ATRP/OMRP/CCT Interplay in Styrene Polymerization Mediated by Iron(II) Complexes: A DFT Study of the α‐Diimine System

Abstract

A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R(2) substituent in the atom-transfer radical polymerization (ATRP) of styrene catalyzed by [FeCl2 (R(1) N=C(R(2))C(R(2))=NR(1))] complexes. All model systems used R(1) =CH3 in place of the experimental Cy and tBu substituents and 1-phenylethyl in place of the polystyrene (PS) chain. A mechanistic investigation of 1) ATRP activation, 2) radical trapping in organometallic-mediated radical polymerization (OMRP), and 3) pathways to the hydride CCT intermediate was conducted with a simplified system with R(2)=H. This study suggests that CCT could occur by direct hydrogen-atom transfer without any activation barrier. Further analysis of more realistic models with R(2)=p-C6 H4 F or p-C6 H4 NMe2 suggests that the electronic effect of the aryl para substituents significantly alters the ATRP activation barrier. Conversely, the hydrogen-atom-transfer barrier is essentially unaffected. Thus, the greater ATRP catalytic activity of the p-NMe2 system makes the background CCT process less significant. The DFT study also compares the [FeCl2 (R(1) N=C(R(2))=C(R(2))=NR(1))] systems with a diaminobis(phenolato) derivative for which the CCT process shows even greater accessibility but has less incidence because of faster ATRP chain growth and interplay with a more efficient OMRP trapping. The difference between the two systems is attributed to destabilization of the Fe(II) catalyst by the geometric constraints of the tetradentate diaminobis(phenolato) ligand.

Understanding Organometallic-Mediated Radical Polymerization with an Iron(II) Amine–Bis(phenolate)

“…This behavior suggests that CCT is the predominant termination mechanism, rather than bimolecular termination. This mechanism has been previously observed for both β-ketiminate 8 and α-diimine 29,30…”

The Interplay of ATRP, OMRP and CCT in Iron-Mediated Controlled Radical Polymerization

“…6b, 8b and 7b (left to right) compared (32,47) These findings were found to accord with those of Johansson and Swart, who also recently published a computational study of this Fe-α-diimine system (48). Johansson and Swart also argued that the electron-withdrawing/donating parasubstituents of the 2,3-diarylated α-diimine complexes act to modulate the relative energies of the ATRP and CCT pathways by charge-transfer effects.…”

“…Poli and co-workers recently reported a computational investigation of the mechanism of CRP mediated by this family of Fe II -α-diimines (47). In contrast to a previous report (32), they found that the ligands themselves do not control the spin-state of the various complexes.…”

“…Poli and co-workers sought to further explore the intimate mechanism of the CCT reaction observed in certain systems and investigate the origin of the mechanistic differences between various 2,3-diarylated α-diimines (47). Contrary to what was suggested in previous work (36), where the CCT reaction was proposed to occur via a β-hydride elimination pathway from an OMRP resting state, DFT calculations here indicated that a direct H-transfer from the propagating radical is the first step in the CCT mechanism.…”

New Phenomena in Organometallic‐Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers

“…Indeed, a computational study has indicated that the preferred pathway to CCT, starting from the Fe III OMRP dormant species, is homolytic bond breaking followed by direct hydrogen‐atom transfer (Scheme , path a), which occurs essentially without any overbarrier 25. For both α‐diimine and diaminobis(phenolato) systems ( 6 and 7 ), the Fe II complex (Mt x /L) has a four‐coordinate distorted tetrahedral structure and a high‐spin state ( S =2), whereas the CCT intermediate (HMt x +1 /L) and the OMRP dormant species (P n Mt x +1 /L) have an S =3/2 ground state.…”