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The Effect of Trifluoroacetic Anhydrideon Poly(ϵ-caprolactone) (PCL) OligomersJosé E. Báez a b , Aurelio Ramírez-Hernández b & Ángel Marcos-Fernández aa Departamento de Química y Tecnología de Elastómeros , Institutode Ciencia y Tecnología de Polímeros (CSIC) , Madrid, Spainb Universidad del Papaloapan (UNPA), Campus Tuxtepec , Tuxtepec,Oaxaca, MexicoPublished online: 11 Aug 2011.

To cite this article: José E. Báez , Aurelio Ramírez-Hernández & Ángel Marcos-Fernández (2011) TheEffect of Trifluoroacetic Anhydride on Poly(ϵ-caprolactone) (PCL) Oligomers, International Journal ofPolymer Analysis and Characterization, 16:6, 377-389, DOI: 10.1080/1023666X.2011.596264

To link to this article: http://dx.doi.org/10.1080/1023666X.2011.596264

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THE EFFECT OF TRIFLUOROACETIC ANHYDRIDE ONPOLY(e-CAPROLACTONE) (PCL) OLIGOMERS

Jose E. Baez,1,2 Aurelio Ramırez-Hernandez,2 andAngel Marcos-Fernandez11Departamento de Quımica y Tecnologıa de Elastomeros, Instituto de Ciencia yTecnologıa de Polımeros (CSIC), Madrid, Spain2Universidad del Papaloapan (UNPA), Campus Tuxtepec, Tuxtepec,Oaxaca, Mexico

Trifluoroacetic anhydride (TFAA) and asymmetric telechelic a-hydroxyl-x-(carboxyl

acid) poly(e-caprolactone) (HA-PCL) were used to obtain in situ a-trifluoroacetate-

x-trifluoroacetanhydride poly(e-caprolactone) (TF-PCL). After reaction, the polymer

was characterized by 1H and 13C NMR, MALDI-TOF, FT-IR, DSC, and TGA. Under

the influence of TF-PCL and its terminal groups, parasitic reactions as transesterifications

and inter- and intramolecular condensation are favored. The reactivity of x-trifluoroacetan-

hydride end group with a-hydroxyl-x-methoxy polyethyleneglycol (MPEG) was used for

the synthesis of poly(e-caprolactone-b-ethylene glycol) (PCL-b-PEG) diblock copolymer.

Keywords: Asymmetric telechelic; End group; Oligomer; Poly(e-caprolactone); Trifluoroacetic anhydride

INTRODUCTION

Poly(e-caprolactone) (PCL) 1 (Scheme 1) is an aliphatic polyester with biode-gradable properties.[1] Typically, the method for PCL synthesis is via ring-openingpolymerization (ROP) of the e-caprolactone (CL).[2–4] PCL is linear polyester,usually with two end groups, and can be classified as a telechelic polymer. Generally,these end groups (–OH, �COOH, �NH2, �COCl, etc.) in the PCL are used inothers reactions, for example, the most famous PCL end group is the a-hydroxyl(PCLOH) or a,x-dihydroxyl (HOPCLOH), used as precursor of diblock or triblockcopolymers[5–7] and polyurethanes.[8–10]

Submitted 1 May 2011; accepted 13 May 2011.

J.E.B. thanks Programa del Mejoramiento al Profesorado (PROMEP) for financial support

(PROMEP=103.5=07=2740), Universidad del Papaloapan (UNPA), and Instituto de Ciencia y Tecnologıa

de Polımeros (CSIC). The authors are indebted to Dr. Placido Galindo Iranzo for obtaining MALDI-TOF

spectra and Q.F.B. Karla Barrera Rivera for her assistance in the preparation of some polymer samples for

NMR.

Correspondence: Jose E. Baez, Departamento de Quımica y Tecnologıa de Elastomeros, Instituto

de Ciencia y Tecnologıa de Polımeros (CSIC), C=Juan de la Cierva No. 3, 28006 Madrid, Spain. E-mail:

jebaez14@yahoo.com.mx

International Journal of Polymer Anal. Charact., 16: 377–389, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 1023-666X print=1563-5341 online

DOI: 10.1080/1023666X.2011.596264

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In previous work, we synthesized one asymmetric telechelic a-hydroxyl [HO�]-x-(carboxyl acid) [�(C¼O)�OH] poly(e-caprolactone) (HA-PCL) 2.[11] In this sense,one easy method for the end group derivatization of HA-PCL (and characterization by1H NMR) is the reaction with trifluoroacetic anhydride [F3C�(C¼O)�O�(C¼O)�CF3] (TFAA), producing a-trifluoroacetate [CF3�(C¼O)�O�]-x-trifluoroacetan-hydride [�(C¼O)�O�(C¼O)�CF3] poly(e-caprolactone) (TF-PCL) 3.

In organic chemistry, the order of reactivity toward addition-elimination (alcoho-lysis) is: acid chloride> anhydride> ester.[12] Due to high reactivity of acid chloride(�COCl), in polymer chemistry the reaction of acid chloride end group withalcohol[13–15] or amine[16] for the functionalization[13] and synthesis of block copolymeris common.[14–16] On the other hand, it is evident that anhydride (�CO�O�CO�)functional group has intermediate reactivity between the acid chloride and ester groups.However, the effect of anhydride as TFAA or mixed anhydride (R�CO�O�CO�CF3)end group on oligomers (such as PCL) has not been reported in polymer chemistrystudies.

Our interest in this study is exploring the effect of TFAA on HA-PCL oligo-mer. Additionally, the reaction of a mixed anhydride, x-trifluoroacetanhydride[�(C¼O)�O�(C¼O)�CF3] PCL, with monohydroxyl asymmetric telechelic poly-mer [a-hydroxyl-x-methoxy polyethyleneglycol (MPEG)] was used for the synthesisof a diblock copolymer. The homopolymer was characterized by nuclear magneticresonance (NMR), Fourier transform-infrared (FT-IR) spectroscopy, matrix-assisted laser desorption=ionization-time of flight (MALDI-TOF), differential scan-ning calorimetry (DSC), and thermogravimetric analysis (TGA) and the blockcopolymer by NMR and FT-IR.

Scheme 1. Poli(epsilon-caprolactone).

Scheme 2. Derivatization of end groups in the HA-PCL by TFAA.

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EXPERIMENTAL SECTION

Materials

e-Caprolactone (CL), chloroform-d (CDCl3), trifluoroacetic anhydride(TFAA), and a-hydroxyl-x-methoxy polyethyleneglycol (MPEG) were purchasedfrom Aldrich Chemical Company and used without further purification. Ammoniumheptamolybdate tetrahydrate (NH4)6[Mo7O24] � 4H2O was purchased from Riedel-deHaen and used without further purification. Methanol (CH3OH) and chloroform(CHCl3) were purchased from Alyt Reactivos Analıticos.

Synthesis of a-Hydroxyl-X-(Carboxylic Acid) Poly(e-Caprolactone)(HA-PCL) Homopolymer

Typical polymerization was carried out in a 100mL round-bottom flask pre-viously dried. e-Caprolactone (CL, 0.541mol, 60mL), ammonium heptamolybdatetetrahydrate (NH4)6[Mo7O24] � 4H2O (Hep, 0.027mmol, 33.3mg), and water (H2O,27.7mmol, 500 mL) were charged and heated to reflux by stirring them in an oil bathat 150�–160�C for 2 h. Ammonium decamolybdate (NH4)8[Mo10O34] was obtainedin situ in a solid state by thermal decomposition of ammonium heptamolybdate(NH4)6[Mo7O24].

[4,17] The polymer obtained was precipitated from chloroform=methanol, recovered by filtration, and left to dry at room temperature for one night.Number-average molecular weight was monitored by 1H NMR (Mn(NMR)¼ 1950),and the yield was calculated based on monomer and water at the feed (yield¼ 76%).IR (cm�1): 2944 (nas, CH2), 2865 (ns, CH2), 1720 (n, C¼O), 1471 (ds, CH2), 1170(nas, C�(C¼O)�O), 1043 (nas, O�C�C), 731 (q, CH2). NMR data for HA-PCLhomopolymer (room temperature): 1H NMR (300MHz, CDCl3, ppm): d 4.06 (t,2H, [�CH2O�]), 3.63 (t, 2H, [�CH2OH]), 2.36 (t, 2H, [�CH2CO2H]), 2.30 (t, 2H,[-CH2CO2�]), 1.63 (m, 4H, [�(CH2)2�]), 1.36 (q, 2H, [�CH2�]). In the case ofTable II, HA-PCL with Mn(NMR)¼ 2640 was used.

Preparation In Situ of a-Trifluoroacetate-X-Trifluoroacetanhydride PCL(TF-PCL)

The reaction was carried out in a 50mL round-bottom flask previously dried.To a solution of HA-PCL (4.48 g, 2.29mmol, Mn(NMR)¼ 1950) with CHCl3(20mL), an amount of trifluoroacetic anhydride (0.14mL, 1mmol) was added.The mixture was vigorously stirred at room temperature for 12min. The crudewas added in 250mL of cold water, and stirring continued until the entire polymerwas precipitated. After 3.5 h the polymer was filtered, washed with distilled waterand cold methanol, and left to dry at room temperature for one night. Number-average molecular weight (Mn) was obtained by 1H NMR (Mn(NMR)¼ 2330),and the yield was calculated based on polymer at the feed (yield¼ 65%). 1H NMR(300MHz, CDCl3, ppm): d 4.07 (t, 2H, [�CH2O�], cyclic PCL), 4.05 (t, 2H,[�CH2O�], PCL), 3.64 (t, 2H, [�CH2OH], PCL), 2.36 (t, 2H, [�CH2CO2H],PCL), 2.30 (t, 2H, [�CH2CO2�], PCL), 1.64 (m, 4H, [�(CH2)2�], PCL), 1.37 (q,2H, [�CH2�], PCL).

EFFECT OF TFAA ON PCL 379

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Reaction from a-Trifluoroacetate-X-(Trifluoroacetanhydride)Poly(e-Caprolactone) (TF-PCL) with a-Hydroxyl-X-MethoxyPolyethyleneglycol (MPEG)

A typical reaction was carried out in a 50mL round-bottom flask previouslydried. To a solution of a-hydroxyl-x-(carboxylic acid) poly(e-caprolactone) (HA-PCL)(4.48 g, 1.69mmol,Mn(NMR)¼ 2640) with CHCl3 (10mL) an amount of trifluoroace-tic anhydride (TFAA) (0.55mL, 4mmol) was added. After 5min of reaction, 32mmol(24 g) of a-hydroxyl-x-methoxy polyethylene glycol (MPEG) dissolved in CHCl3(10mL) was injected in the round-bottom flask using a syringe. The mixture was vigor-ously stirred at room temperature for 30min. Cold methanol was added at crude, andstirring continued until the entire polymer was precipitated. The polymer was filteredand left to dry at room temperature for one night. Number-average molecular weightwas obtained by 1H NMR, and the yield was calculated based on polymer at the feed(yield¼ 78%). IR (cm�1): 2944 (nas, CH2, PCL), 2865 (ns, CH2, PCL), 1720 (n, C¼O,PCL), 1471 (ds, CH2, PCL), 1170 (nas, C�(C¼O)�O, PCL), 1106 (vas, C�O�C,PEG), 1043 (nas, O�C�C, PCL), 731 (q, CH2, PCL). NMR data for sample no. 3(Table II) diblock copolymer (room temperature), 1H NMR (200MHz, CDCl3,ppm): d 4.48 (t, 2H, [�CH2�O�], PEG), 4.20 (t, 2H, [�CH2�O�], PEG), 4.03 (t,2H, [�CH2�O�], PCL), 3.61 (s, 4H, [�CH2�CH2�O�], PEG), 3.35 (s, 3H,[CH3�O�], PEG), 2.64 (s, 1H, [H�O�], PCL), 2.27 (t, 2H, [�CH2CO2�], PCL),1.61 (m, 4H, [�(CH2)2�], PCL), 1.35 (q, 2H, [�CH2�], PCL).

Characterization

Solution 1H and 13C NMR spectra were recorded at room temperature withCDCl3 as solvent on a Varian Gemini 200 (200MHz 1H and 50MHz 13C) andVarian Unity Plus 300 (300MHz 1H and 75.47MHz 13C). Spectra were referencedto the residual solvent protons at d 7.26 for CDCl3 in the 1H NMR spectrum, andthe residual solvent carbons at d 77.0 for CDCl3 in the 13C NMR spectrum.Matrix-assisted laser desorption=ionization time-of-flight (MALDI-TOF) spectrawere recorded in the linear mode by using a Voyager DE Pro time-of-flight massspectrometer (Applied Biosystems) equipped with a nitrogen laser emitting at337 nm with a 3 ns pulse width and working in positive-ion mode and delayed extrac-tion. A high acceleration voltage of 20 kV was employed. 2,5-Dihydroxybenzoic acid(DHB) at a concentration of 10mg=mL in acetonitrile was used as matrix. Sampleswere dissolved in acetonitrile and mixed with the matrix at a molar ratio of approxi-mately 1:100. FT-IR spectra were obtained with the attenuated total reflectancespectroscopy (ATR) technique on powder deposited over a diamond on a zinc selen-ide (ZnSe) crystal in a PerkinElmer Precisely Spectrum 100 FT-IR spectrometer. Dif-ferential scanning calorimetry (DSC) was performed in a Mettler Toledo DSC822einstrument. Samples were sealed in aluminum pans. Two scans [PCL (25� 75�Cand �90� 75�C)] were performed with an initial heating rate of 10�C=min and thefastest cooling rate of the instrument between runs under nitrogen purge. The melt-ing points (Tm) are given as the maximum of the endothermic, and the data reportedare taken from the second scan. A Thermal Analysis TA Q500 instrument was usedfor thermogravimetric analysis (TGA). Samples with weights between 10 and 30mg

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were scanned in Hi-Resolution mode with an initial heating rate of 10�C=min undera flux of nitrogen.

RESULTS AND DISCUSSION

a-Trifluoroacetate-X-(Trifluoroacetanhydride) Poly(e-Caprolactone)TF-PCL

Ring-opening polymerization of e-caprolactone (CL) was conducted withwater and ammonium decamolydate as initiator and catalyst, respectively; thispolymerization was carried out at 150�–160�C (see Experimental Section), anda-hydroxyl-x-(carboxyl acid) poly(e-caprolactone) (HA-PCL) 2 (no.1, Table I) isobtained after 2 h (yield¼ 76%). HA-PCL was used in the reaction with trifluoro-acetic anhydride (TFAA) for obtaining in situ a-trifluoroacetate-x-(trifluoroacetan-hydride) poly(e-caprolactone) (TF-PCL) 3. This specie was observed by 1H NMRspectrum; in Figure 1 both end groups a [�CH2�O�CO�CF3, d 4.35] and h[�CH2�CO�O�CO�CF3, d 2.65] can be seen.

The effect of TFFA on HA-PCL toward production of TF-PCL was studiedwith different reaction times of 12min and 180min (Table I); after the reactionthe sample was precipitated in water excess. Due to electrophilic character in the car-bonyl groups for both end groups of TF-PCL, it is expected that water acted as anucleophile, and the polyester obtained may be recovered with the original functio-nalization a-hydroxyl-x-(carboxyl acid) in its terminal groups.

In Figure 2 (top panel), the 1H NMR spectrum for HA-PCL is shown. A tripletassigned to methylene close to oxygen e [d 4.05, �CH2�O�] in the PCL main chainis seen. On the other hand, when HA-PCL reacted with TFAA for 180min (no. 3,Table I), a new signal e0 [d 4.07, �CH2�O�] attributed to cyclic species of PCL isclearly seen (Figure 2, bottom panel); this evidence indicates a parasitic reaction inthe formation process of TF-PCL. Iversen and coworkers prepared HA-PCL byROP from CL catalyzed by lipase, and the same type of the cyclic species wasobserved by 1H NMR and MALDI-TOF.[18]

In MALDI-TOF analysis for HA-PCL (Figure 3(a)), unimodal distribution isshown. However, when HA-PCL reacted with TFAA for 12min (Figure 3(b); no. 2,Table I), a multimodal distribution and an increase in the high molecular weightspecies were observed; a similar phenomenon with a HA-PCL after 3 h in the

Table I. Reactions between TFAA and HA-PCL

No. TFAA addition Time (min) Yield (wt.%)a Mn(NMR)b Tmc(�C) Td

d (�C)

1 Not — — 1950 56 363

2 Yes 12 65 2330 57 —

3 Yes 180 80 2540 58 364

aWith respect to the polymer feed.bCalculated by 1H NMR analysis.cDetermined by DSC.dDetermined by TGA.

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Figure 2. 1H NMR spectra in CDCl3 (300MHz), methylenes close to oxygen (-CH2-O-). Effect of trifluoro-

acetic anhydride (TFAA) on poly(e-caprolactone) (PCL): top, PCL before TFAA (no. 1, Table I), and

bottom, PCL after 3 h in presence of TFAA (no. 3, Table I).

Figure 1. 1H NMR spectrum (300MHz) for a-(trifluoroacetyl)-x-(trifluoroacetanhydride) poly(e-caprolactone) (TF-PCL) in CDCl3 at 25

�C.

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presence of TFAA (no. 3, Table I) is evidenced. In comparison with Figure 3(a),high molecular weight species are seen now in Figure 3(b). Species with high degreeof polymerization (DP¼ 68–80) have been attributed to intermolecular conden-sation reactions between PCL�OH and PCL-CO�O�CO�CF3 end groups(Scheme 3). An expanded view in the spectrum of HA-PCL (Figure 4(a); no. 1,Table I) shows three different peaks (Scheme 4) assigned to HA-PCL (A) dopedwith Naþ and Kþ and x-carboxilated species (B); this end-group specie is a conse-quence of the ionization process.[11] Moreover, other two HA-PCL samples(Figures 4(b) and 4(c); nos. 2 and 3, respectively) after reaction with TFAA showedthe formation of cyclic (C) and terminal trifluorinated (D) species (Scheme 4). Thiscyclic species has also previously been observed by 1H NMR. PCL cyclic speciescan be product of: (1) intramolecular condensation between a-hydroxyl and

Figure 3. MALDI-TOF spectra of (a) HA-PCL (no. 1, Table I) and (b) HA-PCL after 12min in presence

of TFAA (no. 2, Table I).

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x-(trifluoroacetanhydride) end groups (Scheme 5(a)) and (2) intramolecular trans-esterification catalyzed by acidic (þH) conditions (Scheme 5(b)).

In DSC thermograms for the PCL oligomers (nos. 1–3) an endothermic peak ineach sample (from 56� to 58�C, Table I) was observed. TGA for HA-PCL with orwithout addition of TFAA was obtained, and only one inflection temperature at363� (no. 1, Table I) and 364�C (no. 3, with TFAA, t¼ 180min) was observed forboth samples. Additionally, in the HA-PCL sample no. 3 (Table I), the effect of cyc-lic species (observed by MALDI-TOF and 1H NMR) on the melting point (Tm) andthermal decomposition (Td) was not observed.

Exploring the Synthesis of Diblock Copolymer Using TF-PCL

Poly(e-caprolactone-b-ethyleneglycol) (PCL-b-PEG) is an important diblockcopolymer due to its amphiphilic characteristics and is a potential candidate forbiomedical applications.[19,20] It is well known in organic chemistry that a mixedanhydride such as trifluoroacetanhydride (R�CO�O�CO�CF3) can produce a newester group (R�CO�O�R0) by reaction with an alcohol group (R0�OH).[12,21,22] Inthis sense, one interesting purpose for our work was to observe the reactivity ofx-(trifluoroacetanhydride) terminal group toward synthesis of PCL-b-PEG usinga-hydroxyl-x-methoxy poly(ethylene glycol) (MPEG).

Reaction of TF-PCL (obtained in situ by HA-PCL and TFAA) with a-hydroxyl-x-methoxy poly(ethylene glycol) (MPEG) was carried out at room tem-perature in CHCl3 for 30min with a MPEG=HA-PCL feed molar ratio from 9 to18 (nos. 1–3, Table II) in order to shift equilibrium to the right side (Le Chatelier’sprinciple).

FT-IR spectra for MPEG and HA-PCL precursors and the product betweenTF-PCL and MPEG (no. 3, Table II) were obtained. Usually, PCL and PEG arecharacterized by prominent absorptions at 1720 cm�1(n, C¼O) and 1106 cm�1 (n,C�O�C), respectively.[19] In the FT-IR spectrum for sample no. 3, band character-istics for both species were clearly seen.

To verify the chemical nature of sample no. 3 (Table II), the polyester wasanalyzed after the reaction with a new derivatization of the end groups using trifluor-oacetic anhydride (TFAA) and observed by NMR. In Figure 5, the 1H NMR

Scheme 3. Intermolecular condensation in the PCL between hydroxyl (�OH) and mixed anhydride

(�CO�O�CO�CF3) end groups.

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spectrum for sample no. 3 after TFAA addition is shown. Signals for three differentspecies at d 4.20 [c, �CH2�O�, PCL-b-PEG], 4.12 [g, �CH2�O�, TF-PCL], and3.73 ppm [b, �CH2�O�, TF-PEG] are clearly seen. Signal i at 2.65 ppm indicates

Figure 4. MALDI-TOF spectra, expanded view for: (a) HA-PCL (no. 1, Table I), (b) HA-PCL after 12min

in presence of TFAA (no. 2, Table I), and (c) HA-PCL after 3 h in presence of TFAA (no. 3, Table I).

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Scheme 5. Intramolecular condensation (a) and intramolecular transesterification catalyzed by trifluoace-

tic acid (b) in the PCL.

Scheme 4. Different species observed in MALDI-TOF (Figure 4).

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that the complete reaction from x-trifluoroacetanhydride TF-PCL [�CO�O�CO�CF3] mixed anhydride end group with a-hydroxyl MPEG [�OH] does not occurunder these conditions. On the other hand, signal j at 4.48 ppm was attributed toa-trifluoroacetate PEG (MF-PEG), thus, part of MPEG does not react with TF-PCLto form PCL-b-PEG. Hence, sample no. 3 is a blend of three species: (a) PCL-b-PEGand (b) MPEG and HA-PCL precursors (without reaction). Similar behaviors wereobserved for samples 1 and 2 (Table II). The percentage of PEG-b-PCL in sampleno. 3 was obtained from integrals observed by 1H NMR as h [�CH2�O�CO�CF3, d 4.35] and i [�CH2�CO�O�CO�CF3, d 2.65] end groups. Only from 20

Figure 5. 1H NMR (200MHz) spectra for the a-trifluoroacetate-x-methoxy (PEG-b-PCL) and

a-trifluoroacetate-x-trifluoroacetanhydride (TF-PCL) obtained after derivatization of sample 3 (Table

II) with TFAA in CDCl3 at 25�C.

Table II. Preparation of PCL-b-PEG diblock copolymer by reaction of TF-PCL (in situ) with MPEG

Feed (mmol) DP (NMR)a Mn(NMR)a

Sample HA-PCL MPEG PCL PEG PCL PEG Diblock %a

1 1.69 16 16 14 1820 610 27

2 1.69 24 19 16 2160 700 20

3 1.69 32 23 13 2620 570 29

aCalculated from 1H NMR analysis.

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to 29% of the x-trifluoroacetanhydride, PCL (�CO�O�CO�CF3) reacted withMPEG producing PEG-b-PCL (Table II).

Based on our experience and on the data reported in the literature, ROP of CLin the presence of a a-monohydroxyl polymer (for example, PEG[19,20] or poly-ethylene (PE)[23]) as macroinitiator and using a catalyst is a better method for thesynthesis of diblock copolymer with respect to the route by mixed anhydridePCL�CO�O�CO�CF3 presented here.

CONCLUSION

The asymmetric telechelic polyester a-trifluoroacetate-x-(trifluoroacetanhy-dride) poly(e-caprolactone) (TF-PCL) was prepared in situ by reaction froma-hydroxyl-x-(carboxylic acid) poly(e-caprolactone) (HA-PCL) with trifluoroaceticanhydride (TFAA). In the preparation of TF-PCL, cyclic species of PCL andincrease of Mn(NMR) were observed; there is an evidence that parasitic reactionsas transesterifications and inter- and intramolecular condensation occur. Mixedanhydride x-(trifluoroacetanhydride) [PCL�CO�O�CO�CF3] end group in theTF-PCL was used as precursor of a diblock copolymer poly(e-caprolactone-b-ethyl-ene glycol) (PCL-b-PEG) by the reaction with a-monohydoxyl telechelic polymeras: a-hydroxyl-x-methoxy poly(ethylene glycol) (MPEG). However, the reactionwas less favored and the parasitic reactions inherent to this pathway make it aninefficient route.

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