Loubna Najemia

Thomas Jeanmaireb

Amar Zerroukhib

Mustapha Raihanec

a Ingénierie des MatériauxPolymères/LRMP,UMR CNRS 5223,Université Jean Monnet,Saint-Etienne, France

b Département de Chimie,Université Jean Monnet,Saint-Etienne, France

c Faculté des Sciences etTechniques Guéliz, Laboratoirede Chimie Bioorganique etMacromoléculaire,Université Cadi Ayyad,Marrakech, Morocco

Isocyanate-free Route to Starch-graft-Polycaprolactone via Carbonyldiimidazole(CDI)-mediated End Group Conversion

This paper reports on the synthesis of biodegradable starch-graft-poly e-caprolactone(starch-g-PCL) in two steps. First, poly e-caprolactone (PCL) was modified on the ter-minal groups by using carbodiimidazole (CDI) as coupling agent. Then, the starch graftcopolymers with PCL were prepared through a carbonate bond using carboimidazolefunction. Monofunctional and bifunctional reactive end-group PCL terminated withcarboimidazole groups were prepared and reacted with hydroxyl groups of starch. Theresulting starch-g-PCL copolymers were characterized by FT-IR, NMR, HR-MAS-NMR, DSC, MEB and XRD.

Keywords: Coupling agent; Graft copolymers; Polycaprolactone; Starch

90 Starch/Stärke 62 (2010) 90–101

1 Introduction

Due to environmental pollution, the development of bio-degradable materials increases with the aim to replacesome petroleum-based plastics. Among the biopolymer,starch is the most studied because of its low price andnatural abundance [1]. However, pure starch basedmaterials are hydrophilic and their mechanical propertiesdecrease with the water intake [2-3]. To overcome thisbehaviour, starch can be blended [4-9], plasticized [10-18] or chemically modified with hydrophobic compounds.Such compounds could be polyester like poly (lactic acid)(PLA) [19-20], poly (e-caprolactone) (PCL) [21-24] etc. andlead to both biodegradable and processable starch-graft-polyesters.

So in order to make the graft with controlled length,grafting onto technique is the most effective. This tech-nique falls in two parts, first activation of polyester endchain, then coupling with starch.

Many coupling agent are described in literature, like diiso-cyanates [25], dianhydrides [26-28], diacid chlorides [29-33] and carbonyl-diimidazole (CDI) [34-38]. CDI appears tobe a versatile reagent with a good water tolerance andpresents a great activity toward polysaccharides like dex-tran, chitosan and cellulose [39-41]. Besides, such cou-pling agent is usually employed to graft onto starch almostsmall molecular weight molecules [36, 42].

In this work, PCL was chosen because of its goodmechanical properties and because it is one of the morecommercially available biodegradable polymers. In ourcase, monofunctional or difunctional PCL is activated byCDI and then grafted onto starch.

The structures and morphology of the starch-graft-polye-caprolactone (starch-g-PCL) copolymers are describedin detail using several techniques, HR-MAS NMR, NMR,FT-IR, and also MEB, XRD and DSC.

2 Materials and Methods

2.1 Materials and reagents

Benzyl alcohol (purity 99%), Tin (II)-2-ethyl hexanoate(Sn(Oct)2, purity 95%), Carbodiimidazole (purity 97%, mp115-122oC), N-methylimidazole (NMI, purity 98%),Toluene (purity 97%), Tetrahydrofuran (THF, purity 99%),Phenetylamine (purity 99%) and dimethylsulfoxide(DMSO, purity 98%) were shipped from Sigma-Aldrichand were used as received. e-Caprolactone (CL) waspurchased from Solvay Company (Warrington, UK) andwas stirred over CaH2 for 48 h and then distilled underreduced pressure prior to use. PCL diol 1000 g.mol-1

(CAPA 2100A) and 2000 g.mol-1 (CAPA 2201A) were kindlydonated by Solvay. 2% Acetylated corn starch (GtissalysG115) was kindly provided by Roquette.

2.2 Characterization

1H-NMR, 13C-NMR and 1H-HR-MAS-NMR spectra wereobtained on Bruker (400-MHz) spectrometer. DMSO D6

Correspondence: Amar Zerroukhi, Université Jean Monnet,Département de Chimie, 23 rue du Dr Paul Michelon, 42023Saint-Etienne, Cedex 2, France. Tel: 0033477485100, e-mail:amar.zerroukhi@univ-st-etienne.fr

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DOI 10.1002/star.200900192

Starch/Stärke 62 (2010) 90–101 Starch-graft-Polycaprolactone via End Group Conversion 91

and Tetramethylsilane were used as a solvent and internalstandard respectively. FT-IR spectra were obtained with aresolution of 4 cm21 in the 500–4000 cm21 range with aNicolet NEXUS Fourier Transform Infrared Spectrometer.DSC analyses were performed using TA instrument Q10with heating rate of 107C/min. The MEB measurementswere performed at room temperature on a Hitachi S-3000N. XRD analyses were done on a Siemens D5000 X-ray system.

2.3 Synthesis of monohydroxyl-terminated poly(�-caprolactone) (Bz-PCL-OH).

Monohydroxyl-terminated PCL (Bz-PCLOH) was synthe-sized via the ring-opening polymerization (ROP) of e-CLwith benzyl alcohol as the initiator and Sn(Oct)2 was usedas the catalyst according to literature [8-9].

In our case, benzyl alcohol (0.54 g, 5 mmol) and e-CL(11.414 g, 0.1 mol) were charged to a 100 mL round-bot-tom flask equipped with a magnetic stirring bar andSn(Oct)2 (1% wt with respect to e-CL).

The reactive mixture was degassed and then immersed ina thermostated oil bath at 1507C for 5 h. The product isthen purified by dissolution in toluene and precipitation inmethanol. Polymers were dried under vacuum until con-stant weight.

2.4 Synthesis of monocarboimidazole-terminated PCL (Bz-PCL-CI).

5.0 g of the above PCL (2.5 mmol) in DMSO or THF(10 mL) were added in a 150 mL round-ground flaskwith a magnetic stirrer containing CDI (0.446 g, 2.75mmol), NMI (11.288mg, 0.1375 mmol) and DMSO orTHF (50 mL). Then, the flask was kept under stirring for16h at room temperature. The solution was stripped ofsolvent under vacuum.

2.5 Reactivity of Bz-PCL-CI with starch

A solution containing 1.5 g (9.26 mmol of anhydroglucoseunit) of Starch and 0.75 g (9.26 mmol) of NMI in 15 mL ofDMSO was added to the mixture of Bz-PCL-CI in DMSO.

After 96 h (807C) or 3h (1307C) of reaction, the crudeproduct was then purified by precipitation in methanol(400 mL) and stripped under vacuum. Then, the solid wascrushed under liquid nitrogen and PCL homopolymer wasextracted by toluene (400 mL, 96 h). The PCL graft starchwas isolated as a powder by filtration.

2.6 Synthesis of biscarboimidazole-terminatedPCL (PCL10-CI, PCL20-CI ).

In a 150 mL round-ground flask with a magnetic stirrercontaining 0.892 g (5.5 mmol) or 1.783 g (11 mmol) ofCDI in DMSO was added a solution of 5.0 g of commer-cial PCL diol (5 mmol or 2.5 mmol for Mn=1000 g.mol-1

and 2000 g.mol-1 respectively) in DMSO (10 mL) .Then,the flask was kept under stirring for 16 h at room tem-perature.

2.7 Reactivity of PCL10-CI, PCL20-CI with starch

A solution containing 1.5 g (9.26 mmol of anhydroglucoseunit) of starch and 0.75 g (9.26 mmol) of NMI in 15 mL ofDMSO was added to various quantities of the Starch/PCLmixture ( wt Starch/wt PCl =13%, 23% or 33%).

After 96 h (807C) or 3 h (1307C) of reaction, the crudeproduct was then purified by precipitation in methanol(400 mL) and stripped under vacuum. Then, the solid wascrushed under liquid nitrogen and PCL homopolymer wasextracted by toluene (400 mL, 96 h). The PCL graft starchwas isolated as a powder by filtration.

3 Results and Discussion

3.1 Synthesis and characterization of Bz-PCL

Polycaprolactone Bz-PCL was synthesized according toliterature [43-44] by ring-opening polymerization of e-caprolactone, using stannous octanoate as catalyst andbenzyl alcohol as initiator (Scheme1).

The reaction is carried out in bulk at 1507C with a initialratio [e-CL]/[BzOH]= 20.

The degree of polymerization (DPn), which is theoreticallyequal to the ratio [e-CL]/[BzOH], is given by equation (1)and is determined by 1H-NMR.

DPnðRMNÞ ¼Ie þ Iet

Iet(1)

Where Ie is assigned to methylene integral –CH2-O(2mH, d= 4 ppm) and Iet is assigned to terminal hy-droxymethylene integral –CH2-OH (2H, d = 3.4 ppm)(Fig. 1).

Application of equation (1) gives a DPn of 19.5 which is ingood agreement with theory. Therefore, benzyl end groupfunctionality could be determined around 98% using aro-matic protons (5H, 7.34 ppm) and terminal hydroxy-methylene integral –CH2-OH (2H, d = 3.4 ppm).

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92 L. Najemi et al. Starch/Stärke 62 (2010) 90–101

Scheme 1: ROP of e-caprolactone monomer

Fig. 1. 1H NMR –DMSO D6 spectrum of Bz-PCL-OH (DPn=20).

3.2 Characterization of commercial PCL

Both commercial PCL diol (Mn=1000 g.mol-1 and 2000g.mol-1) are initiated by neopentylglycol (NG) and contain amixture of x% of symmetric (B) and y% of non symmetric(A) PCL (Fig. 2).Values were determined by 1H NMR (Fig. 3).

1H NMR spectroscopy displays characteristic peaks at1.3-1.5, 2.3, 3.4 and 4 ppm related to PCL and shows thatsignals ratio a 1at/e1et /(bgd1bt gt dt) is in a good agree-ment with the theoretical value 1/1/3. However, et (CH2-CH2OH; 3.4 ppm) and c (2C(CH3)2CH2OH, 3.19 ppm)intensities are not equal and this difference explains bythe presence of symmetric and non symmetric PCL.

These integrals are used to evaluate polymer compositionwith relation (2).

2x 1 4y = et (2)

2x = c

The results indicate that the PCL diol 1000 g.mol-1 andPCL diol 2000 g.mol-1 contains 30% and 15% of nonsymmetric structure respectively.

3.3 Preparation of PCL-CI

Carboimidazole terminated PCLs were prepared by thereaction between carbodiimidazole (CDI) and hydroxylgroups of PCL at ambient temperature.

The general reaction is shown in scheme 2.

Confirmation of carbo-imidazole end group formationwas accomplished through 1H NMR spectroscopy ofmodified mono functional Bz-PCL (Fig. 4) and commercialPCL diol (Fig. 5).

By comparison between Figs. 4 and 5, a quantitativecrude yield was determined using characteristic shift ofhydroxmethylene signals (CH2-CH2OH) from 3.4 ppm to4.4 ppm after carboimidazole modification (CH2-CH2O-CO-Im). Furthermore, benzyl aromatic signals (5H; 7.33ppm) and carboimidazole signals (3H; 8.16, 7.51, 7.04ppm) were in good intensity ratio meaning that PCL wasbenzyl capped on one side and carboimidazole cappedon the other side.

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Starch/Stärke 62 (2010) 90–101 Starch-graft-Polycaprolactone via End Group Conversion 93

Fig. 2. Structure of commercial PCL diol.

Fig. 3. 1H NMR –DMSO D6 spectrum of PCL diol (DPn=10).

Scheme 2. Preparation of PCL-CI

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94 L. Najemi et al. Starch/Stärke 62 (2010) 90–101

Fig. 4. 1H NMR –DMSO D6 spectrum of modified Bz- PCL-CI.

Fig. 5. 1H NMR –DMSO D6 spectrum of modified PCL diol (DPn=10).

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3.4 Preparation of starch-g-PCL

Starch grafted PCL copolymers were prepared by react-ing well defined mono and difunctional carboimidazolePCL with starch in DMSO using N methyl-imidazole (NMI)as catalyst. The general reaction is briefly shown inscheme 3.

The results from different grafting reaction experimentswith varying the ratio PCL-CI/starch are listed in Tab. 1.

The effect of temperature on grafting parameters wasstudied by changing the reaction temperature from 807Cto 1307C and keeping the other reaction condition con-stant. It can be seen from Tab. 1 (samples 1 and 2) thatwith the rise of temperature, a decreasing reaction time isobserved for a same PCL grafting efficiency.

A study with difunctional PCL was also carried out. Theconcentration ratio was varied by increasing weightstarch content from 13 to 33% (samples 3 to 5) in thesame experimental conditions. Interestingly, it appearsthat the yield of starch-g-PCL increases with decreasingthe molar amount of reactive PCL. As demonstrated insample 3 and 5, a decrease of the molar ratio PCL/anhy-droglucose unit from 2.16 to 0.65 leads to increase thegrafting efficiency from 13 to 70%. All these results con-

firm the low reactivity of starch hydroxyl groups towardcarboimidazole and the need to keep a large excess ofstarch. With a difunctional PCL, a polymer networkstructure was obtained and for PCL with DPn 20 the pu-rification was very difficult.

3.5 Spectral characterization of starch-g-PCL

The characterization of starch-g-PCL was conducted byusing FTIR, 13C NMR, 1H-NMR and HR-MAS-1H-NMR forreticulated structure.

First, the FT-IR spectra of starch-g-PCL were recorded formonofonctional graft PCL (Fig. 6) and difonctional graftPCL (Fig. 7). These spectra provide both evidence ofgrafting by the presence of the PCL ester carbonyl groupstretch at 1728 cm-1.

The 13C-and 1H-NMR spectra are shown in Figs. 8 and 9,respectively and are in good agreement with Choi et al.[45]. 13C-NMR presents typical signals of PCL a, b, g, dand e (33.57; 25.01; 24.15 and 63.55 ppm ) and also ter-minal methylene signal et at 60.831 ppm. Starch anhy-droglucose carbons C2 to 6 (73.39; 72.31; 79.11; 71.88;60.83 ppm) and anomeric carbon C1 (100.18 ppm) werealso present with low intensities.

Scheme 3. Preparation of starch-g-BzPCL.

Tab.1. Results of PCL grafting onto starch.

Sample PCLDPn

PCLfunctionality

% (Wt)Starch

PCL/AGUa

(mole/mole)

bGraftingefficiency (%)

cYield(%)

T(7C)

Time(h)

1 20 Mono 23 0.27 106 32 80 962 20 Mono 23 0.27 109 33 130 33 10 Di 13 2.16 86 13 80 964 10 Di 23 1.08 140 42 80 965 10 Di 33 0.65 144 70 80 966 10 Di 23 1.08 140 42 130 37 20 Di 23 0.54 100* 31* 80 96

a ratio initial molar ratio [PCL]/ anhydroglucose unit]b Determined by gravimetry = [M1 – M] / M where M and M1 are initial starch and starch-g-PCL weight respectively.c Determined by gravimetry = [M1 – M] / M0 where M0 is initial PCL weight.* Important loss during purification.

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96 L. Najemi et al. Starch/Stärke 62 (2010) 90–101

Fig. 6. FTIR spectra of starch-g-PCL (Sample 1).

Fig. 7. FTIR spectra ofstarch-g-PCL (Sample 5).

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Starch/Stärke 62 (2010) 90–101 Starch-graft-Polycaprolactone via End Group Conversion 97

Fig. 8. 13C NMR –DMSO D6 spectrum of starch-g-PCL (Sample 1).

The 1H-NMR spectra also confirm the grafting of PCLonto starch (Fig. 9).

This spectra showed peaks of the PCL hydrocarbon chainb, g, d (1.53-1.30 ppm), a (2.26 ppm) and PCL benzylgroup f and w (7.34 and 5.07 ppm). Unfortunately, PCL esignal was hidden by anhydroglucose H2 to 6 protons.Also, PCL et signal and anhydroglucose OH groups weretogether around 4.5 ppm.

For difunctional PCL grafting, we obtain reticulatedstructures which can not be analyzed by “classical” NMRexperiment, also High Resolution-Magic Angle SpinningNuclear Magnetic Resonance technique (HR-MAS NMR)was used for swelled samples.

As described in Fig. 9, typical signals from PCL and starchwere presents which prove grafting. With integrals valuesof b, g, d protons (6nH16H) and b protons (3H13H), DPnof PCL could be determined around 4.6 while DPn beforegrafting was around 10. This difference can be explainedby the fact that in HR-MAS only species swollen in DMSO

solvent become observable. Also, the whole chain of PCLwas not detected and the DPn appears to be lower aftergrafting.

3.6 DSC and MEB analysis.

Thermal behaviour of starch-g-PCL was investigated byDSC measurements on monofunctional and difunctionalgraft PCL onto starch and also pure PCL before grafting(Figs. 10 and 11, respectively).

In this study, the melting point of the PCL chain in graftedcopolymer was determined around 407C while the meltingpoint of PCL before grafting was higher (587C). Glasstransitions were also determined at –54 and –587C forPCL and starch-g-PCL respectively (not shown here).

Furthermore, the thermogram curves were broadenedafter grafting meaning that PCL was not in the samechemical environment after being grafted on starch.

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98 L. Najemi et al. Starch/Stärke 62 (2010) 90–101

Fig. 9. 1H NMR –DMSO D6 spectrum of starch-g-PCL (Sample 1).

DSC results for starch-g-PCL with difunctional PCL wereslightly different than monofunctional graft (Fig. 11). Thisdid not surprise us since the structure was reticulated andleads to a loss of PCL chain mobility. This phenomenaimply a complete loss of PCL crystallinity after graftingwhile PCL before grafting have a melting point around 40-507C.This result was confirmed by XRD analysis.

The MEB images of starch granules (a) and starch-g-PCL(b) were represented in Fig. 12. According to the resource,native starches have dimensions ranging from 20 to 70microns. The grafted PCL can be seen clearly on the sur-face of the starch in Fig. 12b. Indeed, a large amount ofthe coating covers the starch granules by the grafted PCL

and attests for the covalent grafting of PCL chains ontoStarch.

4 Conclusions

In this paper, the synthesis of starch-graft-PCL withcontrolled graft length of PCL is described. This “graft-ing onto” approach using carbodiimidazole (CDI)involves two steps. First, hydroxyl end group of poly-caprolactone (synthetic or commercial diol) was con-verted to reactive carboimidazolide end group. Then,the polyester chains were coupled onto starch througha carbonate linkage. The grafting reaction was high-

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Starch/Stärke 62 (2010) 90–101 Starch-graft-Polycaprolactone via End Group Conversion 99

Fig. 10. DSC analysis of monofunctional PCL and starch-g-PCL (Sample 1).

Fig. 11. DSC analysis ofdifunctional PCL and starch-g-PCL (Sample 5).

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100 L. Najemi et al. Starch/Stärke 62 (2010) 90–101

Fig. 12. MEB analysis of starch granules (a) and starch-g-PCL(b) (Sample 1).

lighted by FTIR, NMR and also HR-MAS-NMR for retic-ulated structure.

DSC analysis shows the loss of crystalline starch granulesafter the grafting step in polymer network. Furthermore,DSC reveals a decrease of melting point temperature formonofunctional PCL graft (from 47 to 407C).

All these analysis show that the rate of grafted chain ispredetermined from the ratio PCL/starch and the goodsuitability of CDI as coupling agent.

The starch/PCL composite compatibilized by starch-g-PCL can be one of the good candidates in the potentialapplication as biodegradable plastic.

In a forthcoming paper, the biodegradability of this mod-ified starch will be described.

Acknowledgements

This project was financially supported by: Comité MixteFranco-Marocain (programme Volubilis) and RégionRhône-Alpes.

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(Received: June 29, 2009)(Revised: September 3, 2009)(Accepted: September 8, 2009)

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