Ring Opening Polymerization of Tetrahydrofuran Catalysed by Maghnite-H+

Chinese Journal of Polymer Science Vol. 30, No. 1, (2012), 5662 Chinese Journal of Polymer Science © Chinese Chemical Society Institute of Chemistry, CAS Springer-Verlag Berlin Heidelberg 2012 RING OPENING POLYMERIZATION OF TETRAHYDROFURAN CATALYSED BY MAGHNITE-H+ Khadidja Benkenfoud, Amine Harrane* and Mohammed Belbachir Laboratoire de Chimie des Polymères, Département de Chimie, Faculté des Sciences, Université d'Oran Es-Senia, BP No 1524 El M'Naouar, 31000 Oran, Algeria Abstract The cationic ring-opening polymerization of tetrahydrofuran using maghnite-H+ is reported. Maghnite-H+, is a non-toxic solid catalyst issued from proton exchanged montmorillonite clay. Polytetrahydrofuran, also called “poly(butandiol) ether”, with acetate and hydroxyl end groups was successfully synthesized. Effects of reaction temperature, weight ratio of initiator/monomer and reaction time on the conversion of monomer and on the molecular weight are investigated. A cationic mechanism of the reaction was proposed. This chemistry can be considered as a suitable route for preparing poly(THF) as a soft segment for thermoplastic elastomers. Keywords: Montmorillonite; Maghnite; Ring opening polymerization; THF. INTRODUCTION Significant advances have been made to prepare block and graft copolymers with known structures. This is often achieved by preparing a telechelic polymer with a functional end group capable of acting as an initiator or monomer and thus producing active sites for second block[1]. Polytetrahydrofuran (poly(THF)), with hydroxyl end groups, well known as poly(tetramethylene glycol), is a very important soft segment for producing thermoplastic elastomers such as polyester (Hytrel®) and polyurethane (Spandex®)[2]. THF reacts with cationic initiators to give poly(THF). Such reaction has been the subject of a number of papers[25] since Meerwein reported trialkyloxonium salt as initiator[6]. The polymerization for their production is initiated by electrophilic agents, such as, Brønsted acids (HCl, H2SO4, HClO4, etc.) and Lewis acids (AlCl3, BF3·OEt2, TiCl4, etc.). However, the protonic acid catalysts used are very noxious and corrosive. As for Lewis acids, it is known that their use requires large amounts to achieve acceptable yields of polymers. Increasing environmental concerns in recent years have resulted in a demand for more effective catalytic processes. In this regard, studies have been carried out on the development of solid acids, such as “Keggin-type Heteropolycompounds”[7] or acidic clays[8, 9], to replace aggressive and dangerous homogeneous acids to overcome the problems of separating the catalyst from the products and the disposal of solid/liquid wastes. In recent years, our group has explored new modified natural clay catalysts or initiators for polymerization of vinylic and hetero-cyclic monomers[1016]. In this article we report the ring-opening polymerization of tetrahydrofuran using a proton-exchanged montmorillonite clay, called maghnite-H+, as a non-toxic catalyst, in the presence of acetic anhydride to produce polytetrahydrofuran with acetate end groups. The saponification of this product with NaOH leads to polytetrahydrofuran with hydroxyl end groups. Techniques such as 1H-NMR, GPC and viscosimetry, were used to characterize the products of the * Corresponding author: Amine Harrane, E-mail: [email protected] Received January 21, 2011; Revised March 18, 2011; Accepted March 28, 2011 doi: 10.1007/s10118-012-1094-6 + Ring Opening Polymerization of Tetrahydrofuran Catalysed by Maghnite-H 57 reaction. The effects of the proportions of acetic anhydride and catalyst and reaction temperature and time on monomer conversion and on the average molecular weight of the resulted polymer were also examined. EXPERIMENTAL Materials Butanone (98%), acetic anhydride (98%) and ethanol (95%) were used as received. Tetrahydrofuran (THF 99% grade Aldrich Chemical) was distilled over CaH2 under argon atmosphere before use. Methanol was dried over magnesium sulphate MgSO4 and distilled. Dichloromethane was dried over CaH2 and distilled on the day of experiment. Raw-maghnite: Algerian montmorillonite clay was procured from “BENTAL” (Algerian Society of Bentonite). Preparation of Maghnite-H+ Maghnite-H+ was prepared according to the process reported in our previous study[10, 11]. Raw-maghnite (20 g) was crushed for 20 min using a prolabo ceramic balls grinder. It was then dried for 2 h at 105°C. The maghnite was placed in an Erlenmeyer flask together with 500 mL of distilled water. The maghnite/water mixture was stirred using a magnetic stirrer and combined with 0.25 mol/L sulfuric acid solution, until saturation was achieved over 2 days at room temperature, the mineral was then washed with distilled water to became sulfate free and then dried at 105°C. Kinetics Procedure The ring opening bulk polymerization of THF was carried out in sealed tubes. Each tube contains a mixture of 1 g of THF, acetic anhydride and an amount of maghnite-H+. The mixtures were kept in thermostat at 30°C and stirred with a magnetic stirrer under dry nitrogen. The reaction was terminated by methanol. The resulting polymer was extracted with dichloromethane, precipitated in methanol, washed for several times, dried at 40°C in vacuum and weighed. The monomer conversion was determined by weighing the precipitated polyether chains. Saponification of Di-acetate End Groups of Poly(THF) Poly(THF) with hydroxyl end groups was obtained by refluxing 2 g of the resulted polyether with a mixture of saturated aqueous NaOH solution (20 mL) and ethanol (20 mL) for 30 min. The resulting polymer was extracted with butanone, precipitated into methanol, washed for several times, dried at 40°C in vacuum and weighed. Polymer Characterization 1 Measurements of H-NMR spectra were conducted in CDCl3 solution under ambient temperature on an AM 300 FT Bruker spectrometer using tetramethylsilane (TMS) as internal standard. Viscosity measurements were carried out with an Ubbelohde capillary viscosimeter (viscologic TI1, version 3-1 Semantec). Intrinsic viscosity, [η] (mL/g), was measured at 25°C in toluene solution. Gel-permeation chromatography (GPC) was performed with a Spectra-Physics chromatograph, equipped with four columns connected in series and packed with Ultrastyragel 102, 103, 104, 105 nm. THF was used as solvent and the instrument was calibrated to a first approximation with polystyrene of known molecular weights. RESULTS AND DISCUSSION The use of acid treated clays as a solid source of protons in many industrial significant reactions continues because they constitute a widely available, inexpensive solid source of protons, e.g. they were employed as cracking catalysts until the 1960s[17], and are still used actually in industrial processes such as the alkylation of phenols and the dimerization and polymerization of unsaturated hydrocarbons[18]. The present study is also concerned with polymerization and examines the catalytic activity of Algerian proton exchanged montmorillonite clay for THF cationic ring opening polymerization. The structure and the composition of the catalyst were reported in previous works[10, 11]. It was demonstrated that there is an excellent correlation between the acid treatment and the catalytic activity of maghnite[11, 16]. It was reported that best 58 K. Benkenfoud et al. values of monomer conversions were obtained with maghnite-H+ which has been produced by treatment of raw- maghnite with 0.25 mol/L sulfuric acid solution. This treatment leads to a complete saturation of montmorillonite with protons without destruction of the catalyst structure[19, 20]. Montmorillonites have both Brönsted and Lewis acid sites and when exchanged with cations having a high charge density, as protons, produce highly active catalysts for acid-catalyzed reactions[18]. Intercalated organic molecules are mobile and can be highly polarized when situated in the space between the charged clay layers. These exchanged montmorillonites have been successfully used as catalysts for the reactions of polymerization[21]. Telechelic Poly(THF) Synthesis The cationic polymerization of THF was examined in the presence of maghnite-H+ powder and acetic anhydride in bulk at 30°C (Scheme 1). Scheme 1 Polymerization of THF using maghnite-H+ 1H-NMR measurements (Fig. 1) confirm the structure of poly(THF) that resulted from the reaction of polymerization (Scheme 1). As shown in 1H-NMR spectrum of the product, the signal assigned to the protons of methylene bonded to oxygen poly(THF) appeared at = 3.49. The signal due to other methylene protons was observed at = 1.57. Besides these signals, characteristic of the polymer chains, overlapping resonances assignable to the methyl ester end group was observed ( = 2.2, s). The signal at = 4.2 was attributed to methylene end groups (CH2―OCOCH3). Fig. 1 1H-NMR spectrum of poly(THF) with di-acetate end-groups The saponification of the resulted di-ester poly(THF) leads to the telechelic dihydroxyl poly(THF) (Scheme 2). The proof of this reaction is reported in Fig. 2. As reported in Fig. 2, 1H-NMR measurements of the resulted product show that the signal of methyl-ester end group at = 2.2 (Fig. 1) disappeared as a result of its consumption during the saponification. + Ring Opening Polymerization of Tetrahydrofuran Catalysed by Maghnite-H 59 Scheme 2 Saponification of di-acetate end-groups of poly(THF) Fig. 2 1H-NMR spectrum of poly(THF) with di-hydroxyl end-groups Effect of Temperature Figures 3 and 4 show the experimental results for THF polymerization

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