The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs.

The effect of viscosity on the diffusion efficiency (Fdif ) of an organic radical pair in a solvent cage and the termination mechanism, that is, the selectivity of disproportionation (Disp) and combination (Comb) of the geminated caged radical pair and the diffused radicals encountered, were investigated quantitatively by following the photolysis of dimethyl 2,2'-azobis(2-methylpropionate) (V-601) in the absence and presence of PhSD. Fdif and Disp/Comb selectivity outside the cage [Disp(dif) /Comb(dif) ] are highly sensitive to the viscosity. In contrast, the Disp/Comb selectivity inside the cage [Disp(cage) /Comb(cage) ] is rather insensitive. The difference in viscosity dependence between Disp(cage) /Comb(cage) and Disp(dif) /Comb(dif) is explained by the spin state of the radical pair inside and outside the cage and the spin state dependent configurational changes of the radical pair upon their collision. Given that the configurational change of the radicals associates the displacement and reorganization of solvents around the radicals, the termination outside the cage, which requires larger change than that inside the cage, is highly viscosity dependent. Furthermore, while the bulk viscosity of each solvent shows good correlation with Fdif and Disp/Comb selectivity, microviscosity is the better parameter predicting Fdif and Disp(dif) /Comb(dif) selectivity regardless of the solvents.

Control of the Termination Mechanism in Radical Polymerization by Viscosity: Selective Disproportionation in Viscous Media.

The termination mechanism of radical polymerization, that is, disproportionation (Disp) versus combination (Comb), determines the chain length and end-group structure of the resulting polymer as well as polymer properties, and yet factors governing the mechanism are still unclear. Furthermore, no attempts have been made to control the mechanism. Here, the effects of temperature and viscosity on the termination of methyl methacrylate (MMA) and styrene (St) polymerization were elucidated by using small molecular model-radicals and the corresponding polymer radicals in various solvents. The results showed that Disp was preferred over Comb if the temperature was decreased and the viscosity of the media was increased for all the radicals examined. Although the temperature effect on the Disp/Comb selectivity is counterintuitive because Disp should be favored entropically over Comb considering the decrease in the number of polymer chains in Comb, the results clearly showed that the observed inverse temperature effect was a result of the viscosity effect. Disp was favored over Comb at lower temperatures and in more viscous solvents because the transition state leading to Disp is more flexible than that for Comb. Because of the significant viscosity effect, Disp selectively occurred in highly viscous solvents; the Disp/Comb selectivity was 97/3 in both MMA and St termination. For the first time, the termination mechanism was intentionally controlled and such a high Disp selectivity was observed. In particular, the termination mechanism in St is described as Comb in textbooks, but nearly complete inversion of the selectivity from Comb to Disp is realized by simply changing the viscosity of the media.

Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene.

The mechanism of the Cu(I)/Cu(0)-mediated reductive coupling reactions of bromine-terminated polymer chain-end radicals, so-called atom-transfer radical coupling (ATRC), is studied. Poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), and polystyrene (PSt), prepared by atom-transfer radical polymerization (ATRP), were activated by an excess amount of Cu(I)Br and Cu(0) in the presence of tris[2-(dimethylamino)ethyl]amine (Me6TREN), and the structural analyses of the resulting polymer products and deuterium-labeling experiments unambiguously determined the mechanism. While PMMA and PSt reacted by a radical mechanism as expected, PMA-bromide was reduced to an anionic species, which was most likely an organocopper species. Trapping experiments with TEMPO suggested that the polymer chain-end radicals were generated in all cases by the reduction of the bromine-terminated polymers by low-valent Cu species. However, the PMA chain-end radical was further reduced to the anionic species from which coupling products formed in low yield.