Poly(phenylene ethynylene) Conjugated Polyelectrolytes Synthesized

May 10, 2019 - This study examines the use of Pd(0)-catalyzed chain-growth Sonogashira coupling to prepare a series of poly(phenylene ethynylene) ...
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Poly(phenylene ethynylene) Conjugated Polyelectrolytes Synthesized via Chain-Growth Polymerization Pradeepkumar Jagadesan and Kirk S. Schanze* Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States

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S Supporting Information *

ABSTRACT: This study examines the use of Pd(0)-catalyzed chain-growth Sonogashira coupling to prepare a series of poly(phenylene ethynylene) (PPE)-type conjugated polyelectrolytes that feature alkyl sulfonate (−R-SO3−)-solubilizing groups. The polymerization used an AB-type monomer, I− Ar−CCH, with pendant sulfonate ester units (−O− CH2CH2CH2−SO3R), wherein the ester serves as a protecting group for the sulfonate groups. The conjugated polyelectrolytes were produced by hydrolysis of the ester-protecting groups post polymerization. As a point of reference, PPE-type polymers with nominally the same structures were prepared by step-growth polymerization of the AB monomer under conventional Sonagashira conditions, as well as via AA + BB polymerization of disubstituted monomers I−Ar−I and HCC−Ar−CCH. The ester-protected polymers were characterized by gel permeation chromatography in tetrahydrofuran (THF) solution. Samples prepared by chain growth have Mn values ranging from 4.6 to 7.5 kDa and have comparatively low polydispersity indices, Đ ∼ 1.1−1.2. The step-growth polymers have Mn ranging from 11.3 to 13.5 kD, with higher dispersity, Đ ∼ 1.5−2.3. The photophysical properties of the samples were compared, as both the ester-protected forms (in THF solution) and the polyelectrolyte forms (in MeOH and water). In general, the polymers prepared by chain growth have higher fluorescence quantum yields and better-resolved spectra, suggesting that the chains are comparatively defect-free and do not aggregate in solution.

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INTRODUCTION Over the past two decades, conjugated polyelectrolytes (CPEs)1−4 have generated extensive interest due to their exceptional optical and electronic properties, leading to potential applications in chemo-5−7 and biosensing,8−10 bioimaging,11 organic photovoltaics,12,13 and light-emitting diodes.14−16 Due to the extended π-conjugation of the backbone, CPEs posses excellent light-harvesting properties and display enhanced fluorescence quantum yields when compared to their monomer and oligomer congeners. In addition, the fluorescence emission of CPEs can be quenched considerably even at very low analyte concentrations, a phenomenon called “amplified quenching” that make them attractive as sensory materials.17 The key to this amplifiedquenching effect is the spatial delocalization and mobility of excitons in the polymer that enhances the efficiency of electron/energy transfer from the CPE to a quencher. Poly(phenylene ethynylene) (PPE)-type conjugated polyelectrolytes are widely studied due to their ease in synthesis and the possibility to enrich the structural diversity by modifying the side-chain units, which confer them the potential for multiple applications.18−22 PPEs are most frequently synthesized by AA/BB copolymerization of a 1,4dihaloarene with a 1,4-diethynylbenzene using Pd-catalyzed, Sonogashira-type polycondensation that proceeds via a stepgrowth mechanism. While this reaction is versatile, polymers © XXXX American Chemical Society

prepared by the above method display relatively high polydispersity and lack end-group fidelity.23 Hence, a polymerization method that can offer precision in the control of the fundamental characteristics such as molecular weight, polydispersity, and end-group functionality may yield a polymer with improved optical and electronic properties. In this context, the controlled chain-growth polymerization methods introduced by Yokozawa24 and McCullough25 proceed via initial complex formation between a metal catalyst and an ABtype monomer through oxidative addition, which further grows into a polymer, as the catalyst intramolecularly transfers to the chain end subsequently upon reductive elimination in a repetitive fashion. The intramolecular catalyst migration during the chain-growth process leads to well-defined and fully conjugated polymers. Several examples of conjugated polymers including polyphenylenes,26 polythiophenes,27 and polyfluorenes28 synthesized by the controlled chain-growth method have been reported. Bielawski and co-workers were the first to utilize a chaingrowth method to obtain low dispersity PPE polymers using a bis-alkyloxy-substituted 1-ethynyl-4-iodobenzene monomer.29,30 Their studies revealed that an intermediate molecular Received: February 10, 2019 Revised: April 24, 2019

A

DOI: 10.1021/acs.macromol.9b00288 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Sulfonate-Substituted PPEs by (a) Chain-Growth and (b) Step-Growth Approachesa

(a) PtBu3Pd(Ph)Br (1−5 mol %), CuI, PPh3, dichloromethane (DCM)/diisopropylamine (DIPA), 65 °C, 1−5 days; (b) NaOH, 1,4-dioxane/ H2O, reflux, 1 day; and (c) Pd(PPh3)4, CuI, DCM/DIPA, reflux, 65 °C, 1−2 days.

a

other polar solvents. Water-soluble CPEs with sulfonate groups are frequently synthesized directly from their (ionic) sulfonated monomers.31−33 However, to allow the characterization of the molecular weights of the polymers by organicphase gel permeation chromatography (GPC), a precursor approach34 was adopted whereby an ester-protected AB-type monomer (8, Scheme 1) was used to prepare organic-soluble sulfonate ester-substituted polymers. The synthetic scheme and detailed procedures for monomer 8 are provided in the Supporting Information (SI). The choice of α-trifluoromethylbenzyl (TFMB) as the esterprotecting group for the sulfonate units was made on the basis of its stability under the polymerization conditions and the ability to cleave it by treatment with either trifluoroacetic acid (TFA)35 or a strong base. The ester-protected polymer cg-AB was synthesized by reacting AB monomer 8 with PhPd(tBu3P)Br in the presence of CuI and PPh3 in a CH2Cl2/ diisopropylamine mixture under reflux. Polymerization was carried out using various PhPd(t-Bu3P)Br (catalyst/initiator) to 8 (monomer) molar ratios and with various reaction durations (1−5 days). (The molar fraction of CuI (10%) and PPh3 (20%) used in all of the reactions was based on the previous work by Bielawski and co-workers, where the polymerization conditions were optimized for degree of

weight and low polydispersity polymer can be achieved using the PhPd(t-Bu3P)Br catalyst/initiator in the presence of CuI and PPh3. Since a chain-growth method may deliver defect-free polymers, one would expect an enhancement in the photophysical properties of these polymers compared to analogous structures synthesized by a step-growth approach. Nevertheless, the photophysical properties of the chain-growth polymerized PPE polymers have not been previously investigated. In addition, water-soluble conjugated polymers synthesized via the chain-growth method have not been reported. Herein, we report the first PPE-type, water-soluble, conjugated polyelectrolytes synthesized by a controlled chaingrowth method. The work reveals the potential for the chaingrowth polymerization method to synthesize PPE-type CPEs with improved photophysical properties.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Conjugated Polymers. An AB-type monomer featuring both terminal acetylene and iodo-groups was synthesized and subjected to the chain-growth polymerization to achieve the water-soluble conjugated polyelectrolytes (Scheme 1). The sodium alkyl sulfonate (R-SO3− Na+) moiety was chosen as the pendant ionic group for this study due to its good solubility in H2O and B

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Macromolecules Table 1. Polymerization Conditions and GPC Data for Ester-Protected Polymers polymer

catalyst:monomer

cg-AB1 cg-AB2 cg-AB3 cg-AB4 cg-AB5 sg-AB sg-AABB1 sg-AABB2c

5:100 5:100 5:100 2:100 1:100 5:100 5:100 5:100

Mna

Mwa

Đa

DP (n)b

yield (%)

6220 7520 6360 7270 4650 11 330 13 450 7966

7550 9020 7210 8280 4940 17 870 31 540 10 444

1.21 1.20 1.13 1.14 1.06 1.58 2.35 1.31

8 10 9 10 6 15 18 10

88 87 84 93 64 88 88 62

reaction duration 1 2 5 2 2 2 2 2

day days days days days days days days

a

Determined by GPC in tetrahydrofuran (THF) against polystyrene standards. bCalculated based on Mn and monomer mass. cDP controlled by the addition of 10 mol % of ethynyl benzene as an end-capping agent.

polymerization (DP) and polymer yield.)29 The results are summarized in Table 1. Higher molecular weight and yields (84−93%) were obtained for cg-AB (1−4) synthesized with 2−5% catalyst, as determined by GPC analysis, and the highest degree of polymerization (DP) was calculated to be ∼10. However, sample cg-AB5 derived from reaction with a 1% catalyst loading displayed the lowest molecular weight with only 64% yield. As expected for polymers prepared by the chain-growth method, the polydispersity indices (Đ) determined for cg-AB (1−5) are narrow and, in fact, comparatively narrower than for the organic-soluble PPEs synthesized by chain growth reported by Bielawski.29 Initial attempts to deprotect the ester groups of cg-AB in neat trifluoroacetic acid (TFA) as reported previously35 resulted in the degradation of the polymer. This could be due to the sensitivity of the alkyne units to the strong acid. Further efforts to utilize TFA/1,4-dioxane mixtures (1,4dioxane = 10−50%) for the deprotection also proved unsuccessful. Luckily, it was found that the TFMB-sulfonate ester-protecting groups could be hydrolyzed in the presence of NaOH in H2O/1,4-dioxane mixture to yield the water-soluble conjugated polyelectrolytes, cg-P. To allow comparative studies of the photophysical properties of polymers synthesized by the chain-growth and stepgrowth methods, two different approaches were utilized to synthesize the ester-protected polymers by the step-growth methods using Pd(PPh)4 catalyst. (Note that nominally the structure of these polymers is the same, but they differ in their method of synthesis.) First, polymerization was carried out with AB monomer (8) to yield sg-AB (Scheme 1b). The second approach involved the synthesis of sg-AABB1 via the AABB-type polymerization of 5 and 9 (Scheme 1b). (The synthetic scheme and detailed procedures for monomers 5 and 9 are provided in the Supporting Information.) The purity of all of the ester-protected polymers was confirmed by 1H NMR analysis (Figures S15−S20, SI). Consideration of the data in Table 1 reveals some important trends. First, in general, the molecular weights for the polymers obtained from the two step-growth reactions are generally larger than any of the samples obtained by chain growth. Unfortunately, the degree of polymerization for samples obtained by chain growth is limited to ∼10, despite using different reaction conditions. This suggests that the catalyst is limited to a turnover number of ∼10 under the conditions of the reactions reported here.36 However, the good news is that the dispersity of the chain-growth samples is considerably less than for the samples prepared by the step-growth methods (Table 1). Further, we purposely synthesized a step-growth polymer with DP ∼10 (sg-AABB2) to allow comparison with

the chain-growth polymers of comparable DP. The DP for sgAABB2 (the last entry in Table 1) was controlled by the addition of 10 mol % of ethynyl benzene as an end-capping agent. The final water-soluble conjugated polyelectrolytes sgAB-P and sg-AABB-P1 were obtained following hydrolysis of the respective ester-protected polymers, using NaOH in H2O/ 1,4-dioxane mixture (Scheme 1b). Photophysical Characterization of the Ester-Protected Polymers. To understand the effect of the polymerization method on the photophysical properties of the resulting polymer samples, the absorption and photoluminescence properties of both types of ester-protected polymers and CPEs were compared. Since the ester-protected polymers are very soluble in THF, their photophysical properties were first studied in this solvent. Since cg-AB (1−4) displayed similar photophysical properties (Figure S27a), the absorption and fluorescence spectra of only cg-AB2 were chosen as the representative example and are depicted in Figure 1a. The UV−visible absorption spectra of cg-AB2 displayed a broad absorption band that ranges from 350 to 480 nm with λmax ≈ 440 nm (Figure 1a). The λmax for cg-AB5 was 420 nm (Figure S27a), which is blue-shifted to ∼20 nm compared to the other samples. This is attributed to the reduced conjugation length in cg-AB5 as a result of its relatively low degree of polymerization

Figure 1. Normalized UV−visible absorption and fluorescence spectra of (a) cg-AB2 and (b) sg-AB, sg-AABB1, and sg-AABB2 in THF. Polymer concentration = 5 μm in all cases. C

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Macromolecules (n ∼ 6). The finding is consistent with the previous studies of PPE oligomers, which suggest that the optical properties define a conjugation length of ∼10 phenylene ethynylene repeats.37 The fluorescence spectra of cg-AB2 reveal a narrow emission band with λmax ≈ 469 nm accompanied by a well-defined vibronic structure (Figure 1a). This spectrum is typical for PPE-type polymers in good solvents.38 The emission of cgAB5 displayed a similar emission with λmax slightly blue-shifted to 463 nm (Figure S27b). The absorption and fluorescence maxima for sg-AB, sg-AABB1, and sg-AABB2 lie at 452, 472, and 470 nm (Figure 1b), respectively. The observed red shift in the case of sg-AB and sg-AABB1 when compared to cg-AB (1−4) is attributed to the presence of extended conjugation due to the higher molecular weights of the step-growth samples. The fluorescence of PPE-type polymers and oligomers is inherently efficient, with quantum yields (ϕf) in excess of 0.8 reported.39,40 Fluorescence lifetimes (τf) are typically less than 1 ns, and the lifetimes often decrease with increasing ϕf, consistent with excited-state relaxation dominated by radiative decay.40 Factors that might be expected to lead to reduced quantum yields include defects in the main chain, halogen end groups, and aggregation. In addition, based on studies of PPE oligomers, it is expected that the ϕf will increase with the degree of polymerization due to an increased radiative decay rate.40,41 To determine whether the synthetic approach used to prepare the polymers influences their photophysics, ϕf and τf values were determined for the ester-protected polymers prepared by the three approaches described above. The complete data sets, including ϕf values for all samples and τf values for selected samples, are in the Supporting Information (Table S1), and important data are shown in a graphical format in Figure 2. These results generally show that all ester-

process may give rise to increased integrity of the phenylene ethynylene polymer backbone structure (e.g., minimizing butadiyne and other structural defects).42−44 A shortcoming of copper-mediated Sonagashira coupling is the formation of butadiyne defects resulting from acetylene homocoupling.45 Moreover, a previous report indicated that poly(pphenylenebutadiynylene)s (PPBs) obtained from homocoupling of 1,4-diethynylbenzene derivatives were only sparingly soluble.43 However, the UV−visible absorption and fluorescence spectra of these PPBs were similar to those of typical PPE-type polymers. This literature precedent suggests that the presence of butadiyne defects in the polymer structure can lead to decreased solubility and increased interchain aggregation, which can subsequently lower the fluorescence quantum yields. A possible explanation for the reduced ϕf values from the stepgrowth samples could be the presence of butadiyne defects that promote nonradiative decay. Photophysical Characterization of the Conjugated Polyelectrolytes. The water-soluble conjugated polyelectrolytes obtained by hydrolysis of the ester precursors are of immense interest with respect to the sensor and biological applications.46 Thus, we investigated the photophysical properties of the CPEs derived by hydrolysis of the ester groups of their corresponding protected polymers. The optical properties of all of the samples were compared in methanol and water because in the previous work, it was found that methanol is a comparatively “good” solvent for sulfonated PPE-type conjugated polyelectrolytes, whereas in water, the polymers have a greater tendency to aggregate.33 The absorption and photoluminescence spectra of cg-P2, sgAB-P, sg-AABB-P1, and sg-AABB-P2 in methanol and water and trends in the fluorescence quantum yields for the different samples in the two solvents are shown in Figure 3. The spectra

Figure 2. Fluorescence quantum yield values of ester-protected PPE polymers cg-AB2, cg-AB5, sg-AB, sg-AABB1, and sg-AABB2 in THF solution.

protected polymers display relatively high fluorescence yields, with ϕf > 0.74. However, interestingly, the fluorescence quantum yields obtained for the samples prepared by chain growth (cg-AB1−4) are consistently higher compared to the samples prepared by step growth (sg-AB, sg-AABB1, and sgAABB2, see Figure 2 and Table S1). The substantial difference in ϕf values observed for the ester polymers synthesized by different methods is an intriguing result. Because the structure of the polymers is nominally identical, the difference in photophysical properties must arise from differences in backbone or end-group structures (or defects) that result from the difference in polymerization method. An important feature of the chain-growth polymerization is that the catalyst from the initial catalyst−monomer complex transfers along the growing polymer chain, and this

Figure 3. (a) Normalized UV−visible absorption spectra and normalized fluorescence spectra of cg-P2, sg-AB-P, sg-AABB-P1, and sg-AABB-P2 in methanol (left) and water (right). CPE concentration = 5 μm in all cases. (b) Fluorescence quantum yields of cg-P2, cg-P5, sg-AB-P, sg-AABB-P1, and sg-AABB-P2 in methanol (red) and H2O (blue). D

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Macromolecules

water. While it is unclear what is the origin of this effect, it may be due to proton transfer from water to the singlet state of the polymer, possibly involving the −CC− bond. Previous studies of phenylene ethynylene oligomers revealed the presence of an irreversible photochemical reaction that is initiated by the attack of water on the acetylene unit of the excited-state molecule.50 Femtosecond−picosecond transient absorption spectra of cg-P2 in methanol and water give very similar spectral results, with the only notable difference being accelerated excited-state decay (Figure S37). Amplified Fluorescence Quenching. As noted in the introduction, the primary interest in conjugated polyelectrolytes is their use as high-sensitivity fluorescence sensors based on the amplified-quenching effect.2,8,17,51 In particular, electron or energy acceptors that are charged opposite to the CPE give rise to fluorescence quenching with Stern−Volmer (SV)quenching constants (KSV) in excess of 105 M−1, allowing turnon and turn-off sensing of analytes in the μM−pM concentration range. In the previous work, we and others have demonstrated that there is a correlation between CPE chain length (DP), charge density, and aggregation state on the amplified fluorescence-quenching effect.47,52−58 In particular, the quenching efficiency (as reflected by the KSV value) generally increases with DP, polyelectrolyte charge density, and when the polymers are aggregated. To further characterize the PPE-type CPEs prepared by using the chain-growth and step-growth methods, the SVquenching efficiency was measured for the series of polymers, using the cationic electron acceptor N,N′-dimethyl-4,4′bipyridinium (MV2+). Since cg-P1 to cg-P4 possess similar characteristics and fluorescence properties, we selected cg-P2 that has the highest molecular weight among the four polymers along with cg-P5, sg-AB, and sg-AABB1 and carried out the fluorescence-quenching experiments in methanol and H2O. The Stern−Volmer-quenching constants for each of these polymers are summarized in the bar graph presentation in Figure 4, and typical SV plots are shown in the Supporting Information (Figures S33−S36). For all of the CPE samples, the SV plots are approximately linear at low [MV2+], but they curve upward at higher concentrations. The KSV values are extrapolated from the linear region at low concentration. Figure 4 includes also the [MV2+] needed to quench 90% of the fluorescence (Q90). Several trends are notable with respect to the quenching data. First, in each case, the quenching constant is >106 M−1 in both MeOH and water. This quenching efficiency is typical for the quenching of anionic PPE-type CPEs and is consistent with the amplified-quenching effect.48 Second, KSV is generally larger in H2O compared to that in methanol, and the difference is more pronounced for the sg-AABB-P1 and sg-AB-P. Greater quenching efficiency in water has been seen previously and attributed to an increased aggregation in water. Third and more interesting is the fact that in water, KSV follows the trend cg-P5 < cg-P2 < sg-AABB-P1 ∼ sg-AB-P. This mirrors the trend in DP for the polymers; several previous studies showed that for anionic CPEs, the quenching efficiency by a cationic quencher increased with DP, and the current results can be explained similarly.40,55 Of further note is the fact that for each CPE listed in Figure 4, the Q90 values in H2O track closely with the polymer chain concentration, which was calculated based on the molar mass of the polymer repeat units (PRUs) and is given by [PRU]/DP, where [PRU] = 5 μm. This indicates that

are generally similar to those seen for the ester-protected polymers in THF, with some important differences. First, for the chain-growth polymer cg-P2, the absorption and fluorescence in methanol and water are similar in almost every respect, including the wavelength maxima and band shape. They are also similar to the spectra of the ester precursor in THF. (The findings are the same for the other chain-growth samples with the exception of cg-P5, which has the lowest Mn, see the SI.) However, one important difference is that the ϕf values are significantly lower in water compared to those in methanol (Figure 4 and Table S3). This trend is

Figure 4. Stern−Volmer (SV)-quenching constants of cg-P2, cg-P5, sg-AB-P, and sg-AABB-P1 with MV2+ as the quencher in methanol (red) and H2O (blue). The numbers at the top of the bar graph are KSV values (×106 M−1), and the [MV2+] required for 90% fluorescence quenching (Q90) is displayed in parentheses. Polymer concentration = 5 μm in repeat units.

typically observed for CPEs in water and has often been attributed to aggregation.33,47−49 As discussed more below, we do not believe that the chain-growth samples aggregate in water, and therefore there must be another underlying reason for the reduced ϕf in water. By contrast, the absorption and fluorescence of the stepgrowth polymers sg-AB-P, sg-AABB-P1, and sg-AABB-P2 exhibit significant differences compared to the spectra of the chain-growth samples and corresponding ester-protected polymers in THF. In particular, in both methanol and water, sg-AB-P and sg-AABB-P1 exhibit a pronounced shoulder on the red side of the main absorption band. Their fluorescence emission is also broader in water, which is especially noticeable for sample sg-AB-P. The ϕf values in water are also noticeably lesser than in methanol, but in this respect, the trends are similar for the step-growth and chain-growth samples. Taken together, the results obtained on the chain-growth samples sgAB-P and sg-AABB-P1 strongly suggest that these samples may be aggregated in both methanol and water, and the aggregation leads to changes in the optical properties. While the origin of the different solution properties between the chain-growth and step-growth samples cannot be definitively proved, it is possibly related to the presence of defects in the polymer backbone structures, which result from the stepgrowth polymerization process. By contrast, the overall spectroscopic properties of the chain-growth samples are to a great extent preserved in methanol and water, suggesting that the chain-growth method gives rise to PPE chains with higher fidelity. As noted above, all of the CPEs exhibit significantly reduced ϕf in water compared to that in methanol. The decreased emission yields are accompanied by the corresponding reduction in the fluorescence lifetimes (Table S3), indicating that the nonradiative decay rates of the CPEs are larger in E

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in aqueous solution on average, a single MV2+ is able to quench an entire polymer chain.

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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.9b00288. Complete details concerning the synthesis and structural characterization of the monomers and polymers; additional photophysical data, including absorption spectra, fluorescence spectra, and lifetimes, and femtosecond− picosecond transient absorption spectra (PDF)

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REFERENCES

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CONCLUSIONS This study examined the use of the chain-growth polymerization to prepare a series of PPE-type conjugated polyelectrolytes. The approach relied on the use of an AB-type monomer featuring sulfonate-solubilizing chains, in which the sulfonate groups were protected as α-trifluoromethylbenzyl (TFMB) esters. The polymers obtained by chain growth displayed very low polydipersity but relatively lower molecular weight compared to structurally analogous polymers synthesized by the conventional step-growth Sonogashira-catalyzed polymerization. Despite multiple trials to optimize the chaingrowth polymerization, the highest molecular weight obtained was ∼7.5 kD, corresponding to a number average DP of 10. Photophysical analysis of the PPE-type polymers, both as the ester-protected, organic-soluble forms and as the water-soluble conjugated polyelectrolytes, demonstrated that the chaingrowth samples displayed superior properties, with respect to absorption and fluorescence spectra and fluorescence quantum yields. Although not emphasized above, another significant aspect of this work is the use of the TFMB ester as a protecting group for the sulfonate moieties during polymerization. This study is the first to report the synthesis of a sulfonated PPE-type conjugated polyelectrolyte via an ester-precursor approach, and it has the distinct advantage of affording an organic-soluble ester polymer that can be fully characterized by NMR and GPC in organic solution prior to the release of the polyelectrolyte via ester hydrolysis. This new approach will allow future work to be carried out on more highly functionalized conjugated polyelectrolytes, taking advantage of the powerful water-solubilizing capability of the −SO3− moiety.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pradeepkumar Jagadesan: 0000-0002-5319-1395 Kirk S. Schanze: 0000-0003-3342-4080 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Welch Foundation through the University of Texas at San Antonio (UTSA) Welch Chair (Grant No. AX-0045-20110629) and start-up funding from UTSA. F

DOI: 10.1021/acs.macromol.9b00288 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00288 Macromolecules XXXX, XXX, XXX−XXX