An experimental and modelling investigation of a tetrafunctional initiator designed for free radical polymerizations is presented. Multifunctional initiators are believed to provide two advantages over traditional monofunctional initiators. With a higher number of functional sites per molecule, they are able to increase polymer production while simultaneously maintaining or increasing polymer molecular weight. Examination of the literature indicates the majority of academic and industrial published studies have investigated difunctional initiators with most focusing on styrene. In this thesis, a tetrafunctional initiator, JWEB50, was systematically investigated for a variety of monomer systems in order to develop a better understanding of the behaviour of multifunctional initiators in free radical polymerizations. <br /><br /> A kinetic study comparing the tetrafunctional initiator to a monofunctional counterpart, TBEC, demonstrated that the impact of a multifunctional initiator is dependent upon monomer type. Regardless of the homo- or copolymer system examined, it was observed that the tetrafunctional initiator could produce higher rates of polymerization due to the greater number of labile groups per initiator molecule. However, the influence of the tetrafunctional initiator on the polymer molecular weight was dictated by the polymerization characteristics of the system in question. In the case of styrene, the tetrafunctional initiator maintained similar molecular weights compared to the monofunctional initiator while for methyl methacrylate (MMA), switching from a mono- to a tetrafunctional initiator actually decreased the polymer molecular weight. Other monomers such as butyl acrylate and vinyl acetate and copolymers of MMA and styrene or alpha-methyl styrene were examined to study the effect of initiator functionality in free radical polymerizations. <br /><br /> Subsequent to the kinetic investigation, polystyrene and poly(methyl methacrylate) samples produced with the tetrafunctional initiator were characterized in detail in order to examine the effects of initiator functionality on polymer properties. Samples generated with the monofunctional initiator were used for comparison purposes. Chromatographic and dilute solution methods were able to detect significant levels of branching in the polystyrene sample produced with JWEB50, while poly(methyl methacrylate) samples showed no evidence of branching. Rheological tests involving a combination of oscillatory and creep shear measurements were completed in order to detect differences between samples. The presence of branching using rheological techniques was clearly observed for both polystyrene and poly(methyl methacrylate) samples produced with the tetrafunctional initiator. <br /><br /> In order to explain the experimental results observed in the kinetic and polymer properties studies, a reaction mechanism for polymerizations initiated with a tetrafunctional initiator was proposed and used in the development of a mathematical model. Reactions involving the fate/efficiency of functional groups are properly accounted for, while in the past this had been ignored by modelling work in the literature. Based on model predictions, di-radical concentrations were estimated to be several orders of magnitude smaller than mono-radical concentrations and their contribution in the reaction mechanism was found to be negligible. Modelling results also demonstrated that the concentration and chain length of various polymer structures (i. e. , linear, star or coupled stars) depend upon monomer type and reaction conditions.

An experimental and modelling investigation of a tetrafunctional initiator designed for free radical polymerizations is presented. Multifunctional initiators are believed to provide two advantages over traditional monofunctional initiators. With a higher number of functional sites per molecule, they are able to increase polymer production while simultaneously maintaining or increasing polymer molecular weight. Examination of the literature indicates the majority of academic and industrial published studies have investigated difunctional initiators with most focusing on styrene. In this thesis, a tetrafunctional initiator, JWEB50, was systematically investigated for a variety of monomer systems in order to develop a better understanding of the behaviour of multifunctional initiators in free radical polymerizations. <br /><br /> A kinetic study comparing the tetrafunctional initiator to a monofunctional counterpart, TBEC, demonstrated that the impact of a multifunctional initiator is dependent upon monomer type. Regardless of the homo- or copolymer system examined, it was observed that the tetrafunctional initiator could produce higher rates of polymerization due to the greater number of labile groups per initiator molecule. However, the influence of the tetrafunctional initiator on the polymer molecular weight was dictated by the polymerization characteristics of the system in question. In the case of styrene, the tetrafunctional initiator maintained similar molecular weights compared to the monofunctional initiator while for methyl methacrylate (MMA), switching from a mono- to a tetrafunctional initiator actually decreased the polymer molecular weight. Other monomers such as butyl acrylate and vinyl acetate and copolymers of MMA and styrene or alpha-methyl styrene were examined to study the effect of initiator functionality in free radical polymerizations. <br /><br /> Subsequent to the kinetic investigation, polystyrene and poly(methyl methacrylate) samples produced with the tetrafunctional initiator were characterized in detail in order to examine the effects of initiator functionality on polymer properties. Samples generated with the monofunctional initiator were used for comparison purposes. Chromatographic and dilute solution methods were able to detect significant levels of branching in the polystyrene sample produced with JWEB50, while poly(methyl methacrylate) samples showed no evidence of branching. Rheological tests involving a combination of oscillatory and creep shear measurements were completed in order to detect differences between samples. The presence of branching using rheological techniques was clearly observed for both polystyrene and poly(methyl methacrylate) samples produced with the tetrafunctional initiator. <br /><br /> In order to explain the experimental results observed in the kinetic and polymer properties studies, a reaction mechanism for polymerizations initiated with a tetrafunctional initiator was proposed and used in the development of a mathematical model. Reactions involving the fate/efficiency of functional groups are properly accounted for, while in the past this had been ignored by modelling work in the literature. Based on model predictions, di-radical concentrations were estimated to be several orders of magnitude smaller than mono-radical concentrations and their contribution in the reaction mechanism was found to be negligible. Modelling results also demonstrated that the concentration and chain length of various polymer structures (i. e. , linear, star or coupled stars) depend upon monomer type and reaction conditions.

This dissertation presents studies of the synthetic processes and applications of siloxane-based materials. Kinetic investigations of bridged organoalkoxysilanes that are precursors to organic-inorganic hybrid polysilsesquioxanes are a primary focus. Quick gelation despite extensive cyclization is found during the polymerization of bridged silane precursors except for silanes with certain short bridges. This work is an attempt to characterize and understand some of the distinct features of bridged silanes using experimental characterization, kinetic modeling and simulation. In addition to this, the dissertation shows how the properties of siloxane- materials can be engineered for drug delivery and adsorption. The phase behavior of polymerizing mixtures is first investigated to identify the solutions that favor kinetic characterization. Microphase separation is found to cause gradual loss of NMR signal for certain initial compositions. Distortionless Enhancement by Polarization Transfer 29Si NMR is employed to identify the products of polymerization of some short-bridged silanes under no signal loss conditions. This technique requires knowing indirect 29Si-1H scalar coupling constants which sometimes cannot be measured due to second-order effects. However, the B3LYP density functional method with 6-31G basis set is found to predict accurate 29Si-1H coupling constants of organoalkoxysilanes and siloxanes. The scalar coupling constants thus estimated are employed to resolve non-trivial coupled NMR spectra and quantitative kinetic modeling is performed using the DEPT Si NMR transients. In order to investigate the role of the organic bridging group, the structural evolution of bridged and non-bridged silanes are compared using Monte Carlo simulations. Kinetic and simulation models suggest that cyclization plays a key role right from the onset of polymerization for bridged silanes even more than in non-bridged silanes. The simulations indicate that the carbosiloxane rings formed from short-bridged precursors slow down but do not prevent gelation. The tuning of siloxane-based materials for adsorption technologies are also discussed here. In the first example, antioxidant enzyme loading is investigated as a means to reduce oxidative stress generated by silica nanoparticle drug carriers. Materials are engineered for promising enzyme loading and protection from proteolysis. Second, the potential of copper sulfate impregnation to enhance adsorption of ammonia by silica is explored by molecular simulation.