Ring-Opening (ROP) versus Ring-Expansion (REP) Polymerization of ε-Caprolactone To Give Linear or...

Ring-Opening (ROP) versus Ring-Expansion (REP) Polymerization ofε‑Caprolactone To Give Linear or Cyclic PolycaprolactonesJose A. Castro-Osma,† Carlos Alonso-Moreno,‡ Joaquín C. García-Martinez,‡ Juan Fernandez-Baeza,†

Luis F. Sanchez-Barba,† Agustín Lara-Sanchez,*,† and Antonio Otero*,†

†Facultad de Ciencias y Tecnología Química de Ciudad Real, Departamento de Química Inorganica, Organica y Bioquímica,Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain‡Facultad de Farmacia de Albacete, Departamento de Química Inorganica, Organica y Bioquímica, Universidad de Castilla-LaMancha, 02071 Albacete, Spain

*S Supporting Information

ABSTRACT: Macromolecular engineering of cyclic polycaprolac-tones has been carried out by a ring-expansion procedure catalyzedby a series of alkyl-organoaluminum initiators. The single-sitenature of the initiators allows a very well-controlled macro-lactonization process to give moderate to high molecular weightcyclic polymers with narrow polydispersities. Cyclic architecturesare supported by a combination of techniques such as viscositymeasurements, NMR, and MALDI-TOF MS analysis.

■ INTRODUCTION

The past two decades of the twentieth century have witnessed aparadigm shift from biostable to biodegradable biomaterials formedical and related applications.1 In fact, this current trend haspromoted a spectacular advance in new polyester materials andthe development of therapeutic devices such as scaffolds fortissue engineering and as drug delivery vehicles.2 Among them,polycaprolactone (PCL), polylactide (PLA), and their copoly-mers have represented potential candidates to replace tradi-tional olefin-based polymers, since they are biodegradable andthe nontoxic products can be reabsorbed or excreted by thehuman body.3 Ring-opening polymerization of cyclic esters(ROP) has been the most versatile method employed toprepare a variety of polymers with control of the majorvariables that affect polymer properties.4 A plethora oforganometallic initiators supported by a judicious selection ofancillary ligands are now available for the ROP of cyclic esters.5

Among them, complexes bearing scorpionate ligands haveproved to be highly productive and selective initiators for thepolymerization of lactides and lactones.6

In this field, macromolecular engineering of large-ring cyclicpolyesters is a redoubtable challenge; for example, in polymerchemistry macrolactones present a broad range of interestingmedicinal properties.7 Although several elegant routes for thesynthesis of cyclic polymers have been reported, there is still aneed for a suitable method for the synthesis of high molecularweight cyclic polymers with narrow polydispersities.8 The mostcommon strategy relies on the coupling of linear chains by adifunctional agent9 or by reactive hetero end groups.10 Despitethe fact that this route has been significantly improved,

undesired polycondensation to give a mixture of linear andcyclic polymers, the need to use a highly dilute medium, anddifficulties in controlling the process are severe limitations forthe production of high-molecular-weight cyclic polymers. Thekinetically controlled synthesis of cyclic polyesters is anotherelegant route that has been reported. In this approach, acontrolled ring-expansion polymerization (REP) involving theuse of cyclic tin alkoxides has been developed,11 albeit with asignificant toxicity limitiation.12 Zwitterionic polymerization hasbeen reported as an easy way to synthesize long cyclicoligomers by Kricheldorf et al.13 and cyclic polymers byWaymouth et al.,14 both with a defined molecular weight andnarrow molecular weight distribution. However, the organo-catalytic process requires high initiator-to-monomer ratiosalong with the addition of an aprotic trapping agent for theorganocatalyst to terminate the reaction. It is worth high-lighting a recent publication concerning the first aluminumcatalyst capable of undergoing REP to give cyclic polylactidearchitectures of defined molecular weight.15

As summarized above, a significant number of syntheticmethodologies to give cyclic polyesters are being explored inthe field of catalysis. Herein, we would like to add the first seriesof alkyl-organoaluminum complexes to the ever-expandingbody of catalysts for the well-controlled REP of cyclic esters.7

The alkyl-organoaluminum complexes 1−6 show a very rareability to generate controlled cyclic polylactones.

Received: June 13, 2013Revised: July 29, 2013Published: August 16, 2013

Article

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■ RESULTS AND DISCUSSIONAs reported previously, a straightforward one-step alkaneelimination reaction was used to obtain the monoalkyl-

organoaluminum complexes 1−66b and the dialkyl-organo-aluminum complexes 7−126c as fine colorless powders (seeScheme 1). The complexes 1−12 are stable in the presence ofair in the solid state for several days at room temperature (seeFigures S1−S3 in the Supporting Information).Drawing upon previous studies in which the aforementioned

complexes were shown to act as efficient and versatile initiatorsin ROP,6b,c we were initially interested in completing our initialstudies by the systematic evaluation of complexes 1−12 in theROP of ε-caprolactone (ε-CL). First, it is worth noting that theevaluation of complexes 1−12 in the ROP of rac-LA showedthem to be single-site initiators capable of affording PLAs withrelatively narrow polydispersities and molecular weights ingood agreement with calculated values. NMR and MALDI-TOF MS analysis provided evidence that both series ofinitiators led to linear polyesters through a coordination/insertion mechanism.6b,c When complexes 1−12 wereevaluated as initiators in the polymerization of the monomerε-CL, the results observed on using complexes 1−6 wereintriguing. The molecular weight (Mw), molecular weightdistribution (Mw/Mn), and yield of PCLs obtained on usingcompounds 1−12 as initiators are compiled in Table 1. Asobserved previously, the polymerization activity of compounds1−12 is consistent with a well-defined catalyst. GPC (gelpermeation chromatography) analysis of the polymers (seeFigures S4−S15 in the Supporting Information) supported thisproposed situation, with a narrow Mw/Mn (entries 1−12 inTable 1) and a monomodal distribution (see Figure 1 asexample GPC traces for entries 1 and 7 in Table 1).

Scheme 1. Molecular Structures of Compounds 1−12

Table 1. Polymerization of ε-CL Catalyzed by Compounds1−12a

entry initiator time (min) yieldb (%) Mw(exp)c (Da) Mw/Mn

1 1 20 97 21 140 1.172 2 20 99 20 540 1.213 3 20 91 22 930 1.194 4 20 98 22 510 1.195 5 30 98 22 190 1.156 6 30 99 23 256 1.187 7 15 99 36 220 1.218 8 16 97 30 360 1.229 9 15 97 29 990 1.2010 10 14 96 32 250 1.1811 11 22 98 30 490 1.1712 12 20 97 28 810 1.1613d 1 25 98 32 060 1.3514e 1 25 95 52 020 1.3715f 1 9 98 27 830 1.2516g 1 16 h 99 18 690 1.02

aPolymerization conditions: 4.5 × 10−3 M of initiator, 110 °C, [CL]:[Al] = 200, 20 mL of toluene as solvent. bYield (%) = (dried masspolymer/theoretical mass polymer) × 100. cDetermined by GPCrelative to polystyrene standards in tetrahydrofuran. d[CL]:[Al] = 350.e[CL]:[Al] = 500. f130 °C. g70 °C.

Figure 1. GPC spectra of linear PCL and cyclic PCL showing the shiftin retention time of the cyclic polymer due to the smallerhydrodynamic volume.

Figure 2. 1H NMR spectra in CDCl3 at room temperature of low-molecular-weight PCLs (a) having a number-average molecular weightMn of 2100 g mol−1, prepared with 1, and (b) having a number-average molecular weight Mn of 2210 g mol−1, prepared with 7.

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dx.doi.org/10.1021/ma401216u | Macromolecules 2013, 46, 6388−63946389

The results in Table 1 are indicative of the single site natureof the initiators and a well-controlled growth of the polymerchain on the metal center (see Table S1 in the SupportingInformation). Complexes 1−6 and 7−12 were designed tohave different steric and electronic features on the metal center,and it was anticipated that some differences would be found intheir behavior in the polymerization of ε-CL.The values of Mw differ markedly when the same [CL]:[Al]

ratio was employed for the two classes of complexes (cf. entries1 and 7 in Table 1). Furthermore, when the low-molecular-weight samples were characterized, in the first instance by 1HNMR spectroscopy (see Figure 2 as a representative example),evidence was observed for chains capped by −COMe and−OH for polymers from initiators 7−12, whereas evidence forthe presence of end groups was not observed for polymersobtained with initiators 1−6.MALDI-TOF mass spectra supported the NMR experimen-

tal evidence, revealing well-defined cyclic structures whencomplexes 1−6 were used as initiators in the polymerization. Asa representative example, the MALDI-TOF MS spectra oftypical samples of PCL obtained with complexes 1 and 7 usingDCTB and NaI as matrix and cationization agent, respectively,are shown in Figure 3. Both of these spectra display amonomodal distribution of peaks corresponding to a uniquewell-defined polymer structure with equal peak separationcorresponding to the repeating unit of ε-CL. In addition to thesimilarities, a shift of ca. 16 Da is observed on comparing the

two spectra (inset Figure 3). This difference is consistent withthe incorporation of a methyl initiator group in the polymerobtained with complex 7. Moreover, and as a representativeexample, the MALDI-TOF spectrum of polycaprolactoneobtained with complex 1 shows a peak at 2989.63 Da (Figure3a), and this can be assigned to the cyclic polymer containing26 repeat units. On the other hand, the peak at 3004.21 Da(Figure 3b) matches with a linear polymer containing the samenumber of repeating units but with methyl as the initiatorgroup. The MALDI-TOF results unambiguously show that thechemical composition of both polymers is consistent with amacrolactonization process to give cyclic polymers whenmonoalkyl-organoaluminum complexes 1−66b are used and anormal ROP process to give linear polymers when thepolymerization is catalyzed by dialkyl-organoaluminum com-plexes 7−12. Thermal characterization of a cyclic PCL verifiesthe expected decrease in glass transition temperature whencomparison with linear analogous is carried out (see FiguresS16 and S17 in the Supporting Information).16

It is well-known that ring polymers have smaller hydro-dynamic volumes than their linear analogues.9 Thus, cyclicpolymers exhibit a range of unique dilute solution properties,and these enable their fundamental physical properties to bemeasured.9 As a result, further evidence for the cyclic structureof these PCLs can be provided by GPC in conjunction withlight-scattering and viscometer detectors. Comparison of thelogarithm of absoluteMw versus elution volumes of PCLs of the

Figure 3. MALDI-TOF mass spectra of (a) a low-molecular-weight PCL having a number-average molecular weight Mn of 3500 g mol−1, preparedwith 1, and (b) a low-molecular-weight PCL having a number-average molecular weight Mn of 3000 g mol−1, prepared with 7. (c) and (d)correspond to PCLs obtained by 1 and 7, respectively, but with the spectra expanded in the region from 2985 to 3020 m/z.



same Mw (obtained from 1 and 7) shows that lower elutionvolumes were obtained for PCLs from complex 7 (Figure 4b).This observation is again indicative of a cyclic architecture forpolymers obtained with 1−6 as a smaller hydrodynamic radiusresults in longer GPC elution times relative to linearcounterparts.9

The cyclic topology was finally confirmed by comparing thedilute solution viscosities of polymers obtained with 1 and 7.An increase in the intrinsic viscosity was observed on increasingthe molecular weight for both types of complexes (Figure 4a).PCL samples obtained by 1 as initiator showed lower intrinsicviscosities compared to the linear counterparts due to the lossof the link at each end of a linear polymer chain to produce acyclic polymer.Bearing in mind the results discussed above, it is clear that

complexes 1−6 are the first alkyl-organoaluminum initiatorsthat are capable of polymerizing ε-CL to give moderate to highmolecular weight cyclic polymers with narrow polydispersities.Encouraged by these results, we decided to obtain more detailsabout the catalytic performance of these systems. Polymer-izations with complexes 1 and 7, as representative examples ofthe two series of complexes, were monitored over time byregular manual sampling followed by 1H NMR analysis todetermine the degree of monomer conversion. The semi-logarithmic plots of ln([CL]0/[CL]t) versus reaction time areshown in Figure 5a, where [CL]0 is the initial caprolactonemonomer concentration and [CL]t the lactone concentration ata given reaction time t. In both cases the linearity of the plotshows that the propagations were first order with respect tolactone monomer. Furthermore, an induction period was not

observed, and this indicates the presence of a reactive speciesfrom the outset. The linearity of the plots also illustrates thattermination reactions did not occur during polymerizationswith complexes 1 and 7. The kapp values for both initiators areof the same order of magnitude, but the monoalkyl derivative 1presents the lowest kapp value under the same reactionconditions, probably due to the steric hindrance caused bythe two polydentate ligands on aluminum retarding theinteraction between the Al center and lactone.Complex 1 was also able to polymerize ε-CL in a well-

controlled manner on using [CL]:[Al] ratios of 350 and 500(see entries 13 and 14 in Table 1), but as expected, highermolecular weight polymers with higher molecular weightdistributions were obtained. In addition, complex 1 showshigher activity and gives higher molecular weight distributionfor the resulting polymers on employing higher temperatures(see entries 15 and 16 in Table 1).The polymerization involving complex 1 exhibits several

remarkable features that are typical of living behavior: (1) thepolymerization rates are rapid, with complete conversionachieved within minutes; (2) narrow molecular weightdistributions of polymers are obtained; and (3) the molecularweight of the polymers increases with monomer conversion(see Figure 5b). It is worth noting that these features imply thatthe rate of macrolactonization to generate the cyclic polyesterarchitectures is much slower than propagation.Finally, the proposed REP for the polymerization with

initiators 1−6 is shown in Scheme 2. It is known that dialkyl-organoaluminum complexes follow a well-known nucleophilicroute, in which polymerization is initiated by the transfer of an

Figure 4. (a) Plot of intrinsic viscosity versus molecular weight forPCLs obtained with compounds 1 and 7. Mw is determined by lightscattering technique. (b) Plot of logarithm of molecular weight versuselution time for PCLs obtained with compounds 1 and 7.

Figure 5. (a) First-order kinetic plots for ε-CL polymerizations intoluene at 110 °C with [CL]/[Al] = 200 and [Al] = 4.5 × 10−3 molL−1; 1, kapp = 6.3 × 10−4 s−1 (linear fit, R = 0.986); 7, kapp = 9.2 × 10−4

s−1 (linear fit, R = 0.986). (b) Plot of Mn and Mw/Mn versusconversion for the polymerization of ε-CL with 1. [Al] = 4.5 × 10−3

M, 110 °C, [Cl]:[Al] = 200, 20 mL of toluene as solvent.



Scheme 2. Proposed REP for the Polymerization of ε-CL by Initiators 1−6

Figure 6. (a) 1H NMR spectrum for complex 3 in toluene-d8 at room temperature. (b) 1H NMR spectrum of the mixture of complex 3 with CL in aratio [CL]:[AL] = 10 in toluene-d8 at room temperature. (c) 1H NMR spectrum of the ring-expansion reaction of CL with complex 3, in a ratio[CL]:[ AL] = 10, after 2 min, in toluene-d8 at 110 °C, the CL monomer still remains. (d) 1H NMR spectrum of the ring-expansion reaction of CLwith complex 3, in a ratio [CL]:[AL] = 10, after 10 min, in toluene-d8 at 110 °C, the CL monomer is consumed.



alkyl ligand to the monomer, with cleavage of the acyl−oxygenbond and formation of the metal alkoxide propagatingspecies.6c This coordination/insertion mechanism in the ROPof ε-CL is supported, in our studies, by the alkyl end groupsobserved by MALDI-TOF and 1H NMR spectroscopy. On theother hand, the preparation of cyclic PCLs with 1−6 shouldfollow a tailored organometallic catalyzed ring-expansionprocess (REP). A cyclic catalyst would be generated bynucleophilic attack by the scorpionate ligand, and macro-lactonization could occur without the involvement of the alkylgroup directly attached to the aluminum center. Repeatedmonomer insertion would generate macrometallacycles which,after an intramolecular chain-transfer reaction, would producecyclic polylactones and regenerate the monoalkyl-organo-aluminum initiator.

1H NMR spectroscopic data support the proposed REP of ε-CL by initiators 1−6. As a representative example of thesestudies, REP of ε-CL with 3 monitored by 1H NMRspectroscopy is shown in Figure 6 (see also full-page imageswith peaks integration for the 1H NMR spectra in Figures S18−S21 of the Supporting Information). Signals for the remainingmonomer can be observed in Figures 6b,c, whereas the end ofthe polymerization reaction is evident in Figure 6d sinceevidence for remaining monomer is not observed. A singletresonance for the Al−Me group can be observed in Figures 6a−d at around at δ = −0.2 ppm, and this might indicate theexclusion of the methyl ligand in the activation reaction. Thefact that the methyl ligand is not transferred to the growingchain and is retained by the organoaluminum compoundprovides sufficient evidence to support a macrolactonizationprocess in which initiators 1−6 take part. However, due to thehigh lability of the Al−N(pyrazole) bond,6c the displacement ofthe pyrazole ring by other strong donors cannot be ruled out.

■ CONCLUSIONIn summary, we report highly efficient single-site initiators forthe well-controlled synthesis of cyclic polycaprolactones withmoderate to high molecular weight and narrow polydispersities.In contrast, organoaluminum counterparts with only onepolydentate ligand coordinated to the metal center undergoROP of ε-CL to give linear PCLs. Cyclic polymericarchitectures are supported by NMR and MALDI-TOF studiesas well as by comparison of the intrinsic viscosities and elutiontimes of the cyclic structures with the linear analogues.The synthesis of cyclic PCLs by polymerization with

monoalkyl-organoaluminum compounds 1−6 as initiators isparticularly attractive because (1) it is well-controlled andefficient, (2) it is highly productive, (3) it does not generatelinear intermediates, (4) it does not require time-consumingpurification procedures, (5) only small quantities of initiator arerequired, and (6) cyclic polymers with defined moderate tohigh molecular weights and narrow polydispersities areobtained.Further studies are underway to examine thoroughly the

effect on the polymerization behavior of changing both themetal center and the substituent of the ligand framework in aneffort to expand the scope of the initiators in the synthesis ofnew cyclic polylactone architectures.

■ EXPERIMENTAL SECTIONGeneral Procedures. All manipulations were performed under

nitrogen using standard Schlenk techniques. Solvents were predriedover sodium wire (toluene, n-hexane) and distilled under nitrogen

from sodium (toluene) or sodium−potassium alloy (n-hexane).Deuterated solvents were stored over activated 4 Å molecular sievesand degassed by several freeze−thaw cycles. Microanalyses werecarried out on a PerkinElmer 2400 CHN analyzer. 1H and 13C NMRspectra were recorded on a Varian Inova FT-500 spectrometer andreferenced to the residual deuterated solvent. ε-Caprolactone waspurchased from Alfa-Aesar. The organoaluminum complexes wereprepared according to literature procedures.6b,c ε-Caprolactone wasdried by stirring over fresh CaH2 for 48 h, then distilled under reducedpressure, and stored over activated 4 Å molecular sieves. Gelpermeation chromatography (GPC) and viscosity measurementswere performed on a Viscotek GPCmax VE-2001 TDA 302 Detectorsinstrument equipped with a Waters Styragel column with tripledetection, i.e., LALS (low angle light scattering), RALS (right anglelight scattering), and viscosity detector. The GPC column was elutedwith THF at 50 °C at 1 mL/min and was calibrated using eightmonodisperse polystyrene standards in the range 580−483 000 Da.The MALDI-TOF spectra were acquired using a Bruker Autoflex IITOF/TOF spectrometer using DCTB and NaI as matrix andcationization agent, respectively. Samples cocrystallized with matrixin a ratio of 100:1 on the probe were ionized in positive reflectormode. External calibration was performed by using Peptide CalibrationStandard II (covered mass range: 700−3200 Da) and ProteinCalibration Standard I (covered mass range: 5000−17 500 Da).

General Polymerization Procedure. Schlenk tubes were chargedin the glovebox with the required amount of monomer and initiator,separately, and then attached to the vacuum line. The initiator andmonomer were dissolved in the appropriate amount of solvent, andtemperature equilibration was ensured in both Schlenk flasks bystirring the solutions for 15 min in a bath. The appropriate amount ofinitiator was added by syringe, and polymerization times weremeasured from that point. Polymerizations were stopped by injectinga solution of acetic acid (5 vol %) in methanol. Polymers wereprecipitated in a mixture of methanol/hexane, filtered off, washedthree times with 20 mL of methanol, and dried in vacuum to constantweight.

Polymerization Kinetics. Kinetic experiments were carried out inflasks at 110 °C on the Schlenk line with polymerizations using stocksolutions of the reagents. Specifically, at appropriate time intervalssamples were removed by syringe and quickly quenched into 1 mLvials containing 0.6 mL of undried “wet” CDCl3. The quenchedaliquots were analyzed by 1H NMR spectroscopy. For ε-CLpolymerization, the [CL]0/[CL]t ratio was determined by integrationof the peaks for ε-CL (4.2 ppm for the OCH2 signal) and PCL (4.0ppm for OCH2 signal) according to the equation ([CL]0/[CL]t) =(A4.2 + A4.0)/A4.2.

Apparent rate constants (kapp) were extracted from the slopes of thebest-fit lines to the plots of ln([monomer]0/[monomer]t) vs time.

■ ASSOCIATED CONTENT*S Supporting InformationNMR, GPC, and DSC data for polymers obtained bycomplexes 1−10. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Fax (+ 34) 926-295-318; e-mail [email protected](A.O.) or [email protected] (A.L.-S.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge financial support from the Minister-io de Economia y Competitividad (MINECO), Spain (GrantNo. CTQ2011-22578 and Consolider-Ingenio 2010 ORFEOCSD 00006-2007).



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