Copolymerization of Isoprene with p-Alkylstyrene Monomers

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Copolymerization of Isoprene with p‑Alkylstyrene Monomers: Disparate Reactivity Ratios and the Shape of the Gradient Philipp von Tiedemann,†,‡ Jan Blankenburg,†,‡ Kamil Maciol,† Tobias Johann,†,§ Axel H. E. Müller,*,† and Holger Frey*,† †

Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128 Mainz, Germany § Max Planck Graduate Center with the Johannes Gutenberg University, Staudinger Weg 6, 55128 Mainz, Germany Downloaded via WESTERN SYDNEY UNIV on January 12, 2019 at 14:50:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: The statistical copolymerization of isoprene with p-ethyl- (p-ES), p-isopropyl- (p-iPS), and p-tert-butylstyrene (ptBS) initiated by sec-butyllithium in cyclohexane was investigated with respect to kinetics, reactivity ratios, and formation of tapered block copolymers with pronounced monomer gradient. An efficient synthetic route to the monomers was developed on a multigram scale, relying on the precipitation of the side-product triphenylphosphine oxide at low temperature. The copolymerization kinetics and resulting molecular weight distributions were analyzed. The dispersity, Đ, of the copolymers depends on the p-alkyl substituent, the the degree of polymerization Pn and the comonomer mole fraction, X. In situ 1H NMR kinetics characterization revealed a strong gradient structure for all three copolymer systems (rI = 21.9, rp‑ES = 0.022; rI = 19.7, rp‑iPS = 0.027; rI = 19.8, rp‑tBS = 0.022). The rate of crossover from a polyisoprenyllithium chain end (I) to a p-alkylstyrene (S) unit relative to the alkylstyrene homopolymerization, kIS/kSS (in 10−3 (L mol−1)−1/4), decreases in the order p-MS (19.1) > p-ES (11.3) > p-iPS (5.71) ≈ p-tBS (5.63), supporting the observed, increasingly bimodal character of the molecular weight distributions and the higher dispersity. Thermogravimetric analysis revealed that all poly(p-alkylstyrene) homopolymers are stable up to 300 °C.

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INTRODUCTION Styrene derivatives1−8 are highly established monomers, particularly for the living anionic polymerization. Despite the immense developments in the area of controlled radical polymerization techniques9,10 within the past decades, among all controlled polymerization techniques,11 the classical anionic living polymerization12,13 still represents a widely established and reliable strategy for block copolymer architectures with precisely adjustable material properties.14,15 The variety of existing and prospective applications based on these materials reflects their potential for many further years of research.16 Besides block copolymer synthesis based on sequential addition of two monomers, the direct statistical copolymerization of a monomer mixture can be used to prepare so-called “tapered” block copolymers, showing a monomer gradient.17−24 Commercial products like Styrolux and Styroflex consisting of styrene and butadiene demonstrate the © XXXX American Chemical Society

importance of this class of materials within the chemical industry.25 When two monomers with strong reactivity difference are combined, the respective tapered copolymers exhibit a steep gradient and may undergo phase separation like sequential block copolymers,1,19,23,26 resulting in the formation of different nanosegregated morphologies determined by the volume fractions of the constituents.24 In apolar media like cyclohexane, isoprene and styrene exhibit highly disparate reactivity ratios, leading to tapered copolymers, i.e., copolymers with a steep composition gradient along the polymer chains.27,28 We recently introduced a rapid in situ monitoring technique for living anionic copolymerizations based on realReceived: October 23, 2018 Revised: December 13, 2018

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DOI: 10.1021/acs.macromol.8b02280 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules time 1H NMR kinetics measurements that enables to analyze the microstructure of the growing copolymer chains in great detail.29−32 By use of in situ NMR, the copolymerization behavior of styrene with 1,1-diphenylethylene derivatives has recently also been investigated.33 Relying on this technique, the reactivity ratios of isoprene and styrene in cyclohexane were determined to be rI = 11.5 and rS = 0.044,1 close to literature values previously reported.27,28 Surprisingly, changing from styrene to p-methylstyrene (p-MS) yielded polymers with an even stronger gradient structure (rI = 25.4 and rpMS = 0.007).1 This demonstrates how a subtle change in the chemical nature of styrene monomers can drastically affect their copolymerization behavior, which is explained by higher electron density at the vinyl group and the carbanionic chain end. These findings sparked our interest regarding the statistical copolymerization of isoprene with styrene monomers bearing largerand especially branchedalkyl groups in general. A general synthetic access to substituted styrene derivatives based on inexpensive starting materials that can be scaled up, ideally circumventing chromatographic purification procedures, is a challenge. Functional phenyl aldehydes are suitable starting materials for such syntheses as they are commercially available at low cost. The classical Wittig reaction34 represents a suitable route to a variety of substituted styrene monomers. Nevertheless, on a multigram-scale challenging purification protocols are unavoidable, in particular to remove triphenylphosphine oxide as the side product. On a small scale this can easily be managed by column chromatography, but on larger scale the Wittig reaction and other synthetic routes (Appel reaction,35 Mitsunobu36 reaction, or aza-Wittig reaction37) providing triphenylphosphine oxide are known to involve challenging purification protocols. Alternative methods38,39 were developed to remove triphenylphosphine oxide without column chromatography. However, the respective procedures usually require strict conditions such as very specific solvents40 or large amounts of toxic compounds like oxalyl chloride.41 Within this work an improved purification procedure is introduced, avoiding column chromatography or additional reagents.

ization of p-ethylstyrene (p-ES), p-isopropylstyrene (p-iPS), and p-tert-butylstyrene (p-tBS). Homopolymers of p-ES synthesized by free radical polymerization using AIBN as an initiator were studied regarding their photodegradation induced via exposure to 254 nm radiation several decades ago.49 Similar experiments were performed for p-iPS,50 and its homopolymers’ dilute solution properties51 and peroxidation behavior52 were investigated. Among these three monomers only p-tBS was employed for the synthesis of block copolymers by Fetters et al. and later by Catala et al.53,54 None of these monomers have ever been copolymerized with isoprene. Besides the use of p-tBS in homogeneous catalysis55 it was shown that poly(p-tBS) also forms miscible blends with polyisoprene.56 Living anionic polymerization is often praised for its high level of control and access to high molecular weight polymers with very narrow molecular weight distributions. However, the synthesis of well-defined polymers with low dispersity might not always be the ultimate goal when it comes to precise adjustment of the materials’ properties in the desired application.57−65 A fundamental understanding of different molecular weight distribution phenomena like asymmetry, broadening, or tailing in the anionic copolymerization is key for further development of well-defined copolymers. The present work aims at a general, in-depth understanding of the kinetics and scope of the statistical anionic copolymerization of isoprene and p-alkylstyrene monomers (Scheme 2) with disparate reactivity ratios. To this end, the copolymers’ microstructure was investigated by in situ 1H NMR kinetics characterization. Homopolymerization kinetics (rate constants) and molecular weight distributions (MWD) with respect to dispersity, Đ, as a function of the degree of polymerization, Pn, and comonomer mole fraction, X, were studied in detail for all three copolymer systems and compared to data for the copolymerization of isoprene with styrene and p-methylstyrene.

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EXPERIMENTAL PART

Terminology. Styrene, p-ethylstyrene, p-isopropylstyrene, p-tertbutylstyrene, and isoprene are hereinafter abbreviated as S, p-ES, piPS, p-tBS, and I, respectively. Styrene and isoprene were purchased from Sigma-Aldrich. Instrumentation. 1H NMR spectra (400 MHz) were recorded on a Bruker Avance II 400 spectrometer equipped with a 5 mm BBFOSmartProbe with z gradient and ATM as well as a SampleXPress 60 sample changer. All spectra are referenced internally to residual proton signals of the deuterated solvent. For in situ 1H NMR kinetics measurements a Bruker Avance III HD 400 spectrometer equipped with a 5 mm BBFO SmartProbe (Z-gradient probe) and an ATM as well as a SampleXPress 60 autosampler was used. Size exclusion chromatography (SEC) measurements were performed using an Agilent Series 1100 equipped with a SDV column set from PSS (SDV 103, SDV 105, SDV 106). Tetrahydrofuran (THF) was used as the

Scheme 1. Synthesis of p-Alkylstyrene Monomers

As compared to the polymerization of p-methylstyrene,42−48 only few works have been published concerning the polymer-

Scheme 2. Copolymerization of Isoprene with p-Alkylstyrene Monomers

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DOI: 10.1021/acs.macromol.8b02280 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules mobile phase (flow rate 1 mL min−1) and as the solvent. Poly(styrene) standards were provided by PSS for calibration. The measurements were performed at 30 °C with an RI and UV (275 nm) detector. The molecular weight (Mn) and MWD of the copolymers were determined using the copolymerization module within the PSS WinGPC UniChrom (V 8.31, Build 8417) software provided by PSS Polymer Standards Service GmbH (for details see the Supporting Information). Thermogravimetric analysis (TGA) was performed with a PerkinElmer Pyris 6 instrument with a nitrogen or oxygen flow rate of 10 K min−1. Reagents. Chemicals and solvents were purchased from commercial suppliers (Acros, Sigma-Aldrich, Fisher Scientific, and Alfa Aesar). Deuterated solvents were obtained from Deutero GmbH. Isopropyl alcohol and methanol were used as received without further purification. Cyclohexane was purified by stirring over diphenylhexyllithium (adduct of sec-butyllithium and 1,1-diphenylethylene), vacuum-transferred, and degassed by four freeze−pump−thaw cycles prior to use. Isoprene, p-ES, p-iPS, and p-tBS were purified by stirring over CaH2 overnight followed by distillation. All reagents were degassed by four freeze−pump−thaw cycles prior to use. Monomer Synthesis. p-Ethylstyrene (p-ES). p-ES was prepared in a one-step synthesis following a modified literature procedure reported by Casanova and co-workers. 66 Methyltriphenylphosphonium bromide (43.16 g, 120.8 mmol) was added to a solution of sodium hydride (3.81 g, 158.9 mmol) in 150 mL of freshly distilled dry THF in a 250 mL Schlenk flask under argon at 0 °C, and the mixture was vigorously stirred (1000 rpm, big stirring bar) for 35 min at this temperature. Subsequently, p-ethylbenzaldehyde (17.0 mL, 112.5 mmol) was added slowly at 0 °C via a syringe through a septum. After complete addition the reaction mixture was warmed up to room temperature and left to stir overnight. The conversion was checked by TLC (pure hexanes, Rf (product) = 0.8). When quantitative conversion was reached, the reaction mixture was cooled to −78 °C with an ethanol/liquid nitrogen bath and quenched dropwise with water, until no more exothermicity was observed (∼10 mL) to precipitate most of the triphenylphosphine oxide (TPPO). The solution was filtered (P3 coarse frit), and the filtrate was concentrated under reduced pressure (600−400 mbar) at 50 °C to remove the solvent. To the yellow viscous residue 50 mL of cold (0 °C) diethyl ether was added, and the suspension was filtered rapidly after gentle shaking. The filtrate was distilled under reduced pressure (10 mmHg at 80 °C) to give p-ES as a colorless liquid. Yield: 73%. 1 H NMR (400 MHz, chloroform-d1): δ (ppm) = 7.36 (d, J = 8.1 Hz, 2H, d), 7.18 (d, J = 8.2 Hz, 2H, e), 6.72 (dd, J = 17.6, 10.9 Hz, 1H, c), 5.73 (dd, J = 17.6, 1.0 Hz, 1H, a), 5.21 (dd, J = 10.9, 1.0 Hz, 1H, b), 2.66 (q, J = 7.6 Hz, 2H, f), 1.26 (t, J = 7.6 Hz, 3H, g). 13C NMR (100 MHz, chloroform-d1) δ (ppm) = 144.17 (c), 136.86 (b), 135.22 (f), 128.16 (d), 126.33 (e), 112.96 (a), 28.76 (g), 15.70 (h). For spectra see Figures S1 and S2. p-iPS and p-tBS were synthesized in an analogous manner with comparable yields. p-Isopropylstyrene. Yield: 70%, colorless liquid. 1H NMR (400 MHz, chloroform-d1) δ (ppm) = 7.36 (d, J = 8.2 Hz, 2H, a), 7.20 (d, J = 8.2 Hz, 2H, b), 6.71 (dd, J = 17.6, 10.9 Hz, 1H, c), 5.72 (dd, J = 17.6, 1.0 Hz, 1H, d), 5.20 (dd, J = 10.9, 1.0 Hz, 1H, e), 3.02−2.80 (m, 1H, f), 1.26 (d, J = 7.0 Hz, 6H, g). 13C NMR (100 MHz, chloroformd1) δ (ppm) = 149.09 (c), 137.16 (b), 135.69 (f), 127.02 (d), 126.65 (e), 113.31 (a), 34.34 (g), 24.41 (h). For spectra see Figures S3 and S4. p-tert-Butylstyrene. Yield: 71%, colorless liquid. 1H NMR (400 MHz, chloroform-d1) δ (ppm) = 7.44−7.31 (m, 4H, d+e), 6.74 (dd, J = 17.6, 10.9 Hz, 1H, a), 5.75 (dd, J = 17.6, 1.3 Hz, 1H, b), 5.24 (dd, J = 10.9, 1.3 Hz, 1H, c), 1.36 (s, 9H, f). 13C NMR (100 MHz, chloroform-d1) δ (ppm) = 150.98 (c), 136.73 (b), 134.97 (f), 126.07 (d), 125.56 (e), 113.13 (a), 34.70 (g), 31.44 (h). For spectra see Figures S5 and S6. Sample Preparation for in Situ 1H NMR Kinetics Studies. The monomer/solvent mixtures (20 wt % in cyclohexane-d12) were prepared in an argon-filled glovebox (MBraun UNILAB, p-ES ≃ p-iPS > p-tBS, this order being the same as that of the monomer electron densities. Because of the observed strong taper formation, the three copolymer systems hold great promise for the synthesis of block-like gradient copolymers. Since the resulting copolymers remain living after full monomer conversion, they also facilitate rapid access toward more complex polymer architectures, as they can be precisely end-functionalized or used as a macroinitiator for successive polymerization steps.

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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.8b02280. Additional data and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.F.). *E-mail: [email protected] (A.H.E.M.).

Figure 6. Thermogravimetric analysis (TGA) under N2 (a) and under pure O2 (b) of 5000 g mol−1 poly(p-alkylstyrene) sample compared with polystyrene.

ORCID

Axel H. E. Müller: 0000-0001-9423-9829 Holger Frey: 0000-0002-9916-3103

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Notes

CONCLUSIONS The statistical copolymerization of isoprene with three different p-alkyl-substituted styrene monomers (p-ES, p-iPS, and p-tBS) initiated by sec-butyllithium in apolar media (cyclohexane) was investigated in detail to evaluate the effect of the alkyl group on the monomer gradient and molecular weight distributions. The resulting molecular weight distributions (dispersity and shape) of the copolymers were found to be governed by three parameters: (i) the size of the p-alkyl substituent of the styrene monomer, (ii) the desired degree of polymerization, Pn, of the resulting copolymer, and (iii) the comonomer mole fraction, X. A less sterically demanding p-alkyl substituent yielded narrower molecular weight distributions. Similarly, an increase in the overall Pn resulted in copolymers with lower dispersity. A higher comonomer mole fraction, X, of the p-alkylstyrene reduced the bimodal character of the SEC traces. In situ 1H NMR kinetics showed that all three statistical copolymerizations yielded tapered block copolymers with a strong gradient microstructure (rI = 21.9, rp‑ES = 0.022; rI = 19.7, rp‑iPS = 0.027; rI = 19.8, rp‑tBS = 0.022). Thus, an alkyl substituent in the para position of styrene enhances rI dramatically. This effect is most pronounced for the smallest substituent from our previous studies p-methyl1 (rI = 25.4, rp‑MS = 0.007) and decreases as the number of carbon atoms at

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Nadine Schenk for technical assistance. P.V.T. and J.B. acknowledge the Graduate School of Excellence MAINZ for financial support. T.J. thanks the MPGC Mainz and the Gutenberg Academy for financial support.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b02280 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02280 Macromolecules XXXX, XXX, XXX−XXX