Isospecific Copolymerization of Cyclohexene Oxide and Carbon

Feb 1, 2018 - These simple achiral catalysts showed good activity for the isoselective copolymerization, with high yield and high carbonate-linkage co...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Isospecific Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by Dialkylmagnesium Compounds Swarup Ghosh,† David Pahovnik,‡ Udo Kragl,§ and Esteban Mejía*,† †

Leibniz Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany Department of Polymer Chemistry and Technology, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia § Institute of Chemistry, University of Rostock, Albert-Einstein-Str. 3a, 18059 Rostock, Germany ‡

S Supporting Information *

ABSTRACT: Herein we report the synthesis of isotactically enriched poly(cyclohexene carbonate) (PCHC) by the bulk alternating copolymerization of meso-cyclohexene oxide and carbon dioxide using dialkylmagnesium compounds as catalyst. These simple achiral catalysts showed good activity for the isoselective copolymerization, with high yield and high carbonate-linkage contents, in a controlled manner under mild conditions and atmospheric CO2 pressure. The triad sequences in the 13C NMR spectra of these polymers were assigned using Bernoullian statistical methods in order to determine the tacticity of the copolymer. Differential scanning calorimetry analysis confirmed the formation of highly isotactic polycarbonate. Furthermore, kinetic studies and matrix-assisted laser desorption/ionization time-offlight mass spectrometry investigations allowed us to shed light on the reaction mechanism of the polymerization process.

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unexploited.42 However, various well-defined catalytic systems for the synthesis of isotactic poly(cyclohexene carbonate (isoPCHC) have been reported by several groups.43−51 In 1999, the enantioselective copolymerization of CHO and CO2 was reported for the first time by Nozaki and co-workers, using a 1:1 mixture of ZnEt2 and a chiral amino alcohol, (S)-α,αdiphenylpyrrolidine-2-yl-methanol.43 Shortly after, in 2000, Jacobsen et al. used an enantiopure (R,R)-salen Cr(III) complex as a catalyst in a selective copolymerization of CO2 with the (S)-enantiomer of racemic 1,2-epoxyhexane to produce a polycarbonate.52 Lu and co-workers reported a remarkable enantiopure dinuclear Co(III) chiral catalyst system with a rigid bridging biphenyl linker which demonstrated outstanding activity and unprecedented enantioselectivity for the alternating copolymerization of CO2 with meso-epoxides under mild conditions.50 Furthermore, Coates and co-workers introduced a variety of highly active zinc catalysts, which showed good activity and selectivity for the copolymerization of CO2 and CHO at mild reaction conditions. Their investigation on the reaction mechanism showed that the active catalysts existed as “loosely associated” dimers under polymerization conditions.17,53−55 The first example of an efficient dinuclear magnesium complex for the alternating copolymerization of CHO and CO2 under mild condition was made by Ding et al. in 2006.56 Later, Williams et al. reported the synthesis of dinuclear complexes of macrocyclic ancillary ligands coordinated to Zn, Co, Mg, or Fe with prominent activity and high

INTRODUCTION Over the past two decades, the alternating copolymerization of CO2 and epoxides catalyzed by metal complexes has been established as an environmentally benign alternative for the synthesis of biodegradable polycarbonates, partially (or solely) from renewable resources.1−11 Because of their attractive properties such as high transparency, heat resistance, high tensile strength, lightness, durability, and their useful electrical insulating ability, polycarbonate-based materials have found many applications in areas like coatings, electronics, ceramics, films, packaging, and engineering.11−13 In a seminal report in 1969, Inoue et al. disclosed the synthesis of high molecular weight polycarbonates from carbon dioxide and epoxides using heterogeneous zinc catalysts, prepared from diethylzinc and water.14 Subsequent investigations focused on the development of homogeneous catalysts for the copolymerization of CO2 and cyclohexene oxide (CHO) emphasizing on the substantial benefits over the traditional heterogeneous system.15−22 More recent developments have shown the importance of both the metal center(s) and the ligand scaffold in the activity and selectivity of the alternating copolymerization of epoxides and CO2.1,6,8,23−40 Stereoselective polymerization is a potent synthetic method to gain access to high performance materials, since the controlled stereochemistry of the polymer microstructure where successive stereocenters are of the same (isotactic) or alternating (syndiotactic) relative configurations usually affords higher crystallinity compared to their atactic analogues and hence improved mechanical and thermal properties. 41 Compared to the tremendous amount of research on the stereoselective polymerization of olefins, the stereoselective epoxide polymerization and copolymerization remain largely © XXXX American Chemical Society

Received: November 21, 2017 Revised: January 17, 2018

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DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Copolymerization of Cyclohexene Oxide with CO2 Using Dialkylmagnesium Compounds 1−3 as Catalysts

analysis was performed using the ATR-IR spectrometer Nicolet 6700 (Thermo Electron), spectral range of 4000−400 cm−1 and maximum resolution 0.5 cm−1. Matrix-assisted laser desorption/ionization timeof-flight mass spectrometry (MALDI-TOF MS) measurements were carried out on a Bruker UltrafleXtreme MALDI-TOF mass spectrometer (Bruker Daltonik, Bremen, Germany). Samples were dissolved in THF (10 mg/mL) and mixed with a solution of sodium trifluoroacetate in THF (10 mg/mL) and solution of matrix 2,5dihydroxyacetophenone in THF (20 mg/mL), in a volume ratio of 1:3:10. 0.4 μL of thus-prepared solution was spotted on the target plate (dried-droplet method). The reflective positive ion mode was used to acquire the mass spectra of the samples. The calibration was done externally with the poly(methyl methacrylate) standards using the nearest-neighbor positions. General Procedure for Copolymerization of CO2 and Cyclohexene Oxide. Copolymerization reactions of CO2 and CHO using dialkylmagnesium (1, 2, and 3) were performed in a 100 mL stainless steel Parr reactor equipped with a motorized 4-fold bladed impeller for stirring. The reaction temperature was regulated by a copper heating mantle block and a thermocouple connected to a Parr 4848 controller unit. The reactor was equipped with a syringe port valve for aliquot sampling and liquid addition, a gas inlet valve for CO2 introduction, and an additional valve for applying vacuum or inert gas as well as a pressure gauge and safety rupture disk. In a typical experiment, first the empty reactor was heated to 100 °C under vacuum for 4 h and then cooled under vacuum to 23 °C. Then, 1 bar of CO2 was introduced to the autoclave. After 5 min, 20 g of CHO was added to the reactor via the syringe port valve and stirred slowly. The required pressure of CO2 was attained by stepwise pressure increase until the solution was saturated with the gas and the pressure was stable. The required amount of catalyst was added to the reaction mixture via the injection port while maintaining the desired CO2 pressure. The stirring rate was increased, and the autoclave was heated to 100 °C. After the required reaction time the autoclave was allowed to cool down to room temperature. A small aliquot of the resulting polymerization mixture was removed from the reactor and carefully dissolved in chilled CDCl3 (0 °C) in an NMR tube equipped with a Young valve. The NMR tube was sealed immediately and analyzed by 1 H NMR for the determination of reaction conversion. The remaining contents were dissolved in minimum quantity of CH2Cl2 and quenched with 5% HCl solution in methanol and precipitated by addition of methanol. The precipitate was collected through filtration. The pure polycarbonates were collected by repeating the precipitation process several times. Determination of the Polymerization Kinetics with Respect to Concentration of CHO ([CHO]). In order to determine the reaction kinetics with respect to [CHO], the polymerization was carried out in the monomer-to-catalyst ratio [CHO]0/[1]0 = 100 at 100 °C under 1 bar of CO2 pressure in a 100 mL stainless steel Parr reactor as described above. The general polymerization procedure was followed, using 0.3 mol of CHO and 3 mmol of 1 (1.0 M solution in heptane). At different time intervals, 0.3 mL aliquots were taken from the reaction mixture through the syringe port valve rapidly under a countercurrent of CO2 in order to avoid contamination by air or water,

selectivity toward the alternative block copolymerization of CO2 and epoxides at CO2 pressures as low as 1 bar.57−63 We recently reported the catalytic activity of simple di-nbutylmagnesium for the isoselective living ring-opening polymerization (ROP) of racemic propylene oxide (rac-PO) to obtain isotactic PO (mm triad >99%) with high yield and high molecular weight.64 Herein, we explore the alternative bulk copolymerization of cyclohexene oxide and carbon dioxide with the same type of catalysts, namely nBu2Mg (1), nBuEtMg (2), and nBunOctMg (3). Interestingly, these simple achiral catalysts also exhibited excellent activity in the stereoselective lowpressure bulk copolymerization of meso-CHO and CO2. The produced iso-PCHC displays up to 80% isotacticity (probability of meso diads, Pm) and can be made with high yield and high number-average molecular weight (Mn) with a carbonate linkage up to 92%. To the best of our knowledge, this is the first achiral catalyst system able to desymmetrize meso-CHO and catalyze its copolymerization with CO2.

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EXPERIMENTAL SECTION

General Experimental Details. All the copolymerization reactions were performed with rigorous exclusion of moisture and air in a 100 mL Parr autoclave under CO2 pressure. nBu2Mg was purchased from Sigma-Aldrich as a 1.0 M solution in heptane and used as received. nBuEtMg and nBunOctMg were kindly supplied by LANXESS Organometallics as a 20% solution in heptane and used as received. Cyclohexene oxide (CHO) was purchased from SigmaAldrich and was dried over calcium hydride and distilled prior to use. Concentrated hydrochloric acid, NaOH, and all the solvents used for the experiments were purchased from Sigma-Aldrich. CO2 (99.99%) was purchased from Linde and used as received. 1H and 13C NMR spectra were recorded in CDCl3 and CD2Cl2 using a Bruker Avance 300 spectrometer with a QNP probe head (1H: 300 MHz; 13C: 75 MHz) or a Bruker Avance 400 (1H: 400 MHz; 13C: 100 MHz). The calibration of the spectra was carried out using residual solvent shifts (CDCl3, 1H = 7.26, 13C = 77.16 and CD2Cl2, 1H = 5.33, 13C = 54.24) and were reported as parts per million relative to SiMe4. All the NMR samples were measured at 297 K. GPC measurements were performed on an HP1090 II chromatograph with a DAD detector (HewlettPackard) at 40 °C with a flow of 0.9 mL/min and THF as the eluent. The columns used for the measurements were three consecutively connected PSS SDV gel columns with molecular weight ranges of 102, 103, and 104 g/mol. Number-average molecular weights (Mn) and molecular weight distributionss (Mw/Mn) of polymers were measured relative to polystyrene standards. Differential scanning calorimetry (DSC) measurement was performed on a DSC 1 STARe device from Metler Toledo. The heating and cooling rate was 10 °C/min. The ramp method was used for these analyses, and 10−15 mg samples were taken. The optical rotation of the polymer was measured at room temperature with a tungsten halogen lamp using a modular circular polarimeter (Anton Paar, MCP-200), cell length 1 dm, and measurement accuracy 0.001° in THF solution. Infrared spectroscopy B

DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Copolymerization of Cyclohexene Oxide and CO2 Using nBu2Mg (1) entry

[M]0/[C]0

press. (bar)

time (h)

conva (%)

TONb

TOFc (h−1)

carbonated (%)

Mn(obs) e (kg/mol)

Mw/Mne

P mf

1 2 3 4 5 6

100:1 200:1 500:1 100:1 200:1 500:1

1 1 1 10 10 10

15 19 24 5 8 12

89 88 85 99 99 96

89 176 425 99 198 480

6 9 18 20 25 40

85 84 81 82 80 80

7.6 13.6 25.6 10.8 18.1 32.2

2.0 2.2 2.5 2.4 2.6 2.8

82 82 81 80 80 78

a

Estimated by 1H NMR. bTurnover number = number of moles of epoxide consumed per mole of catalyst. cTurnover frequency = TON/h. Determined by comparison of the integrals of signals arising from the methylene protons in the 1H NMR spectra due to carbonate linkages (4.6 ppm), ether linkages (3.4 ppm), and the signals due to cyclic carbonate byproduct (4.0 ppm). eDetermined by GPC in THF at 40 °C, calibrated with polystyrene standards. fPm for m-centered tetrads, determined by 13C NMR spectroscopy (CDCl3, 100 MHz). d

and the CO2 pressure was re-established. The aliquots taken from the reaction mixture were analyzed through 1H NMR as described before. The reaction order of CHO for the block copolymerization was determined by the plot of ln[CHO]0/[CHO]t versus time. Determination of the Polymerization Kinetics with Respect to Concentration of Catalyst ([Catalyst]). In order to determine the reaction kinetics, with respect to catalyst, the polymerizations were carried out using a defined quantity (0.6, 0.9, 1.2, and 1.5 mmol) of 1, with 29.4 g (0.3 mol) of CHO at 100 °C under 1 bar CO2 pressure in a 100 mL stainless steel Parr reactor following the general polymerization procedure. At different time intervals, 0.3 mL aliquots were taken from the reaction mixture and analyzed through 1H NMR as described above. Initial rates depending on [catalyst] were determined by the plots of [copolymer] versus time (Supporting Information, Table S1). Reaction order of catalyst for the block copolymerization of CHO was determined by the plot of the initial rate versus the concentration of catalyst.

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RESULTS AND DISCUSSION Copolymerization Experiments. The copolymerization of CHO with CO2 using three different dialkylmagnesium catalysts (1−3) at different pressures and monomer/catalyst ratios ([M]0/[C]0) was performed (Scheme 1). The results obtained with catalyst 1 are depicted in Table 1. At only 1 bar pressure of CO2, the catalyst showed remarkable performance toward the polycarbonate formation producing the copolymer with up to 89% CHO conversion at monomer to catalyst ratio of 100:1 (Table 1, entry 1). The epoxide conversions of the copolymers were determined through 1H NMR analysis by integrating the significant peaks. The peak at 4.6 ppm (broad signal) corresponds to the methyne protons in poly(cyclohexene carbonate) (PCHC). The peaks at 4.41 ppm correspond to the methyne protons on the end group. The peaks at 1.15− 2.1 ppm correspond to the remaining aliphatic protons in the PCHC. The broad signal at δ 3.4−3.5 ppm corresponds to polyether linkages. The corresponding cyclic carbonate was verified by the signal at δ 4.0 ppm (see Supporting Information). The percentage of carbonate (CO2 incorporation) and ether linkage or cyclic carbonate in the copolymer was also determined by integration of the corresponding peaks in the 1H NMR spectrum. IR spectroscopy was also used to verify the CO2 incorporation by ν(CO) stretch for the polycarbonate through the appearance of a broad signal at 1748 cm−1 and for the cyclic carbonate at 1804 cm−1. The formation of the PCHC chain was also confirmed by MALDI-TOF analysis (see Figure 1 and Figure S23) of the polycarbonates where the difference between two successive peaks is 142 units, which corresponds to the total mass of both CHO and CO2 monomers. The resulting copolymer contains 85% carbonate linkages and 15% ether linkages, which were calculated by the

Figure 1. MALDI-TOF mass spectrum of the isotactic poly(cyclohexene carbonate) obtained from a reaction between CHO and CO2 using nBu2Mg as a catalyst in 100:1 ratio at 1 bar pressure of CO2. Measured monoisotopic signals are denoted in the mass spectrum together with the calculated exact masses ionized with sodium ion (in parentheses) for the proposed structures for a specific number of repeating units (n).

integration of the corresponding peaks in 1H NMR spectrum as described in the Experimental Section (see also Figure S1). The polymer was recrystallized several times by dissolving it in CH2Cl2 followed by precipitation with methanol to remove any polyether homopolymer. Consequently, the percentage of carbonate linkages increased only slightly (to 87%) while the ether linkages decreased accordingly (down to 13%) with respect to that in crude polymer as shown by 1H NMR. This observation strongly suggests that the obtained copolymer is made of both polycarbonate and polyether repeating units and that the polyether homopolymer is formed only to a limited extent. This observation was further confirmed by MALDITOF analysis of the copolymer (see Figure 1 and Figure S23). Upon increasing the monomer-to-catalyst ratio from 100:1 to 500:1 the Mn of the copolymer also increased from 7.6 to 25.6 kg/mol while Mw/Mn also increased from 2.0 to 2.5 (Table 1, entries 2 and 3). On increasing the CO2 pressure from 1 to 10 bar, the rate of copolymerization was around 2 times faster than that at 1 bar. Under these conditions almost quantitative conversions were achieved. The Mn of the obtained copolymers were also comparatively higher, between 10.8 and 32.2 kg/mol. The values of Mw/Mn were also higher, rising to 2.8 (Table 1, entries 5 and 6). Notably, 1H NMR analysis showed that the copolymer obtained at 10 bar of CO2 contains 82% carbonate C

DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Table 2. Copolymerization of Cyclohexene Oxide and CO2 Using Catalysts 2 and 3 at a Monomer-to-Catalyst Ratio of 200:1 entry

catalyst

press. (bar)

time (h)

conva (%)

TONb

TOFc

carbonated (%)

Mn(obs) e (kg/mol)

Mw/Mne

P mf

1 2 3 4

2 3 2 3

1 1 10 10

19 9 8 8

92 90 99 99

184 180 198 498

10 20 25 62

88 86 92 90

13.1 12.9 10.2 11.3

1.8 1.9 2.2 2.1

80 79 79 78

a

Estimated by 1H NMR. bTurnover number = number of moles of epoxide consumed per mole of catalyst. cTurnover frequency = TON/h. Determined by comparison of the integrals of signals arising from the methylene protons in the 1H NMR spectra due to carbonate linkages (4.6 ppm) and ether linkages (3.4 ppm). eDetermined by GPC in THF at 40 °C, calibrated with polystyrene standards. fPm for m-centered tetrads, determined by 13C NMR spectroscopy (CDCl3, 100 MHz). d

linkages and 5% ether linkages together with 13% of transcyclohexene carbonate (trans-CHC), which corresponds to the signal at δ = 4.02−4.05 ppm (Table 1, entry 5, and Figure S4). Formation of trans-CHC was also confirmed by the FTIR absorption band of CO vibration mode at ν = 1804 cm−1, a characteristic for the cyclic carbonate (see Figure S18). Interestingly, with the use of catalysts 2 and 3, comparable results to 1 were obtained (Table 2 and Table S2), with the important difference that no cyclic carbonate (trans-CHC) was observed in any case, as evidenced by NMR (Figures S10, S11, S14, and S15) and IR analysis (Figures S19 and S20). Considering that for the three different catalysts the active species shall be the same (see below), the formation of the cyclic side product (trans-CHC) exclusively with 1 might be due to the influence of the triethylaluminum present in the commercial nBu2Mg (1 wt %), commonly used as viscosity reducer. Conversely, the asymmetric, bulkier dialkylmagnesium compounds 2 and 3, having a lower tendency to form aggregates in solution, do not require the use of viscosity modifiers; hence, the amount of triethylaluminum present in the commercial samples is negligible. Stereochemical Characterization of Poly(cyclohexene carbonate). The stereochemistry of the copolymer was determined by 13C{1H} NMR analysis of the carbonyl region. The tacticity of the polycarbonate was resolved based on the signals at δ 153.7 ppm for isotactic diads and at δ 153.1−153.3 ppm for syndiotactic diads.65 Previously, Coates and Nozaki assigned the peaks in the 13C NMR shifts in the carbonyl region of various stereoregular PCHC triads.43,44,55,65 Furthermore, by synthesizing model poly(cyclohexene carbonate) oligomers, Lu and Nozaki assigned the resonances corresponding to [mmm] and [rrr] tetrad arrangements using Bernoullian statistical methods.51,65 All [mmm], [mmr], and [rmr] tetrads were correlated to one resonance at 153.7 ppm while the remaining r-centered tetrads appear within the 153.3−153.1 ppm range. Therefore, isotactic PCHC is characterized by a strong resonance at 153.7 ppm with a smaller set of resonances around 153.3−153.1 ppm due to stereoerrors. Statistically, the most important stereoerrors in isotactic PCHC are [mrr] and [mmr] tetrads, with little contribution from the remaining [rrr], [rmr], and [mrm] tetrads. The carbonyl region in the 13C NMR spectrum of the polycarbonate obtained by copolymerization of CHO and CO2 using nBu2Mg at 100 °C under 1 bar CO2 pressure (Table 1, entry 1) is presented in Figure 2 (for Table 1, entry 4, and Figure S16). As described above, the resonances at 153.78 ppm were assigned to [mmm + mmr] tetrads and the resonances at 153.14−153.32 ppm assigned to [mrm], [rrm], [rrr], and [rmr] tetrads, resulting in an isotactic enrichment (Pm) up to 82%.

Figure 2. Carbonyl region of 13C NMR spectrum of the poly(cyclohexene carbonate) obtained from CHO and CO2 using nBu2Mg as a catalyst at 1 bar pressure of CO2. The tacticity of the polycarbonate was resolved based on the signals at δ 153.7 ppm for isotactic diads (assigned to [mmm + mmr] tetrads) and the resonances at δ 153.1−153.3 ppm for syndiotactic diads (assigned to [mrm], [rrm], [rrr], and [rmr] tetrads). Therefore, isotactic PCHC is characterized by a strong resonance at 153.7 ppm with a smaller set of resonances around 153.3−153.1 ppm due to stereoerrors.

This is a remarkable feature considering that the used dialkylmagnesium catalysts are achiral. The isotactic nature of the polycarbonate was further confirmed by DSC analysis, where the Tg and Tm were found at 120 and 220 °C, respectively (Figures S21 and S22). These values are in good agreement with those previously reported for isotactic PCHC.44,54 The asymmetric alternating copolymerization of CHO and CO2 was first reported by Nozaki using a chiral organozinc amino-alcoholate catalysts.43 Later on, Coates et al. developed a discrete chiral zinc catalysts for asymmetric alternating copolymerization of CHO and CO2.44 The same group also introduced a route for the synthesis of highly isotactic polycarbonates using enantioselective β-diiminate Zn catalysts54 and a series of (salen)Co(III) complexes for the synthesis of syndiotactic poly(cyclohexene carbonate).55 D

DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Recently, Liu et al. reported formation of isotactic multiblock polycarbonates using a racemic bimetallic cobalt catalyst from meso-epoxides and carbon dioxide.51 To the best of our knowledge, the present contribution represents the first example of the synthesis of isotactic poly(cyclohexene carbonate) via sereoselective copolymerization of meso-CHO and CO2 using a simple achiral catalysts (dialkylmagnesium compounds 1−3) without any chiral additive or cocatalyst. Moreover, the copolymerization reaction proceeds smoothly under only 1 bar CO2 pressure. Kinetic Studies. Reaction Order with Respect to [CHO]. The reaction order with respect to CHO concentration was determined by the integrated reaction rate law method. The copolymerization reaction was performed in a monomer-tocatalyst ratio [CHO]0/[1]0 = 100 at 100 °C under 1 bar CO2 pressure. Aliquots were taken from reaction mixture at different time intervals. The samples were analyzed using 1H NMR spectroscopy. A plot of ln[CHO]0/[CHO]t vs time evidently demonstrated a linear dependence (Figure 3) without any induction period which clearly suggests that the copolymerization reaction rate is first-order with respect to CHO concentration.

Figure 4. Plot of the initial rate vs catalyst concentration for the synthesis of PCHC using 1. Copolymerization conditions: [CHO]0 = 0.3 mol, 100 °C, 1 bar of CO2.

protons while the peaks at 1.38−2.1 ppm correspond to the remaining aliphatic protons in PCHC. The broad signal at δ 3.35 ppm belongs to the polyether linkages. The absence of the undesired cyclic carbonate was verified by the absence of its characteristic signal at δ 4.0 ppm. From integration of the 1H NMR signals, it was observed that resultant copolymer contains 85% carbonate linkages and 15% ether linkages (Table 1, entry 1). The incorporation of CO2 was further verified by IR through observation of the carbonyl stretch for the polycarbonate at ν(CO) 1748 cm−1 (Figure S17). The formation of the PCHC chain was also confirmed by MALDI-TOF analysis of the obtained polycarbonates where the difference between two successive peaks is 142 units, which corresponds to the combined mass of one molecule of CHO and one molecule of CO2 (see Figure 1 and Figure S23). The results clearly show that the polymer chains are predominantly end-capped by cyclohexyl and hydroxyl groups. This observation also suggests that the obtained copolymer contains mainly polycarbonate repeating units with small amount of ether linkage as well, which is in good agreement with the NMR results. The above observations indicate that the copolymerization reaction may proceed through a coordination−insertion mechanism (see Scheme 2). The initiation step starts with the formation of a metal hydride (4) via β-hydride elimination, followed by coordination of CHO to the magnesium center. Then, hydride transfer from the metal to the electrophilic carbon center of CHO results in opening of the oxirane ring, forming an alkyl−alkoxy intermediate (5) which should rapidly yield a dialkoxymagnesium complex (6). DFT calculations from our previous investigations have shown that a Mg−dialkoxy complex is more than 27 kcal/mol more stable than its alkyl−alkoxy precursor; hence, both alkyl groups shall undergo peroxide insertion in the initiation step.64 It has been shown that dialkoxymagnesium complexes act as a catalytically active species preferably in the form of multimetallic aggregates rather than as discrete monometallic species.64,66,67 We can thus infer that the actual active species in our system should be of multimetallic character (7). The chain propagation step proceeds via alternative coordination and insertion of CHO and CO2 molecules (8). Finally, the hydrolysis of metal−oxygen bond (Mg−O) from 9 leads to the termination of the polymerization forming hydroxyl end chain

Figure 3. Semilogarithmic plot of CHO conversion vs time for the copolymerization with CO2 (1 bar) initiated by 1. [M]0/[C]0 = 100, 100 °C.

Reaction Order with Respect [Catalyst]. For the determination of reaction order with respect to the catalyst concentration, the kinetic studies were performed using 1 over the concentration range 0.6−1.5 mmol with 0.3 mol of CHO at 100 °C, under 1 bar of CO2, following the same procedure as described above. The initial rates were calculated from the slopes of the plots [PCHC] vs time (Table S1). The 1 H NMR analysis showed an apparent linear relationship between the initial rate and the catalyst concentration (Figure 4), which suggests that the copolymerization reaction is firstorder on catalyst concentration. Mechanistic Studies. For a better understanding of the reaction mechanism, the obtained polycarbonates were thoroughly characterized using NMR and IR spectrometry and MALDI-TOF mass spectrometry. The copolymer composition was determined through 1H NMR by integrating the corresponding peaks (see Figures S1, S3, and S5). The peak at δ 4.60 ppm (broad signal) corresponds to the methyne E

DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 2. Proposed Mechanism for the Block Copolymerization of CHO with CO2 Using Dialkylmagnesium Compounds

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copolymer. The proposed reaction mechanism is depicted in Scheme 2.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02463. Spectroscopic characterization of the obtained polymers including 1H and 13C NMR spectra, IR spectra; MALDITOF spectra as well as DSC thermograms (PDF)

CONCLUSIONS In summary, we have demonstrated that simple achiral dialkylmagnesium compounds are efficient catalysts for the isoselective copolymerization of cyclohexene oxide and carbon dioxide, yielding isotactic polycarbonate with high yields and high carbonate linkage (92%), in a controlled manner. The system works effectively under low CO2 pressures (1 bar) and without the addition of any cocatalyst, additive, or solvent. Increasing the CO2 pressure from 1 to 10 bar results in an increase of the catalytic activity of copolymerization without noticeable difference on the isotacticity of the resulting PCHC. DSC analysis results confirmed the isotactic enrichment of products. Kinetic studies analysis of the of the copolymerization at 100 °C and 1 bar pressure of CO2 with 1 demonstrated that the reaction has a first order dependence on both the cyclohexene oxide and catalyst concentrations. With the help of MALDI-TOF MS analysis and based on our previous investigations,64 we proposed a plausible reaction mechanism. To the best of our knowledge, this is the first example of a simple achiral catalyst inducing isoselectivity in the block copolymerization of CHO and CO2 even at atmospheric CO2 pressure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (E.M.). ORCID

David Pahovnik: 0000-0001-8024-8871 Esteban Mejía: 0000-0002-4774-6884 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the University of Rostock is gratefully acknowledged. We thank Dr. Thorsten Holtrichter-Rößmann from LANXESS Organometallics for his valuable insights and for kindly providing us catalysts samples. F

DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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(39) Decortes, A.; Haak, R. M.; Martin, C.; Belmonte, M. M.; Martin, E.; Benet-Buchholz, J.; Kleij, A. W. Macromolecules 2015, 48, 8197. (40) Kissling, S.; Lehenmeier, M. W.; Altenbuchner, P. T.; Kronast, A.; Reiter, M.; Deglmann, P.; Seemann, U. B.; Rieger, B. Chem. Commun. 2015, 51, 4579. (41) Coates, G. W. Chem. Rev. 2000, 100, 1223. (42) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Chem. Rev. 2014, 114, 8129. (43) Nozaki, K.; Nakano, K.; Hiyama, T. J. Am. Chem. Soc. 1999, 121, 11008. (44) Cheng, M.; Darling, N. A.; Lobkovsky, E. B.; Coates, G. W. Chem. Commun. 2000, 2007. (45) Nakano, K.; Nozaki, K.; Hiyama, T. J. Am. Chem. Soc. 2003, 125, 5501. (46) Xiao, Y.; Wang, Z.; Ding, K. Chem. - Eur. J. 2005, 11, 3668. (47) Nishioka, K.; Goto, H.; Sugimoto, H. Macromolecules 2012, 45, 8172. (48) Nakano, K.; Nakamura, M.; Nozaki, K. Macromolecules 2009, 42, 6972. (49) Wu, G.-P.; Ren, W.-M.; Luo, Y.; Li, B.; Zhang, W.-Z.; Lu, X.-B. J. Am. Chem. Soc. 2012, 134, 5682. (50) Liu, Y.; Ren, W.-M.; Liu, J.; Lu, X.-B. Angew. Chem., Int. Ed. 2013, 52, 11594. (51) Liu, Y.; Ren, W.-M.; Zhang, W.-P.; Zhao, R.-R.; Lu, X.-B. Nat. Commun. 2015, 6, 8594. (52) Jacobsen, E. N.; Tokunaga, M.; Larrow, J. F. US6262278B1 ed.; President and Fellows of Harvard College, 2001. (53) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003, 125, 11911. (54) Ellis, W. C.; Jung, Y.; Mulzer, M.; Di Girolamo, R.; Lobkovsky, E. B.; Coates, G. W. Chem. Sci. 2014, 5, 4004. (55) Cohen, C. T.; Thomas, C. M.; Peretti, K. L.; Lobkovsky, E. B.; Coates, G. W. Dalton Trans 2006, 237. (56) Xiao, Y.; Wang, Z.; Ding, K. Macromolecules 2006, 39, 128. (57) Jutz, F.; Buchard, A.; Kember, M. R.; Fredriksen, S. B.; Williams, C. K. J. Am. Chem. Soc. 2011, 133, 17395. (58) Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K. Angew. Chem., Int. Ed. 2009, 48, 931. (59) Kember, M. R.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2009, 48, 9535. (60) Kember, M. R.; Jutz, F.; Buchard, A.; White, A. J. P.; Williams, C. K. Chem. Sci. 2012, 3, 1245. (61) Kember, M. R.; White, A. J. P.; Williams, C. K. Macromolecules 2010, 43, 2291. (62) Buchard, A.; Kember, M. R.; Sandeman, K. G.; Williams, C. K. Chem. Commun. 2011, 47, 212. (63) Kember, M. R.; Williams, C. K. J. Am. Chem. Soc. 2012, 134, 15676. (64) Ghosh, S.; Lund, H.; Jiao, H.; Mejia, E. Macromolecules 2017, 50, 1245. (65) Nakano, K.; Nozaki, K.; Hiyama, T. Macromolecules 2001, 34, 6325. (66) Turova, N. Y.; Popovkin, B. A.; Novoselova, A. V. Z. Anorg. Allg. Chem. 1969, 365, 100. (67) Thoms, H.; Epple, M.; Viebrock, H.; Reller, A. J. Mater. Chem. 1995, 5, 589.

REFERENCES

(1) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618. (2) Chisholm, M. H.; Zhou, Z. J. Mater. Chem. 2004, 14, 3081. (3) Sugimoto, H.; Inoue, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5561. (4) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.; Billodeaux, D. R. Acc. Chem. Res. 2004, 37, 836. (5) Ochiai, B.; Endo, T. Prog. Polym. Sci. 2005, 30, 183. (6) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388. (7) Nakano, K.; Kosaka, N.; Hiyama, T.; Nozaki, K. Dalton Trans. 2003, 4039. (8) Kember, M. R.; Buchard, A.; Williams, C. K. Chem. Commun. 2011, 47, 141. (9) Klaus, S.; Lehenmeier, M. W.; Anderson, C. E.; Rieger, B. Coord. Chem. Rev. 2011, 255, 1460. (10) Darensbourg, D. J.; Poland, R. R.; Strickland, A. L. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 127. (11) Luinstra, G. A. Polym. Rev. 2008, 48, 192. (12) Koning, C.; Wildeson, J.; Parton, R.; Plum, B.; Steeman, P.; Darensbourg, D. J. Polymer 2001, 42, 3995. (13) Hongfa, C.; Tian, J.; Andreatta, J.; Darensbourg, D. J.; Bergbreiter, D. E. Chem. Commun. 2008, 975. (14) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Part B: Polym. Lett. 1969, 7, 287. (15) Lu, X.-B.; Wang, Y. Angew. Chem., Int. Ed. 2004, 43, 3574. (16) Super, M.; Berluche, E.; Costello, C.; Beckman, E. Macromolecules 1997, 30, 368. (17) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998, 120, 11018. (18) Sarbu, T.; Beckman, E. J. Macromolecules 1999, 32, 6904. (19) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C.; Reibenspies, J. H. J. Am. Chem. Soc. 2000, 122, 12487. (20) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.; Holmes, A. B. Macromolecules 2000, 33, 303. (21) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738. (22) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew. Chem., Int. Ed. 2002, 41, 2599. (23) Decortes, A.; Castilla, A. M.; Kleij, A. W. Angew. Chem., Int. Ed. 2010, 49, 9822. (24) Darensbourg, D. J. Inorg. Chem. 2010, 49, 10765. (25) Lu, X.-B.; Darensbourg, D. J. Chem. Soc. Rev. 2012, 41, 1462. (26) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. Acc. Chem. Res. 2012, 45, 1721. (27) Li, F.; Xia, C.; Xu, L.; Sun, W.; Chen, G. Chem. Commun. 2003, 2042. (28) Li, C.-H.; Chuang, H.-J.; Li, C.-Y.; Ko, B.-T.; Lin, C.-H. Polym. Chem. 2014, 5, 4875. (29) Yu, C.-Y.; Chuang, H.-J.; Ko, B.-T. Catal. Sci. Technol. 2016, 6, 1779. (30) Lin, P.-M.; Chang, C.-H.; Chuang, H.-J.; Liu, C.-T.; Ko, B.-T.; Lin, C.-C. ChemCatChem 2016, 8, 984. (31) Tsai, C.-Y.; Cheng, F.-Y.; Lu, K.-Y.; Wu, J.-T.; Huang, B.-H.; Chen, W.-A.; Lin, C.-C.; Ko, B.-T. Inorg. Chem. 2016, 55, 7843. (32) Xu, Y.; Lin, L.; He, C.-T.; Qin, J.; Li, Z.; Wang, S.; Xiao, M.; Meng, Y. Polym. Chem. 2017, 8, 3632. (33) Huang, L.-S.; Tsai, C.-Y.; Chuang, H.-J.; Ko, B.-T. Inorg. Chem. 2017, 56, 6141. (34) Lee, T.-Y.; Lin, Y.-J.; Chang, Y.-Z.; Huang, L.-S.; Ko, B.-T.; Huang, J.-H. Organometallics 2017, 36, 291. (35) Ho, C.-H.; Chuang, H.-J.; Lin, P.-H.; Ko, B.-T. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 321. (36) Abbina, S.; Chidara, V. K.; Bian, S.; Ugrinov, A.; Du, G. ChemistrySelect 2016, 1, 3175. (37) Martinez, J.; Castro-Osma, J. A.; Lara-Sanchez, A.; Otero, A.; Fernandez-Baeza, J.; Tejeda, J.; Sanchez-Barba, L. F.; RodriguezDieguez, A. Polym. Chem. 2016, 7, 6475. (38) Zhang, D.; Zhang, H.; Hadjichristidis, N.; Gnanou, Y.; Feng, X. Macromolecules 2016, 49, 2484. G

DOI: 10.1021/acs.macromol.7b02463 Macromolecules XXXX, XXX, XXX−XXX