Reaction Kinetics, Mechanisms and Catalysis

, Volume 99, Issue 2, pp 463–470

β-Pinene cationic polymerization using Keggin heteropolyacid catalysts

Authors

  • Hualong Zhu
    • College of Chemistry and Ecological EngineeringGuangxi University for Nationalities
    • College of Chemistry and Ecological EngineeringGuangxi University for Nationalities
    • College of Chemical EngineeringNanjing Forestry University
  • Taishun Zhang
    • College of Chemistry and Ecological EngineeringGuangxi University for Nationalities
  • Wei Zeng
    • College of Chemistry and Ecological EngineeringGuangxi University for Nationalities
  • Xinnan An
    • College of Chemical EngineeringNanjing Forestry University
  • Fuhou Lei
    • College of Chemistry and Ecological EngineeringGuangxi University for Nationalities
Article

DOI: 10.1007/s11144-009-0144-8

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

Abstract

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.

Keywords

β-PineneHeteropolyacidCationic polymerization

Introduction

β-pinene (https://static-content.springer.com/image/art%3A10.1007%2Fs11144-009-0144-8/MediaObjects/11144_2009_144_Figa_HTML.gif , 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 [710], 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 [1319]. Burrington et al. [20] have published on the use of a heteropolyacid salt as catalyst for the cationic polymerization of isobutylene. Dianyu Chen et al. [21] 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.

Experimental

Catalyst preparation

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.

Catalysis test

The polymerization reactions were carried out in a three-necked flask under N2 atmosphere with mechanical stirring. The flask was cooled to the required temperature and then 5 mL PI and 5 mL solvent were added. HPAs (5% with respect to PI) were outgassed with N2 for 15 min prior to the addition of PI. After 9 h of reaction, the contents of the flask were poured into 20.0 mL of cold methanol. The precipitated polymer was washed by ethanol, and then dried under vacuum at 60 °C. Overall PI conversions were determined by GC 14-B equipped with a FID detector and a capillary column (DB-1). The PI conversion and polymer yield were calculated according to the following equations:
$$ \begin{gathered} {\text{PI conversion}} = \left( {{\text{m}}_{\text{in}} - {\text{m}}_{\text{out}} } \right)/{\text{m}}_{\text{in}} \hfill \\ {\text{Polymer selectivity}} = {\text{m}}_{\text{polymer}} /\left( {{\text{m}}_{\text{in}} - {\text{m}}_{\text{out}} } \right) \hfill \\ \end{gathered} $$
where min and mout are the amounts (weights) of PI fed in and remaining in the solution after reaction, and mpolymer is the amount (weight) of the product after drying under vacuum for 24 h.

Catalyst recovery

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.

Characterization techniques

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

Table 1 displays the catalytic performance of different HPA catalysts. It was observed that among the catalysts tested, PW12 dehydrated at 200 °C showed the best catalytic performance. The PI conversion was up to 96.53%, and the polymer selectivity was 63.04%. At the same time, Mn values of all of the polymer products were not very large, polydispersities were narrow, that is, HPA catalysts had the function to stabilize the active center. As PW12 is the strongest acid in the Keggin structure, it can be inferred that the acid property of the HPA catalysts played a very important role in the polymerization reaction. PI conversions and polymer selectivities increased with increasing acid strength of the catalyst. As PW12 was the most efficient catalyst, we used PW12 in the following discussion.
Table 1

Catalytic activities of different HPAs on PI polymerization

Catalyst

PI conversion (%)

Polymer selectivity (%)

Polymer yield (%)

Mn

Da

PW12

96.53

63.04

60.85

815

1.29

SiW12

87.64

32.80

28.75

741

1.24

PMo12

64.25

15.97

10.26

682

1.30

Polymerization carried out at −10 °C with 5 mL PI and 5 mL (CH2Cl)2

[HPAs] = 5% (pretreated at 200 °C). Reaction time: 22 h

aPolydispersity of the polymer

IR characterization

From the IR spectrum of the catalyst of PW12 (Fig. 1), four characteristic IR absorption peaks in the range from 700 to 1,100 cm−1, corresponding to the stretching vibrations of P–O, W=O and W–Oc/e–W bonds of the Keggin units were observed [22].
https://static-content.springer.com/image/art%3A10.1007%2Fs11144-009-0144-8/MediaObjects/11144_2009_144_Fig1_HTML.gif
Fig. 1

The IR spectrum of PW12 (a) undehydrated; (b) before reaction; (c) after reaction

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.

XRD characterization

XRD patterns of PW12 are displayed in Fig. 2. The XRD pattern of solid PW12 has intense diffraction peaks in the range 7–11, 14–26, 27–35 and 36–40°, which are in line with the pattern of the Keggin structure. Fig. 2 shows that the catalyst, both before and after the reaction, had the same X-ray diffraction intensity changes within the theta of 10–50°. Therefore, the crystal structure of the heteropoly anion was not destroyed during reaction, which was also proved by IR spectrums studied above.
https://static-content.springer.com/image/art%3A10.1007%2Fs11144-009-0144-8/MediaObjects/11144_2009_144_Fig2_HTML.gif
Fig. 2

The XRD patterns of PW12 (a) undehydrated; (b) before reaction; (c) after reaction

1H-NMR characterization of the PW12 catalyst

As discussed before [23, 24], the bulk proton sites in crystalline hexahydrate PW12 are represented as diaquahydrogen ions (H5O2+). In the dehydrated PW12, the protons are predominantly localized on the terminal oxygens (W=O–H+–O=W). 1H-NMR (in Fig. 3) showed that the chemical shift of the proton of undehydrated PW12 was 4.50 ppm, which was attributed to the chemical structure of H5O2+. After dehydration, the proton shifted quickly to terminal oxygens (peak intensity decreased at 5.86 ppm). But after reaction, the bands all disappeared, in other words, the protons all took part in the polymerization, which could also be seen in IR spectrums. It was indicated that the protons in the catalyst PW12 were active, and played an important role in the polymerization procession.
https://static-content.springer.com/image/art%3A10.1007%2Fs11144-009-0144-8/MediaObjects/11144_2009_144_Fig3_HTML.gif
Fig. 3

1H-NMR spectroscopy of PW12 (a) undehydrated; (b) before reaction; (c) after reaction

1H-NMR characterization of the polymer

Fig. 4. shows the 1H-NMR spectrum of a typical sample of poly (β-pinene) obtained with PW12. The polymer gave the characteristic absorptions of the endo-olefinic protons (a) of the ring-opening isomerization of the β-pinene unit at 5.5–6.0 ppm [25, 26].
https://static-content.springer.com/image/art%3A10.1007%2Fs11144-009-0144-8/MediaObjects/11144_2009_144_Fig4_HTML.gif
Fig. 4

1H-NMR spectroscopy of the polymer

XPS characterization

The chemical species present on the catalyst surfaces before and after the reaction were evaluated by XPS. From the results in Table 2, two peaks for the original W appeared. After polymerization, no new peaks arose, which indicated that W had the same element oxidation states before and after polymerization. Part of O 1s at 531.15 and 532.53 eV was shifted up to 532.23 and 534.13 eV, and part of C 1s at 285.99 eV was shifted down to 285.15 eV, which could indicate that the electron was deviated from C and led to O, and the weak force of the cationic active center might be formed between the atoms of C and O.
Table 2

The XPS results of the catalyst of PW12

Atom

Before reaction

After reaction

W 4f

36.02

38.16

36.97

39.13

O 1s

531.15

532.53

532.23

534.13

C 1s

284.62

285.99a

284.60

285.15

aRefers to C 1s reference energy. The unit: eV

Possible reaction scheme

As discussed above, the possible polymerization scheme would be described as Fig. 5. In step 1, the dissociated protons were combined with the monomer to obtain a carbonium ion. Step 2 was the initiating step in β-pinene polymerization. The cationic active center was formed in this step by ring opening isomerization. Step 3 was the chain-propagating step, and in this step, poly(β-pinene) was produced. In this procession, PW12 played two important roles: polymerization initiator, and the counter-anion of the growing cation center.
https://static-content.springer.com/image/art%3A10.1007%2Fs11144-009-0144-8/MediaObjects/11144_2009_144_Fig5_HTML.gif
Fig. 5

Reaction scheme of β-pinene polymerization

Conclusions

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.

Acknowledgments

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).

Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2010