Article pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Quantitative Measurements of Polymer Hydrophobicity Based on Functional Group Identity and Oligomer Length Nayanthara U. Dharmaratne,† Terra Marie M. Jouaneh,† Matthew K. Kiesewetter,† and Robert T. Mathers*,‡ †
Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, United States Department of Chemistry, The Pennsylvania State University, New Kensington, Pennsylvania 15068, United States
‡
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S Supporting Information *
ABSTRACT: A combined experimental and computational investigation revealed a hydrophobicity trend for oxygen-containing functional groups commonly encountered in monomers and polymers. Based on solvatochromatic dye experiments, HPLC retention times, and theoretical LogP values, the arrangement of the three oxygen atoms in carbonates results in more hydrophobicity than other permutations like anhydrides. Another trend emerged for functional groups with two oxygen atoms (acetals > esters). Overall, when comparing aliphatic polymers with similarly sized monomers, hydrophobicity decreased as follows: carbonates > acetals > esters > anhydrides. These trends have important implications for degradation, conductivity, and many other applications.
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INTRODUCTION Many scientists who study polymers from the perspective of surface chemistry,1 biochemistry,2 biology,3 or physics4 routinely use words like “hydrophobic” and “hydrophilic”.5 These valuable concepts provide a necessary framework for describing physical properties of glycoproteins,6,7 block copolymers,8 nanostructures,9 multicompartment micelles,10 microemulsions,11 and polyelectrolytes.12 While such constructs can facilitate a nice qualitative comparison of similar functional groups, like a series of lactones with different ring sizes or acrylates with various alkyl substituents, using the term “hydrophobic” has limitations when ranking dissimilar polymers. To some extent, ambiguity arises due to the instinctual perspective of counting carbon atoms in functional groups or calculating a carbon:oxygen (C:O) ratio for monomers. Considering that a number of oxygen-containing monomers with ketones, ethers, esters, acetals, anhydrides, and carbonates may have similar C:O ratios, structural diversity creates a dilemma when evaluating homopolymers and copolymers. For instance, while poly(tetrahydrofuran) (PTHF) is viewed as “hydrophobic” relative to poly(N-isopropylacrylamide) (PNIPAM),13 other researchers view PTHF as “hydrophilic” relative to polycaprolactone (PCL).14 Because the C:O ratio for PTHF (4:1) is larger than PCL (3:1) but less than PNIPAM, this perspective illustrates the confusion when counting atoms in a monomer unit. In addition, various PCL derivatives have been designated as “hydrophobic”,15 “moderately hydrophobic”,16 and “highly hydrophobic”.17 To further compound the uncertainties present with homopolymers, copolymers with carbonate units are more hydrophobic than esters in some cases but less hydrophobic than esters in other copolymers.18 © XXXX American Chemical Society
Because terms like hydrophobic have potential for ambiguity when ranking polymers with oxygen-containing functional groups, quantitative metrics offer a potential pathway to reduce misperception. As such, a number of computational strategies like octanol−water partition coefficients (LogPoct)19,20 and molecular dynamics (MD) simulations of water contact angles21 and solubility parameters (δ)22 greatly assist efforts to understand hydrophobicity. These computational approaches complement experimental methods based on contact angle measurements, 23 swelling experiments,13,24 liquid chromatography,25 and solvatochromatic dye experiments.26 Recently, combined experimental and computational approaches with LogPoct values have demonstrated the ability to discern hydrophobicity of anhydride monomer libraries,27 polyacrylate homopolymers,28 star polymers,29 postpolymer modification,24 polyester electrolytes,30 solubility of B vitamins in melt polymerizations,31 and diblock copolymers for crystallization driven self-assembly (CDSA).32 The wideranging usefulness of partition coefficients stems from LogPoct values being proportional to the Gibbs free energy (ΔG).33,34 Consequently, LogPoct values have potential to create a more accurate picture of hydrophobicity than calculations involving solubility parameters (δ), like Hildebrand and Hansen values, which depend on enthalpy (ΔH) and molar volume (Vm). While the concept of δ values is noteworthy, normalizing LogPoct values with surface area (SA) rather than molar volume (V m) accommodates common architectural features in monomers and polymers as SA more effectively accounts for Received: August 14, 2018 Revised: October 2, 2018
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DOI: 10.1021/acs.macromol.8b01747 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules branching and cyclization.35 Interestingly, a number of publications in areas of chemistry (physical,35 medicinal,36 and analytical37), molecular dynamics simulations,38 and chemical engineering39 also indicate SA is a key contributor to hydrophobicity. As the structural diversity of petroleum40−43 and renewable44−51 monomers continues to expand, understanding and predicting hydrophobicity for various permutations of oxygen atoms leads to challenges. Consequently, an opportunity exists to rank common functional groups, like ethers,52 carbonates,40 esters,31 anhydrides,27 and acetals.53−55 We hypothesize that functional groups with oxygen atoms exert a major influence on hydrophobicity and suggest that using the C:O ratio or size of alkyl substituents oversimplifies assessment when comparing structurally diverse monomers. For instance, will functional groups with the same number of oxygen atoms, like esters and acetals, have differing amounts of hydrophobicity? What about carbonates and anhydrides, which each have three oxygen atoms? With these questions in mind, we have begun to investigate the connection between oxygen-containing functional groups, like acetal, ester, ether, carbonate, and anhydrides, and the hydrophobicity of monomers and polymers. To answer these questions, a combination of experimental and computational methods were investigated for acyclic model compounds, cyclic and tricyclic monomers, and some selected polymer examples.
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HPLC analysis was investigated with a Shimadzu HPLC system equipped with two pumps (20AT), an autosampler (SIL-20), degasser, column oven (20HT), and a Waters UV/vis detector. Acetonitrile (0.15 mL/min) was eluted through a Phenomenex C18 column (3 mm × 100 mm, 3 μm particles) at 30 °C. Samples (1 mg/ mL) were prepared in acetonitrile and injected (1 μL) via an autosampler. The UV detector was set at 254 nm. Photopolymerization of neat butyl acrylate was conducted at ambient temperature. Initially, filtering butyl acrylate (BA) through neutral alumina removed inhibitor. The photoinitiator (DMPA) was crushed to a fine powder with a spatula. Then, BA (2.4 g) and DMPA (0.5 wt %) were added to a cuvette, capped with a septum, and vortexed until homogeneous. After bubbling the solution with nitrogen for 3 min, the capped cuvette was immediately irradiated in a UV oven (Electro-Lite Corporation, Model ELC-500) at 365 nm for 1 min. Based on previous studies involving DMPA and BA,28 an appropriate light intensity (7.7 ± 0.2 mW/cm2) at 365 nm was confirmed by a ThorLabs optical power and energy meter (Model PM100D) with photodiode power sensor (Model S120VC). After the exothermic polymerization cooled to ambient temperature, Nile Red (∼0.05 mg) powder was added. The solution stirred for several minutes before taking a UV/vis spectrum. General Procedure for Oligomerization of VL and CL. To a 20 mL vial, CL (2.000 g, 17.5 mmol) and benzyl alcohol (0.1890 g, 1.75 mmol) were added, and the contents were stirred to mix. To a second 7 mL vial, TBD (0.0240 g, 0.175 mmol) and acetone (2.0 mL) were added and agitated until homogeneous. The contents of the second vial were transferred to the first vial via a Pasteur pipet, and the contents were agitated to mix. The reaction was quenched in 1 h by the addition of benzoic acid (0.0430 g, 0.345 mmol). Polymer was purified using a silica column where 100% dichloromethane was used as the mobile phase. PCL was removed of volatiles under high vacuum prior to characterization. The yield was 97%, and the Mn value (1200 g/mol) was determined by NMR.
EXPERIMENTAL SECTION
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Acetic anhydride (Acros Organics, 99+%, AA), acrolein diethyl acetal (Sigma-Aldrich, 96%, ADA), n-butyl acrylate (Sigma-Aldrich, >99%, BA), diethyl ether (Fisher Scientific, laboratory grade, DE), diethyl carbonate (Sigma-Aldrich, 99%, DEC), dimethyl carbonate (SigmaAldrich, 99%, DMC), dimethoxymethane (Sigma-Aldrich, 99%, DMM), 2,2-dimethoxy-2-phenylacetophenone (Sigma-Aldrich, 99%, DMPA), ethyl acetate (EMD Chemicals, >99.5%, EtAc), methyl succinic anhydride (Sigma-Aldrich, 98%, MSA), Nile Red (Acros Organics, 99%), (propylene carbonate (Sigma-Aldrich, 99.7%, PC), 3pentanone (Acros Organics, 98%, 3P), and γ-valerolactone (SigmaAldrich, 99%, GVL) were used as received. δ-Valerolactone (Acros Organics, 99%, VL) and ε-caprolactone (Sigma-Aldrich, 97%, CL) were distilled from calcium hydride under high vacuum. UV/vis spectroscopy measurements were conducted with a Shimadzu UV-1650PC spectrophotometer at ambient temperature. Samples were analyzed in semi-micro UV cuvettes (path length 10.0 mm) (Brand GMBH, Germany). The λmax values of Nile Red were taken from maximum height of absorbance. Reported values are an average of five experiments. A typical standard deviation was ±0.5 nm. For consistency, Nile Red concentrations were adjusted to achieve absorbance values of esters. To create a comprehensive picture of functional groups in different chemical environments, both the size and architecture of model compounds varied in Figures 1−3. Understandably, as the size of the small molecules in Figure 1 increased in Figures 2 and 3, physical properties changed from liquids to a mixture of liquids and solids. To accommodate differences in physical properties, multiple characterization techniques created a complementary understanding. Accordingly, Nile Red experiments quantified liquids and low melting waxes while HPLC analysis and LogPoct calculations compared liquids and solids with higher Tm values. In Figure 3, HPLC analysis reinforced the concept of a hydrophobicity trend and demonstrated a correlation between experimental retention time and computational LogPoct/SA values. As such, the five-membered acetal (i.e., 1,3-benzodioxole) was more hydrophobic than the corresponding ester and anhydride. In addition to mirroring the trends of fivemembered rings in Figure 2, this data had reasonable agreement with other molecules, like benzaldehyde and styrene oxide. Unexpectedly, a caveat emerged when attaching two large groups, like benzyl groups, to a carbonate or ester. As such, these large groups overwhelmed the influence of a single functional group and caused HPLC retention times to C
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Figure 3. Correlation between experimental HPLC retention times and computational LogPoct values normalized by surface area (SA). Line represents linear regression.
converge. Additionally, electronic effects (Figure S1) on the functional group were observed when attaching phenyl groups. Moving from small molecule trends in Figures 1−3 toward polymers, a number of unanswered questions exist. As such, this investigation clarifies a number of critical questions: What occurs to hydrophobicity upon the ring-opening of cyclic monomers (Figure 4a)? How much influence would vinyl groups exert on hydrophobicity (Figure 4b)? How relevant is the C:O ratio in determining hydrophobicity for polyesters (Figure S2), polyacetals (Figure 5a), and polyethers? Do the trends from Figures 1−3 apply to polymers over a range of
Figure 5. Comparison of hydrophobicity values for (a) ethers and acetals and (b) acetals, esters, and carbonates versus number of monomer units. Examples shown include polytetrahydrofuran (PTHF) (yellow ●), polyoxymethylene (POM) (■), polyoxetane (×), poly(ethylene oxide) (PEO) (yellow ▲), poly(propylene carbonate) (purple ◆), and poly(γ-valerolactone) (red ●). For comparison, an analogous polyacetal (□) in part b is given. The dashed lines represent logarithmic regression.
oligomer sizes (Figure 5)? Would the influence of end groups alter hydrophobicity trends (Figure 5b)? To investigate what happens during a polymerization, Figure 4a considers ring-opening polymerization (ROP) while Figure 4b explores whether the addition of an alkene will compete with the influence of esters and acetals. Figure 4a revealed cyclic monomers, like PC, GVL, VL, and MSA, exhibit less hydrophobicity than the ring-opened model compounds. This may arise from the increased H-bond basicity (i.e., larger dipole moment) of lactones versus the open, s-trans, esters.64,65 Surprisingly, this observation was most evident for carbonates like DMC. Contact angle measurements (Figure S3) confirm the polarity difference between AA and MSA is smaller than the difference between PC and DMC. These remarks about Figure 4a are important because cyclic monomers would underestimate hydrophobicity of corresponding polymers obtained by ROP. For example, cyclic VL (λmax = 546.0 nm) is less hydrophobic than an oligomer of polyvalerolactone (DP = 5, λmax = 542.7 nm). Although cyclic structures resulted in significantly less hydrophobicity than linear analogues, the effect of adding an alkene (Figure 4b) to DMM and EtAc is less pronounced. Similarly, switching the alkyl length in ethyl acrylate (EA) to a butyl group has minimal effect on the dominant influence on the ester group. As a result, photoinitiated polymerization of neat butyl acrylate (BA) (λmax = 525.1 nm) with DMPA produced poly(butyl acrylate) with a similar Nile Red absorbance (λmax = 523.1 nm). A comparison of BA with an acetal containing identical numbers of carbon and oxygen atoms, like ADA, indicates the acetal is more hydrophobic than the ester.
Figure 4. UV/vis spectroscopy data for solvatochromatic behavior of Nile Red comparing (a) cyclic carbonates, esters, and anhydrides with the corresponding acyclic molecule and (b) evaluation of functional groups with and without vinyl groups. Lower λmax values indicate an increase in hydrophobicity. D
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experimental results in Figure 6 correlate well with computational results in Figure 5. For clarity, Table 1 summarizes results from Figures 1−6 and corresponding functional group trends. Although addi-
The intuitive process of examining the C:O ratio has some relevance for qualitatively assessing hydrophobicity of polymers with similar functional groups. For example, the hydrophobicity trend for polyesters (Figure S2) like PCL, PVL, and PGBL clearly follows the different C:O ratio’s in the repeat unit. However, the situation becomes more complex when entertaining structurally diverse monomers or comparing different functional groups. For instance, Figure 5a highlights an example where the C:O ratio fails to account for the differences between polyacetals and polyethers. Remarkably, in Figure 5a, the acetal units in POM contain a minimal number of carbon atoms (C:O = 1:1) but exhibit more hydrophobicity than PEO (C:O = 2:1), polyoxetane (C:O = 3:1), and slightly more than polytetrahydrofuran (PTHF) (C:O = 4:1). The large degree of hydrophobicity for POM probably results from influence of anomeric interactions.66 As such, to approximate the hydrophobicity of the acetal in POM, a polyether would need a C:O ratio ≥4:1. Furthermore, the computational results in Figure 5a correlate to experimental contact angle measurements observed for POM (θ = 69°,67 θ = 67°68), poly(ethylene oxide) (PEO) (θadv = 34°−45°),56 and lower molecular weight analogues like poly(ethylene glycol) (PEG).69 While some polyethers, like PEO, are widely recognized as hydrophilic, Figure 5a represents the first systematic comparison as a function of oligomer length. To understand whether small molecule trends from Figures 1−3 apply to a range of oligomer sizes, a few ring-opening polymerizations (ROP) were investigated. In Figure 5b, a series of oligomers made with carbonate-, acetal-, and esterbased monomers from Figure 2 were constructed. Then, computational LogPoct/SA values for these oligomers were compared. Overall, the functional group trends for polymers, like poly(PC), poly(GVL), and poly(PA), had good agreement with the monomers. In Figure 5, OH chain ends exhibit a strong influence on hydrophobicity when the degree of polymerization (DP) is below ∼8 monomer units. To compare these theoretical results with experimental values, a series of oligomers ranging from DP 2 to DP 10 were synthesized via ROP of VL and CL. Based on analysis by GPC, thermogravimetric analysis (TGA) (Figures S4 and S5), and differential scanning calorimetry (DSC) (Figures S6 and S7), the oligomer lengths of DP 2, DP 5, and DP 10 represent average values. In Figure 6, solvatochromatic experiments with these oligomers indicated chain end influence on hydrophobicity diminished as molecular weight increased from DP 5 and DP 10. These
Table 1. Summation of Hydrophobicity Trends for Small Molecules and Polymers scenario
data
aliphatic acyclic model compounds cyclic and tricyclic monomers HPLC retention time of model compounds cyclic versus acyclic influence of vinyl groups electronic effect of phenyl polyethers versus polyacetal ring-opening polymerization influence of OH chain end
Figure 1
trend
Figure 2 Figure 3 Figure Figure Figure Figure
4a 4b S1 5a
Figure 5b Figure 6
acetal > carbonate > ester > anhydride carbonate > acetal > ester > anhydride acetal > ester > anhydride carbonate > ester > anhydride acetal > ester carbonate > ester > anhydride polyacetals (POM) > PTHF > polyoxetane > PEO polyPC > polyacetal > poly(γvalerolactone) PCL > PVL; hydrophobicity increases as DP increases
tional examples were contemplated, the substantial number of model compounds (10 linear alkyl, 9 linear aromatic), monomers (16 cyclic, 4 tricyclic), and polymers (10) sufficiently addressed the questions posed in the Introduction. Consequently, the trends in Table 1 validated the hypothesis that functional groups exert a major influence on hydrophobicity and provide an alternative to counting the C:O ratio.
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CONCLUSION Computational LogPoct values, Nile Red (NR) solvatochromatic experiments, HPLC retention times, and contact angle measurements confirmed hydrophobicity depends upon the number of oxygen atoms and the presence of alkyl groups or aromatic rings. As such, among functional groups with three oxygen atoms, aliphatic carbonates are more hydrophobic than anhydrides. Based on model compounds, the electronic effect of phenyl rings (Figure S1) narrows the hydrophobicity gap between carbonates and anhydrides. Although the difference in hydrophobicity for carbonates and anhydrides in Figure 2 is substantial, the gap between acetals and esters was smaller. When comparing similar C:O ratios, acetals are more hydrophobic than esters. As a general trend, symmetry of oxygen atoms increases hydrophobicity. For instance, Figure 1 highlights how DMC is more hydrophobic than EtAc and 3P. Interestingly, sulfurbased analogues in Scheme 1 also demonstrate hydrophobicity trends that depend on symmetry of the oxygen atoms. As such, symmetry of two oxygen atoms in phenylmethyl sulfone results in more hydrophobicity compared to the single oxygen atom in the corresponding sulfoxide. This perspective on hydrophobicity helps explain conductivity trends for polymers containing sulfoxide, sulfone, and sulfide groups.70 Considering computational LogPoct values span 5−6 orders of magnitude, this metric assesses the concept of hydrophobicity on a large continuum and complements experimental techniques, like contact angle measurements and solvatochromatic experiments. As a result, LogPoct values may offer valuable insight in regards to understanding experimental studies on polymer degradability of polyesters,71 polyacetals,72 and polycarbonates.55 Furthermore, the nuances of copolymers
Figure 6. UV/vis spectroscopy data for solvatochromatic behavior of Nile Red comparing oligomers of PVL and PCL initiated with benzyl alcohol. Mn values measured by NMR end-group analysis. E
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(6) Graham, B.; Fayter, A. E. R.; Houston, J. E.; Evans, R. C.; Gibson, M. I. Facially Amphipathic Glycopolymers Inhibit Ice Recrystallization. J. Am. Chem. Soc. 2018, 140, 5682. (7) Congdon, T.; Notman, R.; Gibson, M. I. Antifreeze (Glyco)protein Mimetic Behavior of Poly(vinyl alcohol): Detailed Structure Ice Recrystallization Inhibition Activity Study. Biomacromolecules 2013, 14, 1578. (8) Figg, C. A.; Carmean, R. N.; Bentz, K. C.; Mukherjee, S.; Savin, D. A.; Sumerlin, B. S. Tuning Hydrophobicity To Program Block Copolymer Assemblies from the Inside Out. Macromolecules 2017, 50, 935. (9) Ricarte, R. G.; Li, Z.; Johnson, L. M.; Ting, J. M.; Reineke, T. M.; Bates, F. S.; Hillmyer, M. A.; Lodge, T. P. Direct Observation of Nanostructures during Aqueous Dissolution of Polymer/Drug Particles. Macromolecules 2017, 50, 3143. (10) Moughton, A. O.; Hillmyer, M. A.; Lodge, T. P. Multicompartment Block Polymer Micelles. Macromolecules 2012, 45, 2. (11) Vadrucci, R.; Monguzzi, A.; Saenz, F.; Wilts, B. D.; Simon, Y. C.; Weder, C. Nanodroplet-Containing Polymers for Efficient LowPower Light Upconversion. Adv. Mater. 2017, 29, 1702992. (12) Parelkar, S. S.; Chan-Seng, D.; Emrick, T. Reconfiguring polylysine architectures for controlling polyplex binding and non-viral transfection. Biomaterials 2011, 32, 2432. (13) Guan, Y.; Ding, X.; Zhang, W.; Wan, G.; Peng, Y. Polytetrahydrofuran Amphiphilic Networks, 5. Synthesis and Swelling Behavior of Thermosensitive Poly(N-isopropylacrylamide)-l-polytetrahydrofuran Networks. Macromol. Chem. Phys. 2002, 203, 900. (14) Vaikkath, D.; Anitha, R.; Sumathy, B.; Nair, P. D. A simple and effective method for making multipotent/multilineage scaffolds with hydrophilic nature without any postmodification/treatment. Colloids Surf., B 2016, 141, 112. (15) Cai, L.; Lu, J.; Sheen, V.; Wang, S. Lubricated Biodegradable Polymer Networks for Regulating Nerve Cell Behavior and Fabricating Nerve Conduits with a Compositional Gradient. Biomacromolecules 2012, 13, 358. (16) Tse, K.-H.; Sun, M.; Mantovani, C.; Terenghi, G.; Downes, S.; Kingham, P. J. In vitro evaluation of polyester-based scaffolds seeded with adipose derived stem cells for peripheral nerve regeneration. J. Biomed. Mater. Res., Part A 2010, 95A, 701. (17) Jahani, H.; Jalilian, F. A.; Wu, C.-Y.; Kaviani, S.; Soleimani, M.; Abassi, N.; Ou, K.-L.; Hosseinkhani, H. Controlled surface morphology and hydrophilicity of polycaprolactone toward selective differentiation of mesenchymal stem cells to neural like cells. J. Biomed. Mater. Res., Part A 2015, 103, 1875. (18) Weiser, J. R.; Zawaneh, P. N.; Putnam, D. Poly(carbonateester)s of Dihydroxyacetone and Lactic Acid as Potential Biomaterials. Biomacromolecules 2011, 12, 977. (19) Petrauskas, A. A.; Kolovanov, E. A. ACD/Log P method description. Perspect. Drug Discovery Des. 2000, 19, 99. (20) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv. Drug Delivery Rev. 2001, 46, 3. (21) Liu, H.; Li, Y.; Krause, W. E.; Rojas, O. J.; Pasquinelli, M. A. The Soft-Confined Method for Creating Molecular Models of Amorphous Polymer Surfaces. J. Phys. Chem. B 2012, 116, 1570. (22) Belmares, M.; Blanco, M.; Goddard, W. A.; Ross, R. B.; Caldwell, G.; Chou, S.-H.; Pham, J.; Olofson, P. M.; Thomas, C. Hildebrand and Hansen solubility parameters from Molecular Dynamics with applications to electronic nose polymer sensors. J. Comput. Chem. 2004, 25, 1814. (23) Gao, L.; McCarthy, T. J. Teflon is Hydrophilic. Comments on Definitions of Hydrophobic, Shear versus Tensile Hydrophobicity, and Wettability Characterization. Langmuir 2008, 24, 9183. (24) Waggel, J.; Mathers, R. T. Post polymer modification of polyethylenimine with citrate esters: selectivity and hydrophobicity. RSC Adv. 2016, 6, 62884.
Scheme 1. Comparison of Sulfur-Based Functional Groups with Ether and Ketone Analogues
often create confusion. For example, opposite degradation trends are observed for poly(TMC-co-LA) and poly(LA-coDHA) copolymers.18 Therefore, why do esters provide more hydrolytic degradation than carbonates in one case and less in another? The answer lies in recognizing a hydrophobicity trend: TMC > lactic acid (LA) > dihydroxyacetone (DHA).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01747. LogPoct calculations, contact angle data, TGA, and DSC (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (R.T.M.). ORCID
Matthew K. Kiesewetter: 0000-0001-5475-1246 Robert T. Mathers: 0000-0002-0503-4571 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.K.K. thanks the NSF for a CAREER Award (CHE 1554830). This research features data acquired on a 400 MHz NMR spectrometer purchased thanks to the support of the NSF (1531963). R.T.M. thanks the Penn State Institute for CyberScience and Materials Research Institute for providing access to Materials Studio.
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DOI: 10.1021/acs.macromol.8b01747 Macromolecules XXXX, XXX, XXX−XXX
