Theoretical Mechanistic Investigation into Metal-Free Alternating

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Theoretical Mechanistic Investigation into Metal-Free Alternating Copolymerization of CO2 and Epoxides: The Key Role of Triethylborane Dan-Dan Zhang,†,‡ Xiaoshuang Feng,*,† Yves Gnanou,*,† and Kuo-Wei Huang*,†,‡ †

Division of Physical Sciences and Engineering and ‡KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia

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ABSTRACT: The copolymerization of carbon dioxide (CO2) and epoxides has received much attention during the past decades for the production of aliphatic polycarbonates. Remarkably, the green synthesis of polycarbonates was recently demonstrated by copolymerization of CO2 with epoxides under metalfree conditions. In this work, the reaction mechanism of this highly selective polymerization was further investigated using DFT calculations. Four steps were studied: step I describes the epoxide ring-opening by the chloride anion in the presence of the Lewis acid triethylborane (TEB); step II is related to the subsequent insertion of CO2; step III corresponds to the alternating insertion of an epoxide facilitated by TEB; step IV is characterized by the leaving of TEB followed by a new round of polymerization. The high selectivity to form alternating polycarbonates and the suppression of backbiting and homopolymerization that respectively generate cyclic carbonates and polyethers were confirmed by the difference of energy barriers. The key role of TEB at every step was also elucidated. Our theoretical results support the proposed experimental outcomes and provide the fundamental mechanistic insights.

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INTRODUCTION The significant advances made in the copolymerization of epoxides with carbon dioxide (CO2) have been translated into the industrial production of various aliphatic polycarbonates.1−9 Sophisticated ligands have been designed, and a series of homogeneous catalysts based on transition metals Zn(II),10−15 Co(III),16−22 and Cr(III)23−28 have been developed that exhibit both high activity and selectivity. Typically these transition metal systems bring about chain growth via a coordination− insertion mechanism which involves a first coordination step of the epoxides to the metal complex followed by their insertion to the growing chain.29−33 Cheap and environment-friendly earthabundant main group metals such as iron,34,35 aluminum,36,37 and magnesium38,39 were also employed to copolymerize CO2 with epoxides by the similar coordination−insertion mechanism. However, the drawbacks of these metal-based catalytic system are manifold: the preparation of the catalyst/ligand is a complicated process which usually requires multistep syntheses;40−44 the coloration and toxicity of the polycarbonate samples obtained go against green chemistry, and a postsynthesis purification step is necessary;45−47 finally, the carbonate contents cannot be readily varied in the polymers formed.48,49 To address these issues, the effective copolymerization of CO2 with epoxides was recently achieved for the first time under metal-free conditions (Scheme 1).50 With the choice of TEB as Lewis acid to activate epoxides and onium salts as initiator, alternating copolymers made of CO2 and propylene oxide (PO) or cyclohexene oxide (CHO) could be obtained, while a higher © XXXX American Chemical Society

activity was observed for CHO (TON 4000) than that for PO (TON 500). In this work, we report our theoretical mechanistic investigations to elucidate the role of TEB and how PO or CHO can be effectively copolymerized with CO2 in an alternating manner under metal-free conditions.

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COMPUTATIONAL DETAILS All the calculations were performed in the Gaussian 09 program package.51 The geometric structures of all the species were first optimized without any geometry or symmetry constraint, using M06-2x52−55 functional which shows the high accuracy for organic systems, with the all-electron basis set 6-31G(d,p)56 for N, O, H, C, and B. Each stationary point was classified as minima or transition state by analytical calculation of the frequencies. The IRC57 (intrinsic reaction coordinate) calculations at the same level were carried out to confirm the right connections among a transition state and its forward and backward minima. The energy of each species was refined by a single-point calculation at the M06-2x/6-311++g(d,p) level coupled with SMD58 solvent model (THF). The single-point energies were further corrected to free energies at 358.15 K and 1 atm based on the gas-phase M06-2x/6-31g(d, p) harmonic frequencies. Unless otherwise mentioned, all energy values in the text Received: March 5, 2018 Revised: June 27, 2018

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DOI: 10.1021/acs.macromol.8b00471 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Metal-Free Alternating Copolymerization of Epoxides and CO2, with TEB as Activator50

Scheme 2. Schematic Illustration of the Overall Mechanism for the Copolymerization of CO2 with Epoxides (PO and CHO)

PO: The Ring-Opening (Step I) and the Addition of CO2 (Step II). For steps I and II with PO used as a model epoxide monomer, the profiles are shown in Figure 1, and the key optimized structures are presented in Figure 2. Transition state TS1 corresponds to the direct opening of the PO ring without the participation of TEB, while transition state TS2 involves the complexation of TEB with PO followed by the ring-opening by the chloride (Figure 1). The key kinetic facilitation effect of TEB on the first step can be clearly observed from the remarkable decrease of energy barriers from 45.5 kcal/mol for TS1 to 23.6 kcal/mol for TS2 after complexation of TEB with PO, although the combination of PO with Lewis acid TEB gives the intermediate IN1, which is 2.8 kcal/mol higher related to the initial substrate PO and forms a new O−B bond with the length of 1.781 Å. Transition state TS2 describes the attack of Cl− at the CH2 of PO. Alternatively, Cl− also can attack the hindered CH; however, the corresponding transition state I-TS2 (Figure S1) exhibits a slightly higher energy barrier (24.4 kcal/mol) than that of TS2 due to the steric hindrance of methyl group connected with CH. In the process from IN1 to TS2, the C−O bond is lengthened from the single bond (1.433 Å) to 1.939 Å and the

stand for the Gibbs free energy at the M06-2x/6-311++g(d,p) (SMD, THF) theory level, and the potential energy surfaces are located in the singlet ground state (S0).

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RESULTS AND DISCUSSION As shown in Scheme 2, four steps were considered in this study on the alternating copolymerization of CO2 with epoxides. Both PO and CHO were chosen as the epoxide substrates. Step I is the ring-opening of epoxide by the chloride (Cl−). Step II corresponds to the competitive insertion of CO2, epoxide, and epoxideTEB into the product of step I. In step III, four possible reaction channels were investigated: (i) the intramolecular cyclization to give a five-membered cyclic carbonate compound to end the polymerization; (ii) the ring-opening of the TEB-activated epoxide by the carbonate ester group; (iii) the ring-opening of the epoxide without complexation with TEB by the carbonate ester group; (iv) the nucleophilic attack on the C atom of CO2 by the alkyl carbonate group. Step IV is a new round of copolymerization reaction by successive insertions of CO2 and epoxide-TEB. For each step, the epoxides activated by TEB (epoxide-TEB) were used in order to investigate the effect of TEB in the copolymerization. B

DOI: 10.1021/acs.macromol.8b00471 Macromolecules XXXX, XXX, XXX−XXX

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describes the CCO2−O1 bond along with the migration of TEB from O1 to O2 (B−O1, 2.774 Å, and B−O2, 2.713 Å). In the vibrational mode of TS3, the bonds CCO2−O1 and B−O2 are shortened, while the bonds CCO2−O1 and B−O1 are lengthened, indicating the formation of a new CCO2−O1 bond is accompanied by migration of TEB with a barrier of 17.2 kcal/mol relative to IN3. After the complete insertion of CO2, intermediate IN4 is produced, and its energy is further decreased to −5.3 kcal/mol. The insertion of PO or PO-TEB to IN2 was also evaluated in order to theoretically confirm the selectivity of alternating copolymerization. Compared to TS3, both transition state TS4 and TS5 have higher energy barriers, 24.6 and 33.4 kcal/mol relative to IN3, respectively. Thus, our calculations confirm that the insertion of CO2 is kinetically and thermodynamically favorable at step II to give carbonate product IN4. PO: The Alternating Adding of Epoxide-TEB (Step III). At step III, the insertion of PO-TEB or PO is desired for the alternating copolymerization reaction, while the insertion of CO2 and the intramolecular cyclization are occurring as side reactions. The corresponding profiles are shown in Figure 3, and

Figure 1. Profiles of both steps I and II for the PO model (kcal/mol).

Figure 2. Key optimized structures (bond length/Å) for the profiles in Figure 1. Atoms C, O, B, and Cl are shown in gray, red, yellow, and green, respectively. Figure 3. Profiles of step III in the PO model (kcal/mol).

O−C−C angle is increased from 59.0° to 84.9°. In intermediate IN2, the PO ring has been completely opened with the angle of 111.1°, and the newly formed carbon chloride bond is 1.816 Å. The intermediate IN2 is −1.0 kcal/mol relative to initial reactants, indicating the first ring-opening is slightly thermodynamically favorable. On the other hand, the B−O bond is considerably shortened from 1.781 Å (IN 1) to 1.529 Å (IN2), which may increase the stability of IN2. In step II, the insertion of substrate CO2 is desired for the alternating copolymerization. The approaching of CO2 on the O1 of IN2 gives the intermediate IN3 which has the weak CCO2−O1 bond (2.580 Å) and is more stable than IN2 (−3.3 kcal/mol vs −1.0 kcal/mol). All attempts to find the intermediate with CCO2− O1 single bond (1.290 Å) gave the intermediate IN3. When the CCO2−O1 bond was scanned from 3.300 to 1.290 Å, the energies directly increased with the shortened bond length because the formation of the CCO2−O1 bond requires the breaking of O1−B bond (see Figure S2). Using the initial structure with the lengthened O1−B bond and the shortened CCO2−O1 bond, we found transition state TS3 in which the vibrational mode

the key optimized structures are presented in Figure 4. The carbonate O2 of intermediate IN4 interacts weakly with the C2 of PO-TEB to give the intermediate IN5 (−0.6 kcal/mol) (Figure 4). In the process from intermediate IN5 to transition state TS6, the O1−C2 bond is lengthened from 1.456 to 1.841 Å, and the corresponding O1−C1−C2 angle increases from 59.9° to 79.6°, indicating the opening of the second PO ring. At the same time, the C2−O2 bond is shortened from 2.778 to 1.932 Å, indicating a new single bond is being formed. This process contributes to the energy barrier of transition state TS6, 22.5 kcal/mol relative to IN4. Compared to TS6, transition state TS7 that describes the ring-opening insertion of PO without TEB has a considerably high energy barrier of 34.6 kcal/mol, indicating that TEB significantly promotes the ring-opening of PO. Transition state TS6 with a smaller angle of O1−C1−C2 and a shorter bond length O1−C2 compared to those of TS7 (79.6° vs 94.3° and 1.841 Å vs 2.085 Å) is closer to the geometry of intermediate IN5, suggesting TS6 is an early transition state accounted for the lower activation barrier. In addition, the bond C

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Figure 4. Key optimized structures (bond length/Å) for the profiles in Figure 3. Atoms C, O, B, and Cl are shown in gray, red, yellow, and green, respectively.

length of O1−B2 is shortened from 1.689 to 1.588 Å in the process from IN5 to TS6, which may further stabilize transition state TS6. One notes that the ring-opening step from IN4 (−5.3 kcal/mol) to T-IN6 (−3.8 kcal/mol) is slightly thermodynamically endergonic by 1.5 kcal/mol, but it can be followed by the dissociation of one TEB to give a more stable intermediate IN6 (−9.6 kcal/mol), suggesting that the TEB may leave away after the addition of the alkyl bicarbonate to an epoxide (Figure 3). For the intramolecular cyclization, we located transition state TS8 with a high energy barrier, 44.9 kcal/mol. Because of the serious structure rotation in the presence of TEB, the formation of the new C−O3 bond and the breaking of the C−Cl bond to form the cyclic carbonate are not energetically favored. The competitive insertion of CO2 was also excluded by the potential energy surface (PES) scan calculation (Figure S3) which shows that the approach of CO2 to the carbonate ester group leads to increasing energy. The energy of the highest point in the PES scan when CO2 approaches the carbonate ester group is 54.2 kcal/mol higher than the energy of intermediate IN6. Our calculations demonstrate that the reactant PO in the ringopening reaction should exist as PO-TEB rather than a free PO and TEB can activate the epoxide monomer to remarkably decrease the energy barrier. Moreover, it was found that the initial B1−O3 bond is continually lengthened in the process from IN5 (1.599 Å) to TS6 (1.632 Å) and then to IN6 (1.698 Å), suggesting the first TEB may leave away after enchainment of carbonate (vide infra). PO: A New Round of Alternating Copolymerization Reaction (Step IV). Step IV is a new round of monomer addition process in the alternating copolymerization where it is expected that CO2 is first added followed by PO-TEB. For the insertion of CO2, the profiles are shown in Figure 5, and the key optimized structures are displayed in Figure 6. Starting from intermediates T-IN6 and IN6, we located two different pathways (blue and green in Figure 5) in order to investigate the dissociation of the initial TEB complexed with carbonate. The insertion of CO2 into intermediate T-IN6 gives the slightly more stable intermediate T-IN7 with an elongated initial B−O bond (1.698 Å (T-IN6) to1.711 Å (T-IN7)). The following dissociation of the initial TEB then affords intermediate IN7 (−11.3 kcal/mol). Relative to the lowest point IN7, the transition state TS9 without the initial TEB has the 16.1 kcal/mol energy barrier while that of

Figure 5. Profiles of the insertion of CO2 in a new round of copolymerization in the PO model (kcal/mol). The black numbers are the corresponding bond lengths (Å).

the corresponding transition state TS10 with the initial TEB is 20.1 kcal/mol. The energy barriers further favor the elimination of the initial TEB. Compared to transition state TS10, both O2−B2 and O2−C1 bonds of TS9 are shortened from 2.759 Å and 1.451 Å to 2.732 Å and 1.448 Å, respectively. For the intermediates, IN8 (−14.2 kcal/mol) is more favorable than T-IN8 (−9.1 kcal/mol). The advantages of intermediate IN8 can be geometrically analyzed from two aspects. On one hand, the dissociation of the TEB facilitates the bending of the polymer chain to stabilize the molecular structure. As shown in Fig. 6, the dihedral O1−C3−C2−O2 highlighted inside the circle in IN8 is 72.1°, smaller than 80.7° of T-IN8. On the other hand, compared to T-IN8, both O3−B2 and O2−C1 bonds of IN8 are shortened from 1.605 Å and 1.390 Å to 1.599 Å and 1.387 Å, respectively. In addition, the insertions of PO and PO-TEB into the intermediate IN6 are also excluded due to the unfavorable energy barriers in profiles (TS11, 21.8 kcal/mol, and TS12, 34.0 kcal/mol), consistent with the alternating nature of copolymerization from experimental results. For the subsequent insertion of PO-TEB, the profiles are shown in Figure 7, and the key optimized structure are displayed in Figure 8. For the green pathway, the adding of the PO-TEB into IN8 is slightly endergonic by 0.9 kcal/mol to produce intermediate IN9 (−13.3 kcal/mol). For the blue pathway where T-IN8 holds the initial TEB, the direct addition of POTEB to give intermediate T-IN9 is endergonic by 6.6 kcal/mol, indicating the unfavorable thermodynamics. Further inspection of B−O bond length shows that the old B−O bond is gradually lengthened along with the formation of the new B−O bond. In the process from T-IN8 to T-IN9, the first B−O bond and the second one are lengthened from 1.724 and 1.594 Å to 1.736 and 1.609 Å, respectively. In the process from IN8 to IN9, the only D

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Figure 8. Key optimized intermediate structures (bond length/Å) for the profile in Figure 7. Atoms C, O, B, and Cl are shown in gray, red, yellow, and green, respectively.

However, the corresponding transition state TS14 with three TEB groups has the higher energy barrier, 33.5 kcal/mol relative to IN9. In addition to the energy cost for lengthening B−O bonds, transition state TS14 has the larger O1−C1−C2 angle (81.1° vs 80.5°) and the longer O1−C2 bond length (1.869 Å vs 1.861 Å) than that of TS13 (Figure S5), which is consistent with the previous discussion that the serious changed transition state structure corresponds to the higher activation energy barrier. The process from the intermediate IN8 to IN10 is exergonic by 6.7 kcal/mol, while the process from intermediate T-IN8 to T-IN10 is endergonic by 5.0 kcal/mol. Therefore, our calculations confirm the leaving of the initial TEB along with the proceeding of the copolymerization is both thermodynamically and kinetically favorable. Selective key optimized intermediate structures are shown in Figure 8. In addition, in the process IN9 → TS13 → IN10, the trends of changes in two O−B bonds are opposite, and the length of the new O3−B1 bond gradually decreases, 1.686 Å → 1.589 Å → 1.539 Å, while the old O1−B2 increases, 1.604 Å → 1.645 Å → 2.425 Å. Similarly, the TEB on the carbonyl group then leaves to give the more stable intermediate IN11 (−23.1 kcal/mol). We envision that the dissociated TEB can activate another epoxide for the next round of addition event to start. Noticeably, two TEB molecules are connected on the propagating chain end in the copolymerization process, in agreement with the feeding ratio of 2:1 for TEB to initiator in the experiment where the excess amount of TEB will hinder the leaving of the TEB to decrease the reaction selectivity. CHO Case. Besides PO, excellent experimental polymerization results were also obtained when applying the same conditions to epoxide monomer CHO. DFT calculations were also carried out using CHO as the substrate. The corresponding profiles are summarized in Figure 9. The green pathway describes a favorable route for the alternating copolymerization of CO2 with CHO. Consistent with the PO model, the possible competitive reactions (blue lines) at each step are all unfavorable because of the considerably high energy barriers. It is noteworthy that the energy barriers of the favorable green pathway in the CHO model are all slightly lower than those of the PO model. The energy barrier for CHO ring-opening initiated by chloride anion (step I) is 22.4 kcal/mol, slightly lower than that of the PO model (23.6 kcal/mol). Similarly, the energy barriers of steps II and III in the CHO model are 16.5 and 21.5 kcal/mol, which are

Figure 6. Key optimized structures (bond length/Å) for the profile in Figure 5. Atoms C, O, B, and Cl are shown in gray, red, yellow, and green, respectively.

Figure 7. Profiles of the insertion of PO-TEB in a new round of copolymerization in the PO model (kcal/mol). The black numbers are the corresponding bond lengths (Å).

one B−O bond is lengthened from 1.590 to 1.604 Å. Transition state TS13 describes the ring-opening insertion of PO-TEB with the energy barrier of 19.2 kcal/mol relative to intermediate IN9. E

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (X.F.). *E-mail [email protected] (Y.G.). *E-mail [email protected] (K.-W.H.). ORCID

Yves Gnanou: 0000-0001-6253-7856 Kuo-Wei Huang: 0000-0003-1900-2658 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work is supported by King Abdullah University of Science and Technology (KAUST). Figure 9. Profiles of CHO model (kcal/mol).

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all lower than 17.2 and 22.5 kcal/mol of the PO model, respectively. Consistent with the experimental results, our calculations indicate that CHO is a more active monomer than PO for the alternating copolymerization of CO2 with epoxides. Inspection on the structures suggests a better activation of CHO than PO by TEB. For instance, the B−O bond of C-TS2 is 1.578 Å, which is shorter than 1.582 Å of TS2, while the B−O bonds of C-TS6 are 1.582 and 1.626 Å, which are shorter than 1.588 and 1.632 Å of TS6, respectively.

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CONCLUSION On the basis of experimental results related to the metal-free alternating copolymerization of CO2 with epoxides, the detailed computational mechanism studies were performed by DFT calculation with the M06-2x functional. Four reaction steps were investigated by locating the key intermediates and transition states. In step I, the epoxide is activated by the Lewis acid, TEB, followed by the ring-opening by the chloride anion. In step II, the alternating insertion of CO2 occurs, and the competing side reaction of insertion of epoxide ring can be excluded because of the high energy barriers. For step III, the intramolecular cyclization has a significantly high energy barrier, and the insertion of CO2 is also unfavorable based on the PES scan. Step IV is a new round of alternating copolymerization where the elimination of the initial TEB is thermodynamically and kinetically favorable for subsequent reactions, and the dissociated TEB should activate another epoxide for the next round of addition event to proceed. Our calculations also suggest that only 2 equiv of TEB to the initiator in experiment is needed while excess TEB will result in higher TS energies and thus hinder the copolymerization steps. The CHO model under similar reaction conditions shows the lower energy barriers than those of the PO model, in agreement with the enhanced reactivity for the CHO and CO2 copolymerization. In accordance with experimental results, the remarkable decrease of barriers in the presence of TEB confirms the activation role of the Lewis acid on the epoxides.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00471. Figures S1−S6 and computed Cartesian coordinates (PDF) F

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DOI: 10.1021/acs.macromol.8b00471 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00471 Macromolecules XXXX, XXX, XXX−XXX