Disproportionation or Combination? The Termination of Acrylate Radicals in ATRP

Abstract

The termination of acrylate radicals in atom transfer radical polymerization (ATRP) can involve either conventional bimolecular radical termination (RT) or catalytic radical termination (CRT). These processes were investigated using a poly(methyl acrylate)-Br macroinitiator under different initial conditions tuned to change the RT/CRT ratio. The polymers, obtained from alkyl halide chain-end activation by [Cu I (L)] + (L = tris[2-(dimethylamino)ethyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA), or tris(3,5dimethyl-4-methoxy-2-pyridylmethyl)amine (TPMA* 3)) in the absence of monomer, were analyzed by size exclusion chromatography (SEC). RT-promoting conditions resulted in the increase of a shoulder with double molecular weight (MW) relative to the macroinitiator distribution, indicating that RT occurred predominantly via radical combination. Conversely, when CRT was promoted, the macroinitiator distribution did not shift, indicating a disproportionation-like pathway. The termination reactions for the TPMA system were further analyzed via PREDICI simulations, which showed the significant impact of mid-chain radicals, arising from backbiting, on the overall termination profile. In all cases, CRT and cross termination between secondary chain-end and tertiary mid-chain radicals contributed the most to the overall amount of terminated chains.

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

“…In this study, preset amounts of the [Br‐Cu II (L)] + deactivator were also added. While the deactivator plays no direct role in CRT, it lowers the radical concentration by shifting the ATRP activation equilibrium and thus skews the termination process in favor of CRT . Using the Me 6 TREN system, while the product distribution obtained in the absence of Cu II essentially matched the previously published one, an incremental addition of Cu II decreased the double mass distribution, unambiguously proving that the Comb product derives predominantly from the spontaneous bimolecular termination and not from CRT.…”

“…A subsequent investigation, however, revealed that part of the above conclusions were based on a wrong assumption. This new investigation made use of TPMA, TPMA* 3 and Me 6 TREN in carefully dried MeCN at room temperature and in the absence of Cu 0 , in order to avoid any possible contribution of Cu 0 ‐CRT and to allow an accurate determination of the extent of terminations by measurement of [Cu II ] (halogen conservation principle).…”

“…Neither did this Disp/Comb ratio change significantly in the presence of [Cu I (Me 6 TREN)] + (in CD 3 CN) or CuBr/Cu 0 /Me 6 TREN (in C 6 D 6 ), when no H‐X was present. The molar amounts used in these experiments were chosen to mimic those of the previous CRT studies in MeCN and toluene, respectively, but in addition, these experiments were done in thoroughly dehydrated solvents. This result is not surprising, because the stabilized tertiary cyanoisopropyl radical should not favourably interact with L/Cu I to generate a sufficiently stable L/Cu II ‐C(CN)Me 2 intermediate, thus no significant impact of CRT is anticipated.…”

Why Do We Need More Active ATRP Catalysts?

“…Although the mechanism and product distribution of CRT are still under investigation, some reports have shown that CRT results in saturated chain ends . Thus another decomposition pathway for R−Cu II /L, called reductive radical termination (RRT), is plausible.…”

Pseudo-Homopolymerization Approach To Predict the Molecular Weight Distribution in the Copolymerization via Activator Regenerated by Electron Transfer Atom Transfer Radical Polymerization

“…However, the addition of a low amount of BA in the system gives rise to a remarkable decrease in the M n with higher dispersity values. 26 Recently, Ribelli et al 27 reported the contribution of the bimolecular reaction termination and the catalytic radical termination (CRT) in the polymerization of methyl acrylate (MA) via ATRP using Cu 2 Br (pMA-Br) with several ligands: TPMA, tris [2-(dimethylamino)ethyl]amine (Me 6 TREN), and tris(3,5-dimethyl-4-methoxy-2-pyridylmethyl) (TPMA* 3 ). By using size exclusion chromatography (SEC), the MWD of the polymers obtained from the activation of the pMA-Br macroinitiator in more conventional radical conditions exhibited a bimodality, concluding that the higher molecular weight population corresponds to dead chains predominantly obtained by termination by combination for acrylate systems.…”