Multiscale Modeling and Characterization of Radical-Initiated Modification of Molten Polyolefins

Multiscale approaches for peroxide-initiated grafting of vinyl alkoxy silane (VAOS) monomers to polyolefins have been investigated to develop a mechanistic model for the synthesis of functional copolymers by melt-phase processing. By investigating the reaction kinetics through both molecular modeling and model compound studies, a comprehensive mechanistic view of the complex radical-mediated reactions is developed. This mechanism indicates that grafting of single monomers via intramolecular H transfer is much more probable than single grafts in close proximity, oligomeric grafts, or chain branching under typical melt processing conditions. These results clearly show that the overall mechanism is dominated by repeated grafting of single monomers of vinyl silane to a hydrocarbon substrate as a consequence of very rapid intramolecular hydrogen transfer, and the resulting radicals are terminated primarily via combination to form polymer cross-links. With the kinetic rates evaluated from a hybrid quantum calculation procedure as well as the analyses of the NMR spectrum and GC/MS data of model compounds, a fundamental kinetic model is established to depict the general chemistry involving all the critical reactions in modification of the molten polymer and their relationships to processing conditions. Predictions of the model are confirmed by Fourier transformation infrared spectroscopy (FTIR), linear rheology, and high-temperature gel permeation chromatography (HT-GPC) analysis of 30 polyolefin elastomer (POE) formulations. This allows quantitative estimation of both the grafting yield and the cross-linked polymer fraction during the synthesis of polyolefin graft copolymers in the molten state.