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Polymer 52 (2011) 5255e5261

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Polymer

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The polymerization mechanism of lactide initiated with zinc (II) acetylacetonatemonohydrate

Malgorzata Pastusiaka, Piotr Dobrzynskia,b,*, Bozena Kaczmarczyka, Janusz Kasperczyka,c, Anna Smolaa

a Polish Academy of Sciences, Centre of Polymer and Carbon Materials, 34 Sklodowska-Curie Street, 41-800 Zabrze, Polandb Jan Dlugosz University in Czestochowa, Institute of Chemistry, Environmental Protection and Biotechnology, 13 Armii Krajowej, 42-218 Czestochowa, PolandcMedical University of Silesia, School of Pharmacy, Department of Biopharmacy, 1 Narcyzow Str., 41-200 Sosnowiec, Poland

a r t i c l e i n f o

Article history:Received 21 May 2011Received in revised form1 September 2011Accepted 14 September 2011Available online 19 September 2011

Keywords:LactideZinc complexesPolymerization

* Corresponding author. Polish Academy of ScienCarbon Materials, 34 Sklodowska-Curie Street, 41-800271 60 77; fax: þ48 32 271 29 69.

E-mail address: piotr.dobrzynski@cmpw-pan.edu.p

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.09.024

a b s t r a c t

The paper presents the mechanism of lactide polymerization initiated with zinc (II) acetylacetonatemonohydrate. However, the actual initiator of this reaction is the complex containing a metaleoxygenbond, formed by the exchange reaction of acetylacetonate ligand with deprotonated lactide derivative.The described reaction results in the release of free acetylacetonate and formation of transitional zinccomplex with metaleoxygen bond connecting the zinc atomwith derivative of lactide, incorporated e asa new, active in polymerization ligand. Polylactide chain propagation process, which constitutes thefollowing stage of the reaction, is caused by a typical, well known, coordination-insertion ring openingpolymerization. The proceeding polymerization maximum yield at the applied conditions does notexceed about 70% in benzene solution and 90% at bulk.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years considerable interest in syntheses of polymericmaterials for medical applications has been recorded. Amongthose, the bioresorbable aliphatic polyesters, polycarbonates, aswell as poly(esters-co-carbonates) based on lactides, lactons andselected cyclic carbonates are gaining great importance. Thesepolymers are synthesized with cyclic esters or carbonatesaccording to ring opening polymerization (ROP), which has beenwidely investigated by numerous researchers. Application inmedicine requires that the applied materials are biocompatiblewith body tissue, whereas tin compounds conventionally used asinitiators for synthesis of these polymers are undeniably toxic.Taking into account the fact that it is practically impossible tocompletely eliminate the initiator from the reaction product, theuse of tin compounds is at least controversial in the case ofmaterials applicable for medicine. Therefore, a need has arisen tofind new effective, low-toxic initiators, satisfactorily capable ofinitiating such reactions. Thus the requirement of biocompatibilitywould be satisfied. Consequently, many compounds andcomplexes of nontoxic metals or biometals were examined in

ces, Centre of Polymer andZabrze, Poland. Tel.: þ48 32

l (P. Dobrzynski).

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terms of their suitability as initiators in polymerization and co-polymerization of lactides, lactones and cyclic carbonates [1].

Literature reports show many examples of application in ROPcompounds of such metals as lead, zinc, aluminum, iron, yttrium,bismuth, which effectively initiated polymerization of lactones, butnot always are a biocompatible or nontoxic. Their usage them inform of oxides, chlorides, alkoxides or salts of monocarboxylic acidswas considered. However most frequently, these are complexeswith a complicated structure, difficult to synthesize, or hygroscopiccompounds, prone to hydrolysis and therefore a highly problematicin a practical, industrial application. So far, from this kind of initi-ators only nontoxic zinc lactate is used successfully in industrialsynthesis of polylactide and its copolymers [2].

From a technological point of view, acetylacetonates emergedvery attractive in considerations of ROP polymerization, especiallydue to the fact that in general acetylacetonates are soluble in thesekinds of molten monomers and relatively very stable and easy tostore. Kowalski et al. [3] described polymerization of the lacide and3-caprolactone applying as an initiator the aluminum acetylaceto-nate, whereas Nijenhuis et al. [4] used the tin acetylacetonate inpolymerization of polylactide. Our team has also proved that ace-tylacetonates of low-toxicity metals such as calcium, magnesium,iron, zinc and zirconium, compounds relatively simple andsusceptible to synthesis constitute interesting initiators of ROPpolymerization of cyclic esters and carbonates [5e8]. Many of thesecompounds are effective initiators, capable of replacing commonly

M. Pastusiak et al. / Polymer 52 (2011) 5255e52615256

used tin compounds. Until now, the mechanism of initiation oflactide with zirconium (IV) acetylacetonate polymerization hasbeen studied [5]. The compound capable of deprotonation (whichwas lactone and lactides) followed the exchange of acetylacetonateligand with formation of metaleoxygen bond, which finallyenabled the further start of polymerization [6,7].

In another work [3] a slightly different mechanism of initiationof lactide polymerization in the presence of aluminum acetylacet-onate and alcohols was proposed. In that case, according to theauthors, the active complex was the complex formed as a result ofexchange of acetylacetonate group by the alcoholate group.

Recently we also published the results of investigation of poly-merization of cyclic trimethylene carbonate with presence of zinc(II) acetylacetonate monohydrate. We found that this compoundwas very effective as a catalyst for this polymerization. In this case,the initiation stage and further extending phenomenon of chaingrowth proceeded unexpectedly through the activation of mono-mer, not on typical insertions-coordination way [9]. Therefore, aninteresting issue emerge whether the initiation of lactide poly-merization reaction proceeds with this compound, analogically tothe examined TMC polymerization, or rather according to thepolymerization course, described earlier while applying zirconiumacetylacetonate and aluminum acetylacetonate initiators, throughthe creation of an active actual initiator by preliminary ligandexchange reaction.

The structure of zinc acetylacetonate has been studied since1960s by many researchers [10e13]. Zinc acetylacetonate isa complex compound, which may appear in monohydrate, dihy-drate or anhydrous forms. While hydrous forms are built from twochelating acetylacetonate rings and one or two water moleculescoordinated to zinc atom, the structure of anhydrous form is morecomplicated. Crystallographic studies proved that it appeared astrimer containing three zinc atoms and six acetylacetonate ligands,thus creating a three-core structure [11e13]. It could be a reasonthat anhydrous acetylacetonate is a slower initiator than themonohydrate complex [9].

Our preliminary investigations indicate that themonohydrate ofzinc (II) acetylacetonate is exceedingly active, for instance, in thereaction of cyclic carbonates polymerization [5]. Due to the fact thatit also initiates lactide polymerization, it appears worthwhile todiscover the mechanism of these reactions. Explanation of themechanism will help us evaluate the efficiency of the polymeriza-tion reaction and will allow to choose proper composition of theinitiator/catalysts system. In a nutshell, it will make it possible totake control over the reaction. This emphasizes the importance ofthe reaction from the practical point of view.

2. Experimental

2.1. Materials

L-lactide (LA) was purchased from Purac (Holland) and recrys-tallized from dry ethyl acetate and then was dried in vacuum ovenat room temperature. Zinc (II) acetylacetonate monohydrate (AlfaAesar) in short Zn(acac)2 � H2O was used as received. Anhydrousbenzene (Aldrich) was dried over CaH2 before distillation.

2.2. Polymerization procedure

The model oligomerization of L-lactide was carried out underargon, in sealed glass Erlenmayer flask (50 mL), equipped witha magnetic stirrer. Dry benzene 20 mL was weighed into the reac-tion vessel, and then 1 mmol of L-lactide was added and immersedin a thermostatically controlled oil bath at 60 �C. Then, initiator ezinc (II) acetylacetonatemonohydrate, in amount depending on the

type of experiment was added (Table 2). The reactions were con-ducted for more than 24 h.

The solution polymerization of L-lactide was conducted ina larger Erlenmayer flask (150 mL) equipped with the same devicesunder the same conditions for 4 days. A dry benzene (60 mL), L-lactide or TMC (50 mmol), 1 mmol of Zn(Acac)2 � H2O wereweighed into the reaction vessel. In the kinetics investigations,several samples were taken out with a syringe at various times.These samples with aqueous acetic acid solution to stop the reac-tion and to extract the initiator were treated. The obtainedmixtureswerewashedwith cold water, and the organic phases, including thepolymer, were separated and concentrated by evaporation for NMRand gel permeation chromatography analysis.

To evaluate practical usefulness of the Zn(acac)2 monohydrate,L-lactide polymerization in bulk was also conducted. In argonatmosphere, L elactide monomer (150 mmol) with the initiator(0.188 mmol) have been charged into dried glass ampoules whichhave then been sealed. The ampoules have been conditioned in anoil bath equipped with a periodically working shaker at 130 �C.After the selected reaction time the ampoules have been quicklyquenched to room temperature and the obtained polymers havebeen discharged.

2.3. Measurements

The conversion of the reaction and structure of obtained prod-ucts was determined by 1H NMR spectroscopy. The 1H NMR spectrawere recorded at 600MHz with Avance II Bruker TM at 25 �C. Driedbenzene-d6 was used as solvent and tetramethylsilane was appliedas the internal standard. The spectra were obtained with 64 scans,2.65 s acquisition time and 11 ms pulse width.

The number-average and weight-average molar masses (Mn andMw, respectively) and dispersity indexes (Mw/Mn) of the oligomerswere determined by gel permeation chromatography with a Vis-cotek RImax chromatograph. Chloroform was used as the eluent,the temperature and the flow rate were 35 �C and 1 mL/min,respectively, two PL Mixed E columns with a Viscotek model 3580refractive index detector and injection volume equal to 100 mL wereused. The molecular weights with polystyrene standards werecalibrated. We determined the correct polylactide Mn value, basedon GPC dates and calibrationwith polystyrene standards, accordingto the method described by Kowalski et al. [14].

Infrared spectra were acquired on a DIGILAB FTS-40A Fouriertransform infrared spectrometer in the range of 4000e400 cm�1 ata resolution of 1 cm�1 and for an accumulated 32 scans. Sampleswere analyzed in a form of pellets in potassium bromide.

3. Results and discussion

In order to explain the mechanism of initiation of ROP poly-merization of lactide using a single core complex of zinc (II) a seriesof model reactions of this compound with L-lactide were carriedout. The main aim of this study was to examine whether themechanism of initiation of polymerization, as well as the growth ofthe chain is analogous to the mechanism of polymerization oflactidewith complex of zirconium (IV) acetylacetonate [6] or does ithave a completely different course, e.g. similar to initiation of cycliccarbonate by activation of monomer [9]. The aforementionedmodel reactions were conducted in a solution of anhydrousbenzene, which practically eliminates the possibility of an addi-tional effect of the complexation of central metal by the solventused in the reactions. The reaction was carried out at relatively lowtemperature (60 �C), which ensured the stability of theZn(Acac)2 � H2O complex [15]. Potential exchange of acetylaceto-nate ligand of this complex by alcohols or water traces, which was

Table 1Chemical Shifts in the 1H NMR spectra of L-lactide reaction (benzene e d6).

Signal Origin d (ppm)

1 CH in Zn(Acac)2 5.05 (s)2 CH3 in Zn(Acac)2 1.75 (s)3 CH of Acac in Zn(Acac)Lac 5.17 (s)4 CH3 of Acac in Zn(Acac)Lac 1.82 (s)5 CH3 in enol isomer Hacac 1.62 (s)6 CH in enol isomer Hacac 4.94 (s)7 CH2 in keto isomer Hacac 2.83 (s)8 CH3 in keto isomer Hacac 1.73 (s)9 CH in unreacted L-lactide monomer 4.18 (m)10 CH3 in unreacted L-lactide monomer 1.19 (d)11 CH3 in deprotonated L-lactide 1.68 (s)12 CH3 in deprotonated L-lactide 1.24 (d)13 CH in deprotonated L-lactide 3.78 (m)14 CH in the lactidyl chain 5.25 (m)15 CH3 in the lactidyl chain 1.44 (d)

M. Pastusiak et al. / Polymer 52 (2011) 5255e5261 5257

reported as the cause of initiating of lactide polymerization withaluminum (III) acetylacetonate [3], could be excluded because ofthe known relatively high stability of Zn(Acac)2 � H2O in aqueousand alcoholic solutions in a large range of pH and temperature[15,16]. Our tests confirmed the stability of this compound inbenzene solution containing about 10 wt% ethyl alcohol too.

For the purpose of monitoring the initial stage of forming of anactive zinc complex, being the initiator of studied lactide poly-merization, a series of samples was prepared, with changingcomposition of the starting reaction mixture (molar ratio; L-lactide/Zn(Acac)2 as 1:1, 1:5 and 1:15). The resulting products present inboth the solid part and volatile compounds in distilled benzenesolution were analyzed in detail with the NMR and FTIR spectros-copy. The 1H NMR spectra of obtained complexes of zinc, producedin the conducted reaction are shown in Fig. 1. In the case of allperformed reactions of Zn(Acac)2 with L-lactide the presence of freeacetylacetone (Hacac) in obtained benzene distillate was proven onthe basis of characteristic signals assigned to enol and keto forms ofthis compound present in the NMR spectrum (Table 1). We foundtraces of this compound in the evaporated solid products too(Fig. 1). In addition, 1H NMR spectrum of the reaction products of1 mol of hydrated zinc (II) acetylacetonate with 1 mol of L-lactide(Fig. 1, A) showed a significant decrease in intensities of protonsignals originated from acetylacetonate ligand with regard torespective signals of started Zn(Acac)2 monohydrate complex.Detailed analysis of the obtained spectra allowed the assignment ofthe recorded signals to appropriate chemical groups of resulting

Fig. 1. 1H NMR spectra (in benzene-d6) of reaction products of Zn(Acac)2 � H2O withL-lactide conducted with M/I molar ratio as; (A) �1:1, (B) �5:1, (C) 15:1.

compounds (Table 1). Previously published data describing similarcomplexes of zirconium [6] proved helpful in proper interpretationof the spectra. In the studied reactions of zinc (II) acetylacetonate(Fig. 1, Table 1), beside the doublet signals typical for CH3 lactidegroup e signal 12, only slightly shifted in comparison to signals ofCH3 group from initial monomeric lactide (signal 10), formation ofproton singlet signal 11 from CH3 group of reacted lactide wasobserved. This proves a running of lactide deprotonation as nocoupling with the proton of CH groups indicates a lack of proton inthe neighborhood of this group (Fig. 2).

All of the above observations showed that mechanism offormation and the final structure of the complex of zinc (II) acety-lacetonate with L-lactide were very similar to those previouslydescribed [6]. This mechanism consists of creating a reactivecomplex (Fig. 2) which constitutes the appropriate initiator of thestudied polymerization by means of acetylacetonate ligandexchange on the ligand formed from deprotonated enol derivativeof lactide with simultaneous release of acetylacetone (Hacac). Alsofurther detailed analysis of the resulting data confirmed thesepreliminary observations.

For 1H NMR spectrum of the reaction products of 1mol hydratedzinc (II) acetylacetonate with 1 mol of L-lacide (Fig. 1, A) a decreaseof about 50% in the intensity of proton signals arising from acety-lacetonate ligand was observed, compared to the initial Zn(Acac)2complex. The signals originating from acetylacetonate ligand(Fig. 1A, signal 3 and 4) were slightly shifted in comparison to theseof the initial Zn(Acac)2 � H2O complex, which confirms theassumption that a new complex with a slightly altered structure isformed. The resulting reaction mixture contained only a smallamount of Zn(Acac)2 � H2O (signals 1, 2, about 10 mol%) and therewas practically no unreacted lactide monomer. The calculations

Fig. 2. Optimized geometries of complex obtained in reaction of 1 molZn(Acac)2 � H2O with 1 mol of lactide (type1 complex). Performed with HyperChem7.51 (PM3� method).

Table 2Dependence of the composition of the reaction mixture on the concentration ace-tylacetonate groups, unsaturated bonds and lactidyl chains length in the obtainedproducts.

No RatioM/I

Time ofreaction

Conv.[%]

C0acac[%]

Ckacac

[%]Ctacac

[%]CCH3ðsÞ[%]

CtCH3ðsÞ[%]

Cchain

1 1 24 w100 66.7 52 50 48 50.0 0.12 5 24 w100 28.6 14 16.7 18 16.7 43 15 36 w99 11.8 w2 6.3 9 6.3 164 50 96 69 e e e e e 38 or 36* (GPC)5 800 72 87 e e e e e 562* (GPC)

Where; row 1 e 4 e lactide oligomerization conducted in benzene solution at 60 �C,row 5 e lactide polymerization conducted in bulk at 130 �C.Conv. e lactide conversion. C0

acac initial content of acetylacetonate groups ¼ [acac]o/([acac]o þ [lactide or TMC]). Ckacac content of acetylacetonate groups in obtainedcomplex ¼ [acac]/([acac] þ [lactide or TMC origin]). Ct

acac calculated content ofacetylacetonate groups in obtained mixture when only one ligand was changed.CCH3ðsÞ content of CH3 groups (nearby unsaturated bonds) in deprotonated lactideand lactide ligands ¼ [signal 11]/([signal11] þ [signal12] þ 1/2 [signal15]). Ct

CH3ðsÞtheoretical calculated content of CH3 groups (nearby unsaturated bonds) indeprotonated lactide and lactide ligands when only one ligandwas changed. Cchaineaverage amount of lactidyl units calculated when chain’s propagation take place atonly one ligand¼ 1/2[CH3(d) signals15]/[CH3(s) signals11] or obtained based on theGPC measurement e with asterisk.

M. Pastusiak et al. / Polymer 52 (2011) 5255e52615258

presented in Table 2, based onmeasurements of signal intensities ofcorresponding protons, confirmed that major part of the resultingreaction mixture was a complex of zinc (II), containing one acety-lacetonate ligand and one ligand derived from deprotonated lac-tide. This complexwas formedwith a deprotonation of lactide, withproton transfer to acetylacetonate ligand, which finally resulted inthe release of free Hacac and formation of a ligand from deproto-nated lactide derivative (Scheme 1). The concentration of acetyla-cetonate groups in the obtainedmixture determined on the basis of

Scheme 1. Proposed mechanism of lactide polymerization initiated by zinc (II) acetylacetona

the NMR measurements suggested the presence of more acetyla-cetonate ligands, than expected assuming complete exchange ofhalf of the acetylacetonate groups by deprotonated lactide (Table 2,row 1). The reason was, however, that a part of the starting lactidemolecules did not participate in ligand exchange reaction because itwas included in the competitive ROP reaction. As a result,a complex containing a short lactidyl chain was created. Suchphenomenon is evidenced by the presence of weak signals 15 and14, typical for the protons of CH3 and CH lactidyl chain groups. Inobserved NMR spectrum, number of singlet CH3 groups, present inthe obtained product, was also very close to the theoretical value,calculated for the type 1 complex.

The presence of such complex is also confirmed by infraredspectra. While in the region originated from the stretching vibra-tions of the C]O ester groups, for PLA one band with maximum at1759 cm�1 and for monomer, a broad band with a series of maximaare observed, for the 1:1 ratio LA/Zn(Acac)2 complex four stronglyoverlapped bands with maxima at 1770, 1754, 1742 and 1703 cm�1

(Fig. 3a) appear. The bands at 1770 and 1703 cm�1 could arise fromC]O and C]C groups in deprotonated lactide, respectively [17].Relatively high frequencies of these bands are expected due to thefact that alkenyl groups bonded to the ether oxygen of C(]O)eOone increase the frequency of the carbonyl stretching mode. On theother hand also ester groups attached to the C]C groups shift theirabsorbance bands to higher wavenumbers. The presence of theshort lactidyl chain supports the bands at 1754 and 1742 cm�1.Their relatively lower intensities prove their smaller content inrelation to the deprotonated lactide ring. The changes are alsoobserved for the Zn(Acac)2 characteristic absorbance bands. Due tothe delocalization of the p-electrons of the C]O and C]C bonds inthe Zn(Acac)2 three bands at 1605 and a doublet at 1521, 1511 cm�1

ascribed to the C]O C]C stretching mode appear in that case.

te monohydrate e numbers for protons correspond to 1H NMR signals given in Table 1.

Fig. 3. FTIR spectra of Zn(Acac)2 � H2O, 1:1 LA/Zn(Acac)2 complex, 1:15 LA/Zn(Acac)2 complex, LA and PLA in the region of 1850e1500 cm�1 (a) and 1500e800 cm�1 (b).

M. Pastusiak et al. / Polymer 52 (2011) 5255e5261 5259

In the 1:1 ratio LA/Zn(acac)2 complex instead of a band at1605 cm�1, a broad, high intensity band with maximum at1592 cm�1 appears, with a distinct shoulder at about 1613 cm�1.The band at 1511 cm�1 disappears and intensity of the band at1521 cm�1 is distinctly diminished. These quantitative changesconfirm the decrease in the acetylacetonate group contents whilethe shifts and changes in the shapes of bands prove the forming ofa complex with a new ligand. The absorbance bands attributed towater molecule appear at about 3200 cm�1 in Zn(Acac)2 and in the1:1 LA/Zn(Acac)2 complex at 3450 cm�1 (Fig. 4). This phenomenonprobably indicates the weakening of H2OeZn interactions ina newly formed complex. However this signal was still clearlyvisible, which also indicates that in the conditions where thepolymerization was conducted, the water molecule coordinatedwith the zinc atom did not participate in this reaction. Otherwise,this signal would disappear, as was observed in the case of poly-merization initiated with TMC and catalyzed with Zn (acac)2 � H2O[8]. In the region corresponding to the deformation, vibrations ofthe CH and CH3 groups are slightly shifted and a little change intheir shape is also recorded (Fig. 3b). A new band appearing at1422 cm�1 can prove that the surroundings of CH3 groups are

Fig. 4. FTIR spectra of Zn(Acac)2 � H2O (a), 1:1 LA/Zn(Acac)2 complex (b), 1:15 LA/Zn(Acac)2 complex (c), LA (d), PLA (e) in the region of 3850e2600 cm�1.

changed. The band at 1261 cm�1 attributed to the stretchingvibration of CeCH3 and groups, similarly as that at 1022 cm�1

ascribed to CH3 group rocking vibrations in the Zn(Acac)2 ispreserved, while their intensities are smaller than in pure Zn(A-cac)2, which confirms their diminishment in the acetylacetonateligand contents. The region from 1250 to 1030 cm�1 is character-istic for the CeOeC grouping stretching and CH3 group rockingvibrations in the LA and PLA. The band at 1244 cm�1 originatingfrom monomer confirms the presence of CeOeC lactide structurewhile a new bands at 1141, 1096 and 1054 cm�1 prove the formingof polylactide chain although relations between them differentthan in PLA spectrum indicate the presence of rather short chains(Fig. 3a).

In the case of reaction of zinc (II) acetylacetonate with morelactide amount (molar ratio as 1:5), unreacted initial complex ofzinc (II) acetylacetonate occurred, as expected, only in traceamounts (Fig. 1B, signal 1, 2). Detailed analysis of the intensities ofparticular signals revealed that in that case half of the amount ofthe starting ligands of acetylacetonate groups has been replaced. Asa result of the reaction a type 2 complex is formed (Fig. 5), con-taining a chain with an average of four lactydyl units ended witha deprotonated lactide group and the remaining second acetyla-cetonate ligand. In the case of large excess of lactide (reaction 1 molof Zn(Acac)2 with 15 mol of L-lactide) the mixture of lactide oligo-mers was obtained, some of which did not contain acetylacetonategroups (Table 2, Row 3). This means that during the polymerizationconducted with a sufficient excess of lactide, in some of thecomplexes, the second acetylacetonate ligand is also released andreplaced by deprotonated lactide molecule. Average number offorming chain units, calculated from the relative intensity of signalsof end CH3 groups of deprotonated lactide to the intensity of CH3group signal of polylactide chain, was very close to the theoreticalvalue (Table 2, Row 3).

Fig. 5. Optimized geometries of complex obtained in reaction of 1mol Zn(Acac)2 � H2Owith 5 mol of lactide (type 2 complex). Performed with HyperChem 7.51 (PM3�method).

Fig. 7. Relationship betweenMn, Mw/Mn, and the degree of lactide conversion, reactioncarried out in benzene at 70 �C, with M/I molar ratio as 50:1.

M. Pastusiak et al. / Polymer 52 (2011) 5255e52615260

FTIR spectrum of the 15:1 ratio LA/Zn(Acac)2 (Fig. 3) complex ismore similar to the PLA one, both with regard to the positions ofbands and to their shapes. However, the splitting of the bands at theregions attributed to the C]O and C(]O)eO ester groups provesthe presence of a small amount of monomer derivative in thesample investigated. Also several bands with small intensity,similar in shape to these for the 1:1 LA/Zn(acac)2 complex aredetected in the region of 1680e1500 cm�1. It follows that in thecase of the 15:1 ratio LA/Zn(ac)2 lactide oligomers are forming inwhich the 1:1 complex described above constitute the end groups.

In summary it was proved, that at the first stage of initiation ofthe aforementioned lactide polymerization reaction the complexcontaining metaleoxygen bond was formed through the exchangeof Acac ligand by deprotonated lactide. That complex was the actualinitiator of further oligomerization reaction. The following mainpropagation reaction occurs in accordance with the typical ROPcoordination-insertion mechanism. With a sufficient excess of themonomer, competitive reaction of exchange of the remaining Acacligand to the lactide derivative was observed too. It runs in parallelto the main reaction of chain growth. Of course, the question stillremains openwhether the propagation of polylactide chain runs onone or both of the ligands of Zn complex created that way andwhether the water molecule coordinated with Zn atom participatesin the course of the reactions. The answer to these questions wasgiven by the results of the later conducted model polymerization oflactide. It was carried out in benzene solution at 60 �C, with themolar ratio of monomer/initiator M/I as 1:50. After 72 h of thereaction the polylactide with 70% monomer conversion was ob-tained (Fig. 6). Extending the reaction duration to over 72 h did notcause a significant increase in the value of monomer conversion,which indicated that the investigated lactide polymerization hadrather an equilibrium character, with a limit of about 70% conver-sion of monomer. Analyzing the dependence of the monomerconversion degree on the averagemolecular weight of the obtainedpolylactide (after separation of the initiator), it should be noted thatthe average molecular mass of product was very close to the theo-retical molecular mass calculated assuming a growth of chain onlyon one ligand of initiation complex (according to the formula:Mn ¼ ([Lactide]/[I]) � 144 � conversion). This dependence is illus-trated in Fig. 7. This indicates that the propagation of polylactidechain proceeded generally on a single initiator ligand and that thewater molecules coordinated with the central atom of the complexdid not participate in the chain propagation process. Otherwise, thewater molecule or the second Acac ligand of the initiating complexwould be the next center of the chain growth. Consequently, themeasured actual average molecular mass of obtained polylactide

Fig. 6. Conversion of L-lactide as a function of polymerization time, reaction carriedout in benzene at 70 �C, with M/I molar ratio as 50:1.

would have to be at least less than calculated. In the initial phase ofthe studied polymerization process highmolecular mass dispersionMw/Mn > 3 was observed, which over time (with a degree ofmonomer conversion above 15%) fell to about 1.3. The observedphenomenon indicates that the step of ligand exchange andformation of an active initiator was much slower than the initiatedreaction of polylactide chain propagation. Further relatively smallincrease in mass dispersionwas observed after several dozen hoursof the process, which was caused by intermolecular trans-esterification, a phenomenon well known in this type of reactions.

Additionally, we performed lactide polymerization initiatedwith Zn(acac)2, conducted in bulk at 130 �C (Table 2, row 5). Thehighmolecular polylactide (Mn ¼ 81 kDa e based on NMRmeasurement) with molecular mass dispersion Mw/Mn ¼ 1.7 wasobtained. However, despite the long reaction time, the final yield ofreaction was relatively low, below 90%.

4. Conclusion

The investigated lactide polymerization reaction demonstratesa completely different course to previously studied polymerizationof cyclic carbonates initiated with the same complex of Zn(acac)2 � H2O. This initiation and chain propagation mechanism isvery similar to the previously presented mechanism of initiatingthis type of monomers by zirconium acetylacetonate. The realinitiator of this reaction is complex containing a metaleoxygenbond, formed by the exchange reaction of acetylacetonate ligandwith deprotonated lactide derivative. The next propagation of thepolylactide chain is caused by the coordination-insertion ROPpolymerization.

A certain problem in the application of zinc (II) acetylacetonateas an initiator of lactide polymerization can be observed not toohigh, not exceeding 90%, yield and rather low activity incomparableless than the activity of this same initiator in cyclic carbonatepolymerization [9]. Most probably, side reactions such as; intra-molecular transesterification and thermal degradation of polylac-tidewas themain cause of the observed lower than the theoreticallyexpected yield. This means that the reported polymerization israther not a strictly living process, at the applied conditions.

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