A quantum mechanical study of methacrylate free-radical polymerizations

Theoretical calculations at the semiempirical and ab initio levels of theory have been completed for a series of methacrylate compounds reported in the literature as measured by pulsed-laser initiated polymerization in conjunction with size-exclusion chromatography (PLP-SEC). Modeling includes calculation of the Gibb's free energies (ΔG(‡)) and activation energies (E(a)). These results were then compared to experimental results. Semiempirical ΔG(‡) using AM1-CI calculations successfully predicted relative activation energies (R(2) = 0.89). HF and DFT methods more accurately predicted absolute activation energies, but the relative values were less reliable. Accurate quantitative structure property relationship (QSAR) models for propagation rate coefficients, k(p), were developed using AM1-CI and DFT. The semiempirical model included two charge descriptors, partial negatively charged surface area (PNSA), and minimum net atomic charge for oxygen (R(2) = 0.959). The DFT information, which included two quantum chemical descriptors (1-electron reactivity index for carbon and point charge component of the molecular dipole) calculated from the ground state structure, had improved statistics (R(2) = 0.979). A second DFT model is reported for 10 hydrocarbon methacrylate structures based on the 1-electron reactivity index for carbon (R(2) = 0.979). Theoretical results were also analyzed to provide an explanation for the unexpectedly large experimental k(p) values observed in the case of larger methacrylate monomers.