Catalyzed Radical Termination in the Presence of Tellanyl Radicals

Abstract

The decomposition of the diazo initiator dimethyl 2,2'-azobis(isobutyrate) (V-601), generating the Me C (CO Me) radical, affords essentially the same fraction of disproportionation and combination in media with a large range of viscosity (C D , [D ]DMSO, and PEG 200) in the 25-100 °C range. This is in stark contrast to recent results by Yamago et al. on the same radical generated from Me C(TeMe)(CO Me) and on other X-TeR systems (X=polymer chain or unimer model; R=Me, Ph). The discrepancy is rationalized on the basis of an unprecedented RTe -catalyzed radical disproportionation, with support from DFT calculations and photochemicaL V-601 decomposition in the presence of Te Ph .

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

“…Both Disp (cage) /Comb (cage) and Disp (dif) /Comb (dif) increased with the increase in bulk viscosity,b ut the magnitude was significantly different. The results clearly show that the limited viscosity effect of the Disp (sum) /Comb (sum) selectivity observed in the previous [14] and this studies is due to the marginal viscosity effect on the Disp (cage) /Comb (cage) selectivity and high viscosity dependence of F dif .T he results also demonstratet he irrelevance of the use of azo compounds for clarifying the termination mechanism of radicalp olymerization, as all polymer-end radicals are diffusedradicals. The difference between Disp (cage) / Comb (cage) and Disp (dif) /Comb (dif) becomes more significant in [D 6 ]DMSO and PEG400 (runs 5-12).…”

“…Please keep in mind that the structures of 9d, 10 d,a nd 11 d are identicalt ot hose of 3b, 4b,a nd 5b,r espectively.T wo significant conclusions have been drawn from the analyses of molecular weights and endgroup structures:1 )the termination of ap olyacrylate-end radical predominantly proceeds by Disp under ambient temperature, whereas major textbooks describe the termination of acrylate polymerization as Comb. [14] They proposed the tellanyl radical generated from 8c abstracts the b-hydrogen of 7c in as olvent cage, resulting in 9c and a tellurol, which furtherr eacts with either 7c or 8c,g iving 10 c (Scheme2,p ath B). [17] In contrast, Matyjaszewski and Poli have proposed that the tellanyl-radical-mediated chain-transfer pathway is responsible for the formation of Disp products in our study from the mar-ginal Disp/Comb selectivity on the viscosity by using V-601.…”

“…[13] However,t he impact of the Disp/Comb selectivity of caged and diffused radicals having polar functional groups, 2, which mimicst he polymethacrylate end radical, has not been reported so far.V ery recently, Matyjaszewski and Poli have studied the viscosity effect on the termination reaction of radical 2b generated from an azo compound, V-601 (1b). Althought hey observed marginal viscosity effects on the Disp/ Comb selectivity (Scheme 1, R = CO 2 Me), [14] no cage effect was considered.…”

Impact of Catalyzed Radical Termination (CRT) and Reductive Radical Termination (RRT) in Metal‐Mediated Radical Polymerization Processes

“…In light of this study, it is quite clear that the spontaneous termination of acrylate radicals proceeds primarily by Comb. The observation of prevalent Disp when these radicals were generated from photolysis of PhTe‐PMA is an interesting phenomenon and a possible interpretation has been advanced . However, the nature of the product(s) derived from CRT is still uncertain, because all the above studies were carried out using an over‐stoichiometric amount of L/Cu I .…”

“…The observation of prevalent Disp when these radicals were generated from photolysis of PhTe-PMA [25] is an interesting phenomenon and a possible interpretation has been advanced. [28] However, the nature of the product(s) derived from CRT is still uncertain, because all the above studies were carried out using an over-stoichiometric amount of L/Cu I . While this ensured an efficient radical trapping with minimization of the competing bimolecular termination process, it also evidenced the presence of reductive radical termination (see next section), which, like Disp, yields a molar mass distribution identical to that of the macroinitiator.…”