Development of New Methods and Polyphosphazene Chemistries For Advanced Materials Applications

Open Access
Author:
Hindenlang, Mark David
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 03, 2010
Committee Members:
  • Harry R Allcock, Dissertation Advisor
  • Harry R Allcock, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Alan James Benesi, Committee Member
  • James Patrick Runt, Committee Member
Keywords:
  • proton conducting
  • radio opaque
  • biocompatible
  • Polyphosphazene
Abstract:
The work described within this thesis focuses on the design, synthesis, and characterization of new phosphazenes with potential in advanced materials applications. Additionally, these unique polymers required the development of novel reaction methods or the investigation of new phosphazene chemistry to achieve their synthesis. Chapter 1 lays out some of the basic principles and fundamentals of polymer chemistry. A historical overview of phosphazenes is also included along with an indepth look at the chemistry of polyphosphazenes and how they relate to advanced materials for power generation and biomedical applications. Chapter 2 investigates the use of iodinated polyphosphazenes as x-ray opaque materials. Single-substituent polymers with 4-iodophenoxy or 4-iodophenylanaline ethyl ester units as the only side groups were prepared. Although a single-substitutent polymer with 3,5-diiodotyrosine ethyl ester groups was difficult to synthesize, probably because of steric hindrance, mixed-substituent polymers that contained the non-iodinated ethyl esters of glycyine, alanine, or phenylalanine plus a corresponding iodinated substituent, could be synthesized. Multinuclear NMR spectroscopy was used to follow the substitution of side groups onto the phosphazene back bone and judge the ratio of substituents. Heterophase studies of the hydrolysis of iodo-amino acid/non-iodinated amino acid side group species in deionized water at 37 ºC followed a bulk hydrolysis profile, with the rates dependent on the structure of the side groups. The effectiveness of these polymers as X-ray opaque materials was examined by the use of the poly(organophosphazenes) and conventional organic polymers as filters for copper K&#945; or rhenium-tungsten-molybdenum radiation. The phosphazene polymers that contained iodine in the side groups were opaque to X-rays, whereas the conventional organic polymers were essentially transparent to the same radiation. Chapter 3 details the initial investigation into 3,4-dihydroxy-L-phenylalanine ethyl ester and dopamine substituted polyphosphazenes that could be applied to a number of applications. L-DOPAEE was acetonide protected to prevent crosslinking reactions by the catechole functionality. Cyclic small molecule studies and macromolecular substitution reactions on the linear high polymer were conducted with the protected L-DOPA. After isolation of the substituted phosphazene, the acetonide protecting group was removed in acidic conditions. Attempts to acetonide protect dopamine by the same method as L-DOPA yielded 6,7-dihydroxy-1,1-dimethyl-1,2,3,4-tetrahydro-isoquinolinium (DTTQ) by an acid catalyzed Pictet-Spengler reaction. DTTQ was utilized in chlorine substitution reactions at both the trimer and polymer levels. Weather it was obtained as a single substituent or glycine, alanine, or phenylalanine ethyl ester cosubstituted polymer, the hydrolytic stability of DTTQ polyphosphazenes was evaluated. As a result or oxidative instability or coordinated hydrochloric acid, the DTTQ substituted phosphazenes readily hydrolyzed when exposed to deionized water. Continuing studies into protection of the dopamine catechol have elucidated a viable method for the synthesis of amino-linked dopamine polymers. Chapter 4 describes a method for the synthesis of phosphazenes with quaternary amine complexes as potential antibacterial agents. Replacement reactions of pyridine alkoxides and chlorophosphazenes were first attempted at the small molecule level to study the reactivities of pyridine alkoxides. The formation of an insoluble product indicated the participation of pyridine alkoxides in macromolecular substitution, but a co-substituent was necessary for the formation of a soluble product. In order to obtain a soluble product at the polymer level, a typical two-step addition, side group exchange reactions between poly[(trifluoroethoxy)phosphazene] and 4-pyridine propoxide, and a three-step addition were attempted. Only the three-step addition where the solubilizing co-substituent is added first and last yielded soluble pyridine propoxy polymers. Pyridine methoxy substituted polymers were noted to be more soluble than their pyridine propoxy analogues and a homosubstituted pyridine methoxy polyphosphazene was obtained. Quaternization of the pyridine nitrogen was possible when an excess amount of 1-bromohexane was added to the reaction. Anti-bacterial studies with low polymer loadings (<900 &#956;g/mL) did not show any therapeutic activity. Higher loadings (&#8805; 1500 &#956;g/mL) should be evaluated during future studies. Chapter 5 evaluates the potential for functionalization of polyphosphazenes by “click” chemistry with the intent of forming pendent 1-H-[1,2,3]-triazoles. Alkynoxides were co-substituted on the cyclic trimer with trifluoroethoxide as a proof of concept. The co-substitution was translated to the high polymer with both methoxyethoxyethoxide and trifluoroethoxide. Two different methods of “click” chemistry were utilized. The first method was the use of triphenylmethyl azide as an activated species and subsequent deprotection of the trityl-protected triazole. A second method utilized TMS-azide which directly produces the 1-H-[1,2,3]-triazole in relatively high yield. This method has prospective utility in the production of proton-conducting membranes, metal coordination chemistry, and many other areas. Chapter 6 examines the hydrophobicity and reactivity of a phosphazene system with engineered branches of well defined length occurring at precise intervals along the polyphosphazene backbone. Poly[(hexachlorophosphazo)tetrachlorophosphazene] was synthesized by the thermal ring opening polymerization of its monomer which is prepared by the treatment of hexachlorocyclotriphosphazene with ammonia followed by phosphorous (V) chloride. The polymer was treated with trifluoroethoxide and the hydrophobicity of fiber mats obtained by electrospinning were evaluated. The increased fluorine loading found in this polymer does not give an enhancement of the hydrophobic character when compared to linear trifluoroethoxy substituted polyphosphazenes. Cyclo-linear phosphazene polymers were synthesized from phosphazo precursors by adapting methods developed for the living cationic polymerization. Polymers contained 27, 2, or 1 P=N unit between each ring in the structure. Phosphazene-organic hybrid polymers were also prepared from the phosphazophosphazene cyclic monomer and were connected by reacting with a diamine. Molecuar weight analysis of these cyclo-linear polymers show that with optimization, it may be possible to obtain high molecular weight species.