Copolymerization of CO2 and epoxides mediated by zinc organyls

Herein we report the copolymerization of CHO with CO2 in the presence of various zinc compounds R2Zn (R = Et, Bu, iPr, Cy and Ph). Several zinc organyls proved to be efficient catalysts for this reaction in the absence of water and co-catalyst. Notably, readily available Bu2Zn reached a TON up to 269 and an initial TOF up to 91 h−1. The effect of various parameters on the reaction outcome has been investigated. Poly(ether)carbonates with molecular weights up to 79.3 kg mol−1 and a CO2 content of up to 97% were obtained. Under standard reaction conditions (100 °C, 2.0 MPa, 16 h) the influence of commonly employed co-catalysts such as PPNCl and TBAB has been investigated in the presence of Et2Zn (0.5 mol%). The reaction of other epoxides (e.g. propylene and styrene oxide) under these conditions led to no significant conversion or to the formation of the respective cyclic carbonate as the main product.


Introduction
CO 2 is the by-product from combustion of fossil resources and chemical processes and its increasing concentration in the atmosphere is linked to global climate change. 1 Thus, the utilization of the greenhouse gas CO 2 as a C1-building block 2-4 has attracted much attention due to its low cost, availability and the potential to substitute fossil fuel based feedstocks. Reductive transformations of CO 2 to produce basic chemicals, e.g. formic acid 5 or methanol require stoichiometric amounts of reductants such as silanes, boranes or hydrogen. 6 In contrast the addition of CO 2 to strained rings, such as oxetanes or epoxides, is a nonreductive process. 7 The catalytic coupling of CO 2 with epoxides 1 to generate cyclic carbonates 2 or polycarbonates 3 is an atom economic reaction (Scheme 1). The thermodynamically favored product of this reaction is the cyclic carbonate 2. 8 Lower reaction temperatures and suitable catalysts allow kinetic control, thus the polycarbonates might be favored.
Over the past two decades signicant efforts have been made in industry and academia to develop efficient catalysts for the selective formation of either cyclic carbonates [9][10][11][12] or polycarbonates from epoxides and CO 2 . [13][14][15][16][17][18] Polycarbonates produced by this reaction are used even on industrial scale e.g. for the production of polyurethanes. [19][20][21][22][23] Current industrial processes for the production of polycarbonates are based on the condensation of diols with highly toxic phosgene. In 1969 Inoue and co-workers were the rst to report the synthesis of poly(propylene carbonate) from CO 2 and propylene oxide utilizing partially hydrolyzed Et 2 Zn to initiate the polymerization. 24,25 Since this pioneering work many catalysts have been developed for the copolymerization of CO 2 with epoxides. [13][14][15][16][17][18] Especially zinc based catalyst systems were shown to be efficient. A variety of initiating systems based on different zinc species alone 26,27 as well as in combination with additives e.g. ZnO and diprotic activators (e.g. glutaric acid) 28 have been reported. [29][30][31] Moreover, transition metal complexes based on zinc proved to be highly efficient and selective. In this context, Darensbourg et al. developed zinc phenoxide catalysts, which exhibit high turnover capabilities for the copolymerization of cyclohexene oxide (CHO) and CO 2 . 32-34 Coates and co-workers developed zinc-b-diiminate-complexes for the synthesis of monodispersed, highly alternating copolymers with high molecular weight. [35][36][37][38] The group of Williams demonstrated a zinc-based macrocyclic bimetallic catalyst, which showed remarkable activity even at atmospheric pressure of carbon dioxide. [39][40][41] More recently, Rieger et al. [42][43][44] reported dinuclear zinc-b-diiminate complexes and Dinjus and co-workers 45 complexes based on the N 4 -N,N-bis(2-pyridinecarboxamide)-1,2-benzene chelating ligand for the copolymerization of CO 2 and CHO.
Notably, most of these systems require halogen containing compounds, such as ammonium and phosphonium salts which are commonly employed co-catalyst for the synthesis of cyclicas well as polycarbonates from epoxides and CO 2 . We are generally interested in the reaction between epoxides and CO 2 . [46][47][48][49][50][51][52][53][54] Most recently we reported a zinc based binary catalytic system for the synthesis of cyclic carbonates. 55 Herein we report the efficient copolymerization of CHO and CO 2 in the presence of organozinc compounds in catalytic amounts under cocatalyst and halogen-free conditions.

Measurements
1 H and 13 C spectra were recorded with a Bruker 300 Fourier (300 MHz), Bruker AV 300 (300 MHz) and Bruker AV 400 (400 MHz). Shis d are stated in ppm. The spectra were calibrated to the rest signal of the applied solvent. CDCl 3 : 1 H d ¼ 7.27 ppm, 13 The experiments were carried out under increased pressure in a Multiple Reactor System 5000 and Compact Micro Reactor 5000 from Parr. The molar masses and dispersities were analyzed employing size exclusion chromatography (SEC) 1100 GPC from Agilent Technologies with a refraction index detector at 40 C. The measurements were performed at a constant temperature of 40 C using three columns with a polyester copolymer network as stationary phase (PSS GRAM 1000 A, 5 mm particle size, 8.0 Â 300 mm; PSS GRAM 100 000Å, 5 mm particle size, 8.0 Â 300 mm; PSS GRAM 1 000 000Å). For calibration polystyrene standards from Polymer Standards Service (PSS) were used. Unstabilized THF (HPLC grade) was applied as the mobile phase with a ow rate of 1 mL min À1 . For this purpose around 10 mg of the sample were dissolved in 1 mL THF. Ethylene glycol was used as reference peak. For the recording and the evaluation of the measurement the soware PSS WINGPC 6® UniChrom from PSS was used.

General procedures for the copolymerization with zinc organyls
In a 45 cm 3 stainless-steel autoclave a solution of R 2 Zn (0.25 mmol, 0.5 mol%) was added dropwise to a solution of cyclohexene oxide (4.91 g, 50 mmol) in 2 mL toluene. The reactor was sealed and charged with 0.5-5.0 MPa CO 2 at 60-100 C. The reaction mixture was stirred for 1-48 h. Subsequently, the reactor was cooled to #20 C in an ice bath and CO 2 was released slowly. Aer the removal of all volatile components in a vacuum the polymer was solved in 20 mL of CH 2 Cl 2 and precipitated with 50 mL of a solution of MeOH and 5% HCl. The precipitate was ltered off and dried to yield a polymer as a colorless solid.

General procedure for the copolymerization with Et 2 Zn
In a 45 cm 3 stainless-steel autoclave Et 2 Zn (0.25 mmol, 0.23 mL, 15 wt% in toluene, 0.5 mol%) was added dropwise to a solution of cyclohexene oxide (4.91 g, 50 mmol) in 2 mL toluene. The reactor was sealed and charged with 2.0 MPa CO 2 at 100 C. The reaction mixture was stirred for 16 h. Subsequently, the reactor was cooled to #20 C in an ice bath and CO 2 was released slowly. Aer the removal of all volatile components in vacuum the polymer was solved in 20 mL of CH 2 Cl 2 and precipitated with 50 mL of a solution of MeOH and 5% HCl. The precipitate was ltered off and dried to yield 5.28 g polymer (66%) as a colorless solid.

Results and discussion
Initially we studied different readily available organozinc compounds as catalysts under standard reaction conditions (0.5 mol% R 2 Zn, 100 C, 2.0 MPa, 16 h, Table 1). In the absence Table 1 Screening of readily available zinc organyls R 2 Zn for the copolymerization of CHO and CO 2 shows the alternating nature of the polycarbonate with repeating units for the monomer of 142 g mol À1 (Fig. S1 †). Subsequently, we studied the inuence of various reaction parameters (p(CO 2 ), T, t) for all of the tested zinc compounds since all of them showed similar activity and selectivity as well as promising results in respect to the polymer properties such as amount of incorporated CO 2 and M n .
First the inuence of the CO 2 -pressure on the copolymerization of CHO and CO 2 was investigated ( Table 2). For all of the tested zinc compounds R 2 Zn an increase of the pressure to 5.0 MPa led to higher turnover numbers and increased isolated polymer yields compared to 2.0 MPa ( Since the polycarbonate is the kinetic product of the reaction of CO 2 and CHO, we subsequently studied the inuence of the reaction temperature. 8 Thus the reaction was carried out at 60 C in the presence of the different organozinc compounds ( Table 3). As expected in all cases the formation of the cyclic   (Table  3, entry 1 vs. Table 1, entry 2). In the presence of the other zinc compounds ZnR 2 (R ¼ Bu, iPr, Cy or Ph) polymers obtained at 60 C showed similar CO 2 content compared the products which were obtained at 100 C (Table 3, entries 2-5 vs. Table 1, entries 3-6). Interestingly, for iPr 2 Zn and Ph 2 Zn the M n at lower temperature was about three times higher compared to 100 C ( Table 3, entries 3 and 5 vs. Table 1, entries 4 and 6). The observation that with decreasing reaction temperature the M n increases is in accordance with work previously reported by Soga and co-workers. 56 They investigated the effect of the temperature on the copolymerization of propylene oxide and CO 2 using an alumina-supported diethylzinc catalyst. In general the observed dispersities of the products were higher compared to the polymers obtained at 100 C ( Table 3, entries 1-5 vs. Table  1, entries 2-6). Consequently we investigated the inuence of the reaction time on the copolymerization for three different dialkyl zinc compounds (R 2 Zn, R ¼ Et, Bu, iPr). Bu 2 Zn showed the highest initial activity with 45% conversion of the starting material aer 1 h (Fig. 1). 47,56 In the presence of Et 2 Zn and iPr 2 Zn the conversion of CHO was 31% and 25% aer 1 h, respectively. Notably, aer 4 h more than 50% of the starting material was converted. Subsequently the reaction rate decreased in all cases which resulted in a maximum conversion of only 86% for iPr 2 Zn aer 16 h. The exponential formation of the polycarbonate was accompanied by a linear increase in the cyclic carbonate content of the reaction mixture which is in accordance with previously reported work by Górecki and Kuran. 47,57 Notably, the CHC-content in all cases did not exceed 10% aer 16 h.
In the presence of Et 2 Zn the M n of the obtained polycarbonate showed a linear increase from 8.9 to 28.3 kg mol À1 during the rst 8 h (Fig. 2). Subsequently the M n decreased to 12.2 kg mol À1 aer 16 h. This might be addressed to trans-esterication reactions as suggested by Vandenberg and Tian. 58 This result is comparable to the molecular weights obtained by Williams et al. using mixed salts prepared from Et 2 Zn and acetic acid as a catalyst. 59 Notably, no clear trend was observed for the other zinc compounds (R 2 Zn, R ¼ Bu and iPr). 47,56 However, in all cases the CO 2 content of the obtained polymer was approximately constant (85%) over time. In further investigations, the inuence of different co-catalysts was tested ( Table 4). The copolymerization was performed with Et 2 Zn (0.5 mol%) under standard reaction conditions (2.0 MPa, 100 C, 16 h) in the presence of commonly used co-catalysts. 60-66 Fig. 1 Influence of the dialkyl zinc compounds (ZnR 2 , R ¼ Et, Bu, iPr) on the conversion of CHO (copolymerization of CHO and CO 2 , observed CHC #10%) in dependence on the reaction time. Reaction conditions: 50 mmol CHO, 0.5 mol% R 2 Zn, 2 mL toluene, p(CO 2 ) ¼ 2.0 MPa, T ¼ 100 C, t ¼ 1-16 h. The data points are the results for separate batches.  Notably, in all cases the utilization of a co-catalyst led only to the formation of the cyclic carbonate and/or low molecular weight products/oligomers. Tetra-n-butylammonium bromide (TBAB) and bis(triphenylphosphine)iminium chloride (PPNCl) are known to catalyze the addition of CO 2 to epoxides yielding cyclic carbonates. 67,68 Both co-catalyst provided similar results mainly producing the cyclic carbonate in 63% and 82% yield respectively (entry 1 and 2). Known as catalysts for ring-openingpolymerization (ROP) of cyclic carbonates, 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) and triazabicyclodecene (TBD) were used as co-catalyst to decrease the cyclic side product. 64,69 In the presence of DBU the formation of an oligomer (M n ¼ 510 g mol À1 ) with a polycarbonate content of 55% was observed (entry 3). A similar result was obtained with TBD as the co-catalyst (entry 4) while 4-dimethylaminopyridine (DMAP) showed no signicant conversion (entry 5). It is generally discussed that TBD hinders the depolymerization of the polycarbonate through deprotonation of the in situ formed hydroxyl end-group. 70,71 Additionally, we studied the conversion of various other epoxides with CO 2 . In the presence of Et 2 Zn (0.5 mol%) under our standard reaction conditions (2.0 MPa, T ¼ 100 C, t ¼ 16 h, Table 5). For the simple propylene oxide a conversion of 8% and low weight polymer with high polyether content was reached (entry 1). This is a lower polymer yield then previous reported by Inoue where 5.0-6.0 MPa CO 2 -pressure and the Et 2 Zn/H 2 O system was used. 24,25 For the conversion of butylene oxide only 7% of the oligomeric ether and 5% of the respective cyclic carbonate were observed (entry 2). The reaction of styrene oxide led to higher TON of 59 and 20% yield of an oligomer with a M n < 500 g mol À1 (entry 3) which is higher than the obtained yield of 2% by the Et 2 Zn/H 2 O system by Endo and co-workers. Epichlorohydrin showed a poor yield of the corresponding cyclic carbonate of only 2% (entry 4) which is lower than the result of Endo and co-workers. 24 Notably, other cyclohexane based epoxides could not be converted to the corresponding polycarbonates (entries 5-8).
In the case of limonene oxide this might be addressed to steric hindrance which was observed before by Coates 71 and Anwander 72 (entry 5). The alkoxy silyl functionalized substrates showed no conversion which might be addressed to a reaction of these groups with the catalyst. 72,73 The methyl substituted cyclohexene oxide was partially converted to an oligomeric ether (entry 8).

Conclusions
Zinc organyls (R 2 Zn, R ¼ Et, Bu, iPr, Cy and Ph) efficiently mediate the copolymerization of CO 2 and CHO. Under the standard reaction conditions (100 C, 2.0 MPa) an initial TOF of up to 91 h À1 (for Bu 2 Zn) and TONs up to 269 aer 16 h were achieved. Polycarbonates with molecular weights up to 79.3 kg mol À1 and a CO 2 content up to 97% were obtained. The effect of various parameters on the reaction outcome has been investigated. An increase of the pressure to 5.0 MPa led to higher turnover numbers and increased isolated polymer yields compared to 2.0 MPa. However, at this pressure also the highest dispersities were observed. Higher molecular weight products could be isolated at lower reaction temperature of 60 C. Several commonly employed readily available co-catalysts were studied in combination with Et 2 Zn. However, the co-catalysts facilitated rather the formation of the cyclic carbonate than the production of the polycarbonate. Moreover different epoxides were tested in the copolymerization with CO 2 , unfortunately only low conversions and/or the formation of the respective cyclic carbonates were observed as the major product.

Conflicts of interest
There are no conicts to declare.