Reversible-deactivation radical polymerization in the presence of metallic copper. kinetic simulation

摘要

Reversible-deactivation radical polymerization (RDRP) of methyl acrylate in DMSO in the presence of Cu~0 was studied by kinetic simulations. Kinetic simulations give access to the rates and contributions of all reactions, including those of activation of alkyl halides by Cu~I and Cu~ 0 species, disproportionation of Cu~I species, and comproportionation between Cu~(II) and Cu~0. Every relevant reaction was quantified by experimentally measured rate coefficients. The rates and contributions allow the exact roles of Cu~0 and Cu~I species to be evaluated. These simulations show that the control over the polymerization is due to the atom transfer radical polymerization (ATRP) dynamic equilibrium with Cu~I as the major activator and Cu~(II) as the major deactivator. The ATRP equilibrium is maintained throughout the entire process. The simulations confirmed earlier experimental findings that in dimethyl sulfoxide (DMSO) with tris[2-(dimethylamino)ethyl]amine (Me_ 6TREN) ligand comproportionation between Cu~0 and Cu~ (II) species dominates disproportionation of Cu~I species, with both reactions being relatively slow. The contribution of Cu~0 activation of alkyl halides to the overall reaction is very small, and plays only a supplemental role, since alkyl halides are predominantly activated by Cu~I species. The effect of Cu~0 activity on polymerization rate and livingness were also studied by a series of simulations. In all cases, the rate of supplemental activation by Cu~0 was similar to the rate of radical termination, with both being relatively low in order to preserve the livingness of the chains. Cu~0 not only acts as a supplemental activator (SA), but also as a reducing agent (RA) and it is able to regenerate Cu~I from Cu~(II), through comproportionation. Simulations based on experimentally measured rate coefficients showed that Cu~0 acts as a supplemental activator and reducing agent (SARA) and the results of an RDRP in the presence of Cu~0 are consistent with the SARA ATRP mechanism, and in direct conflict with the single electron transfer-living radical polymerization (SET-LRP) mechanism. The kinetic analysis also revealed that the contribution of disproportionation of Cu~I to the polymerization kinetics is negligible, and that the Cu~I species are predominantly involved in activation reactions. The effect of the surface area of Cu~0, the effect of initially added Cu~(II) species, and other reaction parameters are discussed in light of SARA ATRP.