Highly Regioselective and Alternating Copolymerization of Racemic

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Highly Regioselective and Alternating Copolymerization of Racemic Styrene Oxide and Carbon Dioxide via Heterogeneous Double Metal Cyanide Complex Catalyst Ren-Jian Wei, Xing-Hong Zhang,* Bin-Yang Du, Xue-Ke Sun, Zhi-Qiang Fan, and Guo-Rong Qi MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

T

nucleophile, which activated CH2 of SO due to less steric effect at this site (i.e., PPNY acts as a “polar” group of SO, as shown in Scheme 1). The positive charges of CH and CH2 groups would be of equal energy, resulting in the random attack of propagating species of −COO− to both carbons in SO. Therefore, we could audaciously suppose that a catalyst system without using such organic base (i.e., just nucleophilic propagating species of −O− and −COO− exist in the reaction system) might lead to a regioregular PSC if it could catalyze CO2−SO copolymerization with 100% alternating degree. In this work, we tested and confirmed such a hypothesis. We report the synthesis of PSC with 99.4% alternating degree, 96.0% head-to-tail (HT) content, and “pure” two end hydroxyl groups, which was successfully achieved via heterogeneous catalysis of nanosized zinc−cobalt(III) double metal cyanide complex (Zn−Co(III) DMCC) catalysts at 35−40 °C without using any cocatalysts. With the cheap price of SO and high Tgs, such regioregular PSCs have promising applications as new degradable materials. Zn−Co(III) DMCC is a highly active heterogeneous catalyst for epoxides−CO2 copolymerization.2c,11 It has a threedimensional backbone in which the zinc and cobalt atoms are linked by cyanide bridges. The catalytic center of this catalyst is confirmed to be a tetrahedral Zn ion with one OH group (inset in Scheme 1).11d,12 CO2−SO copolymerization had been attempted by using two other kinds of DMCC catalysts;9,13 however, only poly(ether−carbonate)s with considerable amounts of cyclic carbonate were obtained. In both cases, the catalysts exhibited low activities and required high reaction temperatures, which resulted in massive side products. It should be noted that Zn−Co(III) DMCC catalysts prepared with the traditional method11c,f could not catalyze CO2−SO copolymerization at 60 °C within 10 h (entry 2, Table S1). Presumably, its relative big conglomeration (hundreds of nanometers, Figure S1) and low surface areas (230 m2/g) made it less available active sites for the copolymerization.11e Herein, Zn−Co(III) DMCC catalysts with nanolamellar structures, which could catalyze CO2−PO copolymerization at 40−60 °C,12 were applied for CO2−SO copolymerization. Nanolamellar catalysts (Cat. 1−3) were synthesized according to our reported procedure11d using tert-butanol,

he transformation of CO2 into polycarbonates has attracted much attention in the past decades due to the growing concerns of CO2 accumulation in the atmosphere and the clean usage of “waste” gas. Since the first discovery of ZnEt2/H2O system for CO2/propylene oxide (PO) copolymerization by Inoue et al.,1 much progress has been made in developing various catalyst systems for CO2 copolymerization,2 which were summarized by several review articles.3 However, compared to the impressive achievements of alternating copolymerization of CO2 with terminated epoxides with electron-donating groups such as PO and cyclohexene oxide (CHO),4 only limited success was achieved for the copolymerization of CO2 with terminated epoxides anchoring an electron-withdrawing group (“polar” epoxides),5−9 such as styrene oxide (SO)5 and epichlorohydrin (ECH).6 A βdiiminate−zinc complex was applied as the first homogeneous catalyst for CO2−SO copolymerization, but it suffered from low polymer selectivity of 35%.7 Heterogeneous catalysts such as ZnEt2/H2O system,1 zinc glutarate,8 and hybrid sol−gel double metal cyanide catalysts9 could catalyze the CO2−SO copolymerization; however, considerable amounts of ether linkages and cyclic carbonates were concomitantly produced. Lu and Daresnbourg et al. reported a perfect alternating CO2−SO copolymerization using a cobalt-based complex with organic bases as cocatalysts (i.e., binary catalyst, Scheme S1) at 25 °C, which afforded poly(styrene carbonate) (PSC) with >99% carbonate linkages.5a Recently, a regio- and stereoregular PSC from chiral styrene oxide in perfectly alternating structure and about 96% head-to-tail (HT) linkages were reported by the same groups.5b It remains a big challenge to synthesize highly regioregular copolymers from CO2 with racemic “polar” epoxides. Moreover, there is a deficiency in synthesizing full alternating copolymers of CO2 with “polar” epoxides via heterogeneous catalysis. For the copolymerization of CO2 with a “polar” epoxide of SO, it was proposed that the nucleophilic attack of propagating species to SO would occur predominantly at the methine bond (CH−O) (route 1, Scheme 1) rather than the methylene bond (CH2−O).5a,10 As a result, the resultant PSC should exhibit highly regioregular structure, i.e., high HT content. However, the copolymer from racemic SO (rac-SO) and CO2 catalyzed by the rac-salenCo(III)X/PPNY (X and Y = 2,4-dinitrophenoxy; PPN+ = bis(triphenylphosphine)iminium, Scheme S1) system gave a HT content of 51%,5a which meant the attacks of propagating species to CH and CH2 of SO had equal probability. It was understandable because PPNY was a strong © 2013 American Chemical Society

Received: March 5, 2013 Revised: April 9, 2013 Published: April 18, 2013 3693

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Scheme 1. Supposed That CO2−SO Copolymerization Was Full Alternating, the Attack of −COO− to CH or CH2 of SO (Step 1 or 2) Will Determine the Regioselectivity (M = Metal Ion; :uN = Organic Base); Inset: Typical SEM Image of Heterogeneous Zn−Co(III) DMCC Catalysta

a

The tetrahedral (CN)2Zn−OH structure is proposed as the initiating group of this catalyst (CA represents the complexing agent used during the preparation of the catalyst).11d,12

Table 1. Copolymerization of Styrene Oxide and CO2 by Zn−Co(III) DMCC Catalysta

entry

temp (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17f 18g

90 60 50 40 35 30 35 35 35 35 35 35 35 35 35 35 60 40

catalyst Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat. Cat.

1 1 1 1 1 1 1 1 1 1 1 1 2 2 3 3 1 1

solvent (mL)

THF (0.8) THF (2.0) CH2Cl2 (2.0) CH2Cl2 (2.0) CH2Cl2 (2.0)

press. (MPa)

FCO2b (%)

H−T contentc (%)

WSCb (wt %)

Mnd (kg/mol)

PDId

TOFe (h−1)

4.0 4.0 4.0 4.0 4.0 4.0 5.0 3.0 2.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

44.1 91.1 93.2 96.1 97.8 ND 98.2 97.2 97.0 98.8 99.2 99.0 97.4 99.4 97.6 99.1 99.0 96.2

85.3 87.5 91.4 94.4 ND 94.5 93.6 93.1 95.0 95.7 95.4 94.0 96.0 94.2 95.4 80.5 92.0

60.3 4.4 3.2 1.4 1.3 ND 1.1 1.2 1.2 0.4 0.2 0.4 1.3 0.1 1.3 0.3 0 1.6

3.5 5.8 11.7 25.0 26.4 ND 25.8 22.4 19.8 7.5 4.4 4.8 13.6 3.6 10.8 3.4 12.3 18.4

2.8 3.2 3.0 2.3 2.2 ND 2.6 2.4 2.4 2.8 3.0 3.0 2.7 3.1 2.6 3.2 3.1 2.5

58.5 104.7 85.0 54.0 42.4 ND 43.6 41.2 32.8 32.1 12.4 35.0 22.4 13.5 20.6 12.7 53.4 47.2

a

Reaction conditions: rac-styrene oxide: 3.0 mL, Zn−Co (III) DMCC: 3.5 mg, reaction time 10 h. bFCO2 (%) indicates the molar fraction of carbonate linkages in the produced polymer, and Wsc (wt %) indicates the weight percentage of styrene carbonate in the total product. Both of them were calculated by integrating the 1H NMR peak area: FCO2 (%) = (A5.85−5.60 + A4.40−4.25 − 2A4.80)/[(A5.85−5.60 + A4.40−4.25 − 2A4.80) + A3.80−3.40]; WSC (%) =164 × 3A4.80/[164(A5.85−5.60 + A4.40−4.25 + A4.80) + 120A3.80−3.40]. cHT content of PSC was determined by 13C NMR spectra. dDetermined by gel permeating chromatography in THF, 35 °C, calibrated with standard reference polystyrene. eTurnover frequency of styrene oxide to polymer (excluding the amount of SC), mol SO/mol Zn/h. fThe 2-benzyloxirane−CO2 copolymerization. gThe (R)-SO−CO2 copolymerization.

80 nm and high BET surface areas of 561−653 m2/g that were resulted from nanopores and piled pores in the catalyst. These Zn−Co(III) DMCCs with high surface areas could accommodate more active sites than the traditional one.12 Initially, bulk CO2−SO copolymerization was carried out at 90 °C using Cat. 1 (entry 1, Table 1). The productivity of Cat. 1 was excellent with a nearly complete SO conversion;

tetrahydrofuran (THF), and ethylene glycol monomethyl ether as complexing agents, respectively. Cat. 1−3 were characterized by elemental analysis, scanning electron microscopy (SEM, see Scheme 1 and Figures S2−S4), and N2 absorption and desorption tests (Figure S5), and the corresponding structural parameters are summarized in Tables S2 and S3. Cat. 1−3 exhibited nanolamellar structures with thicknesses of about 20− 3694

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All copolymers were then characterized by 13C NMR. The results showed that PSCs from solution copolymerization had 95.0−96.0% head-to-tail (HT) contents (Table 1), which was even higher than that of PSC (82%) from the copolymerization of an optically active (S)-SO (98% ee) with CO2 via the catalysis of chiral (1R,2R)salenCo(III) complex/PPNY system.5 For example, the 13C NMR spectrum of PSC with a FCO2 of 99.0% presents a high HT content of 96% (Figure 1A). Herein,

however, the polycarbonate selectivity (FCO2) was only 44.1%, and a massive production of styrene carbonate (SC) of 60.3 wt % was observed. Several bulk CO2−SO copolymerizations were then tested using Cat. 1 at 30−60 °C (entries 2−6, Table 1 and Figures S6, S7). One can see that the lower the temperatures were, the higher FCO2 and molecular weight (MW) were. Gratifyingly, the copolymerization went well even at 35 °C with a rather low SC weight percentage (WSC) of 1.3%, high FCO2 of 97.8%, and good activities (entry 5, Table 1). However, no products were collected at 30 °C within 10 h (entry 6, Table 1). These results strongly indicated that the productions of ether unit and SC were sharply suppressed at low reaction temperatures. Enhancing CO2 pressure from 2.0 to 5.0 MPa resulted in a slight increase of FCO2 from 97.0% to 98.2% and TOF from 32.8 to 43.6 (entries 5, 7−9, Table 1), while WSC kept pretty low. Increasing the amounts of Cat. 1 from 3.5 to 10.0 mg led to slight variation of FCO2 (from 98.2 to 97.1%), Mns (25.2−26.3 kg/mol), and WSC (%) (1.1−1.2) (Table S4). However, low catalyst loading (1.0 mg of Cat. 1 for 3.0 mL of SO) led to a sharp decrease of Mn (14.3 kg/mol) and FCO2 (95%). Moreover, the Mn and polydispersity index (PDIs) of these copolymers were 5.8−26.4 kg/mol and 2.2−3.2, respectively. A relatively broad PDI was caused by multiple active sites since Zn−Co(III) DMCC was a typical heterogeneous catalyst. The FCO2 values of the resultant PSCs were clearly higher than those reported.9,13 However, there were still about 2−3% ether linkages in the copolymers. A kinetic study of CO2−SO copolymerization catalyzed by Cat. 1 showed that the ether linkages were predominantly produced at the early stage of the copolymerization (Figure S8), which was consistent with the results of the copolymerization of CO2 with epoxides anchoring electron-donating group, such as PO, CHO catalyzed by the same catalyst.11d,12 Based on our previous observation of solvent-assisted selectivity for inhibiting ether units in Zn−Co(III) DMCCcatalyzed CO2 polymerization,11e,14 CO2−SO copolymerization catalyzed Cat. 1 was performed at 35 °C in the presence of different solvents. As we expected, the resultant PSCs with FCO2 of 99.2 and 99.0% were obtained when 2.0 mL of THF and dichloromethane was introduced (entries 11 and 12, Table 1). Cat. 2 and 3 were also tested for CO2−SO copolymerization. FCO2 values of the resultant PSCs were 97.4 and 97.6% in bulk copolymerization, respectively (entries 13 and 15, Table 1). Furthermore, adding dichloromethane could lead to higher FCO2 (99.4 and 99.1%, entries 14 and 16, Table 1 and Figure S9). The SC contents of these samples from solution copolymerization were rather small (0.1−0.4%) based on 1H NMR spectra (Table 1 and Figure S10), which suggests that the diluted reaction system caused relative low SC content, kinetically due to the decrease of depolymerization rate through the backbiting route. However, the copolymers from solution copolymerization presented relatively low Mns due to the chain transfer reaction to trace amount of water or other impurities.12 The above results showed that PSCs with >99% alternating degree could be successfully obtained by using Zn−Co(III) DMCC catalyst. According to the assumption that the propagating species could only attack the CH site with an electron-withdrawing phenyl group, it was expected that the resultant PSCs had high HT contents in our catalytic system.

Figure 1. Carbonyl region of 13C NMR (125 MHz) of PSC (curve A, entry 14, Table 1) and CO2−2-benzyloxirane copolymer (Mn: 12 300; PDI: 3.1, curve B, entry 17, Table 1).

(R)-SO (97% ee) was employed for the copolymerization using Cat. 1 at 40 °C and 4.0 MPa (entry 18, Table 1). The HT content of the resultant copolymer (FCO2: 96.2%) was 92.0% (Figure S11), which is close to that of rac-SO−CO2 copolymer from entry 4 (91.4%). Increasing reaction temperatures (entries 2−5) or decreasing CO2 pressures (entries 5, 7−9) resulted in relatively low HT contents and FCO2, which suggested that more amounts of ether units in the copolymer chain caused low HT content. This is also supported by the fact that FCO2 and HT content increased with increasing reaction time (Figure S12). Since the ether linkages were produced by consecutive insertion of SO into propagating species of −O−Zn, the attack of −O− to both carbon atoms of SO might be random (it has stronger nucleophilic ability than −COO−). However, even for those resultant PSCs with relative low FCO2 (91.1−98.2%), they still presented high HT contents of 85.3−94.5% (Table 1). The HT contents of PSCs with nearly full alternating structure (entries 11−12, 14, and 16, Table 1) were independent of solvents, the complexing agents used for preparing catalysts. When the CO2−SO copolymerization was nearly full alternating, as described in Scheme 1, the propagating species for attacking SO should be −COO−Zn, which was formed after CO2 insertion. Because the CH site in SO anchored an electron-withdrawing phenyl group and was hence more electrophilic, the propagating species of −COO− should predominantly attack the CH site of SO. For further clarifying this point, a control experiment was carried out. 2-Benzyloxirane was copolymerized with CO2 with Cat. 1 as a catalyst. 2-Benzyloxirane was derived from the oxidation of 1-allylbenzene (Scheme S2). A copolymer with a FCO2 of 99.0% (Figure S13) was obtained. The HT content of this copolymer was 80.5% (Figure 1B) and clearly less than those of PSCs obtained here. The results were consistent with the order of the electron-withdrawing ability of phenyl > benzyl 3695

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although both groups had nearly the same steric hindrance. Therefore, the electron-withdrawing effect of the phenyl group was the key factor for regioselective CO2−SO copolymerization by Zn−Co(III) DMCC catalysis. Since such CO2−SO copolymerization was initiated by Zn− OH,11d,12 the resultant PSC should contain two terminal hydroxyl groups. One was −CH2OH and the other was −CH(Ph)−OH, and the molar ratio of the protons of CH2OH to CH(Ph)−OH should be 2/1 for the copolymer with nearly 100% HT content. Figure 2 shows the 1H NMR spectrum of a

AUTHOR INFORMATION

Corresponding Author

*Tel +86-571 87953732; e-mail [email protected] (X.-H.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Science Foundation of the People’s Republic of China (No. 21074106 and 21274123) and the Science and Technology Plan of Zhejiang Province (No. 2010C31036).

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REFERENCES

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Figure 2. 1H NMR spectrum (600 MHz, with a scanning number of 500) of the purified PSC with FCO2 of 99.1% (entry 16, Table 1).

nearly full alternating copolymer (FCO2: 99.1%, Mn: 3400; entry 16, Table 1). The peaks at 3.9 and 5.0 ppm were ascribed to the protons of CH2OH and CH(Ph)−OH groups, respectively. The integral area ratio of the two peaks was 1.98/1.00. Moreover, the end −OH groups were also detected by FT-IR (Figure S14) and ESI-MS spectra of the PSC with low Mn, of which all captured copolymers had two hydroxyl groups (Figure S15 and Table S5). Moreover, these regioregular PSCs exhibited high glass transition temperatures (Tgs, Table S6). For example, the obtained PSC with a Mn of 25.8 kDa had a Tg of 82 °C (Figure S16) although it still contained 1.8% random ether linkages. In summary, the highly regioselective rac-SO−CO2 copolymers with HT contents of ca. 96% and alternating degree >99% were successfully obtained by using heterogeneous nanosized Zn−Co(III) DMCC catalysts. A simple catalytic pathway was presented to control the stereochemistry during the copolymerization of CO2 with a “polar” epoxide. In the current catalytic system studied here, only two kinds of nucleophilic species (i.e., −O− and −COO−) made the system simple, achieving a better stereochemistry control. Further investigation on intensive mechanistic study of this catalyst system and development of highly active Zn−Co (III) DMCC catalyst is underway.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and tables giving general experimental procedures and characterization data for epoxide−CO2 copolymers. This material is available free of charge via the Internet at http:// pubs.acs.org. 3696

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