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. 2023 Jun 29;145(27):14756–14765. doi: 10.1021/jacs.3c03253

Living/Controlled Anionic Polymerization of Glycolide in Fluoroalcohols: Toward Sustainable Bioplastics

Pengfei Zhang 1, Viko Ladelta 1,*, Nikos Hadjichristidis 1,*
PMCID: PMC10347546  PMID: 37382584

Abstract

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Ring-opening polymerization (ROP) is a promising approach to accessing well-defined polyesters with superior (bio)degradability and recyclability. However, the living/controlled polymerization of glycolide (GL), a well-known sustainable monomer derived from carbon monoxide/dioxide, has never been reported due to the extremely low solubility of its polymer in common solvents. Herein, we report the first living/controlled anionic ROP of GL in strong protic fluoroalcohols (FAs), which are conventionally considered incompatible with anionic polymerization. Well-defined polyglycolide (PGA, Đ < 1.15, Mn up to 55.4 kg mol–1) and various PGA-based macromolecules are obtained at room temperature for the first time. NMR titration and computational studies revealed that FAs simultaneously activate the chain end and monomer without being involved in initiation. Low-boiling-point FAs and PGA can be recycled through simple distillation and sublimation at 220 °C in vacuo, respectively, providing a promising sustainable alternative for tackling plastic pollution problems.

Introduction

The global economy as well as human life have benefited from petroleum-based plastics for many decades. However, ∼82% of plastics on the market are of single use and not recycled, creating extremely negative impacts on the health and the environment. The design and use of biodegradable and chemically recyclable polymers is a promising approach for the circular economy of plastics and for achieving a sustainable world.1−4 This approach has been successfully applied in polyesters,5−14 polythioesters,15−18 polyacetals,19 polycarbonates,20 and polyolefins.21 Among them, aliphatic polyesters are an important class of biocompatible, biodegradable commodity polymers usually obtained by living anionic ring-opening polymerization (ROP).22 Therefore, the development of a methodology for the polymerization and depolymerization of polyesters is of great interest.

Polylactides (PLAs) are the most common and extensively used biodegradable polyesters. However, the high ceiling temperature (>600 °C in bulk,23 or 275 °C with the tin catalyst24), i.e., the temperature where polymerization and depolymerization reach an equilibrium, and slow degradation kinetics limit its recycling and reuse process.25 In addition, the chiral center, which is crucial to maintain PLA’s crystallinity and mechanical properties, creates difficulties during both polymerization and depolymerization. PLA can easily undergo epimerization and β-elimination reactions at elevated temperatures during recycling by pyrolysis,26 resulting in racemization and a mixture of stereoisomers, as well as other byproducts, thereby significantly reducing its value (Supporting Information, Figure S1).

On the contrary, polyglycolide (PGA) has no chiral center and possesses better crystallinity, gas barrier property, and degradability than PLA.27 Therefore, it is more suitable for biomedical, bio-absorbable sutures and single-use packaging applications.28 The monomer, glycolide (GL), is the cyclic dimer of glycolic acid, the simplest α-hydroxyl carboxylic acid that can be extracted from plants or synthesized from petroleum feedstocks and carbon monoxide/dioxide. However, unlike lactide (LA) or other lactones, living/controlled polymerization of GL at room temperature has never been reported. This is mainly because even low-molecular-weight PGAs are insoluble in any common organic solvent, causing considerable difficulty in solution polymerization and characterization. Currently, PGA homopolymers are usually prepared by bulk ROP, which generally requires high temperatures (150–230 °C) to achieve high conversion and a high molecular weight.29,30 More importantly, bulk polymerization of GL does not show “livingness” (dispersity, Đ from 1.5 to >3.0) and is incompatible with other monomers, except for oligomers at a lower temperature.30 PGA synthesized by ROP in supercritical carbon dioxide (scCO2),31,32 high-boiling-point solvents, or polycondensation of glycolic acid33,34 shares similar drawbacks of high dispersity, low molecular weight, and absence of livingness, in addition to the high-temperature requirement. Due to its exceptionally high melting transition temperature among the polyester family and its high crystallinity, the crystallization study of PGA-based macromolecules is of great interest to polymer physicists. Still, few examples of PGA-based multiblock polymers have been reported so far due to the difficulties discussed above.35

Achieving polymers with low dispersity, well-defined (micro)structures, and controllable molecular weight has been of great interest to synthetic chemists.36−38 Living/controlled anionic ROP of cyclic esters/ethers is devoid of chain transfer and termination reactions, enabling the synthesis of well-defined polyesters, polyethers, etc. with unprecedented precision.39 However, due to the high reactivity of the propagating species (oxyanion), anionic ROP necessitates strict requirements of the reaction medium (i.e., solvent) and is particularly sensitive to protic sources (e.g., moisture and alcohol). Only a few examples of proton-tolerant anionic polymerization are observed in aza-anion-propagating ROP40 or ROP of O-carboxylanhydrides.41 The active chain end (oxyanion) can be readily quenched through protonation by moisture or alcohols and thus terminate the polymerization. At the same time, water and alcohol can trigger a separate polymerization and thus significantly decrease the molecular weight or even totally impede propagation.

In this work, by the judicious selection of bulky and highly fluorinated alcohols (FAs) as (co)solvents, we realized, for the first time, anionic ROP in a strong protic medium and achieved the first living/controlled ROP of GL in solution at room temperature. This strategy was also successfully applied to other common cyclic monomers, paving the way to the one-pot synthesis of diverse PGA-based macromolecular architectures (e.g., X-b-PGA-b-Y-type triblock terpolymer) with well-defined structures. Furthermore, the low-boiling-point FA can be readily recycled by simple distillation with a high yield. Interestingly, although monomers with a lower ring strain are generally considered to yield polymers with better recyclability,42 high-molecular-weight PGA, thanks to its distinct catalytic degradability, can be chemically recycled back to monomers1 almost quantitatively at 220 °C under vacuum.

Results and Discussion

Reaction Condition Optimization

ROP of GL was first carried out in pure dichloromethane (DCM) with a weak phosphazene base tBuP1, which is considered a mild condition for its analog, LA. As expected, due to its high propagation rate and poor solubility, a white solid (PGA) precipitated almost immediately after the base was injected, which was further promoted by PGA crystallization. The fast reaction reached >99% conversion within 3 min (Table 1, entry 1). The resulting PGA has a high dispersity as determined by size exclusion chromatography (SEC, Đ = 1.61, Figure S2), indicating uncontrolled polymerization. In contrast, when pure hexafluoroisopropanol (HFIP, pKa = 9.3, by potentiometric titration method (PTM) in H2O43), a common FA, was used as the solvent, the ROP proceeded much more gently compared to that in pure DCM and reached 95.5% conversion in 10 min (Table 1, entry 2). However, it was accompanied by severe transesterification, as evidenced by SEC (Đ = 1.28, Figure S3a) and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS, odd-numbered PGA population generated by competitive transesterification, see Figure S3b). MALDI-TOF MS also revealed that the PGA was initiated exclusively from HFIP, and the molecular weight was much less than expected (Figure S3b).

Table 1. Fluoroalcohol/Base-Mediated ROP of Cyclic Monomersa.

entry mon. feed ratiob [I]\[M]\[base]\[TU4] base FA co-solvent time (min) conv. (%) Mn,theockg mol–1 Mn,NMRdkg mol–1 Đe
1 GL 1:40:1:0 tBuP1   DCM 3 >99 4.6 3.8 1.61
2 GL 1:40:1:0 tBuP2 HFIP   10 95.5 4.4 1.1i 1.28
3 GL 1:40:1:0 tBuP1 HFAB DCM 10 98.8 4.6 4.6 1.22
4 GL 1:40:1:0 tBuP1 HFIB DCM 5 96.4 4.5 3.3 1.37
5 GL 1:40:1:0 tBuP1 HFPP DCM 5 >99 4.6 4.6 1.27
6 GL 1:40:1:0 TMG HFAB DCM 5 84.2 3.7 3.7 1.27
7 GL 1:40:1:0 DIEA HFAB DCM 5 >99 4.6 4.7 1.23
8 GL 1:40:1:0 tBuP1 HFAB toluene 5 97.9 4.5 4.4 1.20
9 GL 1:40:1:0 tBuP1 HFAB benzene 5 89.7 4.2 4.3 1.21
10 GL 1:40:1:2 tBuP1 HFAB toluene 10 78.9 3.7 3.7 1.16
11 GL 1:40:1:2 tBuP2 HFAB toluene 5 >99 4.6 3.9 1.15
12f GL 1:40:1:2 tBuP2 HFAB toluene 5 >99 3.7 3.5 1.13
13 GL 1:80:1:2 tBuP2 HFAB toluene 15 >99 9.4 9.8 1.08
14 GL 1:200:1:2 tBuP2 HFAB toluene 30 >99 23.2 18.8 1.10
15 GL 1:500:1:2 tBuP2 HFAB toluene 360 >99 58.0 55.4 1.19
16g LA 1:40:1:0 tBuP2 HFAB toluene 24 h 93.7 5.3 5.3 1.09
17g VL 1:50:1:0 tBuP2 HFAB toluene 24 h 61.8 3.0 3.0 1.07
18h PO 1:80:1:3 tBuP2 HFAB toluene 24 h >99 4.7 1.6i 1.05
19h EO 1:120:1:3 tBuP2 HFAB toluene 1 h >99 5.3 1.7i 1.05
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a

All reactions were carried out at room temperature unless otherwise stated. [GL]0 = 0.5 M, [LA]0 = 1.0 M, [VL]0 = 4.6 M, [PO]0 = 6.3 M, and [EO]0 = 9.0 M.

b

Molar feed ratio.

c

Determined by ([M]0/[ini]0) × Mw(M) × conv. % + Mw(I).

d

Mn,NMR was determined by 1H NMR spectroscopy.

e

Đ was determined by SEC at 40 °C in HFIP as the eluent with poly(methyl methacrylate) standards.

f

Ethyl 2-hydroxyacetate (EthylGL) was used as the initiator.

g

Performed at 45 °C.

h

Triethylborane (Et3B) was used60 instead of TU4.

i

Initiated from solvent.

We then carried out the ROP of GL with tBuP1 as the catalyst in a much bulkier FA, 1,3-bis(2-hydroxyhexafluoroisopropyl)benzene (HFAB), which was reported as a hydrogen bonding (co)catalyst for ROP of cyclic esters44 and N-carboxyanhydrides.45 HFAB was mixed with DCM as a co-solvent (50% v/v) to reduce its viscosity. To our delight, the HFAB did not initiate the reaction (MALDI-TOF MS, and 1H NMR, Figures S4 and S5). The extent of transesterification is slightly less than that of pure HFIP as indicated by a lower Đ value of 1.22 (Table 1 entry 3). The co-solvent ratio was then optimized to find the best conditions. When using 25% v/v HFAB, the polymer started to precipitate after 3 min, indicating insufficient solubilization of PGA. When the HFAB ratio was increased from 37.5 to 75% (v/v), no precipitation was observed but the propagation kinetics became slower (Table S1, see detailed discussion in the NMR Titration and Kinetics Study section). The ratio of HFAB to the co-solvent was then fixed at 37.5% v/v. Various FAs were then screened under similar conditions (Figure S6). The ROP of GL in 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HFIB, pKa = 9.55, PTM46) and 1,1,1,3,3,3-hexafluoro-2-phenyl-2-propanol (HFPP, pKa = 8.8, extrapolated value in H2O47) was found to perform well in terms of reaction rate and controllability (Table 1, entry 4–5). Nonafluoro-tert-butyl alcohol (NFTB, pKa = 5.1, PTM43) and hexafluoro-2,3-bis(trifluoromethyl)-2,3-butanediol (HFBDO, pKa = 5.95, PTM48) were found to inhibit the reaction (<1% in 10 min) probably because of their stronger acidity. The reaction achieves 35.7% conversion in HFBDO when a stronger base tBuP2 was used. A mono −CF3 substituted alcohol with less acidity, trifluoroethanol (TFE, pKa = 12.8, PTM43), and acetic acid (pKa = 4.76, PTM49) cannot fully dissolve PGA and were therefore not tested for further polymerization (refer to Table S2, solubility test). The use of a strong acid, trifluoroacetic acid (TFA, pKa = −0.2650) resulted in the degradation of the polymer and was also not used for further experiments. The optimized pKa range for achieving both good solubility and fast kinetics is 8.8 ≤ pKa ≤ 9.55. The pKa values for common protic and aprotic compounds (especially the FAs used here) are summarized and compared in Figure 1a.

Figure 1.

Figure 1

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(a) pKa “spectrum” for common protic and aprotic compounds. The pKa values are determined by the PTM in H2O unless otherwise stated (from the “iBonD” database, original references are given for each value).49†Conductometric method.51 *Extrapolated value in H2O.47 **Obtained by PTM in DMF.52 ***Estimated in H2O.53 (b) Gibbs free energy profile of monomer activation by FAs with different acidity. (c) Gibbs free energy profile of ROP of GA in FAs with different acidity.

Since the acidity of the solvent is important in the selection of a suitable solvent, the acidity and kinetics relationship was studied by density functional theory (DFT). NFTB and its homologs with different numbers of −CF3 substitutions were used as model FAs. On one hand, more acidic FA can form a more stable hydrogen bonding complex with the monomer (Figure 1b) and increase the Gibbs free energy barrier (ΔG‡, refer to the Mechanism Investigation section). On the other hand, in more acidic FA, more electronegative fluorine atoms can disperse the charge in the transition state and thus stabilize it (Figure 1c), which will decrease the ΔG‡.

Compared to the HFIB path, NFTB will stabilize the reactant more than the TS, thus increasing the ΔG‡. TFIB has a weaker stabilization effect for the monomer, but a weaker stabilization for TS as well compared to the HFIB path. The net effect is the increase in the ΔG‡ value. The ΔG‡ follows the order: NFTB (6.5 kcal mol–1) > TFIB (5.4 kcal mol–1) > HFIB (4.6 kcal mol–1). This supports the experimental findings that in NFTB or HFBDO, the reaction is much slower.

Various classes of organobases were also screened, and the results are summarized in Figure S6. All phosphazene bases and 2-tert-butyl-1,1,3,3-tetramethylguanidine (tBu-TMG, Table 1 entry 6) used in this work showed fast kinetics (>95% conversion within 10 min). Triethylamine, 4-dimethylaminopyridine (DMAP), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) showed rather slower kinetics, probably due to their low basicity. However, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), which is known as an efficient catalyst for the ROP of LA,54,55 exhibited slower kinetics (40.8%, 10 min), presumably due to the interference of FA’s strong hydrogen bonding to the bifunctional activation mechanism.56 Notably, diisopropylethylamine (DIEA), an inexpensive base commonly used in industry, showed remarkably fast propagation comparable to or even superior to phosphazene bases (Table 1 entry 7). Despite the fast reaction rate, all catalysts used here with 37.5% v/v HFAB and DCM show some extent of transesterification (1.2 < Đ < 1.3, Figures S7–S11), indicating mediocre chain-end selectivity.

Toluene and benzene were then used as the co-solvent since nonpolar solvents induce tight ion pair formation between the propagating oxyanion and its counterion, thus reducing the chain-end reactivity.57 The transesterification was significantly suppressed, as observed in the MALDI-TOF MS spectra with a minimal, odd-numbered PGA population (Figures S12 and S13, Table 1, entries 8 and 9) and a lower Đ value of 1.20 (toluene) or 1.21 (benzene). The dispersity was further reduced when thiourea (TU4; for screening of thioureas, see Figures S14–S20), was used as the cocatalyst with tBuP1 (Đ = 1.16, Table 1, entry 10), at the expense of slower kinetics (78.9% conversion in 10 min) and a sign of slow initiation. The reaction rate was improved as the stronger tBuP2 was used without broadening the distribution (Đ = 1.15, Table 1 entry 11, and Figure S20). The slow initiation was eventually avoided by using substituted glycolate as the initiator (e.g., ethyl glycolide and ethylGL), which possesses the same “chain end” as the propagating species and was assumed to have similar reactivities (Đ = 1.13, Table 1 entry 12, Figure 2a–e). MALDI-TOF MS shows a symmetrical spectrum having a single population and minimum transesterification (Figure 2e). The wide-angle X-ray diffraction (XRD) shows clearly the (110) and (020) diffraction planes of PGA at 22.1 and 28.7°, respectively (Figure 2b). The degree of crystallinity based on XRD was calculated to be 46.7%. Differential scanning calorimetry (DSC) clearly shows a strong and sharp crystallization peak at 192.6 °C (Figure 2d). The melting peak is bimodal (209.1 and 218.9 °C) for the second heating–cooling cycle. This is possibly due to the melt-recrystallization phenomena, which is very common in the crystallization of polyesters.58,59

Figure 2.

Figure 2

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Molecular characterization of PGA (Table 1, entry 12). (a) SEC trace, HFIP, 40 °C, PMMA standards. (b) Wide-angle XRD pattern. (c) 1H NMR spectrum (600 MHz, 298 K, CDCl3/HFIP). (d) DSC scans (10 °C min–1). Onset temperatures for melting and cooling (Tonset) are indicated. The enthalpy for melting (ΔHmelting) and enthalpy for crystallization (ΔHcrys) are also reported. (e) MALDI-TOF MS spectrum. Calculated residue group mass (EthylGL + Na+): 127.09, found: 128.63; calculated monomeric unit mass (GL): 116.07, found: 116.33.

The livingness of the ROP of GL catalyzed by organobase tBuP2 and TU4 in diluted FA was demonstrated by the sequential addition of monomers (chain-extension experiment). The reaction time was prolonged to 15 min to ensure quantitative conversion before adding the same equivalent monomer. The shift of the SEC trace (Figure S21) and low dispersity (Đ = 1.08, Table 1, entry 13) proved that GL polymerized in a living and controlled manner (n1 = n2 = 40, conv.1 > 99%, conv.2 > 99%). The linear relationship of Mn,NMR and conversion is also demonstrated for further evidence of livingness (Figure S22). The feed ratio was then modified to obtain higher-molecular-weight PGA (Table 1, entry 14–16). PGA up to 55.4 kg mol–1, comparable to or higher than the commercialized PGA samples with a much narrow dispersity of 1.10–1.19, was obtained in 6 h, demonstrating the remarkable improvement compared to that observed with the commercial sample.

To demonstrate its universality, the ROP using a FA solvent was expanded to other cyclic monomers such as LA, valerolactone (VL), and even epoxides. Even though coupled with strong base tBuP2, which could cause severe side reactions in common organic solvents (e.g., pure DCM or toluene), LA and VL showed slower kinetics but excellent control with nearly no transesterification, as evidenced by the narrow dispersity (1.07 < Đ < 1.09, Table 1 entry 16–17, Figure S23) and MALDI-TOF MS spectrum (Figure S24). The polyepoxides showed narrow distribution but lower molecular weight due to the initiation from FAs (entries 18–19, Figures S25 and S26).

NMR Titration and Kinetics Study

In order to probe the plausible reaction mechanism, NMR titration and kinetics experiments were conducted. Mixing GL with an excess of HFAB results in a shift of 0.261 ppm for the −C(CF3)2OH peak of HFAB to a low field in the 1H NMR spectrum (Figure 3a), indicating that strong hydrogen bonding is formed between the monomer and the FA. The downfield shift of 1.279 ppm for the carbonyl carbon of GL [−CH2(C=O)O−] and of 0.215 ppm for the α-methylene carbon of the carbonyl group [−CH2(C=O)O−] in the 13C NMR spectrum also confirms the hypothesis that FA can activate cyclic ester monomers (Figure 3b–c). The notable downfield shift [0.596 ppm, −(CF3)2OH] of HFAB when mixed with tBuP2 suggests a strong interaction between HFAB and tBuP2 base (Figure 3d). The downfield shift (0.48 ppm, PhCH2–OH) and the upfield proton shift (0.049 ppm, Ph–CH2−) of the initiator when mixed with HFAB and tBuP2 implies that the initiator can be activated via the [HFAB–tBuP2] complex (Figures 3e and S27). The activation or deactivation of the initiator is strongly dependent on the type of FAs used (Figure 3f), while all FAs can activate the monomer (Figure 3g; for more discussions, see the Supporting Information, Monomer/Initiator Activation section, and Figures S28 and S29).

Figure 3.

Figure 3

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NMR titration studies. (a–e) 1H or 13C NMR spectra recorded at 500 MHz or 125 MHz (CDCl3, 298 K). (f) Chemical shift changes of PhCH2OH recorded by 600 MHz 1H NMR (CDCl3, 298 K). (g) Chemical shift changes of the carbonyl group recorded by 125 MHz 13C NMR (CDCl3, 298 K).

The affinity of FA for cyclic esters (simulated by VL rather than GL to avoid polymerization) and polymer main chain esters (simulated by ethyl acetate) was also determined through 13C NMR titration (Figure S30). The association constant (Kass) of FA and cyclic ester was measured to be 6.2 M–1, which is 1.7 times higher than that for main chain esters (3.6 M–1), indicating a selective activation of monomers (s-cis esters) over polyesters (s-trans esters) by FA. Under similar conditions, the Kass value for the living chain end (simulated by tert-butanol/tBuP2/FA complex, Figure S31, details in the Supporting Information, NMR Titration for Determining Binding Constants section) and cyclic esters is measured to be 31.1 M–1, which is ∼2 times higher than that for open-chain esters (15.0 M–1), indicating that the chain end can also selectively bind the monomer over the polymer. These titration results strongly imply that the FA and the living chain end can selectively bind to/activate the cyclic ester (monomer) and thus explain experimentally the selectivity of the FA and the minimal transesterification observed despite the highly reactive monomer used.

The solvent effect was further revealed by kinetics studies. The apparent reaction rate (kobs) decreases from 0.482 to 0.059 min–1 as the HFAB content increases from 25 to 75% (v/v, Figure 4a–d), suggesting that FA may participate in the proton exchange with the living chain end and therefore slows down the reaction. An increase in the viscosity might be another reason for the slow kinetics. The reaction rate was promoted by crystallization (solid precipitates in ∼3 min) when HFIB was used (kobs = 0.619 min–1) instead of HFAB at the same ratio (37.5% v/v), probably because the monoalcohol is insufficient to completely solubilize the product at a similar concentration (Figure 4e). The reaction kinetics also improved when the nonpolar solvent benzene (kobs = 0.530 min–1, Figure 4f) and toluene (kobs = 0.556 min–1, Figure 4g) were used as co-solvent instead of DCM. However, the kinetics significantly slowed down when a polar solvent (e.g., acetonitrile, Figure 4h) was used as a co-solvent. This result is unusual for an SN2 reaction, and further investigation is undergoing.

Figure 4.

Figure 4

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Kinetics studies. (a–h) Kinetics study of the ROP of GL catalyzed by tBuP1 with different solvent combinations and volume ratios at room temperature. [GL]0 = 0.5 M.

Mechanism Investigation

DFT calculations were performed to explore the complete picture of protic solvent-mediated ROP. By calculations of the Gibbs free energy change (ΔG), it was concluded that the monomer is activated either by HFIB (−3.2 kcal mol–1) or by a simplified analog (TU) of TU4 (−5.4 kcal mol–1) (Figure 5a,b). Direct activation (5.5 kcal mol–1) or deprotonation (18.0 kcal mol–1) of the initiator by tBuP1 was observed to be a non-spontaneous process (Figure S32). However, the HFIB (solvent) readily deprotonated from the phosphazene base tBuP1 through a barrierless proton exchange reaction and formed an activated initiator with an advantageous ΔG value of −4.2 kcal mol–1, allowing us to further simplify this system (Figure S33, further discussion is available in the Supporting Information, Formation of Activated Initiator section). The FA-activated initiator (IN1, living chain-end) first binds with an FA-activated monomer (AM1) to form the reactant complex IN2 (Figure 5c). It then forms a tetrahedron intermediate (IN3) through the first transition state (TS1), with a ΔG‡ of 4.6 kcal mol–1. It then undergoes a hydrogen bond rearrangement (IN3 → IN4 → IN5) with a net ΔG gain of 1.4 kcal mol–1 to generate the reactant complex (IN5) for the second step.

Figure 5.

Figure 5

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Computational studies of the living ROP of GL. (a,b) Monomer activation through HFIB and TU. (c) Energy profile for ROP of GL mediated by HFIB (black) and TU (green). HFIB and methanol (MeOH) were used to imitate the FA and the initiator/chain end to improve the calculation efficiency. (d) Gibbs free energy barrier of the rate-determining step in HFIB- and HFIP-mediated ROP of GL. (e) Equilibrium of the living chain-end and deprotonated HFIB. (f) 3D visualization of MD simulation results. Only HFAB and the PGA main chain are shown here for clarity. (g) Radial distribution function of HFAB and toluene molecules around the PGA main chain (details are presented in the Methods section, Supporting Information).

IN5 then opens its ring to form IN6 through TS2 with a ΔG‡ of 7.9 kcal mol–1, completing the propagation of one monomer. IN6 can also bind with another activated monomer to form IN7, with a significant Gibbs free energy drop of −32.0 kcal mol–1, consistent with the NMR titration experiments that the living chain-end can selectively bind the monomer over the open growing chain. The six-membered ring of IN7 will be opened to form the next tetrahedron intermediate IN8, and the propagation is continued.

The larger Gibbs free energy drop for the nucleophilic attack of the chain end to the activated monomer was observed for TU-involved propagation (Figure S34, IN1 → IN2TU, −26.4 kcal mol–1, and IN9TU → IN10TU, −39.7 kcal mol–1), which implies a better selectivity for TU/base-catalyzed ROP. This is consistent with our results (Table 1 entry 11–16) and Waymonth’s pioneering work on living/controlled ROP of LA and lactones.61

In order to gain more insight into which factor determines whether the solvent participates in the initiation step, the Gibbs free energy barrier for HFIB (TS3) and HFIP-initiated (TS4) ROP of GL was calculated. The additional methyl group in HFIB increases the ΔG‡ of the rate-determining step from 11.7 to 14.3 kcal mol–1, slowing down this step by ∼75 times as calculated from transition state theory (Figure 5d). The steric hindrance is likely to contribute the most, as shown by the interaction region indicator (IRI)62 by Multiwfn63 (Figures S35 and S36, see the Supporting Information, IRI Analysis section).

The equilibrium of the living chain-end and deprotonated HFIB was also explored by DFT (Figure 5e). The hydrogen bonding complex is calculated to be the most stable state with the lowest energy. This clearly supports our hypothesis that the chain end can be activated through deprotonated FA and also alleviates our concern that FA might have quenched the reactive chain-end.

The distribution of the FA in the HFAB/toluene/PGA system was simulated through molecular dynamics (MD, Figures 5f,g and S37–S40, refer to the MD Simulation section in the Supporting Information). The FAs were found to bind to the PGA main chain much tighter than toluene via hydrogen bonding, as indicated by the higher FA intensity on the radical distribution function (Figure 5g) and the number of hydrogen bonds formed (Figure S39). Three layers of HFAB were found near the PGA main chain. This microheterogeneity of FA also indicates that FA concentration in the solution is lower than expected, as more FAs are surrounded by the main chain, thus further hindering their possibility of initiation.

PGA-Based Block Copolymers

The livingness of this polymerization can be translated into the one-pot synthesis of various well-defined PGA-based polymers via sequential monomer addition. VL and additional tBuP2 were added to the living solution of PGA (tBuP2/TU4 in 37.5% v/v HFAB/toluene). The reaction was kept at 60 °C for 24 h before being quenched and precipitated in methanol. The resultant poly(glycolide)-b-poly(valerolactone) (PGA-b-PVL) was characterized/confirmed via 1H NMR (Figure 6a). Besides solvent signals, only one diffusion coefficient (6.11 × 10–11 m2 s–1) was found in the DOSY spectrum corresponding to the block copolymer of PGA and PVL, while the DOSY spectrum for the mixture of PGA and PVL homopolymers, with similar molecular weight and composition, showed two distinct diffusion coefficients (Figure S41). This confirms that instead of a mixture of two homopolymers, there is only one type of polymer (PGA-b-PVL) in the system. DSC also shows clearly two sets of melting and crystallization peaks, demonstrating a PGA-based semicrystalline block copolymer (Figure S42).

Figure 6.

Figure 6

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PGA block copolymer and recycling of solvent and monomer. (a) 1H NMR spectrum and DOSY spectrum of PGA-b-PVL (600 MHz, 298 K, CDCl3/HFIP). (b) Solvent recycling after polymerization (600 MHz, 298 K, CDCl3). (c) Monomer recycling of PGA homopolymer (600 MHz, 298 K, CDCl3).

Recycling

Recycling of reagents and polymer products is a promising way toward the achievement of a truly sustainable society. Due to the low boiling point of FA, the solvent system can be easily recycled by simple vacuum distillation. For example, the 50% HFIB–toluene solvent system can be recycled within a few minutes with a near-quantitive yield (>96%) after polymerization, without affecting its composition (Figure 6b). After simple drying with 3 Å molecular sieves, the recycled solvent can be readily reused to conduct the next polymerization.

Recycling of PGA homopolymer was carried out in sublimation apparatus. As a representative example, a low-molecular-weight PGA was directly recycled at 220 °C, under vacuum (∼10 mbar) in 5 min without a catalyst (crude purity: 90.3%, Figure 6c). The crude product was purified by recrystallization from ethyl acetate (purified purity: >99%, Figures S43 and S44). It can then be used to generate new PGA after freeze-drying with anhydrous dioxane.

Conclusions

In summary, we report, for the first time, the efficient living/controlled ROP of a commercialized bioderived monomer (GL), using strong protic FAs as (co)solvents. The bulky FA not only solubilizes the polymeric product but also serves as an activator for monomer and living chain-end, as elucidated by DFT and NMR titration, achieving selective ROP to afford polymers with a tunable molecular weight, well-defined structures, low dispersity, and minimum transesterification. Furthermore, it is easy to incorporate other blocks such as polyethers, polyesters, polycarbonates, etc., with PGA and create novel PGA-based complex macromolecular architectures with unprecedented properties. The PGA product can be chemically recycled into monomers with high selectivity and high yield, providing a sustainable alternative to conventional “non-recyclable” synthetic polymers (e.g., single-use plastics) that cause tremendous environmental issues. This work also deepens our understanding of the mechanism of conventional anionic ROP, where protic sources are considered detrimental impurities that negatively affect polymerization, hinting that by a wise choice of the solvent (i.e., bulkiness, acidity, solubility to the product, etc.), various anionic ROP reactions can be realized without sacrificing its feasibility and selectivity while maintaining the beneficial properties conferred by the solvent (e.g., product or catalyst solubility, photo-responsive properties, etc.).

Acknowledgments

King Abdullah University of Science and Technology (KAUST) baseline funding is gratefully acknowledged. KAUST Supercomputing Laboratory (KSL) is acknowledged for computing resources. Prof. Alejandro J. Müller from University of the Basque Country UPV/EHU and Dr. Jiaxi Xu from KAUST are also acknowledged for their helpful discussions. This paper is dedicated to the memory of the late Professor Lew Fetters, a Titan of Polymer Science. Lew was an inspiring mentor and friend (N.H.).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c03253.

  • NMR spectra, SEC traces, MALDI-TOF MS spectra, DFT calculation details, DFT-optimized structures, and MD simulation details (PDF)

Author Contributions

All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ja3c03253_si_001.pdf (4.9MB, pdf)

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