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eric fossum, ph.d - research

Fossum Group Research Areas

The majority of research in our group is focused on utilizing first principles to solve real world problems ranging from proton exchange membranes for hydrogen fuel cells to "smart" drug delivery agents. The ability to control the macroscopic properties of polymers by "designing in", at the molecular level, the topology and functionality is absolutely critical. There are three main areas in which we apply this philosophy including: 1) synthesis, characterization, and utilization of hyperbranched polymers with controlled molecular weights and defined levels of branching, 2) tailoring the physical and chemical properties of linear poly(arylene ether) systems by taking advantage of the "unusual" reactivity of a new class of monomer species to afford polymers with truly pendant functional groups, and 3) investigations of amphiphilic, thermo- and pH-responsive polymer systems as agents for drug and gene vectors, catalyst recovery, and sequestering of spent or excess reagents. The following sections are short synopses of ongoing and proposed research projects in my group.

Reactivity Controlled Polycondensation – RRCP: Tailored Branching

The overall theme of this project is to utilize the concept of reactivity ratio controlled polycondensation reactions, RRCP, to prepare branched polymers with defined MWs, and controlled placements and degrees of branching, DB. The ability to control the level of branching is critical because the viscosity of a polymer varies not only with molecular weight, but also the degree of branching. As depicted in Figure 1, for comparable materials, the viscosity of a linear polymer is always greater than that of a hyperbranched polymer, which, in turn, is more viscous than the perfectly branched dendrimer. The viscosity of polymeric materials plays a major role in their processing ability.



Figure 1. Relationship between a) viscosity and molecular weight and b) mechanical properties and degree of branching in linear, hyperbranched and dendritic polymers.

The mechanical properties display an inverse relationship with the level of branching as dendrimers typically have poor mechanical properties while linear analogues tend to possess quite good mechanical properties. With the ability to tune the branching in a straightforward manner one can prepare materials with a delicate balance between viscosity and mechanical properties.

scheme oneThe concept of RRCP relies on the inherent differences in reactivity, toward NAS reactions, that are designed into the individual monomers (kAB and kAB’sub> in Scheme 1).  For example, our initial explorations began with 3,4,5-trifluoro-4’-hydroxydiphenyl sulfone, 4, which possesses three unique electrophilic sites, one in the para position and two in the meta positions (Figure 2).

Scheme 1. Comparison of incorporating the proton conducting group on the backbone or in a truly pendant position.

Figure 2. Comparison of incorporating the proton conducting group on the backbone or in a truly pendant position.

Model reactions have shown that the most reactive electrophilic site is located in the para position and, while the two meta positions are initially equivalent, once the first meta fluorine atom is displaced the reactivity of the second meta position is significantly decreased. Therefore, low polymerization temperatures should lead to a more linear polymer, whereas a higher reaction temperature should provide a more branched structure. Indeed polymerization reactions carried out at 100 °C provided nearly linear polymers while reactions carried out at 180 °C afforded, essentially, hyperbranched polymers.

Linear Poly(arylene ether)s with Truly Pendant Functional Groups and Tailored Properties – Application to Proton Exchange Membranes

Scheme 2. Synthesis of PAEs from 3,5-difluoro aromatic systems.

We have recently developed a route to a poly(arylene ether)s, PAEs, that possess truly pendant functional groups.  The polymers are prepared via the NAS polycondensation reaction of 3,5-difluoro aromatic systems in which a strongly electron withdrawing group, located in the meta position, provides sufficient activation to allow for the synthesis of high molecular weight materials (Scheme 2).  The nucleophilic components include a variety of well-known bisphenols that are converted to the corresponding phenolates with K2CO3.  The resulting PAEs possess a structure analogous to copolymers of meta and para poly(phenylene oxide), PPO, but carry a pendant phenyl sulfonyl, benzoyl, or diphenyl phosphoryl group. The glass transition temperatures (Tg) range from 111 (13c) to 175 °C (15d) while the 5 % weight loss temperatures range from 451 (13c) to 526 °C (15c) in air.  These data suggest that these new PAEs, with pendant benzoyl, phenyl sulfonyl, and diphenyl phosphoryl groups, should provide a versatile, relatively low cost, and chemically robust platform to introduce a wide variety of functional groups such as sulfonic acids, phosphonic acids, N-heterocycles, bromides, iodides, carboxyl acids, etc.  In addition, the straightforward synthesis of graft copolymers of PPO-like structures is envisioned

Our initial efforts have been directed toward the synthesis of the sulfonated (or phosphorylated) pendant phenyl sulfonyl analogues for use as proton exchange membranes, PEM. The advantages of this new system, illustrated in Figure 3, include: 1) proton conducting moieties located in truly pendant positions allowing for a decoupling from the backbone and providing a more accessible site, 2) lower pKa value of the acid due to the absence of electron donating groups, 3) very stable materials due to the lack of a highly electron withdrawing group in the backbone of the polymer, 4) high loadings of conducting groups, and 5) a unique and universal platform that will allow for comparing the various proton conducting moieties on the exact same site pendant to the backbone.

Figure 3. Comparison of incorporating the proton conducting group on the backbone or in a truly pendant position.



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