Controlled Metathetic Depolymerization of Natural Rubber in Ionic

Research Article pubs.acs.org/journal/ascecg

Controlled Metathetic Depolymerization of Natural Rubber in Ionic Liquids: From Waste Tires to Telechelic Polyisoprene Oligomers Ali Mouawia,† Arnaud Nourry,‡ Annie-Claude Gaumont,† Jean-François Pilard,‡ and Isabelle Dez*,† †

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Normandie Université, UNICAEN, ENSICAEN, Laboratoire de Chimie Moléculaire et Thioorganique, CNRS, 6 boulevard du Maréchal Juin, 14050 Caen, France ‡ LUNAM Université, Institut des Molécules et des Matériaux du Mans (IMMM), Equipe Méthodologie et Synthèse des Polymères, UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans, Cedex 9, France S Supporting Information *

ABSTRACT: A controlled degradation process enabling Natural Rubber (NR) depolymerization and using the olefin metathesis reaction in ionic liquid medium is reported. Using trihexyl-(tetradecyl)phosphonium chloride (Cyphos101) and N,N-dioctylimidazolium bromide (C8C8ImBr), low-dispersity telechelic polymers are produced. With N,N-dioctylimidazolium bromide (C8C8ImBr), the degradation process could be performed for five consecutive cycles with excellent control. This degradation process was successfully applied to waste tires. KEYWORDS: Polyisoprene, Cross-metathesis, Controlled depolymerization, Functional polymers, Ionic liquid, Catalyst recycling

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INTRODUCTION Due to the large production of polymers, the chemical recycling of polymer waste, which would offer a sustainable polymer reusing technology, is of major importance.1 This process consists of the conversion of polymer waste into monomers or reactive functional polymers or oligomers for a further use in the development of new high value polymers. In this regard, the recycling of natural rubber (NR, poly(cis-1,4-isoprene)), which is used in many applications of importance (tires, joints, belts, pipes, etc.) is a major challenge, as the annual production (2014) amounts to 11.8 × 106 tons. Furthermore, its depolymerization enables the formation of reactive functional polymers through the reactivity of the isoprene double bond units. The few known methods of degradation/depolymerization of NR include ozonolysis,2 biodegradation,3 photodegradation (sunlight and UV irradiation),4 ultrasonic irradiation,5 and chemical degradation, mainly through oxidative reactions.6−10 The main limitations of these processes are related to the various side-reactions, which are responsible for a low controlled depolymerization process and limit the synthesis of polymers having a defined microstructure. Moreover, these conditions do not allow the custom synthesis of functional polymers, amenable to further uses.11−13 Contrarily, the recently reported ruthenium-metathetic degradation of NR allows a simultaneous decrease in the molar mass of the starting NR and functionalization of the obtained polyisoprene oligomers by the use of various chain transfer agents during the depolymerization process.14−16 The process, which consists of a cross-metathesis reaction (chain transfer) between the propagating center and a functionalized allylic chain transfer agent (CTA), enables a choice of the chain-end functionality. © 2016 American Chemical Society

The commercial availability and the high functional tolerance of the ruthenium catalysts are additional advantages.17−19 Previous reports in this field deal with the metathetic degradation of NR to synthesize terpene or bistrithiocarbonyl-terminated oligomers with high yield (>75%) in homogeneous or heterogeneous conditions.20,21 Although the benefit of using ionic liquid (IL) media for various polymerization processes, particularly for ring-opening metathesis polymerization (ROMP), is recognized in the literature, so far it has never been applied to the depolymerization of NR. The merits in the application of IL solvents in ROMP have been demonstrated, allowing for the synthesis of polymers in high yield, in a controlled way and with a low metal contamination; moreover, it affords the recycling of the catalytic phase.22−24 Continuing our work on the development of an ecofriendly controlled depolymerization process of NR, we propose in this article the first depolymerization process of NR in ionic liquid, for the single-step synthesis of acetoxy telechelic polyisoprenic oligomers.25 The aim was to develop an efficient and controlled metathetic depolymerization process and investigate the possibility of catalyst recycling.

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RESULTS AND DISCUSSION Optimization of the Conditions for a Depolymerization Process in ILs. The degradation process of NR should Received: August 25, 2016 Revised: October 12, 2016 Published: October 20, 2016 696

DOI: 10.1021/acssuschemeng.6b01777 ACS Sustainable Chem. Eng. 2017, 5, 696−700

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Metathetic Degradation of SR in Cyphos101

obtained with a high yield (95%), an average molecular weight Mn of 14 000 g·mol−1, and a dispersity of 1.74. The proposed process exhibiting high efficiency on the synthetic SR and TSR10 polyisoprene samples, metathetic degradation, was next applied to a natural rubber sample (NRCV60), which is mainly constituted of polyisoprene (94%) but also contains proteins, lipides, glycosides, etc. (6%) that are known to limit the metathetic reaction.8 Performed on a NR ground powder characterized by a Mn of 462 000 g·mol−1 and a Đ of 2.37, a high rate of degradation was reached (yield of 97%). The resulted polymers have a Mn of 26 000 g·mol−1 with a dispersity of 1.83 (Figures S1 and S2 in the Supporting Information). Ru contamination, determined by ICP-OES, was very low (3.6 ppm), emphasizing another benefit of the IL medium.27 In comparison, when the degradation process was performed in toluene, the ruthenium contamination is higher than 100 ppm, which strictly forbids an industrial application. Decreasing the reaction time to 1 h led to a lower and less controlled degradation with polymers having a Mn of 74 000 g· mol−1 (Đ = 2.26). NMR and IR analyses confirmed the endfunctionalization of the oligomers by the acetoxy groups (Figures S3 and S4 in the Supporting Information). The molar mass value of the telechelic polymers is too large to afford MALDI-TOF analysis. The reaction was next performed with different PI concentrations (Table 1). Decreasing the [PI]LI from 5.14 to

fulfill the following criteria: efficiency, controlled degradation allowing the synthesis of oligomers with low Đ, and high functionalization of the degradation product. For this, the selected ILs should be able to dissolve and stabilize the Ru catalyst and the CTA and to afford the swelling of NR or its partial solubility. Compared to common imidazolium-based ILs, long-chain phosphonium-based Ils, which have high hydrophobic character, fulfill the requested criteria. In addition, they have increased thermal and chemical stabilities, allowing wider processing temperatures.26 NR being partially soluble in trihexyl(tetradecyl)phosphonium chloride (Cyphos101), this IL was selected to set up the conditions of the reaction. The degradation process was first developed from synthetic polyisoprene (SR), which is characterized by a Mn of 114 000 g· mol−1 and a Đ of 2.91. Cis-1,4-diacetoxy-2-butene (DBA) was chosen as the CTA since it can be easily deprotected to give hydroxyl groups, which are ideal end groups due to their flexibility for further functional group transformation and their potential to be used directly in subsequent polymerization reactions such as polyaddition or polycondensation. Typically, the reaction was performed according to Scheme 1. In a first step, the Grubbs II (GII, 1 mol %) catalyst was reacted with DBA (2 mol %) used as the chain transfer agent in Cyphos101. These conditions were chosen according to previous work.21 The catalytic solution was added to ground powders of SR under a nitrogen atmosphere. Under these conditions, the SR concentration in IL reached 5.14 mol·L−1. The mixture was heated under stirring at 45 °C and the reaction stopped after 3 h by adding a large excess (200 equiv) of ethyl vinyl ether (terminating agent) in the reaction mixture, to remove the Ru-alkylidene from the polymer. The telechelic polymers were precipitated in acetone and recovered by simple filtration. Their average molecular weights were measured by SEC analyses. In these conditions, the degradation proceeded to completion. No trace of the starting material was observed in the chromatogram, and a high yield of telechelic PI polymers was obtained (99%). The acetoxy telechelic PI polymers were evidenced by 1H NMR analysis through the characteristic cis and trans chain-end ethylenic protons at 5.78 ppm (cis HC CH) and 5.58 ppm (trans HCCH), the isoprenic ethylenic chain-end protons at 5.31 ppm (−C(CH3)CH), the isoprenic ethylenic backbone protons at 5.1 ppm (−(CH3)CCHCH2−), and the chain end CH2 at 4.59 and 4.50 ppm (−C(CH3)CH−CH2OC(O)−). IR analyses showed the characteristic peak of the carbonyl group acetoxy telechelic PI at 1739 cm−1. The polymeric products displayed an average molecular weight Mn of 17 000 g·mol−1 and a Đ of 1.77. These results showed that the degradation proceeded efficiently in Cyphos101 and that acetoxy telechelic PI was obtained with a low dispersity, evidencing the high control of the polymeric degradation. The depolymerization process was then extended to TSR10, which is a NR grade without any traces of hydroxylamine sulfate, widely used in the tire and rubber industry. The TSR10 used is characterized by a Mn of 180 000 g·mol−1 and a Đ of 3.09. It was treated according to the conditions previously described. The telechelic polymers were

Table 1. Influence of [PI]LI on the Metathetic Degradation of NRa 1 2 3

[PI] (mol L−1)

Mn (g mol−1)b

Mw (g mol−1)b

Đb

yield (%)

5.14 1.71 1.02

26 000 35 000 38 000

48 000 71 000 88 000

1.83 1.99 2.26

97 97 96

Reaction performed at 45 °C in cyphos 101 with DBA and Grubbs II (1 mol %) catalyst for 3 h. bExperimental average molecular weight measured by size exclusion chromatography (SEC) calibrated with polystyrene standards. Đ: Dispersity measured by SEC. a

1.71 mol·L−1 afforded telechelic polymers with a Mn of 35 000 g·mol−1 (Đ = 1.96; Table 1, entry 2). Lowering the concentration to 1.02 mol·L−1 led to telechelic polymers having Mn = 38 000 g·mol−1 (Đ = 2.26; Table 1, entry 3). Interestingly, the decrease of [PI]LI resulted in an increase of the obtained polymers average molecular weight, highlighting the fact that the molecular weight of the telechelic polymers can be modulated upon varying the concentration. These results further highlighted the benefit of the IL medium in the PI degradation process affording a high and controlled degradation of the starting PI (even for natural crude materials such as NR), the functionalization of oligomer end chains, and the low Ru contamination of the products. On the basis of this study, we investigated the efficiency of the catalytic degradation protocol toward more complex substrates such as waste tires. The waste rubber tires, coming from truck tires, are mainly composed of polyisoprenic (38%) 697

DOI: 10.1021/acssuschemeng.6b01777 ACS Sustainable Chem. Eng. 2017, 5, 696−700

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filtration, displayed an average molecular weight Mn of 23 000 g·mol−1 and a Đ = 1.89. These results are similar to those obtained in Cyphos101. A temperature of 50 °C was thus chosen to perform the degradation of NR in C8C8ImBr. The recycling of the IL phase was performed after evaporation of acetone, and washing of the catalytic phase with pentane to remove the excess of 2-isopropoxystryrene. A low degradation of NR was observed with the recycled catalytic phase leading to telechelic polymers with a Mn of 52 000 g·mol−1 and a dispersity of 2.25. To avoid the fast deactivation of the catalytic phase, acetone was replaced by ethanol in the precipitation step, after the cross-metathesis reaction.22 Under these conditions, the resulting telechelic polymers have a Mn of 34 000 g·mol−1 with Đ = 2.46 (entry 1, Table 2).

and polybutadienic structures (PB, 18%) and also contain carbon black (27%). Typically, the reaction was carried out on waste tire powder under the developed conditions using GII and DBA in Cyphos101. At the end of the reaction, the PI degradation products cannot be recovered by precipitation in acetone, after separation of black carbon, due to the low average molecular weight of the resulted PI polymers. Evaporation of acetone was performed. However, the degradation product cannot be extracted from the IL phase because of the high miscibility of Cyphos101 with most of the organic solvents. The recovery of the resulted telechelic PI required the use of an IL having a lower miscibility with some organic solvents. An imidazolium based IL was selected for this purpose; the results obtained will be depicted at the end of the next section. IL Catalytic Phase Recycling. In the second part of this study, we focused on the recycling of the catalyst and the IL phase to reduce the cost of the process. The problems to tackle were a little bit different. Indeed, to afford the recycling of the Ru catalyst and the easy separation of the IL catalytic phase from the products but also from the used additives such as the terminating agent, a new protocol had to be developed taking into account the following points: (1) A more robust and stable catalyst such as the Hoveyda−Grubbs (HG) catalyst had to be used.22 (2) The recovery of the HG catalyst required the use of a particular terminating agent, 2-isopropoxystyrene, which corresponded to the oxygenated ligand of the HG catalyst.28 (3) The elimination of the excess of the terminating agent had to be accounted for; to this purpose the ionic liquid catalytic phase had to be washed before reuse. Since Cyphos 101 is miscible with most of the organic solvents, another IL was selected for this purpose. The choice of the IL (N,Ndioctylimidazolium bromide (C8C8ImBr)) was driven, on the one hand, by its ability to solubilize the HG catalyst, the CTA and the NR (or, at least, its ability to allow the swelling of NR) and, on the other hand, by its immiscibility with the extracting organic solvent selected for the elimination of the excess of terminating agent. The selected long-chain imidazolium-based IL has a high degree of hydrophobicity, which offers a partial solubility of NR and a very poor miscibility with common organic solvents, such as toluene, diethyl ether, and hydrocarbon solvents. It was synthesized according to the procedure described in the literature.29,30 The degradation process was performed with NR in C8C8ImBr using the HG catalyst (1 mol %) and DAB as a chain transfer agent (2 mol %). The mixture ([PI]LI= of 5.14 mol·L−1) was heated under stirring at 45 °C. Then, the reaction was stopped after 3 h by adding 2-isopropoxystyrene (1.2 equiv) to remove the Ru-alkylidene from the polymer. The addition of acetone in the reaction mixture resulted in the precipitation of telechelic PI, which was recovered by filtration. The acetoxy telechelic PI polymers, obtained in high yield (98%) have a Mn of 45 000 g·mol−1 and a Đ of 2.10. IR and NMR analyses confirmed the functionalization of the depolymerized PI. In comparison with the results obtained in Cyphos101, the depolymerization process conducted in C8C8ImBr is less efficient (higher Mn and Đ). This is presumably due to the higher viscosity of the C8C8ImBr leading to a slightly less controlled degradation. Increasing the reaction time from 3 to 5 h did not allow a decrease in the molar mass of telechelic PI. To lower the viscosity, the reaction was next performed at 50 °C instead of 45 °C. Under these conditions, the acetoxy telechelic PI, recovered after the addition of acetone and

Table 2. Recycling of HG Catalyst in the Metathetic Degradation of NR in C8C8ImBra 1 2 3 4 5 6 7 8 9

recycling

Mn (g mol−1)b

Mw (g mol−1)b

Đb

yield (%)

0 1 2 3 4 5 6 7 8

34 000 45 000 50 000 57 000 60 000 62 000 61 000 70 000 116 000

86 000 108 000 95 000 124 000 126 000 147 000 165 000 193 000 285 000

2.46 2.36 1.85 2.18 2.09 2.38 2.70 2.73 2.46

91 90 90 89 90 90 91 90 92

Reaction performed at 50 °C in C8C8ImBr with DBA (2 mol %) and HG (1 mol %) catalyst for 3 h. [PI] = 5.14 mol·L−1 bExperimental average molecular weight measured by size exclusion chromatography (SEC) calibrated with polystyrene standards. Đ: Dispersity measured by SEC. a

The second run (first recycling), allowed obtaining the telechelic polymers in 90% yield (entry 2, Table 2), with only a slight increase of the Mn values (45 000 g·mol−1). The third run also resulted in high polymer yields up to 90% with a Mn of 50 000 g·mol−1 and Đ = 1.85 (entry 3, Table 2). Although a slight increase of the average molecular weights of the resulting polymers was observed along the cycles, the IL phase containing the HG catalyst could at least be reused for six cycles. In the following cycles, the efficiency of the catalytic solution was low (seventh recycling, Mn of 70 000 g·mol−1 (entry 8, Table 2); eighth recycling, Mn of 116 000 g·mol−1 (entry 9, Table 2)). The slight deactivation of the IL catalytic phase along the cycles can be related to the progressive extraction of the HG catalyst from the IL phase during its washing with pentane (light green coloration observed). Finally, we investigated the efficiency of the catalytic C8C8ImBr phase toward the degradation of waste tires. Typically, the reaction proceeded according to Scheme 1. The reaction was performed on waste tire ground powders using GII (1 mol %) catalyst, DBA (2 mol %) in C8C8ImBr (1 mL). After separation of the fillers such as carbon black, the acetoxy telechelic PI oligomers were obtained with a yield of 26%. Their structure was evidenced in 1H NMR analysis by the characteristic methylene proton peaks of the acetoxy end chain at 4.59 and 4.50 ppm. They display an average molecular weight of 400 g·mol−1 (determined by 1H NMR, Figure S5 in the Supporting Information). 698

DOI: 10.1021/acssuschemeng.6b01777 ACS Sustainable Chem. Eng. 2017, 5, 696−700

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times with acetone. Finally, the isolated polymer was dried under a vacuum. The product was obtained in quantitative yield (>95%). Degradation Procedure in C8C8ImBr with Hoveyda-Grubbs II Catalyst (HG) (General Procedure). In a 100 mL Schlenk flask, 32.1 mg (0.0512 mmol, 0.01 equiv) of Hoveyda Grubbs II catalyst (HG) and 17.6 mg (0.1022 mmol, 0.02 equiv) of DAB are introduced in 1 mL of C8C8ImBr. The Schlenk was then evacuated under a vacuum and placed under nitrogen, and the mixture was stirred for 40 min at room temperature. A total of 350 mg of NR (5.14 mmol, 1 equiv) was added to the mixture and stirred for 3 h at 50 °C under nitrogen. The metathesis reaction was quenched by adding 10.2 mg (0.0614 mmol, 1.2 equiv) of 2-isopropoxystyrene (IPS) into the reaction solution under a nitrogen atmosphere. Then, 50 mL of ethanol was added to the reaction mixture to precipitate the bifunctional telechelic polyisoprene. The mixture (ethanol+IL+HGII+IPS) was transferred to another Schlenk flask under nitrogen. The degradation product was washed several times with ethanol and dried under vacuum to remove any traces of solvent. The product was obtained in quantitative yield (>90%). The ionic liquid catalytic phase was then recycled. First, the ethanol was evaporated under reduced pressure, and the excess of 2isopropoxystryrene was eliminated from the catalytic IL phase with 5 mL of dry pentane. After evaporation under vacuum for 15 min, a new charge of DAB was added to the catalytic IL phase, and the solution was stirred for 40 min. Finally, a new portion of NR was added to proceed to a new depolymerization step, following the previously described procedure.

CONCLUSION The depolymerization of NR in hydrophobic IL was carried out successfully according to a controlled pathway using the olefin metathesis reaction. This process was efficient in the IL phase under soft conditions (low temperature, low IL quantity, short reaction time). High yields of acetoxy telechelic PI polymers with low Ru contamination were obtained. Moreover, the degradation rate can be controlled with the PI concentration used in the reaction. The catalytic IL phase could be recycled five times, allowing in each cycle an efficient and controlled depolymerization of NR leading to acetoxy telechelic PI polymers with a Mn in the range of 45 000 g·mol−1 to 62 000 g·mol−1. Highlighting the potential of this process, the metathetic degradation reaction proceeded successfully on waste tires to provide telechelic oligomers that are currently important, as they are key intermediates in the elaboration of innovative materials (block copolymer compatibilizers, thermoplastic elastomers, etc.). Even if the yield of oligomers issued from waste tires is low, these first results are encouraging and lead us to further pursue the study on other elastomer wastes.

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EXPERIMENTAL SECTION

Starting materials were purchased from Sigma-Aldrich and used without further purification. Cyphos 101 [trihexyl(tetradecyl)phosphonium chloride] (93%; Strem chemicals) was purified by passing it through a short pad of silica gel. 2-isopropoxystyrene (IPS) was prepared according to the procedure reported elsewhere.31 N,N-Dioctylimidazolium bromide (C8C8ImBr) was synthesized according to the procedure described elsewhere.29,30The cis-but-2-ene-1,4-diacetate (DBA) was synthesized according to the procedure described elsewhere.32 NMR spectra were recorded on a Bruker Avance Spectrometer for 1 H NMR (500 MHz). Chemical shifts (δ) were reported in parts per million downfield from tetramethylsilane (TMS). ICP-OES analyses were performed with an Agilent apparatus. Calibration was done with Ru standards containing 0.1, 0.5, 1.0, and 5.0 ppm. The numberaverage molecular weights (Mn) and molecular weight distribution (Đ) were measured using size exclusion chromatography (SEC) on a Waters system (515 HPLC pump, 410 differential refractometer, and 996 photodiode array detector) with two styragel water columns (HR5E THF 7.8 × 300 mm from 2000 to 4 × 106 g mol−1 and HR1 THF 7.8 × 300 mm from 100 to 4000 g·mol−1). The SEC analyses were performed at 35 °C, using tetrahydrofuran (THF) as the eluant with a flow rate of 1 mL min−1. The calibration was performed using a series of narrow molecular weight linear polystyrene standards (ranging from 1.27 × 103 g mol−1 to 3.04 × 106 g mol−1). The molecular weights of PI were corrected by the Benoit̂ factor of 0.67 for PI according to the known formula.33 Natural rubber (cis-1,4-polyisoprene with high molecular weight) was used without any further purification and grounded with a SPEX sample prep Freezer/Mill 6770. The waste tire materials were donated by Delta Gom Society (60400 Noyon, France), and their characterization was published in a previous work.10 All manipulations were performed by standard Schlenk techniques under a N2 atmosphere. Acetone was distilled from anhydrous potassium carbonate under argon. Degradation Procedure in CYPHOS 101 with Grubbs II Catalyst (General Procedure). In a 100 mL Schlenk flask, 43.6 mg (0.0512 mmol, 0.01 equiv) of Grubbs second generation catalyst (GII) and 17.6 mg (0.1022 mmol, 0.02 equiv) of DAB are introduced in Cyphos 101 (1 mL); the mixture was then evacuated under a vacuum, placed under nitrogen, and stirred for 40 min at room temperature. A total of 350 mg (5.14 mmol, 1 equiv) of NR was then added and stirred for 3 h at 45 °C under nitrogen. Then, the reaction was stopped by adding 1 mL (0.0104 mol, 200 equiv) of ethyl vinyl ether. Then 70 mL of acetone was added to the reaction mixture to precipitate the telechelic polymers. The precipitate was recovered and washed several

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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/acssuschemeng.6b01777. SEC analysis of telechelic polymer after NR degradation; SEC analyses of NRCV60 and telechelic polymer and two mixtures of both; 1H NMR spectrum of telechelic polymer after NR degradation; IR spectrum of telechelic polymer after NR degradation; 1H NMR spectrum of acetoxy telechelic PI oligomers issued from waste tires ground powders (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was promoted by the “Agence de l’Environnement et de la Maitrise de l’Energie” (ADEME) and the “Region Basse Normandie,” that are gratefully thanked for their financial support. Maine University, ENSICAEN and UNICAEN, CNRS, and Labex EMC3 are also thanked for their support.

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REFERENCES

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Research Article

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DOI: 10.1021/acssuschemeng.6b01777 ACS Sustainable Chem. Eng. 2017, 5, 696−700