Random Copolymerization of ε‑Caprolactone and Lactides Promotedby Pyrrolylpyridylamido Aluminum ComplexesGang Li,† Marina Lamberti,† Daniela Pappalardo,‡ and Claudio Pellecchia*,†

†Dipartimento di Chimica e Biologia, Universita di Salerno, I-84084, via Ponte don Melillo, Fisciano, Salerno, Italy‡Dipartimento di Scienze per la Biologia, la Geologia e l’Ambiente, Universita del Sannio, via dei Mulini 59/A, I-82100, Benevento,Italy

*S Supporting Information

ABSTRACT: The monomethylaluminum complexes 1 and 2,bearing pyrrolylpyridylamido as dianionic [−NNN−] tridentate ligandswith general formula [NNN]AlMe, were synthesized and tested asinitiators in the ring-opening polymerization (ROP) of ε-caprolac-tone, L-lactide, and D,L-lactide. In the presence of 1 equiv of alcohol,compounds 1 and 2 were highly active initiators in the ROP of ε-CL(TOF up to 4000 molCL molAl

−1 h−1), and they showed moderateactivity in the ROP of lactides (TOF up to 1.7 molLA molAl

−1 h−1).The polymerization processes proceeded with a living mechanism;moreover, the obtained PLAs resulted isotactic-enriched with Pmvalues up to 76%. More interestingly, this class of catalysts promoted the random copolymerization of ε-caprolactone andlactides. In particular, compound 1 allowed excellently controlled random copolymerization of ε-caprolactone and D,L-lactide asindicated by both the values of the reactivity ratios of the two monomers (rLA = 1.17; rCL = 1.36) and the average lengths of thecaproyl and lactidyl sequences (LCL = 2.0; LLA = 2.5).

■ INTRODUCTION

The synthesis of aliphatic polyesters, such as polycaprolactone(PCL), poly(lactic acid)s (PLA), and their copolymers, is atopic of wide interest in both academic and industrial researchbecause of their important applications in agriculture, pack-aging, and the biomedical field.1 The ring-opening polymer-ization (ROP) of the cyclic esters, such as ε-caprolactone and L-or D,L-lactide, promoted by metal coordination catalysts canprovide polymers of high molecular weight, low polydispersity,and controlled microstructure.2

The physical properties of PCL and PLA are quite differentand somehow complementary. In detail, PCL has remarkabledrug permeability, good elasticity, and thermal properties. Onthe contrary, PLA has good mechanical properties but poorelasticity, and it is hardly permeable to most drugs. Moreover,the PCL’s degradation rate (1 year in vivo) is much slower thanthat of PLA (a few weeks in vivo).1,3 Obviously, thecopolymerization of ε-CL and LA could enable the preparationof materials having improved features in comparison with theparent homopolymers. Consequently, the synthesis of PCL andPLA copolymers has attracted increased attention in thepharmaceutical and medical fields.4

The fine-tuning of the materials, in terms of mechanical andthermal properties, biodegradation behavior, and drugsolubility, could be achieved by controlling the compositionand distribution of the two comonomers through the polymerchain to meet the requirements of various applications.Growing interests have been devoted to the search of

efficient catalysts for the copolymerization of ε-CL and LA, and

different metal systems have been tested and reported, rangingfrom the traditional stannous octoate,4f to aluminumcompounds,4a,c to rare earth catalysts,4b and to zinc catalysts.4i

Nevertheless, truly random copolymers of ε-CL and LA havebeen rarely obtained. As a matter of fact, the rates ofhomopolymerization of ε-CL and LA are substantially different,with the ε-CL being faster; in ε-CL/LA copolymerization, thisorder is inverted, and the LA comonomer is consumed first. Asa result of the substantially different reactivity ratio, block(PLA-b-PCL) or gradient poly(LA-grad-CL) copolymers wereobtained. In other cases, copolymers having “randomized”structure were produced, as a consequence of transesterificationreactions occurring at high conversion and/or high temper-ature. For instance, the copolymers produced by Al(acac)3,

4a

Al(OiPr)3,4c rare earth catalysts,4b stannous octoate,4f and zinc

catalysts4i showed quite broad polydispersities and presented,in the carbonyl region of the 13C NMR spectra, signalscorresponding to the triads including one single “lactic” esterunit between two CL units, clear evidence of transesterificationreactions. Some of us succeeded in the preparation ofcopolymers having a tendentially random structure in theabsence of transesterification reactions.5

So far, rigorously random ε-CL/LA copolymers have beenrarely obtained. In the elegant work of Florczak and Duda, thenet reactivities of the ε-CL and LA comonomers were

Received: September 20, 2012Revised: October 12, 2012Published: November 2, 2012

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controlled by the configuration of the active species andresulted very close in the presence of a chiral binaphthyl-bridged salen aluminum alkoxide initiator.6 With a differentapproach, Nomura et al. described strictly random copoly-merization of ε-CL/LA with Bernoullian distribution of thecomonomers in the presence of a homosalen aluminumcomplex.7 In this case, the reactivity of lactide was reducedby increasing the bulkiness of the substituents in the orthopositions of the phenoxide groups.In this context the understanding at molecular level of the

origin of the differences in the reactivities of ε-CL and LAtoward the copolymerization is a very stimulating issue. Inorder to unravel the factors responsible of the copolymeriza-tion, it would be valuable to extend the class of initiators tocomplexes other than those bearing salen-type ligands.8

Recently, we reported the synthesis of pyrrolylpyridylamidodianionic [−NNN−] tridentate ligands and the preparation ofthe related pentacoordinate group 4 metal complexes, active inthe polymerization of ethylene and α-olefins.9 The same classof ligands has been used for the synthesis of group 3 complexes,resulting in very active initiators for the ROP of D,L-lactide toheterotactic PLA.10 Herein, we report on the synthesis of twonovel Al complexes bearing pyrrolylpyridylamido ligand andtheir application in the ring-opening homo- and copolymeriza-tion of ε-caprolactone, L,L-lactide, and D,L-lactide.

■ RESULTS AND DISCUSSION

The proligands have been synthesized following a previouslypublished synthetic procedure.9b Complexes 1 and 2 weresynthesized in toluene by the alkane elimination reactionbetween the corresponding proligands and 1 equiv of AlMe3. Atroom temperature the pyrrolylpyridylamido compoundscoordinate to the aluminum atom as monoanionic ligands,yielding the dimethyl compounds A (Scheme 1). In the 1HNMR spectra two sharp peaks for the methyl protons bound tothe aluminum atom and a single peak for the proton of theanilino moiety (NH) were observed. Compounds A werecompletely converted to the desired complexes by heating thesolutions at 90−100 °C overnight. Complexes 1 and 2 weresubsequently recovered, by evaporation of the solvent in vacuo,as yellow powders in good yields (1: 83%; 2: 98%). The

monomethyl complexes 1 and 2 were fully characterized byNMR spectroscopy and elemental analysis.In the 1H and 13C NMR spectra of complexes 1 and 2 each

proton of the ancillary ligands displays different signals,indicative of a C1 symmetry in solution. In the 1H NMRspectrum of complex 1 a sharp singlet at −0.72 ppm is observedfor the methyl protons of the Al−CH3, and two multiplets andfour doublets were observed respectively for the methine andthe methyl protons of isopropyl groups, whereas the protons ofthe bridging methylene appear as an AB pattern at 4.01 ppm(see Figure S1 in the Supporting Information). In the 1H NMRspectrum of complex 2, three multiplets of methine protonsand six doublets of the methyl protons of the isopropyl groupsand a sharp singlet of methyl protons of Al−CH3 at −0.69 ppmwere observed. NMR analysis of complex 2 revealed thepresence of two sets of similar signals in a ratio of 70:30 (seeFigure S3). Any attempt to separate the two species wasunfruitful. Variable temperature 1H NMR spectra acquiredfrom 298 to 353 K did not show significant changes.

Homopolymerization of ε-Caprolactone and Lactides.Complexes 1 and 2 were tested in the ring-openingpolymerization of ε-caprolactone. The main results aresummarized in Table 1. Complex 1 resulted highly active inthe ROP of ε-caprolactone at 70 °C converting 200 equiv of ε-caprolactone within 3 min (TOF: 4000 molCL molAl

−1 h−1).GPC analysis of the obtained PCL samples displayed anexperimental molecular weight higher than the theoretical one,suggesting a low initiation efficiency and a broad molecularweight distribution (run 1 in Table 1). However, a polymer-ization experiment carried out by adding 1 equiv of 2-propanol11 showed the same activity, a good agreementbetween the experimental and the theoretical Mn as well as anarrow molecular weight distributions (PDI = 1.23) (run 2 inTable 1). High activity and a good control over thepolymerization process were retained also at room temperature(run 3). These activity data compare favorably with those ofquinoline-based [N,N,N] aluminum complexes under similarreaction conditions.12

Complex 2 showed a lower activity (TOF: 34.4 molCLmolAl

−1 h−1) with respect to complex 1, indicating that theintroduction of bulky substituent on the carbon between thepyridine and anilido moieties is detrimental in the ring-opening

Scheme 1. Synthetic Route for Complexes 1 and 2

Table 1. Ring-Opening Polymerization of ε-Caprolactone Promoted by Complexes 1 and 2a

run [I0] [I0]/[ε-CL]/[ol] time (min) conv (%) Mn(th)b (103) Mn(GPC)

c (103) Mw/Mn

1 1 1/200/0 3 100 22.8 75.0 4.162 1 1/200/1 3 97 22.1 28.0 1.233d 1 1/200/1 60 100 22.8 18.5 1.124 2 1/200/1 300 86 19.6 17.2 1.19

aGeneral conditions: I0 = initiator, 10 μmol; toluene, 2 mL; ol = 2-propanol, 10 μmol; temperature, 70 °C. bCalculatedMn of PLA (g/mol) = 114.14× ([ε-CL]/[I0]) × conversion (ε-CL). cExperimental Mn values were determined by GPC analysis in THF using polystyrene standards andcorrected by the equation: Mn = 0.58Mn(GPC).

dPolymerization temperature: 25 °C.

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process. The polymerization reaction proceeded in a controlledfashion, resulting in a polymer with the expected molecularweight (17.2 kg/mol) and a narrow molecular weightdistribution (PDI = 1.19).Next, we turned to the ROP of lactides, carrying out the

reactions in toluene solution at 70 °C in the presence of 1 equivof 2-propanol. The main results are listed in Table 2. Complex1 was able to polymerize both L-LA and D,L-LA with the sameactivity and a good control of the molecular weight. The lineardependence of Mn versus the conversion and the narrow Mw/Mn ratio (runs 3−6 in Table 2 and Figure 1) are consistent witha living polymerization model.

Compound 2 showed a relatively lower activity in thepolymerization of D,L-LA (run 7, TOF: 1.3 molLA molAl

−1 h−1)with respect to that of compound 1 (run 6, TOF: 1.7 molLAmolAl

−1 h−1), the same trend which was observed in thepolymerization of ε-CL. However, compounds 1 and 2 showed,in the ROP of lactides, activity typical of tetracoordinatedaluminum complexes.13 The replacement of 2-propanol bybenzyl alcohol as cocatalyst resulted in a slightly lower activity.GPC analysis of the PLA samples produced by the compound 2displayed monomodal curve and narrow molecular weightdistributions, although the PDI values of these polymers werehigher than those obtained with complex 1.The tacticity of the obtained polymers was determined by

inspection of the methine region of the homonuclear decoupled1H NMR and 13C NMR spectra. The peaks were assigned to

the appropriate tetrads in accordance with the shifts reported inthe literature.15 Compounds 1 and 2 resulted able topolymerize D,L-LA to isotactic-enriched PLAs. Slightly higherPm values were obtained with the more encumbered catalyst 2.The homonuclear decoupled 1H NMR analysis of thestereosequences distributions of the obtained polylactidesamples produced by both 1 and 2 showed a 2:1:1:1 ratio ofthe [mrm]:[rrm]:[mrr]:[rmr] tetrads (see Figure 2), inagreement with an “enantiomorphic sites” mechanism of thesteric control.14

End-group analysis, performed by 1H NMR spectroscopy, ofthe obtained PLA samples showed that the polymer chainswere end-capped with an isopropyl ester and a hydroxyl group.

Copolymerization of ε-Caprolactone and Lactide.Copolymerizations of ε-CL and lactides were performedunder the same experimental conditions used for thehomopolymerizations (i.e., in the presence of 1 equiv of 2-propanol, in toluene solution, at 70 °C), by varying the ε-CL/lactides molar ratio. The obtained polymer samples werecharacterized by 1H and 13C NMR spectroscopy, GPC, andDSC analyses. The main results are summarized in Table 3.Polymerization runs initiated by compound 1 showed activity

typical of aluminum based ROP initiators and reached almostequal conversion of 200 equiv of ε-CL and lactides in 3−4 days.

Table 2. Ring-Opening Polymerization of Lactides Promoted by Complexes 1 and 2a

run [I0] monomer [I0]/[LA]/[ol] time (days) conv (%) Mn(th)b (103) Mn(GPC)

c (103) Mw/Mn Pmd

1 1 L-LA 1/100/1 3 88 12.7 15.9 1.152 1 D,L-LA 1/100/1 3 85 12.3 11.1 1.11 0.713 1 D,L-LA 1/200/1 1 40 11.5 8.8 1.12 0.704 1 D,L-LA 1/200/1 2 66 19.0 13.4 1.09 f5 1 D,L-LA 1/200/1 3 74 21.3 16.4 1.17 f6 1 D,L-LA 1/200/1 4 82 23.6 19.7 1.12 0.687 2 D,L-LA 1/200/1 4 61 17.6 8.9 1.33 0.738e 2 D,L-LA 1/200/1 4 54 15.6 8.6 1.30 0.76

aGeneral conditions: I0 = initiator, 10 μmol; toluene, 2 mL; ol = 2-propanol, 10 μmol; temperature, 70 °C. bCalculatedMn of PLA (g/mol) = 144.14× ([LA]/[I0]) × conversion (LA). cExperimental Mn values were determined by GPC analysis in THF using polystyrene standards and corrected bythe equation: Mn = 0.56Mn(GPC).

dPm is the probability of forming a new m-dyad, determined from the methine region of the homonucleardecoupled 1H NMR spectrum.14 eBenzyl alcohol, 10 μmol, was used instead of 2-propanol. fNot evaluated.

Figure 1. Dependence of Mn and polydispersity index (Mw/Mn) ofpolylactide on monomer conversion using complex 1 for D,L-LApolymerization in toluene at 70 °C (see runs 3−6 in Table 2).

Figure 2. Methine region of the homonuclear decoupled 1H NMR ofthe PLA sample obtained by complex 2/benzyl alcohol in toluene at70 °C (run 8 in Table 2).

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The relative contents of each monomer in the copolymerswere determined by 1H NMR spectroscopy in CDCl3, throughthe ratio of the integrated values of the methylene signals of theε-CL units around 4.0 ppm and the methine signals of LAaround 5.2 ppm.The copolymer composition parallels the ε-CL/LA molar

ratio in the feed (see Table 3). While this feature is obviouslyexpected when the full conversion is approaching, it was alsonoticed at lower conversion. For instance (see run 3, in Table3), the copolymerization of ε-CL/D,L-LA (100/100) at 74%conversion of ε-CL and 83% conversion of D,L-LA produced acopolymer having 53% of “opened” ε-CL units. The sameexperiment, carried out with equimolar ε-CL/D,L-LA feed ratio(100/100) but at lower conversion (46% for D,L-LA and 49%for ε-CL) also resulted in 55% of opened ε-CL in thecopolymer (run 8 in Table 3). These data suggested that the

two monomers were homogeneously incorporated during thecopolymerization process.Analogous results were obtained in the presence of

compound 2, having a bulkier substituent; with a lowerpolymerization rate (5 days), a 43% conversion of ε-CL and54% conversion of LA were obtained with 37% of opened ε-CLunits in the copolymer (Table 3, run 7).Moreover, substantially no effect on the rate of copoly-

merizations was observed by comparing, for compound 1, theL-lactide/CL copolymerization rate and that of D,L-lactide/CL(cf. runs 3 and 6 in Table 3).A detailed characterization of the copolymers chain micro-

structures was performed by 1H NMR and 13C NMR analysis atdiads and triads level. The percentage of CL−LA heterodiadswere calculated by comparing, in the 1H NMR spectrum, theintensity of the signals of the methylene protons close to the

Table 3. Copolymerization of Lactide and ε-Caprolactonea

conv (%)

run I0time(days)

CL:lactide (mol:mol) inthe feed

CL in the copolymer(mol %) LCL

b Llactideb

yield(g) CL LA

Mn,GPCc

(×103) PDI Tg (°C) Tgd

1 1 4 40:160 25 1.2 7.5 0.195 82 85 43 1.25 30 15.02 1 4 80:120 40 1.9 2.7 0.177 83 81 40 1.26 1.0 −0.643 1 3 100:100 53 2.0 2.5 0.142 74 83 27 1.13 −3.8 −15.04 1 3 120:80 71 3.3 1.8 0.187 88 85 31 1.24 −25 −33.25 1 3 160:40 85 5.4 1.0 0.167 82 93 34 1.23 −46 −39.36 1 3 100:100e 55 2.8 3.2 0.167 82 87 49 1.31 −5.5 −12.77 2 5 100:100 37 1.9 4.9 0.105 43 54 17 1.07 −11 1.78 1 1 100:100 55 2.1 1.7 n.d. 46 49 15 1.08 n.d. n.d.

aGeneral conditions: initiator: 10 μmol; toluene: 2 mL; 2-propanol: 10 μmol; D,L-lactide: 2 mmol; temperature: 70 °C. bAverage sequences length ofthe caproyl unit and of the lactidyl unit as determined by 13C NMR analysis. cExperimental Mn values were determined by GPC analysis in THFusing polystyrene standards. dTheoretical values calculated by Fox equation, by using for the Tg of the homopolymers the following literature values:PCL: −60 °C; PLLA: 57 °C; PDLLA: 45 °C.4a eL-Lactide was used instead of D,L-lactide.

Figure 3. Carbonyl range of 13C NMR spectra (CDCl3, 25 °C) of copolymers of runs 1−5 in Table 3.

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carbonyl of the CL−LA heterosequences with the samemethylene protons for the CL−CL homosequences. Increasingthe amount of lactide in the feed, the amount of CL−LAheterodiads increased, thus indicating a random copolymeriza-tion behavior (see Figure S6 in the Supporting Information).The analysis of the chain microstructures at triads level was

performed by inspection of the carbonyl region range (from165 to 175 ppm) of the 13C NMR spectra of the obtainedcopolymers (Figure 3). As expected for the case of a binarycopolymerization, eight triads were observed. According to theliterature, the four peaks between 173.02 and 173.74 ppmderived from the caproyl unit, while the peaks between 170.57and 169.54 ppm can be attributed to the lactidyl unit.4b

It is worth noting that the signal at 171 ppm, related to thetriad having one single “lactic” ester unit between two CL units,was never detected. That triad is indicative of the occurrence oftransesterification reactions because it cannot result from theinsertion of the lactide monomer into the chain.4a The absenceof transesterification reactions is further confirmed by the PDIdata, determined by GPC; as matter of fact, values between 1.1and 1.3 were observed, indication of a “controlled” polymer-ization.The average length of the caproyl (LCL) and lactidyl (LLA)

sequences can be calculated from the integrals of the triadssequences signals following previously reported methods.4b

Obviously, both the LCL and LLA increased as the relativemonomer amount in the feed increased. Moreover, for a 1:1ratio of the monomers in the feed, the LCL and LLA values were∼2, not only approaching full conversion (see run 3 in Table3), but also at lower conversion (46% for D,L-LA and 49% for ε-CL) (see run 8, Table 3). Again, a random copolymerizationbehavior is evidenced by these experimental features.The determination of the reactivity ratios rLA and rCL was

performed by using the nonlinear least-squares (NLLS)method carrying out the copolymerizations of the monomerwith different compositions (LA:CL ≅ 10:90, 30:70, 50:50,70:30, and 90:10) at a low conversion (see SupportingInformation).7 The values rLA = 1.17 and rCL = 1.36 indicatedan ideal random copolymerization.Thermal analysis of the copolymers was carried out by

differential scanning calorimetry (DSC) in the range from−100 to 200 °C. The obtained copolymers were amorphous. Inall cases, the copolymer samples displayed unique glasstransition temperature (Tg) with values intermediate between−60 °C of poly(caprolactone) and 45 °C of poly(D,L-LA), anddecreasing as the percentage compositions of ε-CL in thecopolymer increased. The experimental values of Tg were ingood agreement with the theoretical values calculated by theFox equation16 (Figure 4). These data are a further support tothe random structure of copolymers.

■ CONCLUSIONSIt is well established that the copolymerization of ε-CL and LAproduces materials having improved properties in comparisonwith the parents homopolymers. In this regards the search forefficient initiator for the synthesis of copolymers havingcontrolled composition and architecture is a very stimulatingfield. The pyrrolylpyridylamido compound 1 promotedcontrolled homopolymerization of ε-CL and LA, producingpolymers having narrow molecular weight distribution, and inthe case of D,L-lactide, a living behavior was also assessed. Thesame initiator promoted the random copolymerization of ε-CLand D,L-LA, as indicated by the reactivity ratio values (rLA =

1.17; rCL = 1.36). Moreover, the narrow molecular weightdistribution and the absence of deleterious transterificationreaction indicated that the controlled behavior was kept alsoduring the copolymerization. Coherently, the thermal charac-terization of the copolymers disclosed amorphous materialswhose Tg values were modifiable in a linear fashion between−60 and 45 °C by adjusting the relative proportions of LA andε-CL content. We can envisage that these results should beuseful in applications were modulated thermal, physical, anddegradation properties of PCL/PLA-based materials arerequired.

■ EXPERIMENTAL SECTIONGeneral Procedures. All manipulations of air- and/or water-

sensitive compounds were carried out under a nitrogen atmosphereusing standard Schlenk or glovebox techniques. All solvents werepurchased from Carlo Erba; toluene was refluxed over sodium/benzophenone and then distilled under nitrogen before use. Thepyrrolylpyridylamido proligands were synthesized according to thepreviously reported procedures.9b D,L-Lactide and L-lactide (Aldrich)were purified by crystallization from dry toluene. 2-Propanol andbenzyl alcohol were purified by distillation over sodium. ε-Caprolactone (Aldrich reagents) was dried with CaH2 for 24 h atroom temperature and then distilled under reduced pressure. All otherchemicals were purchased from Sigma-Aldrich and used as receivedunless otherwise stated.

Instruments and Measurements. The NMR spectra wererecorded on Bruker Avance 400 spectrometer (1H, 400.00 MHz;13C, 100.62 MHz) at 25 °C, unless otherwise stated. Deuteratedsolvents were purchased from Cambridge Isotope Laboratories, Inc.,degassed, and dried over activated 3 Å molecular sieves prior to use.Chemical shifts (δ) are listed as parts per million and couplingconstants (J) in hertz. 1H NMR spectra are referenced using theresidual solvent peak at δ 7.16 ppm for C6D6 and 7.27 ppm for CDCl3.13C NMR spectra are referenced using the residual solvent peak at δ128.39 ppm for C6D6 and 77.23 ppm for CDCl3.

The molecular weights (Mn) and the molecular mass distribution(Mw/Mn) of polymer samples were measured by GPC at 30 °C, theuse of THF as solvent, flow rate of eluant 1.0 mL/min, and narrowpolystyrene standards as reference. The measurements wereperformed on a Waters 1525 binary system equipped with a Waters2414 RI detector using four Styragel columns (range 1000−1 000 000Å). Every value was the average of two independent measurements.

Glass transition temperatures (Tg) and melting points (Tm) of thepolymers were measured by differential scanning calorimetry (DSC)

Figure 4. Experimental () and theoretical (···) Tg of the CL/LAcopolymers as a function of the mole fraction of ε-CL unit.

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using a DSC 2920 (TA Instruments) in nitrogen flow with a heatingand cooling rate of 10 °C min−1 in the range −100 to 200 °C. Glasstransition temperatures and melting temperatures were reported forthe second heating cycle.Synthesis of Complexes. Complex 1. A solution of AlMe3 (43

mg, 0.6 mmol) in toluene (1 mL) was added dropwise into a stirredsolution of H2L

1 (200 mg, 0.6 mmol) in toluene (4 mL). The yellowsolution was heated at 90−100 °C and stirred overnight. All volatileswere removed under vacuum to yield a yellow solid. The crudeproduct was washed with pentane. The light yellow solid was obtainedin 83% yield (186 mg, 0.5 mmol). 1H NMR (400 MHz; C6D6): δ−0.72 (3H, s, AlCH3), 1.04 (3H, d, J = 8 Hz, CH(CH3)2), 1.06 (3H, d,J = 8 Hz, CH(CH3)2), 1.14 (3H, d, J = 8 Hz, CH(CH3)2), 1.28 (3H, d,J = 8 Hz, CH(CH3)2), 2.92−3.08 (2H, m, CH(CH3)2), 3.83 (1H, d, J= 20 Hz, CHH), 4.17 (1H, d, J = 20 Hz, CHH), 6.03 (1H, d, J = 8 Hz,Ar-H), 6.45 (1H, m, pyrrole-H), 6.61 (1H, m, pyrrole-H), 6.78 (1H, t,Ar-H), 7.04 (1H, m, pyrrole-H), 7.10−7.19 (3H, m, Ar-H), 7.31 (1H,d, J = 8 Hz, Ar-H) ppm. 13C NMR (100.62 MHz; C6D6): δ −13.73,24.42, 24.70, 26.14, 27.45, 27.77, 59.55, 112.05, 116.56, 118.54,123.38, 123.56, 124.52, 125.18, 129.34, 131.05, 132.28, 139.01, 146.07,147.78, 148.79, 156.00, 163.60 ppm. Elemental analysis: Calcd forC23H28AlN3 (Fw = 373.21): C, 73.97; H, 7.56; N, 11.25%. Found: C,74.15; H, 7.68; N, 11.65%.Complex 2. A solution of AlMe3 (19 mg, 0.25 mmol) in toluene (1

mL) was added dropwise into a stirred solution of H2L1 (115 mg, 0.25

mmol) in toluene (4 mL). The yellow solution was heated at 90−100°C and stirred overnight. All volatiles were removed under vacuum toyield a yellow solid. The crude product was washed with pentane. Thelight yellow solid was obtained in 98% yield (121 mg, 0.24 mmol). 1HNMR (400 MHz; C6D6): δ −0.69 (3H, S, AlCH3), 0.13 (3H, d, J = 8Hz, CH(CH3)2), 0.64 (3H, d, J = 8 Hz, CH(CH3)2), 0.69 (3H, d, J = 8Hz, CH(CH3)2), 1.12 (3H, d, J = 8 Hz, CH(CH3)2), 1.18 (3H, d, J = 8Hz, CH(CH3)2), 1.31 (2H, d, J = 8 Hz, CH(CH3)2), 2.67 (1H, m,CH(CH3)2), 3.06 (1H, m, CH(CH3)2), 3.19 (1H, m, CH(CH3)2),5.59 (1H, d, J = 8 Hz, Ar-H), 5.84 (1H, s, CHN), 6.17 (1H, t, Ar−H),6.39 (1H, m, pyrrole-H), 6.58 (1H, m, pyrrole-H), 6.71 (1H, d, J = 8Hz, Ar-H), 6.87 (1H, t, Ar-H), 6.98−7.10 (5H, m, Ar-H), 7.30 (1H, d,J = 8 Hz, Ar-H) ppm. 13C NMR (100.62 MHz; C6D6): δ −9.77 (Al−CH3), 22.67, 24.24, 25.37, 26.58, 26.72, 27.53, 27.84, 28.58, 67.31,112.55, 121.23, 121.27, 124.15, 124.61, 125.17, 125.62, 125.93, 127.07,127.69, 130.37, 133.13, 133.51, 141.44, 141.56, 144.66, 148.29, 148.49,149.43, 156.63, 168.58 ppm. Elemental analysis: Calcd for C32H38AlN3(Fw = 491.29): C, 78.17; H, 7.79; N, 8.55%. Found: C, 78.87; H, 7.99;N, 8.20%.Compounds A. By reaction of AlMe3 with 1 equiv of each proligand

in the same conditions described for complexes 1 and 2 but operatingat room temperature, compounds A were isolated.Compound A1. 1H NMR (400 MHz; tol-d8): δ −0.37 (3H, s,

AlCH3), −0.31 (3H, s, AlCH3), 1.1 (12H, d, J = 8 Hz, CH(CH3)2),3.05 (2H, m, CH(CH3)2), 4.34 (1H, d, J = 8 Hz, CH2), 4.42 (1H, br,NH), 6.36 (1H, d, J = 8 Hz, Ar-H), 6.49 (1H, m, pyrrole-H), 6.68 (1H,t, Ar-H), 6.76−6.68 (1H, m, Ar-H), 6.95−7.05 (4H, m, Ar-H) ppm.Compound A2. 1H NMR (400 MHz; C6D6): δ −0.29 (3H, S,

AlCH3), −0.02 (3H, S, AlCH3), 0.70 (6H, d, J = 6.64 Hz, CH(CH3)2),0.99 (6H, d, J = 6.72 Hz, CH(CH3)2), 1.14 (6H, br, CH(CH3)2), 2.51(2H, m, CH(CH3)2), 2.92 (1H, br, CH(CH3)2), 3.05 (1H, s, NH),5.24 (1H, d, J = 8 Hz, Ar-H), 5.75 (1H, d, J = 8 Hz, Ar-H), 6.06 (1H,d, J = 8 Hz, Ar-H), 6.71−6.77 (2H, m, CHN, Ar-H), 6.88−7.01 (7H,m, Ar-H), 7.11 (1H, d, J = 8 Hz, Ar-H), 7.43 (1H, m, Ar-H) ppm.General Polymerization Procedure. In a Braun Labmaster

glovebox a 10 mL Schlenk tube was charged sequentially with asolution of monomer in 1.5 mL of dry toluene, 10 μmol of 2-propanol,and a solution of the aluminum complex (10 μmol) in 0.5 mL of drytoluene. The mixture was stirred at the desired temperature. After thedesired time, an aliquot of the reaction mixture was sampled with apipet for determining the monomer conversion by 1H NMRspectroscopy (CDCl3, 400 MHz). The reaction mixture was quenchedby adding wet n-hexane. The polymer was filtered and dried in avacuum oven at 60 °C overnight.

General Copolymerization Procedure. In a typical preparation,the reactor vessel was charged sequentially with D,L-lactide, thealuminum complex (10 μmol), 2-propanol (10 μmol), 2 mL oftoluene, and ε-caprolactone. The mixture was thermostated at 70 °C,and after the desired time, the polymerization solution was quenchedin hexane. The copolymer was purified by redissolving in CH2Cl2 andprecipitated from rapidly stirring methanol. The polymer wasrecovered by filtration and dried at 40 °C in a vacuum oven.

■ ASSOCIATED CONTENT*S Supporting Information1H and 13C NMR spectra of complexes 1 and 2; 1H NMRspectra of the copolymer samples; copolymerization data forthe determination of the reactivity ratios. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Fax (+39) 089969603; e-mail cpellecchia@unisa.it.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank Dr. Patrizia Iannece for elemental analyses,Dr. Patrizia Oliva for NMR assistance, and Dr. MariagraziaNapoli for GPC measurements. This work was supported bythe University of Salerno (FARB Grant).

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