Effect of the Substituent Position on the Anionic Copolymerization of Styrene Derivatives: Experimental Results and Density Functional Theory Calculations

In a combined synthetic, kinetic and theoretical study, the living anionic copolymerization of styrene and its ring-methylated derivatives ortho-, meta-, and para-methylstyrene (MS) was examined by real-time ¹H NMR spectroscopy in the nonpolar solvents toluene-d₈ and cyclohexane-d₁₂ as well as by density functional theory calculations. Based on the NMR kinetics data, reactivity ratios for each comonomer pair were determined by the Kelen–Tüdős method and numerical integration of the copolymerization equation (Contour software). The reaction pathway was modeled and followed by density functional theory (DFT) calculations to validate and predict the experimentally derived reactivity ratios. Unexpectedly, two of the three styrene derivatives showed completely different reactivities in copolymerization, governed by the position of the methyl group. While para-MS is considerably less reactive than styrene, resulting in gradient copolymers (r_S = 2.62; r_pMS = 0.37), ortho-MS showed the opposite behavior and is more reactive than styrene (r_S = 0.44; r_oMS = 2.47), leading to a reversal of the copolymers’ gradient. The substitution in the meta-position had nearly no influence on monomer reactivity, and copolymers with close to random comonomer distribution were formed (r_S = 0.81; r_mMS = 1.21). In all cases, the theoretical calculations showed good to excellent agreement with the experimental data. Monomer reactivities correlate with the chemical shifts of the β-carbon signals in ¹³C NMR spectra that are predictive for the gradient structure. The results demonstrate the possibility of tailoring and validating the polymer structures of functional polystyrene copolymers by the choice of the substitution pattern of styrene derivatives, using both experimental and theoretical approaches.