Cellulose 10: 283296, 2003. 2003 Kluwer Academic Publishers. Printed in the Netherlands. 283

Unconventional cellulose esters: synthesis, characterizationand structureproperty relations

Thomas Heinze1,2,3, Tim F. Liebert1, Katy S. Pfeiffer1 & Muhammad A. Hussain21Institut fr Organische Chemie und Makromolekulare Chemie, Friedrich-Schiller-Universitt Jena, Lessing-strasse 8, D-07743 Jena, Germany2Fachbereich 9 (Chemie), Bergische Universitt Wuppertal, Gau Strasse 20, D-42097 Wuppertal, Germany3Author for correspondence (E-mail: thomas.heinze@uni-jena.de)Received 26 November 2002; accepted 30 March 2003

Key words: Cellulose ester, Cellulose solvents, In situ activation, Structure, Synthesis

Abstract

This paper summarizes selected results obtained during a two-year research project in the framework of thefocus program Cellulose and cellulose derivatives (SPP 1011), sponsored by the German Science Foundation(DFG). New synthesis paths for the preparation of the most important cellulose ester, cellulose acetate, wereinvestigated. In contrast to conventional methods, cellulose was converted in a homogeneous phase reaction withacetyl chloride in the presence of different bases, including polyvinyl pyridine and cross-linked polyvinyl pyridine.Moreover, results of the conversion in the new solvent dimethyl sulfoxide/tetrabutylammonium fluoride trihydrateare discussed. The structures obtained were analyzed both on the level of the anhydroglucose unit (AGU) andalong the polymer chain. It was found that the addition of a base can significantly change the selectivity of thereaction and thereby the properties of the products (e.g., solubility). No signs of a non-statistical distribution ofthe acetyl groups along the polymer chains were observed. Furthermore, reactivity and selectivity of the acylationreactions, using in situ activation with p-toluenesulfonyl chloride (Tos-Cl), were studied for different long-chaincarboxylic acids (capric-, caprylic-, decanoic-, lauric-, palmitic-, stearic acid). The thermogravimetric analysisof these derivatives showed that the decomposition temperature increased with an increasing number of carbonatoms, starting from 292 C (cellulose caprate) to 318 C (cellulose stearate). New cellulose derivatives weresynthesized, for example, cellulose adamantoyl ester. For this purpose cellulose was converted homogeneouslyin N,N-dimethylacetamide/LiCl with free acids in the presence of activating reagents, for example, Tos-Cl or1,1-carbonyldiimidazol.

Introduction

The development of new reaction paths for polymeranalogous modification is one of the most impor-tant tools for the design of cellulosics with tailoredproperties. In recent years we have investigated al-ternative paths for the carboxymethylation of cellulose(Liebert et al. 1996; Liebert and Heinze 1998a,b;Heinze et al. 1999). A new concept was established,including the conversion of cellulose dissolved in N,N-dimethylacetamide (DMA)/LiCl with sodium mono-chloroacetate in the presence of solid NaOH particles(Liebert and Heinze 1998a,b). This path yielded de-

rivatives with high degrees of substitution (DS) anda completely new distribution of substituents on thelevel of the repeating units and along the polymerchain, compared with carboxymethyl cellulose (CMC)prepared in the industrially applied slurry process.Thus, it was found that the alternative CMCs showeda preferred functionalization at position 6 compared tocommercial samples. SEC analysis after polymer frag-mentation with endoglucanase indicated a block-likedistribution of the carboxymethyl functions along thepolymer backbone (Saake et al. 2000). Atomic forcemicroscopy (AFM) revealed a new superstructure.The alternatively prepared CMC forms a network-like

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system in solution, while commercially preparedsamples show fringed micelles (Liebert and Heinze2001). These molecular and supermolecular featuresresulted in a number of amazing new macroscopicproperties, for example, different rheological and col-loidal behavior (Ktz et al. 2001).

In one of our basic research projects, interest isfocused on the search for new tools for the preparationof cellulose esters, including the application of cellu-lose solvents, the in situ activation of carboxylic acidsand the use of polymeric bases. The esterificationof cellulose in DMA/LiCl has been extensively stud-ied during the last decade (Dawsey 1994; El Seoudet al. 2000). The conversion of the polymer withfree acids after in situ activation was applied besidesacetylation with acid chlorides and acid anhydrides.In situ activation of the carboxylic acids is possiblewith p-toluenesulfonyl chloride (Tos-Cl). It was firstapplied for the preparation of cellulose acetates(Shimizu and Hayashi 1988). The extension of thispath on the homogeneous derivatization of cellu-lose with waxy carboxylic acids was studied. It wasshown that cellulose esters, having alkyl substitu-ents in the range from C12 (laurylic acid) to C20(eicosanoic acid), could be obtained with almostcomplete functionalization of the OH groups (DSvalues 2.82.9; Sealey et al. 1996). It was exten-ded to the preparation of water-soluble oxocarboxylicacid esters of cellulose (Heinze and Schaller 2000).Moreover, the very powerful condensation agent N,N-dicyclohexylcarbodiimide (DCC) in combination with4-pyrrolidinopyridine (PP) was exploited for the syn-thesis of cellulose esters, starting from free carboxylicacids (Samaranayake and Glasser 1993a,b). This ap-proach can be used to efficiently prepare derivativeswith low DS.

This paper deals with results concerning differ-ent new paths for the esterification of cellulose, in-cluding the application of both alternative cellulosesolvents and polymeric bases, as well as the exploi-tation of 1,1-carbonyldiimidazole (CDA) as activatingagent. The influence of the esterification path on theproperties of the esters obtained was also studied.

Experimental

Materials

Avicel (Fluka, Avicel PH-101, degree of poly-merization DP = 260) was used as a starting poly-mer. Non-crosslinked polyvinyl pyridine had a Mw

of 200,000 g/mol. LiCl was dried for 6 h at 105 Cin vacuum prior to use. Cross-linked polyvinylpyri-dine, tetrabutylammonium fluoride trihydrate (TBAF),acetyl chloride, adamantoyl chloride (AdCl), CDA,Tos-Cl, dimethyl sulfoxide (DMSO), DMA, and thecarboxylic acids, supplied by Fluka, were used asreceived.

Methods

Dissolution of cellulose in DMA/LiCl (solution S1)For a typical preparation, 1.0 g (6.2 mmol) of driedcellulose and 40 mL DMA were kept at 130 C for 2 hunder stirring. After the slurry had been allowed tocool to 100 C, 3 g of anhydrous LiCl were added. Thecellulose was completely dissolved by cooling downto room temperature under stirring.

Acetylation of cellulose with acetyl chlorideA solution S1 (see above) was kept in an ice bath for15 min. To this cooled solution was carefully added2.2 mL acetyl chloride (5 mol/mol AGU). The systemwas heated to 80 C for 2 h and kept at room tempera-ture for 24 h. Isolation was carried out by precipitationinto 200 mL ethanol, washing with ethanol and dryingin vacuum at 50 C (sample A4).Yield: 1.5 g (84.9%).DSAcetate = 2.96 (determined by means of 1H NMRspectroscopy after perpropionylation).FTIR (KBr): 3502 (OH), 2890 (CH), 1750(C==OEster) cm1.13C NMR (DMSO-d6): 169.2169.9ppm (C==O),60.3102.5ppm (cellulose backbone).If a base was applied, it was added before the addi-tion of the acetyl chloride. If polyvinyl pyridine wasused as a base the products were reprecipitated fromDMSO.

Acetylation of cellulose in DMSO/TBAFFor a typical conversion, a solution of 1 g (6.2 mmol)cellulose in 33 mL DMSO and 6.6 mL TBAF wastreated with 1.14 mL (14.2 mmol) of vinyl acetate for70 h at 40 C (for other reagents see Table 1). Theproduct was isolated by precipitation into 200 mLisopropyl alcohol, adding 50 mL water (removal ofinorganic impurities, no signals for TBAF in NMRspectra) and filtration. After washing with 200 mL iso-propyl alcohol, the product was dried in vacuum at50 C (sample B2).Yield: 1.0 g (80.3%).

285Table 1. Summary of reaction conditions and results of acetylation of cellulose dissolved in DMA/LiCl withacetyl chloride.

No. Molar ratio Partial DSAc a in position " Solubilityb(acetyl chloride/AGU)

6 2,3 DMSOc Acetone CHCl3

A1 1.0 0.77 0.44 1.21d + A2 3.0 0.90 1.95 2.85 + +A3 4.5 1.00 1.94 2.94 + eA4 5.0 1.00 1.94 2.96 + +

a DS of the ester obtained, determined via 1H NMR spectroscopy after perpropionylation.b+ Soluble; insoluble.c Dimethylsulfoxide.d This stoichiometrically impossible value may result from fractionation during work up.e The insolubility cannot be explained by structural features.

DSAcetat = 1.04 (determined by means of 1H NMRspectroscopy after perpropionylation).FTIR (KBr): 3490 (OH), 2905 (CH), 1752(C==OEster) cm1.13C NMR (DMSO-d6): 169.1169.9ppm (C==O),60.3102.5 ppm (cellulose backbone).

Reaction of cellulose with AdCl in DMA/LiClAdCl (3.7 g, 18.6 mmol) and 1.8 mL (22.3 mmol)pyridine were added to a solution S1 and stirred for24 h at 80 C. The homogeneous reaction mixture waspoured into 250 mL of ethanol. After filtration, thepolymer was washed with ethanol and dried in vacuumat room temperature, product D13.Yield: 2.3 g (78.6%).DSAd= 1.92 (determined by means of 1H NMR spec-troscopy after perpropionylation).FTIR (KBr): 3457 (OH), 2909, 2854 (CH), 1720(C==OEster) cm1.13C NMR (CDCl3): = 176.5 (CO), 103.0 (C-1),100.9 (C-1), 81.3 (C-2,3s, C-4), 77.0 (C-3, C-5), 73.6(C-2), 61.2 (C-6s), 40.9 (-C), 39.0 (-CH2), 36.4(-CH2), 27.9 ( -CH) ppm.

Reaction of cellulose with adamantane carboxylicacid (AdOH)/CDAAdOH (3.4 g, 18.6 mmol) was dissolved in 20 mLDMA and 3.0 g (18.6 mmol) CDA was added. Thismixture was combined with a solution S1 and stirredfor 24 h at 80 C. The mixture was precipitated in300 mL of ethanol, filtered off, washed with ethanoland dried in vacuum at room temperature (productD26).Yield: 1.8 g (76.7%).DSAd = 1.31 (determined by means of 1H NMR spec-troscopy after perpropionylation).

FTIR (KBr): 3458 (OH), 2910, 2855 (CH), 1728(C==OEster) cm1.13C NMR (DMSO-d6): = 176.4 (CO), 102.6(C-1), 99.5 (C-1), 78.8 (C-4), 73.4 (C-3, C-5, C-2),62.9 (C-6s), 61.6 (C-6), 40.1 (-C), 38.8 (-CH2),36.4 (-CH2), 27.8 ( -CH) ppm.

Esterification of cellulose with lauric acid/Tos-ClTos-Cl (35 g, 12.5 mmol) was added to a solution S1followed by 2.47 g (12.5 mmol) of lauric acid understirring. The reaction mixture was stirred for 24 hat 80 C under N2. The homogeneous reaction mix-ture was precipitated in 800 mL buffer solution (7.14 gK2HPO4 and 3.54 g KH2PO4 per liter of H2O) andthe polymer was collected by filtration. After washingthe polymer with 800 mL water three times, Soxhletextraction with ethanol was carried out for 24 h. Thepolymer was dried at 50 C under vacuum to yieldproduct C4.Yield: 2.1 g (77.0%).DSLaur= 1.55 (determined by means of 1H NMRspectroscopy after peracetylation).FTIR (KBr): 3486 (OH), 2925, 2855 (CH), 1238(COCEster), 1753 (=COEster) cm1.13C NMR (CDCl3): = 173.8 (CO), 104.0 (C-1),102.6 (C-1), 72.3 (C-2), 73.3 (C-3), 82.0 (C-4), 75.1(C-5), 20.634.0 (CMethylene), 13.9 (CMethyl) ppm.

Typical example for perpropionylationof a cellulose ester for DS determinationA mixture of 6 mL pyridine, 6 mL propionic acidanhydride and 50 mg 4-(dimethylamino)pyridine wasadded to 0.3 g of the adamantoyl cellulose D13. After24 h at 80 C, the reaction mixture was cooled toroom temperature and precipitated in 50 mL ethanol.

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For purification, the isolated product was reprecipi-tated from chloroform into 50 mL ethanol, filtered off,washed with ethanol and dried in vacuum at room tem-perature.Yield: 0.7 g (67%).DSAd= 1.92, DSProp= 1.08 (both determined bymeans of 1H NMR spectroscopy).FTIR (KBr): no (OH), 2910, 2854 (CH), 1758,1737 (C==OEster) cm1.13C NMR (DMSO-d6): =177.0173.1 (CO), 100.262.6 (C atoms of the modified anhydroglucose unit(AGU)), 41.1 (CH2-propionate), 39.4 (-C), 39.0(-CH2), 36.8 (-CH2), 28.2 ( -CH), 9.4 (CH3-propionate) ppm.1H NMR (CDCl3): = 5.10 (H-3), 4.68 (H-2), 4.38(H-1, 6), 3.99 (H-6), 3.56 (H-4, 5), 2.18 (CH2-2,3-propionate), 2.03, 1.95, 1.88, 1.73 (H-adamantane),1.03 (CH3-2, 3-propionate) ppm.

Measurements

13C NMR spectra were acquired on a Bruker AMX400 MHz spectrometer. The cellulose esters weremeasured in DMSO-d6, CDCl3 and THF-d8 at 40 and70 C, respectively. The number of scans was in therange from 5000 to 20,000.

1H NMR spectra of the esters were acquiredin CDCl3 after perpropionylation of the unmodifiedhydroxyl groups (Heinze and Schaller 2000) to de-termine the DS-values. FTIR spectra were measuredon a Bio-Rad FTS 25 PC, using the KBr pellettechnique.

Thermal decomposition temperatures (Td) of thecellulose esters were determined by thermogravimet-ric analysis (TGA) on a Mettler Toledo TC 15 MettlerTG 50 Thermo balance. The Td was reported as theonset of significant weight loss from the heated sample(Sealey et al. 1996). Samples (10 mg) were measuredunder air with a temperature increase of 10 C/minfrom 35 C up to 600 C.

Elemental analyses were performed by CHNS 932Analyzer (Leco).

For GPC analysis, JASCO equipment was usedincluding degasser (DG-980-50), pump (PU-980), RI-detector (RI-930) and UV-detector (UV-975) workingat 254 nm. THF was used as eluent (30 C, 1 mL/min).The separation was carried out using columns frompolymer standards service (Mainz, Germany) with1000, 10,000 and 1,000,000 . Polystyrene standardswere used for calibration.

The HPLC analysis of the Sisal cellulose sampleswas carried out as described for cellulose derivatives(Liebert and Heinze 2001).

For the methylation, 0.5 g of the starting cellu-lose ester was dissolved in 30 mL trimethylphosphate.Methyl trifluoromethane sulfonate (4 mol/mol re-maining hydroxyl group) and 2,6-di-tert-butylpyridine(3 mol/mol hydroxyl group) were added. This mixturewas stirred for 4 h at 60 C and 16 h at room tempera-ture using argon as protective gas. Isolation was car-ried out by precipitation into ethanol. For completedepolymerization the methyl cellulose ester was treat-ed with 2 N TFA for 4 h at 120 C. The acid and thewater were removed by distillation. The HPLC experi-ments were carried out as described (Erler et al. 1992).

The Karl Fischer titration was carried out with aMettler-Toledo Coulometer DL 37 using Hydranal Aand Hydranal C (Sigma-Aldrich) as reagents.

Results and discussion

Cellulose acetate influence of baseson the reaction

Different paths for the homogeneous synthesis of cel-lulose acetates are known. Thus, cellulose was acet-ylated in DMA/LiCl using acetic anhydride (Marsonet al. 1999; El Seoud et al. 2000). We studied theacetylation of cellulose dissolved in DMA/LiCl withacetyl chloride without an additional base and in thepresence of different pyridine derivatives. In a pre-liminary set of experiments, cellulose dissolved inDMA/LiCl was converted homogenously with acetylchloride. The experimental details and the values ofthe DS of the products are summarized in Table 1.The reaction succeeds with almost complete conver-sion of the reagent, that is, it can be controlled bystoichiometry. 1H NMR experiments of the perpropi-onylated samples show a preferred functionalizationof the primary hydroxyl group.

In addition to the NMR spectroscopic experiments,the structure of sample A1 was studied by HPLC afterpermethylation and depolymerization. For this pur-pose, the product A1 was permethylated with methyltrifluoromethane sulfonate in trimethyl phosphate inthe presence of 2,6-di-tert-butylpyridine, to convertthe pattern of substitution of the acetate into an in-verse methyl ether pattern (Figure 1). After completesaponification of the ester functions and degradationof the polymer with aqueous trifluoroacetic acid, the

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Figure 1. Analytical path for the determination of the functionalization pattern of cellulose esters by means of HPLC after permethylation anddegradation.

Table 2. Summary of reaction conditions and results of the acetylation of cellulose dissolved in DMA/LiCl with acetylchloride in the presence of pyridine.

No. Molar ratio Partial DSa in position " Solubilityb

Acetyl chloride/AGU Pyridine/AGU 6 2,3 DMSOc Acetone CHCl3

A5 1.0 1.2 0.63 0.37 1.00 + A6 3.0 3.6 0.94 1.62 2.56 + +A7 5.0 6.0 0.71 2.0 2.71 + + +A8 5.0 10.0 0.46 2.0 2.46 + + +

a DS of the ester obtained determined via 1H NMR spectroscopy after perpropionylation.b + Soluble; insoluble.c Dimethylsulfoxide.

mixture of methyl glucoses obtained can be separatedby means of HPLC (Erler et al. 1992). A DSAcetate of1.16 was calculated from the chromatogram, which isin good agreement with the DS obtained by 1H-NMRspectroscopy (DS= 1.21). No evidence for ester groupmigration during the procedure was found, but it can-not be completely excluded. A comparison of theresults obtained using this analytical strategy with sta-tistical calculations was performed in the same way asfor the analysis of CMC (Heinze et al. 1999). No sig-nificantly increased amounts of glucose or trimethylglucose were found by means of HPLC. Thus, glu-cose was determined to be 3% (calculated 5.7%) and21% trimethyl glucose was found (calculated 23.1%).Consequently, cellulose acetates prepared via this pathhave a statistically even distribution of substituentsalong the polymer chain.

In another set of experiments the influence of abase on the course of the reaction and on the distri-bution of substituents was studied. An amazing resultwas that the application of pyridine as base leads toproducts of a decreased DS (Table 2). Comparisonof samples A2 and A6 or samples A4 and A7 showsa decrease of DS of about 0.3. It is even more pro-nounced if the amount of base is increased (see sampleA8). Moreover, 1H NMR spectroscopy of the productsreveals less preferred substitution in position 6. Thisselectivity is diminished by an increased concentra-

tion of the base. Thus, sample A8 shows a partialDSO-6 of 0.46 versus an overall DS of 2.46, thatis, all the secondary OH groups are acetylated. Thiscould be a first hint for a preferred deacetylation at the6-O-position.

GPC was applied to investigate hydrolytic deg-radation of the polymer chain during the reaction.It was found that the depolymerization was rathersmall without a base. All derivatives were preparedwith Avicel as starting polymer, having a DP of 260.Product A7 possesses a DP of 256. However, the DPdecreases to 103 during the reaction under comparableconditions but using pyridine as base (sample A8).

One possible explanation for the degradation mightbe the formation of the acidic pyridinium hydrochlo-ride in the case of the base-catalyzed reaction. Mostof the HCl formed is liberated from the system if noadditional base is applied. It needs to be mentionedthat the influence of the acidic pyridinium hydrochlo-ride yields a product with a different solubility. Thus,sample A8 dissolves completely in acetone in contrastto sample A4 (prepared with no base, see Table 1).Permethylation, degradation and HPLC as describedabove did not show any hints for a non-statistical dis-tribution of the substituents along the polymer chain.Consequently, the different solubility is only due to thedifferent distribution of substituents on the level of theAGU.

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Table 3. Conditions and results of the acetylation of cellulose dissolved in DMA/LiCl with acetyl chloride in the presenceof crosslinked polyvinyl pyridine.

No. Molar ratio Partial DSa in position " Solubilityb

Acetyl chloride/AGU Base/AGU 6 2,3 DMSOc Acetone CHCl3

A9 1.0 1.2 0.35 0.13 0.48 + A10 2.0 2.4 0.82 0.51 1.33 + A11 3.0 3.6 0.91 0.65 1.56 + A12 4.5 4.5 1.0 1.24 2.24 + + A13 5.0 6.0 1.0 1.62 2.62 + A14d 5.0 10.0 A15e 5.0 10.0 0.97 1.31 2.28 + +

a DS of the ester obtained determined via 1H NMR spectroscopy after perpropionylation.b + Soluble; insoluble.c Dimethylsulfoxide.d Product isolation not possible because insoluble polymer is fixed on base surface.e Non-crosslinked base was used.

For the first time, polymer-bound bases like cross-linked polyvinyl pyridine were applied for the pre-paration of cellulose carboxylic acid esters. Table 3summarizes the reaction conditions and results. Again,a significant decrease of overall DS values can be rec-ognized in comparison to reactions applying no base.Thus, for sample A2 (see Table 1) an almost completesubstitution (DS = 2.85) was found if a molar ratio of3 mol acetyl chloride per mol AGU is used. The DSreached was 1.56 in a comparable experiment (sampleA11) applying polyvinyl pyridine. High selectivity ofthe acetylation at position 6 was observed, in contrastto the acetylation reactions with pyridine as base. Adrastic decrease of both the DS values and the yield aswell as a different solubility of the product is observedif a large surplus of polymer-bound base is used.Thus, sample A13 is soluble in DMSO only, evenwith a high DS of 2.62. If the molar ratio base/AGUis in the range >10, product isolation is almost im-possible (sample A14). Extraction of the precipitate,which consists mainly of polyvinylpyridine and cel-lulose acetate, as can be confirmed by FTIR (signalsat 1595 and 3060 cm1 for the polyvinylpyridine andsignals at 1019 and 1740 cm1 for the cellulose ace-tate), yields only traces of the product, in the rangeof 1%. If the extraction was carried out with DMSO,9g cellulose acetate was recovered for an exper-iment with 1 g of cellulose as starting material. IfTHF was used, 11g were isolated. 1H-NMR spec-troscopy was applied for structure determination butno DS calculation was possible because of the pooryield. The alternative solubility of these cellulose ace-tates is comparable to p-toluenesulfonic acid esters

of cellulose (cellulose tosylate), with a non-statisticaldistribution of substituents along the polymer chainprepared in a reactive microstructure, that is, by con-version of cellulose regenerated from solution on solidNaOH particles (Einfeldt et al. 2002).

An acetylation experiment was carried out usingsoluble, non-cross-linked polyvinyl pyridine (sampleA15; Table 3) to obtain a cellulose acetate that canbe isolated from the polymeric base. No regenerationor precipitation of the polymers occurred during thecompletely homogeneous reaction. A cellulose acetatewas obtained with a DS of 2.28, determined by 1HNMR spectroscopy, which is easily soluble in acetone.HPLC analysis gave a DS of 2.30 and showed no in-creased values for non- or fully-substituted repeatingunits. It may be assumed that during the conversionthe cellulose is not permanently fixed to the dissolvedpolymeric base and an even distribution of substituentsresulted from an equilibrium reaction.

It should be mentioned that in the framework ofthis study, acetylation experiments were carried outusing in situ activation of acetic acid with CDA (forthe mechanism see below). DSAcetate values of 0.7, 1.5and 2.1, respectively, were achieved if molar ratios of1:2:2, 1:5:5 and 1:10:10 (AGU/acid/CDA) were ap-plied. However, this new path did not yield polymerswith a new pattern of functionalization.

Acylation in the new cellulose solventDMSO/TBAF

A mixture of DMSO/TBAF represents an efficient cel-lulose solvent. It dissolves cellulose completely with a

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Table 4. Esterification of different types of cellulose in DMSO/TBAF. Summary of reaction conditions and results.

No. Cellulose Acetylating agent Molar %TBAF in Time (h) Temp. (C) DSb Solubilitytype ratioa DMSO

B1 Avicel Acetic anhydride 1:2.3 16 70 40 0.83 InsolubleB2 Avicel Vinyl acetate 1:2.3 16 70 40 1.04 DMSOB3 Avicel Vinyl acetate 1:10.0 16 70 40 2.72 DMSOB4 Avicel Vinyl butyrate 1:2.3 16 70 40 0.86 InsolubleB5 Avicel Vinyl laurate 1:10.0 16 70 40 2.60 Pyridine, THF, CHCl3B6 Avicel Vinyl benzoate 1:2.3 16 70 40 0.95 InsolubleB7 Sisal Vinyl laurate 1:2.3 11 3 60 1.24 InsolubleB8 Sisal Acetic anhydride 1:2.3 11 3 60 0.3 InsolubleB9 Sisal Acetic anhydride 1:2.3 8 3 60 0.96 DMSO, pyridineB10 Sisal Acetic anhydride 1:2.3 7 3 60 1.07 DMSO, pyridineB11 Sisal Acetic anhydride 1:2.3 6 3 60 1.29 DMSO, DMF, pyridine

a Mol AGU/mol acylation reagent.b DS of the ester obtained determined via 1H-NMR.

DP of up to 650 without pretreatment within 15 min.In 13C NMR spectra signals appear only at 102.7(C-1), 78.4 (C-4), 75.6 (C-5), 75 (C-3), 73.5 (C-2)and 59.9 ppm (C-6) (Heinze et al. 2000). This clearlyshows that the cellulose is dissolved without covalentinteractions, as can be concluded from the compar-ison of the chemical shifts with values of cellulosedissolved in DMA/LiCl, which is a typical so-callednon-derivatizing cellulose solvent. This new solventwas exploited for a number of acylation reactions. Asummary of reaction conditions and DS values of theproducts obtained is given in Table 4.

The dissolved cellulose was treated with acetic an-hydride for 70 h at 40 C. A cellulose acetate with aDS value of 0.83 was obtained if a molar ratio of2.3:1.0 (acylation reagent/AGU) was applied. Com-parable conditions were used for the reaction withvinyl acetate as acylating reagent. In case of the samemolar ratio, a DS of 1.04 can be achieved, which isdue to the formation of acetaldehyde during this con-version, shifting the equilibrium towards the productside. On the other hand, the lower DS in the case of theapplication of acetic anhydride is caused by the com-parably fast hydrolysis of the reagent, due to the watercontent of the solvent. A variety of vinyl carboxylicacid esters can be exploited for this type of conversion(see Table 4). The DS values can be controlled via theamount of reagent added. A remarkable result was aDS as high as 2.6 for cellulose laurate, indicating thatthis homogeneous esterification path is highly efficientfor the preparation of fatty acid esters of cellulose.

Experiments were carried out with Sisal cellulose,which represents fast-growing lignocellulosic material

comparable to sugarcane bagasse and linters (El Seoudet al. 2000; Marson et al. 2000; Ass and Frollini 2001;Sun et al. 2001). The starting material had a DP of 650,a crystallinity index (Ic) of 77%, and contained about14% hemicellulose, as confirmed by 13C NMR spec-troscopy (Figure 2) and HPLC analysis after completedepolymerization (Figure 3). The conditions suitablefor dissolution of cellulose materials (Avicel, woodpulp) in DMSO/TBAF discussed above did not yieldoptically clear solutions in the case of Sisal cellulose.This is obviously due to the presence of hemicelluloseand the fibrous structure of Sisal cellulose. However, itwas found that Sisal cellulose dissolves completely inthe mixture DMSO/TBAF after 30 min at room tem-perature and 60 min at 60 C (Ciacco et al. 2000).Nevertheless, static light-scattering experiments of thesolution showed a fairly high amount of aggregation.The values of the molecular weights determined werein the range of 20 to 50 106 g/mol (Figure 4).

Sisal celluloses were esterified homogeneously inDMSO/TBAF using acetic anhydride and vinyl laurateas acylating reagents. Reaction conditions, results andsolubility of the esters obtained are listed in Table 4.Transesterification with vinyl laurate yields the corre-sponding ester with a DS of 1.24, while conversionwith acetic anhydride gave an acetate with DS of 0.30.The concentration of TBAF in the solution was variedfrom 6 to 11% to study this influence on the dissolu-tion and the product features. All mixtures gave clearsolutions. The DS values of the cellulose acetates pre-pared in these different solvent mixtures decrease withincreasing TBAF concentration (see Table 4). As thesalt is hydrated, the amount of water in the medium

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Figure 2. 13C-NMR spectrum of Sisal cellulose (cell) in DMSO/TBAF, also showing peaks due to the presence of xylose (xyl).

Figure 3. HPL-chromatogram of a completely depolymerized Sisalcellulose sample. (A chiral detector signal, B refraction indexdetector (RI) signal, dr detector response, RT retention time, 1 inorganic salts, 2 glucose, 3 xylose).

increases with the salt concentration. This in turn in-creases the rate of hydrolysis both of the anhydride andprobably of the ester moieties formed as well. Further-more, the interactions of the water with the cellulosicOH groups may hinder the access of acetic anhydride,resulting in a lower DS.

Figure 4. Berry plot of Sisal cellulose and alkali treated Sisalcellulose in DMSO/TBAF.

Experiments directed towards removal of the waterin the solvent were carried out. The water content wasanalyzed by means of Karl Fischer titration. The addi-tion of molecular sieves did not significantly influencethe water content. Because the treatment of the solventwith strong dehydrating reagents like sodium hydridewould produce the undesired dimsyl ions as a by-product, we studied the dewatering of DMSO/TBAF,DMSO/TBAF/Sisal and DMSO/TBAF/Avicel by va-cuum distillation.

A mixture of 60 ml DMSO and 6.6 g TBAF andcomparable mixtures containing cellulose (between1.4 and 2.6%, w/w) were distilled stepwise (stepsof 0.6 mL). The first sample obtained for pure

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DMSO/TBAF contained 55% water. In the case ofa solution of Avicel/DMSO/TBAF 22% water wasfound and 5% for a solution of Sisal/DMSO/TBAF,after the first distillation step. These data lead to theassumption that the water is strongly involved in thesolution complex of cellulose. After distillation of atotal amount of about 6 mL of water (in the case of thecellulose-containing solution) a drastic increase in theviscosity occurred. Therefore, it is useful to preparea mixture of DMSO/TBAF, distill the majority of thewater by removing about 30% (v/v) of the mixturein vacuum and dissolve the cellulose in the resultingsolvent mixture. Solutions prepared in this manner stillgave optically clear systems after the heat treatmentdescribed above, which was also used for the acetyla-tion of cellulose. The reactions in the solvent with areduced water content lead to products with a signifi-cantly higher DS under comparable conditions. TheDSAcetate increases from 0.30 (see Table 4, sample B8)to 1.15. Thus, this path is more efficient for the acet-ylation of Sisal, applying acetic anhydride. 1H NMRanalysis of the perpropionylated product shows a dis-tribution of the acetyl groups at the reactive sites in theorder C-6>C-2>C-3. No hints for a non-statisticaldistribution of substituents along the polymer chainwere found. A reaction of cellulose dissolved in an-hydrous DMA/LiCl, applying the same molar ratio ofacetic anhydride, yields a product with a DS of 1.0(Ciacco et al. 2000).

Studies on the acylation of cellulose with carboxylicacids in situ activated with Tos-Cl

An interesting new path for cellulose ester preparationis homogeneous acylation after in situ activation ofcarboxylic acids with Tos-Cl (Figure 5). It was shownthat cellulose esters, having alkyl substituents in the

Figure 5. Schematic plot of the conversion of cellulose withcarboxylic acid applying in situ activation with Tos-Cl.

range from C12 to C20, could be obtained with al-most complete functionalization of the accessible OHgroups (Sealey et al. 1996). A variety of differentcellulose esters was successfully synthesized via thispath, however, without the use of an additional base(Koschella et al. 1997; Heinze and Schaller 2000).

Considering these results, the question arises if thereaction conditions (time, molar ratio of the reagents)and the application of an additional base, for example,pyridine, influence the DS, the molecular weight andother structural features of the products. These studieswere performed with long-chain fatty acids becausethe efficiency of this particular system for the prep-aration of the corresponding esters had been shown(Heinze and Liebert 2001).

Thus, cellulose dissolved in DMA/LiCl was al-lowed to react with two equivalents, carboxylic acid(capric-, caprylic-, decanoic-, lauric-, palmitic- andstearic acid, respectively) and Tos-Cl without an addi-tional base for 24 h at 80 C. The corresponding esters(polymers C16, Table 5) show two characteristicpeaks in FTIR spectra typical for the ester moieties atabout 1240 cm1 (COCEster) and about 1750 cm1(C==OEster). Elemental analysis reveals the absence ofsulfur in the samples, showing that there is no remark-able introduction of tosylate groups, either covalentlybounded or as impurity.

The 13C NMR spectrum of C4, for example, re-corded in CDCl3, shows the characteristic signals at= 173.8 (CO), 104.0 (C-1), 102.6 (C-1), 72.3 (C-2),73.3 (C-3), 82.0 (C-4), 75.1 (C-5), 62.5 (C-6), 13.9(CH3) ppm. The signals of the methylene groups ofthe lauric acid appear in the range of 22.634.0 ppm(Figure 6). The peak for C-6 bearing an ester group ap-pears at = 62.5 ppm. The acylated primary OH groupexhibits a downfield shift of about 3 ppm comparedwith the corresponding carbon of the CH2OH func-tion. Again DS values were determined by means of1H-NMR spectroscopy after peracetylation of the re-maining OH groups. A representative 1H-NMR spec-trum of cellulose acetate laurate (synthesized fromsample C4) recorded in CDCl3 is shown in Figure 7.The protons of the laurate moiety appear at 2.3 (H-8),1.21.6 (H-10-17) and 0.8 (H-18) ppm. The acetatemethyl group leads to the signal at 1.9 (H-20) ppm.These results are in very good agreement with valuesreported for a cellulose acetate laurate synthesized inthe new solvent DMSO/TBAF, applying vinyl laurateand acetic anhydride (see above). It was found thatthe DS increased with the increasing carbon numberof the carboxylic acid. Thus, a DS of 0.6 was found

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Table 5. Conditions and results of esterification of cellulose dissolved in DMA/LiCl mediated with Tos-Cl with different carboxylic acids.

No. Carboxylic acid Molar ratioa Time (h) DSb Solubility

C1 Capric 1:2:2:0 24 1.31 DMFC2 Caprylic 1:2:2:0 24 1.40 DMSOc, DMF, CHCl3C3 Decanoic 1:2:2:0 24 1.48 DMF, CHCl3, tolueneC4 Lauric 1:2:2:0 24 1.55 Toluene, CHCl3C5 Palmitic 1:2:2:0 24 1.60 Toluene, CHCl3C6 Stearic 1:2:2:0 24 1.76 Toluene, CHCl3C7 Caprylic 1:2:2:4 24 1.76 DMF, CHCl3C8 Lauric 1:2:2:4 24 1.79 CHCl3C9 Palmitic 1:2:2:4 24 1.71 CHCl3C10 Stearic 1:2:2:4 24 1.92 CHCl3C11 Caprylic 1:1:1:0 24 0.63 DMSO, DMFC12 Lauric 1:1:1:0 24 0.36 InsolubleC13 Palmitic 1:1:1:0 24 0.46 InsolubleC14 Caprylic 1:4:4:0 24 2.56 Toluene, CHCl3C15 Lauric 1:4:4:0 24 2.56 Toluene, CHCl3C16 Palmitic 1:4:4:0 24 2.54 Toluene, CHCl3C17 Caprylic 1:2:2:0 4 1.27 DMF, CHCl3C18 Lauric 1:2:2:0 4 1.55 CHCl3C19 Palmitic 1:2:2:0 4 1.50 CHCl3C20 Caprylic 1:2:2:0 1 1.25 DMSO, DMFC21 Lauric 1:2:2:0 1 1.36 InsolubleC22 Palmitic 1:2:2:0 1 1.36 CHCl3

a Mole AGU/mol carboxylic acid/mol Tos-Cl/mol pyridine.b DS calculated by 1H NMR spectroscopy after peracetylation.c Dimethylsulfoxide.

Figure 6. 13C-NMR spectrum of cellulose laurate C4 (DS = 1.55) recorded in CDCl3 at 40 C, index means influenced by a functionalizationof the neighbor position (number of scans 11,000).

for the cellulose caprate C1, while cellulose caprylateC2 possesses a DS of 1.4. Under comparable condi-tions, a cellulose stearate C6 with a DS of 2.0 wasaccessible.

The cellulose esters possess a different solubil-ity depending on their DS and the chain length ofthe carboxylic acid (Table 5). In general, cellulosefatty acid esters having DS values higher than 1.4 are

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Figure 7. 1H-NMR spectrum of cellulose acetate laurate (starting polymer C4) recorded in CDCl3 at 40 C (16 scans were accumulated).

soluble in CHCl3, independently of the chain length ofthe carboxylic acid. Polymers with DS values higherthan 2.3 are soluble in toluene.

In another series of experiments the influenceof an additional base was investigated. Cellulosewas reacted with two equivalents of carboxylic acidand Tos-Cl and four equivalents with pyridine asbase. Thus, polymers C710 were obtained bearingcaprylic- (C7), lauric- (C8), palmitic- (C9) and stearicester (C10) functions. It was found that the DS valueswere higher than the samples prepared without base(C16). For instance, a DS of 1.55 was found for thecellulose laurate C4, synthesized without base. Theaddition of base increases the DS to 1.79 (C8) (seeFigure 8). Elemental analysis revealed the absence ofsulfur. Therefore, it can be concluded that Tos-Cl actsonly as activating reagent. No tosylation occurs.

GPC was applied to investigate hydrolytic de-gradation of the polymer chain during the reaction.Cellulose palmitate C5, synthesized in the absence ofbase, yielded a polymer with DP 41, whereas cellulosepalmitate C9 obtained in the presence of base, yiel-ded a DP value of 69. Similar results were obtainedfor cellulose stearate C6 (without base, DP= 45) andC10 (with base, DP = 61). Compared with the DP ofthe starting cellulose Avicel (DP 260), a fairly drasticdegradation occurred in every case.

Td were obtained by TGA for cellulose caprate(292 C), caprylate (300 C), decanoate (301 C),laurate (302 C), palmitate (306 C) and stearate(318 C). Cellulose esters C16 showed increasing

Figure 8. DS of cellulose esters synthesized in N,N-dimethyl ace-tamide/LiCl using in situ activation with the Tos-Cl dependent onthe carboxylic acid and the addition of pyridine (!) and withoutpyridine (").

stability with an increase in chain length from C-6 toC-18. The effect of the chain length on that behavioris more pronounced than the influence of the rathersmall increase in DS of the samples discussed. Theminimum Td value of cellulose laurate C4 was 292 C.The maximum Td value for cellulose stearate C6 was318 C. The results of TGA were comparable with thereported behavior of long-chain fatty acid esters ofcellulose (Sealey et al. 1996).

Synthesis and characterization of adamantoylcellulose prepared via different paths

The esterification of cellulose with AdOH was stud-ied because this ester moiety has found considerable

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Table 6. Summary of reaction conditions and results of the homogeneous reaction of cellulose with AdCl in DMA/LiCl.

No. Molar ratioa Time (h) Temperature (C) DSAdb Solubility

D2 1.0 24 20 0.24 InsolubleD3 1.3 24 20 0.51 DMSOc, pyridineD4 1.5 24 20 0.65 DMSO, pyridineD5 1.7 24 20 0.76 DMSO, pyridineD6 2.0 5 20 0.08 DMSO, pyridineD7 2.0 24 20 0.90 DMSO, pyridineD8 3.0 24 20 1.37 DMSO, pyridineD9 1.0 24 80 0.51 DMSO, pyridineD10 1.5 24 80 0.87 DMSO, pyridineD11 2.0 5 80 1.21 DMSO, pyridine, THFd

D12 2.0 24 80 1.71 Pyridine, THF, CHCl3D13 3.0 24 80 1.92 Pyridine, THF, CHCl3D14 4.0 24 80 1.94 Pyridine, THF, CHCl3D15 5.0 24 80 2.12 Pyridine, THF, CHCl3D16 2.0 24 35 1.19 DMSO, pyridine, THFD17 2.0 24 50 1.38 DMSO, pyridine, THFD18 2.0 24 65 1.57 DMSO, pyridine, THF

a Mole AdCl per mol AGU.b DS of adamantoyl functions determined after perpropionylation by 1H NMR spectroscopy.c Dimethylsulfoxide.d Tetrahydrofurane.

interest as a base-sensitive protecting group indeoxyribonucleoside chemistry (Greene and Wuts1991). On the other hand, incorporation of adamantoylfunctions leads to products with interesting biolog-ical activities, such as antimicrobial and antibacterialactivity (Orzeszko et al. 2000; Perrakis et al. 1999) aswell as antitumor activity (Gerzon and Kau 1967).

For the activation of the carboxylic acid, differentmethods were studied, namely acid chloride as wellas in situ activation with Tos-Cl and CDA were ap-plied. All reactions were carried out homogeneouslyin DMA/LiCl. Thus, cellulose dissolved in DMA/LiClwas allowed to react with AdCl in the presence ofpyridine (Grbner et al. 2002).

The conversion of Avicel with AdCl leads to theadamantoylated products D218 (Table 6), whichshow the typical IR spectra. Absorption bands ofthe cellulose backbone were found and, additionally,a signal at 1758 cm1, (C==OEster), indicating thepresence of the ester moiety. As can be seen fromTable 6, DS values up to 2.12 were accessible via thispath. With regard to the reaction efficiency, that is,the amount of carboxylic acid bound to the polymerrelated to the molar ratio, the use of 2 mol AdCl permol AGU is most effective. About 85% of the AdClreacts with the cellulose backbone (sample D12).

The reaction of the dissolved cellulose with AdOHin the presence of Tos-Cl leads to correspondingcarboxylic acid esters with a rather high DS of 1.50and 1.75 at a molar ratio of AGU/AdOH/TosCl of1/2/2 and 1/3/3, respectively within 24 h at 80 C re-action temperature (samples D20 and D21, Table 7).Surprisingly, conversion at a molar ratio of 1/1/1(sample D19) at 80 C as well as at room temperaturedoes not yield sufficient DS.

Comparable results were obtained by the conver-sion of cellulose with AdOH in the presence of CDA.This method is especially suitable for cellulose modi-fication because the pH is not drastically changedduring the conversion and consequently diminishedchain degradation can be guaranteed. The by-productsformed during the reaction (Figure 9) are only CO2and imidazole, and none of the reagents and by-products are toxic. At room temperature, even at amolar ratio of 1/3/3 (AGU/AdOH/CDA), no celluloseester was formed. On the other hand, at the compa-rable molar ratio of 1/3/3, however at 80 C, productD26 with a DS of 1.31 is obtained, which is rather lowcompared to product D13 (DS = 1.92), synthesizedwith AdCl under the same conditions. It is interestingthat the increase in the amount of cellulose from 1 to10 g of starting material results in higher DS values

295Table 7. Conditions and results of the reaction of cellulose dissolved in DMA/LiCl with AdOH after in situactivation with Tos-Cl (method A) or CDA (method B).

No. Method Molar ratioa Time (h) Temperature (C) DSb

D20 A 1:2:2 24 80 1.50D21 A 1:3:3 24 80 1.75D22 Bc 1:3:3 6 80 0.62D23 Bc 1:3:3 8 80 0.90D24 B 1:1:1 24 80 0.54D25 B 1:2:2 24 80 0.98D26 B 1:3:3 24 80 1.31D27 Bc 1:3:3 24 80 1.42

a Mole AGU/mol AdOH/mol Tos-Cl or CDA.b DS of adamantoyl functions determined after perpropionylation by 1H NMR spectroscopy.c Amount of cellulose 10 g.

Figure 9. Schematic plot of the conversion of cellulose withcarboxylic acid applying in situ activation with CDA.

under comparable conditions. Thus, a product with aDS of 1.42 was synthesized (sample D27, Table 7).

The structures of the synthesized adamantoyl cel-luloses were confirmed by means of 13C NMR- and 1HNMR spectroscopy, including two-dimensional meth-ods (Grbner et al. 2002). Adamantoyl celluloses D7,D10, D11, D17 and D21, of different DSAd, were se-lected for screening for biological activity. It is knownthat adamantoylated nucleosides are biologically act-ive because the adamantoyl moiety binds precisely toa complementary hydrophobic receptor region of pro-tein molecules (Gerzon and Kau 1967). Thus, the an-timicrobial, antioxidant and anti-inflammatory effectsof the cellulose derivative were tested. All samples in-vestigated did not show antioxidant or anti-microbialeffects. However, polymers D7, D11 and D21 pos-sessed a strong anti-inflammatory effect while D10and D17 showed no effect. Obviously, the effect doesnot only depend on the DSAd, because samples D7and D10 or D11 and D17 have similar DS, however,

polymer D10 and D17 are not active at all. Furtherstudies about the molecular weight, its distributionas well as about the biological dependence of theadamantoyl cellulose activity on the physico-chemicalproperties are needed to gain clear structurepropertyunderstanding.

Conclusions and outlook

The results of our work show that polymeric bases,such as polyvinyl pyridine and the new cellulosesolvent DMSO/TBAF, are new tools for the prepa-ration of cellulose esters. Furthermore, the efficiencyof the in situ activation of the carboxylic acids withCDA and Tos-Cl, respectively, has been shown. Thus,a broad variety of cellulose esters with tailored prop-erties, for example, solubility, biological activity orthermal behavior can be synthesized. These featuresare adjustable by the type of substituent introducedand by the pattern of substitution, which can be modi-fied by application of one of the methods described.

Unfortunately, the synthesis of cellulose esterswith an unconventional (non-statistical) distributionof substituents along the polymer chain has still notbeen achieved. Therefore, we continue our work onthe application of polymeric reagents for ester syn-thesis. On the other hand, the analysis of the patternof substitution on this level is not yet satisfactory.The HPLC analysis, via methylation and depolymer-ization, is connected to a number of errors, mainlyundermethylation, migration of the substituents duringthe second substitution and incomplete depolymeriza-tion. Thus, at present, a comparable method is under

296

investigation, applying percarbanilation of the cellu-lose ester as the second derivatization step.

Acknowledgements

Financial support for this study (project HE2054/5-3)was provided as part of the focus research program(Schwerpunktprogramm) on Cellulose and cellulosederivatives molecular and supramolecular structur-al design by the Deutsche Forschungsgemeinschaft(DFG) [described in the editorial commentary ofProf G. Wenz in Cellulose 10-1] and by the Fondsder Chemischen Industrie. The authors would like tothank former co-worker D. Grbner, and PhD studentG.T. Ciacco (Instituto de Quimica des Sao Carlos,Universidade de Sao Paulo, Brazil) having stayedfor six months in my group; they were included inthe basic research program on the acylation of cel-lulose. Furthermore, we thank Dr W. Radosta andDr W. Vorwerg (Fraunhofer Institut fr AngewandtePolymerforschung, Golm, Germany) for GPC studies.Moreover, the authors wish to thank M. Ktteritzschfor technical assistance.

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