Theoretical Investigation of Lactide Ring-Opening Polymerization

Nov 14, 2013 - College of Chemistry and Chemical Engineering, Lanzhou University, ... University of British Columbia, 2036 Main Mall, Vancouver, Briti...
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Theoretical Investigation of Lactide Ring-Opening Polymerization Induced by a Dinuclear Indium Catalyst Jian Fang,†,‡ Insun Yu,§ Parisa Mehrkhodavandi,*,§ and Laurent Maron*,† †

INSA, UPS, CNRS, LPCNO, Université de Toulouse, 135 avenue de Rangueil, 31077 Toulouse, France College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China § Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada ‡

S Supporting Information *

ABSTRACT: A DFT study of the ring-opening polymerization of lactide (LA) induced by a dinuclear indium catalyst supported by a chiral diamino phenoxy ligand, [(NNHO)InCl]2(μ-Cl)(μ-OEt) (1), is reported. The nature of the active catalyst, mononuclear vs dinuclear, was investigated and was shown to be dinuclear because of the high energetic cost of its dissociation. The selectivity of the system was investigated for the polymerization of LA with the dinuclear (R,R/R,R)-1 catalyst. In complete agreement with experimental results we observed that (1) selectivity is controlled by the nucleophilic addition of LA to the alcoholate, resulting in the chain-end control of polymerization, (2) a slight kinetic preference for the polymerization of L-LA over D-LA is found that translates to a krel value of ∼14, which is identical with the experimental value, and (3) when rac-LA is used, no clear preference for D- vs L-LA insertion is found, leading to isotactic PLA.

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INTRODUCTION The use of poly(lactic acid) (PLA), a biodegradable polyester derived from renewable sources, has been growing in a variety of applications ranging from bulk disposables and packaging to medical devices and drug delivery.1 The most efficient method for the generation of PLA is the ring-opening polymerization (ROP) of the six-membered cyclic ester lactide (LA) with a variety of metal-based catalysts or organocatalysts.2 Metal-based systems have received greater attention, due to their greater reactivity and control of polymer micro- and macrostructures.3 These metal-based catalysts are Lewis acidic and display varying degrees of aggregation in solution and in the solid state. The Mehrkhodavandi group has reported a family of dinuclear indium-based catalysts for the ring-opening polymerization of lactide (Chart 1).4 These compounds were the first indium-based catalysts for lactide polymerization and showed excellent control over the living polymerization of lactide and other cyclic esters.5 A thorough experimental exploration of the mechanism of polymerization of lactide with these catalysts was published recently.6 The parent catalyst in the family is a chiral ethoxy- and chloro-bridged dinuclear indium complex, [(NNHO)InCl]2(μ-Cl)(μ-OEt) (1). Complex 1 was shown to be dinuclear both in the solid state and in solution.6 Two possible mechanisms for the ring-opening polymerization of lactide by catalyst 1 were proposed (Scheme 1). The first is a mechanism similar to that reported for other dinuclear trivalent metal catalysts involving the dissociation of the dinuclear complex 1 to yield the active mononuclear propagating species [(NNHO)In(Cl)(OEt)] (1mono) and the inactive complex (NNHO)InCl2 (Scheme 1A).7 In the other © 2013 American Chemical Society

option, complex 1 does not dissociate and the propagating species is dinuclear (Scheme 1B).8 Experimental results strongly support the mechanism involving two metal centers that can stabilize the propagating polymer chain, which explains the highly living character of the catalyst. Extensive studies of the stereoselectivity of the catalyst for the ROP of rac-, L-, and D-LA also support this mechanism. Theoretical methods have proven their ability to efficiently describe and explain the ring-opening polymerization of cyclic esters, such as lactones and lactide.9 In order to obtain a more nuanced picture of the various processes involved and understand the mechanism of this catalyst more thoroughly, we have recently undertaken theoretical calculations for this system, the details of which shall be discussed herein.

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RESULTS AND DISCUSSION In order to gain more insights on the reaction mechanism, DFT calculations (B3PW91) were carried out on the two first insertions of lactide with the dinuclear catalyst [(NNHO)InCl]2(μ-Cl)(μ-OEt) (1) and the putative mononuclear propagating species [(NNHO)In(Cl)(OEt)] (1mono). For all calculations and for sake of simplicity (as both enantiomers have the same energy), only the R,R/R,R enantiomers of the catalysts 1 and 1mono were considered. To minimize the effect of entropy, energy profiles are given in enthalpy at room temperature. All calculations were carried out in the gas phase, as solvent inclusion by means of a continuum model Received: May 7, 2013 Published: November 14, 2013 6950

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Chart 1. Dinuclear Indium Catalysts Developed by the Mehrkhodavandi Group

Scheme 1. Two Possible Pathways for the ROP of LA by [(NNHO)InCl]2(μ-Cl)(μ-OEt) (1)

(dichloromethane, experimental solvent) did not lead to any substantial differential effects in this case (average differential effect of 0.2 kcal mol−1 only). The computational work will probe (1) a mononuclear vs a dinuclear mechanism and (2) the second insertion products and the origin of selectivity in catalyst 1. Mononuclear vs Dinuclear Mechanism. We first examined both the mononuclear and dinuclear pathways for polymerization of D- and L-LA by the catalysts 1 and 1mono. In complete agreement with experimental data, the dissociation of dinuclear catalyst 1 to 1mono and (NNHO)InCl2 is computed to be endothermic by 32.1 kcal mol−1 (Figure 1). The reaction sequence between the mononuclear species 1mono and D-LA is shown in Scheme 2. Similarly, the energy profile with the dinuclear indium complex 1 and the related reaction sequence are presented in Figure 2 and Scheme 3, respectively. As can be seen in Schemes 2 and 3, the reaction sequences are similar for 1 and 1mono. However, in a striking difference the reaction with 1mono is strongly endothermic by up to 27.5 kcal mol−1, which is mainly due to the energetic cost of the dissociation 1 to yield complex 1mono (Figure 1). This result correlates strongly with experimental results, which show that the ethoxy-bridged indium complexes are dinuclear in solution.6 Moreover, experimental results show that, although these compounds are reactive, in all cases the thermodynamic product of the reactions is dinuclear. Analysis of Figures 1 and 2 in more depth shows that the different microsteps (NA and RO) of the first insertion are similar for 1 and 1mono. The barriers (relative to the D-LA

Figure 1. Enthalpy profiles of the first insertion of L- and D-LA with the mononuclear species [(NNHO)In(Cl)(OEt)] (1mono) at room temperature: NA TS, nucleophilic addition transition state; RO TS, ring-opening transition state.

adduct) for nucleophilic addition with complex 1mono and 1 are in the same range (1.8 and 7.5 kcal mol−1, respectively). The same holds true for the ring-opening step with barriers of 7.3 and 1.9 kcal mol−1 from the tetrahedral intermediate for 1mono and 1, respectively. Interestingly, the heights of the barriers for the two microsteps of the first insertion are inverted between the monomer and the dimer. The nucleophilic addition step is more facile for 1mono than for 1 due to a higher Lewis acidity of 6951

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Scheme 2. Schematic Representation of the Reaction Sequence with 1mono and D-LA

From this adduct, the system reaches the nucleophilic addition transition state (NA TS). Nucleophilic addition involves two indium centers: one activates the lactide upon coordination, and the second provides the nucleophile (alkoxide) involved in the nucleophilic attack. Following the intrinsic reaction coordinate leads to a tetrahedral intermediate (TI1D) from which the ring opening occurs through the ring-opening transition state (RO TS). Again, the ring-opening process involves two indium centers, as one indium interacts with the

the metal center in the former. Conversely, the ring-opening step is easier for 1 than for 1mono, due to a reduced steric hindrance in the former around the reactive metal center. Due the strongly endothermic nature of the reaction with 1mono, the reaction mechanism will only be described for the dinuclear compound 1 in the following discussion (Figure 2 and Scheme 3). The reaction with the dinuclear indium catalyst 1 begins by the coordination of D-LA to one of the indium centers, yielding complex ad1D (respectively ad1L). L-LA would react similarly. 6952

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Figure 2. Enthalpy profiles of the first insertion of L- and D-LA with dinuclear complex [(NNHO)InCl]2(μ-Cl)(μ-OEt) (1) at room temperature: NA TS, nucleophilic addition transition state; RO TS, ring-opening transition state.

to the other after each insertion, as shown by Coates et al. with chiral aluminum salen catalysts.11 The enthalpy profile for the second insertion is similar to that of the first insertion (Figure 3). The microsteps, namely nucleophilic addition and subsequent ring opening, of the second insertion are the same as those described for the first insertion in Scheme 3. In particular, the nucleophilic addition (NA) step is slightly more complicated than the RO step, although both barriers are nearly isoenergetic within the precision of the methods used.12 The selectivity is controlled by the nucleophilic addition of the lactide to the alcoholate; thus, the nucleophilic addition transition state (NA TS) is rate determining for the polymerization. The same conclusion is valid on the basis of the free energy profiles, although the computation of the entropy in solution is still problematic (see the Supporting Information). The overall activation barriers, starting from [1]-D, are thus 27.9 (22.0 + 5.9) kcal mol−1 and 29.5 (23.6 + 5.9) kcal mol−1 for the formation of [1]-DD and [1]-DL, respectively. In contrast, the barriers for the second insertions in [1]-L are 24.2 (20.4 + 3.8) kcal mol−1 for [1]-LL and 27.5 (23.7 + 3.8) kcal mol−1 for [1]-LD. Thus, within the precision of this theoretical method, the barriers for the insertion of L- and D-LA from a given product of the first insertion are comparable. Therefore, no clear tacticity, either syndio- or heterotactic, of the polymer is expected and formation of an atactic polymer is anticipated, in line with the experimental observation.6 A deeper analysis of the computed enthalpy profiles for formation of the second insertion products leads to two further conclusions (Figure 3). First, on the rate-determining step (NA), we observe a slight kinetic preference (by 1.6 kcal mol−1) for the formation of [1]-LL over [1]-DD. Applying the Arrhenius law, this corresponds to a krel value of ∼14, which is in agreement with the experimental results. Second, regarding the formation of atactic PLA with (R,R/R,R)-1 when rac-LA is used, as discussed above, no clear kinetic and thermodynamic insertion preference is observed, in line with the formation of atactic polymer. Indeed, all the profiles fall within a 3 kcal mol−1 range, which is within the precision of the method used. This is in very good agreement with experimental observations.

formal carbonyl oxygen and the other with the intracyclic oxygen adjacent to the activated carbonyl. The ring opening leads to the formation of complex [1]-D, where the growing polymer chain remains in a cyclic conformation (the cyclic constraints partially remain). For this intermediate, labeled [1]-D with chain constraint, it is possible to remove this constraint (imposed by RO TS) to obtain a pure aliphatic chain (complex [1]-D). This last step is highly favorable and leads to an overall favorable step thermodynamically. Selectivity. Complex 1 was somewhat isoselective for the polymerization of rac-LA; however, that selectivity was highly dependent on the chirality of the catalyst used.6 The reaction of (±)-1 with rac-LA formed slightly isotactic PLA (Pm = 0.61). In contrast, reaction of the enantiopure catalyst (R,R/R,R)-1 with rac-LA resulted in atactic PLA. A comparison of the rates of polymerization of (R,R/R,R)-1 with L- and D-LA showed a kL/ kD value of ∼14, which is on par with some of the most isoselective chiral aluminum catalysts reported for the polymerization of rac-LA.10 In order to probe these results further, the selectivity of the enantiopure catalyst (R,R/R,R)-1 was investigated at the DFT level. A thorough study of selectivity requires a consideration of the second insertion of LA. For the sake of simplicity, only the dinuclear catalyst 1 was considered in the study of selectivity. Experimentally, the enantiopure catalyst (R,R/R,R)-1 has a much faster rate of ROP for L-LA than for D-LA (krel ≈ 14). Computationally, a comparison of the first insertion step of Lor D-LA does not reveal either a kinetic or a thermodynamic preference for the reactivity of one enantiomer over the other; the first insertion with 1 is predicted to be thermodynamically favorable for both L- and D-LA. As found in previous studies of the ROP of lactide or lactones,9a,b it is often necessary to study up to the second (and sometimes third) insertion to reproduce the experimental selectivity. Therefore, we investigated the second insertion of Dor L-LA into [1]-D or [1]-L, thus considering either a heterotactic ([1]-LD/DL) or isotactic ([1]-LL/DD) enchainment. As noted above, the dimeric pathway requires a migratory polymerization (or back-side-type insertion) where the growing polymer chain is flipping from one indium center 6953

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Scheme 3. Schematic Representation of the Reaction Sequence with Complex 1 and D-LA

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mol−1), making the monomeric pathway strongly endothermic. This is in perfect agreement with experimental observations. The selectivity of the system was investigated for the polymerization of LA with the dinuclear (R,R/R,R)-1 catalyst. Our first observation was that the selectivity is controlled by the nucleophilic addition of LA to the alcoholate, resulting in the chain-end control of polymerization. Second, using the R,R/R,R version of the catalyst, a slight kinetic preference for the polymerization of L-LA over D-LA is found. This slight

CONCLUSIONS

In this study, a theoretical investigation of the reaction mechanism of lactide polymerization catalyzed by [(NNHO)InCl]2(μ-Cl)(μ-OEt) has been carried out at the DFT level (B3PW91) by computing the first and second insertion enthalpy profiles at room temperature. The mechanism is proposed to involve the dinuclear species, because the dissociation energy of the dimer is too high (32.1 kcal 6954

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Figure 3. Enthalpy profile of the second insertion of L- and D-LA with catalyst 1 at room temperature: NA TS, nucleophilic addition transition state; RO TS, ring-opening transition state.

preference (1.6 kcal mol−1) translates to a kL/kD value of ∼14, which is identical with the experimental value. Finally, when rac-LA is used, no clear preference for D- vs L-LA insertion is found, so that atatic polymer formation is predicted. This is again in line with the experimental observations.

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*E-mail for L.M.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

EXPERIMENTAL SECTION

The authors declare no competing financial interest.

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In view of the good performance of density functional theory (DFT), we performed DFT calculations at the B3PW91 level of theory on all stationary points of the potential energy surfaces (PES) we studied using the GAUSSIAN09 program suite.13 The equilibrium and transition structures were fully optimized by Becke’s three-parameter hybrid functional14 combined with the nonlocal correlation functional provided by Perdew/Wang.15 RECP (augmented by an f polarization function, α = 1.0) was used to represent indium.16 For the rest of the nonmetal atoms the 6-31G(d,p) basis set was used.17 In all computations no constraints were imposed on the geometry. Full geometry optimization was performed for each structure using Schlegel’s analytical gradient method,18 and the attainment of the energy minimum was verified by calculating the vibrational frequencies that result in the absence of imaginary eigenvalues. The nature of the stationary points (local minima, transition states) was identified by the number of imaginary frequencies. The vibrational modes and the corresponding frequencies are based on a harmonic force field. This was achieved with SCF convergence on the density matrix of at least 10−9 and an rms force of less than 10−4 au. All bond lengths and bond angles were optimized to better than 0.001 Å and 0.1°, respectively. Enthalpies were obtained at T = 298.15 K within the harmonic approximation. Intrinsic reaction paths (IRPs)19 were traced from the various transition structures to ensure that no further intermediates exist.

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ACKNOWLEDGMENTS L.M. is grateful to the Institut Universitaire de France. CalMip (CNRS, Toulouse, France) and CINES (CNRS, Montpellier, France) are acknowledged for calculation facilities. J.F. acknowledges financial support from the Chinese Scholarship Council (CSC) and the Scholarship Council of Lanzhou University. P.M. acknowledges the NSERC for partially funding this work.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving free energy profiles and tables giving Cartesian coordinates of all optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for P.M.: [email protected]. 6955

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