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Ring-Opening (ROP) versus Ring-Expansion (REP) Polymerization of ε‑Caprolactone To Give Linear or Cyclic Polycaprolactones José A. Castro-Osma,† Carlos Alonso-Moreno,‡ Joaquín C. García-Martinez,‡ Juan Fernández-Baeza,† Luis F. Sánchez-Barba,† Agustín Lara-Sánchez,*,† and Antonio Otero*,† †

Facultad de Ciencias y Tecnología Química de Ciudad Real, Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain ‡ Facultad de Farmacia de Albacete, Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, 02071 Albacete, Spain S Supporting Information *

ABSTRACT: Macromolecular engineering of cyclic polycaprolactones has been carried out by a ring-expansion procedure catalyzed by a series of alkyl-organoaluminum initiators. The single-site nature of the initiators allows a very well-controlled macrolactonization process to give moderate to high molecular weight cyclic polymers with narrow polydispersities. Cyclic architectures are supported by a combination of techniques such as viscosity measurements, NMR, and MALDI-TOF MS analysis.

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INTRODUCTION The past two decades of the twentieth century have witnessed a paradigm shift from biostable to biodegradable biomaterials for medical and related applications.1 In fact, this current trend has promoted a spectacular advance in new polyester materials and the development of therapeutic devices such as scaffolds for tissue engineering and as drug delivery vehicles.2 Among them, polycaprolactone (PCL), polylactide (PLA), and their copolymers have represented potential candidates to replace traditional olefin-based polymers, since they are biodegradable and the nontoxic products can be reabsorbed or excreted by the human body.3 Ring-opening polymerization of cyclic esters (ROP) has been the most versatile method employed to prepare a variety of polymers with control of the major variables that affect polymer properties.4 A plethora of organometallic initiators supported by a judicious selection of ancillary ligands are now available for the ROP of cyclic esters.5 Among them, complexes bearing scorpionate ligands have proved to be highly productive and selective initiators for the polymerization of lactides and lactones.6 In this field, macromolecular engineering of large-ring cyclic polyesters is a redoubtable challenge; for example, in polymer chemistry macrolactones present a broad range of interesting medicinal properties.7 Although several elegant routes for the synthesis of cyclic polymers have been reported, there is still a need for a suitable method for the synthesis of high molecular weight cyclic polymers with narrow polydispersities.8 The most common strategy relies on the coupling of linear chains by a difunctional agent9 or by reactive hetero end groups.10 Despite the fact that this route has been significantly improved, © 2013 American Chemical Society

undesired polycondensation to give a mixture of linear and cyclic polymers, the need to use a highly dilute medium, and difficulties in controlling the process are severe limitations for the production of high-molecular-weight cyclic polymers. The kinetically controlled synthesis of cyclic polyesters is another elegant route that has been reported. In this approach, a controlled ring-expansion polymerization (REP) involving the use of cyclic tin alkoxides has been developed,11 albeit with a significant toxicity limitiation.12 Zwitterionic polymerization has been reported as an easy way to synthesize long cyclic oligomers by Kricheldorf et al.13 and cyclic polymers by Waymouth et al.,14 both with a defined molecular weight and narrow molecular weight distribution. However, the organocatalytic process requires high initiator-to-monomer ratios along with the addition of an aprotic trapping agent for the organocatalyst to terminate the reaction. It is worth highlighting a recent publication concerning the first aluminum catalyst capable of undergoing REP to give cyclic polylactide architectures of defined molecular weight.15 As summarized above, a significant number of synthetic methodologies to give cyclic polyesters are being explored in the field of catalysis. Herein, we would like to add the first series of alkyl-organoaluminum complexes to the ever-expanding body of catalysts for the well-controlled REP of cyclic esters.7 The alkyl-organoaluminum complexes 1−6 show a very rare ability to generate controlled cyclic polylactones. Received: June 13, 2013 Revised: July 29, 2013 Published: August 16, 2013 6388

dx.doi.org/10.1021/ma401216u | Macromolecules 2013, 46, 6388−6394

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RESULTS AND DISCUSSION As reported previously, a straightforward one-step alkane elimination reaction was used to obtain the monoalkylScheme 1. Molecular Structures of Compounds 1−12

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

initiator

time (min)

yieldb (%)

Mw(exp)c (Da)

Mw/Mn

1 2 3 4 5 6 7 8 9 10 11 12 13d 14e 15f 16g

1 2 3 4 5 6 7 8 9 10 11 12 1 1 1 1

20 20 20 20 30 30 15 16 15 14 22 20 25 25 9 16 h

97 99 91 98 98 99 99 97 97 96 98 97 98 95 98 99

21 140 20 540 22 930 22 510 22 190 23 256 36 220 30 360 29 990 32 250 30 490 28 810 32 060 52 020 27 830 18 690

1.17 1.21 1.19 1.19 1.15 1.18 1.21 1.22 1.20 1.18 1.17 1.16 1.35 1.37 1.25 1.02

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

organoaluminum complexes 1−66b and the dialkyl-organoaluminum complexes 7−126c as fine colorless powders (see Scheme 1). The complexes 1−12 are stable in the presence of air in the solid state for several days at room temperature (see Figures S1−S3 in the Supporting Information). Drawing upon previous studies in which the aforementioned complexes were shown to act as efficient and versatile initiators in ROP,6b,c we were initially interested in completing our initial studies by the systematic evaluation of complexes 1−12 in the ROP of ε-caprolactone (ε-CL). First, it is worth noting that the evaluation of complexes 1−12 in the ROP of rac-LA showed them to be single-site initiators capable of affording PLAs with relatively narrow polydispersities and molecular weights in good agreement with calculated values. NMR and MALDITOF MS analysis provided evidence that both series of initiators led to linear polyesters through a coordination/ insertion mechanism. 6b,c When complexes 1−12 were evaluated as initiators in the polymerization of the monomer ε-CL, the results observed on using complexes 1−6 were intriguing. The molecular weight (Mw), molecular weight distribution (Mw/Mn), and yield of PCLs obtained on using compounds 1−12 as initiators are compiled in Table 1. As observed previously, the polymerization activity of compounds 1−12 is consistent with a well-defined catalyst. GPC (gel permeation chromatography) analysis of the polymers (see Figures S4−S15 in the Supporting Information) supported this proposed situation, with a narrow Mw/Mn (entries 1−12 in Table 1) and a monomodal distribution (see Figure 1 as example GPC traces for entries 1 and 7 in Table 1).

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

Figure 1. GPC spectra of linear PCL and cyclic PCL showing the shift in retention time of the cyclic polymer due to the smaller hydrodynamic volume.

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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, prepared with 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.

two spectra (inset Figure 3). This difference is consistent with the incorporation of a methyl initiator group in the polymer obtained with complex 7. Moreover, and as a representative example, the MALDI-TOF spectrum of polycaprolactone obtained with complex 1 shows a peak at 2989.63 Da (Figure 3a), and this can be assigned to the cyclic polymer containing 26 repeat units. On the other hand, the peak at 3004.21 Da (Figure 3b) matches with a linear polymer containing the same number of repeating units but with methyl as the initiator group. The MALDI-TOF results unambiguously show that the chemical composition of both polymers is consistent with a macrolactonization process to give cyclic polymers when monoalkyl-organoaluminum complexes 1−66b are used and a normal ROP process to give linear polymers when the polymerization is catalyzed by dialkyl-organoaluminum complexes 7−12. Thermal characterization of a cyclic PCL verifies the expected decrease in glass transition temperature when comparison with linear analogous is carried out (see Figures S16 and S17 in the Supporting Information).16 It is well-known that ring polymers have smaller hydrodynamic volumes than their linear analogues.9 Thus, cyclic polymers exhibit a range of unique dilute solution properties, and these enable their fundamental physical properties to be measured.9 As a result, further evidence for the cyclic structure of these PCLs can be provided by GPC in conjunction with light-scattering and viscometer detectors. Comparison of the logarithm of absolute Mw versus elution volumes of PCLs of the

The results in Table 1 are indicative of the single site nature of the initiators and a well-controlled growth of the polymer chain on the metal center (see Table S1 in the Supporting Information). Complexes 1−6 and 7−12 were designed to have different steric and electronic features on the metal center, and it was anticipated that some differences would be found in their 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. entries 1 and 7 in Table 1). Furthermore, when the low-molecularweight samples were characterized, in the first instance by 1H NMR 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 for the presence of end groups was not observed for polymers obtained with initiators 1−6. MALDI-TOF mass spectra supported the NMR experimental evidence, revealing well-defined cyclic structures when complexes 1−6 were used as initiators in the polymerization. As a representative example, the MALDI-TOF MS spectra of typical samples of PCL obtained with complexes 1 and 7 using DCTB and NaI as matrix and cationization agent, respectively, are shown in Figure 3. Both of these spectra display a monomodal distribution of peaks corresponding to a unique well-defined polymer structure with equal peak separation corresponding to the repeating unit of ε-CL. In addition to the similarities, a shift of ca. 16 Da is observed on comparing the 6390


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

Figure 5. (a) First-order kinetic plots for ε-CL polymerizations in toluene at 110 °C with [CL]/[Al] = 200 and [Al] = 4.5 × 10−3 mol L−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 versus conversion 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.

same Mw (obtained from 1 and 7) shows that lower elution volumes were obtained for PCLs from complex 7 (Figure 4b). This observation is again indicative of a cyclic architecture for polymers obtained with 1−6 as a smaller hydrodynamic radius results in longer GPC elution times relative to linear counterparts.9 The cyclic topology was finally confirmed by comparing the dilute solution viscosities of polymers obtained with 1 and 7. An increase in the intrinsic viscosity was observed on increasing the molecular weight for both types of complexes (Figure 4a). PCL samples obtained by 1 as initiator showed lower intrinsic viscosities compared to the linear counterparts due to the loss of the link at each end of a linear polymer chain to produce a cyclic polymer. Bearing in mind the results discussed above, it is clear that complexes 1−6 are the first alkyl-organoaluminum initiators that are capable of polymerizing ε-CL to give moderate to high molecular weight cyclic polymers with narrow polydispersities. Encouraged by these results, we decided to obtain more details about the catalytic performance of these systems. Polymerizations with complexes 1 and 7, as representative examples of the two series of complexes, were monitored over time by regular manual sampling followed by 1H NMR analysis to determine the degree of monomer conversion. The semilogarithmic plots of ln([CL]0/[CL]t) versus reaction time are shown in Figure 5a, where [CL]0 is the initial caprolactone monomer concentration and [CL]t the lactone concentration at a given reaction time t. In both cases the linearity of the plot shows that the propagations were first order with respect to lactone monomer. Furthermore, an induction period was not

observed, and this indicates the presence of a reactive species from the outset. The linearity of the plots also illustrates that termination reactions did not occur during polymerizations with complexes 1 and 7. The kapp values for both initiators are of the same order of magnitude, but the monoalkyl derivative 1 presents the lowest kapp value under the same reaction conditions, probably due to the steric hindrance caused by the two polydentate ligands on aluminum retarding the interaction between the Al center and lactone. Complex 1 was also able to polymerize ε-CL in a wellcontrolled manner on using [CL]:[Al] ratios of 350 and 500 (see entries 13 and 14 in Table 1), but as expected, higher molecular weight polymers with higher molecular weight distributions were obtained. In addition, complex 1 shows higher activity and gives higher molecular weight distribution for 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) the polymerization rates are rapid, with complete conversion achieved within minutes; (2) narrow molecular weight distributions of polymers are obtained; and (3) the molecular weight of the polymers increases with monomer conversion (see Figure 5b). It is worth noting that these features imply that the rate of macrolactonization to generate the cyclic polyester architectures 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 dialkylorganoaluminum complexes follow a well-known nucleophilic route, in which polymerization is initiated by the transfer of an 6391


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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 a ratio [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 CL with complex 3, in a ratio [CL]:[AL] = 10, after 10 min, in toluene-d8 at 110 °C, the CL monomer is consumed. 6392


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from sodium (toluene) or sodium−potassium alloy (n-hexane). Deuterated solvents were stored over activated 4 Å molecular sieves and degassed by several freeze−thaw cycles. Microanalyses were carried out on a PerkinElmer 2400 CHN analyzer. 1H and 13C NMR spectra were recorded on a Varian Inova FT-500 spectrometer and referenced to the residual deuterated solvent. ε-Caprolactone was purchased from Alfa-Aesar. The organoaluminum complexes were prepared according to literature procedures.6b,c ε-Caprolactone was dried by stirring over fresh CaH2 for 48 h, then distilled under reduced pressure, and stored over activated 4 Å molecular sieves. Gel permeation chromatography (GPC) and viscosity measurements were performed on a Viscotek GPCmax VE-2001 TDA 302 Detectors instrument equipped with a Waters Styragel column with triple detection, i.e., LALS (low angle light scattering), RALS (right angle light scattering), and viscosity detector. The GPC column was eluted with THF at 50 °C at 1 mL/min and was calibrated using eight monodisperse polystyrene standards in the range 580−483 000 Da. The MALDI-TOF spectra were acquired using a Bruker Autoflex II TOF/TOF spectrometer using DCTB and NaI as matrix and cationization agent, respectively. Samples cocrystallized with matrix in a ratio of 100:1 on the probe were ionized in positive reflector mode. External calibration was performed by using Peptide Calibration Standard II (covered mass range: 700−3200 Da) and Protein Calibration Standard I (covered mass range: 5000−17 500 Da). General Polymerization Procedure. Schlenk tubes were charged in the glovebox with the required amount of monomer and initiator, separately, and then attached to the vacuum line. The initiator and monomer were dissolved in the appropriate amount of solvent, and temperature equilibration was ensured in both Schlenk flasks by stirring the solutions for 15 min in a bath. The appropriate amount of initiator was added by syringe, and polymerization times were measured from that point. Polymerizations were stopped by injecting a solution of acetic acid (5 vol %) in methanol. Polymers were precipitated in a mixture of methanol/hexane, filtered off, washed three times with 20 mL of methanol, and dried in vacuum to constant weight. Polymerization Kinetics. Kinetic experiments were carried out in flasks at 110 °C on the Schlenk line with polymerizations using stock solutions of the reagents. Specifically, at appropriate time intervals samples were removed by syringe and quickly quenched into 1 mL vials containing 0.6 mL of undried “wet” CDCl3. The quenched aliquots were analyzed by 1H NMR spectroscopy. For ε-CL polymerization, the [CL]0/[CL]t ratio was determined by integration of the peaks for ε-CL (4.2 ppm for the OCH2 signal) and PCL (4.0 ppm 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 the best-fit lines to the plots of ln([monomer]0/[monomer]t) vs time.

alkyl ligand to the monomer, with cleavage of the acyl−oxygen bond and formation of the metal alkoxide propagating species.6c This coordination/insertion mechanism in the ROP of ε-CL is supported, in our studies, by the alkyl end groups observed by MALDI-TOF and 1H NMR spectroscopy. On the other hand, the preparation of cyclic PCLs with 1−6 should follow a tailored organometallic catalyzed ring-expansion process (REP). A cyclic catalyst would be generated by nucleophilic attack by the scorpionate ligand, and macrolactonization could occur without the involvement of the alkyl group directly attached to the aluminum center. Repeated monomer insertion would generate macrometallacycles which, after an intramolecular chain-transfer reaction, would produce cyclic polylactones and regenerate the monoalkyl-organoaluminum initiator. 1 H NMR spectroscopic data support the proposed REP of εCL by initiators 1−6. As a representative example of these studies, REP of ε-CL with 3 monitored by 1H NMR spectroscopy is shown in Figure 6 (see also full-page images with peaks integration for the 1H NMR spectra in Figures S18− S21 of the Supporting Information). Signals for the remaining monomer can be observed in Figures 6b,c, whereas the end of the polymerization reaction is evident in Figure 6d since evidence for remaining monomer is not observed. A singlet resonance for the Al−Me group can be observed in Figures 6a− d at around at δ = −0.2 ppm, and this might indicate the exclusion of the methyl ligand in the activation reaction. The fact that the methyl ligand is not transferred to the growing chain and is retained by the organoaluminum compound provides sufficient evidence to support a macrolactonization process in which initiators 1−6 take part. However, due to the high lability of the Al−N(pyrazole) bond,6c the displacement of the pyrazole ring by other strong donors cannot be ruled out.

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CONCLUSION In summary, we report highly efficient single-site initiators for the well-controlled synthesis of cyclic polycaprolactones with moderate to high molecular weight and narrow polydispersities. In contrast, organoaluminum counterparts with only one polydentate ligand coordinated to the metal center undergo ROP of ε-CL to give linear PCLs. Cyclic polymeric architectures are supported by NMR and MALDI-TOF studies as well as by comparison of the intrinsic viscosities and elution times of the cyclic structures with the linear analogues. The synthesis of cyclic PCLs by polymerization with monoalkyl-organoaluminum compounds 1−6 as initiators is particularly attractive because (1) it is well-controlled and efficient, (2) it is highly productive, (3) it does not generate linear intermediates, (4) it does not require time-consuming purification procedures, (5) only small quantities of initiator are required, and (6) cyclic polymers with defined moderate to high molecular weights and narrow polydispersities are obtained. Further studies are underway to examine thoroughly the effect on the polymerization behavior of changing both the metal center and the substituent of the ligand framework in an effort to expand the scope of the initiators in the synthesis of new cyclic polylactone architectures.

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ASSOCIATED CONTENT

S Supporting Information *

NMR, GPC, and DSC data for polymers obtained by complexes 1−10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax (+ 34) 926-295-318; e-mail [email protected] (A.O.) or [email protected] (A.L.-S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministerio de Economiá y Competitividad (MINECO), Spain (Grant No. CTQ2011-22578 and Consolider-Ingenio 2010 ORFEO CSD 00006-2007).

EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under nitrogen using standard Schlenk techniques. Solvents were predried over sodium wire (toluene, n-hexane) and distilled under nitrogen 6393


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REFERENCES

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