Editorial Board

Supported by NSFC

Honorary Editor General ZHOU GuangZhao (Zhou Guang Zhao)

Editor General ZHU ZuoYan Institute of Hydrobiology, CAS, China

Editor-in-Chief LI LeMin Peking University, China

Associate Editor-in-Chief CAO Yong South China University of Technology, China TIAN ZhongQun Xiamen University, China CHEN HongYuan Nanjing University, China XUE Zi-Ling University of Tennessee, USA FENG ShouHua Jilin University, China YUAN Quan Dalian Institute of Chemical Physics, CAS, China LIN GuoQiang Shanghai Institute of Organic Chemistry, CAS, China

Members

BAO XinHe Dalian Institute of Chemical Physics, CAS, China BU XianHe Nankai University, China CHAI ZhiFang Institute of High Energy Physics, CAS, China CHAN Albert S C Hong Kong Polytechnic University, China CHEN Xian University of North Carolina-Chapel Hill, USA CHEN XiaoMing Sun Yat-Sen University, China CHEN Yi Institute of Chemistry, CAS, China CUI ZhanFeng Oxford University, UK DUAN Xue Beijing University of Chemical Technology, China

FEI WeiYang Tsinghua University, China FENG XiaoMing Sichuan University, China GAO ChangYou Zhejiang Universtiy, China GAO Song Peking University, China GUO ZiJian Nanjing University, China

HAN BuXing Institute of Chemistry, CAS, China

HE MingYuan Research Institute of Petroleum Processing, SINOPEC, China HONG MaoChun Fujian Institute of Research on the Structure of Matter, CAS, China HUANG PeiQiang Xiamen University, China HUANG Zhen Georgia State University，USA

JIANG GuiBin Research Center for Eco-Environmental Sciences, CAS, China

JIANG Long Institute of Chemistry, CAS, China

JIAO Kui Qingdao University of Science and Technology, China JU HuangXian Nanjing University, China KONG Wei Oregon State University, USA LI QianShu South China National University, China LI YaDong Tsinghua University, China

LIAN TianQuan Emory University, USA

LIANG WenPing National Natural Science Foundation of China, China LIN JianHua Peking University, China LIU GuoJun Queen’s University, Canada LIU Jun O Johns Hopkins Medicine Institute, USA LU FengCai Institute of Chemistry, CAS, China NIE ShuMing Georgia Institute of Technology and Emory University, USA PAN CaiYuan University of Science and Technology of China, China PU Lin University of Virginia, USA QIAO JinLiang SINOPEC Beijing Research Institute of Chemical Industry, China SHAO YuanHua Peking University, China SHEN ZhiQuan Zhejiang University, China SHUAI ZhiGang Tsinghua University, China SUN LiCheng Royal Institute of Technology (KTH), Sweden TANG Ben Zhong Hong Kong University of Science & Technology, China TIAN He East China University of Science & Technology, China TONG Liang Columbia University, USA TUNG ChenHo Technical Institute of Physics and Chemistry, CAS, China WAN LiJun Institute of Chemistry, CAS, China WANG MeiXiang Institute of Chemistry, CAS, China WANG ShiQing University of Akron, USA WANG ZhenGang California Institute of Technology, USA WANG ZhongLin Georgia Institute of Technology, USA WU YunDong Hong Kong University of Scence & Technology, China XIE ZuoWei Chinese University of Hong Kong, China

XIONG RenGen South East University, China XU ChunMing China University of Petroleum, China YAM Vivian Wing-Wah University of Hong Kong, China YAN DeYue Shanghai Jiao Tong University, China YANG Bai Jilin University, China YANG DongSheng University of Kentucky, USA YANG PengYuan Fudan University, China YANG WeiTao Duke University, USA YANG XueMing Dalian Institute of Chemical Physics, CAS, China YANG YuLiang Fudan University, China YAO ShouZhuo Hunan University, China YAO ZhuJun Shanghai Institute of Organic Chemistry, CAS, China YOU XiaoZeng Nanjing University, China YU LuPing University of Chicago, USA ZHANG HongJie Changchun Institute of Applied Chemistry, CAS, China ZHANG JinSong University of California, Riverside, USA ZHANG JinZhong University of California, Santa Cruz, USA ZHANG John ZengHui New York University, USA ZHANG LiHe Peking University, China ZHANG Xi Tsinghua University, China ZHANG YuKui Dalian Institute of Chemical Physics, CAS, China ZHAO XinSheng Peking University, China ZHAO YuFen Xiamen University, China ZHENG LanSun Xiamen University, China ZHOU QiLin Nankai University, China ZHU Tong Peking University, China ZHU Julian X Université de Montréal, Canada

Editorial Staff ZHU XiaoWen (Director) SONG GuanQun ZHANG XueMei

SCIENCE CHINA Chemistry

© Science China Press and Springer-Verlag Berlin Heidelberg 2012 chem.scichina.com www.springerlink.com

Contents Vol.55 No.8 August 2012

COVER Zinc has been widely used as anti-corrosive coatings in the electroplating industry. However, the conventional processes for preparing zinc coatings suffer from a series of inherent dis-advantages, such as hydrogen embrittlement, pollutant release and high energy consumption. There-fore, it’s necessary to develop high-efficiency and green electrolytes for the electrodeposition of zinc. In this work, it’s first found that the solubilities of ZnO in imidazolium chloride-based ionic liquids are remarkably enhanced by the addition of urea. Shining, dense and well adherent zinc coatings with good purity were electrodeposited from 0.6 M solution of ZnO in 1:1 [Amim]Cl/urea at 323.2−343.2 K. Unlike conventional methods, the deposition process is environmentally friendly, low energy consumption and easy to handle at mild conditions. It’s expected that the solutions of ZnO in imidaz-olium chloride/urea have the potential to replace the traditional electrolytes for low-temperature zinc electroplating and synthesis of functional zinc materials (see the article by ZHENG Yong, DONG Kun, WANG Qian, ZHANG SuoJiang, ZHANG QinQin, LU XingMei on page 15871597).

SPECIAL ISSUE: Ionic Liquid and Green Chemistry Preface: An International Look at Ionic Liquids

ROGERS Robin D., ZHANG SuoJiang & WANG JianJi Sci China Chem, 2012, 55(8): 1475–1477

REVIEWS

The preparation of supported ionic liquids (SILs) and their application in rare metals separation

ZHU LiLi, GUO Lin, ZHANG ZhenJiang, CHEN Ji & ZHANG ShaoMin

Sci China Chem, 2012, 55(8): 1479–1487

Cyano-containing ionic liquids for the extraction of aromatic hydrocarbons from an aromatic/aliphatic mixture

MEINDERSMA G. Wytze & DE HAAN André B.

Sci China Chem, 2012, 55(8): 1488–1499

ii

Ionic liquids: Efficient solvent and medium for the transformation of renewable lignocellulose

LONG JinXing, LI XueHui, WANG LeFu & ZHANG Ning

Sci China Chem, 2012, 55(8): 1500–1508

ARTICLES

The physicochemical properties of some imidazolium-based ionic liquids and their binary mixtures

NING Hui, HOU MinQiang, MEI QingQing, LIU YuanHui, YANG DeZhong & HAN BuXing

Sci China Chem, 2012, 55(8): 1509–1518

Liquid-liquid interfacial tension of equilibrated mixtures of ionic liquids and hydrocarbons

RODRÍGUEZ Héctor, ARCE Alberto & SOTO Ana

Sci China Chem, 2012, 55(8): 1519–1524

Structure-property relationships in ILs: A study of the alkyl chain length dependence in vaporisation enthalpies of pyridinium based ionic liquids

ZAITSAU Dzmitry H., YERMALAYEU Andrei V., EMEL’YANENKO Vladimir N., VEREVKIN Sergey P., WELZ-BIERMANN Urs & SCHUBERT Thomas

Sci China Chem, 2012, 55(8): 1525–1531

iii

Sweet ionic liquids-cyclamates: Synthesis, properties, and application as feeding deterrents

PERNAK Juliusz, WASIŃSKI Krzysztof, PRACZYK Tadeusz, NAWROT Jan, CIENIECKA-ROSŁONKIEWICZ Anna, WALKIEWICZ Filip & MATERNA Katarzyna

Sci China Chem, 2012, 55(8): 1532–1541

Chlorogallate(III) ionic liquids: Synthesis, acidity determination and their catalytic performances for isobutane alkylation

XING XueQi, ZHAO GuoYing & CUI JianZhong

Sci China Chem, 2012, 55(8): 1542–1547

Computational studies of the structure and cation-anion interactions in 1-ethyl-3-methylimidazolium lactate ionic liquid

HE HongYan, ZHENG YanZhen, CHEN Hui, ZHANG XiaoChun, YAO XiaoQian & ZHANG SuoJiang

Sci China Chem, 2012, 55(8): 1548–1556

Understanding the interactions between tris(pentafluoroethyl)-trifluorophosphate-based ionic liquid and small molecules from molecular dynamics simulation

ZHANG XiaoChun, LIU ZhiPing & LIU XiaoMin

Sci China Chem, 2012, 55(8): 1557–1565

iv

CO2 capture through halogen bonding: A theoretical perspective

LI HaiYing, LU YunXiang, ZHU Xiang, PENG ChangJun, HU Jun, LIU HongLai & HU Ying

Sci China Chem, 2012, 55(8): 1566–1572

All-atom and united-atom simulations of guanidinium-based ionic liquids

LIU XiaoMin, ZHANG XiaoChun, ZHOU GuoHui, YAO XiaoQian & ZHANG SuoJiang

Sci China Chem, 2012, 55(8): 1573–1579

Ionic liquid assisted synthesis of flowerlike Cu2O micro-nanocrystals

ZHAO Yang, GUO LiPing, SUN Xin & WANG JianJi

Sci China Chem, 2012, 55(8): 1580–1586

v

Electrodeposition of zinc coatings from the solutions of zinc oxide in imidazolium chloride/urea mixtures

ZHENG Yong, DONG Kun, WANG Qian, ZHANG SuoJiang, ZHANG QinQin & LU XingMei

Sci China Chem, 2012, 55(8): 1587–1597

Conversion coatings of Mg-alloy AZ91D using trihexyl(tetradecyl) phosphonium bis(trifluoromethanesulfonyl)amide ionic liquid

HOWLETT P. C., GRAMET S., LIN J., EFTHIMIADIS J., CHEN X. B., BIRBILIS N. & FORSYTH M.

Sci China Chem, 2012, 55(8): 1598–1607

Composite electrolytes based on poly(ethylene oxide) and binary ionic liquids for dye-sensitized solar cells

YU YingHao, JIANG Peng, WANG FuRong, WANG LeFu & LI XueHui

Sci China Chem, 2012, 55(8): 1608–1613

Iron catalyzed Michael addition: Chloroferrate ionic liquids as efficient catalysts under microwave conditions

VASILOIU Maria, GAERTNER Peter & BICA Katharina

Sci China Chem, 2012, 55(8): 1614–1619

vi

Zinc-assisted synthesis of imidazolium-tetrazolate bi-heterocyclic zwitterions with variable alkyl bridge length

DRAB David M., SHAMSHINA Julia L., SMIGLAK Marcin, COJOCARU O. Andreea, KELLEY Steven P. & ROGERS Robin D.

Sci China Chem, 2012, 55(8): 1620–1626

Development of sequential type iron salt-catalyzed Nazarov/Michael reaction in an ionic liquid solvent system

IBARA Chie, FUJIWARA Masamune, HAYASE Shuichi, KAWATSURA Motoi & ITOH Toshiyuki

Sci China Chem, 2012, 55(8): 1627–1632

High viscosity of ionic liquids causes rate retardation of Diels-Alder reactions

KUMAR Anil & PAWAR Sanjay S.

Sci China Chem, 2012, 55(8): 1633–1637

Properties of alkylbenzimidazoles for CO2 and SO2 capture and comparisons to ionic liquids

SHANNON Matthew S., HINDMAN Michelle S., DANIELSEN Scott. P. O., TEDSTONE Jason M., GILMORE Ricky D. & BARA Jason E.

Sci China Chem, 2012, 55(8): 1638–1647

vii

CO2 Capture technologies: Current status and new directions using supported ionic liquid phase (SILP) absorbers

KOLDING Helene, FEHRMANN Rasmus & RIISAGER Anders

Sci China Chem, 2012, 55(8): 1648–1656

Enhanced refolding of lysozyme with imidazolium- based room temperature ionic liquids: Effect of hydrophobicity and sulfur residue

BAE Sang-Woo, CHANG Woo-Jin, KOO Yoon-Mo & HA Sung Ho

Sci China Chem, 2012, 55(8): 1657–1662

The effect of hydrogen bond acceptor properties of ionic liquids on their cellulose solubility

STARK Annegret, SELLIN Martin, ONDRUSCHKA Bernd & MASSONNE Klemens

Sci China Chem, 2012, 55(8): 1663–1670

viii

Novel acid initiators for the rapid cationic polymerization of styrene in room temperature ionic liquids

VIJAYARAGHAVAN R. & MACFARLANE D. R.

Sci China Chem, 2012, 55(8): 1671–1676

Density estimated physicochemical properties of alanine- based ionic liquid [C7mim][Ala] and its application in selective transesterification of soybean oil

FANG DaWei, LI Meng, GE RiLe, ZANG ShuLiang, YANG JiaZhen & GAO YanAn

Sci China Chem, 2012, 55(8): 1677–1682

Reactivity of N-cyanoalkyl-substituted imidazolium halide salts by simple elution through an azide anion exchange resin

DRAB David M., KELLEY Steven P., SHAMSHINA Julia L., SMIGLAK Marcin, COJOCARU O. Andreea, GURAU Gabriela & ROGERS Robin D.

Sci China Chem, 2012, 55(8): 1683–1687

Infrared spectroscopic study on chemical and phase equilibrium in triethylammonium acetate

LV YiQi, GUO Yan, LUO XiaoYan & LI HaoRan

Sci China Chem, 2012, 55(8): 1688–1694

NEWS & COMMENTS

Three international conferences on ionic liquids held in Beijing in 2012

WANG Qian & LU XingMei

Sci China Chem, 2012, 55(8): 1695–1696

SCIENCE CHINA Chemistry

© Science China Press and Springer-Verlag Berlin Heidelberg 2012 chem.scichina.com www.springerlink.com

*Corresponding author (email: Douglas.MacFarlane@monash.edu)

• ARTICLES • August 2012 Vol.55 No.8: 1671–1676

· SPECIAL ISSUE · Ionic Liquid and Green Chemistry doi: 10.1007/s11426-012-4658-y

Novel acid initiators for the rapid cationic polymerization of styrene in room temperature ionic liquids

VIJAYARAGHAVAN R. & MACFARLANE D. R.*

Department of Chemistry, Monash University, Clayton Campus, Victoria 3800, Australia

Received February 27, 2012; accepted April 22, 2012; published online June 25, 2011

Cationic polymerization of styrene has been achieved using several novel acidic initiators in room temperature ionic liquids (ILs) under mild reaction conditions to obtain polymers of low molecular weight with narrow polydispersity. Both strong pro-tic acids such as bis(trifluoromethanesulfonyl) amide acid (HTFSA) and a moderately weak acid such as bisoxalato phospho-rous acid (HBOP) have been studied as initiators. It has been observed that HTFSA initiates the polymerization rapidly even at room temperature and below, as compared to HBOP which produces a slower polymerization requiring elevated temperatures to complete. The relative difference in reactivity of the initiators as compared to the previously described HBOB initiator is discussed in terms of the difference in their proton acidity and the consequential basicity of the anions. The efficiency of dif-ferent ILs as the reaction solvent is also presented.

cationic polymerization, HTFSA, HBOP, ILs, polydispersity

1 Introduction

Styrene is a well known, commercially available vinyl monomer that undergoes polymerization via cationic [1] as well as radical [2], anionic [3] and coordination pathways [4]. In the cationic polymerization of styrene the lack of strongly electron-donating groups renders the growing car-bo-cation unstable and hence it suffers from side reactions such as chain-transfer accompanied by β proton elimination and Friedel-Crafts alkylation on the phenyl ring of the monomer unit [5]. Hence its cationic polymerization has been considered to be difficult, despite the recent develop-ments made in the field of cationic polymerization generally. There have been recent reports, involving different Lewis acids, salts and solvents to stabilize the carbocation, to re-duce side reactions and produce living polymers [6, 7]. Studies pertaining to controlled polymerization of vinyl monomers at different temperatures in conventional sol-

vents have also been investigated [8–10]. However, the handling and disposal of the conventional Lewis acid initi-ators and the chlorinated solvents used remains an issue [11, 12].

In recent years, ILs have emerged as a potentially green alternative to organic solvents in traditional chemical pro-cesses [13]. ILs are organic salts that are liquid at room temperature, and in many cases ILs display low vapor pres-sures, eliminating the possibility of solvent vapour emis-sions [14]. The use of ILs in different polymerization reac-tions [15–20] has been investigated. In our recent commu-nication, we have shown that a cationic polymerization of styrene is possible, producing controlled molecular weight polymers [21]. A major advantage of the IL solvent is that the polymerisation can be initiated by bisoxalato boric acid (HBOB), a mild Bronsted acid, under ambient conditions. The objectives of the present work were to synthesise an-other novel initiator (HBOP) in the bis-oxalato family to determine and investigate its role in the cationic polymeri-zation of styrene in ILs, while the effect of strong acids such as HTFSA on the polymerization was also studied. The

1672 Vijayaraghavan R, et al. Sci China Chem August (2012) Vol.55 No.8

polymers obtained by these initiators were characterized for thermal, spectral and molecular weight properties.

2 Experimental

2.1 Materials

Analytical grades of styrene (99.5%) (MERK), oxalic acid (99%) (Aldrich), and phosphorous acid (97%) (MERK) were used. HTFSA (99%) (Mortia Chemical Industries, Japan) was used as received. The monomer was washed with 10% aqueous sodium hydroxide and then with distilled water and then distilled under reduced pressure prior to use. The synthesis of N-butyl-N-methyl pyrrolidinium bis(triflu- oromethane sulfonyl)amide abbreviated as [C4mpyr][NTf2], 1-ethyl-3-methyl imidazolium bis(trifluoromethane sul-fonyl)amide abbreviated as [C2mim][NTf2] and trihex-yl(tetradecyl) phosphonium bis(trifluoromethane sulfonyl) amide, abbreviated as [P6,6,6,14][NTf2] ILs followed the pro-cedures in the literature [22–24].

2.2 Measurements

1H NMR spectra of polystyrene samples were recorded us-ing Bruker 300 FT-NMR instrument with CDCl3 as solvent and TMS as internal standard. 13C NMR spectra of polysty-rene samples were recorded at room temperature on a Bruker 400 FT-NMR instrument. The protons were decou-pled by broadband irradiation. The molecular weights and their distributions were determined by means of gel perme-ation chromatography (GPC) at room temperature in a setup comprising of WATERS pump equipped with PLGEL mixed column (particle size 10 m, dimension 7.5 mm × 600.00 mm with porosity ranging from 50 to 106 Å) cali-brated with different polystyrene standards and a differen-tial refractometer detector using tetrahydrofuran as eluent with a flow rate of 1.0 mL/min. The glass transition temper-ature (Tg) was determined by the slope measurement at the onset temperature using Thermal analysis (TA) software of differential scanning calorimetry (DSC). A standard sample of Indium was used to calibrate temperature and power measurements in the DSC over the temperature range from 30 to 170 °C. Typically 5–10 mg of the sample was taken in DSC aluminium pan and a heating rate of 10 °C/min was used. The carrier gas was helium and a flow rate of 40 mL/min was maintained during the analysis. Thermal sta-bility was determined by thermo gravimetric analysis (TGA). Polymer samples of 4–10 mg were analysed at a heating rate of 10 °C/min under nitrogen purge at a flow rate of 50 mL/min. The organo borate acids were character-ized by Electrospray Ionisation Mass Spectroscopy which we found to be a useful method of characterizing the anion of the acid (dissociation of the proton takes place in the electrospray process).

2.3 Synthesis of bis(oxalatophosphinic acid), HBOP

The synthesis involves dehydrating 2 moles of aqueous so-lutions of oxalic acid with one mole of phosphorous acid under vacuum to produce a dry white solid in 98% yield. Electrospray Ionization Mass Spectroscopy was carried out to confirm the identity of the anion and the purity of the product (m/e = 208 for the BOP anion). The chemical structures of the IL and the initiators used are shown in Fig-ure 1.

2.4 Synthesis of polystyrene

The polymerization was carried out by dissolving the acid, for example HTFSA (0.02 g, 0.07 mmoles) into the IL ([P6,6,6,14][NTf2]) medium (0.40 g, 0.94 mmoles) and to the homogenous reaction mixture, 0.18 g (1.7 mmoles) of sty-rene was added and the mixture held at the desired reaction temperature. Then the reaction mixture was quenched with excess methanol. The IL and initiator dissolve in the meth-anol and the polymer precipitates indicating that there was no oligomer formation. The polymer was then washed sev-eral times with fresh methanol and dried in a vacuum oven at room temperature. In the example used here, where T = 60 °C and t = 2 h, the yield was calculated to be 95%. GPC (THF); Number average molecular weight (Mn) = 2720 g/mol, Weight average molecular weight (Mw) = 4390 g/mol, Mw/Mn = 1.6; 1H NMR (300 MHz, CDCl3, δ (ppm) relative to TMS) of polystyrene: 7.26–6.59 (5H, C6H5); 1.83–1.87 (1H, CH); 1.56–1.27 (2H, CH2). Thus 1H NMR spectra of polystyrene samples reveal the absence of [P6,6,6,14][NTf2] IL suggesting that the polymer contained no IL impurity at the level detected by this method.

3 Results and discussion

3.1 Cationic polymerization of styrene

Preliminary experiments on cationic polymerization of sty-rene were carried out using HTFSA in a conventional sol-vent, dichloromethane (DCM), and also in an IL at different

Figure 1 Chemical structures of IL and initiators.

Vijayaraghavan R, et al. Sci China Chem August (2012) Vol.55 No.8 1673

temperatures (0–60 °C). Polymerization occurs in both the solvents rapidly at all temperatures, however better reaction control was observed when the IL was employed as solvent. The polymerization initiated by HTFSA in DCM was found to be a highly exothermic reaction and was observed to boil the solvent with violent bumping even at 0 °C. Since the IL used has relatively low vapour pressure and is stable at high temperatures, better reaction control was observed. Hence the IL is recommended for process safety reasons in this reaction.

In the case of the HBOP initiated reaction, the polymeri-zation did not occur at temperatures between 0 and 60 °C in conventional solvents (DCM and chloroform), whereas good yields were obtained in the IL ([C2mim][NTf2]), me-dium at 60 °C in 2 h, as observed previously for the HBOB initiated system [20]. This could be due to better dissocia-tion of the HBOP in IL solvent compared to traditional sol-vents at that temperature. Hence this reaction temperature (60 °C) and IL have been chosen for the remainder of the studies reported here.

Attention was then focused on the concentration de-pendence of initiator (HTFSA and HBOP) in the cationic polymerization of styrene, at fixed reaction time. The con-centration of styrene used for the HTFSA initiated system was 1.7 mmoles while the reaction time and temperature were maintained at 120 min and 60 °C. In the case of the HBOP initiated system all conditions were the same except for the reaction time which was increased to 180 min. The results are shown in Figure 2. The polymerization initiated by HFTSA shows that the polymer yield increases rapidly with increase in concentration of HTFSA and the polymer yield goes to completion above 0.11 mmoles, while for HBOP initiated system polymer yield initially increased (up to 77%) and then did not increase further with increase in concentration of HBOP.

Experiments were then designed to optimize the reaction time. In the case of the polymerization initiated by HTFSA, the reaction was carried out at room temperature using 0.11 mM HTFSA and the results are shown in Figure 3. The re-

Figure 2 Cationic polymerization of styrene. Effect of initiator concen-tration on polymer yield. Styrene = 1.7 mmoles; T = 60 °C; HTFSA- reaction time = 120 min; HBOP-reaction time = 180 min.

Figure 3 Cationic polymerization of styrene. Effect of time (t) on poly-mer yield; styrene used = 1.7 mmoles.

sults indicate that polymerization was rapid with almost 90% conversion produced in 30 min of reaction time. This is considerably faster than our earlier observations with HBOB as initiator and supports the basic hypothesis that the Bronsted acidity is one of the key factors in this reaction [20]. On the other hand for the HBOP initiated system at 60 ºC, the results show that the polymer yield increased with time up to a certain value and then did not vary much. It was found that polymer yield only increased from 79 to 85% from 3 h to 24 h reaction time; this slowing trend of yields could be due to the gelation of the reaction mixture by the polymer formed in the IL.

The effect of styrene concentration on polymer yield was also investigated (for HTFSA and HBOP systems) at 60 ºC and the results are shown in Figure 4. In the case of the HTFSA system, the results show that almost complete polymer conversion occurred at all the monomer concentra-tions studied. This indicates that the propagating species are active during the entire polymerization, consistent with our earlier observation. A completely different trend was ob-served for HBOP initiated system. The results indicate that

Figure 4 Cationic polymerization of styrene. Effect of monomer concen-tration on polymer yield; HTFSA = 0.11 mmoles; T = 60 °C, t =120 min; HBOP = 0.14 mmols; T = 60 °C; t = 180 min.

1674 Vijayaraghavan R, et al. Sci China Chem August (2012) Vol.55 No.8

the polymer yields increased up to 1.7 mmoles styrene and then dropped with further increase in styrene concentration. This may be the result of the relatively weak acid, initiating only a limited number of polymer chains which eventually grow to high molecular weights, causing gelation and thereby limiting further chain growth.

In order to show the linear dependence of molecular weight on concentration of initiators, which is necessary for a true living polymerization, experiments with increase in molar ratio of monomer/initiator (HTFSA/HBOP) ratios were performed as shown in Figure 5. The straight line in Figure 5 represents the predicted molecular weights and the results show that the actual molecular weights determined by GPC deviate upwards from this line and then appear to reach a limit. In the case of the HTFSA system, given that this is a very strong acid, we presume that rapid dissociation of the acid produces a proton concentration which is the same as the original acid concentration. Thus the higher than expected molecular weights must be an indicator of loss of cationic species, perhaps due to the effect of residual water absorbing the protons. In all these cases the polydis-persity values remained narrow (Table 1).

In the case of HBOP system, it can be seen from Figure 5 that the actual molecular weights are higher than the predicted value and that they remain relatively constant over the range of monomer: initiator concentrations studied; the polymerization cannot be said to be living in this case. Pre-sumably the weak acidity in this case means that only a small fraction of the acid dissociates. It would therefore appear, not surprisingly that a certain minimum level of acidity in the IL is necessary for initiation of this type of cationic polymerization and that in view of the success of the HBOB acid in our previous work [21], it would appear that this minimum lies somewhere between HBOB and HBOP. In water the acidity of these (both relatively weak) acids differs by about 1 pKa units.

Table 1 The effect of initiator concentration on molecular weights

Nature of initiator Ratio of moles of monomer to initi-

ator

Mw (g mol1)

Mn (g mol1)

PDI

HBOP 35.8 2018 1599 1.3

17.9 1769 1437 1.2

11.9 1795 1412 1.3

8.9 1797 1388 1.3

7.2 1810 1383 1.3

5.1 1803 1372 1.3

HTFSA 25.6 4390 2719 1.6

17.1 4594 2645 1.7

12.8 3912 2348 1.6

10.2 2950 1507 1.9

Mw = weight average molecular weight; Mn = number average molecu-lar weight; PDI = polydispersity indices.

Figure 5 Cationic polymerization of styrene. Effect of ratio of styrene to initiator on number average molecular weights (Mn).

3.2 Evidence for cationic mechanism

To demonstrate that the proton is necessary for the observed polymerization to take place, experiments were carried in which the H+ of the initiator was substituted with other cat-ions such as pyrrolidinium, imidazolium and phosphonium and, consistent with the hypothesis of proton initiated cati-onic polymerization, there was no polymer formation ob-served. In another set of experiments methyl methacrylate (MMA), a monomer not known to undergo cationic polymerization, was used and the results showed no poly-mer formation. These observations confirm that proton acidity is one of the pre-requisites for cationic initiation in these cases. The mechanistic details are similar to those presented in our previous work [21]. It is discussed gener-ally as follows:

The initiator HX (exists as ion pairs of H+ and X–), where X can be either TFSA or BOP anions, interacts with the monomer (M) to generate the initiation event (eq. (1)). The protonated monomer (HM+) then propagates to form an oligomeric species (eq. (2)) and the acid anion becomes progressively “lost” within the IL medium of other cations and anions.

H+X + M HM+ + X (1)

HM+ + M HMM+ (2)

The ionic environment of the carbocation will be pre-dominantly the anion of the IL. The action of the IL in this process is unlikely to involve direct deprotonation of the HX since this particular IL is known to be a member of the exceedingly weak base category of ILs [25]. Also as a con-sequence of the IL anion being a very weakly basic species, the proton in HX is largely unsolvated. Thus interaction between an HX molecule and the monomer, which by comparison is a distinctly nucleophilic species, can be ex-pected to produce a rapid proton transfer reaction and thus initiate the reaction. Thus the proton transfer must take

Vijayaraghavan R, et al. Sci China Chem August (2012) Vol.55 No.8 1675

place directly from the HX to the monomer as shown in reaction (1). The role of the IL in this respect then is to pre-sent an extremely weakly basic environment in which direct interaction between the monomer and the acid molecule can take place.

3.3 Effect of other ILs on cationic polymerization

Experiments were also undertaken to study the effect of other ILs in the cationic polymerization of styrene using HBOP as initiator. The ILs investigated in this study include hydrophilic ILs such as N-propyl-N-methyl pyrrolidinium dicyanamide ([C3mpyr][DCA]) and trihexyl(tetradecyl) phosphonium xylene sulfonate ([P6,6,6,14][XS]). The results are summarised in Table 2, which shows that the polymeri-zation takes place if the anion is [NTf2], independent of other cations, but when the anion is changed to other, more basic anions [25], the polymerization did not occur. In the case of the phosphonium ([P6,6,6,14][NTf2]) salt the yield is lower, perhaps due to the bulkiness of the cation which re-stricts the mobility. In the case of xylene sulfonate anions, ester formation with the growing carbocation can be ex-pected to dominate. In the case of recycled IL ([C2mim] [NTf2]) the polymerization takes places as for fresh IL with only a small loss of yield.

3.4 Characterization of polymers

3.4.1 Thermal characterization

Glass transition temperatures (Tg) of the polystyrene ob-tained here were determined by Differential Scanning Calo-rimetry (DSC). The HTFSA and HBOP samples exhibited Tg (onset) values of 67 and 62 °C respectively and is shown in Figure 6. Values of Tg around 60 °C are typical for low molecular weight polystyrenes [26]. Samples were also characterised for thermal stability by Thermo gravimetric analysis (TGA). The polymers were stable up to 300 °C with a very slight decomposition and a weight loss of 20% obtained at 375 °C. The complete decomposition of poly-mer occurred at around 430 °C.

3.4.2 Tacticity—A comparison of nature of initiators

The tacticity of polystyrenes produced with different initia-tors (HTFSA and HBOP) synthesized in ILs was studied.

Table 2 Effect of other ILs on polymer yield

S.No IL used Yield (%)

1 [C4mpyr][NTf2] 80

2 [C3mpyr][DCA] trace

3 [C2mim] [NTf2] 85

4 [P6,6,6,14] [NTf2] 50

5 Recycled

[C2mim] [NTf2] 75

6 [P6,6,6,14][XS] trace

Figure 6 DSC traces for polystyrene synthesized in ILs.

Figure 7 Expanded 13C NMR spectrum of quaternary carbon signal of polystyrene sample prepared by cationic mechanism using HTFSA in IL.

Figure 8 Expanded 13C NMR spectrum of quaternary carbon signal of polystyrene sample prepared by cationic mechanism using HBOP in IL.

The determination of sequences followed the literature pro-cedure [27]. The isotactic triad sequences (mm), syndiotac-tic sequences (rr) and atactic sequences (mr) are respective-ly observed between 145.5 and 147 ppm on the quaternary carbon signal of polystyrene. The expanded 13C NMR spec-tra for HTFSA and HBOP systems are respectively given in Figures 7 and 8. As in the case of polystyrene made using HBOB in our previous studies [21], the polymers of the present initiating systems also indicate a mixture of triad sequences (syndiotactic, isotactic and atactic), however a

1676 Vijayaraghavan R, et al. Sci China Chem August (2012) Vol.55 No.8

predominance (43%) of syndiotactic sequences was noted. The results also show that the tacticity varies with the nature of the initiating species and on the solvents used.

4 Conclusions

It has been successfully demonstrated that cationic polymerization styrene, employing a strong acid such as HTFSA and a much weaker acid, HBOP, as initiators, can be carried out in room temperature IL. The contrasting reac-tivity of the initiators could be due to the difference in their acidity. The polymerization conditions were optimized and it was shown that relatively mild reaction conditions are sufficient for cationic polymerization in IL. The resultant polymers were characterized for molecular and thermal properties and the results reveal that the polymers are of narrow polydispersity and possess good thermal stability.

The authors thank Australian Research Council and Centre for Green Chemistry, Monash University for the research grants. The authors also thank Masahiro Yoshizawa for providing HTFSA sample.

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