The Self-Reactions of 1-Methoxycarbonyl-1-methylethyl and Higher Ester Radicals: Combination vs Disproportionation and Oligomeric Products from Secondary Reactions

Abstract

The geminate self-reactions of the title methyl, ethyl and butyl ester radicals-(2a-c), formed by decomposition of the corresponding azo precursors (1a-c) in the presence of stable nitroxide radical scavengers, were found on the basis of product analysis to comprise combination and disproportionation in the ratios 56 : 44 (methyl), 58 : 42 (ethyl) and 47 : 53 (butyl). In the absence of radical scavengers, extensive oligomerization is observed. Hydrogenation and degradation were used in conjunction with g.l.c.-m.s . to deduce the identities of the dimeric, trimeric and tetrameric products, which were in most cases subsequently confirmed by isolation and n.m.r . analysis. Of particular interest is the highly regioselective disproportionation of radical (3) to give dimethyl 4-methylpent-1-ene-2,4-dicarboxylate (8), and the further reaction of (8) with (2a) to form branched oligomers (10) and (15).

Pentacarbonylmanganese enolate and dienolate complexes. Preparative and mechanistic considerations

“…A similar pattern of products was found for the THF solvent system. These products are in good agreement with those previously reported for radical reactions of ( c H~)~~' COOEt (13), where the radical is produced by a completely different process.…”

Determination of the Mode of Radical Termination from Pulsed Laser Polymerization Experiments in the Presence of Retardation and Chain Transfer to Agent

“…We have found that our value is an intermediate one among the values published in the literature for n‐ BMA polymerization. For termination of oligomeric polyBMA radicals at 80 °C, Bizilj et al have estimated a lower value of the termination mode: δ = 0.53. At the same time, they have pointed out that disproportionation may be slightly more important (higher value of δ ) due to analytical difficulties preventing a precise determination of the disproportionation products.…”

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

“…[9][10][11][12] The termination of the propagating polymer-endr adicalu ndergoes disproportionation (Disp)a nd combination (Comb)t oy ield dead polymers, whichc orrespond to 3 and 4 for Disp, 5 for Comb,a nd 6 fort he CÀNc ombination [Comb (C-N) ]w hen the electron-withdrawing group (EWG) = CN in the model system. [9][10][11][12] The termination of the propagating polymer-endr adicalu ndergoes disproportionation (Disp)a nd combination (Comb)t oy ield dead polymers, whichc orrespond to 3 and 4 for Disp, 5 for Comb,a nd 6 fort he CÀNc ombination [Comb (C-N) ]w hen the electron-withdrawing group (EWG) = CN in the model system.…”

“…Azo initiators are also used as model compounds for clarifying the termination reactiono fr adical polymerization. [9][10][11][12] The termination of the propagating polymer-endr adicalu ndergoes disproportionation (Disp)a nd combination (Comb)t oy ield dead polymers, whichc orrespond to 3 and 4 for Disp, 5 for Comb,a nd 6 fort he CÀNc ombination [Comb (C-N) ]w hen the electron-withdrawing group (EWG) = CN in the model system. Considering that the molecular weighta nd end-group structure affect the physical properties of polymers, an accurate understanding of the termination mechanism,t hat is, Disp/Comb selectivity is as ignificant importance.H owever,t he method wouldh ave inherent problems because, although both caged and diffused radicals generatet he same termination products, 3, 4, 5,a nd 6,t he selectivity among Disp, Comb,a nd Comb (C-N) wouldb edifferent.…”