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|Title:||Experimental and Modeling Studies of Diffusion-Controlled Reactions in Atom Transfer Radical Polymerization at High Conversion|
|Authors:||Mohammad Rabea, Ali|
|Abstract:||Controlled radical polymerization (CRP), especially atom transfer radical polymerization (ATRP), has attracted great attention in the recent years. It provides a powerful tool for synthesis of functional polymers with a predetermined molecular weight and narrow molecular weight distribution (MWD). Although tremendous effort has been made in this area over the past decade, there are still many challenges that limit its industrial applications. High monomer conversion is one of the most important critical issues for commercial exploitation of CRP processes. Another important task is to eliminate organic solvents in polymerization. Most solvents are volatile organic compounds (VOCs), and are of environment and health concerns. Separating the polymer product from the solvent and removal of residual monomer are both energy intensive and costly. This thesis focused on the study of bulk ATRP at high conversion experimentally and through performing modeling. The main objectives are to elucidate the mechanism of ATRP at high conversion and to develop strategies for reaching high conversion in bulk while maintaining livingness and control of the polymerization. This sandwich-type thesis is organized based on five refereed-journal papers first-authored by the candidate. Methyl methacrylate (MMA) was polymerized in bulk via atom transfer radical polymerization (ATRP) using initiator for continues activator regeneration (ICAR) method. “Gel effect” started at medium conversion and it was accompanied with molecular weight distribution broadening. A dramatic increase in polydispersity was observed at high conversion. The loss of control over molecular weight was attributed to diffusion-controlled deactivation. Model simulation of bulk ATRP revealed that it is diffusion-controlled deactivation that causes auto-acceleration in the rate (“gel effect”) and loss of the control. At high conversion, radical chains are “trapped” because of high molecular weight and high viscosity. However, the migration of radical centers through propagation (residual deactivation) can still facilitate catalytic deactivation of radicals, which improves the control of polymer molecular weight to some extent. It was found that ICAR agents could significantly affect the rate of catalyst regeneration and consequently the rate and control of polymerization. By optimizing the ICAR agent concentration and employing step temperature profile, MMA was polymerized with ppm concentration of catalyst up to very high conversion with good livingness and control. Furthermore, it was demonstrated that employing ultrasonic waves could be helpful in order to keep the control and livingness up to high conversion through improving homogeneity of the polymerization medium and diffusivity of the reactants during the polymerization, especially after the onset of diffusion-controlled deactivation.|
|Appears in Collections:||Open Access Dissertations and Theses|
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