β-Pinene cationic polymerization using Keggin heteropolyacid catalysts
- First Online:
- Cite this article as:
- Zhu, H., Liu, Z., Zhang, T. et al. Reac Kinet Mech Cat (2010) 99: 463. doi:10.1007/s11144-009-0144-8
- 95 Views
New and efficient Keggin heteropolyacid (HPA) catalysts were explored for β-pinene (PI) cationic polymerization. Among them, 12-phosphotungstic acid (PW12) dehydrated at 200 °C exhibited high catalytic activity. The overall PI conversion was up to 96.53%, and the obtained polymer product yield was 60.85%. In order to study this new catalyzed reaction, special techniques of FT-IR, 1H-NMR, XRD, and XPS were used in this paper, and it was shown that the crystal structure of the heteropoly anion was not destroyed during the reaction. The protons dissociating from the catalyst played an important role in the polymerization and the HPAs had two important functions: polymerization initiator, and the counter-anion of the growing cation center.
β-pinene ( , PI), a bicyclic aliphatic monomer abundantly found in natural turpentine, readily undergoes cationic polymerization via both conventional [1, 2] and living processes [3, 4] to give polymers. The polymers are called terpene resins, have a molecular weight probably around 1,000, and can be used in many industrial applications, particularly as pressure-sensitive adhesives, hot-melt coatings, tackifying agents, and additives in rubber [5, 6]. PI polymerization can be initiated with Ziegler-Natta, free-radical, and cationic catalysts as well as by high-energy radiation [7–10], among which halide Lewis acid catalysts such as AlCl3, BF3, TiCl4 are the most effective and therefore commonly used in the commercial production. But these halide catalysts have severe problems of corrosive effects and environmental hazards. In addition, the separation of the Lewis acids from the reaction products produces a large volume of acidic waste. Therefore, the investigation of new, more environmental friendly catalytic systems for the polymerization of PI attracts special industrial interest.
Heteropolyacids (HPAs) are polyoxometalates incorporating anions with metal oxygen octahedra as the basic structural unit. Using HPAs (salts) as catalysts have been paid more and more attention in organic synthesis and in the petrochemical industry [11, 12]. HPAs have many advantages such as strong acidity, high catalytic activity, high thermal and oxidative stability, commercial availability, non-corrosive and non-polluting nature. HPAs as acidic catalysts, at present, are those with Keggin structure, having the general formula Xn+M12O40(8 − n)−, where X is the central atom (Si, P, etc.) and M represents the coordinate atoms (W, Mo, etc.).
In recent years, Keggin HPAs had also been introduced into the polymerization of oxygen heterocycle compounds such as tetrahydrofuran. In these reactions, the aimed catalysts acted as polymerization initiators, and the reaction mechanism was accepted commonly as cationic polymerization [13–19]. Burrington et al.  have published on the use of a heteropolyacid salt as catalyst for the cationic polymerization of isobutylene. Dianyu Chen et al.  have used H3PMo12O40 (PMo12) to carry out controlled polymerization of styrene, thus obtaining a high molecular weight product. All the above showed the HPAs have excellent catalytic activities in polymerization. But to our knowledge, they have not been previously reported to catalyze PI polymerization. Thus, introduction of HPAs into the catalytic system of PI cationic polymerization is very important. Here we present the results of a study on PI cationic polymerization by the Keggin HPAs, whereby the efficiency of this catalyst was studied.
Commercially available H3PW12O40 (PW12), H4SiW12O40 (SiW12), H3PMo12O40 (PMo12) were purchased from Shanghai Chem. Reagent Co. (China). All solvents and reagents were used with further purification. All catalysts were dehydrated at 200 °C for 2 h before use.
Pretreatment of β-pinene
Before polymerization, the monomer of PI, provided by Wuzhou Richeng Forest Chemicals Co., was pretreated as follows: dried in the presence of anhydrous calcium chloride for 24 h, and then distilled under reduced pressure over CaH2 with temperature of 110 °C.
After the completion of the reaction, 20 mL toluene was added into the three-necked flask, then the system was left to settle statically for 24 h. The precipitated catalyst was washed by toluene three times, and then dried under vacuum at 60 °C.
The IR spectra of the catalysts were recorded on a BRUKER VECTOR33 spectrophotometer in the wavenumber range from 4,000 to 500 cm−1 using KBr pellet technique.
Powder X-ray diffraction (XRD) patterns of the catalyst samples were recorded on PGENERAL XD-3 X-ray diffractometer. Copper Kα radiation was used with a power setting of 40 kV and 34 mA, the scanning range was 10–50° with a scan rate of 4°/min.
The XPS spectra were recorded on an Axis Ultra DLD spectrometer, using the Mg Kα X-ray source (1,486.6 eV). The reference energy was the C 1s signal at 284.6 eV.
1H-NMR spectra were recorded in Dimethyl Sulfoxide-d6 (catalyst) and CDCl3 (polymer) at room temperature on a Varian INOVA spectrometer.
The molecular weights and polydispersities were measured with a Waters-Breeze gel permeation chromatograph at 35 °C in THF on three polystyrene gel columns (Waters Styragel HR1, HR3, and HR4) connected with an RI-2414 refractive-index detector with polystyrene as a standard.
Results and discussion
Catalytic efficiency of different HPAs
Catalytic activities of different HPAs on PI polymerization
PI conversion (%)
Polymer selectivity (%)
Polymer yield (%)
Compared with the band changes discussed above for Fig. 1, although there were nearly no H–O absorption peaks after the reaction, the corresponding catalyst bands still appeared between 700 and 1,100 cm−1. It could be concluded that the protons took part in the polymerization and the crystal structure of PW12 was not destroyed during polymerization.
1H-NMR characterization of the PW12 catalyst
1H-NMR characterization of the polymer
The XPS results of the catalyst of PW12
Possible reaction scheme
HPAs were efficient, environmental friendly catalyst for PI cationic polymerization. PW12 dehydrated at 200 °C showed the best catalytic performance. The protons dissociating from the catalyst played an important role in polymerization. In this reaction procession, the aimed catalyst played two important roles: polymerization initiator, and the counter-anion of the growing cationic center.
This work was supported by China Postdoctoral Science Foundation funded project (20070421021); Open Fund of Key Laboratory of Development and Application of Forest Chemicals of Guangxi, China (GXFC08-12) and the Innovation Project of Guangxi University for Nationalities (gxun-chx2009078).