Tapered Bottlebrush Polymers: Cone-Shaped Nanostructures by

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Tapered Bottlebrush Polymers: Cone-Shaped Nanostructures by Sequential Addition of Macromonomers Scott C. Radzinski,† Jeffrey C. Foster,† Samantha J. Scannelli, Jeffrey R. Weaver, Kyle J. Arrington, and John B. Matson* Department of Chemistry and Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *

ABSTRACT: Tapered (cone-shaped) bottlebrush polymers were synthesized for the first time by ring-opening metathesis polymerization (ROMP) using a sequential-addition of macromonomers (SAM) strategy. Polystyrene macromonomers with molecular weights that increased from 1 to 10 kg mol−1 were polymerized in sequence to high conversion, yielding tapered bottlebrush polymers that could be visualized by atomic force microscopy (AFM).

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unlock new applications of nanostructured materials.10 We envisioned that asymmetric polymers with a variety of shapes might be synthesized in a bottom-up approach from bottlebrush polymers. Bottlebrush polymers, also called molecular brushes, are large macromolecules comprised of a polymer backbone with pendent polymer side chains.4,11 The bottlebrush topology leads to unique properties stemming from the dense packing of side chains, causing bottlebrush polymers to adopt persistent nanostructures in solution.12,13 The synthesis of bottlebrush polymers has been studied extensively by our group and others.14−16 In particular, bottlebrush polymer synthesis by grafting-through polymerization, also known as the macromonomer (MM) method, has the advantages of excellent sidechain control and “perfect” grafting density.17 Ring-opening metathesis polymerization (ROMP) is particularly well suited for bottlebrush polymer synthesis by grafting-through due to its excellent functional group tolerance, its rapid polymerization rates, and its capability to realize near quantitative conversion of MMs. Based on these advantages, we imagined that ROMP grafting-through using sequential-addition of macromonomers (SAM) could be employed to prepare asymmetric polymer topologies (Figure 1). ROMP grafting-through has been exploited to access bottlebrush polymers with complex topologies because different MMs can be consecutively polymerized due to the nature of ROMP, which has living characteristics. For example, Wooley and co-workers prepared dumbbell-shaped bottlebrush polymers,18 Rzayev and co-workers prepared asymmetric bottlebrush polymers by varying the side chain MW of two MM blocks,19 and Cheng and co-workers prepared Janus bottle-

ature has guided the evolution of a broad variety of biological constructs with unique and specific functions, and from proteins to nanoparticles to cells, these functions are intimately tied to the object’s shape. For example, the shape of a red blood cell, a concave disc, allows it to travel smoothly throughout the bloodstream, while the conical shape of the HIV viral capsid is vital for infectivity.1 Polymer chemists have translated natural shape variation to synthetic polymers with complex topologies, including branched and hyperbranched polymers,2 cyclic polymers,3 graft polymers,4 and many others. Introducing another level of complexity, polymer self-assembly has facilitated the construction of spheres, cylinders, bilayers, and other shapes.5,6 Despite the variety of methods available to polymer chemists for preparing precise polymers and nanostructures, the polymers themselves and their selfassembled structures are often highly symmetric. Even simple asymmetric shapes (e.g., cones) are remarkably difficult to access due to the specific challenges that exist in the various polymerization techniques. Appreciating that asymmetric structures may be useful in studying the physical and biological roles of shape, scientists have devised a few clever ways to produce asymmetric polymers and polymer assembles. For example, nearly two decades ago, Stupp and co-workers reported on mushroom-shaped structures through the self-assembly of rigid biphenyl domains connected to cross-linkable polyisoprene units.7 In a more recent example, DeSimone and co-workers used a top-down lithographic printing approach to probe how shape affects cellular internalization.8 Winnik and Manners have developed strategies to prepare noncentrosymmetric micelles via crystallization-driven self-assembly.9 These successes show that nanostructures with shape asymmetry can be constructed, but that routes to such particles are rare and limited to specific systems. New strategies to produce nanostructures with complex, asymmetric shapes could enable systematic studies on the role of shape in physical and biological systems and may © XXXX American Chemical Society

Received: September 17, 2017 Accepted: September 26, 2017

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DOI: 10.1021/acsmacrolett.7b00724 ACS Macro Lett. 2017, 6, 1175−1179

Letter

ACS Macro Letters

by employing a 1:1 ratio of Cu2+ to Cu+. These conditions limited radical addition to the norbornene olefin and the incidence of termination reactions. We also found that complete removal of residual styrene was vital for success in the subsequent ROMP reactions. Table 1 summarizes sizeexclusion chromatographic (SEC) and 1H NMR spectroscopic analyses of the MMs used in this study. Table 1. Characterization of PS MMs Prepared by ATRP

Figure 1. Synthesis of symmetric and asymmetric bottlebrush polymers using a sequential-addition of macromonomers (SAM) strategy.

MM

[Sty]0/[I]0

Mn,theora (kg mol−1)

Mn,NMRb (kg mol−1)

Mn,SECc (kg mol−1)

Đc

MM1k MM3k MM5k MM8k MM10k

100 300 500 700 900

1.0 2.5 5.2 7.2 9.2

1.2 2.5 4.8 7.7 9.0

0.9 2.7 5.0 7.5 9.5

1.06 1.16 1.01 1.02 1.02

a Calculated based on conversions measured by 1H NMR spectroscopy. bDetermined via end group analysis. cAbsolute MW measured by light scattering. All polymerizations were conducted at [I]/[CuBr]/ [CuBr2]/[PMDETA] = 1:0.5:0.5:2 and 50 v/v% monomer in DMF at 70 °C and stopped at ∼10% conversion.

brush polymers with discrete poly(lactic acid) (PLA) and poly(styrene) (PS) domains.20 Shape asymmetry in bottlebrush polymers has also been achieved by varying grafting density along their long axis.17,21−23 These examples illustrate the potential that bottlebrush polymers hold in the synthesis of asymmetric shapes, but access to complex shapes would require the consecutive polymerization of several distinct MMs with virtually no loss of the living nature of the polymerization. To date, ROMP has been used to synthesize block polymers with up to seven blocks for small molecule monomers,24 but to our knowledge, there exist no reports on consecutive polymerization of more than 3 MMs.25 In fact, even in bottlebrush triblock polymers, substantial catalyst death often occurs, leading to the formation of polymer species that do not possess the desired topology (e.g., diblocks instead of triblocks). Therefore, preparation of multiblock bottlebrush polymers by ROMP grafting-through represents a significant scientific and technical challenge. We previously reported that the anchor group portion of a MM, the collection of atoms connecting the polymerizable unit (e.g., norbornene) to the polymer chain, has a dramatic effect on the rate of polymerization.26 We also found that the anchor group affects how many MM units a single catalyst can successfully polymerize before undergoing catalyst death. Using this knowledge, in this contribution we describe a synthetic route to prepare multiblock bottlebrush polymers with precisely defined, cone-shaped, three-dimensional structures. To prepare tapered bottlebrush polymers, we first synthesized a series of five polystyrene (PS) MMs via atom-transfer radical polymerization (ATRP) with MWs ranging from 1 to 10 kg mol−1 (Scheme 1). A norbornene-functionalized initiator

In order to conduct sequential-addition of macromonomers by ring-opening metathesis polymerization (SAM-ROMP), the amount of time between additions required optimization. Of critical importance was the fact that full conversion of each MM was vital to ensure efficient blocking, but that catalyst death could occur in the absence of MM. Therefore, using Grubbs’ third generation catalyst ((H2IMes)(pyr)2(Cl)2RuCHPh, G3) as an initiator, kinetic analyses of ROMP grafting-through of the five MMs were conducted under air in EtOAc with an initial [MM]/[G3] ratio of 10:1 and an initial MM concentration of 100 mg mL−1. All five MMs reached near complete conversion in 2 min (Figure S4); therefore, each block was allowed to polymerize for 2 min between additions in all subsequent experiments. We next sought to evaluate the efficiency of SAM-ROMP using different MMs. Toward this end, decablock bottlebrush polymers were synthesized from each of the five MMs via 10 sequential additions of the same MM under similar conditions to those described above. The [MM]/[G3] ratio was held constant at 10 for each block, and each charge of MM was allowed to polymerize for 2 min prior to the next addition. Aliquots were removed between additions to monitor conversion and MW evolution. This process was repeated until a total of 10 blocks had been added to give a final bottlebrush polymer with a theoretical maximum degree of polymerization (DP) of 100 (Figure 2A). For each MM, SEC analysis revealed a decrease in retention time after each block addition, indicating an increase in MW of the bottlebrush polymer (SEC data for MM5k are shown in Figure 2B). The bottlebrush polymer traces were narrow and symmetrical, with dispersity values 99% conversion (as indicated by 1H NMR spectroscopy), the MW of the bottlebrush polymer side chains necessarily changes in a stepwise fashion between 1 and

Figure 2. Decablock bottlebrush polymer synthesis via SAM-ROMP: (A) Schematic representation of decablock synthesis strategy; (B) Representative SEC traces for MM5k showing a decrease in retention time after each block addition; (C) Observed Mn values (orange circles) as measured by SEC compared to expected values calculated from MM conversions (black line) for each aliquot in the decablock synthesis using MM5k. The small peaks at retention time ∼17 min correspond to residual linear polymer that did not polymerize, likely due to the absence of norbornene end groups.

Figure 3. Tapered bottlebrush polymer synthesis via SAM-ROMP: (A) schematic representation of tapered bottlebrush polymer synthesis strategy (“reverse” synthesis shown); (B) representative SEC traces of “forward” tapered bottlebrush polymer synthesis showing a decrease in retention time after each block addition (order of MM addition was MM1k (blue trace), MM3k (red trace), MM5k (orange trace), MM8k (green trace), MM10k (purple trace)); (C) experimentally measured Mn (colored circles) compared to expected Mn values calculated from MM conversions after addition of each block (black line); (D) representative SEC traces of “reverse” tapered bottlebrush polymer synthesis showing a decrease in retention time after each block addition (order of MM addition was MM10k, MM8k, MM5k, MM3k, MM1k); (E) experimentally measured Mn (colored circles) compared to expected Mn values calculated from MM conversions after addition of each block (black line). The small peaks in panels B and D at long retention time correspond to residual linear polymer that did not polymerize, likely due to the absence of norbornene end groups. The slight discrepancy in ultimate Mn between the final points in panels C and E likely results from termination in the form of catalyst death. 1177

DOI: 10.1021/acsmacrolett.7b00724 ACS Macro Lett. 2017, 6, 1175−1179

Letter

ACS Macro Letters 10 kg mol−1. The resulting bottlebrush polymer is therefore conical, as depicted visually in Figure 3A. Finally, a tapered bottlebrush polymer sample with a backbone DP = 100 was prepared for imaging by atomic force microscopy (AFM). Direct visualization of the tapered bottlebrush polymers revealed cone-shaped nanostructures (Figure 4A−C). The tapered bottlebrush polymers were wider on one end than the other (Figure 4D), confirming the successful formation of cone-shaped single macromolecules.

Funding

This work was supported by the Army Research Office (W911NF-14-1-0322) and the American Chemical Society Petroleum Research Fund (54884-DNI7). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Stephen McCartney for assistance with AFM and helpful discussions. We thank Materia for catalyst.

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Figure 4. (A−C) AFM phase images of tapered bottlebrush polymers on HOPG. Samples were drop cast from a 0.1 mg mL−1 solution of bottlebrush polymer in CHCl3. Scale bars are 20 nm. (D) Height profiles of a tapered bottlebrush polymer at the wide (blue) and narrow (green) ends obtained by AFM. The data were processed with a sliding average smoothing function.

In summary, we demonstrated here the potential of the SAM strategy for making unique polymer topologies. We reported the preparation of decablock bottlebrush polymers by SAMROMP for MMs with Mn values as high as 10 kg mol−1, affording bottlebrush polymers with Mn values exceeding 1000 kg mol−1. To further illustrate the transformative potential of this technique, we synthesized polymers with a simple asymmetric topology, tapered (cone-shaped) bottlebrush polymers, and directly visualized them using AFM. The success of this synthetic technique is likely due to a combination of factors, including the ideal anchor group for ROMP of MMs, extremely pure macromonomers, and optimized reaction conditions allowing for fast propagation. We envision that the SAM technique can be broadly applied to access more complex architectures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00724. Synthetic and experimental details and characterization data (PDF).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeffrey C. Foster: 0000-0002-9097-8680 John B. Matson: 0000-0001-7984-5396 Author Contributions †

These authors contributed equally. 1178

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