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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01777 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Sustainable Chemistry & Engineering

Controlled metathetic depolymerization of natural rubber in ionic liquids: from waste tires to telechelic polyisoprene oligomers Ali Mouawiaa, Arnaud Nourryb, Annie-Claude Gaumonta*, Jean-François Pilardb*, and Isabelle Deza* a

Normandie Université; UNICAEN; ENSICAEN; Laboratoire de Chimie Moléculaire et Thioorganique ;

CNRS; 6 boulevard du Maréchal Juin, 14050 Caen, FRANCE. [email protected] b

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.

Abstract: A controlled degradation process enabling Natural Rubber (NR) depolymerization and using 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 5 consecutive cycles with an excellent control. This degradation process was successfully applied to waste tires.

Keywords: polyisoprene, cross-metathesis, controlled depolymerization, functional polymers, ionic liquid, catalyst recycling. 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 in 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…) is a major challenge, as the annual production (2014) amounts to 11.8x106 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 limitation of these processes are related to the various side-reactions, which are

ACS Paragon Plus Environment

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responsible for a low controlled depolymerisation 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 to simultaneously decrease the molar mass of the starting NR and to functionalize the obtained polyisoprene oligomers by the use of various chain transfer agents during the depolymerization process [14-16]. The process, which consists in a cross-metathesis reaction (chain transfer) between the propagating center and a functionalized allylic chain transfer agent (CTA), enables to choose the chain-end functionality. The commercial availability and the high functional tolerance of the ruthenium catalysts are additional advantages [17-19]. Previous reports in this field deals 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 polymerisation processes, particularly for ring-opening metathesis polymerisation (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. Results and discussion Optimisation of the conditions for a depolymerization process in ILs

The degradation process of NR should fulfil the following criteria: efficiency, controlled degradation allowing for the synthesis of oligomers with low PDI, 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 commun imidazolium-based ILs, long-chain phosphonium-based Ils, which have a high hydrophobic character fulfil the requested criteria. In addition, they have increased thermal and chemical stabilities, allowing wider processing temperatures [26]. NR being partially


2

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soluble in trihexyl(tetradecyl)phosphonium chloride (Cyphos101), this IL was selected to set up the conditions of the reaction. The degradation process was firstly developed from synthetic polyisoprene (SR), which is characterized by a

Mn

of 114,000 g.mol-1 and a Ð 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.

Grubbs II (1 mol %)

O

O

O

O

DBA (2 mol %) n

Cyphos 101 45°C

z 90%).


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The ionic liquid catalytic phase was then recycled. Firstly, the ethanol was evaporated under reduced pressure and the excess of 2-isopropoxystryrene was eliminated from the catalytic IL phase with 5 mL of dry pentane. After evaporation under vacuum for 15 minutes, a new charge of DAB was added to the catalytic IL phase and the solution was stirred for 40 minutes. Finally, a new portion of NR was added to proceed to a new depolymerization step, following the previously described procedure.

Supporting Information 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

Acknowledgements 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. References [1] Anastas, P.T.; Warner, J.C.; Green Chemistry: Theory and Practice; Oxford University Press: London, 1998. [2] Nor H.M. and Ebdon J.R. Ozonolysis of natural rubber in chloroform solution – Part 1. A study by GPC and FTIR spectroscopy. Polymer, 2000, 41, 2359-2365. [3] Rose K. and Steinbüchel A. Biodegradation of Natural Rubber and Related Compounds: Recent Insights into a Hardly Understood Catabolic Capability of Microorganisms. Appl. Environ. Microbiol., 2005, 71, 2803-2812. [4] Ravindran T., Gopinathan Nayar M.R. and Francis D.J. Production of hydroxyl terminated liquid natural rubber—mechanism of photochemical depolymerization and hydroxylation. J. Appl. Polym. Sci., 1988, 35, 1227-1239. [5] Magalhães A.S.G. and Feitosa J.P.A. Ultrasonic degradation of natural rubber in toluene: GPC study. Polímeros, 1999, 9, 65-70.


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Telechelic polyisoprene Ionic liquid


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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

A metathetic degradation process in IL of waste tires affords in one step and with high yield telechelic polyisoprene oligomers.


Controlled Metathetic Depolymerization of Natural ... - ACS Publications

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