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1、Anionic polymerization of styrene in ionic liquidsR. Vijayaraghavan *, J.M. Pringle, D.R. MacFarlaneSchool of Chemistry, Monash University, Clayton, Victoria 3800, Australiaa r t i c l e i n f o Article history:Received 24January 2008Received in revised form 14February 2008Accepted 29February 2008Av

2、ailable online 18March 2008Keywords:Anionic polymerization Zwitterions Ionic liquids Co-initiatorThermal stabilitya b s t r a c tFor the rst time anionic polymerization of styrene has been successfully carried out at ambient temperatures in an ionic liquid, providing milder reaction conditions than

3、classi-cal methods. The addition of the zwitterion provides better dissociation of the metal cation based initiators and the IL based reaction allows the use of a much milder Lewis base ini-tiator than is usually required. The present method also eliminates the traditional solvents and rigorous reac

4、tion conditions.Ó2008Elsevier Ltd. All rights reserved.1. IntroductionThe vinyl and acrylic polymers made commercially by classical free radical polymerization processes, often do not provide control over molecular weight and chain branching, although recent advances in controlled free radical

5、processes offer great potential 1. It is well known that the properties of a polymer are strongly inuenced by its architectural features; the suitability of a polymer for a particular application is often determined by its specic structural and molecular parameters such as molecular weights, its dis

6、tribution and the presence of functional groups and their spatial location 2. Living anionic poly-merization provides an excellent route to tailor-make model compounds with well dened polymer architecture 35. These polymers are useful in many applications 6 7such as additives in the oil industry, re

7、sins, impact mod-iers etc. The monomers susceptible to anionic polymer-izations are those having substituents which stabilize a negative charge when incorporated into the active centre. These include non-polar monomers such as styrene andthe dienes, where the stability can be facilitated by charge d

8、elocalisation. The types of initiators required in anionic polymerization depend on the electron afnity of the monomer, which is conveniently determined from polaro-graphic measurements 8. For instance, methyl methacry-late requires compounds like indenyl or uorenyl sodium, which will not polymerise

9、 non-polar monomers. With a gi-ven initiator, the initiation rate is strongly dependent on the solvent 9. However, with a given solvent, large varia-tions in rate can be produced by varying the structure of the initiating carbanion nucleophile 1012. During the polymerization reaction itself, the for

10、mation of ion pairs, free ions and triple ions can all occur during the propaga-tion step and the fraction of ionic species and their reactiv-ity depends on solvent and temperature. Thus the strong solvating power of 1,2-dimethoxy ethane (DMEwas very clear in the living anionic polymerization of sty

11、rene 13in DMEbenzene mixtures in which the formation of highly reactive intermolecular solvent separated triple ions was noted. Recently a new generation of anionic initiators based on trialkylaluminium and alkali metal hydride has been developed for the control of styrene polymerization at high tem

12、peratures 14. Nonetheless the stringent reac-tion conditions (lowwater content remains a limitation to this type of polymerization. European Polymer Journal 44(200817581762Contents lists available at ScienceDirectEuropean Polymer Journaljournal homepage:www.elsevie r. c o m /l o c a t e /e u r o p o

13、 l jThe rst homogenous anionic polymerization of sty-rene, free of termination and chain transfer, was reported by Szwarc 15. The anionic polymerization of hydrocarbon monomers such as styrene in polar solvents 16has since been intensely studied. The mechanism of the anionic polymerization of styren

14、e with lithium as a counter ion in non-polar solvents has been well established 17. In solvents such as benzene 11and cyclohexane 18, a ki-netic order of 0.5with respect to the living end concentra-tion of polystyryl-lithium (PStLiwas found for the observed rate constants. Anionic polymerization of

15、vinyl monomers and dienes initiated by superbases constituted by the association of alkyl lithium and alkali metal alkox-ides has also been widely investigated 1923. Detailed reviews on the past and recent developments in living an-ionic polymerization of vinyl and acrylic monomers using different i

16、nitiating systems has been published 24,25. In recent years, room temperature ionic liquids (ILs have been extensively investigated for use as replacement solvents for clean synthesis in a variety of chemical reac-tions 2628. The major advantage in using these ionic liquids is their ability to disso

17、lve a wide range of organic and ionic compounds to an appreciable extent. Their polar-ity, lack of volatility, and their high thermal stability are also important features. The use of ionic liquids as solvents in polymerization reactions has recently been reported for free radical polymerization 29,

18、 transition-metal medi-ated living free-radical polymerization 30, cationic 31 and charge transfer polymerization 32. However, there are no reports of the use of ionic liquids as solvents in an-ionic polymerization. Ionic liquids in principle should be ideal solvents for anionic polymerization becau

19、se of their ability to stabilize the anionic species. This hypothesis is based on our previous work in which we have found that the ionic liquid solvent provides an excellent medium for polymerization processes involving charged species 31,32. In this work we report the successful anionic poly-meriz

20、ation of a model system (styreneand demonstrate that the hydrophobic IL provides a basis for reaction to take place under milder conditions than would typically be nec-essary. In particular the need for a strong Lewis base cata-lyst along with low water content is lessened.Thus, in this communicatio

21、n, we report the anionic polymerization of styrene using s-butyl lithium as initia-tor in a hydrophobic ionic liquid. The results are com-pared with the polymers made by employing traditional initiating system (underidentical water con-tent and other reaction conditions as for the ionic liquid solve

22、nt in a conventional solvent (THF.In another set of experiments zwitterionic compounds are added to the reaction mixture to assess their role as a dissociation enhancing additive 33,34and the results are compared. We also report the application of a weakly basic salt such as sodium acetate as initia

23、tor in the anionic poly-merization of styrene, with and without the addition of the zwitterion, in the IL solvent. The ionic liquid chosen for this study is trihexyl(tetradecylphosphonium bis(tri-uoromethane sulfonyl amide, abbreviated as P6,6,6,14NTf2Both of the ions are chosen because of their hyd

24、rophobicity. The zwitterion used in this study is butyl imidazolium butane sulfonate. 2. Experimental2.1. Materials and methodsAnalytical grades of the monomer (styrene,s-butyl lithium (1.4M in cyclohexane, anhydrous sodium acetate, tetrahydrofuran (THFand cyclohexane were used. The synthesis of P6,

25、6,6,14NTf2ionic liquid and zwitterions fol-lowed literature procedures 35,36. The chemical struc-tures of the ionic liquid and zwitterion is given in Fig. 1. Typically 0.05g (0.19mmol of zwitterion is dissolved in 0.50g of ionic liquid solvent and either 0.0040g (0.062mmol of s-butyl lithium or sodi

26、um acetate initia-tors (0.036mmol is added. Then to the reaction mixture, 0.30g (2.88mmol of styrene is added, stirred and thermo-stated. After the desired reaction time the polymer is pre-cipitated from methanol, washed and dried. The polymers were characterised by gel permeation chromatography (GP

27、Cand thermal analysis techniques.2.2. CharacterizationThe polymer samples were characterized by gel perme-ation chromatography (GPCto obtain the molecular weight distribution. The instrument was calibrated with different polystyrene standards and tetrahydrofuran (THFwas used as an eluent. Glass tran

28、sition temperatures (T g were recorded by DSC (differentialscanning calorime-try and the instrument was calibrated using Indium as a standard (samplesize 510mg, heating rate 5o C/min,ar-gon atmosphere. Thermogravimetric (TGAcurves were recorded using a thermogravimetric analyser. Polymer samples of

29、410mg were analysed at a heating rate of 10°C/minunder nitrogen purge at a ow rate of 50ml/ min.3. Results and discussionConventionally the anionic polymerization of styrene requires fairly anhydrous reaction conditions. The present study involves relatively routine reaction conditions and ambi

30、ent temperatures, with no extra care to maintain anhydrous conditions. The water contents of the starting R. Vijayaraghavan et al. /European Polymer Journal 44(2008175817621759materials were determined by Karl Fischer analysis as follows:Styrene =0.02%,ionic liquid =0.05%,sodium ace-tate =1%.The ani

31、onic polymerization of styrene in IL is summarized schematically in Scheme 1. When the anionic polymerization of styrene was carried out at 60°C in P6,6,6,14NTf2IL using s-butyl lithium as initiator, poly-merization was successful in producing high molecular weight polymer, however only a low y

32、ield (20%was ob-tained (run1of Table 1 after 70h. Under similar reaction conditions sodium acetate produced only 10%yield (run4. It is well known that the NTf À2anion provides only a very weakly basic environment from the point of view of metal ion coordination 37, hence not providing a donor e

33、nvi-ronment for the alkali metal cation in solution; this tends to not favour the initiator ion-dissociation reaction. To overcome this in Li battery electrolytes based on ionic liq-uids 38,39, the addition of zwitterionic compounds bear-ing a more basic functionality has proved successful in enhanc

34、ing dissociation. Similarly in the present case, the addition of the zwitterion increased the yield of polymer to 75%in the case of initiation by s-butyl lithium and 65%in the case of sodium acetate (runs2and 3, respec-tively. With slight changes in concentrations of the initia-tors (runs2a and 4a,

35、the polymer yields increased further. The polymer yield reached 94%(run2b when the reaction time was increased to 140h.In order to compare with the conventional anionic poly-merization reaction, the polymerization of styrene was carried out at 60°C using s-butyl lithium as initiator and tetrahy

36、drofuran, or cyclohexane as solvents. The reactions produced only traces of product. Trials used a variety of different basic initiators such as s-butyl lithium, sodium acetate and sodium xylenesulfonate, in combination with zwitterions as co-initiators, but even these reactions pro-duced only trace

37、s of product (runs5and 6. Part of the rea-son for this low yield may be due to the presence of relatively more water in the traditional solvents (theTHF contained 0.1%water as determined by Karl Fischer deter-mination as compared to 0.03%water in the ionic liquid. At 0.1%there is sufcient water pres

38、ent to consume a sig-nicant part of the initiator or low molecular weight ionic oligomers in side reactions.The effect of reaction time on the polymer conversion was studied using the conditions of run 2as shown in Fig. 2. It was found that the polymer yield increased with time to levels around 94%.

39、Notably also, there is what ap-pears to be an induction period at the onset of the reaction of around 10h. This is thought to correspond to the lower molecular weight stage of the polymerization. Reactions at these times produce hazy solutions when quenched in methanol, but no precipitate. Thus reac

40、tion is occurring, Table 1Effect of different initiators and their concentrations on polymer yieldS. No. Styrene (mmolZwitterion (mmolInitiator (mmolSolvent Time (hTemperature (°C Yield (%1760R. Vijayaraghavan et al. /European Polymer Journal 44(200817581762but at these early times has not yet

41、proceeded to high molecular weight.The thermal stabilities of polystyrene prepared in both conventional (s-butyllithium and non conventional (Naacetate initiators using zwitterion as co-initiator and ionic liquid as solvent are shown in Fig. 3. The results indicate that there is no difference in the

42、 thermal stabilities beyond 200°C, although a slightly lesser thermal stability was ob-served in the case of polystyrene prepared using non-con-ventional initiator between 130and 190°C. The glass transition temperatures for these polymers were in order (98°C with the conventionally ob

43、tained polystyrene samples. 4. ConclusionsThese studies of anionic polymerization in a hydropho-bic IL show that the reaction can occur under relatively mild conditions. The addition of the zwitterion provides better dissociation of the metal cation based initiators and no doubt other metal chelatin

44、g compounds could play a similar role. Notably, the IL based reaction allows the use of a much milder Lewis base initiator than is usually re-quired. The present method also eliminates the traditional solvents and rigorous reaction conditions. The recovery of the IL from the reaction mixture in this

45、 method of poly-merization is straightforward and can be achieved by evap-orating the methanol after precipitation of the polymer. Studies to further understand and optimise this reaction by employing other initiators and other ILs are underway. AcknowledgementsWe thank ARC Special Research Centre f

46、or Green Chem-istry, Monash University for the nancial support. References1Matyjaszewski K. ACS Symp Ser 1998:685.2Hawker C. Adv Polym Sci 1999;147:113.3Ito M, Ishizone T. J Polym Sci Part A:Polym Chem 2006;44:4832.4Hawker C, Wooley KL. Science 2005;309:1200.Table 2Molecular weights of anionic/ILpol

47、ymerised polystyrene S. No. Zwitterion a (mmolInitiator (g/molM w Â10À4(g/molM n Â10À4g/molPDI 10.19Na acetate72.339.11.8aZwitterion used:butyl imidazolium butane sulfonate. R. Vijayaraghavan et al. /European Polymer Journal 44(20081758176217615Kobayashi M, Uchino K, Ishizone T.

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