Fast and Controlled Ring-Opening Polymerization of Cyclic Esters by

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Fast and Controlled Ring-Opening Polymerization of Cyclic Esters by Alkoxides and Cyclic Amides Chen Tan, Shuoyan Xiong, and Changle Chen* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: Ring-opening polymerization is a powerful method for the synthesis of biodegradable and biorenewable polyesters. In this contribution, we report that the combination of alkali alkoxides and commercially available cyclic amides catalyzes fast and controlled ring-opening polymerization of L-lactide. The constrained cis CN bond in the imidate catalyst is critical for achieving high catalytic activity. By optimizing the basicity of the catalyst, a good balance between activity and control (Mw/Mn < 1.1) is realized. A high amide/initiator ratio is essential for producing narrow dispersities and inhibiting transesterification.

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INTRODUCTION The development of new catalysts and polymerization strategies is crucial for the advancement of polymer

L-lactide

(L-LA, Scheme 1a) was proposed to be the result of Brønsted acid/base bifunctional catalysis (Scheme 1b), which was supported by the observation that the anion of the methylated urea, devoid of the second acidic N−H moiety, exhibited very low catalytic activity and poor control.31 Consistent with this, we observed low activity and broad dispersity in the ROP of L-LA initiated by the linear amide 1a and NaOMe (Table 1, entries 1 and 2) (Scheme 1c). In addition, more acidic linear amide 1b and sulfonamide 1c have no activity (Table 1, entries 3 and 4). Nevertheless, the exploration of the amide system is fascinating and potentially useful because amides are commercially available, cost-effective, and versatile33−35 compared to their thiourea and urea counterparts. Herein, we introduce a constrained cis-configuration strategy for improving the efficiencies of amide-catalyzed ROP reactions; this strategy is based on the hypothesis that the cis-constrained CN bond in a cyclic imidate ion gives rise to a Brønsted base/Lewis acid bifunctional catalyst (Scheme 1d). As such, improved activity and control may be achievable through this cooperativity effect. In contrast, steric hindrance and stereoelectronic effect36 result in trans CN bonds in linear amide anions and prevent the Brønsted base/Lewis acid cooperativity (Scheme S1). Here the ROP reaction of L-LA was employed to test this hypothesis.

Scheme 1. (a) ROP of L-LA to PLA, (b) Catalysis with Thiourea/Urea Anions, (c) Catalysis with Linear Imidates, and (d) Catalysis with Alkali Alkoxides and Cyclic Amides

chemistry.1−10 Ring-opening polymerization (ROP) is a powerful method for the synthesis of biorenewable and biodegradable polymers11−17 such as polylactide (PLA). Moreover, precisely controlled ROP provides a method for the synthesis of well-defined and functional polyesters for biomedical and materials applications.18−20 Despite extensive research efforts and the existence of numerous catalysts,11−17,21−29 the design of a simple and user-friendly ROP system remains a significant challenge.30−32 Recently, Waymouth et al. reported a breakthrough involving the anions of thiourea and urea.31,32 The fast and precisely controlled ROP of © XXXX American Chemical Society

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RESULTS AND DISCUSSION In the ROP of L-LA, a combination of caprolactam (2) (Scheme 2) and NaOMe resulted in a 3 orders of magnitude increase in activity (Table 1, entry 5) over that of the linear Received: December 21, 2017 Revised: February 11, 2018

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Macromolecules Table 1. Alkoxide-Initiated ROP of Cyclic Monomers in the Presence/Absence of Amides entrya

Mb

Ic

h

L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA ε-CL ε-CL ε-CL δ-VL L-LA L-LA L-LA L-LA L-LA

NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaH/BnOHj NaOMe NaOMe NaOMe NaOMe KOMe KOMe KOMe NaOMe IPrm/BnOHj

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19k 20k 21k 22k 23h 24 25l 26l 27l

Ad 1a 1b 1c 2 3 4 5 6 7 8a 9 10 TU 8b 8c 8d 8a 3 3 8b 8b 8a 8a 8a 8a

time

conve (%)

Kobsf (min−1)

40 min 4h 4h 4h 10 s 15 s 30 s 40 s 40 s 40 s 180 s 6h 6h 180 s 90 s 60 s 60 min 180 s 60 s 30 s 1.5 h 10 min 40 min 60 s 30 s 90 s 10 min

94 82 0 0 96 94 94 92 94 95 93 32 16 89 91 90 76 95 95 62 58 60 89 95 98 91 90

0.10(3) 1.1(3) × 10−2 0 0 16(3) 12(2) 5.3(6) 4.2(7) 3.7(7) 3.4(7) 0.90(1) 1.1(1) × 10−3 5.2(2) × 10−4 0.86(1) 1.7(1) 2.3(1) 1.9(3) × 10−2 0.89(8) 2.5(5) 2.5(5) 9.9(3) × 10−3 0.10(1) 7(1) × 10−2 3.0(1) 8.2(3) 1.7(1) 0.24(1)

Mn,GPCg (kDa)

Mw/Mng

19.8 6.3

1.71 1.49

49.3 22.1 6.8 10.9 8.3 9.8 12.0 3.6 N.D.i 12.2 14.1 11.0 18.2 13.9 18.1 10.7 4.7 9.4 18.7 13.8 7.6 6.7 7.3

1.59 1.59 1.57 1.40 1.35 1.34 1.06 1.10 N.D.i 1.12 1.09 1.25 1.47 1.08 1.49 1.45 1.08 1.08 1.63 1.08 1.09 1.08 1.06

General conditions: [initiator]0:[amide]:[monomer]0 = 1:5:100 and [monomer]0 = 0.5 M in DCM at 35 °C (reactions were quenched with PhCO2H). bMonomer. cInitiator. dAmide or amide derivative. eConversion of monomer determined by 1H NMR analysis. fObserved polymerization constants (Kobs) (standard deviations in parentheses) were calculated by Kobs = −ln [1 − (conve/100%)]/time (for representative kinetic plots of ln([M]0/[M]) versus time, see Figure S1). gMn,GPC and Mw/Mn were determined by gel permeation chromatography (GPC) in THF using polystyrene as standard. Mn was corrected by correction factors (0.58 for PLA and 0.52 for PCL). hNo amide was added. iNot detected. jMolar ratio of base/initiator = 1:1. k[Initiator]0:[amide]:[monomer]0 = 1:5:100 and [monomer]0 = 1.0 M in THF at 35 °C. l[Initiator]0:[amide]:[monomer]0 = 1:5:50 and [monomer]0 = 0.5 M in DCM at 35 °C. mIPr is the N-heterocyclic carbene (NHC) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. a

supports our hypothesis of the importance of Brønsted base/ Lewis acid cooperativity (Scheme 1d). The NaH/BnOH initiator exhibited high activity and gave a narrow dispersity (Table 1, entry 18); the end group of the resulting PLA is adjustable through the use of different alcohols. Valerolactam (3)/NaOMe initiated rapidly but catalyzed the uncontrolled ROP of the monomer ε-caprolactone (ε-CL), while 8b/NaOMe initiated a slow but controlled reaction (Table 1, entries 19, 20, and 21). Much higher activity was observed with δ-valerolactone (δ-VL) (Table 1, entry 22). Furthermore, the effect of the alkoxide counterion was explored. KOMe alone mediated slow and uncontrolled ROP of L-LA (Table 1, entry 23), while 8a/KOMe exhibited high activity and produced a narrow dispersity comparable to that of the 8a/NaOMe system (Table 1, entries 24, 25, and 26). In contrast, the imidazolium counterion, a hydrogen-bonding organocatalyst derived from IPr,5,41 exhibited much lower activity (Table 1, entry 27) and formed atactic PLA (Figure S2).42,43 This indicates that a reversible Brønsted acid/base reaction between the N−H and the initiator32 (Scheme S2) generates free IPr that catalyzes the epimerization of L-LA.44,45 A high amide/initiator ratio is essential for achieving the controlled and selective ROP of L-LA (Table 2, entries 1−4). For example, a 1:1 ratio of 8a/NaOMe resulted in poor selectivity (Figure S3), with a series of ions separated by m/z 72, and ions corresponding to cyclic PLA derived through

counterpart 1a. Cyclic amides 3 and 4 also exhibited high activities when combined with NaOMe (Table 1, entries 6 and 7). However, the broad dispersities (Mw/Mn > 1.5) of the PLAs produced in these reactions are suggestive of poor control. To address this issue, the influence of amide basicity on the properties of the catalyst was investigated in order to further optimize the system. With reference to Bordwell’s comprehensive work on the basicities of amides and carbamates in DMSO,37,38 the pKa values of cyclic amides/carbamates 3−10 were used as quantitative references (Scheme 2 and Table 1, entries 6−13). Clearly, both activity and Mw/Mn decrease with decreasing pKa (Figure 1). Markedly, 2-oxindole (8a) exhibited a good balance between activity and control comparable to those of the thiourea/NaOMe system (Table 1, entry 14).32 The activity of 8a was further improved by the introduction of sterically bulky substituents (8b and 8c, Table 1, entries 15 and 16). A narrow dispersity was obtained when 8b was used, highlighting that this system is tunable and versatile. In addition, we measured the pKa values of 8b (19.2 ± 0.2 in DMSO) and 8c (19.9 ± 0.3 in DMSO) by using the spectrophotometric method of overlapping indicators developed by Schreiner39 and Bordwell.40 These pKa values support that both activity and Mw/Mn decrease with decreasing pKa. In contrast, the N-substituted oxindole 8d (pKa = 18.5 in DMSO), devoid of the N−H group, exhibited low activity and produced PLA with a broad dispersity (Table 1, entry 17); this result B

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transesterification, observed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).5,31,32,46,47 In contrast, a 10:1 ratio of 8a/NaOMe exhibited very good selectivity (Figure S4), with significantly decreased levels of ions separated by m/z 72 observed by MALDI-TOF MS. The selectivity can be further improved by lowering the polymerization temperature to 25 °C (Figure 2). The MALDI-TOF MS shows a small amount of amide end group (Figure 2, Figures S3 and S4), indicating ROP processes initiated by amide anions (Scheme S3).48,49 Reversible Brønsted acid/base reactions in the 8a/NaOMe system were noted by Bordwell et al.37 (Scheme 3). To investigate the deprotonation and equilibration in this catalytic system, 13C NMR analysis was performed to identify the dominant species in 8a/NaOMe (1:1 mixture) and 8a/NaH (1:1 mixture) (Figure 3). The amide anion form A (Scheme 3) is the dominant species in 8a/NaOMe (1:1 mixture) and 8a/ NaH (1:1 mixture), while the enolate anion form B is not observed. Different 8a/NaOMe ratios lead to shifting and broadening of the chemical resonances for CH (b) as well as NH (a) (Figure 4, Figures S5−S8). Therefore, there is a dynamic hydrogen exchange between CH (b), NH (a), and OH (methanol) in the system (Scheme 3). Moreover, the integration of NH (a) peak decreases with increasing 8a/ NaOMe ratio. When NaH was used instead of NaOMe, NH (a) signal disappeared and a narrow peak associated with CH (b) was observed. These results indicate the formation of the hydrogen-bonded adduct derived from methanol and the amide anion, and this hydrogen-bonded adduct (A1) (Scheme S4) may be the real active species in the ROP of L-LA.31,32 Since alkali metals are often highly coordinated,50,51 one sodium center may possibly coordinate to two or more amide ligands to afford complexes that serve as the actual active species. If this is the case, then the fluorescence spectrum of a 1:5 (molar ratio) mixture of NaOMe and 3-(1-pyrenylmethyl)2-oxindole (8c) would exhibit a significant broad excimeric emission peak at ∼480 nm.52,53 However, only a narrow emission peak at 434 nm was observed (Figure 5), consistent with the formation of a 1:1 sodium:amide complex. Furthermore, two-dimensional diffusion-ordered NMR spectroscopy (2D-DOSY) experiment shows that the 1:1 mixture of 8a and NaH is a single-component system, and no aggregation with slow diffusion was observed (Figure S9).

Scheme 2. Organic Molecules Used for ROP (pKa Values in DMSO in Parentheses)a

a The catalysts were generated in situ through the deprotonation of N− H with alkoxides.

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Figure 1. Plots of Kobs (min−1) and Mw/Mn as functions of amide/ carbamate pKa in DMSO.

CONCLUSIONS In summary, we demonstrated a simple catalytic system for ROP that involves combining cyclic amides with alkali alkoxides. The cyclic amides used in this work are commercially available and very versatile. The constrained cis CN bond in the cyclic imidate anion is critical for achieving high catalytic activity. A basicity-based optimization approach was used to improve the control of this system. Since a higher basicity of cyclic imidate anion generated a more electronegative oxygen in alcohol end group, nucleophilic ability of the hydroxyl group was enhanced and a more active catalytic species was formed. Moreover, the decreased nucleophilic ability of hydroxyl group resulted in inhibited transesterification and greater control of polymerization. Markedly, a good balance between activity and control was obtained through the use 2-oxindole (8a). A high amide/initiator molar ratio results in a polymer with a narrow dispersity and very little transesterification. Acid−base bifunctional catalysts have been widely used in organic synthesis54,55 as well as ring-opening polymeriza-

Table 2. ROP of L-LA Initiated with 8a at Different Molar Ratios entry

A/Ib

time (min)

convc (%)

Mn,GPCd (kDa)

Mw/Mnd

1 2 3 4 5e

1 2 5 10 10

2 2 1.5 1 1.5

96 96 91 91 92

5.7 7.4 6.7 6.4 7.1

1.56 1.24 1.08 1.07 1.07

a

General conditions: [initiator]0:[monomer]0 = 1:50 and [monomer]0 = 0.5 M in DCM at 35 °C (reactions quenched by PhCO2H). b Amide/initiator molar ratio. cMonomer conversion determined by 1 H NMR analysis. dMn,GPC and Mw/Mn were obtained by GPC in THF using a polystyrene standard. Mn was corrected by a factor of 0.58. e Temperature was 25 °C.

C

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Figure 2. MALDI-TOF MS of a PLA sample (Table 2, entry 5) (C8H6NO is end group derived from 8a).

Scheme 3. Reversible Brønsted Acid/Base Reactions in the 8a/NaOMe System

Figure 4. 1H NMR spectra (in d8-THF, 25 °C) of (a) 8a/NaH (1:1 mixture), (b) 8a/NaOMe (1:1 mixture), (c) 8a/NaOMe (2:1 mixture), (d) 8a/NaOMe (5:1 mixture), and (e) 8a (*THF, **CH2Cl2).

Figure 3. 13C NMR spectra (in d8-THF, 25 °C) of (a) 8a/NaOMe (1:1 mixture) and (b) 8a/NaH (1:1 mixture).

tion.13,17,31,32 We have found that the alkali metal salts of cyclic amides can catalyze some known and simple organic reactions, such as aldol reaction, and believe that this bifunctional strategy as well as this simple, low-cost, and tunable system has great potentials for a variety of polymerization applications as well as other organic transformations.

Figure 5. Fluorescence spectrum of 8c/NaOMe in 1:4 (v/v) THF/ DCM ([NaOMe]:[8c] = 1:5 and [8c] = 5 × 10−4 M). D

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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.macromol.7b02697. Materials, instrumentation, ring-opening polymerization procedures, polymerization data, characterization data, and supplemental figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.C.). ORCID

Changle Chen: 0000-0002-4497-4398 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, 21690071 and 51522306). REFERENCES

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