Structure Analysis of Polyacrylonitrile Polymerized in Ionic Liquids

164Joumal of Donghue University (Eng. Ed.) Vol. 25, No.2 (2008)Structure Analysis of Polyacrylonitrile Polymerized in Ionic LiquidsZHANG Hong-yan (张红燕)，ZHANG Yu-mei (张玉梅)，YANG Ling-ling (杨玲玲), WANG Hua-ping(王华平)College of Material Science and Engineering ,Donghua University, Shanghai 201620, ChinaAbstract: Polyacrylonitriles (PANS) were syonthesized both bywere applied to control the polymerization of acrylonitrileatom transfer radical polymerization (ATRP) and free radial(AN) , such as anionic polymerization, which often involvepolymerization in ionic liquid 1- buty- 3 - methylimidazoliumcomplex catalysis or side reactions with the nitrilechloride ( [bmim]Cl). [bmim]Al demonstrates to be agroupsia]. Living/controlled radical polymerization can bepreferable solvent for ATRP of acrylonitrile (AN). Theused as an alternative such as atom transfer radicalpolymerization maintains the usual advantages of ATRPpolymerization (ATRP)，which involves a fast dynamicwith molecular weight agrees well with theoretical value andequilibrium between dormant species and active radicallow polydispersity (PD[= 1.15). It is also shown the higherspecies to provide controF4t. PAN with predefinedconversion and lower molecular weight dispersion in ionicmolecular weights and narrow polydispersity has beenliquid than in dimethylformamide (DMF). From FTIR andsccessfully synthesized by the use of ATRPC-0.NMR analysis, it is confirmed that the chemical structuresHowever, the solvents used are all organic solvents such asof PANs synthesized in [bmim' ]CI were identical with thatethylene carbonate ( EC), N， N-dimethylformamideobtained in DMF. In atom transfer radical polymerizatio,(DMF). They are volatilizable which can slow thethe methine and cyan carbon atoms in isotactic configurationpolymerization rate dramatically and the catalyst cannot befor PAN produced in [bnim] Cl have a configurationremoved from the polymer efficiently, which leads toconsisting of about 55.5% isotactic diads. It is higher thancontamination of products. Moreover, the removedthat obtained in DMF which is 52.2%. So, ionic liquid hascatalysts also cannot be recycled easily for cost-saving.effet on the stereostructure of PANs. Further analysis ofIonic liquids (ILs), known as green solvents, have been”C NMR spectra indicated that the isotacticity of PANexplored asreaction media in variousorganicsynthesized by free radical polymerization was lower thantransformations. The unique properties exhibited by ILsthat of PAN prepared by ATRP, although both of them werecan lead to significant improvements in the rate, yield, andrandom in stereoregularity. Besides the pentad tacticities ofselectivity of reacionsl1. Furthermore, it was found thatPANs also suggested that the sequence distributions of themthe ILs used can lead to rapid reaction rates and highall obey Bernoulli statstics.Keywords: polyacrylonitrile; ionic liquid; atom transfermolecular weights for polymeriztion[12. Severalliteratures have reported that atom transfer radicalradical polymerization; structurepolymerization with ionic liquids as medium allows easy andCLC number: O 63Document code; Aefficient separation of catalyst from the polyme11. TheArticle ID: 1672 - 5220(2008)02 - 0164 - 06separated catalyst can also be recycled for potentialreuel5s.In this paper, it is the first time to report theIntroductionpolymerization of PAN in ionic liquids by ATRP. ThePolyacrylonitrile (PAN) is a commerially importanteffect of solvents and polymerization process on thepolymer because of its unique and well-known propertiesmolecular weight, chemical structure and stereoregularincluding hardness and rigidity， chemical resistance,structure were investigated in detail.compatibility with certain polar substances, low gaspermeabilityL) and reactive ability for the nove1 Experimentalfunctionality of aromatic tetrazole2]. PAN is usuallyprepared by radical polymerization without control over1.1 Materialsmolecular dimension and structure, while other methodsCuBr (98%), purchased from Acros Organics, was中国煤化工Received date: 2007 -09 - 18Supported by Science and Technology Commissio of Shanghai MunicipaliMYHC N M H Ghai Mumicipal EducationCommission (No. DAWN Project) fundCorrespondence should be addressed to ZHANG Yu-mei, associate professor, E-mail; zhangym@dhu. edu. cnJoumal of Donghua University (Eng. Ed.) Vol. 25, No.2 (2008)165purified by washing with glacial acetic acid, ethanol andFTIR curves were obtained using Nicolet NEXUS- 670diethyl ether successivelya6]. Ethyl 2 - bromiopropionatespectrometer with the distinguishing rate of 0. 09 cm~1.(EPNBr) and pentamethyl-diethylenctriamine ( PMDETA )The polymer was blended with KBr to prepare the samples.were purchased from Aldrich and distilled prior to use. AN"C NMR spectra of the polymer were recorded on a(AR，99%) was purchased from Acros Organics andBruker ARX - 400 spectrometer at room temperature. Thedistiled to remove inhibitors by reducing pressure. ILs, 1-sample concentration was about 5wt % in DMSO- d.butyl - 3 - methylimidazolium chloride ([bmim]Cl) wassynthesized in our laboratory and purified according to the2 Results and Discussionliteratureln. AIBN (AR, 97%) was rerystallized frommetbanol and dried under vacuum, Methanol and N, N-2.1 Polymerization of PANsdimethylformanide (DMF) (AR,> 99. 5%) were usedPolymerization of AN was carried out by ATRP inwithout further purification.[bmim]Cl and DMF. Parallel experiment was done by free1.2 Polymerizationradical polymerization in [bmim]Cl. Polymerization1.2.1 Atom transfer radical polymerizationconditions and the resulted conversion, molecular weightThe polymerization of AN in Cbmim]Cl or N, Nand molecular weight distribution for the three. PANdimethylformanide was carried out using the EPN-Br/samples (1- 3) are summarized in Table 1.CuBr/PMDETA initiating system under the condition ofTable 1 Polymerization data for Acrylonitrile[AN]o/[EPN-Br]o/[CuBr]o/ [PMDETAJo = 100/1/1/2 inin different conditionsnitrogenat 65 c for 3 h. The raw product was precipitatedwith a large excess of a 50 % aquecous methanol solution. ThenSamples SolventPolymerization methods Temperature/Cit was fitered, washed by 50% aqueous methanol for severalPAN1ATRP65times and dried under high vacum to oonstant weight.PAN2 [bmnim]Cl551.2.2 Conventional free radical polymerizationPAN3 Cbmim]alRPSolution of monomer (10 wt %) in [bmim]Cl waspolymerized with AIBN(1wt%, based on the monomer) asSamplesConversion/%M，Mq,a" PDIinitiator under nitrogenat 65 C for 3 h. The raw productwas purified by washing with a large excess of a 50 %PANI31. 4814791 8531.45. PAN264.263 592aqueous methanol solution to remove residual IL andPAN345.7814 450 .2.32monomer. Then it was filtered and dried under highvacuum to constant weight.●RP: conventional fre radical polymerization1.3 Characterization"* Mau = (CAN]o/[ENBr]o) x M(AN) x Coreson+ M,(EPN-Br)The conversion of the monomer was determinedTable 1 shows considerable differences in moleculargravimetrically and calculated from the following equation;weight and polydispersities for polymers synthesized indifferent solvents and different polymerization processes.Conversion(%)= w./W x 10%1)In case of ATRP， the molecular weight of polymerwhere Wp is the weight of the obtained polymer, Wo is thepolymerized in [bmim]Cl is much higher than that inDMF. And the polydispersity (Mw/Mo) of polymer intotal weight of the corresponding polymerization solution and[bmim]Cl are lower than that in DMF. The results areW is the initial concentration of monomer in the solution.The viscosity-average molecular weight (M,) of ANaccordant with Strehmel's report [19] . The higher viscositypolymers was determined by the intrinsic viscosityof [bmim]Cl when compared with DMF presumably ismcasurements. The intrinsic viscosity [切] of AN polymersregarded as one reason for this phenomenon (at 30C, thewas measured at 30C in DMF using an Ubbelohdeviscosities of [bmim]Cl and DMF are 1100 cP^20] andviscometerl8], and the molecular weight was calculated0. 802 cPC20] respectively). Because of the high viscositypolymerization medium, both chain terminations and/orfrom the following equationchain transfers are diminished in viscous system. Compared[]=3.92X10-4M70.s(2)to the polymer synthesized by the free radicalPolydispersity ( Mw/M。) was determined with apolymerization. the Dolvdispersitv of polymer prepared byWaters gel permeation chromatograph (GPC, model 150C)中国煤化工and the molecularusing DMF as the eluent. The experiments were carried outweightC N M H Chcoretical molclarat 40C，with an injection volume of 200 mL and aweight m山al conversion ul 04. co to. It maintains thepumping rate of 1 mL/min. The Column calibrations wereusual●advantages of ATRP leading to well controlledperformed with polystyrene standards.molecular weights and low polydispersities.166Jouma/ of Donghua University (Eng. Ed.) Vol. 25, No.2 (2008)2. 2 Chemical structure of PANclearly demonstrates that ILs and the polymerizationThe chemical structures of the obtained polymers weremethods have no obvious effect on the chemical shifts ofanalyzed by FTIR and "C NMR. Figure 1 shows the FTIRcarbon atom in PaN.spectrums for polymer samples. All the spectrums have the2.3Stereoregularity analysissame bands which indicate that the obtained polymer productsIn addition to the effect on molocular weight andhave the same structure. The FTIR can be identified from thepolydispersity, the higher vsooity and iooic structure of ILsmain absorption peaks as shown in Table 2. The band atwould lead to special interaction between ILs and2 956 cm~1 corresponds to the stretching vibration of themacromolecules, which will influence the steric regularity of theCHs, CH and CH group, the band at 2 243 cm-1 to themacromolecular chain. So the stereostructure of PANs obtainedstretching vibration of the CN group, the band atin [bmnim]a will be discassed in the following section.1 453 cm-1 to the scissoring vibration of the CH group andEnd group of polymers by NMR spectroscopy allowsthe band at 1 077 cm~1 to the skeletal vibration of thefor the direct estimation of the relative concentration ofC- C stretch. It confirms that the chemical structure ofthe different isomers in a mixture and is very sensitive topolymers synthesized under the conditions in Table 1 is thedetails of polymer structure such as end-group, branches,same to that of acrylontrile homopolymer (PAN).head-to-bead and monometer sequencest21,2}. So thestereoregularity of PAN1 - 3 can be analyzed by thWVPAN3expanded "C NMR spectrums of methine (- CH) andcyano (CN) as sbown in Fig.3 and Fig. 4 respectively.The methane carbon gives rise to three well-resolved peaks厂mrr PANcentered at 30. 14 due to triad chemical shift sensitivity.The signals are asigned to isotactic (mm), atactic (mr)and syndiotactic (π) with decreasing magnetic field. Theapproximate triad tacticity content of the stercoregularitycan be obtained from the ratio of integrated intensities of3500 3000 2500 2000 1500 1000 500the three signals in Fig. 3. From the triad content we canWavenumber/ cm~calculate the diad tacticity content for PAN samples. ThFig.1 FTIR spectrum of PAN samplesresults are showm in Table 3. From Table 3, it is shown thatTable2 Analysis for FTIR spectrum of PAN samplePAN synthesized by ATRP has higher isotactic configurationthan that prepared by cooventional free radical polymerizationNo. Wavenumber/cm-1Corresponding groupsand the high viscosity and polarity of ILs can partially2 956C-H of CHs, CH2, CHinfluence the sterco structure of PMMA, which leads to lessstretching vibrationformation of syndiotactic structure.2243C=N stretching vibration1453CH2 sissoring vibration1 077C-C skeletal vibrationm“C NMR analysis of the polymer samples does provideadditional evidence as to the structure of PAN (Fig. 2).The peaks at 122. 84, 35. 40 and 30. 13 correspond to thechemical shifts of CN, - CH2 and - CH- respectively. It3332~302928ppmDMSO-d。(@) PANImmCH2 CHPANICN-PAN2中国煤化工MHCNMHGmww .200500 ppm3 3232928 ppm8Fig.2 "C NMR spectrum of PAN samples(b) PAN2Joumal of Donghua University (Eng. Ed.) Vol.25, No.2 (2008) 167mrmmVmmwWlwwh124.0 123.5 123.0 122.5 122.0 121.5 ppm029 28 ppm(b) PAN2(c) PAN3Fig.3 Expanded uC NMR spectrum of CH region of PANmorTable3 Diads and triads tacticity calculated basedmmmon "C NMR peak of CH regionmrmDiad tacticity/%*Triad tacticity/%SamplesmPAN152.247.27.948.623.5PAN255.544.531.448.1 20.449.450.724.6.49.525.98"Derhad from tiad acictis (m) = +1/2 (mr);(r) =n +1/2 (m)The stereoregularity of PAn is analyzed further fromthe pentads distribution of cyano carbon atom inFig.4 Expanded "C NMR spectrumI*C NMR spettrum. The splitting peak is much moreof CN region of PANcomplicated (shown in Fig. 4). There are ten stereoTable 4 Experimental and calculatedsequences corresponding to the isotactic ( mmmm ),pentads tacticity of Pansyndiotactic (rrr), and atactic structure. It also revealsthat the isotactic structure ( mmmm) increased in PanPAN3polymerized by ATRP in [ bmim ]CI, although all Pan}FMEBFMsamples are predominantly syndiotactic. Therefore,compared with the conventional frec radicalmmmm 0.084 0.089 0.019 0. 116 0.141 0.033 0.078 0.095 0.078polymerization, it appears that in the propagation ommmr 0.126 0.148 0.062 0.188 0.178 0.075 0.133 0.152 0. 154PAN by ATRP mechanism, more stereochemical controlrmmr 0.089 0.061 0.047 0.072 0.056 0.034 0.094 0.061 0.075is operative, and a higher isotactic polymer is obtained.morm 0.147 0.148 0.115 0.104 0.178 0.108 0.148 0.152 0. 188Further calculation data based on the pentad sequencesmorr 0.241 0.123 0.100 0.243 0.113 0.124 0.237 0.122 0.095and statistic models are shown in Table 4. The isotacticmrm 0.136 0.123 0.187 0.115 0.113 0.123 0.135 0.122 0. 185content of PAN1 and PAN2 further confirmed that thehigh viscosity and polarity of [ bmim]Cl have effect onmrr 0.050 0.102 0.152 0.037 0.071 0.113 0.042 0.098 0.093mrrm 0.082 0.061 0.092 0.070 0.056 0.090 0.083 0.061 0. 058the stereo structure of PAN.mrrr 0.037 0.102 0.160 0.048 0.071 0.206 0.039 0.098 0.059rrr 0.009 0.042 0.065 0.008 0.022 0.094 0.011 0.039 0.015mmmr mm-0.50一’一0.761一--0.51-- 0.016 0.025 一0.016 0.0270.015 0.017mmmnE: experimental data; B: Bernulli model; FM: first order Markov modelim中国煤化工-ans the discrepancywMubetwenes and the statisticmodel:YHCNMHG124.0 123.5 123.0 122.25 122.0 121.5 ppmn(a) PANI8= {它x.-Xx)2}"/N(3)168Joumal of Donghua University (Eng. Ed.) Vol. 25, No.2 (2008)where Xi.an and Xi,ak are one of the theory sequencesFunctional Polymers, 2004, 59 (1); 53- 61.and calculated sequences respectively; N is the number of[3] Ono H, Hisatani K, Kamide K. 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