Effect of Substituents on Radical Stability in Reversible Addition Fragmentation Chain Transfer Polymerization:  An ab Initio Study

Abstract

The effects of the R- and Z-substituents on radical stability in the reversible addition fragmentation chain transfer (RAFT) polymerization process have been studied via high level ab initio molecular orbital calculations. Radical stabilization energies (RSEs) of the RAFT-adduct radicals CH3SC•ZSR and corresponding leaving group radicals R• have been calculated for various combinations of Z = H, Cl, C⋮CH, CHCH2, CN, CF3, NH2, CH3, Ph, Bz, naphthyl, OCH3, OCH2CH3, OCH(CH3)2, and OC(CH3)3 and R = CH2CN, C(CH3)2CN, Bz, CH(Ph)CH3, C(Ph)(CH3)2, CH2COOCH3, CH(COOCH3)CH3, CH2OCOCH3, and CH2CH3. The results were used in combination with the corresponding values of the enthalpies of the fragmentation reactions, CH3SC•(Z)SR → CH3SC(Z)S + •R and CH3SC•(Z)SR → •CH3 + SC(Z)SR, to examine the effects of the substituents on the stability of both the RAFT-adduct radicals and the corresponding thiocarbonyl compounds. The RAFT-adduct radicals are stabilized by electron donation from the two sulfur substituents, and this stability can be further enhanced by unsaturated π-accepting substituents (such as CN, phenyl, and naphthyl). In contrast, lone pair donor Z-substituents (such as Cl, NH2, and OCH3) have a much smaller effect on radical stability. The R-group, which can modify the donation ability of the SR-group, has a minimal effect on the stability of the RAFT-adduct as it is buffered by the second sulfur substituent. However, these orbital interactions do affect the strength of the breaking S−R bond, and this provides an important contribution to the trends in the fragmentation enthalpies. Steric effects on radical stability are also important, with bulky R- and Z-groups inducing conformational changes that interfere with these orbital interactions, sometimes with unexpected consequences. The substituent effects on the RAFT agents are qualitatively different; the agents are strongly stabilized by the lone pair donor Z-substituents and strongly destabilized by electron withdrawing groups (such as CN and CF3) in the R- and Z-positions. Moreover, steric effects are generally more significant, with bulky R- and Z-groups destabilizing the RAFT agent more than the corresponding RAFT-adduct radicals. As part of this work, the accuracy of the low-cost RMP2/6-311+G(3df,2p) method for studying addition−fragmentation processes in RAFT polymerization was evaluated.

Bond Dissociation Energies of Organophosphorus Compounds: an Assessment of Contemporary Ab Initio Procedures

“…A profound performance study of this method has however not yet been done for phosphorus-containing species and hence we compute heats of formation for a set of small organophosphorus molecules for which experimental data is available. The excellent performance of G3(MP2)-RAD for other open-shell species is earlier reported, in particular for carbon- 29,58,59 and nitrogen-centered radicals. 60 Phosphorus-containing additives have been shown to be effective in inhibiting coking rates during thermal cracking processes.…”

Chain Transfer in Degenerative RAFT Polymerization Revisited: A Comparative Study of Literature Methods

“…Transfer coeffi cients can then be calculated a posteriori and explicitly. Hitherto, high-level ab initio calculations [ 17,[34][35][36][37] and parameter estimation procedures based on more extensive kinetic modeling results [38][39][40][41][42] have been used to obtain these individual rate coeffi cients. Despite being based on fi rst principles, the former calculations often rely on calculations for smaller compounds to represent the considered system as a compromise between accuracy and computational cost, [ 15,37 ] whereas the parameter estimation methods highly depend on the elementary reactions considered in the kinetic model and the size of the polymerization data set.…”

Molecularly imprinted polymers by reversible addition–fragmentation chain transfer precipitation polymerization for preconcentration of atrazine in food matrices

“…1 shows the common structure of CTA with dithioester. An ideal CTA should satisfy the following requirements [26]: CTA should have a high chain transfer constant; the R-S bond should be a weak single bond because R is a free radical leaving group, and R free radical must be able to reinitiate polymerization, and furthermore, the activity of R free radical should be higher than primary free radical; the role of Z is to modify addition or fragmentation rate.…”