Doctoral Dissertations

Date of Award

5-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Spiro D. Alexandratos

Committee Members

Jeff Kovac, Richard Pagni, Engin Serpersu

Abstract

Polymers can be designed with specific chemical properties for application in organic synthesis and selective metal ion complexation. The chemical properties of the polymer-supported reagent are influenced by the structure of the polymer backbone. The focus of this dissertation is to investigate this relationship in the attempt to optimize the kinetics of the polymer-supported reagent for its specific application.

The influence of the polymer structure on the kinetics of an SN2 displacement mechanism was evaluated by the Hammett correlation. Polymer-supported phenoxide resins were synthesized from polystyrene beads and used in the Williamson ether synthesis. The initial rate of the reaction between the phenoxide resins and benzyl chloride in ethanol decreased with increasing strength of the electron withdrawing group of the substituted phenoxide. A non-linear Hammett correlation of the kinetic data determined a change in the rate limiting step to occur between sigma -0.17 and 1.27. It is proposed that a change in the rate limiting step from ion-pair dissociation to nucleophilic attack is the result of an increased delocalization of the phenoxide electron density. Variations in the polarity of the polymer backbone did not alter the rate of ion-pair dissociation, but did influence the activation energy of nucleophilic attack. The initial reaction rate with polymer-supported p-nitrophenoxide was increased by a factor of 1.4 when the percentage of ionic monomer units was decreased from 100% to 50%. It is proposed that a decrease in the polarity of the local environment around the phenoxide increased the reactivity of the reactants, resulting in an increase in the initial rate. Reactions with partially functionalized p-nitrophenoxide resins containing 50% methyl methacrylate in the copolymer were 1.4 times faster than resins containing 50% styrene. The enhanced reactivity of the methyl methacrylate resin was attributed to a decrease in anion solvation by ethanol which increased the reactivity of the nucleophile.

The yield of the Mitsunobu coupling reaction is significantly decreased when the pKa of the acidic reagent approaches 11. A series of polymer-supported phosphine resins were synthesized to evaluate the influence of the local environment within the macromolecule on the Mitsunobu etherification of m-cresol (pKa = 10.01) -with benzyl alcohol. Resins with phenyl or ester groups neighboring the phosphine produced final yields of 20.8% and 0.0% respectfully. Due to competing side reactions, the ether yield was significantly influenced by the rate of the reaction. Reaction rates were determined to increase with the number of phenyl rings surrounding the phosphine (percent yield at 0.1 h: 4.2% and 15.7% for polymers with 0% and 82% phenyl groups). Similar behavior was observed in the Mitsunobu esterification of benzoic acid with benzyl alcohol. It is proposed that a non-polar microenvironment can be used to enhance the reactivity of ionic intermediates increase the rate of product formation. Etherification kinetics were also determined to be enhanced by an increase in the electron-withdrawing substituent of the polymer-supported phosphine. Resins containing only triphenylphosphine or benzyldiphenylphosphine produced ether yields of 43.7% and 11.0% respectfully.

The selective complexation of metal ions by a polymer-supported reagent is hindered in highly acidic solutions due to a collapse of the micropores. A series of phosphonic acid resins were synthesized with varying levels of crosslinking, and studied for their ability to complex Eu(in). Increased crosslinking of the phosphonic acid resin was unable to prevent pore collapse in 0.50 N nitric acid solutions . The hydrophobicity of crosslink levels between 12% and 25% DVB was determined to increase the matrix sensitivity to the solution pH and decrease the amount of Eu(III) complexed. Pore collapse of the phosphonic acid resin was prevented by sulfonating the ion-exchange resin. In highly acidic solutions, the sulfonic acid ligand of the bifunctional resin provided an access mechanism for metal ions, and prevented collapse of the microporous structure. Selective ion complexation was enabled by the recognition mechanism of the phosphonic acid ligands which selectively coordinated Eu(in) ions once inside the polymer.

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