Well-Defined and Precision-Grafted Bottlebrush Polypentenamers

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Letter Cite This: ACS Macro Lett. 2018, 7, 1080−1086

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Well-Defined and Precision-Grafted Bottlebrush Polypentenamers from Variable Temperature ROMP and ATRP William J. Neary, Brandon A. Fultz, and Justin G. Kennemur* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States

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S Supporting Information *

ABSTRACT: Polypentenamer macroinitiators are synthesized through variable temperature ring opening metathesis polymerization of 3-cyclopentenyl α-bromoisobutyrate, which has sufficient ring strain (ΔHp = −22.6 kJ mol−1) to produce targeted molar mass (1000 kg mol−1) and Đ = 1.33. This system exhibits high synthetic versatility and control with a unique flexible backbone to expand the suite of densely grafted polymers.

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ncreasing the side-chain density of grafted polymers changes their definition to bottlebrush (BB), where significant changes in properties are traversed.1 Inspired by natural proteoglycans,2,3 the BB architecture is best known for its unique combination of large molar mass and low melt viscosities. This is in direct contrast to typical linear polymers, and a viable reason for the prominence of proteoglycans in articular cartilage where lubrication and impact dampening is required.4,5 Synthetic BBs have developed rapidly over the last few decades,1,6−18 however, definitive correlations between physical properties and brush architecture remain elusive. In addition to their biomimetic applications, synthetic BBs and bottlebrush block copolymers (BBCP)s have shown promise in photonic crystals,7 active camouflage,19 lithographic patterning,20 porous membranes,21 drug delivery,22 advanced coatings,23 super elastic/soft gels,24−26 energy storage,27 and sensors.28 Steric interactions, imposed by reducing the number of backbone atoms between each graft (ng) and increasing the side chain degree of polymerization (Nsc), cause the backbone and the side chains to extend into worm-like cylindrical polymers with reduced entanglements.29−31 The divergent set of synthetically modifiable parameters, such as Nsc, ng, and backbone degree of polymerization (Nbb) presents a playground for structure−property relationships experimentally32−41 and theoretically.9,29,42−46 Despite the seemingly limitless selection of graft chemistries, the variety of backbones that permit precise control over high grafting density has been dominated by two types: those derived from vinyl monomers and those from norbornene-based monomers. “Grafting-through” using vinyl-terminated macromonomers, such as polymer-tethered (meth)acrylates47−50 or styrenes,51,52 © XXXX American Chemical Society

result in BBs, often termed polymacromonomers (PMM)s, with one methylene between each graft site (ng = 1). Poly(norbornene)s (PNB) have arguably received the most focus of recent BB and BBCP investigations with many citations listed above and some additional examples provided here.53−66 Norbornene (NB) has a high ring strain, [ΔH] = 113.8 kJ mol−1,67 allowing NB-terminated macromonomers to be “grafted-through” to high conversions and complete grafting fidelity. The PNB BB has a five-carbon repeating unit and typically contains one graft. Therefore, ng = 4 has been accepted as a suitable grafting density to be classified as a BB system. However, the PNB repeating unit also contains a cyclopentane ring that adds an unavoidable enhancement to the backbone rigidity. This is exemplified by the glass transition temperature (Tg) of PNB (∼45 °C)68 which is ∼90 °C higher than its linear five-carbon analog, polycyclopentene (PCP), with Tg = −44 °C.69 Furthermore, many of the ω-NB-macromonomers feature a succinimide ring fused to the NB structure, which exacerbates the backbone rigidity and can raise Tg over 200 °C with only a phenyl group attached to the succinimide nitrogen.70 For comparison, a phenyl branch directly attached to a polypentenamer repeating unit produces a material with Tg = 17 °C.71,72 Many studies focus on the effect that Nsc has on the extension and rigidity of the backbone. Thereby, the rigidity of the PNB BB may be playing a significant role in the correlations of these effects. We Received: July 31, 2018 Accepted: August 20, 2018

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DOI: 10.1021/acsmacrolett.8b00576 ACS Macro Lett. 2018, 7, 1080−1086

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ACS Macro Letters

5 °C increments from 5−25 °C. Here we note that catalyst choice, in theory, should be independent of the thermodynamic parameters.93 The unique chemical shift of the methine protons of CPBIB and poly(3-cyclopentenyl α-bromoisobutyrate) (PCPBIB) allows for integration comparison to determine conversions (Figure S6). Equilibrium monomer concentration, [M]eq, as a function of temperature is shown in Figure 1a and the linear trend is analyzed through the expression for equilibrium polymerization in eq 1:94

hypothesized that a BB made from a polypentenamer backbone would provide a valuable material for further investigations on the structure−property relationships of densely grafted polymers that is void of these rigidity enhancements and would provide access to a more definitive understanding on the behavior of the ng = 4 topology. ROMP of cyclopentene (CP) derivatives has been largely overshadowed due to their low ring strain, [ΔHp] = ∼23 kJ mol−1,73 which reduces polymerization kinetics and limits high conversions due to the thermodynamics of the equilibrium ROMP. However, ROMP of CP and derivatives has received heightened interest within the last several years.71−87 Recently, we reported a method to achieve high conversion, targeted molar mass, and low dispersity (Đ) PCP by altering the temperature during ROMP.77 This method, which we termed variable temperature ROMP (VT-ROMP), initiates polymerization at warm temperature and propagates CP to high conversion by cooling after initiation is complete. Molar masses near targeted (based on conversion), high conversions, and narrow Đ were achieved. It was also discovered that chain transfer is suppressed at cold temperatures, permitting narrow Đ to be maintained during the slower kinetics. The universality of this method should be applicable to any CP derivative with suitable ring-strain. We aimed to design a new CP monomer with a substituent suitable for initiation of grafting polymerizations using the “grafting-from” technique. We note that “grafting-through” is not a viable option; the dilution of the CP monomer, caused by the excess molar mass of the macromonomer, would prohibit appreciable conversions. The strategy to synthesize our targeted monomer, 3-cyclopentenyl α-bromoisobutyrate (CPBIB), is shown in Scheme 1.

ln[M ]eq =

ΔHp RT

−

ΔSp R

(1)

where T is temperature (K) and R is the gas constant (8.314 J K−1 mol−1). The resulting thermodynamic parameters (ΔHp = −22.6 kJ mol−1 and ΔSp = −74.5 J mol−1 K−1) reveal CPBIB has a ring strain similar to CP (ΔHp = −23.4 kJ mol−1)73 and is well suited for VT-ROMP. This is surprising since bulky substituents on CP rings can have deleterious effects on ΔHp due to Thorpe-Ingold effects.95,96 A series of CPBIB polymerizations was performed in THF using G1 catalyst and a VT-ROMP method of warm initiation at 50 °C for 5 min followed by cooling to 0 °C for 5 h prior to cold termination. Table 1 lists the resulting PCPBIB(Nbb) samples determined by multiangle light scattering size exclusion chromatography (MALS-SEC). For all samples, the molar masses were within 5% of theoretical based on conversions and Đ ≤ 1.23 signifying excellent control using the VT-ROMP method. For monomer to initiator ratios ([M]0/[I]0) of 75, 100, and 150 for PCPBIB(53, 68, and 95), respectively, an increase in molar mass is observed, as expected, while maintaining good control. PCPBIB(61, 63, and 68) were performed in triplicate, albeit at slightly different scales, and show good repeatability for the VT-ROMP method. For a comparison to SEC molar mass, PCPBIB(61) and PCPBIB(63) were characterized by 1H NMR to determine molar mass by end-group analysis (Figure S10). Comparative integration of the olefin protons on the phenyl end group (6.45 and 6.16 ppm, confirmed by COSY-NMR Figure S11) to the repeating unit methine protons (4.87 ppm) results in Mn,NMR = 17.0 and 17.7 kg mol−1, respectively. These values are slightly higher than Mn,MALS but within relatively good agreement. Inverse-gated decoupling 13 C NMR determined that PCPBIB(61) and PCPBIB(63) have approximately 83% trans-olefin isomers (Figure S13): a value consistent with previously reported ROMP at low temperature using similar catalysts.71,77 PCPBIB(61) and PCPBIB(63) were chosen to perform a duplicate series of polystyrene (PS) grafting using standard conditions for ATRP.48 Initiator concentration [BIB]0 was determined by dividing PCPBIB mass by the formula weight of the monomer unit (FWCPBIB = 233 g mol−1). The molar ratios of initial styrene [S]0, [BIB]0, and the remaining ATRP reagents were 600:1:0.4:0.03:0.8 for [S]0/[BIB]0/CuBr/ CuBr 2 /N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA). We chose to use CuBr2 and high [S]0, as this has been shown to aid in grafting efficiency.48 ATRP was performed at 80 °C for 8 h using two different methodologies (Table 2). For PCPBIB(61), five individual yet identical polymerizations were performed and each quenched at the respective time. For PCPBIB(63), a singular polymerization was performed with aliquots taken for analysis every 2 h. We feel this illustrates the repeatability of the grafting experiments

Scheme 1. Synthetic Route to a Polypentenamer Bottlebrush System

Monoepoxidization of cyclopentadiene using peroxyacetic acid followed by reduction to 3-cyclopentenol (3CPOH) is straightforward with acceptable yields (∼50% over both steps).88 The homoallylic alcohol was esterified using αbromoisobutyryl bromide (BIBB)89 to produce CPBIB in good yield (93%). The α-bromoisobutyrate (BIB) group is a workhorse initiator used in atom transfer radical polymerizations (ATRP), which would allow subsequent grafting after VT-ROMP.90−92 By placing the BIB functionality on the homoallylic position, a precise five-carbon spacing of these initiation sites along the polypentenamer is achieved. Since the ability to perform ROMP on CPBIB is unknown, the thermodynamics were investigated. Determination of polymerization enthalpy (ΔHp) and entropy (ΔSp) was performed by adding Hoveyda-Grubbs second generation catalyst (HG2) (0.23 mol %) to CPBIB ([M]0 = 2.26 M in toluene-d6) and monitoring ROMP equilibrium by 1H NMR at 1081

DOI: 10.1021/acsmacrolett.8b00576 ACS Macro Lett. 2018, 7, 1080−1086

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Figure 1. (a) Logarithmic equilibrium monomer concentration [M]eq (mol L−1) as a function of inverse temperature (K−1) for ROMP of CPBIB, as determined by 1H NMR analysis. (b) Number-average molar mass (Mn) as a function of ATRP grafting time on PCPBIB(63), as determined by 1 H NMR (triangles), MALS (squares), and RI (circles). Inset shows MALS traces of PCPBIB(63)-g-S showing a decrease in elution time as a function of increasing Nsc. (c) Stacked 1H NMR spectra (CDCl3, 25 °C) of CPBIB, PCPBIB(63), and PCPBIB(63)-g-S(8) highlighting key proton signals of the structures after each chemical transformation. (d) Stacked 13C NMR of PCPBIB(63) and PCPBIB(63)-g-S(8) with inset of the CO signal.

Table 1. Polypentenamer Macroinitiators Produced from VT-ROMPa sample ID

[M]0 (mol L−1)

[M]0/[I]0

convb (%)

Mn,theoc (kg mol−1)

Mn,MALSd (kg mol−1)

Đd

PCPBIB(53) PCPBIB(61) PCPBIB(63) PCPBIB(68) PCPBIB(71) PCPBIB(95)

2.05 2.25 2.25 2.25 2.25 2.25

75 100 100 100 100 150

68.6 62.3 64.4 71.3 71.5 65.5

12.0 14.5 15.0 16.6 16.7 22.9

12.4 14.2 14.6 15.9 16.6 22.1

1.17 1.22 1.19 1.17 1.18 1.23

transe (%)

Mn,NMRf (kg mol−1)

83.0 83.3

17.0 17.7

Polymerizations were conducted using G1 in THF at 50 °C for 5 min and then 0 °C for 5 h. bConversion determined by 1H NMR. cTheoretical molar mass based on monomer-to-catalyst ratio ([M]0/[I]0) and corrected for conversion. dNumber-average molar mass and dispersity determined by MALS-SEC (dn/dc = 0.090 mL g−1). eBackbone trans-olefin percent determined by inverse gated decoupling 13C NMR. fDetermined by 1H NMR end group analysis. a

through two commonly used methods. Determination of Nsc,NMR via 1H NMR was accomplished by integration comparison between the aromatic signals (δ = 6.5−7.5 ppm, 5H/PS repeating unit) to the backbone olefin hydrogens (δ = 5.21 ppm, 2H/PCPBIB repeating unit). 1H NMR was also used to determine the overall Mn,NMR of each BB through the equation: Mn,NMR = Nbb(FWCPBIB) + (NbbNscFWS), where FWS is the formula weight of styrene (104.15 g mol−1). MALS-SEC traces of each aliquot from ATRP of PCPBIB(63) (Figure 1b, inset) and PCPBIB(61) (Figure S15) show a decrease in retention time of monomodal peaks with increasing Nsc and

ATRP reaction time. Figure 1b illustrates the similarity of Mn,NMR and Mn,MALS and also the disparity of Mn,MALS versus Mn,CCC determined by conventional column calibration (CCC) of PS standards. The increasing error between Mn,MALS and Mn,CCC is a trend traditionally observed for BB systems and a possible indicator that backbone conformation is extending with increasing Nsc.16,97 A representative overlay confirming the 1H NMR spectral transformations from CPBIB to PCPBIB(63) to PCPBIB(63)-g-S(8) is shown in Figure 1c, with notable signals highlighted. Near quantitative grafting efficiency was confirmed by 13C NMR performed before and 1082

DOI: 10.1021/acsmacrolett.8b00576 ACS Macro Lett. 2018, 7, 1080−1086

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PCPBIB(61)-g-S(34) is the fact that one of the PS chainends is attached to the BB and therefore has less influence on the reduction of Tg.94 A notable advantage of the “grafting-from” method is the ability to produce BBCPs or core−shell cylindrical polymers (CSCP) with high overall Nsc.1,11 To illustrate this method for our system, we synthesized a new macroinitiator, PCPBIB(82), and performed sequential ATRP using S followed by methyl methacrylate (MMA) to produce PCPBIB(82)-g-[S(21)-blockMMA(100)] (SI). Figure 3a displays the 1H NMR spectrum

Table 2. ATRP Grafting of Styrene from Select PCPBIB Macroinitiatorsa sample ID

rxn time (h)

Mn,MALSb (kg mol−1)

Mn,NMRc (kg mol−1)

Đb

PCPBIB(63)-g-S(8) PCPBIB(63)-g-S(15) PCPBIB(63)-g-S(21) PCPBIB(63)-g-S(28) PCPBIB(61)-g-S(5) PCPBIB(61)-g-S(10) PCPBIB(61)-g-S(18) PCPBIB(61)-g-S(34) PCPBIB(61)-g-S(44) PCPBIB(61)-g-S(49)

2 4 6 8 2 4 6 8 10 12

58.9 102.0 138.5 195.1 48.1 78.9 132.8 227.8 298.0 325.5

67.1 113.0 152.4 198.3 47.5 80.3 123.7 230.2 293.8 319.9

1.30 1.26 1.34 1.28 1.23 1.24 1.26 1.29 1.32 1.29

Polymerizations were conducted at 80 °C at a [S0/BIB0/CuI/CuII/ PMDETA] of [600:1:0.4:0.03:0.8]. bNumber-average molar mass and dispersity determined via MALS-SEC. cDetermined by 1H NMR.

a

after ATRP which reveal a complete shift of the ester carbonyl signal from 171 to 180 ppm (Figure 1d), consistent with the loss of the neighboring bromine. Thermogravimetric analysis (TGA) on PCPBIB(61) and PCPBIB(61)-g-S(49) was performed under argon at a rate of 10 °C min−1 (Figure S15). For PCPBIB, a two-step decomposition is observed. At ∼215 °C, a loss of ∼70% w/ w is equivalent to the loss of BIB functionality and consistent with previous literature.98 After grafting, the first decomposition step is much less prominent (∼3% w/w) and the second decomposition step occurs at an onset temperature of ∼340 °C, consistent with PS degradation. Differential scanning calorimetry (DSC) thermograms of PCPBIB(61) and grafted samples in Table 2, taken upon the second heating at 10 °C min−1, are shown in Figure 2. PCPBIB(61) has a midpoint Tg

Figure 3. PCPBIB(82)-g-[S(21)-b-MMA(100)] characterized by (a) 1 H NMR with relevant signals annotated and normalized SEC traces (top right inset) before and after each sequential grafting step and (b) AFM height images showing a zoomed inset (top right) and cartoon representation of the core−shell BBCP morphology (top left).

Figure 2. Stacked DSC thermograms of PCPBIB(61) and the grafted samples in Table 2 (rate = 10 °C min−1, 2nd heating, exo down). Triangles annotate the midpoint Tg values.

showing the presence of PMMA signals after ATRP with an inset of SEC traces before and after each grafting step. Through 1 H NMR comparative integration with the polypentenamer olefin-H, PS aryl-H, and PMMA−OCH3 signals, we determined the total molar mass to be 1,220 kg mol−1 with Đ = 1.33. Given the large graft molar mass (∼12 kg mol−1), compared to the polypentenamer backbone (∼5.5 kg mol−1), we anticipate this particular architecture to resemble more of a spherical or oval core−shell morphology as depicted in the cartoon on Figure 3b. Atom force microscopy revealed ring-like structures of approximately 30−40 nm in diameter

= 4 °C, which further demonstrates the flexibility of the polypentenamer backbone. As Nsc is increased, a sharp increase in Tg is also observed, which eventually plateaus at ∼97 °C for Nsc = 34 and 44. This suggests the mobility of the BB system quickly becomes dominated by the movement of the side chains (Tg,PS ≈ 100 °C).99 Interestingly, linear PS with N ≈ 34 is unentangled and would be predicted to have a reduced Tg ≈ 75 °C.100 One rationalization for the increased Tg of 1083

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consistent with the size and predicted morphology of this sample. In conclusion, we report a unique polypentenamer BB system that allows control over Nbb, Nsc and block copolymer grafts with a high degree of control and versatility. This new BB system, having a much more flexible backbone, has the opportunity to elucidate how much the PNB backbone is contributing to the rigidity of these systems and continue our understanding of BB structure−property relationships for the ng = 4 topology. Such investigations are currently underway.

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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/acsmacrolett.8b00576.

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Additional experimental details, procedures, calculations, NMR, SEC, TGA, and AFM data (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Justin G. Kennemur: 0000-0002-2322-0386 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. 1750852. We also thank the Donors of the American Chemical Society Petroleum Research Fund (55378-DNI7) and the Florida State University Materials and Energy Hiring Initiative for partial support of this research. GC-EI/MS data was provided by the University of Florida Mass Spectrometry Research and Education Center funded by NIH S10 OD021758-01A1. The authors acknowledge Dr. Banghao Chen and the FSU NMR Facility for guidance and support and Prof. Albert Stiegman for use of the DSC instrument.

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REFERENCES

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DOI: 10.1021/acsmacrolett.8b00576 ACS Macro Lett. 2018, 7, 1080−1086

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