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|Title:||Synthesis and Evaluation of Model Initiator / Reversible Terminating Adducts for the Stable Free Radical Polymerization Process|
|Authors:||Moffat, Ann Karen|
|Advisor:||Stover, Herald D.|
|Abstract:||<p>This thesis describes the synthesis, characterization and evaluation of model initiator/reversible terminating agents, to enhance the fundamental understanding of the stable free radical polymerization process. One portion of this research, involved AM1 and PM3 semi-empirical molecular orbital calculations on a model SFR reaction. The aim of this work was to identify new nitroxyl radicals with a labile C-O bond, but it became apparent that semi-empirical MO calculations were not accurate enough to differentiate between various nitroxyl radicals. To calibrate the MO calculations and determine where the deficiencies in the methods were, the single crystal X-ray structure of MB-TMP was obtained. A series of alkoxy and aryloxy-tetramethylpiperidine compounds, RO-TMP were prepared by the reaction of tri-n-butyl tin hydride with various alkyl or aryl halides and TEMPO. Using the same synthetic route, the 1-phenylethyl carbon radical fragment was trapped by 1,1,3,3-tetramethyl- and 1,1,3,3-tetraethylisoindoline nitroxide to form MB-TMI and MB-TEL. All compounds were isolated, purified and characterized by ¹H and ¹³C NMR spectroscopy. Using ESR spectroscopy, the enthalpy of activation to homolytically cleave the C-O bond in ten of the nitroxide adducts was measured. In the adduct series, MB-TMP, MiP-TMP and EiB-TEMP which mimic the styrene, acrylate and methacrylate monomers respectively, the enthalpy of activation decreased from 131.2 ±7.2, 125.3 ±5.4 to 119.2 ±3.4 kJ/mol, respectively. This indicated that the C-O bond strength between the propagating polymer chain end and TEMPO is the weakest for a methacrylate monomer with increasing C-O bond strength for acrylate and styrene monomers. Steric bulk of the tertiary methacrylate radical carbon contributed to a weaker and more labile C-O bond. In the series of initiating carbon radicals bonded to TEMPO, it was demonstrated that increasing the steric crowding about the carbon radical decreased the enthalpy of activation to break the C-O bond. Also, when steric bulk was increased on the alpha carbons to the nitroxyl functional group in the tetraethylisoindoline nitroxide, the C-O bond in 1-phenylethyl-tetraethylisoindoline adduct, MB-TEI was 4.7 kJ/mol weaker than in MB-TMI. The thermal decomposition of nitroxide adduct MB-TMP was measured as a function of time at 125℃ by ESR and ¹H NMR spectroscopy. As a result of this kinetic study, an alternative mechanism to that of Li et al. was proposed for possible polymer chain termination when the excess nitroxide in the polymerization system builds up. All major products from the thermal decomposition of MB-TMP were identified and the effect of oxygen in the polymerization via autoxidation mechanism was observed. This study has identified that hydrogen radical abstraction by a significant excess of TEMPO can produce a terminal vinyl group at the end of a polystyrene chain. In the presence of oxygen and excess TEMPO, a phenyl ketone functional group can also be produced to terminate a polystyrene chain end. Two nitroxide adducts, MB-TMP and B-TMP and the unimer BST were evaluated as initiators and as the only source of nitroxyl radical in the bulk polymerization of styrene by the stable free radical polymerization process. The C-O bond in B-TMP did not cleave at a fast enough rate to enable a controlled SFR polymerization. Instead, polystyrene chains were self-initiated by the Mayo mechanism resulting in a conventional free radical polymerization process. The initiating efficiency of MB-TMP was found to be equivalent to BST and a controlled "living" stable free radical polymerization with narrow polydispersities demonstrated. The alpha-methyl group on the benzylic carbon in MB-TMP was responsible for increasing the steric bulk around the C-O bond and making the bond more labile. The addition of excess nitroxide decreased the rate constant of propagation and lowered MWD during the initiation period of polymerization.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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