Polymerization of (Macro)Lactones Using Aluminum Salen Catalysts

Article pubs.acs.org/Macromolecules

Kinetic Investigation on the Catalytic Ring-Opening (Co)Polymerization of (Macro)Lactones Using Aluminum Salen Catalysts M. P. F. Pepels, M. Bouyahyi, A. Heise, and R. Duchateau* Laboratory of Polymer Materials, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, The Netherlands S Supporting Information *

ABSTRACT: The kinetic behavior of the catalytic ringopening polymerization (cROP) of a range of macrolactones, including ω-pentadecalaconte (PDL), ambrettolide (Amb), and butylene adipate (BA), and small-ring lactones, including L-lactide (LLA), ε-caprolactone (ε-CL), ε-decalactone (ε-DL), and β-butyrolactone (B-BL), using various aluminum salen complexes was investigated. The cROP rates were shown to be first order both in catalyst and in monomer. The activation energies of the polymerization of PDL and LLA in combination with aluminum salen complexes, with and without tert-butyl groups, were determined, showing that the increase in steric hindrance is negatively affecting the polymerization rate of LLA more than of PDL. Interestingly, an increase of the salen diimine bridge from ethylene to 2,2dimethyl propylene leads to a dramatic increase in rate for the polymerization of small-ring lactones, while it leaves the rate of polymerization of macrolactones practically unchanged. In order to exploit this difference in reactivity, the synthesis of blockcopolymers of ε-CL and PDL was attempted using kinetic resolution. However, all the polymers obtained over time were found to be fully random, which appeared to be the result of fast transesterification. Poly(PDL-b-CL) block copolymers were successfully synthesized applying the high reactivity of ε-CL in a sequential feed strategy. However, these block copolymers rapidly transform into fully random copolymers as a result of transesterification, which was shown to have a similar rate constant as the rate constant of the polymerization of PDL. By carefully tuning the reaction time polymers with block, gradient or random topology can be obtained.

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INTRODUCTION

The catalytic ring-opening polymerization (cROP) of macrolactones to polymacrolactones (PMLs) has great potential since this route allows the formation of high molecular weight products, which possess good ductility and strength, similar to polyethylene (PE) and in contrast to other renewable polymers such as PLA and polyhydroxyalkanoates (PHAs) that are intrinsically brittle.18,22−25 Due to the living or even immortal character of most of the cROP catalysts and their tolerance to a wide variety of functional groups, random-, alternating-, block-, gradient-, and graft-copolymers with tunable molecular weight can be synthesized using for example (di)lactones, cyclic carbonates or the combination of epoxides and CO2 or anhydrides.16,26−37 In this they outperform enzymes, which give high molecular weight PMLs but show poor control over the polymer microstructure as the transesterification mechanism exclusively yields random copolyesters.15,16,38−42 Aluminum salen complexes form a versatile class of catalysts, which show good activity in the cROP of lactones including

Extensive research in the field of polymer science is currently focused on incorporating biobased building blocks in polymers, with the goal to decrease the dependence on fossil feedstock as well as to benefit from the structural features of certain renewable monomers.1−3 The global plastic production in 2010 reached a staggering 265 Mton, of which an increasing amount encompasses biobased polymers. This production is expected to reach a capacity of 800 kton in 2020.4,5 Of all biobased polymers, aliphatic polyesters have probably been investigated most extensively, in which poly(lactic acid) (PLA) takes a leading role since it is biocompatible, biodegradable and it has interesting features regarding stereoregularity.6 More recently, an increasing number of studies has been reported on fatty acid-based polyesters. Interesting examples are the polycondensation of fatty acid-derived diols and diesters, ADMET and thiol−ene chemistry of fatty acid-based α,ω-dienes or the ringopening polymerization of macrolactones, which can be derived from ω-hydroxy fatty acids.7−18 Several groups have shown that it is possible to synthesize the required monomers from inexpensive and abundantly available fatty acids using either chemical or enzymatic routes.8,19−21 © XXXX American Chemical Society

Received: April 9, 2013 Revised: May 8, 2013

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dx.doi.org/10.1021/ma400731c | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Results of the cROP of PDL Using Different Salen Aluminum Catalystsa

[PDL] = 1.00 M, [catalyst] = 0.01 M, [BnOH] = 0.01 M, T = 100 °C. In-graphic footnotes are as follows: (a) Determined using 1H NMR in CDCl3 by comparison of the α-methylene resonances of the monomer and the polymer. (b) t = 37 h. (c) t = 40 h. (d) Catalyst did not fully dissolve during the reaction. a

Figure 1. Overview of the cROP of various (macro)lactones using 1, 2 and 3.

macrolactones.43−47 Although these catalysts have been studied extensively, the cROP field is dominated by studies on catalyst design and the catalysts are generally screened for a limited number of monomers, typically lactide or ε-caprolactone. Recently, we reported the surprisingly effectiveness of salen aluminum complexes as catalysts for the cROP of macrolactones to high molecular weight polymers (Mn > 100 kDa).17 Since copolymers obtained from macrolactones and small-ring lactones can give rise to interesting properties, it is desired to investigate the polymerization kinetics of both classes of lactones using these catalysts. Only few detailed kinetic studies have been reported on these catalysts, which solely consider small-ring lactones.46 Here we report the first detailed kinetic study on the polymerization of macrolactones as well as small-ring lactones using different aluminum salen complexes. In the first section, the focus is on establishing the general polymerization kinetics of macrolactones. Subsequently, the effect of the ancillary ligand structure on the polymerization rate for various macrolactones and small-ring lactones is described. Finally, the copolymerization of pentadecalactone (PDL) and εcaprolactone (ε-CL) is described.

Introducing sterically hindered t-Bu groups in the phenoxy ortho-position (3) significantly reduces the activity.17 Furthermore, the flexibility of the diimine-bridge (1, 2) seems to be crucial for high activity. From these initial studies complexes 1-3 were selected to subsequently investigate the polymerization kinetics of a number of (macro)lactones (Figure 1). Complexes 1 and 3 were chosen in order to investigate the effect of steric hindrance on the reaction rate. To obtain deeper insights into this, the activation energy of 1 and 3 for both PDL and L-lactide (LLA) homopolymerization were determined and compared. Besides this, complexes 1 and 2 were chosen for further investigation, since the catalytic activity of these complexes is known to be very different toward LLA,46 while they show similar reactivity toward PDL (Table 1). General Considerations of Polymerization Kinetics. In order to properly determine and understand the kinetic parameters, several issues have to be considered. Since rate laws are dependent on concentration, it is important to determine the exact reaction volume in which a compound is present at the reaction temperature. To address this issue, the density of mixtures consisting of various ratios of PDL and solvent were determined at various temperatures. It is assumed that the contributions of PDL and PPDL to the density are similar. Hence, the density of the reaction solutions during the polymerization is expected to be constant. For these solutions the density depends linearly on temperature and composition (Supporting Information: Figures S1 and S2). The density values obtained from these experiments were used to determine the reaction volume and concentrations for all experiments reported in this contribution. Second, since it is not unlikely that the insertion of a lactone into the initiating group attached to aluminum shows a different

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RESULTS AND DISCUSSION Initial Activity Determination. To evaluate the influence of ligand variation on the catalytic behavior of the aluminum salen catalysts, several aluminum salen complexes (1−7) were tested for their activity in PDL polymerization (Table 1). Complexes 1, 2, and 7 show the highest activity. Complex 7 did not fully dissolve, which makes it likely that this complex has an even higher intrinsic activity, suggesting that electron withdrawing groups have a positive effect on the activity. However, its low solubility makes it less suitable for kinetic studies. B


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Figure 2. Plot of the conversion vs time (a), and plot of the initial polymerization rate vs the concentration of PDL (b). Polymerization conditions: [1′] = 10 mM, [PDL] = 3.74, 2.01, 1.00, 0.50, and 0.25 M in p-xylene at 100 °C (conversion determined by 1H NMR spectroscopy). Error bars are constructed from the 95% confidence intervals of the kexperimental values determined using a method based on an “asymptotic normal distribution for the parameter estimate”.50

rate than propagation,48 it was decided to start with hexadecanolate (HDO) as initiating group for its resemblance to the growing polymer chain. NMR experiments demonstrated that the protonolysis of 1−3 by hexadecanol (HDOH) is instantaneous at 100 °C and premixing of 1−3 at 100 °C with HDOH prior to the polymerization therefore results in the in situ formed catalysts [salen]AlOHD (1′−3′). Another advantage of using HDOH instead of the commonly used benzyl alcohol as initiator is the increased solubility of the formed complexes. Reaction Rate Order in Monomer. A plot of conversion vs time for the polymerization of PDL using 1 in p-xylene is in agreement with first order kinetics with respect to monomer (Figure S3, Supporting Information) and the rate of PDL consumption is well described by

−

d[M] = k[cat][M] dt

Supporting Information), the simpler eq 1 has been used in the remainder of this paper. To confirm that the reaction rate follows first order kinetics with respect to monomer, five independent experiments have been performed varying the initial concentration of monomer (0.25, 0.50, 1.00, 2.01, and 3.74 M), while keeping the temperature (100 °C) and catalyst concentration ([1′] = 10.0 mM) constant. Figure 2 depicts the concentration of PDL vs time (a) and the initial reaction rates vs the initial PDL concentration (b) for these experiments. The initial reaction rate clearly shows a linear dependence on the concentration of monomer, which proves that the reaction rate has a first order dependence in monomer in the range investigated. Although not investigated as extensively as for 1′, both 2′ and 3′ showed a very similar dependence on monomer concentration. An interesting fact is that for the polymerization of PDL to PPDL Duda and Kowalski reported a monomer equilibrium concentration for PDL of 0.70 mol·L−1 at 100 °C,51 which was calculated from the thermodynamic parameters determined by Lebedev et al.52 Hence, theoretically the polymerization of PDL should not take place when the concentration of PDL is lower than 0.7 mol·L−1. However, as shown in Figure 2, PDL polymerizes over the full range of concentrations, as low as 0.25 M as starting concentration. The reason for this mismatch with the reported thermodynamic data could be the preparation method of PPDL by Lebedev et al., which encompasses the polymerization of PDL and subsequent dissolution in chloroform and precipitation in methanol (three times).52 The precipitation step will, next to PDL, also remove low molecular weight linear and cyclic chains from the polymer, which can comprise a significant fraction of the total product distribution. Hence, it is very likely that the thermodynamics of polymerization favor the formation of PPDL more than the reported values suggest. Indeed, determination of the equilibrium concentration of PDL using crude polymer samples revealed it to be 0.016 M at 100 °C (Supporting Information, Table S2). This is in agreement with the reported conversions exceeding

(1)

where k is the rate constant, [cat] is the catalyst concentration (mol·L−1), [M] is the monomer concentration (mol·L−1), and t is the time (min). The obtained k value is 5.3 L·mol−1·min−1 (95% confidence interval from 5.0 to 5.6 L·mol−1·min−1) at 100 °C. However, in a recent report discussing the complexity of the kinetics of a polymerization through a coordination − insertion mechanism, it has been suggested that by using the Michaelis Menten equation the coordination and the insertion step can both be quantified and consequently the kinetics can be better understood.49 Therefore, the Michaelis Menten equation was also used to fit the same experiment as used for eq 1 −

k [cat][M] d[M] = 2 dt K m + [M]

(2)

where k2 is the rate constant for insertion and Km is the inverse of the binding constant for coordination (kdecoordination/ kcoordination).49 Since the experimental data proved not sensitive enough to distinguish between eqs 1 and 2 (Figure S3, C


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Figure 3. Plot of the conversion vs time for different complex concentrations (a), and plot of the initial polymerization rate vs the concentration of complex (b). Polymerization conditions: [PDL] = 1.00 M, [1′] = 40.2, 20.0, 10.0, 5.0, and 2.6 mM, in p-xylene at 100 °C (conversion determined by 1 H NMR spectroscopy). Error bars are constructed from the 95% confidence intervals of the kexperimental values determined using a method based on an “asymptotic normal distribution for the parameter estimate”.50

Figure 4. Plots showing the reaction rate constant vs temperature and their fit of the Arrhenius equation for PDL and LLA using 1′ (a), and 3′ (b). Error bars are constructed from the 95% confidence intervals of the kexperimental values determined using a method based on an “asymptotic normal distribution for the parameter estimate”.50

impurities deactivating the catalyst. On the basis of the determination of the order of reaction for PDL and 1′, the rate law can be expressed as eq 1 where [cat] = [1′]. Since for 2′ and 3′ a similar dependence on catalyst concentration was observed, the overall second order rate equation eq 1 can also be applied for these catalysts. Activation Energies. In order to be able to understand the influence of steric hindrance on the polymerization kinetics of 1′ and 3′ in more detail, the activation energy for the homopolymerization of PDL and LLA was determined for these catalysts. For this purpose, the homopolymerization of PDL and LLA (1.00 M) using 1′ or 3′ (10.0 mM) in p-xylene was studied at 80, 90, 100, 110, and 120 °C, respectively. The

95% for both bulk and solution polymerizations at 100 °C.17,30,53 Reaction Rate Order in Catalyst. To investigate the order of reaction rate in catalyst, the polymerization of PDL (1.00 M) in p-xylene was studied at 100 °C using 1′ with [1′] = 2.6, 5.0, 10.0, 20.0, and 40.2 mM (Figure 3a). The apparent reaction rates, kapp (= k·[1′]), obtained from the 1H NMR analyses of the aliquots taken at different times show a clear linear relation to the catalyst concentration (Figure 3b), indicating that the reaction rate indeed shows a first order dependence in catalyst, which excludes the existence of a bimetallic mechanism.54 Furthermore, the intersection with the concentration axis is around zero, suggesting the absence of significant amounts of D


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reflects the bulkiness of the lactide. This is ascribed to the fact that CALB has a low reactivity when ester substrates contain a substituent at the α-position.58 Activation energies were not determined for 2′, because the reaction with LLA was so fast, that the experimental error was too high for accurate determination of the activation energy. Rate Constants for Various Monomer−Catalyst Combinations. Since it has been demonstrated that the reactivity of 1′ and 2′ toward LLA shows a large difference, the reactivity behavior for these two complexes toward other lactones was investigated further.46 The various monomers (Figure 1) all have specific structural properties. PDL, Amb, and BA belong to the group of large-ring lactones (macrolactones), which are known to have little to no ring strain.51 εCL, LLA, ε-DL, and β-BL all encompass small-ring lactones, which do have ring strain. Furthermore, these strained monomers, excluding ε-CL, all contain a branch at the αposition adjacent to the ester bond. The reaction rate was studied by polymerization of these monomers (1.00 M) using 1′ and 2′ (10.0 mM) in p-xylene at 100 °C. All reaction rate constants could be determined with high accuracy using eq 1. For the investigated monomers, the monomer equilibrium concentrations ([M]eq) were determined and taken into account for the fitted kinetic model, by substituting [M] with ([M] − [M]eq) in eq 1. Several interesting features for the ROP catalyzed by 1′ can be observed in Figure 5 and Table 3. First of all, all macrolactones have similar and a relatively high reaction rate coefficient ranging from 3.9−5.3 L·mol−1·s−1. Hence, it appears that the small differences in ring-size for these macrolactones do not yield significant differences in rate constant, which can be explained by the fact that all macrolactones have little to no ring strain. The reaction rate is likely to be determined by the steric hindrance the monomer experiences from its own large size while coordinating to the active site. However, for strained rings, the reactivity varies drastically from monomer to monomer. ε-CL polymerizes relatively fast using 1′ with a reaction rate constant of 44 L·mol−1·s−1, while ε-DL, which has the same ring size as ε-CL but contains a butyl branch at the αmethylene position, reacts 2 orders of magnitude slower. The

dependence between the temperature and the reaction rate constant for these experiments all follow an Arrhenius relation (Figure 4). The corresponding activation energies (Ea) are shown in Table 2. It can be seen that the Ea for PDL and LLA Table 2. Activation Energies and A Factors for PDL and LLA Polymerization with 1′ and 3′ monomer

complex

Ea (kJ·mol−1)

PDL PDL LLA LLA

1′ 3′ 1′ 3′

45 52 48 60

A (L·mol−1·min−1) 1.06 0.39 1.98 2.04

× × × ×

107 107 107 107

are comparable (2 kJ·mol−1 difference) for complex 1′. However, for the polymerizations using 3′, the Ea for PDL increases 7 kJ·mol−1 while the Ea for LLA increases 12 kJ·mol−1. This shows that the LLA polymerization experiences significantly more steric hindrance from the bulky tert-butyl substituents on the salen ancillary ligand than the PDL polymerization. It is likely that this difference in activation energy is caused by the combination of steric hindrance induced by the last incorporated monomer connected to the aluminum and the incoming LLA monomer, in line with the chain end control mechanism. However, electronic effects, such as chelate effect of the growing PLLA chain, cannot be excluded. This chain end control mechanism was also shown by Rzepa et al., who showed that the stereocontrol in rac-lactide cROP is mainly governed by the steric hindrance of the ancillary ligands. The coordinating lactide feels the influence of the chain-end, which consists of the methyl at α-posistion as well as chelating carbonyl moiety at β-position, in combination with the ancillary ligand substituents.55 Achiral aluminum salen catalysts are known to be able to synthesize stereoregular PLA from racemic lactide mixtures using this mechanism, where the selectivity increases significantly when bulky substituents are used on the ortho phenoxy position.46,56,57In other work, the inability of Candida Antartica Lipase B (CALB), an enzyme that is generally very active in the ROP of lactones, to polymerize LLA

Figure 5. Polymerization of ε-CL, PDL, Amb, BA, LLA, ε-DL, and β-BL (1.00 M) at 100 °C in p-xylene 1′ (10 mM, a) and 2′ (10 mM, b). E


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resolution. To evaluate this possibility, PDL and ε-CL were copolymerized in a one-pot reaction using [PDL] = [ε-CL] = 0.59 M, [1′] = 13.0 mM at 100 °C. As expected, both monomers show similar kinetics as during homopolymerization (Figure 6), which indeed should result in a blocky structure, consisting of a PCL block, a tapered PCL−PPDL block and a PPDL block, if transesterification does not take place.

Table 3. cROP Rate Constants of Different Cyclic Esters for 1′ and 2′ at 100 °C

a

Determined using a conversion of 0.99 at 30 s.

most logical explanation can be found in the steric hindrance of the butyl branch at the β-position of the growing chain, which hampers an incoming, also branched, monomer from coordinating. The same trend can be seen for β-BL, which is polymerized extremely slowly by 1′. LLA, which has a comparable steric hindrance as β-BL, shows a much higher activity than β-BL. Why this difference is so large is not clear, however, further investigation of this issue falls outside the scope of this paper. When 2′ was used instead of 1′, some significant differences were observed. The most remarkable difference between 1′ and 2′ can be found in comparing the relative reaction rates between small-ring lactones and macrolactones (Figure 5 and Table 3). As can be seen, the reaction rate constants using 2′ for the macrolactones change only slightly compared to 1′. However, the reaction rate constants increase dramatically for the small-ring lactones by at least 1 order of magnitude. For εCL, the rate was so high that injection of the monomer in the vial and subsequent quenching after 30 s yielded a conversion >99%. Therefore, the rate constant was determined using this sole value, and is at least 900 L·mol−1·min−1. Apparently, the extension of the salen diimine-bridge by one carbon atom gives the complex more flexibility resulting in a conformation that causes considerably less steric hindrance for the small rings. It has been shown before that the bridge-size has an influence on the conformation of salen complexes.46,59 For aluminum salen complexes, changing the bridge from ethylene to 2,2- dimethylpropylene tends to change the conformation from meridional to facial. Facial-conformations are known to be more reactive than their meridional counterparts, because there is more room for the monomer to coordinate cis with respect to the growing chain, necessary for insertion. This is very likely to cause the increase in reactivity for small-ring lactones. Copolymerization of PDL and ε-CL. For both catalysts 1′ and 2′ there are significant differences in reactivity between macrolactones and small-ring lactones such as ε-CL. Although reaction rates of monomers during homopolymerizations cannot simply be used to predict copolymerization behavior, based on the similarity of the growing chain structure for ε-CL/ PDL homo- and copolymerization, it is assumed that the reactivity ratios for ε-CL and PDL will be very similar. Hence, based on the large difference in polymerization rate, it might be possible to synthesize block-copolymers consisting of a macrolactone and small-ring lactone block through kinetic

Figure 6. Plot of the conversion vs time for ε-CL and PDL during the ε-CL−PDL copolymerization catalyzed by 1′. Polymerization conditions: [ε-CL] = [PDL] = 0.59 M, [1′] = 13.0 mM in p-xylene at T = 100 °C (conversion determined by GC-FID and 1H NMR) and corresponding homopolymerizations.

The polymers obtained at different reaction times were precipitated twice in methanol and dried under vacuum. To investigate the monomer sequence in the poly(CL-co-PDL) copolymers, 13C NMR was used to determine the ratio of PDL−PDL, CL−CL, CL−PDL, and PDL−CL linkages, by comparing the integration of the diads originating from the αmethylene carbons, since all these linkages have distinct resonances in the 63.9−64.6 region.30,42 From these ratios, the randomness can be determined (for details see Supporting Information). It can clearly be seen that initially a high number of CL−CL diads is present, which is caused by the high conversion of ε-CL in the first minutes of the polymerization (Figure 7). However, as the reaction continues, all diad fractions approach 0.25. Furthermore, from the diad fractions it can be calculated that the randomness (for details of calculations, see Supporting Information) is continuously 1 during the reaction, meaning that, rather than the expected block copolymer, a fully random copolymer is obtained. This is the result of transesterification, the rate of which is at least of the same order of magnitude as the propagation rate of PDL. The complete randomness of the poly(CL-co-PDL) copolymers obtained is furthermore confirmed by the relation between composition and melting point, which practically is a linear combination of the melting point of the two homopolymers (Supporting Information, Figure S8). This is caused by the fact that random copolymers of ε-CL and PDL remain highly crystalline over the whole composition range as a result of cocrystallization and isomorphism phenomena.41,60 A similar copolymerization was performed using 2′, in which [PDL] = 0.50 M, [ε-CL] = 0.51 M, and [2′] = 10 mM. Again, the monomers show similar kinetics as seen in the individual F


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tion is absent or slow. However, when the integrals of the different diads are followed in time (Supporting Information, Figure S9), it can again be seen that next to the CL−CL and PDL−PDL linkages, the number of PDL−CL and CL−PDL linkages also rapidly increases until all the diads reach a fraction of 0.25. When the randomness is calculated from these results, it appears that all copolymers obtained during the polymerization are in a fully random state. This is again confirmed by the melting point of the copolymers (Supporting Information, Figure S10), which show a similar dependence on composition as the copolymers obtained using 1′. Therefore, it can be concluded that also 2′ is a very efficient transesterification catalyst, for which the rate of the transesterification is at least in the same order of magnitude as the rate of polymerization of PDL. Due to the high transesterification rates, it is impossible to obtain block copolymers of ε-CL and PDL in one-pot − singlefeed reactions. However, since ε-CL polymerizes much faster than PDL, especially when 2′ is used, the option to synthesize poly(PDL-b-CL) block copolymers has been explored applying a sequential-feed strategy. Therefore, PDL was first polymerized in six separate crimp cap vials using 2′ ([PDL] = 0.83 M, [2′] = 17.0 mM, T = 100 °C) in p-xylene for 90 min, which resulted in full conversion of the PDL. Subsequently, 500 mg of a solution of ε-CL ([ε-CL] = 1.8 M) was injected into five of the vials and the reactions were terminated at 1 (t1), 5 (t5), 10 (t5), 60 (t60) and 1410 min, respectively. As can be seen in Table 4, all samples practically show full PDL and ε-CL conversion. Analyzing the integrals of the diads of the copolymers obtained at different times revealed that for t1 a randomness of 0.05 is observed, and can thus be regarded as a block copolymer. Copolymers that were allowed to react for a longer time show an increased randomness with increasing reaction time reaching a fully random structure after one hour. Hence, by carefully tuning the reaction time, polymers with block, gradient and fully random topology can be obtained. The randomness of the structure manifests itself also in the melting behavior (Figure 9). Sample t1 shows two melting points, at 90.9 and 59.0 °C, respectively, which is a typical feature of a microphase separated block copolymer. Since the melting point of the PCL-block is higher, and the melting point of the PPDLblock is lower than their homopolymer equivalents, it can be concluded that the PCL-block contains some PDL and the PPDL-block some ε-CL. When the copolymers are further allowed to transesterify, reaching a randomness of 0.22 (t5), only one clear melting peak is observed, with a long low temperature tail. This indicates the presence of a wide variety of PPDL-PCL compositions, which predominantly melt at 85.2 °C. Subsequent longer transesterification times lead to a decrease in melting point, down to 61.7 °C (t1410), which is

Figure 7. Percentage of PDL−CL, PDL−PDL, CL−CL, and CL− PDL diads vs time (left axis) from the copolymers formed using 1′. Randomness calculated from the diads vs time (right axis).

homopolymerizations (Figure 8), where ε-CL is already converted before the reaction mixtures are heated and PDL is gradually polymerized after.

Figure 8. Plot of the conversion vs time for ε-CL and PDL using 2′. Polymerization conditions: [ε-CL] = 0.51 M, [PDL] = 0.50 M, [2′] = 10.0 mM in p-xylene at T = 100 °C (conversion determined by GCFID and 1H NMR) and corresponding homopolymerizations.

Based on this big difference in polymerization rate (and keeping in mind that the chain end structure at the metal center after insertion of ε-CL or PDL is very similar), a perfect block copolymer poly(CL-co-PDL) is expected when transesterifica-

Table 4. Copolymerization of PDL and ε-CL Using a Sequential-Feed Strategy conversion (%)

diad integral ratiosd

Pol (mol %)

sample

t (min)a

PDLb

ε-CLc

CL

PDL

PDL−CL

PDL−PDL

CL−CL

CL−PDL

randomness

t0 t1 t5 t10 t60

0 1 5 10 60

98.2 96.2 98.2 98.2 98.4

− 99 99 99 99

0 0.73 0.73 0.73 0.73

1 0.27 0.27 0.27 0.27

0.0 2.1 5.5 5.8 19.9

100.0 26.3 25.8 22.4 8.8

0.0 71.6 65.2 65.1 52.2

0.0 0.0 3.6 6.7 19.2

− 0.05 0.22 0.30 0.96

a Time after addition of ε-CL (after 90 min homopolymerization of PDL). bDetermined using GC-FID. cDetermined using 1H NMR. dDetermined using 13C NMR.

G


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between PPDL and PCL, it is presumed that the active centers (aluminum connected to a PDL or a CL unit) behave similarly and the ester bonds in the polymer chain (PDL−PDL, CL−CL, PDL−CL, and CL−PDL) all have the same properties. Since PDL is a macrolactone, which has a negligible ring strain, it is reasonable to assume that the transition state of the ROP of PDL is electronically equal to the transition state of transesterification of linear chains, and therefore it can be stated that the activation energy term for both reactions will not differ significantly. For the partitioning term, this is less clear. However, it can be reasonably assumed that the difference will not be extremely large and therefore it will not have a great effect on the rate constants. This hypothesis was tested, by applying the model described above with values of ktrans equal to 0.5 × kPDL, kPDL, and 2 × kPDL, in which kPDL represents the ROP rate constant for PDL using 2′, and comparing the modeled values to the values obtained experimentally (Figure 11). The evolution of the various linkages in the block copolymer, as well as the randomness is well described using these values, especially considering the peak to noise ratio typically obtained for 13C NMR. Indeed, these results confirm that the rate constant of transesterification is in the same order of magnitude as the rate constant of polymerization for macrolactones. This furthermore shows the similarity between ROP of a macrolactone and the transesterification of the formed polymer, which suggests that a large cyclic monomer has similar reaction properties as its corresponding polymer chain. By application of the determined rate constants, the polymerization and transesterification reactions can be combined in a single model, which takes into account both reactions. Using this model, concentration and reaction times can be tuned in order to obtain polymers ranging from block copolymers, to fully random copolymer.

Figure 9. DSC thermograms of the copolymers obtained after several time intervals.

comparable to fully random copolymers with a similar composition. In order to exclude the possibility that the two melting peaks for sample t1 are a result of a blend of two homopolymers, size exclusion chromatography was performed on the PPDL homopolymer (t0) and the copolymer sample t1 (Figure 10).

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CONCLUSIONS This study reports the first detailed kinetic study on the polymerization of macrolactones and small-ring lactones using salen aluminum catalysts. The polymerization’s using 1′−3′ revealed a first order dependence in both monomer and catalyst on the reaction rate in the range investigated, supporting a monometallic mechanism. The steric bulk present in 3′ significantly reduces the polymerization rate, both for the macrolactone PDl ans for the ringstrained LLA. Variation in the diimine-bridge of the aluminum salen complexes has a profound influence on the polymerization rate. Polymerizations of small-ring lactones, i.e. LLA, ε-CL, ε-DL, and β-BL using 2′ all had rates which were at least 1 order of magnitude higher than for 1′. This effect could be ascribed to the spatial conformation of these two complexes (facial versus meridional conformation of the salen ligand), which is more spacious for 2′. However, for the polymerization of PDL and other macrolactones, i.e. Amb and BA, changing from 1′ to 2′ only led to a minor increase in reaction rate. In these cases the ring size is so large that the increasing space resulting from the more flexible salen ligand, only has a minor advantage for the coordination of macrolactones. Furthermore, these monomers all have similar reactivities, which is likely to be caused by the lack of ring strain and comparable steric hindrance of these monomers. Despite the fact that the polymerization rates of both monomers are very different, one-pot − single-feed copolymerizations of PDL and ε-CL using both 1′ and 2′ led to fully

Figure 10. SEC chromatogram of t0 and t1.

The polymer peak clearly shifts to lower elution times, indicating an increase in molecular weight, which supports the formation of a block copolymer. In conclusion, despite the fact that 2′ is a very efficient transesterification catalyst, it was shown that its higher reactivity toward ε-CL allows the synthesis of block-copolymers by sequential feed of ε-CL to PPDL, for which polymerization outruns transesterification. By tuning the reactions time, the block copolymers are gradually transformed into gradient and eventually fully random copolymers. To obtain further insight into the kinetics of transesterification, a situation was modeled in which a perfect block copolymer (for detailed description, see Supporting Information), comprising of 50 PDL units and 133 ε-CL units connected to a metal center, undergoes transesterification with a rate constant of transesterification (ktrans). The composition of this model block copolymer is similar to the synthesized block copolymer (vide supra). Due to the similarity in structure H


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Figure 11. Experimentally determined fraction of PDL−CL, PDL−PDL, CL−CL, and CL−PDL linkages and the randomness calculated from these diads vs time (symbols). Modeled fractions and randomness using ktrans = 0.5 × kPDL, 1 × kPDL, and 2 × kPDL (dotted lines). out on a DSC Q100 from TA Instruments at a heating rate of 10 °C min−1. Second runs were recorded after cooling down to ca. 20 °C. The melting temperatures reported correspond to the melting peaks in the second runs. Density measurements were performed in duplicate on an Anton Paar DMA 500 v4.310.d density meter by premixing the proper ratios of compounds and subsequent injection into the device. Polymerization Procedure. In a typical run, prior to the polymerization a stock solution was prepared in a 20 mL glass crimp cap vial containing equimolar amounts of 1 (498 mg, 1.54 mmol) and HDOH (376 mg, 1.55 mmol) in 12.5 g of p-xylene. The mixture was allowed to react overnight at 100 °C to form 1′. Subsequently, the stock solution (302 mg, containing 11.2 mg (0.035 mmol) of 1′), PDL (842 mg, 3.5 mmol), and p-xylene (2.03 g) were mixed in a vial and distributed over seven small crimp cap vials which were capped, taken out of the glovebox, and put in a carrousel reactor at 100 °C (t = 0). At predetermined times, vials were taken out of the reactor, uncapped, and an aliquot of the crude reaction mixture was taken in order to determine the conversion by 1H NMR. Subsequently, acidic methanol was added to quench the reaction. For some reactions the procedure was slightly adapted, e.g., for the polymerization of LLA using 2′, capped crimp cap vials containing the catalyst stock solution were heated in the reactor after which a hot solution of LLA in pxylene was injected in the vial. For a typical sequential-feed copolymerization, after 90 min of homopolymerization of PDL (100 mg 0.42 mmol), a p-xylene solution containing 25 w% ε-CL (127 mg, 1.11 mmol) was syringed into the crimp cap vial. Samples were taken similarly as in the homopolymerizations. The mixture used for 1H NMR was also used for GC-FID to determine the conversion of PDL, since the PDL and PPDL peaks overlap with PCL. The equilibrium concentration ([M]eq) of the monomers were calculated from the average of the conversions determined by 1H NMR of three identical measurement, using the typical procedure for homopolymerizations in which the mixtures were allowed to react at least five times the time needed to reach 99% conversion (when [M]eq = 0 is assumed). Once no clear monomer peak could be observed anymore in 1H NMR (conversion >99%), [M]eq = 0 was taken to calculate the reaction rate constant for the monomer.

random copolymers over the whole polymerization range, as a result of competitive inter- and intramolecular transesterification reactions. Applying a sequential-feed strategy allowed the synthesis of block copolymers which subsequently transesterify with a rate equal to the ROP of macrolactones. This work shows that understanding the reaction kinetics is a key aspect in the synthesis of various homo- and copolymers and opens the possibility for a wide range of copolymers with different architectures and compositions.

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EXPERIMENTAL SECTION

Reagents and Methods. All solvents and reagents were purchased from commercial sources (Sigma-Aldrich, BioSolve) unless stated otherwise. p-Xylene (99.9%) was dried over sodium and fractionally distilled under nitrogen and degassed prior to use. Amb was kindly received from Symrise. HDOH, PDL, ε-DL, Amb, ε-CL, and β-BL were freshly distilled from CaH2 under nitrogen prior to use. LLA was received from Purac, dried over CaH2 and sublimed prior to use. Butylene adipate (BA) was kindly received from Dr. Felix Scheliga (University of Hamburg). Toluene was passed through purification columns and degassed before use. The aluminum Schiff bases were synthesized using literature procedure (1−3, 5,61 and 4, 6−846). All reactions and preparations were either carried out in an MBraun MB150 GI glovebox or using proper Schlenk techniques. 1 H NMR and 13C NMR spectra were recorded in 5 mm tubes on a Varian Mercury 400 MHz spectrometer equipped with an autosampler at ambient probe temperature in CDCl3. Chemical shifts are reported in ppm vs tetramethylsilane. Copolymerization reactions were followed by gas chromatography (GC) with a Shimadzu GC-2010 equipped with an FID employing a CP-WAX 52 CB, 0.25 mm ×25 m (DF = 0.2 μm) column. Injection and detection temperatures were both set at 270 °C. The internal standard method, taking p-xylene as the internal standard, was used to determine the lactone conversion; all samples were measured using a Shimadzu AOC-20i autosampler. Size exclusion chromatography (SEC) of PPDLs was performed at 160 °C using a Polymer Laboratories PLXT-20 Rapid GPC Polymer Analysis System (refractive index detector and viscosity detector) with 3 PLgel Olexis (300 × 7.5 mm, Polymer Laboratories) columns in series. 1,2,4-Trichlorobenzene was used as eluent at a flow rate of 1 mL·min−1. The molecular weights were calculated with respect to polyethylene standards (Polymer Laboratories). A Polymer Laboratories PL XT-220 robotic sample handling system was used as autosampler. Differential scanning calorimetry (DSC) analyses of PPDL homopolymers and poly(PDL-co-CL) copolymers were carried

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ASSOCIATED CONTENT

S Supporting Information *

Density measurements, additional kinetic data, DSC data of copolymers, calculation of randomness, and the kinetic model I


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used for transesterification. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(R.D.) Telephone: +31 40 247 4918. Fax: +31 40 246 3966. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by SABIC for this work is gratefully acknowledged. Dr. Felix Scheliga of Hamburg University is kindly thanked for providing the BA monomer. Prof. Dr. Cor Koning and Dr. Hans Heuts are acknowledged for the stimulating discussions.

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Polymerization of (Macro)Lactones Using Aluminum Salen Catalysts

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