Katz, Alexander (1999) The synthesis and characterization of molecularly imprinted materials. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-06272005-135319
Current advances in chemical protection strategies, analytical methods such as solid-state NMR and EPR spectroscopies and biological catalysis make it an ideal time to reinvestigate molecular imprinting as a viable method for the synthesis of catalysts for small molecule transformations. In the past, imprinting has been pursued mainly as a method for creating separation media for dealing with issues of adsorption/separation of small molecules. In a few instances, these materials have also been investigated as catalysts. Most imprinted systems to-date have employed non-covalent interactions to achieve functional group positioning, and this type of an approach can lead to other undesired effects such as binding site heterogeneity (as shown in this thesis for one particular system that employs self-assembly to achieve imprinting). Site heterogeneities can be extremely detrimental for selective catalysis. The objective of this work is to determine the origins of site heterogeneity and attempt to overcome these issues in order to synthesize new catalysts via molecular imprinting.
To gain further insight into the causes responsible for site heterogeneity in imprinted materials, the nature of molecular recognition in an imprinted polymer that is known to exhibit strong binding site heterogeneity is investigated. The system is formed by the self-assembly of a binding monomer and an imprint through non-covalent interactions. Based on observations of bulk phase separation in the system investigated, an alternative mechanism for molecular recognition in the imprinted polymer is proposed. This mechanism involves remaining, occluded imprint molecules that provide for binding via imprint-imprint intermolecular interactions. Support for this mechanism is provided from polymers prepared using a combination of imprint and mimic, which remains covalently bound in the polymer and is shown to increase the rebinding of imprint while not significantly affecting the binding of the opposite enantiomer of the imprint. Elucidation of this mechanism provides insight into the nature of site heterogeneity in imprinted polymer systems.
Based on the findings from the self-assembly system, a new molecular imprinting approach is developed that utilizes the controlled distance method. In this approach, silica is used for the inert crosslinking framework instead of an organic polymer due to its significantly greater mechanical rigidity (300 times more rigid and not susceptible to swelling in organic media). Instead of non-covalent interactions as the driving force for functional group positioning, covalent interactions are used. The method positions several amine functionalities (up to three demonstrated), within the three-dimensional porous structure of silica. This imprinting process is characterized by FTIR and solid state NMR spectroscopies. The imprinted silicas show microporosity specifically resulting from the imprinting process, with the amount of microporosity added consistent with the extent of imprint removal. The imprinted amines which reside in the microporous void space are able to bind molecules such as benzoic acid and acetylacetone and also perform shape-selective catalysis. To ascertain the degree of control over functional group positioning with this imprinting approach, fluorescence measurements with a pyrenebutyric acid probe molecule were performed on the imprinted silicas. The results demonstrate that the imprinting process employed gives local functional group ordering for the case of three amines per site and gives well-isolated functional groups for the case of one amine per site. The imprinted silicas thus provide a foundation from which further investigations towards elucidating quantitative distance information between imprinted functionalities in these materials can be developed.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Major Option:||Chemical Engineering|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||10 August 1998|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||27 Jun 2005|
|Last Modified:||26 Dec 2012 02:53|
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