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 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.