Synthesis and application as polymer electrolyte of hyperbranched polyether made by cationic ring-opening polymerization of 3-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxymethyl}-3′-methyl-oxetane

Synthesis and Application as Polymer Electrolyteof Hyperbranched Polyether Made by CationicRing-Opening Polymerization of3-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxymethyl}-30-methyl-oxetane

YE LIN,1 FENG ZENG-GUO,1 ZHAO YU-MEI,1 WU FENG,2,3 CHEN SHI,2,3 WANG GUO-QING2,3

1School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081,People’s Republic of China

2School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081,People’s Republic of China

3Laboratory of National Development Center of Hi-Tech Green Materials, Beijing Institute of Technology,Beijing 100081, People’s Republic of China

Received 5 November 2005; accepted 12 March 2006DOI: 10.1002/pola.21450Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A novel cyclic ether monomer 3-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy-methyl}-30-methyloxetane (HEMO) was prepared from the reaction of 3-hydroxy-methyl-30-methyloxetane tosylate with triethylene glycol. The corresponding hyper-branched polyether (PHEMO) was synthesized using BF3 �Et2O as initiator throughcationic ring-opening polymerization. The evidence from 1H and 13C NMR analysesrevealed that the hyperbranched structure is constructed by the competition betweentwo chain propagation mechanisms, i.e. active chain end and activated monomermechanism. The terminal structure of PHEMO with a cyclic fragment was definitelydetected by MALDI-TOF measurement. A DSC test implied that the resulting poly-ether has excellent segment motion performance potentially beneficial for the iontransport of polymer electrolytes. Moreover, a TGA assay showed that this hyper-branched polymer possesses high thermostability as compared to its liquid counter-part. The ion conductivity was measured to reach 5.6 � 10�5 S/cm at room tempera-ture and 6.3 � 10�4 S/cm at 80 8C after doped with LiTFSI at a ratio of Li:O ¼ 0.05,presenting the promise to meet the practical requirement of lithium ion batteries forpolymer electrolytes. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:

3650–3665, 2006

Keywords: back-biting; cationic ring-opening polymerization; hyperbranched poly-ether; ion conductivity; oxetane; polymer electrolyte

INTRODUCTION

In recent years hyperbranched polymers havereceived much attention due to their unique mo-lecular structure and their ease of preparation ascompared to dentrimers, and various kinds of

Correspondence to: F. Zeng-guo (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 3650–3665 (2006)VVC 2006 Wiley Periodicals, Inc.

3650

hyperbranched polymers have been synthesizedand explored for potential applications.1–12 It isgenerally recognized that the new and improvedmaterial properties can be afforded by furthermodifying and functioning the architecture ofhyperbranched polymers. However, until now, thereports about their application in the area of poly-mer electrolytes for lithium ion battery have beendisclosed.

Since the invention by Wright and Armand in1970s,13–15 polymer electrolytes have been consid-ered to be the suitable replacement of liquid elec-trolytes as long as their ion conductivity reachesthe magnitude of 10�3 S/cm. The main difficultiesencountered currently are, however, how to effi-ciently increase the ion conductivity of polymerelectrolytes since the ion conductivity (10�7 S/cm)of the first generation of PEO–LiX-based polymer

Scheme 1. Synthetic route of the monomer HEMO.

Scheme 2. Chain initiation process in CROP of HEMO.

HYPERBRANCHED POLYETHER MADE BY CROP 3651

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

electrolytes is too low to be put into practicalapplication. Hence, it remains a significant chal-lenge to design and synthesize a polymer-basedelectrolyte system which presents the ion conduc-tivity capable of meeting the requirement of lith-ium ion battery.

As is well known,16–18 one of the most effectiveapproaches to improve the ion conductive per-formance of polymer electrolytes is to lower theirglass transition temperature(Tg) as well as toincrease their amorphous phases. The hyper-branched polymeric structure can remarkablysuppress the chain crystallization so as toenhance the amorphous phases as comparedto the linear polymers. Itoh and coworkers19,20

reported a hyperbranched polyester consistingof di- and triethylene glycols and 3,5-dioxyben-zoate branching units. However, because ofthe poorer motion ability of the aromatic moieties,its ion conductivity (�10�6 S/cm) was unsatisfac-tory to match the practical standard for lithiumion battery. Furthermore, a multistep syntheticpathway of such hyperbranched polyester wastedious.

Recently, a great deal of research work hasbeen focused on new hyperbranched polyethersmade from 3-hydroxymethyl-30-methyloxetane (HMO)and 3-hydroxymethyl-30-ethyloxetane (HEO) bycationic ring-opening polymerization (CROP).1–12

Compared with the hyperbranched polyesteraforementioned, such hyperbranched polyetherscan be synthesized using a one-pot protocol. Moti-vated by this novel synthetic strategy, and alsoas part of the continuing research work to pro-mote the ion conductivity of polyethers afterdoping with lithium salts suitable for polymerelectrolytes, a new cyclic ether monomer 3-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxymethyl}-30-methyl-oxetane (HEMO) was designed and preparedthrough incorporating triethylene glycol unit intoHMO. Since this monomer contains two nucleo-philic sites within the single molecule, that iscyclic ether and hydroxyl group, a novel hyper-branched polyether, referred as PHEMO, wassynthesized in turn by CROP with BF3 �Et2O asinitiator. The motion performance of the resultingpolyether segments is expected to improve signifi-cantly for the lack of stiff groups, such as phenyl

Scheme 3. ACE propagation and back-biting processes.

3652 LIN ET AL.


groups in the backbone, as well as for the amor-phous state inherently derived from the hyper-branched molecular structure.

EXPERIMENTAL

Materials and Measurement

Trihydroxymethylethane was purchased fromFluka (Switzerland) and lithium trifluorometha-nesulfonimide (LiTFSI) was from Aldrich, and

both were used as received. Boron trifluorideetherate (BF3 �Et2O), p-toluenesulfonyl chloride,potassium hydroxide, ethanol, and pyridine wereavailable from Shanghai Chemical Reagent Com-pany, and used without further treatment. THF,triethylene glycol, and CH2Cl2 were purchasedfrom Beijing Chemical Factory, and the CH2Cl2was dried over CaH2 and distilled under N2

atmosphere before use.FTIR spectra were measured with Shimadzu

IR Prestige-21. 1H and 13C NMR spectra wererecorded on Bruker ARX400 using CDCl3 as sol-

Scheme 4. AM propagation and back-biting processes.



vent containing TMS as internal standard. Allthe 13C NMR spectra of the polymers wereobtained by the inverted gate method with a pulsedelay of 10 s. Gel permeation chromatograph(GPC) analysis was made on Waters 2414 withthree Waters styragel HMW columns at 35 8Cusing THF as eluent. From the GPC results, themolecular weight was determined as calibratedwith polystyrene standards. Elemental analysiswas carried out on Vario EL III. Netzsch PC-200was used for an analysis of the thermal behaviorof the polymers at a heating rate of 10 8C/min.The polymer samples (6–10 mg) were heated from�100 to 100 8C in stainless steel pans. The resultsof the second run were used for the Tg investiga-tion. Thermogravimetric analysis (TGA) was con-

ducted with TA 2000 thermogravimeter under N2

atmosphere at a heating rate of 20 8C/min.MALDI-TOF mass spectrometry was performedon Bruker BIFLEX III with 2-cyano-4-hydroxyl-cinnamic acid (CCA) as matrix. The ion conduc-tivity was evaluated using CHI 660A Electro-chemistry Station made in Chen Hua InstrumentCompany (Shanghai, China). The measurementwas carried out from 30 to 80 8C with an intervalof 5 8C.

Synthesis of Monomer

Synthesis of HMO was described in our previouswork.21,22 Its derivative, HEMO was prepared asfollows. A solution of HMO (33.66 g, 0.33 mol) in

Scheme 5. Branching process in CROP of HEMO.

3654 LIN ET AL.


80 mL pyridine was introduced in a round-bot-tomed flask fitted with a dropping funnel. Aftercooling the mixture to 0 8C under constant agi-tation, (125.8 g, 0.66 mol) of tosyl chloride in130 mL pyridine was added dropwise for 90 min.The reaction mixture was then left to attain roomtemperature. After 30 min, this mixture wasdiluted with 200 mL H2O and extracted withether three times. The organic phase was washedwith an aqueous acid solution (15% HCl, 200 mL).The ether phase was dried over Mg2SO4, filtered,and evaporated under reduced pressure to giverise to 3-hydroxymethyl-30-methyloxetane tosylate(HMO-Tosyl)23 (58.3 g, yield 68.7%). FTIR/cm�1:1599,1490, 1463 (benzene), 1357, 1188 (��SO2��), 971 (oxe-tane’s C��O��C); 1H NMR/ppm:1.31 (3H, CH3), 2.46(3H, CH3 in benzene), 4.11 (2H, CH2OSO2), 4.35–4.37 (4H,oxetane’s CH2), 7.36–7.82 (4H, benzene).

A mixture of triethylene glycol (75 g, 0.5 mol)and KOH (9.24 g, 0.14 mol) in THF (200 mL) wasrefluxed until KOH was dissolved. The resulting

solution was cooled to room temperature, towhich HMO-Tosyl (32.8 g, 0.125 mol) was addedand refluxed for 18 h. The solvent was removedunder vacuum, CHCl3 was added, and the organicphase was washed with water (100 mL) threetimes, dried over Na2SO4, concentrated underreduced pressure, and finally dried under highvacuum to yield 3-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxymethyl}-30-methyloxetane (12.3 g, 42%).FTIR/cm�1: 3417 (OH), 1112 (C��O), 976 (oxe-tane’s C��O��C); 1H NMR/ppm: 1.31 (3H, CH3),2.79 (1H, OH), 3.55 (2H, OCH2), 3.60–3.62 (2H,CH2CH2OH), 3.65–3.67 (8H, CH2CH2O), 3.71–3.73 (2H, CH2OH), 4.35–4.54 (4H, oxetane’s CH2);Elemental analysis (%): Theoretical values, C 56.41,H 9.40; Experimental values, C 56.27, H 9.30.

Polymerization of HEMO

CROP of the monomer HEMO proceeded at roomtemperature in solution. The 2 mL CH2Cl2 was atfirst added into a dry round flask under N2 atmos-phere. Then 2.5 g of the monomer was added, ini-tiator BF3 �Et2O was introduced into the flask,and the polymerization was continued for 72 h.The hyperbranched polyether (PHEMO) formedwas purified by precipitation into hexane threetimes, and dried in vacuo.

The Blockage of End Chain ofHyperbrached Polyethers

Three grams of the aforementioned hyper-branched polyether was dissolved in 5 mL THF.Then, 5 g of acetic anhydride was added under N2

atmosphere, and the resultant solution was main-tained at 85 8C for 6 h. Finally, the end-cappedpolyether was purified by precipitation into hex-ane three times, and dried in vacuo.

The resulting end-capped polyether was mixedwith lithium salt LiTFSI in THF solution and driedin vacuo for the ion conductivity measurement.

Figure 1. GPC trace of Sample 4 in Table 1.

Table 1. Polymerization Results of the Monomer HEMO

Sample Code Reaction Time (h) M/I (mol/mol) Mna(Da) Mw/Mn

a DB (%) Yield (%)

1 72 2.5 4463 1.23 29 682 72 8 4541 1.24 20 693 72 15 4369 1.17 18 734 72 25 4345 1.16 16 71

a Measured by GPC.



RESULTS AND DISCUSSION

Synthesis of PHEMO

The synthetic route of the hyperbranched poly-ether is described in Scheme 1. The starting mono-

mer was prepared according to the report that

hexaethylene glycol could react with triethylene

glycol monoether tosylate to give rise to non-

aethylene glycol monoether under continuous re-

flux for 18 h in the presence of KOH.24 Here,

Scheme 6. The exchange reaction between ionized ether and PHEMO.

Figure 2. MALDI-TOF mass spectrum of PHEMO.

3656 LIN ET AL.


HEMO was yielded from the reaction of triethyl-ene glycol with HMO tosylate in THF.

CROP of HEMO was carried out in solution ofmethylene chloride. When boron trifluoride ether-ate was used as initiator, HEMO acted not only asa monomer but also as a coinitiator directly to ini-tiate the polymerization proceeding. For the pur-pose of the mechanistic discussion, the initiationprocess is outlined in Scheme 2. Accordingly, themonomer should firstly complex with BF3 torelease a proton and then this proton added toanother monomer or itself. Thereafter, the poly-mer chain grew either in active chain end (ACE)or active monomer (AM) mechanism as shown inSchemes 3 and 4, respectively.

As also illustrated in Schemes 3 and 4, the chainterminations of both mechanisms most likelyinvolved an intramolecular chain transfer reaction,so-called back-biting process leading to the forma-tion of cyclic fragment owing to the presence of ahigh concentration of the hydroxy group, whichwas evidenced by MALDI-TOF analysis as dis-cussed in the following section. This phenomenonof back-biting in the hyperbranched polyetherpreparation had been reported by Penczek andcoworkers.3–5,10,11 Evidently, the branching processtook place in the reaction of pendant hydroxygroups of the linear chain with activated HEMOmonomers. As a result of continuous branchingprocesses, the hyperbranched structure is con-structed. The formation of hyperbranched poly-ethers is depicted in Scheme 5.

The typical GPC trace of the resulting hyper-branched polyether is displayed in Figure 1, andthe GPC results of all samples are summarized inTable 1. As can be seen here, these polyethersexhibited a narrow and nearly a symmetric molec-ular weight distribution. However, the molecularweights listed in Table 1 seemed to be independentof the monomer to initiator ratio varying from 2.5to 25. The same phenomenon was also observed inour previous work where the linear counterpartwas prepared through CROP of 3-{2-[2-(2-methoxy-ethoxy)ethoxy]ethoxymethyl}-30-methyloxetane(MEMO).21 This independence was possibly due tothe competition between two chain propagationmechanisms in CROP as mentioned above. WhenBF3 �Et2O or benzyltetramethylenesulfonium hexa-fluoroantimonate was used as initiator in CROP of

Figure 4. 1H NMR spectrum of hyperbranched PHEMO.

Figure 3. FTIR spectrum of the hyperbranchedPHEMO.



HEO, the hyperbranched polyethers formed werealso reported1,4,6,10,11 to be uncontrolled by themonomer to initiator ratio.

MALDI-TOF Mass Spectrum

MALDI-TOF is a powerful technique from whichthe cyclic terminal structure of the hyper-branched polyethers can be well defined. The typ-ical MALDI-TOF mass spectrum of PHEMO isshown in Figure 2. Each peak appeared repre-

sents one macromolecule with the certain molecu-lar mass or the degree of polymerization as wellas the type of end group of this specific macromo-lecular species.25,26 Here the peaks can beassigned to a number of peak serials having equalpeak-to-peak mass increment of 234 Da. This in-crement exactly equals to the mass of the repeat-ing unit in PHEMO. Accordingly, all peaks havingthe same increment belong to one homologous se-rial.

At least four different homologous serials areobserved in Figure 2. For example, the peaks at959, 1193, 1427, 1661, 1895 Da and so on belongto one serial, whereas the peaks at 975, 1209,1443, 1677, 1911 etc. are attributed to another se-rial. They are most likely due to the intact macro-molecule cationized by the attachment of Naþ orKþ to form the M þ Naþ or M þ Kþ molecularions, as the polyether can be very easily cation-ized by Naþ and Kþ ions even with natural abun-dance concentration.25

Besides the cationic macromolecular species,the appearance of different homologous serialshaving the same polymer backbone is obviouslythe results of the formation of different endgroups. From the total masses of the macromole-cules and the mass of the polyether backbone,the masses of end groups may be determinedand assigned to a certain chemical structure. Asfor the peak at M þ Naþ ¼ 1193 Da, it was foundthat the mass of the end group was zero aftersubtracting the mass of repeating units indicat-ing that the formation of a cyclic end group dueto inevitable intramolecular chain transfer orback-biting process as mentioned above. The fol-lowing four peaks were used to explain theassignment of four different homologous serialsin the spectrum:

where n represents the degree of polymeriza-tion and 29 equals to the molecular weight ofCH3CH2 as the end group. Since the M þ Naþ

and M þ Kþ peaks are always pair-occurred,25

the four different homologous serials actuallyinvolve two chemical structures of different endgroups of PHEMO. One is due to the back-bitingprocess (Serial 1 and 2), and another is related to

Figure 5. 1H NMR spectra of hyperbranched PHEMObefore (b) and after (a) adding D2O.

3658 LIN ET AL.


the interchange reaction between ionized etherand polyether backbone (Serial 3 and 4). Thechemical structures resulted from Serial 3 and 4are shown in Scheme 6. Actually, the hydroxyend group of the propagation chains can alsoattack at activated or ionized ether moleculesaccording to AM mechanism, because the oxygenatom of ether can be protonated just as the oxy-gen atom of oxetane does. This is also validatedby the 13C NMR spectrum of the resulting hyper-branched polymer with a full chemical shiftrange as shown in Figure 7(c), where the peak la-beled as a corresponds to methyl group and b isattributed to methylene group in ��OCH2CH3,

respectively.There was a significant difference between the

molecular masses determined by MALDI-TOFand GPC analyses. The hyperbranched structureobviously affects the determination of the molec-ular mass of PHEMO. Because the linear poly-styrene is generally used as calibrating stand-ard, the molecular mass of hyperbranched poly-ethers would be underestimated by GPCanalysis. On the other hand, MALDI-TOF tech-nique may also underestimate the sample molec-ular mass owing to the discrimination of highmolecular weight fractions. The similar difficultyof how to exactly determine the molecularweight of PHMO or PHEO was also encounteredby Hult and coworkers1 and Yan and coworkers,7

respectively.

FT-IR Analysis

The IR spectrum of the hyperbranched polymer isgiven in Figure 3. Compared with the IR spec-trum of the monomer, the disappearance of theabsorbance peak at 969 cm�1 indicated thatCROP indeed occurred. The other peaks of theresulting polymer were assigned as follows: 3434(O��H), 2876 (CH2), and 1112 (C��O��C) cm�1.

NMR Identification

Figure 4 displays the 1H NMR spectrum ofPHEMO. Well consistent with the result of FT-IR,the disappearance of quartet peaks appearing at4.3–4.5 ppm indicated that CROP proceeded. Themain resonance peaks were assigned as follows:d 0.89–0.92 (CH3), 3.14–3.29 (CH2 next to thequarternary carbon), 3.45–3.76 (CH2 in triethy-lene glycol units), and 5.24 (OH) ppm. In Figure5, it was found that when an excess of D2O wasadded to the PHEMO sample, the resonance peakcorresponding to the hydroxyl group disappeared,as the OH moiety had exchanged to the corre-sponding OD ones. This clearly implied that theresonance peak at 5.24 ppm is due to the hydroxygroup. It was just the interaction of hydrogenbonds leading this resonance peak to appear insuch low field. A new peak appeared at 4.71 ppmin Figure 5(a) was assigned to the HDO moiety.Figure 6 exhibits the spectrum of PHEMO afteresterified with acetic anhydride. Accordingly, the

Figure 6. 1H NMR spectrum of PHEMO esterified with acetic anhydride.



resonance peak due to the hydroxy group disap-peared, and two new peaks emerged which weredesignated to the methyl group of acetate at 2.01–2.12 and the methylene adjacent to acetate at3.91–4.05 ppm, respectively.

The 13C NMR peaks of quaternary carbon inpolyoxetanes are widely used to characterize thehyperbranched structure.1–12 In the 13C NMRspectrum of linear polyoxtanes, only two signalsare found in the region of quaternary carbonabsorption corresponding to the linear and termi-nal units. As for hyperbranched polyoxetanes,besides those, the signals related to branch unitsare also observed. To demonstrate the hyper-branched structure of PHEMO, its linear counter-part, polymer of 3-{2-[2-(2-methoxyethoxy)ethoxy]ethoxymethyl}-30-methyloxetane (PMEMO) wasalso synthesized according to our previous workas shown in Scheme 7.21,22 The 13C NMR spectraof both linear and hyperbranched polyether aregiven in Figure 7. As can be seen in Figure 7(a),the single resonance peak appearing at 41.3 ppmindicates the quaternary carbon absorption in thelinear structure of PMEMO, whereas the smallpeak at 40.7 ppm corresponds to that in the termi-nal structure. It was found in Figure 7(b) that thespectrum of PHEMO is significantly differentfrom that of PMEMO. It seemed that besides thepeaks assigned to linear and terminal structureunits, the resonance peaks corresponding to thebranched structure also appeared. However, Fig-

ure 7(b) gave rise to an unsatisfactory structuralassignment because of the significant overlap ofresonance peaks. Surprisingly, after the esterifi-cation with acetic anhydride, six well separatedresonance peaks ascribed to corresponding qua-

Figure 7. 13C NMR spectra of linear and hyper-branched PHEMO.

Scheme 7. Themolecular structure of linear PMEMO.

3660 LIN ET AL.


ternary carbons were clearly observed in the 13Cspectrum as shown in Figure 8.

During CROP of the HMO or HEO, the signalsof quaternary carbon were reported1–12 to splitinto three resonance peaks corresponding to thebranched, linear, and terminal units, respectively.Accordingly, the branched degree (BD) of PHMOor PHEO is calculated as follows:

DB% ¼ B=ðBþ Lþ TÞ ð1Þ

where B, L, and T correspond to the integral areaof resonance peaks of branched, linear, and termi-nal quaternary carbons. However, because ofincorporating triethylene glycol into the mono-mer, the distance from pendant hydroxy group tothe quaternary carbon in HEMO is much longerthan that in HMO or HEO. As for either HMO orHEO, the distances from the quaternary carbonto two nucleophilic sites are comparably equiva-lent. On the contrary, these distances are signifi-cantly varied in HEMO so that the much morecomplicated branched molecular structures wereobserved in PHEMO. As shown in Schemes 8–10,there exist ten types of the microstructure unitsassigned to four terminal, four linear, and twobranched quaternary carbons, respectively. Ow-ing to the much longer distance between thehydroxy group and the quaternary carbon inHEMO, whether the hydroxy group reacts withthe oxonium ion or not presumably has little or noinfluence on the chemical shift of the quaternarycarbon. As a result, it was supposed that Type 1and 2 of the macromolecular structures have thesame chemical shifts with Type 7 and 8, whereasType 5 and 6 have the same absorption with Type9 and 10, respectively. After the esterification ofPHEMO with acetic anhydride, there still remainten types of the quaternary carbons correspond-ing to six kinds of separated resonance peaks.Their assignment is marked in Figure 8.

Similar to the hyperbranched polymer of HMOor HEO, the DB of PHEMO can be calculated ifT9 þ T10 are measured accurately:

Figure 8. 13C NMR spectrum of PHEMO after esterified with acetic anhydride.

Scheme 8. The terminal structure of PHEMO.



DB% ¼ ðT9 þ T10Þ=X

Ti ð2Þ

In fact, this requirement is too difficult to meetin the present study. For Type 5 and 6, as thehydroxy group here actually does not participatein the chain propagation, the two structures areconsidered to be linear units. For these molecu-lar structures, however, the length of the sidechain connecting the hydroxy group with thequaternary carbon is close to the length of mainchain. As an expedience, Type 5 and 6 are also

ascribed to the branched units in this study. Inview of this consideration, the DB of PHEMOwas calculated by eq 3, and the correspondingresults are listed in Table 1.

DB% ¼ ðT5 þ T6 þ T9 þ T10Þ=X

Ti ð3Þ

It was reported by Yan and coworkers7 that thehigh feed ratio of monomer to initiator resultedin a mainly linear or slightly branched poly-ether, and the low one gave rise to a more hyper-

Scheme 9. The linear structure of PHEMO.

3662 LIN ET AL.


branched structure. As can be seen in Table 1,when the monomer to initiator ratio changesfrom 2.5 to 25, the calculated DB goes downfrom 29 to 16%. However, these DB values ob-tained from eq 3 seemed to be higher than thetrue values of DB defined by eq 2. So, a furtherstudy should be done for calculating the DBmore exactly.

Thermal Analysis

DSC thermograms of the hyperbranched polyetherPHEMO are shown in Figure 9(a). As depicted inFigure 9(a), no crystalline melting peak was foundand it implied that the resulting hyperbranchedpolymer is perfectly amorphous. The Tg of PHEMOwas determined to be �47.6 8C, indicating theexcellent segment motion performance potentiallybeneficial to ion conductivity.

Figure 9(b) presents the TGA analytical curveof PHEMO after adding with LiTFSI. It wasnoted that this hyperbranched polymer-basedelectrolyte starts to lose the weight at about280 8C (10% loss), and the process completes over450 8C. This revealed that the weight-losing tem-perature of this polymer electrolyte is substan-tially higher than that of liquid electrolytes cur-rently used in lithium ion battery.27 Comparedwith those liquid electrolytes, the weight-losingprocess of the resultant polymer electrolyte isgradual instead of abrupt. As a result, if thishyperbranched polymer is used as a polymer elec-trolyte in the lithium ion battery, it is expected topossess the higher thermostability.

Ion Conductivity

LiTFSI was used to dope the resulting hyper-branched polyether in the present work, which

Scheme 10. The branched structure of PHEMO.

Figure 9. DSC and TGA curves of PHEMO.



had shown to be most beneficial for the ion con-ductivity of the polymer electrolyte in our previ-ous work.21,22 For the measurement, all the hy-droxy end group of the polymer needed to end-capusing acetic anhydride before adding LiTFSI toyield a polymer electrolyte. Figure 10 profilesthe ion conductivity changes at different lithiumsalt concentration versus the reciprocal of tem-perature. The measured data increase with theincrease of temperature as the segment motionperformance of the polymer is progressivelyenhanced. It was found that the ion conductivityreaches 5.6 � 10�5 S/cm at room temperature and6.3 � 10�4 S/cm at 80 8C, respectively. Such val-ues are much higher than that of both the PEO-based system and the hyperbrached polyesterelectrolyte reported by Itoh and coworkers.19,20

Moreover, the polymer electrolyte after dopedwith a higher lithium salt concentration did notpromote the ion conductivity further as shown inFigure 10. The too higher concentration of lith-ium salts possibly damaged the segment motionperformance because of the transient interactionbetween lithium ions and ether oxygen on the poly-ether backbones leading to the decreasing ionconductivity. This phenomenon was also observedelsewhere.21

CONCLUSIONS

A novel hyperbranched polyether was synthe-sized from the oxetane-derived monomer through

CROP with boron trifluoride etherate as initiator.The hyperbranched structure is constructed bythe competition between two chain propagationmechanisms, ACE and AM, respectively. As dem-onstrated by MALDI-TOF analysis, the chainpropagation is terminated by so-called back-bit-ing process to give rise to cyclic fragment. The ionconductivity of the resulting hyperbranched poly-mer reaches 5.6 � 10�5 S/cm at room temperatureand 6.3 � 10�4 S/cm at 80 8C after doping withLiTFSI at a ratio of Li:O ¼ 0.05, respectively. It ispromising for PHEMO to be used as solid polymerelectrolyte matrix in rechargeable lithium ionbattery.

This research work is funded by National Key Proj-ects on Basic Research and Development (‘‘973’’ Pro-gram, Grant Number 2002CB211800).

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