Synthesis and Reaction of Anthracene-Containing Polypropylene: A

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Synthesis and Reaction of Anthracene-Containing Polypropylene: A Promising Strategy for Facile, Efficient Functionalization of Isotactic Polypropylene Deguang Zhang,†,§ Li Pan,*,‡ Yanguo Li,† Bin Wang,‡ and Yuesheng Li‡,∥ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ Tianjin Key Lab of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China § University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China ∥ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China S Supporting Information *

ABSTRACT: A novel anthracene-containing isotactic polypropylene (An-iPP) with high molecular weight (>10 × 104) and satisfying incorporation (5.7 mol %) was synthesized via direct copolymerization of propylene and 9hexenylanthracene. The pendent anthryl group of the resulting An-iPP is quite active, and this provides a facile and efficient avenue to synthesize various functional iPPs. As a typical and important example, maleic anhydride (MA) functionalized polypropylene, was successfully prepared in a highly efficient, catalyst-free, byproduct-free, and controllable way via mild Diels−Alder (D− A) reaction between pendent anthryl groups and MA. More importantly, the D−A functionalization process did not sacrifice the original properties of the An-iPP, as no unfavorable degradation and cross-linking were detected in DSC and GPC analyses. Besides MA, several other dienophiles could also be conveniently used as functional reagents to prepare various functionalized iPPs with distinct properties. The unique fluorescent property of An-iPP was studied and could be used for functionalization process monitoring.

1. INTRODUCTION As one of the most important polymers, isotactic polypropylene (iPP) has enjoyed wide application and commercial success owing to its combination of outstanding physical property, excellent chemical resistance, and especially low cost.1 However, the intrinsic nonpolar property of iPP usually results in low surface energies, poor dyeing properties, incompatibility with polar polymers, and weak adhesion with inorganic materials. These drawbacks of iPP greatly limit its further application in some high value areas where interactive properties are required.2 Therefore, developing functionalized iPP containing polar groups or reactive functional groups has been a hot topic for researchers in the late several years.2−8 In recent years, “reactive polyolefin intermediate” approach pioneered by Chung’s group has attracted considerable attention.2 Compared with postfunctionalization and direct copolymerization with polar comonomers approach,9,10 the “reactive polyolefin intermediate” approach does not need particular catalyst bearing desirable polar group tolerance and can conveniently prepare various well-defined functionalized polymer materials.8,11−23 By taking advantage of the facile, effective chemical transformation of the “reactive group”, welldefined functional PEs or PPs with tunable amount of hydroxyl, amino groups, ester groups, or poly(methyl methacrylate) © XXXX American Chemical Society

chains, etc., have been successfully obtained. All achievements in recent years prompt us to further probe the synthesis of more promising reactive intermediates and the following functionalizations to develop novel functional polyolefin materials with unique properties.19−30 It is well-known that the Diels−Alder (D−A) reaction, between a conjugated diene and a substituted alkene, is particularly useful in synthetic organic chemistry as a reliable method for forming sixmembered systems with good control over regio- and stereochemical properties. A polyolefin containing reactive groups such as (substituted) conjugated diene could be easily transformed into various functional polymers via simple reactions with different substituted alkenes. For instance, anthracene is a relative stable and effective reactant which can easily react with maleic anhydride (MA) via D−A reaction under mild reaction conditions.24 Accordingly, polyolefins containing pendent anthryl groups as the D−A grafting points are expected to show ascendancy of conversion and reactivity as well as byproducts-free and controllable reversibility. Yet at present, applying this facile, efficient, and versatile route to Received: November 25, 2016 Revised: February 28, 2017

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DOI: 10.1021/acs.macromol.6b02550 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules functionalize polyolefin is rarely reported.25 In this contribution, a series of iPPs containing pendent anthryl groups were efficiently synthesized by copolymerization of propylene and 9hexenylathracene (HA) for the first time. As expected, maleic anhydride functionalized iPP (MAPP) and some other functionalized iPPs were synthesized by D−A reaction between the pendent anthryl groups of poly(propylene-co-HA) and dienophiles without any side reaction. Unique fluorescent properties brought by the pendent anthryl groups of iPP chains were studied.

mL of anhydrous toluene was slowly added to the Grignard reagent solution. The mixture was kept at room temperature for 8 h. After that the mixture was acidified with hydrochloric acid (10 vol %), the organic layer was separated, and the aqueous layer was extracted with diethyl ether (2 × 100 mL). The combined organic layer was washed with water and dried by MgSO4, and the solvent was evaporated under vacuum to give the crude product. 46 g of P4O10 and 200 mL of toluene were added to the crude product, and the mixture was stirred for 6 h at room temperature. Then the solid residue was filtered off, and the solvent was evaporated under vacuum. The crude product was purified by column chromatography using hexane to give 33.4 g of pure HA as yellow oily solid (yield 54%). IR (KBr): ν = 3047, 2945, 1620, 1444, 729 cm−1. 1H NMR (400 MHz, δ, ppm, CDCl3): 8.32 (s, 1H), 8.26 (d, J = 8.7 Hz, 2H), 8.00 (t, J = 6.0 Hz, 2H), 7.52−7.42 (m, 4H), 5.85 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), 5.07−4.94 (m, 2H), 3.64− 3.58 (m, 2H), 2.18 (q, J = 7.1 Hz, 2H), 1.84 (ddd, J = 11.7, 9.4, 6.3 Hz, 2H), 1.73−1.64 (m, 2H). 13C NMR (101 MHz, δ, ppm, C2Cl4D2): 139.09 (s, 1C), 135.48 (d, J = 6.4 Hz, 1C), 131.69 (s, 2C), 129.61 (s, 2C), 129.37 (s, 2C), 125.71 (s, 3C), 125.17 (d, J = 6.2 Hz, 2C), 124.65 (s, 2 C), 114.93 (s, 1 C). 2.3. Typical Copolymerization Procedure. Copolymerizations of propylene and HA were performed under atmospheric pressure in a 150 mL glass reactor equipped with a mechanical stirring bar. The reactor was charged with prescribed volume of toluene and comonomer under a nitrogen atmosphere, and then the propylene gas feed was started, followed by the addition of AliBu3 or MMAO. After equilibration at the desired polymerization temperature for 5 min, the polymerization reaction was initiated by adding the prescribed amount of catalyst and cocatalyst. After a desired period, the reactor was vented. The resulting copolymers were precipitated from hydrochloric acid/ethanol (2 vol %), filtered, washed three times with ethanol, then marinated in acetone for 12 h to remove the unreacted comonomer, and then dried under vacuum at 40 °C to constant weight. 2.4. Diels−Alder Functionalization of Poly(propylene-coHA). In a typical reaction, 0.2 g of poly(propylene-co-HA) copolymer was dissolved in 20 mL of toluene at 100 °C under nitrogen in a 100 mL flask with a magnetic stirrer. After cooling the system to 80 °C, an appropriate amount of functional dienophile was added to the reactor, and the reaction was stirred for another 6 h at the temperature before precipitating the reaction mixture in 200 mL of acetone. The resulting grafting copolymer was isolated by filtration, purified by two dissolving−precipitating cycles, and dried under vacuum at 40 °C for 24 h.

2. EXPERIMENTAL DETAILS 2.1. Materials and Instrumentation. All moisture and/or oxygen sensitive manipulations were operated using standard Schlenk techniques or in an MBraun glovebox under a dry nitrogen atmosphere. Anhydrous solvents used in this work were purified by a solvent purification system purchased from Mbraun. rac-Et[Ind]2ZrCl2 and dimethyl(pyridylamido)hafnium precatalyst were sysnthesized using the modified procedures.26,27 Commercial propylene was used directly for polymerization without further purification. Modified methylaluminoxane (MMAO, 10 wt % in toluene), AliBu3, [Ph3C][B(C6F5)4], 6-bromo-1-hexene, P4O10, anthrone, methyl propiolate, vinylene carbonate, N-cyclohexylmaleimide, phenyl vinyl sulfone, fumaronitrile, 4-phenyl-1,2,4-triazoline-3,5-dione, and fullerene were purchased from Sigma-Aldrich and used as received. 9-Vinylanthracene, 9-allylanthracene, and 9butenylanthracene were synthesized according to the literature.28 Maleic anhydride was purchased from Sigma-Aldrich and purified by sublimation before use. All high-temperature NMR spectra were recorded on a Bruker AM-400 instrument in 1,1,2,2-tetrachloroethaned2 at 120 °C. The melting and crystallization temperatures of the polymers were measured by differential scanning calorimetry (DSC, TA Instruments, Model Q2000) with a heating and cooling rate of 10 °C/min. Thermogravimetric analysis (TGA) was carried out under a nitrogen atmosphere using thermogravimetric analyzer (TA Instruments, Model Q50) with a heat rate of 20 °C/min. The molecular weights and molecular weight distributions of the polymer samples were determined at 150 °C by a PL-GPC 220 type high-temperature gel permeation chromatograph (GPC). 1,2,4-Trichlorobenzene (TCB) was employed as elute solvent at a flow rate of 1.0 mL/min, and the calibration was made by polystyrene standard Easi-Cal PS-1 (PL Ltd.). The FT-IR spectra were recorded on a Bio-Rad FTS-135 spectrophotometer using polymer thin films. Polarizing optical microscopy (POM) observation was done on an Olympus BX51 polarizing optical microscopy equipped with a LTS 350 hot stage and a TMS 94 temperature programmer (Linkam). The water contact angel experiment was conducted on a Kruess G10/DSA10 contact angle analyzer. UV−vis spectroscopy was conducted with a SHIMADZU UV-1600PC. A slit of 2 nm was applied, and the absorption was recorded by 0.1 nm at a scan rate of 200 nm/min. Excitation spectra and emission spectra were obtained on a PerkinElmer LS 50B luminescence spectrometer with xenon discharge lamp excitation. Cyclic voltammetry was performed on a CHI600E electrochemical workstation in a standard three-electrode cell under a blanket of argon in a 0.1 M solution of 0.1 M n-Bu4NPF6 at room temperature. Measurement was carried out with platinum working and platinum counter electrodes and an Ag/AgCl reference electrode; the reduction potential was recorded at a scan rate of 100 mV/s and reported with reference to the ferrocene/ferrocenium redox couple. 2.2. Synthesis of 9-Hexenylanthracene (HA). Freshly cut magnesium pieces (7.36 g) were suspended in 200 mL of dry tetrahydrofuran. A few drops of 6-bromo-1-hexene and a pellet of iodine were added into the flask and stirred for 10 min to initiate the reaction. After the color of iodine faded, a total amount of 49 g of 6bromo-1-hexene was added dropwise into the flask at 0 °C. The mixture was stirred at room temperature for 4 h. Then the system was heated to reflux for another 4 h. Subsequently, the system was cooled to 0 °C again, and a solution of anthrone (38.9 g, 200 mmol) in 100

3. RESULTS AND DISCUSSION 3.1. Synthesis of Poly(propylene-co-HA). Since the dimethyl(pyridylamido)hafnium complex (Cat. Hf) and the classical metallocene catalyst rac-Et[Ind]2ZrCl2 (Cat. Zr) displayed excellent performance in the copolymerization of propylene with 4-phenyl-1-butene and p-(3-butenyl)styrene,18 these two catalysts were both adopted for the copolymerizations. To achieve an effective incorporation of anthracenecontaining comonomer and avoid depression of catalytic activity caused by the bulky and conjugated functional group, selecting a promising comonomer is also a matter of great concern. To find out an optimal comonomer candidate, as shown in Scheme 1, a series of 9-substituted anthracene comonomers with different olefin substitutions, including 9vinylanthracene (VA), 9-allylanthracene (AA), 9-butenylanthracene (BA), and 9-hexenylanthracene (HA), were synthesized according to the similar synthetic route reported in the literature and being screened in the copolymerizations.28 Homopolymerizations of all these synthesized anthracenecontaining monomers were tried first but no polymer or oligomer can be obtained. As observed, both Cat. Hf and Cat. Zr failed to efficiently incorporate the anthryl comonomers B

DOI: 10.1021/acs.macromol.6b02550 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route of Functionalized iPP

The representative results of propylene/HA copolymerizations are summarized in Table 1. One can note that Cat. Zr displayed high catalytic activity in the copolymerzations (up to 40 × 105 gpolymer molCat.−1 h−1, runs 4−7). The HA incorporation is not sensitive to reaction temperature but can be remarkably enhanced by HA dosage in feed. As shown in Table 1, a 2-fold increase in HA content is achieved when a 2fold increase in HA dosage applied (run 5 vs run 7). However, the molecular weights (MWs) of the resultant copolymers are below 1.8 × 104 (runs 4−7). On the contrary, surprisingly, besides a high catalytic activity, Cat. Hf can also produce copolymers with much higher MWs (up to more than 10-fold that from Cat. Zr) and HA incorporations (up to 5.7 mol %, runs 10−16). Moreover, the addition of varying amount of AliBu3 in the reaction system helps to tune the MWs of the copolymers, as the AliBu3 serves as an efficient scavenger and also a chain transfer agent. The unimodal distributed MW of the copolymer demonstrates a single site catalyzed copolymerization system. Nonetheless, the copolymers containing 5.7 mol % and higher amount of pendent anthryl groups failed to fully dissolve during GPC tests. The poor solubility might be caused by the intensive intermolecular interaction originated from enhanced π−π stacking between the pendent anthryl groups.31−33 To estimate the copolymerization reactivity ratio of propylene and HA catalyzed by Cat. Hf, a series of copolymerization with low monomer conversion (24 × 105 gpolymer molCat.−1 h−1), molecular weight (>10 × 104), and incorporation ratio (5.7 mol %) have reached a satisfying level simultaneously. The pendent anthryl groups in poly(propylene-co-HA) display high Diels− Alder (D−A) reactivity toward numerous functionalization reagents under mild conditions. Based on this reactive polyolefin intermediate, various functional iPPs including maleic anhydride functionalized iPP with high molecular weight and high functional group content were obtained in a facile way. The employed D−A functionalization strategy possess remarkable features such as operational simplicity, high

381 nm, and the major emission signals are observed at 401, 418, and 444 nm. The shapes of the polymer’s excitation and emission spectra are similar to that of reported anthracene, and the fluorescence intensity stays at a high level.50 The pendent fluorescent anthryl groups can be converted into nonfluorescent D−A adducts after the cycloaddition of MA. As observed, after the MA functionalization, no fluorescence can be detected from the emission spectra. Based on this property, such functional polymers can be utilized in materials’ crack sensoring.51 Additionally, the absorption change of fluorescence spectra and UV−vis spectra after D−A reaction of the pendent anthryl groups makes it possible to monitor the functionalization process of An-iPPs by the decrease of anthryl absorbance. It was also observed from the UV spectra (Figure 4) that AnF

DOI: 10.1021/acs.macromol.6b02550 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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efficiency, ready availability of reagents at low cost, and no damage to iPPs’ excellent performance. The fluorescent properties were also introduced into An-iPPs by pendent anthryl groups and can be used in monitoring the functionalization process. The herein reported efficient conversion of anthracene-containing “reactive polyolefin intermediates” under mild conditions has great potential in modifying and improving iPP’s properties. More studies on the design and synthesis of unique functional polymers with unprecedented performances are in progress.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02550. Typical 1H and 13C NMR spectra, FT-IR spectra, GPC, TGA, DSC, tensile test, and cyclic voltammetry results of some copolymers; detailed calculations about the reactivity ratios of the comonomers in the propylene/ 9-hexenylanthracene copolymerization by using dimethyl(pyridylamido)hafnium complex (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (L.P.). ORCID

Li Pan: 0000-0002-9463-6856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support by the National Natural Science Foundation of China (No. 21234006). REFERENCES

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DOI: 10.1021/acs.macromol.6b02550 Macromolecules XXXX, XXX, XXX−XXX