REGIOSELECTIVITY IN THE ACYLATION OF CELLULOSE WITH 2-BROMOISOBUTYRYL BROMIDE UNDER HOMOGENEOUS CONDITIONS

Transcription

1 CELLULSE CHEMISTRY AND TECHNLGY REGISELECTIVITY IN THE ACYLATIN F CELLULSE WITH 2-BRMISBUTYRYL BRMIDE UNDER HMGENEUS CNDITINS HNGLE WANG, * YINGJUAN FU, * ZEFENG WANG, ** ZHIYNG SHA * nd MENGHUA QIN *,*** * Key Lortory of Pulp nd Pper Science nd Technology of the Ministry of Eduction, Qilu University of Technology, Jinn , Shndong, Chin ** Shndong Ppermking Industry Reserch nd Design Institute, Jinn , Shndong, Chin *** rgnic Chemistry Lortory, Tishn University, Tin , Shndong, Chin Corresponding uthors: Yingjun Fu, fyingjun@163.com Menghu Qin, qmh@qlu.edu.cn Received April 22, 2015 A cellulose-sed mcroinititor, cellulose-bib, ws prepred homogeneously y cyltion of the hydroxyl groups of cotton cellulose with 2-romoisoutyryl romide (BrBiB) in N,N-dimethyl cetmide/lithium chloride (DMAc/LiCl) system. The effects of the molr rtio of BrBiB/nhydroglucose unit (AGU), rection temperture, nd rection time on the degree of sustitution () were investigted in detil. The investigtion ws focused on the rectivity of the three hydroxyl groups t different position during the prtil homogeneous cyltion process. The results reveled tht the of the cellulose-bib incresed with the increse of the molr rtio of BrBiB/AGU nd the rection time. At equivlent ccessiility, the homogeneous cyltion fvored sustitution t the C6 position, showing regioselectivity of C6-H>>C3-H>C2-H for the cotton dissolving pulp. Moreover, the 2-romoisoutyryl romide groups could e regioselectively introduced into the AGU of cellulose y controlling the rection time. Keywords: cellulose-sed mcroinititor, DMAc/LiCl, homogeneous cyltion, 2-romoisoutyryl romide, regioselectivity INTRDUCTIN Cellulose is the most undnt nturl resource in the world nd possesses iocomptiility, iodegrdility, renewility, nd the ility to e modified esily. 1 It hs een considered n idel mteril tht cn replce trditionl non-renewle resources, such s col, fossil oil, nd nturl gs. However, cellulose lso hs certin drwcks, 2 which prevent its wider scle ppliction in vrious fields. Chemicl modifiction of cellulose y etherifiction, esterifiction, or grft polymeriztion llows voiding its intrinsic shortcomings nd endows it with more flexile properties, opening the wy to mny dvnced pplictions. 3 Nowdys, cellulose-sed mterils hve een widely used in food, ioengineering, coting, ppermking, nd other industry res. 4-6 Modifictions y grft polymeriztion provide mens of ltering the physicl nd chemicl properties of cellulose nd incresing its functionlity. 7 Specific molecules re usully grfted onto the cellulose ckone to imprt specific properties onto the cellulose sustrte, without destroying its intrinsic properties. 8 The free rdicl grft copolymeriztion consisting of initition, propgtion nd termintion is the min method to prepre cellulose grft copolymers. However, the trditionl free rdicl grft copolymeriztion hs defect of poor control on the synthetic products, such s moleculr weight nd the polydispersity of the grfted chin. 1 With the development of polymer synthesis chemistry, controlled/ living rdicl polymeriztion Cellulose Chem. Technol., 51 (1-2), 11-21(2017)

2 HNGLE WANG et l. methods hve een put forwrd, which re le to minimize chin trnsfer nd to control the moleculr weight nd polydispersity. 9 Atom trnsfer rdicl polymeriztion (ATRP) is one of the most convenient controlled/ living rdicl polymeriztion methods to prepre well-defined polymers. 10 A lot of investigtions hve een crried out on using ATRP polymeriztion methods to prepre cellulose grft copolymers with chemiclly different grfts, s well s tunle lengths nd densities of grfts. 4,5,10,11 However, in order to prepre cellulose grft copolymers y ATRP, the cellulose or cellulose derivtives must e modified into mcroinititors through ttchment of lkyl romide (or chloride) groups long the cellulose chins. 5,11 The physicochemicl properties of cellulose-sed grft copolymers, such s viscosity, soluility, floccultion ehvior, therml property nd so on, strongly depend on the grfting density of the copolymers, the moleculr prmeters of grfts, 12 nd lso the distriution of the grfts long the cellulose chin. 13 Therefore, the synthesis of cellulose-sed mcroinititors with predetermined numer of initition sites nd different regioselectivity is very importnt for prepring cellulose grft copolymers with well-defined structure y ATRP. Theoreticlly, com-shped grft copolymers cn e chieved y using the cellulose-sed mcroinititor regioselectively sustituted t C6-H s the ATRP inititor sites. If the cellulose-sed mcroinititor is regioselectively sustituted t oth C6-H nd C3-H or t oth C6-H nd C2-H, symmetric centipede-like copolymers cn e otined. Ifuku nd Kdl 10 prepred highly regioselective cellulose mcroinititor (6--romoisoutyryl-2,3-di--methyl cellulose) for the first time, nd then the regioselective copolymeriztion of N-isopropylcrylmide (NIPAM) onto cellulose ws successfully chieved y ATRP using the regioselective C6 cellulosic mcroinititor. Moreover, the synthesis of prticulr regioselectively sustituted cellulose-sed mcroinititor is of criticl importnce for the synthesis of cellulose-sed grft copolymers with superior properties for vrious pplictions. Initition groups in the cellulose-sed mcroinititors re typiclly ttched through cyltion of cellulose hydroxyl groups y suitle orgnic cid derivtives. 12 Due to the lower steric hindrnce of the primry H t C6 nd the greter cidity of the H group t C2, 14 the hydroxyl groups of nhydroglucose unit (AGU) t C2, C3, nd C6 position exhiit different rection ctivities. The rection medium lso hs n importnt effect on the distriution of the sustituent. In solid-phse cetyltion, no regio-selective rectivity ws oserved y Ymmoto et l. 15 mong the three kinds of H groups. The heterogeneous synthesis of cellulose cette in methnol hd prtil order of C3>C2>C6. 16 Under homogeneous conditions, the modifiction of cellulose is much more uniform nd the degree of sustitution nd the sustitution pttern cn e etter controlled. A kind of well-solule cteril cellulose derivtives prepred under homogeneous nd mild rection conditions showed distriution of the cetyl moieties in the order -6 > -3 > Homogeneous cetyltion of cellulose with cetic nhydride t reltively high concentrtion in ionic liquid showed rectivity order of C6-H>C3-H>C2-H. 18 However, different prtil sustitution of C6-H>C2-H>C3-H occurred in the homogeneous cyltion of cellulose with enzoyl chloride in ionic liquid 1-llyl-3-methylimidzolium chloride (AmimCl). 19 Xu et l. investigted the homogeneous cyltions of cellulose with ulky cid chlorides, such s pivloyl chloride nd dmntoyl chloride in DMAc/LiCl nd demonstrted rectivity order of C6-H>C2-H>C3-H. 14 Therefore, it is vitl to choose pproprite rection conditions to synthetize cellulose-sed mcroinititors of certin distriution of sustituent. The 2-romoisoutyryl romide (BrBiB) is the most importnt cyltion regent pplied to prepre cellulose-sed mcroinititors for ATRP. 1,4,5,20,21 However, lthough much reserch hs een devoted to the cyltion of cellulose with BrBiB, 5,6,11 little efforts hve een mde towrds the regioselectivity of the cyltion rection. In the present pper, the hydroxyl groups of the cellulose were treted with BrBiB in DMAc/LiCl to synthesize the cellulose-sed mcroinititors, cellulose-bib. Also, n ttempt ws mde to clrify the rectivity difference of the three hydroxyl groups, C2-H, C3-H, nd C6-H during homogeneous cyltion of cellulose. 12

3 Cellulose EXPERIMENTAL Mterils The cellulose mteril used for the synthesis of the cellulose-sed mcroinititor ws cotton dissolving pulp with degree of polymeriztion (DP) of 500. It ws otined from Shndong Silver Hwk Chemicl Fire Co., Ltd. nd ws dried in vcuum drying oven for 24 h efore use. Lithium chloride (LiCl) ws purchsed from Sigm-Aldrich (Shnghi) Trding Co., Ltd. 2-romoisoutyloyl romide (BrBiB) cquired from Shnghi Bngcheng Chemicl Co., Ltd. ws used without further purifiction. N,N-dimethyl cetmide (DMAc), N,N-dimethyl formmide (DMF), nd triethylmine (TEA) were ll of nlyticl grde. Nitrogen ws otined from Jinn Deyng Specil Gs Co., Ltd. Methods Dissolution of cellulose in DMAc/LiCl An mount of 0.68 g cellulose ws weighed nd put into 250 ml three-necked round-ottom flsk, nd then 100 ml of DMAc ws dded to it. The mixture ws heted t 160 ºC for 0.5 h under mgnetic stirring. Then, the mixture ws filtrted nd the residue ws dried in vcuum drying oven t 60 ºC overnight to otin ctivted cellulose. An mount of 20 g DMAc ws dded into 250 ml three-necked round-ottom flsk, nd then 2 g LiCl ws dissolved completely in the DMAc to otin the DMAc/LiCl solution. The ctivted cellulose ws dded into the DMAc/LiCl solution nd ws heted t 100 ºC for 4 h. The cellulose/dmac/licl solution ws llowed to sty t room temperture for 24 h to otin cler, slightly viscous solution. Acyltion of cellulose with BrBiB in DMAc/LiCl The cellulose solution otined ove ws cooled down to 0 ºC using n ice-wter th. A defined mount of BrBiB ws slowly dropped into the solution vi constnt pressure drop funnel under nitrogen tmosphere. Then the triethylmine with the sme mole s BrBiB ws dded into the mixture. After the flsk ws seled, the rection mixture ws heted to predetermined temperture in n oil th nd llowed to proceed for scheduled time under mgnetic stirring. Then the resulting mixture ws slowly poured into deionized wter with vigorous stirring. The precipitted cyltion product, nmely cellulose-bib, ws redissolved in DMF. After re-precipitting in deionized wter nd re-dissolving in DMF for three times, the cellulose-bib ws dilyzed for three dys in deionized wter nd ws dried in vcuum freeze-drying oven for 48 h. X-ry diffrction nlysis The X-ry diffrction profiles of ntive cellulose, regenerted cellulose, nd cellulose-bib were recorded using n X-ry diffrctometer (D8-Advnce, Germny Bruker AXS Corportion). 22 Ni-filtered Cu Kα rdition (λ = nm) generted t voltge of 40 kv nd current of 40 ma ws utilised, s well s scn speed of 1-2 /min from 5 to 50. FT IR nlysis FT IR spectr of ntive cellulose, regenerted cellulose, nd cellulose-bib were recorded using n IR Prestige-21 FT IR spectrophotometer. The smples were pressed into pellets with KBr efore mesuring. The spectr were collected t resolution of 2 cm -1, in the rnge of 500 nd 4,000 cm H NMR nd 13 C NMR nlyses 1 H NMR spectr of the cellulose-bib were recorded on n AVANCEⅡ400 spectrometer t room temperture in DMS-d6. The sustitution degree () vlue of the cellulose-bib 1 ws clculted from the integrtion rtios of proton resonnces of the methyl of the BrBiB to those of the ckone hydrogens (Eq. (1)) ccording to the method of Goodlett et l.: 23 7 S 6 S = 1 (1) 2 where S 1 is the integrl of the resonnces in the rnge of ppm of the protons of the methyl of the BrBiB, S 2 is the integrl of the resonnces in the rnge of ppm of the protons of the cellulose ckone. The wter pek t 3.3 ppm ws sutrcted mnully y integrtion of the respective pek. 24 The 13 C NMR spectrum of the cellulose-bib ws recorded on the AVANCEⅡ400 spectrometer with minimum of 5000 scns, smple concentrtion of 40 g L -1, sweep width of 80 KHz, nd dely time of 3 s in DMS-d6. The distriution of 2-romoisoutyryl moiety mong the three hydroxyl groups of the AGU ws determined ccording to the method of Kmide nd kjim. 25 The C6, C3, nd C2 of the cellulose-bib were clculted using the following formuls: C6 Totl SC = S S S 6 C3 Totl C 2 C 2 = C 6 C 3 C 2 = S S Totl C3 S = S + S + S (2) where C6, C3, nd C2 re the of corresponding cron toms, Totl is the of the cellulose-bib clculted vi 1 H NMR spectroscopy, nd S C6, S C3, nd S C3 re the integrl of the resonnces of the pek t ppm, ppm, nd ppm, respectively. Thermogrvimetric nlysis A thermogrvimetric nlyzer (TGA Q50, TA Instruments, USA) ws employed for the 13

4 HNGLE WANG et l. thermogrvimetric nlysis of ntive cellulose, regenerted cellulose, nd cellulose-bib. The smples were heted from room temperture to 600 ºC t heting rte of 10 ºC/min under nitrogen tmosphere. The TA universl nlysis softwre ws used for deling with the TG curves. The smples were ll dried t room temperture for 24 h in vcuum drying oven prior to TGA mesurements. RESULTS AND DISCUSSIN The cellulose-sed mcroinititor, cellulose-bib, cn e synthesized y the rection of the hydroxyl groups of cellulose with BrBiB. 1 The rection scheme is shown in Figure 1. Due to the extensive intr- nd inter- moleculr hydrogen onding, crystllinity, nd resulting poor soluility of the dissolving pulp, 26 it is necessry to use powerful solvent system to chieve high level of sustitution nd crete the possiility of fr more selective rection of cellulose. 27 DMAc/LiCl, DMS/TBAF, nd ionic liquid re three importnt clsses of cellulose rection solvents for homogeneous modifiction. 28 The mixed solvent DMAc with LiCl hs een used s n efficient cellulose solvent for over qurter century to dissolve, derivtize nd nlyze the cellulose. 29 In this work, we chose the DMAc/LiCl system, which is true solvent for cellulose to ensure tht the cellulosic hydroxyl groups re redily ccessile. It is necessry to mention tht the ggressive hydrochloric cid s y-product of the cyltion rection will e generted when the hydroxyl groups of AGU rect with BrBiB. To promote the cyltion rection nd limit the cellulose cidic degrdtion, the cid-inding gent triethylmine ws dded to neutrlize HCl s it is formed. Dissolution of cellulose in DMAc/LiCl In order to dissolve the cotton dissolving pulp into the DMAc/LiCl system completely, pretretment (ctivtion) procedure is needed. The ctivtion tretments cn e performed in vrious wys, such s swelling nd solvent exchnge, enhncing the ccessiility nd the soluility of the pulp. Here, three ctivtion procedures re chosen nd their effects on the dissolution of the pulp re shown in Tle 1. As cn e seen, without ctivtion tretment, the cotton dissolving pulp cnnot dissolve in the DMAc/LiCl. H H H H H H H H H DMAc DMAc/LiCl DMAc/LiCl 160, 0.5h 100, 4h BiBBr H Br H H H 3 C Br H H Br H Figure 1: Rection scheme for the synthesis of cellulose-bib in DMAc/LiCl Tle 1 Effects of ctivtion tretments on the dissolution of cotton dissolving pulp in DMAc/LiCl Tretment method Crystlliztion Dissolution Trnsmittnce of Colour of index (%) concentrtion (%) solution (%) solution Untreted insolule - 75% EDA, 40 ºC, 4 h light yellow DMAc, 160 ºC, 0.5 h colourless DMAc+0.2%KMn 4, 160 ºC, 0.5 h yellowish-rown 14

5 Cellulose c d wvenumer(cm -1 ) Figure 2: FT-IR spectr of ntive cellulose (), regenerted cellulose otined fter dissolution in DMAc/LiCl (), cellulose-bib with of (c), nd cellulose-bib with of (d) Ntive cellulose Regenerted cellulose Cellulose-BiB θ( o ) Figure 3: XRD spectr of ntive cellulose, regenerted cellulose, nd cellulose-bib ( = 1.012) A pretretment with 75% ethylene dimine (EDA) t 40 ºC for 4 h or with DMAc t 160 ºC for 0.5 h could fcilitte susequent dissolution of the pulp. The swelling gents cn generlly penetrte into the highly ordered regions nd split the onds etween chins nd firils of cellulose, improving the soluility of the pulp. The chnges of the crystllinity index fter pretretment indicted lower destructivity of the DMAc ctivtion method thn tht of the EDA ctivtion. Moreover, the cellulose solution ctivted y EDA is light yellow nd the ctivtion procedure is not convenient. If 0.2% KMn 4 ws dded during the DMAc ctivtion tretment, the cellulose solution ecme yellowish-rown nd the dissolution efficiency did not improve, mening tht n oxidtion rection of cellulose hppened during the pretretment process. It cn e concluded tht pretreting cellulose in the solution of DMAc, which is convenient nd esy to operte, is the optimum ctivtion method for the cotton dissolving pulp. FT IR is used to determine the chnges of the structure of the cellulose regenerted fter dissolution in DMAc/LiCl solution. Figure 2 illustrtes the FT IR spectr of ntive cellulose () nd regenerted cellulose (). The sorption peks in the two spectr were quite similr, indicting similr structure of ntive cellulose nd regenerted cellulose. This mens tht the chemicl structure of cellulose hs not chnged during the dissolving process, suggesting tht DMAc/LiCl is direct solvent of cellulose nd no derivtiztion rection occurred. Figure 3 shows the X-ry diffrction curves of ntive cellulose, regenerted cellulose, nd cellulose-bib. The diffrction curve of ntive cellulose hd strong chrcteristic peks t round 15, 17, nd 22.5 corresponding to the (110), (11 0), nd (002) plnes of crystls, nd wek chrcteristic pek t out 34.7 corresponding to the (004) plne, which ws coincident with the structure of cellulose І. 30 After dissolution in DMAc/LiCl nd regenertion, the diffrction curve of regenerted cellulose only hd rod chrcteristic pek t round 19.8, 15

6 HNGLE WANG et l. eing typicl of the diffrction pttern of cellulose II. 31 The results reveled tht the crystl form of cellulose І in ntive cellulose hs een converted to cellulose II during the dissolution nd regenertion process. As shown in Figure 3, the degree of crystllinity of regenerted cellulose lmost fdes wy, mening tht most crystlline structure of cellulose hs completely een destroyed during the dissolving process of cellulose in DMAc/LiCl. This mens tht DMAc/LiCl cn provide the possiility to functionlize cellulose in homogeneous solution, nd tht reking intr- nd inter-moleculr hydrogen onds cn drmticlly improve the ccessiility of the cyltion regent with H of AGU susequently. Effects of rection conditions on of cellulose-bib Tle 2 shows the results of cellulose cyltion in the DMAc/LiCl solvent systems. As cn e seen from the tle, the vlues of the cellulose-bib depended on the rection conditions, including the molr equivlents of BrBiB per AGU, rection temperture, nd rection time. At rtio of 3.0 equivlents BrBiB per AGU, low of ws otined with rection time of 24 h t 35 ºC. If the rtio incresed to 4.0, 5.0, nd 6.0 equivlents BrBiB per AGU respectively, the s of 0.677, nd were chieved, respectively. This mens tht the of the cellulose-bib incresed with the molr rtio of BrBiB/AGU. The reson for the increse of the rection efficiency with the molr rtio ws tht the collision proility of BrBiB with H in AGU incresed with incresing the mount of BrBiB, resulting in higher of the cellulose-bib. However, when the molr rtio of BrBiB/AGU reched 8.0:1, the of the resulting cellulose-bib ws only 0.838, much lower thn tht of 6.0:1. The presence of the excess BrBiB incresed the collision proility of the moleculrs of BrBiB nd BrBiB, reducing the vilility of BrBiB with H of AGU. Therefore, in order to prepre the cellulose-sed mcroinititor with high vlue, the pproprite molr rtio of BrBiB/AGU is quite importnt. The influence of the rection temperture on the vlue of the cellulose-bib, pplying molr rtio of 6.0:1 (BrBiB/AGU) nd rection time of 24 h, is shown in Tle 2. The vlues of 0.846, nd were otined when the rection temperture ws 25 ºC, 35 ºC, nd 45 ºC, respectively. It mens tht improving the rection temperture properly could quicken the moleculr motion of rectnts nd ccelerte the cyltion rection rte, nd consequently increse the vlue of the cellulose-bib. However, when the rection temperture elevted to 45 ºC, the of the cellulose-bib decresed. Tle 2 Conditions nd results of the homogeneous rection of cellulose with BrBiB in DMAc/LiCl Smple Temperture (ºC) Time (h) Molr rtio S : S : S : S : S : S : S : S : S : S : S : S : S : S : S : S : Mol BrBiB per mol AGU Degree of sustitution determined y 1 H NMR spectroscopy 16

7 Cellulose The cyltion rection ws n exothermic rection nd ws more rpid t reltively low temperture. n the other hnd, some side effects might occur t higher temperture, leding to lower vlue. A reltively pproprite rection temperture is preferred to otin cellulose-bib with high vlue. The of the cellulose-sed mcroinititor versus rection time is lso illustrted in Tle 2. When the rection ws conducted t 35 ºC with molr rtio of 6.0:1 (BrBiB/AGU), the of the cellulose-bib incresed with the rection time, indicting tht the extent of cyltion incresed pprecily with the rection time. This increment could e due to the increse rte nd time of collision of BrBiB with cellulosic molecules. 32 However, the vlue of of cellulose-bib ws otined t 24 h, of t 36 h, nd t 48 h, showing declining trend. This my e due to the fct tht n incresing mount of HCl would e generted nd the concentrtion of cid-inding gent would ecome insufficient with n extended rection time, proly leding to the hydrolysis of frction of ester groups 29 nd decresing the of the resulted cellulose-bib. Chrcteriztion of cellulose-bib Figure 2 gives the FT IR spectr of the cellulose cetyltion products, the cellulose-bibs, with different (c nd d). Compred with the spectrum of the regenerted cellulose (), the intensity of peks t 3,477 cm -1 (H stretching) nd 1,374 cm -1 (H ending) of the spectr of the cellulose-bibs (c nd d) decresed oviously, nd tht the spectr of cellulose-bibs (c nd d) hd strong sorption nds t 1,748 cm -1 corresponded to the stretching virtion of C= in BiB, 1,6 indicting tht the BiB hs een grfted onto the cellulose ckone successfully fter the cyltion modifiction. The sorption nds t 1,198 cm -1 nd 1,173 cm -1 proved tht the cronyl groups in the cellulose-bibs existed y the form of ester groups. The nd t 1276 cm -1 is ssigned to the skeletl C-C stretching virtions in the C( ) 2 Br groups. 12 The introduction of the stretching virtion sorption pek of - ner 1,464 cm -1, which is the chrcteristic sorption pek of BiB, nd the stretching virtion sorption pek of C-Br t 761 cm -1 in the spectr of cellulose-bib further demonstrted the successful formtion of the cellulose-bib. 12 The structure of the cellulose-bibs is further confirmed y their 1 H NMR spectr, representtive one eing shown in Figure 4. The chemicl shift in the rnge of ppm (sign of ) is ttriuted to the protons of the AGU. The chemicl shift round ppm (sign of ) is scried to the methyl protons of romoisoutyryl group. 1,6 This further demonstrted tht the cyltion regent BrBiB hs een grfted onto the cellulose ckone successfully. Figure 5 (A) shows the full-rnge 13 C NMR spectrum of the cellulose-bib. The shift δ ppm (e) is ssigned to the signl of the cronyl cron region. The pek t ppm (1) is ssigned to C1 djcent to C2 ering n unsustituted hydroxyl group, nd the pek round 98.9 ppm (1 ), which elongs to C1 djcent to C2 ering sustituted hydroxyl group, is very wek. The pek t 78 ppm (4) is ttriuted to C4 djcent to C3 ering n unsustituted hydroxyl group, nd the pek round 76 ppm (4 ) is ttriuted to C4 djcent to C3 ering sustituted hydroxyl group. The resonnce peks of C2, C3 nd C5 hevily overlp s they give only strong cluster round ppm. 18 The chemicl shift t 64.5 ppm (6 ) is ssigned to C6 crons ering sustituted cetyl group. The pek round 60 ppm ssigned to the unmodified C6 33 is not oserved, indicting n lmost complete sustitution of the hydroxyl group t the C6 position. δ 65.9 ppm (c) nd δ 30 ppm () re the typicl chemicl shifts of quternry cron toms nd cron toms of methyl in the BiB, 1,12 respectively. There re not only the resonnce peks of cron toms in the cellulose, ut lso the resonnce peks of cron toms in BiB in the 13 C NMR spectrum, lso demonstrting the successful sustitution of H groups on the cellulose ckone y BrBiB. The cellulose-sed mcroinititor, cellulose-bib, ws solule in rnge of orgnic solvents, such s dimethylsulfoxide (DMS), DMF, N-methyl-2-pyrrolidone (NMP), nd tetrhydrofurn (THF), displying good soluility. The degree of crystllinity of cellulose-bib completely disppered fter the cyltion rection of cellulose with BrBiB shown in Figure 3, further indicting tht the suprmoleculr structure of cellulose is completely destroyed. Therefore, the cellulose-bib cn e used to produce cellulose-sed grft copolymers with well-defined structure in homogeneous systems 17

8 HNGLE WANG et l. y ATRP. Regioselectivity of homogeneous cyltion The regioselective synthesis of cellulose-sed mcroinititor is crucil to prepre cellulose-sed grft copolymers y ATRP. The rectivity differences etween the C6-, C3-, nd C2-H groups during cyltion of cellulose with BrBiB in DMAc/LiCl were evluted y the cronyl cron region of 13 C NMR. The expnded cronyl region spectr of cellulose-bib re shown in Figure 5 (B) nd (C). The sorption peks t ppm, ppm, nd ppm re ttriuted to the cronyl cron linked to C6, C3, nd C2, respectively. The prtil vlues of the romoisoutyryl moiety mong the three H groups re clculted from the integrtion of cronyl cron re of the 13 C NMR spectrum, nd the results re presented in Tle 3. H 2 * H Br H H Br H * n DMS H 3C ppm Figure 4: 1 H NMR spectrum of cellulose-bib ( = 0.795) in DMS-d6 (A) * Br e H 2 H 3 H 1 Br c H 3 C H n * DMS c e 1 1' 4 2,3,5 4' 6' ppm (B) (C) ppm ppm Figure 5: 13 C NMR spectrum of cellulose-bib with of (A), the cronyl cron region of cellulose-bib ((B), = nd (C), = 1.012) After the rection of cellulose with BrBiB for 0.5 h under the molr rtio of 6.0:1 of BrBiB/AGU nd the rection temperture of 35 ºC, the prtil t C6 of the resulted 18

9 Cellulose cellulose-bib (smple S 8, Totl = 0.391) ws equl to the Totl, reveling tht the BrBiB ws selectively introduced into the C6-H of the AGU. When the rection time extended to 1 h, the prtil t C6 of the resulted cellulose-bib, smple S 9 with the Totl equl to ws 0.452, while the t C3 ws 0.027, mening tht the C3-H egn to e sustituted. For the smple S 11 with the Totl of otined t 4 h, the prtil t C6, C3, nd C2 ws 0.640, 0.096, nd 0.032, respectively, reveling tht the C2-H ws lso sustituted. This trend indicted tht t the eginning of the cyltion, the rection occurred lmost exclusively t the C6-H. nly when the mjority of primry hydroxyl groups hve een sustituted, the C3-H nd C2-H egn to e sustituted y BrBiB. The sence of the peks round 60 ppm, ssigned to the unmodified primry hydroxyl crons in cellulose 33 in the 13 C NMR spectroscopy (Fig. 5 (A)) lso indicted tht the C6-H ws lmost completely sustituted. The sustitution distriution of romoisoutyryl groups long the cellulose ckone of the other smple in Tle 3 stisfied the sme order of P C6 >>P C3 >P C2. Most of the sustituents were found t the C6 position, wheres smll proportion of the secondry hydroxyls t C3 nd C2 were lso cylted. It my e concluded tht the cyltion rection occurs preferly t the primry thn t the secondry lcohols, nd the rection ctivity of the C2 hydroxyl groups is somewht lower thn tht of the C3 hydroxyl groups. The result ws similr to tht oserved y Regini et l. 34 BrBiB is voluminous regent molecule. The higher steric hindrnce of the ulky 2-romoisoutyryl sustituents mkes it esier for them to sustitute the primry H t C6 with lower steric hindrnce, ut more difficult to rech the secondry H groups with more steric hindrnce. Moreover, the gret mjority of the inititor groups situted t position C6 of the cellulose-sed mcroinititor will promote the synthesis of cellulose grft copolymer firly regioselectively y ATRP. Tle 3 Distriution of cyl groups mong C6, C3, nd C2 of cellulose-bib Smple Totl C6 P C6 (%) c C3 P C3 (%) c C2 P C2 (%) c S 8 (0.5 h) S 9 (1 h) S 10 (2 h) S 11 (4 h) S 12 (6 h) S 13 (8 h) S 14 (12 h) S 4 (24 h) S 15 (36 h) S 2 (4.0:1) Totl refers to the of cellulose-bib determined y 1 H NMR spectroscopy C6, C3 nd C2 refer to the of the corresponding cron tom c P C6, P C3, P C2 refer to the percentge of of the corresponding cron tom Tle 4 Thermostility of ntive cellulose, regenerted cellulose nd cellulose-bib Smple Tdi (ºC) Tdm (ºC) Tdf (ºC) Residul mss (wt%) Ntive cellulose Regenerted cellulose Cellulose-BiB T di, the initil decomposition temperture, is the temperture t which the decomposition rte results in significnt weight loss; T dm, the mximum decomposition temperture, is the temperture t which the highest decomposition 19

10 HNGLE WANG et l. rte is oserved for the corresponding pttern; T df, the finl decomposition tempertures, corresponds to mximl decomposition of the smple Weight residue (%) Regenerted cellulose Cellulose-BiB Ntive cellulose Temperture ( C) () Weight loss rtio (%/ C) Regenerted cellulose Cellulose-BiB Ntive cellulose Temperture ( C) Figure 6: Thermogrvimetry curves () nd derivtive thermogrvimetry curves () of ntive cellulose, regenerted cellulose, nd cellulose-bib () Therml nlysis of the cellulose-bib Figure 6 shows the thermogrvimetry (TG) nd derivtive thermogrvimetry (DTG) curves of the ntive cellulose, regenerted cellulose, nd cellulose-bib nd their dt of thermostility re summrized in Tle 4. The ntive cellulose strts to decompose t 276 ºC, wheres the regenerted cellulose egins to decompose t 183 ºC. The decomposition temperture with the lrgest weight loss rte is 391 ºC for the ntive cellulose nd 299 ºC for the regenerted cellulose. The initil decomposition temperture (Tdi), mximum decomposition tempertures (Tdm), nd finl decomposition tempertures (Tdf) of the regenerted cellulose ll significntly decresed compred with the ntive cellulose, implying tht the therml stility of cellulose mrkedly declined fter een ctivted nd dissolved. The reson of the decrement ws proly due to prtil hydrolysis nd degrdtion of the cellulose mcromolecule during dissolution, especilly during ctivtion of cellulose t high temperture. n the other hnd, the reking of intr- nd inter-moleculr hydrogen onds in the cellulose chin during the ctivting nd dissolving process gve the chins more moility under the therml condition. 18 The Tdi, Tdm, nd Tdf of the cellulose-bib re lso lower thn tht of the ntive cellulose, ut much higher thn tht of the regenerted cellulose. These trends of the decomposition temperture indicted tht the therml stility of the regenerted cellulose incresed due to the introduction of the romoisoutyryl group. However, the therml stility of cellulose-bib ws still lower thn tht of the ntive cellulose. This is ttriuted to the introduced romoisoutyryl groups reking the hydrogen onds in the cellulose chin, which gve the chins more moility under the therml condition. 18 The residul mss ove 600 ºC ws 8.56% for the ntive cellulose, 31.14% for the regenerted cellulose, nd 12.34% for the cellulose-bib. The pyrolysis residues re primrily indecomposle inorgnic slts. 30 The highest pyrolysis residues of the regenerted cellulose indicted tht more inorgnic slts were involved into cellulose fter dissolution in DMAc/LiCl. It should e noted tht the pyrolysis residue of the cellulose-bib ws lso higher thn tht of the ntive cellulose. The involvement of inorgnic slts in the regenerted cellulose, which prtly remined in the cellulose fter cyltion, ws the min reson. CNCLUSIN The DMAc/LiCl is n pproprite medium for homogeneous cyltion of cellulose with BrBiB in the presence of TEA. The of the cylted product, cellulose-bib, incresed with the increse of the molr rtio of BrBiB/AGU nd the rection time. Moreover, in order to prepre cellulose-bib with different sustitution ptterns, the rection time should e controlled. Under the rection conditions of 6.0:1 molr rtio of BrBiB/AGU, 35 ºC of rection temperture, nd 24 h of rection time, the cellulose-bib with high of ws chieved. The cyltion ws in most cses selective for the primry hydroxyl groups nd the distriution of the cyl 20

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