Note pubs.acs.org/Macromolecules
Rediscovering the Isospecific Ring-Opening Polymerization of Racemic Propylene Oxide with Dibutylmagnesium Swarup Ghosh, Henrik Lund, Haijun Jiao, and Esteban Mejía* Leibniz Institute for Catalysis, University of Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany S Supporting Information *
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INTRODUCTION Aliphatic polyethers have become an important class of materials due to their large variety of applications, e.g., as matrixes for controlled drug delivery systems,1,2 nonionic surfactants,3 lithium polymer batteries,4 polyurethane production, and many others. These polyethers are generally synthesized either by ring-opening polymerization (ROP) of epoxides using metal complexes as initiators or via metal-free polymerization approaches.5−24 Alkali metal derivatives are well-known as efficient initiators for the anionic polymerization of cyclic ethers such as propylene oxide (PO).25 The high basicity of the corresponding propagating species may result in undesired transfer reactions to the monomer, leading to the formation of the corresponding allyl alkoxide and yielding only to moderate polymer molecular weights.26−29 Conversely, through a monomer activation approach, it is possible to obtain high molecular weight polymers under mild conditions, as Deffieux and Carlotti et al. reported, using an anionic polymerization system with trialkylaluminum (e.g., iBu3Al) as catalyst and a tetraalkylammonium salt as the initiator (e.g., [NOct4]Br).25,30 Price and Osgan reported the synthesis of crystalline optically active poly(propylene oxide) from the optically active monomer by using iron(III) chloride or potassium hydroxide as catalyst.31 There are several reports concerning the “asymmetrically selective” polymerization of rac-PO, in which a catalyst system containing an optically active component is employed.32−35 The catalyst systems generally used for this polymerization are multicomponent, based on diethylzinc/ water and an optically active alcohol, ether, or amine as well as iron(III) chloride/water/d-bornyl ethyl ether mixtures.36−38 Inoue and co-workers were the first to report the stereoselective polymerization of epoxides using diethylzinc and enantiopure alcohols such as (+)-borneol and (−)-menthol as additives.34,39−42 Krylov and Livshits used magnesium (+)-tartrate for kinetic resolution polymerization of rac-PO.43 In a benchmarking report, Coates and co-workers introduced an achiral cobalt catalyst for ROP of rac-PO capable to exclusively produce regioregular rac-isotactic poly(propylene oxide) (raciso-PPO). This catalyst is able to produce high molecular weight polymers with >99% mm triads.44 Later on, the same group introduced a homogeneous, bimetallic cobalt catalyst, which showed good activity toward ROP of epoxides when combined with an ionic cocatalyst salt such as bis(triphenylphosphine)iminium acetate, [PPN][OAc]. This bimetallic cobalt complex is the first reported catalyst to accomplish isoselective polymerization of a huge range of epoxides to yield highly isotactic polyethers.45 © XXXX American Chemical Society
The observation of stereospecificity in Lewis-acid-catalyzed ROP of rac-PO without chiral additives dates back to the 50s.46,47 Tsuruta et al. studied the mechanism of PO polymerization with MgEt2 and reported that an amorphous polymer was obtained when nearly racemic PO was used.48 In a brief communication, Vandenberg reported partially stereoselective polymerization of 1-alkene oxides using magnesium− alkyl compounds previously activated with stoichiometric amounts of water.49 Remarkably, we could not find any reports of ROP of epoxides with any dialkylmagnesium derivative in the intervening >40 years in spite of the wide availability of organomagnesium compounds and its reported potential as isospecific catalysts in the ROP of rac-PO. With this in mind, we decided to reexamine this neglected reaction. To our delight, by using di-n-butylmagnesium in the bulk ring-opening polymerization of rac-PO, we produced highly isotactic poly(propylene oxide) with high number-average molecular weight (Mn) in a controlled manner, even at high temperatures.
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RESULTS AND DISCUSSION We explored the catalytic activity of nBu2Mg in the ROP of racPO in argon atmosphere at 40 °C under solvent-free conditions (Scheme 1). This organometallic compound was found to be Scheme 1. Synthesis of rac-Isotactic Poly(propylene oxide) Using nBu2Mg
highly effective. First, we performed the polymerizations at a 200:1 [M]0/[C]0 ratio with rac-PO, and the isolated yield of the resulting PPO was found to be more than 90%. The polymerization results are summarized in Table 1, entry 3. Under all polymerization conditions, the resulting polymers are highly isotactic, as revealed by 13C NMR spectroscopy (Figures S4−S6) and powder X-ray diffraction (Figure S17). The analysis of 13C NMR spectrum shows that the methyl, methylene, and methine carbons peaks correspond to isotactic poly(propylene oxide).44,50−52 The 13C NMR spectra of the Received: August 24, 2016 Revised: December 13, 2016
A
DOI: 10.1021/acs.macromol.6b01830 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Polymerization Data of rac-PO Using nBu2Mg as Catalyst; Variations of Mn with [M]0/[C]0 Ratio at 40 °C under an Argon Atmosphere entry [M]0/[C]0 1 2 3 4e 5 6 7 8
50:1 100:1 200:1 200:1 400:1 600:1 800:1 1000:1
timea (h)
yieldb (%)
Mn(obs)c (kg/mol)
Mn(theo)d (g/mol)
Mw/Mn
8 15 24 56 36 48 59 68
96 95 93 82 87 84 80 75
2.1 7.6 13.7 13.2 25.1 40.2 49.5 64.7
2.9 5.8 11.6 11.6 23.3 34.9 46.5 58.1
1.4 1.7 1.7 1.6 1.8 1.9 2.1 2.3
a
Time required for complete conversion. bIsolated yield of the polymer after quenching. cMeasured by GPC at 40 °C in THF, relative to polystyrene standards with Mark−Houwink corrections. dMn(theo) at 100% conversion = [M]0/[C]0 × mol wt (monomer) + molecular weight of end group. ePerformed at room temperature.
Figure 2. Powder X-ray diffraction pattern of the crystalline iso-PPO obtained from a reaction between rac-PO and nBu2Mg in 200:1 ratio at 40 °C in an argon atmosphere after precipitation in acetone; the polymer crystallizes in an orthorhombic cell with a = 10.47(3) Å, b = 4.71(2) Å, and c = 7.03(2) Å, space group P212121.
polymer obtained after recrystallization showed it to be highly isotactic, with mm triad >99% (Figure 1).
sample after precipitation in acetone); cf. Stanley and Litt: a = 10.40 Å, b = 4.64 Å, c = 6.92 Å). The unit cell of the obtained polymer itself might be slightly enlarged also due to the presence of the butyl group at one chain end (compared to the data of Cesari or Stanley).55,56 However, the powder pattern matches the published P212121 symmetry due to the absence of h00 (h = 2n + 1), 0k0 (k = 2n + 1), and 00l (l = 2n + 1) reflection peaks.53−55 The diffraction pattern of the crude polymer (before precipitation in acetone, Figure S17), along with the sharp and narrow reflection peaks, features a broad peak around 11° 2θ which indicates the presence of an amorphous part in the solid. After the polymerization of rac-PO with nBu2Mg, the unreacted monomer was recovered from the reaction mixture through distillation, and its optical rotation was measured. The recovered monomer showed no optical activity, while the resulting polymer is highly isotactic. When using (R)-PO for the polymerization reaction, the obtained polymer showed optical activity with a specific rotation ([α]23D) of +26.4° (1.5 g/100 mL, THF).31,32 The positive rotation reveals that the ring-opening of the propylene oxide occurred in an isoselective fashion without inversion of the configuration at the methine carbon.45,52 Polymerization experiments were performed at increasing monomer to catalyst ratios for rac-PO at 40 °C in an argon atmosphere. The experimental results are presented in Table 1. A plot of molecular weight (Mn) vs [M]0/[C]0 ratio shows that Mn rises linearly with the increase of monomer-to-catalyst ratios (Figure S12). From the experimental data in Table 1, it was also observed that there is a moderate correlation between Mnobs and the Mntheo at the higher monomer-to-catalyst ratio. Polymerization Kinetics. The kinetic studies for the polymerization of PO using nBu2Mg were performed in the monomer-to-catalyst ratio [PO]0/[nBu2Mg]0 = 200 at 40 °C. The plot of ln[PO]0/[PO]t vs time was found to be linear, suggesting a first-order dependence on the monomer concentration of the rate of polymerization without any induction period (Figure 3). The value of the apparent rate constant (kapp) for the polymerization was found to be 11.223 × 10−2 h−1. The molecular weight (Mn) also showed a linear dependence with
Figure 1. 13C NMR (CDCl3) spectrum of isotactic crystalline poly(propylene oxide) obtained from a reaction between rac-PO and n Bu2Mg in 200:1 ratio at 40 °C in an argon atmosphere.
The glass transition (Tg) temperatures and melting transition temperatures (Tm) of this isotactic polymer were determined by differential scanning calorimetry (DSC) analysis. The Tg was found at −66.8 °C, and the Tm was found at 62.9 °C (Figure S16). All values are consistent with previously reported values for highly isotactic PPO (Tg = −66.8 °C and Tm = 62.6 °C).44,53,54 The sharp endotherm peak of Tm observed for this polymer can be related to the high degree of crystallinity of these polymers.53,54 The crystalline nature of the polymers was also confirmed by powder X-ray diffraction studies.55,56 The result is depicted in Figure 2. The presence of sharp and narrow reflection peaks underlines the crystalline nature of the obtained polymer. Compared to the stored diffraction data of Stanley and Litt (ICDD 00-012-0896),55 a small shift to lower angles of the diffraction peaks was observed. Compared to the stored data, the indexed orthorhombic cell is slightly enhanced (a = 10.47(3) Å, b = 4.71(2) Å, c = 7.03(2) Å (for a crystalline B
DOI: 10.1021/acs.macromol.6b01830 Macromolecules XXXX, XXX, XXX−XXX
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series of large peaks were assigned to Na+ adducts bearing the n-butyl and hydroxyl group at the chain ends. The small series of peaks correspond to proton adducts. The presence of hydroxyl group in the polymer chain was confirmed in IR by the presence of the characteristic band ν(H−O) at 3500 cm−1 (Figure S18). The above results agree with a coordination− insertion mechanism. In order to assess whether both alkyl rests on the Mg center were subject of a monomer insertion (being thus the propagating species dialkoxomagnesium complexes) or only one of the alkyl ligands participates in the reaction, we devised a simple experiment. We performed the reaction using 1 and 2 equiv of rac-PO relative to nBu2Mg, the resulting mixtures were quenched with acidified water and analyzed by NMR and GCMS techniques. In both cases, only heptan-2-ol was obtained, resulting from the insertion of only one PO unit per alkyl rest. The absence of 1-(heptan-2-yloxy)propan-2-ol (the product of insertion of two PO moieties in the Mg−butyl bond), on the one hand, precludes the involvement of Mg(n-butyl)(2propoxy) in the propagation step. On the other hand, it suggests that a magnesium dialkoxylate derivative is most likely the active species.48 From Table 1 it was observed that the Mnobs is close to the Mntheo, which suggests that each catalyst center grows only one polymer chain. This conclusion is supported by the corresponding kinetic studies in terms of the catalyst, where a linear correlation was found between the initial reaction rate and the catalyst concentration, in agreement with a first-order kinetic regime (for details, see the Supporting Information). Further on, the triads’ analysis in the 13C NMR spectra (Figure S5) reveals, apart from large resonance at 75.59 ppm (mm), three small upfield resonances corresponding to mr, rm, and rr triads. This pattern corresponds to single, nonpropagated stereo-defects in the isotactic chain.50−52 This results from an enantiomorphic-site control mechanism, where the chiral active species controls the stereochemistry of the next inserted monomer.52 These mistakes cause a disruption on the crystallinity of the polymer, hence being more pronounced in the amorphous fraction, as evidenced by comparison of the 13C NMR spectra (Figures S2, S5, and S8). DFT Studies. Next, we carried out density functional theory computations (DFT) to elucidate the experimental findings as well as the possible reaction mechanism (Scheme 2). At first, we computed the coordination and insertion of R-PO to
Figure 3. Semilogarithmic plots of PO conversion in time initiated by n Bu2Mg: [M]0/[C]0 = 200 at 40 °C.
the % conversion of monomer (Figure S13). This behavior together with the rapid activation and the irreversible propagation (characteristic for the ROP of strained threemembered monomers) are in good agreement with a living polymerization mechanism. The observed increase on the polydispersity upon increase of [M]0/[C]0 (see Table 1) may be due to rapid, reversible reactions like temporary deactivation and segmental exchange. In spite of these side reactions (which can effectively compete with the chain growth, especially at low monomer concentrations), the reaction still meets the criteria of a living polymerization mechanism. Mechanistic Considerations. For a better understanding of the reaction mechanism, low molecular weight PPO was synthesized using a PO to nBu2Mg molar ratio of 30:1 in bulk at 40 °C. The reaction mixture was stirred for 3 h and then quenched with acidified water. The resulting precipitate was filtered off, washed with water, and dried in vacuo for several hours. The oligomers were thoroughly characterized using NMR and ESI-TOF techniques. The NMR spectrum (Figures S10 and S11) clearly shows that the oligomer chains are endcapped with butyl and hydroxyl groups, suggesting that the ROP is initiated by the n-butyl group. This observation was confirmed by the analysis of ESI-TOF spectrum of the oligomer (Figure 4). It comprises two series of peaks at regular mass intervals of 58 au (corresponding to one monomeric unit). The
Figure 4. ESI-TOF mass spectrum of the crude product obtained from a reaction between PO and nBu2Mg in 30:1 ratio at 40 °C in an argon atmosphere. C
DOI: 10.1021/acs.macromol.6b01830 Macromolecules XXXX, XXX, XXX−XXX
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does not show an energetic difference high enough to account for the observed isoselective chain propagation. Since the DFT results do not support a simple monometallic complex as the active species, our question of “what is the active species for such selectivity at the reaction temperature?” remained unanswered. There are several literature reports of crystal structures of magnesium alkoxide complexes.57−59 In methanol solution, the structure of Mg(OMe)2 is built of residues of four types, namely cubane neutral complexes [Mg 4 (μ 3 -OR) 4 (OR) 4 (ROH) 8 ], dications [Mg 4 (μ 3 OR)4(OR)2(ROH)10]2+, monoanions [(ROH)2H]−, and eight crystallographically independent noncoordinated solvating methanol molecules. All residues are linked into a threedimensional framework by means of a complicated hydrogenbonding system. The basic structural moiety of these crystals is the cubane scaffold, [Mg4(μ3-OR)4(OR)4, containing four μ3 and four terminal alkoxy groups. On the basis of this structural moiety, we have computed a cubane analogue for studying the observed isotactic selectivity of PO insertion and chain propagation. Our preliminary structure optimization showed that it is not possible to coordinate eight R-PO molecules as for the methanol complex due to the larger size of the PO molecule. In order to reduce the computing demand, we used the smaller SVP basis set (BP86/SVP) as well as a small cluster for our study, [Mg4(μ3O-R-2-butyl) 3 (O-R-2-butyl)(μ 3 -OMe)(OMe) 3 (MeOH) 3 ], where one Mg center is coordinated with only one R-PO or SPO. As there are both μ3 and terminal O-R-2-butyl groups, we calculated the insertion of these groups into either R-PO or SPO molecules. The obtained energetic barriers for both insertions are still over 40 kcal/mol, which is not compatible with the low reaction temperatures used in the study. Additionally, the rather small energy difference (less than 1.6 kcal/mol) between the R-PO and S-PO coordination cannot be argued to explain the observed isoselectivity. Although our computational results still cannot enlighten the striking polymerization selectivity, they certainly rule out the most obvious and intuitive assumptions proposed in the historical literature.
Scheme 2. Proposed Mechanism for the Ring-Opening Polymerization of rac-PO Using nBu2Mg
diethylmagneniusm, Et2Mg, which was used to model di-nbutylmagnesium, nBu2Mg, at the B3PW91 level of density functional theory in conjugation with the TZVP basis set. The computational details are given in the Supporting Information. It is found that Et2Mg has a C2v symmetry, where both ethyl groups are parallel on the same side. The coordination of R-PO to the Mg center is found to be exergonic by 1.1 kcal/mol and therefore has a slight thermodynamic preference. Starting with this complex (1), the insertion of one ethyl group into the CH2 carbon of R-PO is found to be very exergonic by 48.3 kcal/mol, hence thermodynamically very favored. In addition, we computed the insertion of the second ethyl group into another coordinated R-PO; this step is also highly exergonic by 47.0 kcal/mol. For comparison, we computed the complex with both R-PO molecules inserted stepwise at only one side, leaving the one ethyl ligand still at the Mg center; this alkyl−alkoxy complex is 27.2 kcal/mol less stable than the dialkoxy (2) complex, despite the fact that the O-R-isopropyl-O-R-2-pentyl chain can form a five-membered chelate ring with the Mg center. Thus, this shows thermodynamically that both alkyl groups shall undergo PO insertion; this is in agreement with our control experiment, where the insertion of only one PO unit per alkyl group was found. Moreover, there is practically no energetic difference between the rac complex (with two OR-2-pentyl ligands) and its meso isomer (with one O-R-2-pentyl and one O-S-2-pentyl groups). This indicates that there is no energetic preference between R-PO and S-PO insertion, hence no enantioselection at this point of the mechanism. Starting from the Mg(O-R-2-pentyl)2 (2), we computed both the third PO coordination and insertion for chain propagation. The coordination (of R-PO) is computed to be exergonic by 7.3 kcal/mol, which is stronger that of the first R-PO coordination to yield 1 (1.1 kcal/mol). It is also noted that the coordination of one S-PO is exergonic by 6.9 kcal/mol. This is again no significant energetic preference to differentiate both PO enantiomers despite the presence of the two R-chiral centers in 2. Starting from both R-PO and S-PO coordinated complexes, we computed the insertion and chain propagation step. It was noted that the computed insertion barriers (30.7 and 29.7 kcal/ mol, respectively) are too high for an exothermic reaction at the rather low temperature used experimentally (40 °C). It also
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CONCLUSION Reinvestigation of a long forgotten reaction, namely the ROP of rac-PO initiated by nBu2Mg, showed that under our conditions, apart from being very efficient, is isospecific, yielding isotactic PO (mm triad >99%) with high yield and high number-average molecular weight (Mn) in a living fashion. Powder X-ray and DSC analysis confirm the high crystallinity of the obtained polymers. Kinetic data analysis shows that the ROP of PO by this system is first order in both monomer and catalysts concentration. Analyses of low molecular weight oligomers revealed that the butyl group is incorporated as one of the end terminal groups in the polymer chain and initiates the polymerization. With the help of DFT calculations we could rule out the role alkyl−alkoxy magnesium complexes as the catalytically active species as well as the involvement of monometallic intermediates and tetrametallic clusters. For the time being, both the active species and the origin of the isotactic selectivity remain elusive. More experimental as well as computational work is indeed required in order to shed light on this chemistry. D
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Nevertheless, to the best of our knowledge, this is the simplest achiral catalytic system yielding the highest isotacticity in the ROP of rac-PO.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01830. Experimental procedures, NMR spectra, kinetic plots, thermal analysis, powder X-ray diffraction analyses and DFT calculations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (E.M.). ORCID
Esteban Mejía: 0000-0002-4774-6884 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Ms. M. Woznicka for the assistance with the DSC analysis. Financial support by Henkel AG & Co KGaA is gratefully acknowledged.
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REFERENCES
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DOI: 10.1021/acs.macromol.6b01830 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b01830 Macromolecules XXXX, XXX, XXX−XXX
