ORIGINAL PAPER

Ring-opening polymerization of ε-caprolactoneinitiated by samarium acetate

J. M. Contreras & D. Medina & F. López-Carrasquero &

R. R. Contreras

Received: 26 February 2013 /Accepted: 11 August 2013 /Published online: 13 November 2013# Springer Science+Business Media Dordrecht 2013

Abstract Poly(ε-caprolactone) (PCL) was prepared by ring-opening polymerization initiated with samarium acetate. Theeffects of reaction temperature, molar ratio of monomer/initiator and reaction time on the conversion of ε-CL to PCLand the molecular weight were investigated. The temperaturewas varied between 80 and 150 °C and the molar ratio ofmonomer to initiator between 107 and 1000 mol/mol. Theresults indicate that the samarium acetate (SmAc3) induces thepolymerization of ε-CL to high conversion and polyesterswith number-average molecular weights of 3.43×103 to12.60×103 were obtained. The polymers were characterizedby Nuclear Magnetic Resonance (1H-NMR), size exclusionchromatography (SEC). Based on the 1H-NMR end-groupanalysis of low-molecular-weight PCL, it was proposed acoordination–insertion mechanism for the polymerization.Kinetics study indicated that the polymerization rate is first-order with respect to monomer concentration.

Keywords Samarium acetate . Ring-openingpolymerization . ε-caprolactone . Poly(ε-caprolactone)

Introduction

The ring-opening polymerization (ROP) of lactones has cur-rently become an attractive and simple method for the

preparation of aliphatic polyesters, since allows synthesizepolymers which exhibit a high molecular weight with narrowmolecular weight distribution [1–4]. This kind of polymers,among which are include poly (ε-caprolactone) (PCL),poly(lactide) (PLA), Poly(glycolide) (PGA) and their copoly-mers, have attracted increasing attention since these can be usedas a synthetic biomaterial due to its good biodegradability andbiocompatibility, properties which often are requested in me-dicinal, pharmacological and agricultural applications [5–8].These materials also become attractive as alternative environ-mentally friendly polymers, which has facilitated its use ascommodity plastics for environmental protection [9]. This rap-idly growing interest in biodegradable materials has also in-creased attention in development of efficient initiators for ROPof lactones, and this can occurs through different mechanisms(anionic, cationic, coordination) each of which affects molecu-lar weight, molecular weight distribution, end group composi-tion and chemical structure of the obtained polymer and copol-ymers [2–4, 9–28].

In the particular case of PCL synthesis, recent research hasfocused his interest towards the development of organometal-lic catalysts. Especially rare earth derivate is an emerging andpromising type of polymerization initiators due their highactivity; low amount used and allowed a controlled polymer-ization of cyclic esters [2–4, 10, 15, 18–20, 29–31]. It has beenreported that these compounds promote a coordination-insertion mechanism, in which the first step is complexationof the monomers at the carbonyl oxygen, the most nucleophil-ic site of a lactone. The next step is a selective cleavage of theacyl-oxygen bond of the lactone, resulting in insertion of thelactone into the metal-oxygen bond [3, 4, 10, 15, 18, 31].

This paper describes the polymerization of ε-caprolactone(ε-CL) initiated by samarium acetate (SmAc3). The character-istics, kinetics and mechanism of the polymerization initiatedby SmAc3 have been reported.

J. M. Contreras (*) :D. Medina : F. López-CarrasqueroGrupo de Polímeros, Departamento de Química, Facultad deCiencias, Universidad de Los Andes, Mérida 5101-A, Venezuelae-mail: jeco@ula.ve

R. R. ContrerasGrupo de Organometálicos, Departamento de Química, Facultad deCiencias, Universidad de Los Andes, Mérida 5101-A, Venezuela

J Polym Res (2013) 20:244DOI 10.1007/s10965-013-0244-z

Experimental

Materials

ε-CL (Aldrich Chemical Co.) was first dried by standing for48 h over calcium hydride, then distilled at reduced pressure(4 mmHg) collecting the 86–88 °C fraction. Samarium acetate(SmAc3) (Aldrich Chemical Co.) was dried at 100 °C for 1 h.

Polymerization

All polymerizations were carried out in bulk in glass ampoulesequipped with a magnetic stirrer. In a typical experiment themonomer and initiator were added into ampoule successivelypurged by dry nitrogen. The ampoule was capped and thenplaced in thermostat oil bath with stirring for the requiredlength of time. After, it was opened at room temperature andthe reaction was quenched by addition of small amount ofmethanol required to precipitate the initiator. The product wasthen dissolved in chloroform and the initiator residues wereremoved by centrifugation. The polymers were isolated byprecipitation in excess methanol. Finally, the purified poly-mers samples were dried at 40 °C in vacuum for 24 h. The

conversion of ε-CL to PCL was obtained by gravimetricmeasurements of the precipitated polyester.

Measurements

Size exclusion chromatography (SEC) was performed with awaters model 150CV chromatograph, operated at 40 °C andequipped with four columns connected in series and packedwith Ultrastyragel 103, 104, 105, 106 Å ; tetrahydrofurane (THF)was used as solvent and the instrument was calibrated to a firstapproximation with polystyrene of known molecular weights.Mn(exp) values were calculated from the linear polystyrenecalibration curve using the correction coefficient previouslyreported (Mn(exp)=0.56*Mn(SEC)) [3]. The

1H and 13C NMRspectra were run in deuterated benzene (C6D6) on a BrukerDLX 400 MHz spectrometer and were referenced to residualprotons in the deuterated solvent (δ=7.15 ppm).

Polymerization kinetics was conducted by the weight mea-suring method, by measuring the values of conversion ofmonomer to polymer at different reaction times.

Table 1 Effect of reaction temperature on the polymerization of ε-CL

T(°C) Conversion(%)a

Mn×10−3b

(Dalton)Mn(exp)×10

−3 c

(Dalton)Mw×10−3b

(Dalton)Mw/Mn

b

60 0.00 – – – –

80 14.89 6.13 3.43 7.35 1.20

100 60.74 6.50 3.64 7.59 1.17

125 64.25 12.13 6.79 22.29 1.84

150 87.69 13.41 7.51 21.36 1.59

[SmAc3]=0.0421 mol/L; [ε-CL]=9.02 mol/L. Reaction time=24 ha Based on initial ε-CLbDetermined by SEC, measured in THF at 40 °CcDetermined by Mn(exp)=0.56*Mn(SEC)

Retention time (min)

Fig. 1 Size exclusion chromatograms (SEC) of PCL obtained usingsamarium acetate a different reaction temperature (reaction time for 24 h)

Table 2 ε-CL polymerization using different ration molar monomer/initiator (ε–CL/SmAc3)

M/I(mol/mol)

Conversion(%)a

Mn×10−3b

(Dalton)Mn(exp)×10

−3

c (Dalton)Mw×10−3b

(Dalton)Mw/Mn

b

107 68.23 12.67 7.10 17.44 1.38

214 70.88 14.43 8.08 19.39 1.34

428 80.23 11.15 6.24 13.50 1.21

856 29.34 9.83 5.50 11.21 1.14

1000 27.17 8.35 4.68 9.82 1.18

Reaction time=36 h. Reaction temperature: 100 °Ca Based on initial ε-CLbDetermined by SEC, measured in THF at 40 °CcDetermined by Mn(exp)=0.56*Mn(SEC)

0 20 40 60 80 1000

20

40

60

80

100

Co

nve

rsio

n (

%)

Time (h)

80ºC100ºC125ºC

150ºC

Fig. 2 Effect of time on the conversion in the polymerization of CL, atvarious temperatures

244, Page 2 of 6 J Polym Res (2013) 20:244

Results and discussion

Temperature effect

Influence of temperature on the bulk polymerization of ε-CLinitiated by samarium acetate (SmAc3) is shown in Table 1.The temperature greatly influences the polymerizations. It wasfound that SmAc3 by itself possesses activity as initiator for ε-CL polymerization at temperatures above 80 °C and proceedsvery slowly at this temperature, where both the yield and Mnof formed PCL are relatively lower compared to thoseobtained at higher temperatures (Table 1). Thus higher reac-tions rates can be achieved by increasing the temperature,being observed that the conversion of ε-CL to PCL increaseswith the reaction temperature.

The Fig. 1 displays the SEC chromatograms of the polyestersobtained. All curves show a monomodal pattern which indicatesthat only one active species is present in the polymerizationsystem [7]. It can noted that the curves show tendency to movetowards the region of high molecular weight, as the reactiontemperature increase, at the same time that the molar massdistribution was broader suggesting the occurrence of intra- and

intermolecular transesterification at the reaction temperature over100 °C. High temperatures, intensifies the transesterificationreactions bringing as result the broadening of molar mass distri-bution [1, 3, 16, 17, 24, 27].

Influence of SmAc3 dosage

The effect of ratio molar of monomer to initiator (M/I) on thepolymerization was studied at 100 °C, and the obtained resultsare shown in the Table 2. For values of M/I below 428 theconversion of ε-CL to PCL and themolecular weight were littleaffected, keeping in around 68 % and 7.10×10−3 g/mol, re-spectively. Conversely, for values of M/I above 428, the con-version to polymer and the molecular weight decrease sharplyby reducing the amount of SmAc3 used in the polymerization.This last fact can be attributed to the polymerization ratedecreases due to the low concentration of initiator, suggestingthat the initiator participates at the active chain-end [24, 32].

Moreover, the decreasing of the amount of SmAc3 usedseems to be favoring a better control over the polymerization,such as it is reflected by the molar mass distribution, i.e. thepolydispersity decreases with increasingmolar ratioM/I [33, 34].

0 20 40 60 80 100

Time (h)

0

5

10

15

20

25

80 ºC

100 ºC

125 ºC

150 ºCM

n x

10-3

(g/m

ol)

Fig. 3 Evolution of molecular weights depending on the reaction time inthe polymerization of ε-CL, at various temperatures

Retention time (min)

Fig. 4 Evolution of the SEC chromatograms of PCL with the reactiontime (polymerization at 125 °C)

0 20 40 60 80 1000,0

0,5

1,0

1,5

2,0

2,5

3,0

ln (

[M] 0

/[M

] t])

Time (h)

80ºC100ºC125ºC150ºC

Fig. 5 First order plot for ε-CL polymerization initiated by samarium acetateat different temperature. Condition: [SmAc3]=0.0421 mol/L; [ε-CL]=9.02 mol/L

-4,5 -4,0 -3,5 -3,0 -2,5 -2,0

-4,8

-4,4

-4,0

-3,6

-3,2

-2,8

ln K

app

ln [SmAc]

Fig 6 The relationship between ln kapp and ln [SmAc3]. Condition: [ε-CL]=9.02 mol/L. and polymerization at 100 °C

J Polym Res (2013) 20:244 Page 3 of 6, 244

Reaction time effect

Polymerizations at various temperatures using a molar ratio of214 were carried out and the reactions were monitored atvarious times. Time-conversion curves (Fig. 2) show that thereaction rate increases with temperature and it can be observedthat conversion of ε-CL to PCL increased with reaction time. At150 °C the reaction proceeds in two steps: a rapid period to 9 hfollowed by a slower. At other temperatures, it can be noted thatregardless of the temperature the reaction appears to proceed inthree periods. At 100 and 125 °C the first period extends up24 h; this is followed by slower step, which goes to 48 h. In thefinal stage of the reaction rate decreased and almost quantitativeconversions (>93 %) were obtained at higher reaction time of72 h. At 80 °C the first period extends up 48 h, followed byslower until 72 h. The reaction rate drastically decreases athigher times of 72 h, which can be related to increased viscosityin the medium of reaction as this advance.

Variation of Mn of PCL with reaction time (Fig. 3) shows asimilar behavior irrespective of the reaction temperature. It

can be observed that molecular weight increased until reach amaximum, and after reaching the highest value, Mn decreasedwith time. This fact could be explained by transesterificationreactions, which occur as side reaction during the polymeri-zation process, especially in the later stages [1, 14, 16, 17, 23,25, 27, 34]. The evolution of the SEC chromatograms wassimilar to the displayed in Fig. 4, and it can be noted the curveshad the same shape with a broadening of the molecular weightdistribution as the reaction time was increased.

Kinetics of the polymerization

The kinetics of ε-caprolactone polymerization with SmAc3 inbulk was investigated by the weight-measuring method atdifferent temperature values. The Fig. 5 shows a straight-linerelationship holds for ln ([M]0/[M]t) as a function of reactiontime. This result indicates that the polymerization is accuratelyfirst-order with respect to monomer concentration and alsoallows the calculation of the apparent rate constant (kapp) forε-CL polymerization initiated by SmAc3 at the temperaturesused [1, 3, 15, 17, 18, 27, 31, 35–38]: kapp=0.010 h−1 at80 °C; kapp=0.037 h−1 at 100 °C; kapp=0.041 h−1 at 125 °Cand; kapp=0.085 h−1 at 150 °C.

The plots of ln kapp in function of ln [SmAc3] is observed astraight line, as is shows in Fig. 6, indicating the polymeriza-tion is of first order with respect to SmAc3 concentration [15,18, 27, 31, 36–38]. Due to the above, the polymerization rateequation can be written as:

Rp ¼ Kp CL½ � SmAc3½ �

The Arrhenius plot of ln kapp versus the reciprocal of absolutetemperature shows a linear trend in the range of temperature

2,3 2,4 2,5 2,6 2,7 2,8 2,9-5,0

-4,5

-4,0

-3,5

-3,0

-2,5

ln k

app

1000/T

Fig. 7 The relationship between ln kapp versus the reciprocal of absolutetemperature.Condition: [ε-CL]=9.02 mol/L. and [ε-CL]/[SmAc3]=214

C6D6 C6D6

A B

a c e

b d f

g

a

b

d

c

fg

e

a’b’

(d’ c’)e’ g’h’

e’c’a’b’ d’h’

g’

Chemical Shift Chemical Shift

Fig. 8 1H (a) and 13C (b) NMRspectrum of the PCL obtainedusing samarium acetate asinitiator (Mn=350 Dalton). Thespectrum was run in deuteratedbenzene

244, Page 4 of 6 J Polym Res (2013) 20:244

studied, see Fig. 7. The activation energy was found to be34.25 kJmol−1 [15, 18, 27, 31, 37, 38].

Mechanistic aspects of polymerization

It had been proposed that the ring-opening polymerization oflactones initiated by rare earth derivative compounds occursby the coordination-insertion mechanism, in which ring open-ing may occur through acyl-oxygen bond or alkyl-oxygenbond [2–4, 10, 15, 18–20, 29, 31, 36–42]. To elucidate thebond cleavage mode of the monomer, analysis by 1H-NMR ofend-group was performed on a sample of low molecularweight PCL (Fig. 8a) [15, 18, 36–38, 40–42]. The signals a,b, c, d and e are assigned to the PCL repeating unit, while thetriplet peak at 3.36 ppm (signal f) was attributed to methyleneproton adjacent to the hydroxyl end group [15, 25] and thesignal g attributed to methoxide group with a chemical shiftthe 3.50 ppm [18, 19]. Both end groups were generated byhydrolysis of the growing chain when methanol was added [3,4, 10, 11, 18, 19, 27, 35–38, 40–42]; 13C NMR spectrumconfirmed these assignments (Fig. 8b). The NMR analysisshowed that the two chain end-groups are CH2-OH andCH3O-, in accord of the mechanism shown in the Scheme 1.

Conclusion

The Samarium Acetate has shown good activity toward thering-opening polymerization of the ε-CL. We have found thatthe polymerization reaction is accurately first-order with re-spect to monomer. The end group analysis of low molecular

weight PCl using NMR spectroscopy indicates that the reactionproceeds via a mechanism coordination insertion, by which themonomer is inserted onto the metal oxygen bond with selectiveacyl-oxygen bond scission of the ε-CL monomer. The weightmolecular distribution curves tend to move at lower weightmolecular zone with the increasing of temperature and the timereaction increasing suggesting the occurrence of thermal deg-radation of the polymers.

Acknowledgments The authors gratefully acknowledge the financialsupport of Consejo de Desarrollo Científico Humanistico, Tecnológico yde las Artes of the Universidad de Los Andes (CDCHTA-ULA) throughgrant numbers C-1752 and C-1744.

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