of NO + O with PI in solution [8]. The same a and g values were observed for block PI samples at 373 K (Figure 6.1b). The initial rates of the accumulation of ARs are proportional to NO concentrations in the gas phase. Kinetic curves have the maximum dependence on NO concentrations. As the concentration increases, the maximum amount of the aminoxyl macroradicals initially grows, reaches a maximum of (4-5) x 10 spin/ sample at [NO ] = (7-8) x 10" mol/l, and then gradually decreases. The time required for accumulating the maximum amount of the radicals increases from 10 minutes to >400 minutes when the [NOj] decreases from 2.310 mol/l to 10 mol/l. 

NMP is historically the first widespread CRP method. In NMP processes, nitroxide derivatives control the polymerization. "" Such persistent aminoxyl radicals couple reversibly at high temperature with macroradicals (i.e., growing polymer chains), which lead to the formation of an equilibrium between a small fraction of active chains and a large fraction of inactive chains (also known as dormant chains) capped by an alkoxyamine moiety. As a consequence, NMPs exhibit a controlled/living behavior and therefore allow wide possibilities of macromolecular engineering. "... 

Polymers with grafted ARs can be also produced in reactions of macroradicals formed by mechanodegradation, photolysis or radiolysis in the presence of aminoxyl biradicals. By this method, for example, AR (XII)... 

The limiting concentration of ARs accumulated in samples does not exceed 1(L mol/ kg, which is much less than the initial concentrations of CAN in PVP. The reason is that the main part of macroradical Rj is consumed in side processes withont aminoxyl formation. That is why there are grounds to conclude that they recombine with OH radicals at the primary stage of radical generation. 

Aminoxyl radicals in pol5rperfluoroalkanes are not formed in these conditions. However, if to carry out a preliminary y-irradiation on air, middle and end peroxide macroradicals appear with conversion into fluoroami-noxyl macroradicals CF2CF(NO )CF2 by the subsequent exposure to NO. Their EPR spectra in oriented films are quintet of triplets with the parameters 4 =0.46 mT, 4 =U1 mT, n=2.006 4=1.12 mT, 4=1.61 mT and 1=2,0071. The following mechanism of formation of aminoxyl radicals in these conditions is offered... 

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