Room-temperature polymerization of β-pinene by niobium and tantalum halides
Graphical abstract
Highlights
► Niobium halides are effective catalysts for cationic β-pinene polymerization. ► Good performances can be obtained at room temperature. ► Tantalum derivatives are less efficient than niobium ones.
Introduction
β-Pinene is the most important distillation product of natural turpentine [1]. Its commercially available polymer (terpene resin) is used in a wide variety of industrial applications (e.g. adhesives and varnishes, bituminous materials, tackifying agents, additives in rubber) [2], [3], due to a combination of desired and outstanding properties: high solubility in hydrocarbon solvents, compatibility with waxes and relatively high softening point (125–135 °C). Moreover this polymer is nontoxic and inert to common chemicals, that makes it suitable for the food packaging and the production of chewing gums [4]. Furthermore, due to the limited fossil resources, terpene resins, derived from renewable resources, have aroused increasing interest in both academic and industrial context as outstanding biomaterials.
A variety of polymerization techniques have been employed for the polymerization of β-pinene in recent years, i.e. the Ziegler–Natta type, the free-radical, the cationic and the radiation-induced cationic polymerization [4], [5]. Lewis acid-promoted cationic polymerization represents the most efficient method for the commercial production of poly(β-pinene). The presence of two alkyl groups on the double bond determines the rapid cationic polymerization of β-pinene with conventional or even living fashion [6]. The resulting polymers have alternating sequences of isobutylene and cyclohexene units (Fig. 1).
Different Lewis acids (AlCl3, SnCl4, ZnCl2, ZrCl4, BF3, etc.) have been used for the polymerization of β-pinene [7], [8], [9], [10], [11], [12], [13], [14] and the main results reported in the literature have been summarized in Table 1.
The catalytic conversion of β-pinene to polymer was studied with a number of Friedel–Crafts catalysts (Runs 4–13, Table 1) at fixed conditions. The results demonstrate that β-pinene polymerizes very fast and in a vigorous way after less than 1% of aluminum chloride is added; otherwise the conversion degree was much lower when SnCl3 and ZnCl2 were employed as catalysts. Very recently the efficient polymerization of β-pinene has been reported in the presence of AlCl3 etherates: relatively high molecular weight polymers (Mn: 9000–14,000 g/mol) were obtained working at room temperature with a catalyst content 2.5–5.5 mM (Runs 14–15, Table 1) [10]. As far as aluminium alkyls are concerned, it has been observed that lower temperatures favour the global yield of the polymerization process using ethylaluminium dichloride [11], while an increase in the temperature of polymerization gave rise to resins of lower molecular weight, lower softening point, and darker color.
More recently, Zhu et al. [15] have stated that Keggin-type heteropolyacids (HPAs) are environmental friendly, have high Brønsted acidity and are efficient catalyst in the cationic polymerization of β-pinene.
Niobium and tantalum pentahalides are strong Lewis acids, easily available and biocompatible metal-based compounds [16] which are attaining increasing popularity as chemical reagents. They have been efficiently employed as catalysts for the living cationic polymerization of isobutyl vinyl ether in the presence of ammonium salts [17].
In this contribution, we report a study on the polymerization reaction of β-pinene mediated by MX5 (M = Nb or Ta; X = halide) [18] or their derivatives. We have found that NbCl5 is an efficient catalyst leading to smooth and quantitative polymerization at room temperature. The effects of the reaction conditions, including the possible use of alkylaluminum co-catalysts, on the NbCl5-mediated polymerization will be discussed.
Section snippets
General
All manipulations of air and/or moisture sensitive compounds were performed under atmosphere of pre-purified Argon using standard Schlenk techniques. The reaction vessels were oven dried at 150 °C prior to use, evacuated (10−2 mmHg) and then filled with argon. NbX5 (X = F, 1a; X = Cl, 1b; X = I, 1d) and TaX5 (X = F, 2a; X = Cl, 2b) were commercial products (Strem) of the highest purity available, stored under argon atmosphere as received. NbBr5 (1c)[19] NbF4(OPh) [20], M(O2CNEt2)5 (M = Nb, Ta) [21], and NbX5
Results and discussion
The results of β-pinene polymerization catalyzed by NbCl5 are reported in Table 2. The adopted reaction conditions, such as very low catalyst concentration (0.6 mM), high monomer concentration (1.2 M), room temperature and toluene as solvent, were selected to be consistent with the industrial requirements. Preliminary experiments were carried out in the presence or in the absence of visible light (Runs 19 and 20, respectively), in order to exclude the possibility of light-induced radical
Conclusion
The polymerization of β-pinene has been conveniently carried out at room temperature by employing NbCl5 as catalyst at low very concentration, and the main reaction parameters (temperature, time, catalyst concentration, solvent) have been optimized. Other Nb(V) halides and TaF5 are also effective as catalysts, although providing lower performances than that of NbCl5. The presence of oxygen ligands in the coordination sphere of the metal centres appears to be detrimental to the polymer
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