DFT Studies on the Polymerization of Functionalized Styrenes Catalyzed by Rare-Earth-Metal Complexes: Factors Affecting C–H Activation Relevant to Step-Growth Polymerization

The polymerization mechanism of functionalized styrene derivatives, viz., p-NMe2 (pMNS)-, o-/p-OCH3 (o/pMOS)-, and p-SCH3 (pMTS)-substituted styrene, catalyzed by cationic rare-earth-metal catalysts has been comparatively studied through density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it is found that large steric hindrance as a main factor prevents oMOS from undergoing o-C–H activation relevant to step-growth polymerization, while the nonoccurrence of C–H activation of pMNS and pMTS can be explained by the relatively less dispersion of charge distribution in the four-center transition states. In addition, the electron-donating pyridine side arm weakened the interaction between pMOS and the metal center and meanwhile increased the steric hindrance, preventing the C–H activation. Therefore, the simultaneous occurrence of C–H activation (step-growth polymerization) and CC insertion (chain-growth polymerization) are affected by multiple factors such as the coordination ability of the heteroatom of monomers, steric hindrance, and the electron-donating ability of the ancillary ligand.