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Organometallic-Mediated Radical Polymerization
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Daniel L. Coward, Benjamin R. M. Lake, Michael Shaver
Small alterations to the acac framework offer little advantage over the initial acac system. Adding a bulky tert-butyl group to acac yields tetramethylheptadionato (tmhd), and the complex [Co(tmhd)2] (10, Figure 6.9) was used alongside V-70 at 30°C.49 Using different ratios of V-70 to 10, polymerizations gave well-controlled polymers with dispersities between 1.1 and 1.5. After six half-lives of V-70, the mechanism of control switches from DT-OMRP to RT-OMRP, albeit with poorer control than [Co(acac)2]. Use of a solvent extends the reaction times, thus slowing down the DT mechanism and promoting the RT mechanism. At long reaction times, deviation from the first-order rate law is observed. This is due to the breakdown of the persistent radical effect and the presence of competing catalytic chain transfer. Further promotion of RT was achieved by decreasing the amount of V-70 used and preventing complete conversion of Co(II) to Co(III), the species required for DT-control. Furthermore, use of an external base, such as water or pyridine, blocks the vacant coordination site on the Co(III)-R species and prevents DT-control. Comparisons between 10 and [Co(acac)2] show that in the absence of an external base the polymerization rate is faster for the former, whereas the latter is five times faster in the presence of an external base. This is due to the competitive steric effects on the Co(III)-R and Co(II)-L bond strengths (where L is the external base) and was supported by 1H NMR and DFT studies.
The interaction of carbon-centered radicals with copper(I) and copper(II) complexes*
Published in Journal of Coordination Chemistry, 2018
Thomas G. Ribelli, Krzysztof Matyjaszewski, Rinaldo Poli
Subsequent work led to the discovery of the CRT phenomenon. Under conditions where excess radicals with respect to the amount of L/CuI were produced, the radical termination rate increased. Various ligands, shown in Figures 7 and 8, were tested and proved to strongly affect the CRT activity (BPMAMe (∼no activity) < BPMA*Pr ∼ TPMA < TPMA*1 < TPMA*2 < TPMA*3) [82, 84] The activity in CRT scales with the electrochemical reduction potential and with the ATRP activity and is not strongly influenced by the ligand denticity (tridentate vs. tetradentate) as demonstrated by the BPMA*Pr-TPMA comparison, although the kinetic analysis revealed that a denticity change altered the rate-determining step of the CRT process under these conditions. The possible involvement of a hydride intermediate, which would lead to termination by disproportionation by a mechanism related to that of catalytic chain transfer [103], was excluded by a computational study [83] while the key role of organocopper(II) species is consistent with the correlation of the CRT activity with the calculated bond strength of the L/CuII−R bond (R = CHMe(COOMe), molecular model of a PMA chain) [84]. However, no clear experimental evidence could be obtained for the generation of this organometallic species in these polymerization studies.