The Mechanism of Stereoregulation in Free-Radical Polymerization of Bulky Methacrylates

Theoretical calculations are performed to explore the origin of inherent tacticity in bulky methacrylates. Geometries and conformer distributions of monomers and oligomeric propagating radicals are calculated to study the impact of steric bulk and π-stacking interactions on the preferences for meso versus isotactic propagation. Consistent with the previous qualitative analyses by Satoh and Kamigaito, we have demonstrated a correlation between the preference for meso propagation and the steric bulk of the ester side chain, where the latter is measured as the volume of the side chain. We have also confirmed that syndiotactic methacrylates prefer linear chains, isotactic methacrylates prefer helical chains and the increasing isotactic preference with chain length can thus be understood in terms of the increasing helical tendency as substituents become more bulky. We also demonstrated that, whilst π-stacking interactions in aryl methacrylates are significant, the extent to which they influence the tacticity depends on their bulkiness and associated helical tendency. We have also provided an explanation for their solvent dependence in terms of the disruption of π-stacking conformations by the formation of inclusion complexes.