Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Chemistry (PhD)

Administrative Home Department

Department of Chemistry

Advisor 1

Shiyue Fang

Committee Member 1

Ashutosh Tiwari

Committee Member 2

Marina Tanasova

Committee Member 3

Qingli Dai

Abstract

This dissertation includes two topics, which are related to each other: (1) Rupture force measurement of single triazole molecule by atomic force microscope (AFM) and (2) Solid phase stepwise synthesis of polyethylene glycols (PEGs).

The reaction of an alkyne and an azide to form a 1,2,3- triazole has found wide applications. However, the stability of triazole under mechanical stress is unclear. We investigated the cycloreversion at the single molecular level using atomic force microscopy. A mica surface functionalized with a molecule containing a 1,2,3-triazole moiety in the middle and an activated ester at the end was prepared. An AFM tip with amino group was ramped over the surface, which temporarily linked the tip to the surface through amide bond formation. During retraction, the triazole broke and the forces were recorded. Most of the forces were below 860 pN. The resulting alkyne were labeled with gold nanoparticles (AuNPs). AFM imaging revealed AuNPs at the predefined locations, proving that 1,2,3-triazole could rupture with force below 860 pN.

We used commercially available functionalized polyethylene glycol (PEGs) for functionalizing of mica surface and AFM tip. These PEGs were polydispersed admixtures with different size of tags and not ideal for many applications. There is a need to develop a new method to synthesize a monodispersed PEGs. We believe that if monodisperse PEGs were used, our data would be much better. In addition, monodispersed PEGs can form single molecular conjugates during chemical reactions which allow precise characterization of all PEGs conjugates. This lead us to conceive an idea to prepare monodisperse PEGs.

Significant efforts have been made in the area, to achieve monodisperse PEGs using solution phase but the drawbacks include the need of multiple column chromatography, low overall yield, and contamination by depolymerized products. To overcome all shortcomings, we have developed a new technology to synthesize monodispersed PEGs on solid support. The Wang resin, which contains the 4-benzyloxy benzyl alcohol function, was used as the support. On this solid support, tetraethylene glycol monomer was added stepwise. Polyethylene glycol (PEG) with eight and twelve glycol units and its derivatives were synthesized. The synthetic cycle consists of deprotonation, Williamson ether formation (coupling), and detritylation. Cleavage of the PEG from solid support was achieved with trifluoroacetic acid. From this method, we prepared highly pure products having different functionalities at the two ends without any chromatographic purification in the entire synthesis.

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