Primary cyclization during the network formation of crosslinking polymers.

摘要

Free radical Polymerization of multifunctional monomers produces crosslinked polymer networks that have a wide variety of applications including protective and decorative coatings, dental restorative materials, superabsorbent materials, contact lenses, and biomedical materials. When designing crosslinked polymer networks for specific applications, understanding of how the choice of monomer, comonomer ratio and reaction conditions affect the network formation and final mechanical properties is imperative. The polymer network structure will dictate many of the material properties. Therefore, with understanding of what controls the network formation, the properties of the material are tailored for the desired application. With a combination of available experimental techniques and modeling, the nature and influence of primary cyclization on network evolution and free radical polymerization kinetics are studied.; This work focuses on developing a first principles model for predicting the network structure as it evolves during free radical polymerization, specifically incorporating the varying pendant reactivity and primary cyclization reactions that occur during the formation of crosslinked polymers. Several experimental systems from highly crosslinked polymeric dental materials to loosely crosslinked acrylic acid hydrogels, were studied to evaluate factors influencing the crosslink density and compared with model predictions. Specifically, the model is utilized to investigate the effects of solvent concentration, crosslinking agent concentration, monomer size, monomer stiffness, and monomer functionality on the network formation. The pendant reactivity, cyclization rate, gel point, and molecular weight between crosslinks are predicted with the model.; Additionally, the model predicts how the evolving network structure affects the mobility of polymer systems and the resulting polymerization kinetics. The effect of primary cyclization on polymerization kinetics was characterized experimentally and evaluated from a theoretical standpoint. A new kinetic model is developed which successfully predicts experimental data for how the extent of primary cyclization and crosslinking influences the polymerization kinetics. Finally, molecular dynamics simulations were used to study the polymer chain conformation during the initial propagation steps. Specifically, the distance between various pendant double bonds and the propagating radical was ascertained. This information is useful for more accurate predictions of primary cyclization rates.