Forces free-radical systems

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

The driving force behind free radical addition to ir-electron systems has been the subject of much study. It is suggested that the rates of interaction should be correlated with the maximum free valence (82) or... 

Force-field methods, calculation of molecular structure and energy by, 13, 1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271... 

The predictions one can make about electrocyclic processes are given in Table 1. Although this is a Table of both allowed and forbidden one-step processes, this does not rule out other reaction paths, e.g. via several steps by free radicals. Furthermore, forcing conditions may provide sufficient energy so that a forbidden path may become allowed. Considering the type of system, there are perhaps more predictions in the Table than experimental facts. Nevertheless, the success of the Woodward-Hoffmann rule has been remarkable. 

Kennedy 67,77 118) studied the ability of w-styryl-polyisobutene macromonomers to undergo free-radical copolymerization with either styrene or butyl or methyl methacrylate. Here, the macromonomers exhibited a relatively high molecular weight of 9000, and the reaction was stopped after roughly 20% of the comonomer had been converted. The radical reactivity ratios of styrene and methyl methacrylate with respect to macromonomer were found to be equal to 2 and to 0.5, respectively. From these results, Kennedy concluded that in the ra-styrylpolyisobutene/styrene system the reactivity of the macromonomer double bond is reduced whereas with methacrylate as the comonomer the polar effect is the main driving force, yielding reactivities similar to those observed in the classical system styrene/MMA. 

The driving force, calculated from the difference in the redox potentials ( + 344 mV for the type-1 copper in ascorbate oxidase (see Table VII) +295 mV for the couple ascorbate/ascorbate-free radical (176)) is 49 mV. In the proposed modeled encounter complex (74), there is a short distance of about 7 A between the two redox centers (distance GUI—01 ASC = 6.8 A distance GUI—02 ASG = 7.5 A) and an effective parallel arrangement of the rings, with good overlap of the TT-electron density systems facilitating a rapid electron transfer (see Fig. 15). 

A similar study performed for CQ-NPG initiator systems [175] showed a rather different kinetic behavior (Figure 35). The rate of polymerization is reduced when the thermodynamic driving force (-ACet) of the process is increased, and this demonstrates inverted-region-like kinetic behavior. The authors explained this variation in reactivity as due to the reactivity of the free radicals formed as a result of processes occurring after electron transfer [123]. 

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