Kinetic Studies of Copolymerization of Cyclohexene Oxide with CO2

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Kinetic Studies of Copolymerization of Cyclohexene Oxide with CO2 by a Diamino-bis(phenolate) Chromium(III) Complex Kaijie Ni and Christopher M. Kozak* Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada S Supporting Information *

ABSTRACT: A diamino-bis(phenolate) chromium(III) complex, CrCl(THF)[L], 1, where [L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate), has been synthesized in high yield and characterized by MALDI-TOF mass spectrometry, elemental analysis, UV−vis spectroscopy and single crystal X-ray diffraction. This complex combined w i t h 4 - d i m e t h y l a m i no p y r i d i n e ( D M A P ) o r b i s (triphenylphosphoranylidene)ammonium chloride or azide salts (PPNCl or PPNN3) shows improved activity over previously reported amine-bis(phenolate) chromium(III) complexes for copolymerization of cyclohexene oxide (CHO) and CO2 to yield poly(cyclohexene) carbonate (PCHC). Kinetic studies of the complex/DMAP system showed the activation energy for polycarbonate formation to be 62 kJ/mol. End group analysis of resulting polycarbonates by MALDI-TOF MS reveals either the chloride of the Cr(III) complex or the external nucleophile initiates the copolymerization reaction.

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INTRODUCTION Carbon dioxide (CO2) is an attractive raw material for the preparation of useful chemicals as it is widely available, nontoxic, cheap, and biorenewable.1 The copolymerization of CO2 with epoxides to yield polycarbonates has become one of the most studied reactions for the chemical transformation of CO2.2−8 A variety of metal complexes typically containing metals such as magnesium,9,10 aluminum,11−13 zinc,14−26 cobalt,27−39 chromium,40−56 iron,57−59 and ytterbium60 have shown activity for coupling or copolymerization of CO2 and epoxides. Well-defined homogeneous metal complexes such as chromium porphyrin compounds,43,56 zinc phenoxides21−24 and β-diiminate zinc alkoxides25,26 have proven highly active. Tetravalent group 4 (Ti and Zr) and 14 (Ge and Sn) metals supported by planar trianionic bis(phenolato) ligands also have shown promising activity.61 By far the most widely studied catalysts for epoxide/CO2 copolymerization have been the Cr(III)44−50,53,54 or Co(III)27,35−39 salen (salicylimine) complexes and more recently salan (reduced-salen) complexes.41,42,55,62 Some of these examples exhibit very high copolymerization activity and, for propylene oxide, good polymer enantioselectivity and stereochemical control.27,42 These complexes typically require an appropriate nucleophile as cocatalyst to achieve high activity. The most commonly used cocatalysts include chloride, bromide or azide paired with bulky cations such as PPN + (PPN = bis(triphenylphosphoranylidene)ammonium) or tetrabutylammonium, or neutral bases such as N-methylimidazole (N-MeIm) or dimethylaminopyridine (DMAP). © XXXX American Chemical Society

The mechanism of epoxide/CO2 copolymerization, particularly the role of these cocatalysts, has been widely studied.62−67 ESI-MS studies by Lu of the binding affinity of neutral cocatalysts to Cr(III) salen or salan complexes have found Cr(III) salen complexes easily bind two DMAP molecules forming a [Cr(III)salen(DMAP)2]+ cation; however, Cr(III) salan complexes tend to bind only one DMAP molecule even in the presence of excess of DMAP.62 This difference in DMAP binding affinity was found to result in a significantly different catalytic activity toward PO/CO2 copolymerization. That is, salenCr(III)/DMAP exhibited a long induction period of up to 2 h; however, a short induction period or none at all was observed for salanCr(III)/DMAP and a much faster reaction rate. It was proposed that DMAP dissociation from the bis-DMAP adduct was difficult, hindering formation of the mono-DMAP adduct, which was thought to be the active species. This was also thought to be the cause of the long induction period for the salenCr(III)/DMAP system. We have developed a series of Cr(III) amino-bis(phenolato) complexes (I−IV, Chart 1), which in the presence of DMAP, PPNCl, or PPNN3 as a cocatalyst show good to excellent activity toward the copolymerization of epoxide and CO2.68−72 We also used MALDI-TOF MS to study the binding affinity of DMAP to a series of amine-bis(phenolate)Cr(III) complexes with different pendent donor groups including oxygencontaining tetrahydrofurfuryl and methoxyethyl moieties or Received: November 20, 2017

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DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

H2[L] with sodium hydride in THF at −78 °C, followed by reaction with CrCl3(THF)3 in THF at −78 °C to afford a purple/pink solid in a 95% yield (Scheme 1). The MALDITOF mass spectrum of complex 1 (Figure S1) shows a peak at m/z 609 corresponding to the radical cation of [CrCl[L]]+·. Radical cations of coordination/organometallic complexes have been observed by others using MALDI-TOF MS.77 The peak at m/z 574 corresponds to [Cr[L]]+ resulting from a loss of a chlorine. The isotopic distribution patterns of these observed ions are in good agreement with the calculated patterns. The UV−vis spectrum of 1 in CH2Cl2 is given in Figure S2. Single crystals of 1 suitable for X-ray diffraction analysis were obtained by slow evaporation of a solution of 1 in THF and hexamethyldisiloxane (HMDSO) under nitrogen in a glovebox. The molecular structure and selected bond lengths and angles are shown in Figure 1, and structural data are given in Table S1.

Chart 1

nitrogen-containing N,N-dimethylaminoethyl or 2-pyridyl moieties.73 Cr(III) amino-bis(phenolato) complexes I, II, and IV showed the ability to bind two DMAP molecules to form a six-coordinate complex ion [LCr(DMAP)2]+, with the exception of N,N-dimethylaminoethyl complex 1 (Scheme 1) Scheme 1. Synthesis of 1

Figure 1. Molecular structure (ORTEP) and partial numbering scheme of 1. Ellipsoids are drawn at 50% probability. Hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Cr(1)−Cl(1), 2.3449(8); Cr(1)−O(1), 1.9305(19); Cr(1)−O(2), 1.9241(19); Cr(1)−O(3), 2.0862(17); Cr(1)−N(1), 2.1025(19); Cr(1)−N(2), 2.147(2); O(3)−Cr(1)−Cl(1), 91.01(5); O(3)− Cr(1)−N(1), 91.88(7); O(3)−Cr(1)−N(2), 174.49(8); O(2)− Cr(1)−Cl(1), 88.08(6); O(2)−Cr(1)−O(3), 87.23(7); O(2)− Cr(1)−N(1), 90.35(7); O(2)−Cr(1)−O(1), 173.09(8); O(2)− Cr(1)−N(2), 94.34(10); N(1)−Cr(1)−Cl(1), 176.63(6); N(1)− Cr(1)−N(2), 82.83(8); O(1)−Cr(1)−Cl(1), 90.12(6); O(1)− Cr(1)−O(3), 86.14(8); O(1)−Cr(1)−N(1), 91.79(7); O(1)− Cr(1)−N(2), 92.45(10); N(2)−Cr(1)−Cl(1), 94.32(6).

even when a 4:1 ratio of DMAP to Cr was used. Furthermore, 1, unlike the other derivatives,74−76 showed no evidence of dimer formation via MALDI-TOF MS, possibly due to the larger steric effect exhibited by the N,N-dimethylaminoethyl pendent donor group. As a result, the polymerization rate of 1 in the presence of 1 equiv of DMAP was found to be faster than that of complex II and other previously reported Cr(III) amino-bis(phenolato) complexes.68,69,71,72 We have not previously reported the synthesis and detailed study of the CO2/ cyclohexene oxide (CHO) copolymerization activities of 1. Herein we report the detailed synthesis and structure of Cr(III) complex 1 and its utilization as a catalyst for CHO/ CO2 copolymerization. The influence of varying reaction surface area, the effect of cocatalyst selection, CO2 pressure, and reaction temperatures are discussed. End group analysis of the resulting polymer by MALDI-TOF MS revealed possible initiation pathways.

The structure of complex 1 shows a distorted octahedral geometry at the Cr(III) center. Similar structures have been reported for yttrium(III)78,79 and chromium(III).80 Four coordination sites are occupied by the [O2NN′] ligand where the two phenolate-oxygen atoms are oriented trans to each other. The other two coordination sites are occupied by the chloride ancillary ligand trans to the central amino donor and a neutral molecule of THF which resides trans to the pendent dimethylaminoethyl group. This ligand arrangement and related geometric parameters are similar to those found in the previously reported structures of similar Cr(III) aminobis(phenolate) complexes. The bond distances between the Cr center and the diamino-bis(phenolato) ligand were, however, slightly longer when compared to that of complex III in Chart 1, which bears methoxy groups instead of tert-butyl groups para

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RESULTS AND DISCUSSION Synthesis and Characterization of Cr(III) Complex. Cr(III) complex 1 was synthesized via reaction of the proligand B

DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry to the phenolate O-donor.72,81 Its composition as CrCl(THF)[L] was also confirmed by elemental analysis. The magnetic moment at 27 °C was found to 3.7 μB, which is consistent with that observed for related complexes having S = 3/2 Cr(III) sites.76,82,83 Copolymerization of CHO and CO2. The copolymerization of CHO and CO2 was carried out using 1 in the presence of DMAP, PPNCl, or PPNN3 cocatalyst at 60 °C and 40 bar CO2. These conditions gave the best activity from our previous work.69 The 1H NMR spectra (Figures S3 and S4) of the resulting products show PCHC is formed with a negligible amount of ether linkages and no cyclic carbonate is produced (Scheme 2). The stereochemistry of produced polymers is atactic based on the 13C{1H} NMR spectrum (Figure S5).

Figure 2. Time profiles of the absorbance at 1750 cm−1 corresponding to the PCHC production by 1 and different cocatalysts: [Cr]: [CHO]:DMAP = 1:500:1, red circles; [Cr]:[CHO]:PPNCl = 1:500:1, black triangles; or [Cr]:[CHO]:PPNN3 = 1:500:1, pink squares; [Cr]: [CHO]:PPNCl = 1:1000:1, blue diamonds.

Scheme 2. Copolymerization of CHO and CO2 Producing PCHC

rates calculated from the slope of absorbance vs time for the linear part showed that ionic cocatalysts were more efficient than DMAP, and PPNN3 exhibited the fastest rate (Figure 3). Initial copolymerization reactions were carried out in a Parr 5500 series 100 mL stainless-steel pressure vessel for 24 h to ensure a complete reaction. The conversion of CHO to PCHC produced by 1 using DMAP, PPNCl, or PPNN3 as the cocatalyst was similar, ranging from 70 to 75% (Table 1, entries 1−3). To trace the formation of PCHC during the reaction, duplicate runs were performed in a 100 mL Parr 4560 mini reactor equipped with bottom-mounted Si-comp ATR probe (Table 1, entries 4−6). It is worth noting that this reaction vessel has the same volume but different dimensions to the Parr 5500 series, having a wider diameter, hence a larger surface area for exposure of the solution to CO2. Interestingly, the conversion of CHO to PCHC was increased by approximately 20%, likely due to the larger surface area (Table 1, entries 4−6). Polymer molecular weights and dispersities were found to be unaffected by shape of pressure vessel. The TON and isolated yields of polycarbonate product are similar to those observed for the well-studied salen, salphen, salan, and salalen chromium(III) binary (i.e., with added onium salt) catalysts.84−87 The in situ infrared reaction profiles for the formation of PCHC using 1 with different cocatalysts are shown in Figure 2. None of the reactions showed an induction period. The initial

Figure 3. Plots of absorbance vs time for the linear portion of polycarbonate formation for the data presented in Figure 2. Straight lines represent best fits of the data for [Cr]:[CHO]:DMAP = 1:500:1, red circles, y = 0.0355x, R2 = 0.9909; [Cr]:[CHO]:PPNCl = 1:500:1, black triangles, y = 0.0566x, R2 = 0.9964; [Cr]:[CHO]:PPNN3 = 1:500:1, pink squares, y = 0.0702x, R2 = 0.9928; [Cr]:[CHO]:PPNCl = 1:1000:1, blue diamonds, y = 0.0201x, R2 = 0.9954.

Table 1. Copolymerization of CHO and CO2 by 1a entryb

[Cr]:[CHO]:[Co-Cat.]

conv. (%)c

yield (%)d

TONd

initial rate (× 10−4/min)e

carbonate linkages (%)c

Mn (g/mol)f

Đ̵ (Mw/Mn)f

1 2 3 4 5 6 7

1:500:1(DMAP) 1:500:1(PPNCl) 1:500:1(PPNN3) 1:500:1(DMAP) 1:500:1(PPNCl) 1:500:1(PPNN3) 1:1000:1(PPNCl)

70 75 71 89 91 92 91

56 64 55 76 77 72 78

350 374 355 445 455 460 910

ND ND ND 355 566 702 201

97 98 97 98 98 98 98

10000 13000 11000 13000 13000 14000 35000

1.06 1.11 1.10 1.10 1.08 1.12 1.12

Reactions were carried out in neat CHO (5 mL) at 60 °C and 40 bar CO2 for 24 h unless otherwise indicated. For all entries, selectivity for polycarbonate was >99% as calculated by 1H NMR. bEntries 1−3 were run in a 100 mL stainless-steel Parr autoclave with a diameter of 3.2 cm; entries 4−7 were run in a 100 mL stainless-steel Parr autoclave with a diameter of 5.2 cm. cCalculated from 1H NMR (Figure S3). dYield = (mass of polymer/molar mass of repeating unit)/mols of starting monomer, TON = turnover number. eSlope of absorbance versus time curves calculated for the linear portion. ND = not determined. fDetermined by triple detection GPC in THF, dn/dc = 0.0701 mL/g. a

C

DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Effect of CO2 Pressure on the Copolymerization of CHO and CO2a entrya

CO2 pressure (bar)

carbonate (%)b

conv. (%)b

TOF (h−1)c

Mn (g/mol)d

D̵ (Mw/Mn)d

1 2 3 4 5e 6

12 20 30 40 40 58

92 95 95 94 95 98

17 28 32 33 76 (4 h) 44

85 140 160 165 95 220

8400 10000 9400 11000 13000 12000

1.25 1.14 1.08 1.09 1.10 1.06

All copolymerization reactions were carried out in neat CHO (5 mL) at 60 °C for 1 h with 0.2 mol % catalyst loading and one equivalent DMAP per Cr. bDetermined by 1H NMR. cTurnover frequency (TOF) is moles of repeating units produced per mol of Cr per h. dDetermined by triple detection GPC in THF, dn/dc = 0.0701 mL/g eReaction was run for 4 h. a

4. Normalized plots of absorbance vs time at different temperatures are given in Figures S10 and S11. No PCHC is

This observation is consistent with a binary system of chromium salen complex with neutral/ionic cocatalyst reported by Darensbourg,49 who showed that the anionic cocatalyst binds to form a well-characterized six-coordinate anionic species, proposed to be the active species for initiating copolymerization.88 No induction period is observed in these ionic cocatalyzed reactions, suggesting that the formation of the active species is a fast process. PCHCs produced by 1 using DMAP, PPNCl, or PPNN3 as the cocatalyst showed similar molecular weights ranging from 13 000 to 14 000 g/mol with narrow dispersities of ∼1.1 (Table 1, entries 4−6). Lower catalyst loading caused a decreased initial rate (Figures 2 and 3);89 however, an increased polymer molecular weight was observed (Table 1, entry 7). A summary of previously reported copolymerization activity of the four chromium(III)chlorido amino-bis(phenolato) complexes shown in Chart 1 is given in Table S2. Entries 1 and 4 in Table S2 were performed in a 5.2 cm diameter, 100 mL Parr 4560 mini reactor with in situ FTIR whereas other entries were run in a 3.2 cm diameter, 100 mL Parr 5500 reactor. 1 shows a higher initial rate than I and II and all catalysts generally showed comparable conversions, but the polymers obtained by I − IV typically had lower molecular weights and broader dispersities than those obtained by 1. Copolymerization of CHO and CO2 was further investigated at different CO2 pressures. As shown in Table 2, the TOF increases with CO2 pressure, suggesting CO2 insertion is the rate-determining step for the formation of PCHC in this catalyst system. At 12 bar CO2, the resulting polymer contained 92% carbonate linkages, which increased to 98% when CO2 pressure increased to 58 bar. The molecular weight of the resulting polymer was found to slightly increase and a narrower dispersity was obtained when higher CO2 pressure was used. It is worth noting that the molecular weight of the polymer changed little as a function of reaction time from 1 to 24 h (Table 2, entry 4, vs Table 1, entry 4), but conversion of CHO to PCHC continues to increase over time showing 76% conversion after 4 h (Table 2, entry 5) and 89% after 24 h (Table 1, entry 4) under the same temperature and pressure conditions. Kinetic Studies. We have previously found that CO2/CHO copolymerization by amine-bis(phenolate) chromium chloride complexes are first-order in metal complex concentration.68 Effect of temperature on the formation of PCHC was monitored by following the νCO of polycarbonate at 1750 cm−1 by in situ infrared spectroscopy wherein the temperature was gradually increased and each temperature maintained for approximately 20 min. Catalyst and cocatalyst loading was 0.2% and P(CO2) was 40 bar at 25 °C. Three-dimensional stack plots of the IR spectra and reaction profile are shown in Figure

Figure 4. (a) Three-dimensional stack plots of IR spectra collected every 60 s during the reaction of CHO and CO2 by the binary system of 1 and DMAP with a 1/CHO/DMAP molar ratio of 1:500:1 and P(CO2) = 40 bar at 25 °C. (b) Time profile of the absorbance at 1750 cm−1 at different temperatures.

produced at room temperature (∼25 °C), but by increasing the temperature to 30 °C, the formation of PCHC could be observed at a very slow rate. Calibration curves show a linear response to absorbance with carbonate concentration in cyclohexene oxide (Figures S6 and S7). The rate of PCHC formation can be observed to increase further with increased temperature (Figure S8). After approximately 90 min, the growth rate of the band at 1750 cm−1 began to slow either because of increasing viscosity of the solution or deposition of polymer on the ATR sensor. When the temperature increased to 85 °C under the same catalyst loading and CO2 pressure, a 5-fold increase in initial reaction rate compared to the reaction D

DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry at 60 °C was obtained (Figure 5a); however, trans-cyclic carbonate was observed, which was identified by a new

End Group Analysis. End group analysis of the obtained polymers was carried out by MALDI-TOF MS to provide insight into the initial CHO ring-opening step during the reaction. All polymers obtained show multiple series of ions separated with a repeating unit of m/z 142 in MALDI-TOF mass spectra. The bimodal distribution observed by mass spectrometry are common in this class of catalyzed polymerization,92−94 and this bimodality is observed by GPC, consistent by previous findings and other reports proposing the bimodality results from chain transfer reactions with adventitious water. The possible end groups of the obtained polymers are shown in Figure 6.

Figure 6. Series of chain end-groups observed in MALDI-TOF MS of the polymers produced by complex 1 with DMAP, PPNCl, or PPNN3 cocatalysts.

The MALDI-TOF mass spectrum of the polymer produced by DMAP as a cocatalyst (Table 1, entry 4) shows hydroxide and chloride end groups in the high mass region (Figure S14). This suggests the incoming DMAP binds to the chromium center displacing the chloride, which then initiates copolymerization by ring-opening of an epoxide monomer, followed by termination by protonolysis of the metal-alkoxide in methanol. At middle mass range (m/z 2000−4000), four series of copolymer chains are observed (Figure 7). Series (a)

Figure 5. (a) Time profiles of the absorbance at 1750 cm−1 corresponding to the PCHC produced by 1 using DMAP as cocatalyst at 60 and 85 °C. (b) Three-dimensional stack plots of the IR spectra.

absorbance at 1825 cm−1 (Figure 5b) and confirmed by 1H NMR with a multiplet at δ 4.0 (Figure S9).23,57 The formation of cyclic cyclohexene carbonate at higher temperatures has been previously observed for Cr salen complexes.47 The Arrhenius plot for PCHC formation by the 1/DMAP system is shown in Figure S12. For the binary 1/DMAP catalyst system, the activation barrier for PCHC formation is 62 kJ/mol, which is slightly higher than the values reported by Darensbourg (47 kJ/mol)47 and Lu (48 kJ/mol)90 for Cr(III) salen and Co(III) salen complexes, respectively. An Eyring plot (Figure S13) results in activation parameters of ΔH⧧ and ΔS⧧ of 59 kJ mol−1 and −96 J K−1 mol−1, respectively, in neat CHO. At 60 °C this gives ΔG⧧ of 91 kJ mol−1 These values agree well with those reported for a tetramethyltetraazaannulene chromium(III) complex; however, that system was studied in dichloromethane solution.91

Figure 7. Lower mass region (m/z 2750−3350, n = 19−22) of the MALDI-TOF spectrum obtained using DMAP as cocatalyst.

corresponds to the sodium ion containing polymer having two hydroxyl end groups [17.0 (OH) + n142.1 (repeating cyclohexene carbonate unit) + 99.1 (C6H10OH)]. Polycarbonate diols are typically observed due to adventitious moisture contamination causing chain transfer.53 Series (b) consists of the expected end groups from chloride initiation and protonation of an alkoxide chain end (Cl− and OH−). Series E

DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

terized. This complex showed improved activity toward copolymerization of CHO and CO2 than the previously reported amine-bis(phenolate) Cr(III) complexes in terms of polymer molecular weight and dispersity. Reaction kinetics indicate a rapid propagation of polymerization with no initiation period when DMAP, PPNCl, or PPNN3 cocatalysts are used. The binding affinities of ionic cocatalysts PPNCl and PPNN3 to 1 are currently under investigation.

(c) corresponds to the presence of one hydroxyl end group and an ether linkage with cyclohexenyl end group [17.0 (OH) + n142.1 (repeating unit) + 180.0 (C12H20O)]. The ether linkage with cyclohexenyl end group can be attributed to the elimination of HCl from the expected chloro-cyclohexanolate chromium species and chain transfer reaction.57 Series (d) contains DMAP and hydroxyl end groups [122.2 (DMAP) + n142.1 (repeating unit) + 99.1 (C6H10OH)], indicating DMAP can also initiate copolymerization by ring-opening of an epoxide monomer. When PPNCl was used (Table 1, entry 5), the MALDI-TOF mass spectrum of the resulting polymer shows the presence of chloride and hydroxyl end groups (Figure 8), demonstrating

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EXPERIMENTAL SECTION

General Materials. Unless otherwise stated, all manipulations were performed under an atmosphere of dry, oxygen-free nitrogen by means of Schlenk techniques or using an MBraun Labmaster DP glovebox. CrCl3(THF)3 was prepared by the reported method using a large excess trimethylsilyl chloride to ensure removal of water from the metal chloride hydrate.98 H2[L] was prepared by a modified literature procedure by using water instead of methanol as the reaction medium,99 and dried over sodium sulfate in tetrahydrofuran. Anhydrous tetrahydrofuran was distilled from sodium/benzophenone ketyl under nitrogen. Cyclohexene oxide was purchased from Aldrich and freshly distilled from CaH2 under nitrogen. Dichloromethane was purified by an MBraun Manual Solvent Purification System. DMAP and PPNCl [PPN+ = bis(triphenylphosphine)iminium] were purchased from Alfa Aesar and used without further purification. PPNN3 was prepared from PPNCl and NaN3 in ethanol. Instrumentation. MALDI-TOF MS was performed using an Applied Biosystems 4800 MALDI TOF/TOF Analyzer equipped with a reflectron, delayed ion extraction and high-performance nitrogen laser (200 Hz operating at 355 nm). Complex samples were prepared in the glovebox and sealed under nitrogen in a Ziploc bag for transport to the instrument. Anthracene was used as the matrix for 1 and 2,5dihydroxybenzoic acid (DHBA) was used as the matrix for the copolymers. Anthracene and 1 were each dissolved in toluene at concentrations of 10 mg·mL−1. The matrix and complex solutions were combined in a ratio of 1:1. DHBA was dissolved in THF at approximately 16 mg·mL−1, and polymer was dissolved in THF at approximately 10 mg·mL−1. The matrix and polymer solutions were combined in a ratio of 4:1. Then, 1 μL aliquots of these samples were spotted on the MALDI plate and left to dry. Images of mass spectra were prepared using mMass software (www.mmass.org). Molecular weight determination of copolymer was performed on an Agilent Infinity HPLC instrument connected to a Wyatt Technologies triple detector system (light scattering, viscometry and refractive index) equipped with two Phenogel 103 Å, 300 × 4.60 mm columns with THF as eluent. Copolymer samples were prepared in THF at a concentration of 4 mg/mL and filtered through 0.2 μm syringe filters. The sample solution was then eluted at a flow rate of 0.30 mL·min−1. The values of dn/dc were calculated online (columns detached) assuming 100% mass recovery using the Astra 6 software package (Wyatt Technologies) giving dn/dc of poly(cyclohexene carbonate) = 0.0701 mL·g−1. 1 H NMR spectra were recorded in CDCl3 at 300 MHz. Elemental analysis was performed at Guelph Chemical Laboratories, Guelph, ON, Canada. In situ monitoring was performed using a 100 mL Parr Instruments stainless-steel reactor vessel with motorized mechanical stirrer and a heating mantle. The vessel was modified with a bottommounted Mettler Toledo SiComp Sentinel sensor, which was connected to a ReactIR 15 base unit through a silver-halide Fiberto-Sentinel conduit. Profiles of the absorbance height at 1750 cm−1 were measured every 60 s. Similar methods for in situ reaction monitoring via FTIR have been reported elsewhere.100,101 A calibration curve to demonstrate a linear response of absorbance to concentrations102 was obtained and given in the Supporting Information. The peak height of the polycarbonate signal at 1750 cm−1 was found to increase linearly with concentration in the neat cyclohexene oxide. The use of peak height to relate to polycarbonate concentration has been practiced by others.100,101 UV−vis spectroscopy was conducted on an Ocean Optics USB4000+ fiber optic

Figure 8. High mass region (m/z 6000−12000, n = 45−77) of the MALDI-TOF spectrum obtained using PPNCl as cocatalyst.

that the chloride from 1 and/or PPNCl initiated the copolymerization reaction. In the low mass region series (a) was observed again (Figure S15). When PPNN3 was used, the MALDI-TOF mass spectrum shows the expected azideinitiated and hydroxyl-terminated polymer chains (Figure S16). Series (a) and (b) were also visible in the low mass region (Figure S17). It is worth noting that MALDI-TOF MS showed the absence of ether linkages in polymers produced using PPNCl or PPNN3 as the cocatalyst. Thermal properties of the polymers were analyzed by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (Figures S21−S23). The glass transition temperature (Tg) at the midpoint for the polymer obtained using PPNN3 as cocatalyst is 109 °C, which is close to the reported Tg of 116 °C.95 For the polymer obtained using DMAP as cocatalyst, a lower midpoint Tg of 90 °C was observed, which can be attributed to the presence of ether linkages.96 The decomposition temperature at 50% weight loss (T50) was found to range between 322 and 332 °C. Similar weight loss T50 values for PCHC have been reported by others.97

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CONCLUSIONS A new amine-bis(phenolate) Cr(III) complex, 1, bearing a dimethylaminoethyl pendent donor was prepared and characF

DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

spectrophotometer. Room-temperature magnetic susceptibility data were collected on a Johnson-Matthey magnetic susceptibility balance. Glass transition temperatures (Tg) were measured using a Mettler Toledo DSC 1 STARe system equipped with a Julabo FT 100 immersion cooling system with a working range of −100 to +20 °C. Samples were weighed into 40 μL aluminum pans and subjected to two heating cycles from 0 to 180 °C at a rate of 10 °C min−1. The glass transition (Tg) temperatures of copolymers were determined from the second heating. Thermogravimetric analysis measurement was performed with a TA Instruments Q500. Samples were loaded onto platinum pans and subjected to dynamic high-resolution scans. Each sample was heated from room temperature to 400 °C at a heating rate of 10 °C min−1. Synthesis of 1. H2[L] (3.50 g, 6.67 mmol) and sodium hydride (0.64 g, 26.67 mmol) were loaded into a Schlenk tube and cooled to −78 °C. THF (50 mL) was added to give a white suspension, which was warmed to room temperature and further stirred for 2 h. This mixture was transferred via filter cannula to a suspension of CrCl3(THF)3 (2.50 g, 6.67 mmol) in THF (50 mL) cooled to −78 °C to give a pink/purple mixture. The resulting mixture was warmed to room temperature and stirred overnight to give a purple solution. The solvent was removed under vacuum, and the residue extracted into toluene and filtered through Celite. The solvent was removed under vacuum to give a purple residue that was washed with pentane and dried to yield 4.33 g (95%) of purple powder. Crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of 1 in THF and hexamethyldisiloxane at room temperature in a glovebox under nitrogen. Anal. Calcd for C38H62ClCrN2O3: C, 66.89; H, 9.16; N, 4.11. Found: C, 66.77; H, 8.89; N, 3.94. MS (MALDI-TOF) m/z (%, ion): 609.3 (50, [CrCl[L]+]), 574.3 (100, [Cr[L]+]). μeff (solid, 27 °C) = 3.7 μB. Copolymerization Conditions. Reactions were carried out in neat CHO (4.85 g), which was added to the catalyst and the appropriate amount of cocatalyst in a glovebox. The reactant solution was added via a syringe to the pressure vessel, which was predried under vacuum at 80 °C. The autoclave was then charged with 40 bar of CO2 and left to stir at 60 °C for 24 h. After the desired time, the autoclave was cooled in an ice bath and vented in a fume hood. An aliquot was taken immediately after opening the reactor for the determination of conversion by 1H NMR. The contents of the reactor were extracted with CH2Cl2 and the polymer precipitated using cold methanol. For the ionic cocatalysts, catalyst and cocatalyst were dissolved in CH2Cl2 first and stirred for 15 min before solvent was removed; the resulting mixture was then dissolved in CHO.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the Canada Foundation for Innovation (CFI), Newfoundland and Labrador Research Development Corporation (RDC), the Natural Sciences and Engineering Research Council (NSERC) of Canada (C.K.) and Memorial University of Newfoundland for a School of Graduate Studies Fellowship (K.N.).

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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.inorgchem.7b02952. Spectroscopic characterization data, tabulated polymerization data (PDF) Accession Codes

CCDC 1554794 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-709-864-8082. ORCID

Christopher M. Kozak: 0000-0001-8205-4130 G

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DOI: 10.1021/acs.inorgchem.7b02952 Inorg. Chem. XXXX, XXX, XXX−XXX