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

A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R² substituent in the atom‐transfer radical polymerization (ATRP) of styrene catalyzed by FeCl₂(R¹N?C(R²)?C(R²)?NR¹)] complexes. All model systems used R¹=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 R²=H. This study suggests that CCT could occur by direct hydrogen‐atom transfer without any activation barrier. Further analysis of more realistic models with R²=p‐C₆H₄F or p‐C₆H₄NMe₂ 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(R¹N?C(R²)?C(R²)?NR¹)] 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.