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.