Synthesis and characteristics of hyperbranched polyradicals

Citation

Lin, Chih-hsiu (1999) Synthesis and characteristics of hyperbranched polyradicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tthb-mt28. https://resolver.caltech.edu/CaltechETD:etd-02212008-095945

Abstract

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The design, synthesis and magnetic characterization of hyperbranched polyradicals based on triphenylmethyl are described. The precursor polymers are synthesized via unimolecular polymerization with a A[...]B type monomer employing palladium catalyzed Suzuki coupling reaction. The required dibromo-boronic acid monomers are synthesized by a newly developed selective lithium-iodo exchange reaction. The polytrityl methyl ethers are first converted to polytrityl trifluoroacetates. They are then transformed into corresponding polyradicals with various reductants. Cobaltocene proves to be the best choice. Magnetic characterization of the polyradicals, using SQUID magnetometry, indicates the samples contain small ferromagnetic-coupled radical clusters with spin concentration near 20 percent. In a futile attempt to minimize the defects in these systems, similar polyradicals with various substitutions are made. However, high-spin coupling is observed in a control system with neighboring antiferromagnetic interactions. This unexpected result is rationalized with a new mode of ferromagnetic interaction unique to hyperbranched or dentrimer systems. The new model provides new insights and implications to the designing of magnetic materials that is not hitherto appreciated.

A polyradical based on phenoxyl radical is also synthesized and characterized. In order to make the system defect-insensitive, the magnetic interaction is transmitted through a conducting polymer backbone. Although this polyradical has a pretty high spin concentration, the S value is only mediocre. Neither does the defect-insensitive property appear.

Thesis Files