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Reports: DNI753195-DNI7: Synthesis of Multivalent and Multimodal Nanoparticles by 'Clicking-To' Multiblock Dendrimers

Jonathan G. Rudick, PhD, State University of New York at Stony Brook

Introduction. The proposed research seeks to develop a "clicking-to" approach to novel multiblock dendrimers. Dendrimers are high-value materials derived from petroleum feedstocks whose development as monodisperse, multivalent and multimodal nanoparticles has been stymied by batch-to-batch variability during synthesis. Monodisperse dendrimers with blocks of orthogonally reactive end group functionalities are anticipated to yield, upon subsequent derivatization, multifunctional dendrimers without sacrificing structural perfection. We hypothesized that orthogonal click reactions would achieve the high reactivity needed to quantitatively functionalize each block of end groups (Figure 1).

Figure 1. Proposed "clicking-to" approach to functionalize triblock dendrimers.

Results and Discussion

Enhanced Reactivity of Dendrons in the Passerini Three-Component Reaction. Triblock dendrimers are prepared in a convergent strategy that culminates in a Passerini three-component reaction. The slow kinetics of the reaction with dendritic components has limited the size of the dendrimers that can be obtained in this manner. For triblock dendrimers to serve as precursors to monodisperse, multivalent and multimodal nanoparticles, we needed to increase the size of the components that successfully undergo the Passerini reaction. Our hypothesis was that short linkers at the apex of the dendrons would result in better reaction kinetics by relieving steric interactions that slow the reaction. Indeed, we have identified linkers that dramatically reduced the required time for the Passerini reaction of dendrons. To our surprise, we found that relief of steric crowding was not the reason for the increased reactivity. Instead, we determined that a previously unobserved electronic effect enhanced the reactivity of the aldehyde component (Figure 2). When X is an oxygen atom, the reactions are fast; whereas when X is a longer aliphatic group, the reactions are as slow as when no linker is present. The substituent effect was further demonstrated through kinetic experiments with small-molecule model reactions. As a result of this unprecedented structure-activity relationship, we have succeeded in synthesizing in a convergent manner three generations of three-component dendrimers.

Figure 2. An unprecedented substituent effect has been observed in the Passerini three-component reaction of dendrons, and been exploited to synthesize dendrimers up to third-generation.

Efficient Syntheses of Star-Branched, Multifunctional Materials. We investigated the direct synthesis of three-arm star-branched mesogens via the Passerini three-component reaction to demonstrate how MCRs circumvent the need to identify and synthesize specialized branched core molecules. Identifying a suitable core through which the arms of the star-branched molecule are joined is a critical challenge when different functional modalities are to be combined. Successful syntheses of non-symmetric star-branched materials have relied heavily on strategies to desymmetrize core molecules with equivalent reactive functional groups. An ABC three-arm star in which all three arms are different from each other has C1‑symmetry that matches the symmetry of a Passerini reaction product. The Passerini reaction can also be applied to the synthesis of star compounds in which two arms (AB2) or all three arms are identical. We have shown that a three-component reaction, the Passerini reaction, makes it feasible to prepare each permutation of three-arm star-branched mesogens directly from mesogen precursors (Figure 3).

Figure 3. A Passerini three-component reaction yielded the first example of an ABC three-arm star-branched compound with calamitic mesogens in the arms.

Broader Impacts. The award has enabled significant progress in developing our approach to prepare this novel class of dendrimers, and has had a tangible impact on three graduate student researchers. One of the students supported under this award successfully defended her PhD thesis. Two of the graduate students each received 50% support and the second received 25% support from the ACS-PRF award. Two of the students are sole authors of recent publications. One student presented her research at the 250th ACS National Meeting, and another presenter her research at the 4th Northeast Complex Fluids and Soft Matter Workshop. All three students have presented at local meetings. Presentations like these are essential for students to develop skills to succeed as a professional researcher. Support of this work by the ACS-PRF was instrumental in giving the students the resources to accumulate the data needed for those presentations.

Concluding Remarks. A key project milestone was completed during the present funding period. We have made advancements in the synthesis of triblock dendrimers via the Passerini reaction that have yielded the largest three-component dendrimers to date. Furthermore, a new research direction investigating multifunctional dendrimers has emerged from our efforts to functionalize triblock dendrimers.