DOI:
10.1039/D5SU00245A
(Paper)
RSC Sustainability, 2025, Advance Article
Polymerization and depolymerization of polyesters and polycarbonates using 1,1,3,3-tetramethyl guanidine as a catalyst for improved resource utilization
Received
4th April 2025
, Accepted 2nd September 2025
First published on 10th September 2025
Abstract
Despite being biodegradable materials whose random disposal has no significant impact on the environment, finding new routes to recycle polyesters and polycarbonates under the current conditions of limited resources is of high importance. This study used 1,1,3,3-tetramethylguanidine (TMG) as a catalyst to test the polymerization of polylactide and poly(hexane-1,6-diol carbonate). In addition, the depolymerization of polylactide, poly-ε-caprolactone, polyvalerolactone, polyethylene terephthalate, and poly(bisphenol A carbonate) using TMG as a catalyst was investigated. In LA polymerization, various alcohols can be employed to initiate the reaction using TMG as a catalyst. When BnOH and poly(ethylene glycol) monomethyl ether-1900 were used as initiators, highly controlled LA polymerizations were observed. In addition, TMG could depolymerize PLA, PCL, PVL, PET, and PBAC in alcohol and water to produce alcoholysis products. Compared to depolymerization using 1,8-diazabicyclo[5.4.0]undec-7-ene and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as catalysts, TMG exhibited an absolute advantage over PCL, PET, and PBAC depolymerization.
Sustainability spotlight
Global annual plastic production surged from 2 million tons in 1950 to 234 million tons in 2000, reaching 460 million tons in 2019. Should alternative strategies remain unimplemented, plastic consumption is expected to reach 1.231 billion tons by 2060. Furthermore, multiple nondegradable petrochemical plastics persist in the environment for centuries due to their durability. Compared with these nondegradable petrochemical plastics, polyesters are biodegradable and environmentally friendly. Although these polyesters and polycarbonates can be decomposed by bacteria under environmental conditions, the resulting products of polymer decomposition such as water and carbon dioxide are of no economic benefit. If these plastics are to be manufactured again, they must be extracted and manufactured from plants or petroleum. Therefore, if used polymers can be transformed into small organic molecules that may be employed as starting materials for these polymers or in other applications, then such a chemical reaction holds economic significance. This study used 1,1,3,3-tetramethylguanidine (TMG) as a catalyst to test the polymerization of polylactide and poly(hexane-1,6-diol carbonate). In addition, the depolymerization of polylactide, poly-ε-caprolactone, polyvalerolactone, polyethylene terephthalate, and poly(bisphenol A carbonate) using TMG as a catalyst was investigated. In LA polymerization, various alcohols can be employed to initiate the reaction using TMG as a catalyst. When BnOH and poly(ethylene glycol) monomethyl ether-1900 were used as initiators, highly controlled LA polymerizations were observed. In addition, TMG could depolymerize PLA, PCL, PVL, PET, and PBAC in alcohol and water to produce alcoholysis products. Compared to depolymerization using 1,8-diazabicyclo[5.4.0]undec-7-ene and 1,5,7-triazabicyclo[4.4.0]dec-5-ene as catalysts, TMG exhibited a significant advantage over PCL, PET, and PBAC depolymerization.
|
Introduction
Plastics are artificial polymers produced by polymerizing diverse organic repeating units called monomers. Given their long-chain structure and high molecular mass, plastics possess outstanding mechanical and chemical strength and durability. Owing to their ease of development and low-cost production, plastics are widely utilized daily in various industrial and consumer applications, thus fostering positive changes in diverse life domains.
Global annual plastic production surged from 2 million tons in 1950 to 234 million tons in 2000, reaching 460 million tons in 2019. Should alternative strategies remain unimplemented, plastic consumption is expected to reach 1.231 billion tons by 2060.1,2 Furthermore, multiple nondegradable petrochemical plastics, including polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), and polystyrene (PS), persist in the environment for centuries due to their durability. Compared with these nondegradable petrochemical plastics, polyesters, including polylactide (PLA) and poly-ε-caprolactone (PCL), are biodegradable and environmentally friendly. Although these polyesters and polycarbonates can be decomposed by bacteria under environmental conditions, the resulting products of polymer decomposition such as water and carbon dioxide are of no economic benefit. If these plastics are to be manufactured again, they must be extracted and manufactured from plants or petroleum. Therefore, if used polymers can be transformed into small organic molecules that may be employed as starting materials for these polymers or in other applications, then such a chemical reaction holds economic significance. There is currently plenty of literature reports on the use of various catalysts for the decomposition of polyesters and polycarbonates. For example, PLA3–13 (Fig. 1A), PCL14–16 (Fig. 1B), polyethylene terephthalate (PET)10e,11,4,17–19,17–20,21–25 (Fig. 1C), and polycarbonates (PCs)13,15,26–28 (Fig. 1D) can be decomposed into small organic molecules through alcoholysis. In general, metal catalysts act as Lewis acids to activate the carbonyl group of polymers, allowing external alcohols to attack the carbonyl group of the polymers (Fig. 1E), leading to the degradation of the polymer and the formation of new ester groups. This reaction is also known as alcoholysis. Analysis revealed that the metal catalyst is stable in alcoholic solvents and under high-temperature conditions. In contrast to metal catalysts, organic catalysts exhibit relatively higher stability and eliminate the concerns of residual toxic metals.
 |
| Fig. 1 Depolymerization of (A) PLA, (B) PCL, (C) PET, (D) PC, and (E) the mechanism of alcoholysis using a metal catalyst. | |
1,1,3,3-Tetramethylguanidine (TMG) is an effective catalyst for urea formation from amine and cyanate,29 carbon dioxide capture,30 ring-opening polymerization of sarcosine-derived N-thiocarboxyanhydride,31 carbonatation of amine and carbon dioxide,32 t-butyldimethylsilylation of alcohols,33 amide synthesis from pantolactone and amino acid,34 Baylis–Hellman reaction,35 epoxidation,36 aldol reaction,37 and advanced synthetic methodologies.38 According to the literature, TMG demonstrated efficiency for carbonyl group activation. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)39 and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)40,41 which are similar to TMG, have been reported to catalyze cyclic ester polymerization. In addition, TMG is stable in the atmosphere, and it may be suitable to catalyze the alcoholysis of the polyesters. Herein, TMG was used to investigate the activity of LA polymerization and poly(hexane-1,6-diol carbonate) synthesis. In addition, TMG was used for the alcoholysis of PLA, PCL, polyvalerolactone (PVL), PET, and poly(bisphenol A carbonate) (PBAC).
Results and discussion
LA polymerization with TMG as a catalyst and various initiators is outlined in Table 1. The optimal conditions for different initiators are detailed in entries 1–3 and 8–13. Notably, the LA polymerization employing benzyl alcohol (BnOH, see entry 1 of Table 1) as an initiator demonstrated a higher polymerization rate, achieving 95% conversion at 25 °C within 70 min. However, lactide polymerization occurred under the conditions specified in Table 1, entry 1, but without the addition of TMG. After one day, no formation of PLA was observed. The molecular mass characteristics for the polymerization (entry 1 of Table 1) resulted in MnGPC-5800 with a dispersity (Đ) value of 1.10. Similarly, the LA polymerization with tetrabutylammonium benzyl alkoxide (NnBu4OBn, entry 2 of Table 1) as an initiator yielded a 95% conversion at 25 °C after 90 minutes, with MnGPC-5000 and Đ = 1.15. This polymerization result suggests that BnOH's proton enhances the catalytic activity of TMG for LA polymerization.
Table 1 L-Lactide polymerization using TMG as a catalysta
Entry |
TMG + initiator initiator= |
Time (min) |
Conv. (%)b |
MnCalc |
MnNMRb |
MnGPCd |
Đd |
kobsb × 104 (error) min−1 |
Reaction conditions: [LA]:[TMG]:[I] = 50 : 1 : 1, 2 mL of CDCl3, [LA] = 0.25 M, at 25 °C. The kinetic data were studied through 1H NMR analysis. MnNMR is calculated from the molecular weight of LA times the ratio (the integration of the peak at 5.2 ppm × 5/the integration of the peak at 7.3 ppm × 2) and adds Mw(BnOH). Calculated from the molecular weight of LA times the ratio of ([LA]0/[BnOH]0), the conversion percentage, and finally adds Mw(BnOH). Obtained through gel permeation chromatography (GPC) analysis and calibration using the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.58. [LA] = 0.5 M in CDCl3, at 25 °C. [LA] = 1 M in CDCl3 without BnOH, at 25 °C. The reaction was performed in the atmosphere. In toluene (2 mL). In tetrahydrofuran (2 mL). In 1,2-dichlorobenzene (2 mL). Neat reaction. LA = 1.44 g (10 mmol), BnOH = 0.011 g (0.1 mmol), Cat. = 0.1 mmol at 110 °C. Neat reaction. LA = 1.44 g (10 mmol), PEG4000 = 0.8 g (0.2 mmol), Cat. = 0.1 mmol at 110 °C. Not determined. |
1 |
BnOH |
70 |
95 |
6900 |
6600 |
5800 |
1.10 |
421 (5) |
2 |
NnBu4OBn |
98 |
95 |
6900 |
6700 |
5000 |
1.15 |
286 (8) |
3 |
BnOD |
1200 |
91 |
6600 |
6300 |
5000 |
1.12 |
20 (1) |
4e |
BnOH |
24 |
93 |
6800 |
6200 |
6000 |
1.12 |
898 (11) |
5e |
NnBu4OBn |
34 |
93 |
6800 |
6500 |
5800 |
1.18 |
542 (7) |
6f |
TMG only |
300 |
94 |
6900 |
6600 |
4700 |
1.18 |
49 (2) |
7g |
BnOH |
85 |
93 |
6800 |
6300 |
5600 |
1.12 |
306 (63) |
8 |
IPA |
480 |
92 |
6700 |
6400 |
5500 |
1.19 |
55 (5) |
9 |
tBuOH |
600 |
90 |
6600 |
4500 |
4400 |
1.41 |
41 (1) |
10 |
PEG4000 |
360 |
89 |
6500 |
7100 |
5000 |
1.14 |
59 (1) |
11 |
MeOPhOH |
210 |
93 |
6800 |
6500 |
4100 |
1.29 |
129 (2) |
12 |
ClPhOH |
900 |
94 |
6900 |
7500 |
5500 |
1.32 |
31 (6) |
13 |
HFIPA |
1560 |
92 |
6700 |
6100 |
4100 |
1.18 |
18 (4) |
14h |
BnOH |
360 |
96 |
7000 |
6500 |
5100 |
1.14 |
101 (4) |
15i |
BnOH |
2160 |
95 |
6900 |
6500 |
4400 |
1.12 |
15 (1) |
16j |
BnOH |
480 |
92 |
6700 |
6400 |
5600 |
1.13 |
55 (1) |
17k |
BnOH |
10 |
93 |
13 500 |
11 000 |
8500 |
1.14 |
—m |
18l |
PEG4000 |
180 |
95 |
10 800 |
9100 |
9300 |
1.10 |
—m |
19k |
DBU + BnOH |
5 |
97 |
14 100 |
12 400 |
9400 |
1.21 |
—m |
20k |
TBD + BnOH |
7 |
95 |
13 400 |
12 400 |
9400 |
1.23 |
—m |
21k |
DMAP + BnOH |
180 |
94 |
13 600 |
11 600 |
8900 |
1.13 |
—m |
Further analysis of the role of H–OBn was conducted using LA polymerization with mono-deuterated benzyl alcohol (BnOD, entry 3 of Table 1), and the lower catalytic activity with BnOD was observed compared to BnOH (entry 1 of Table 1), as indicated by the kinetic isotope effect (KIE) calculated to be kBnOH/kBnOD = 22.21. This result indicates that the bond dissociation of H–OBn occurs at the transition state during the LA polymerization process.
Additionally, the LA polymerization using TMG without BnOH (entry 6 of Table 1) revealed a lower polymerization rate compared to the LA polymerization with BnOH (entry 1 of Table 1). This discrepancy suggests that the polymerization mechanisms differ significantly with and without BnOH, as the rate of LA polymerization using TMG alone is substantially lower. When the LA concentration was increased to 0.5 M (entry 4 of Table 1), the catalytic activity increased by approximately four-fold, achieving a conversion of 93% after 24 minutes. When the LA polymerization was performed in CDCl3 without drying by molecular sieve in the atmosphere (entry 7 of Table 1), the polymerization time increased from 70 to 85 min compared to that in dry CDCl3 (entry 1 of Table 1). The results implied that the presence of moisture slightly reduced the LA polymerization rate with TMG. In addition, various alcohols, including isopropanol (IPA), tert-butyl alcohol, polyethylene glycol 4000 (PEG 4000), 4-methoxyphenol (MeOPhOH), 4-chlorophenol (ClPhOH), and hexafluoro-2-propanol (HFIPA), were used as initiators to investigate their polymerization rate (entries 8–13 of Table 1). The 1H NMR spectra of PLA oligomers synthesized at a ratio of 5
:
1
:
1 (LA
:
TMG
:
initiator) are shown in Fig. S9–S14, and the chain-end signals observed in the 1H NMR spectra demonstrated that these alcohols can act as initiators. The results revealed that the electron-withdrawing group in alcohols decreased the catalytic activity (MeOPhOH > ClPhOH; IPA > HFIPA), and the steric bulk around the alcohols group also decreased the catalytic activity (IPA > tBuOH). The optimization of the reaction solvents, including toluene, tetrahydrofuran (THF), and 1,2-dichlorobenzene, is also studied as summarized in entries 1 and 16–18 of Table 1. The results revealed that TMG exhibited the highest catalytic activity in deuterated chloroform, whereas it dipped to its lowest in THF. This is presumably due to the hydrogen bond between TMG and THF. In adherence to green chemistry principles towards solvent-free reactions, the neat reactions were carried out with BnOH (entry 17 of Table 1) and PEG4000 (entry 18 of Table 1) as initiators. A tremendous catalytic activity of TMG under neat conditions with highly controllable polymerization were observed. Compared with the catalytic activity of LA ROP for other commercially available organocatalysts, including DBU, TBD, and 4-(N,N-dimethylamino)pyridine (DMAP), comparative results are presented in entries 19–21 of Table 1. There is not much difference between the catalytic activities of DBU, TBD, and TMG. DBU has the best catalytic activity (5 min, conversion = 97%) compared with TBD (7 min, conversion = 95%) and TMG (10 min, conversion = 93%). DMAP exhibited the lowest catalytic activity of LA ROP (180 min, 94%).
The results of entries 1 to 7 in Table 2 revealed the highly controlled LA polymerization with narrow dispersity (Đ) values ranging from 1.06 to 1.24, highlighting excellent living properties by using TMG as a catalyst. However, the experimental molecular masses (MnGPC) are always lower than the theoretical masses (MnCal). This phenomenon may be attributed to TMG also acting as an initiator, as supported by the data shown in Table 1, entry 6. To test this hypothesis, TMG was reacted with 5 equivalents of LA in CDCl3, and the resulting mixture was analyzed using MALDI-TOF mass spectrometry (Fig. S17). The results confirmed that TMG can indeed serve as an initiator to attack LA. The lower MnGPC values compared to the MnCal values are likely due to an excess amount of initiators (BnOH + TMG). Considering that TMG alone without BnOH exhibits much lower activity (Table 1, entry 6), increasing the amount of BnOH might suppress the initiation role of TMG, thereby bringing the MnGPC values closer to the MnCal values. Experimental results (Table 2, entries 8–12) supported this hypothesis: when the amount of BnOH was increased to two equivalents, the initiation role of TMG was successfully suppressed, and the MnGPC values of the resulting PLA reached much closer to the MnGPC values, as shown by the linear relationship between MnGPC and [LA]0 × conv./[BnOH], assuming BnOH functions as initiators in the polymerization process (Fig. 2C).
Table 2 Living property of L-LA polymerization using TMG as a catalyst and BnOH as an initiatora
Entry |
[LA] : [TMG] : [BnOH] |
Time (min) |
Conv. (%)b |
MnCalc |
MnNMRb |
MnGPCd |
Đd |
Reaction conditions: [TMG] = 1.15 mM in CDCl3 (2 mL), 72 mg LA for every loading, at 25 °C. Data were obtained through 1H NMR analysis. MnNMR is calculated from the molecular weight of LA times the ratio (the integration of the peak at 5.2 ppm × 5/the integration of the peak at 7.3 ppm × 2) and adds Mw(BnOH). Calculated from the molecular weight of LA times the ratio of ([LA]0/[BnOH]0), the conversion percentage, and finally adds Mw(BnOH). Obtained through gel permeation chromatography (GPC) analysis and calibration using the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.58. |
1 |
50 : 1 : 1 |
70 |
95 |
6900 |
6800 |
5400 |
1.11 |
2 |
100 : 1 : 1 (entry 1 + 50 LA) |
92 |
98 |
14 200 |
9000 |
7300 |
1.06 |
3 |
150 : 1 : 1 (entry 2 + 50 LA) |
102 |
96 |
20 800 |
14 200 |
12 500 |
1.16 |
4 |
200 : 1 : 1 (entry 3 + 50 LA) |
116 |
97 |
28 000 |
19 500 |
17 200 |
1.15 |
5 |
250 : 1 : 1 (entry 4 + 50 LA) |
240 |
98 |
35 400 |
23 800 |
20 700 |
1.18 |
6 |
300 : 1 : 1 (entry 5 + 50 LA) |
420 |
96 |
41 600 |
26 500 |
25 300 |
1.2 |
7 |
350 : 1 : 1 (entry 6 + 50 LA) |
630 |
95 |
47 800 |
31 300 |
30 200 |
1.24 |
8 |
50 : 1 : 2 |
60 |
96 |
3600 |
3500 |
3300 |
1.24 |
9 |
50 : 1 : 3 |
45 |
95 |
2400 |
2300 |
2200 |
1.15 |
10 |
100 : 1 : 2 |
105 |
96 |
7000 |
6900 |
7400 |
1.06 |
11 |
200 : 1 : 2 |
240 |
93 |
13 500 |
12 400 |
12 100 |
1.43 |
12 |
300 : 1 : 2 |
480 |
90 |
19 500 |
18 400 |
18 600 |
1.16 |
 |
| Fig. 2 Linear plots of (A) MnGPC versus ([LA]0 × conv./[BnOH]0) using one eqn BnOH, represented by black squares; (B) MnCal versus ([LA]0 × conv./[BnOH]0) using one eqn BnOH, represented by red dots; (C) MnGPC versus ([LA]0 × conv./[BnOH]0) using two eqn BnOH, represented by green triangles; and (D) MnCal versus ([LA]0 × conv./[BnOH]0) using two eqn BnOH, represented by blue inverted triangles. | |
To support the practicality of using TMG as a catalyst for LA polymerization, TMG was used with poly(ethylene glycol) monomethyl ether-1900 (mPEG1900) to synthesize “polyethylene glycol monomethyl ether-1900-b-poly-L-lactide” (mPEG1900-b-PLA), which is a material commonly used for drug delivery43 as shown in Table 3. The linear relationship between MnGPC and ([LA]0 × conv.)/[BnOH] (entries 1–5 of Table 3) presented in Fig. 3 illustrates the excellent living property of LA polymerization using TMG [Đ values (1.03–1.09)], despite using a large molecular mass mPEG1900 as an initiator. Under the reaction conditions described in entry 2 of Table 3, catalytic data for DBU and TBD (entries 6 and 7 in Table 3) indicate that DBU displays superior catalytic activity (80 min, 96% conversion) compared to TMG; in contrast, TBD is inactive in the LA polymerization when mPEG1900 is used as the initiator.
Table 3 Synthesis of mPEG1900-b-PLA using TMG as a catalyst and mPEG1900 as an initiatora
Entry |
mPEG1900 : LA |
Conv. % |
Time (min) |
Mncalb |
MnNMRc |
MnGPCd |
Đ |
Reaction conditions: DCM, [TMG] = 0.025 M at 25 °C. Calculated from the molecular weight of LA times the ratio of ([LA]0/[BnOH]0), the conversion percentage, and finally adds Mw(mPEG1900). Data were obtained through 1H NMR analysis. MnNMR was calculated from the molecular weight of LA times the ratio (the integration of the peak at 5.2 ppm/the integration of the peak at 3.38 ppm) and adds Mw(mPEG1900). Obtained through GPC analysis and calibration based on the polystyrene standard. Values of MnGPC are the values obtained from GPC molecular weight. Using DBU as an initiator. Using TBD as an initiator. Not available. |
1 |
1 : 30 |
97 |
35 |
6100 |
7000 |
5900 |
1.03 |
2 |
1 : 60 |
90 |
130 |
9800 |
9800 |
9400 |
1.04 |
3 |
1 : 70 |
96 |
180 |
11 600 |
11 300 |
11 200 |
1.09 |
4 |
1 : 80 |
93 |
220 |
12 600 |
12 800 |
12 600 |
1.08 |
5 |
1 : 100 |
80 |
400 |
13 400 |
13 800 |
13 100 |
1.04 |
6e |
1 : 60 |
96 |
80 |
10 200 |
9800 |
11 600 |
1.19 |
7f |
1 : 60 |
0 |
130 |
—g |
—g |
—g |
—g |
 |
| Fig. 3 Possible mechanism of LA polymerization using TMG as a catalyst. | |
Kinetic study of LA polymerization
Experiments with the [LA]0/[TMG]/[BnOH] ratios of 50
:
1
:
1, 50
:
1
:
2, 50
:
1
:
3, 50
:
1
:
4, 50
:
1
:
6, 50
:
2
:
1, 50
:
3
:
1, 50
:
4
:
1 and 50
:
6
:
1 ([LA] = 0.25 M in 2 mL CDCl3 at 25 °C) were performed. As detailed in Tables S1–S2, the kinetic results revealed a first-order dependency on the [LA] concentration, revealing that one LA was initiated in every polymerization cycle according to eqn (1), where kobs is equal to kprop[TMG]x[BnOH]y, and kprop is the propagation rate constant. To determine the order of [TMG] (x), the polymerization conditions of different [TMG] (5, 10, 15, 20, and 30 mM) with the same [LA] (0.25 M) and [BnOH] (5 mM) were conducted (Fig. S4). Since [BnOH] is considered as a constant, it is incorporated into k1 (k1 = kprop[BnOH]y) as described by eqn (2). After ln kobs values were plotted against ln [TMG] (eqn (3), Fig. S6), the k1 value was calculated to be 1.15 M−0.63min−1, and the order of [TMG] (x) is 0.63. Furthermore, various concentrations of [BnOH] (5, 10, 15, 20, and 30 mM) with the same [LA] (0.25 M) and [TMG] (5 mM) (Fig. S5) were screened. When [TMG] is regarded as a constant, it is incorporated into k2 (k2 = kprop[TMG]x) (eqn (4)). After ln kobs values were plotted against ln [BnOH] (eqn (5), Fig. S6), k2 was calculated to be 0.40 M−0.44min−1, and the order of [BnOH] (y) is 0.44. Next, kprop was calculated to be 11.56 M−1.07min−1 by averaging k1/[BnOH]0.44 and k2/[TMG]0.63. LA polymerization using [TMG] and [BnOH] followed an overall kinetic law given by eqn (6). |
−d[LA]/dt = kobs[LA] = kprop [LA][TMG]x[BnOH]y
| (1) |
|
kobs = kprop [TMG]x[BnOH]y = k1[TMG]x
| (2) |
If [BnOH] is a constant |
ln kobs = ln k1 + x ln[TMG]
| (3) |
|
kobs = kprop [TMG]x[BnOH]y = k2[BnOH]y
| (4) |
If [TMG] is a constant |
ln kobs = ln k2 + y ln[BnOH]
| (5) |
|
−d[LA]/dt = 9.91 [LA][TMG]0.42[BnOH]0.61
| (6) |
In summary of the experimental activation parameters, the LA polymerizations using TMG as a catalyst and BnOH as an initiator were investigated at 25 °C, 35 °C, 45 °C, 55 °C, and 65 °C as shown in Table S6. The Eyring plot of ln[k T−1] versus 1 T−1 shown in Fig. S8 revealed an activation enthalpy (△H‡) of 19.40 kJ mol−1 and entropy (△S‡) of −0.21 kJ mol−1 K−1. The estimated Gibbs free energy of activation of 82.0 kJ mol−1 at 298 K is relatively low.
Proposed mechanism of LA polymerization
According to the literature, DBU39 first forms a hydrogen bond with the alcohol, and the hydrogen on this hydrogen bond then forms another hydrogen bond with the carbonyl group of the monomer. Subsequently, the oxygen atom of the alcohol attacks the carbonyl group of the monomer, leading to the formation of a four-membered acetal ring. TBD40 forms a hydrogen bond between the hydrogen atom on its nitrogen and the carbonyl group of the monomer, while another nitrogen atom forms a hydrogen bond with the alcohol. Subsequently, the oxygen atom of the alcohol attacks the carbonyl group of the monomer, resulting in the formation of a six-membered acetal ring. Herein, the study of the interaction between TMG and BnOH was investigated and is shown in Fig. S18. The proton of the methylene group of BnOH shifted to 4.65 ppm (singlet) from 4.68 ppm (doublet, J = 4.0 Hz), and the proton on the nitrogen atom of TMG disappeared. This indicates the formation of a hydrogen bond between BnOH and TMG. Inspired by the literature, the possible mechanism of LA polymerization using TMG as a catalyst is illustrated in Fig. 3. In the beginning, TMG interacts with BnOH through the N⋯H–O hydrogen bond to be RO−TMG+. Then, LA enters to form the reactant complex (RO−TMG+–LA) through another N–H⋯O bond. During the following nucleophilic addition of BnOH to LA, TMG deprotonates BnOH to increase the BnO− nucleophilicity, forming a charge-separated intermediate INT. This unstable charge-separated state “INT” rapidly transforms into a more stable complex (TMG+–LA–OR). Then, the LA ring's ether oxygen interacts with N–H bond's terminal proton to form a new six-membered ring ROLA− = TMG+. Because of the activation of the ether oxygen of the LA ring through the interaction of the Oether–H–N bond, the LA ring opens to afford a new alcohol that will interact with TMG for the next LA ring opening. According to the high KIE value (kBnOH/kBnOD = 22.21 from entry 1 vs. entry 3 in Table 1) for the LA polymerization, the H–OBn bond dissociation/association is likely involved in intermediate INT.
Poly(hexane-1,6-diol carbonate) synthesis from hexane-1,6-diol and dimethyl carbonate polymerization
Poly(hexane-1,6-diol carbonate) is an aliphatic polycarbonate synthesized from 1,6-hexanediol and carbonates such as dimethyl carbonate (DMC) or diphenyl carbonate (DPC) via polycondensation. It functions as the soft segment in thermoplastic poly(carbonate-urethane) elastomers, imparting elasticity and improving durability. This diol contributes to enhanced stability against hydrolysis and oxidation, making it suitable for use in flexible and long-lasting materials like adhesives, coatings, and elastomers. Poly(hexane-1,6-diol carbonate)42 is a valuable component in the synthesis of high-performance polyurethane materials, offering a balance of flexibility and durability.
Based on TMG's strong ability to activate the carbonyl group for esterification, the synthesis of poly(hexane-1,6-diol carbonate) using TMG as a catalyst was investigated. Initially, hexane-1,6-diol and dimethyl carbonate (1
:
1 ratio) are mixed, and then TMG is added as a catalyst at 110 °C in a round-bottom flask equipped with a Dean–Stark apparatus to remove the resulting methanol. However, only hexane-1,6-diyl dimethyl bis(carbonate) (HDMC) was produced after the reaction was refluxed after a day, and no polymer was produced. To improve the polymerization yield, the new polymerization method (Fig. 4) was investigated. The mixture of HDMC (10.00 g, 42.73 mmol), hexane-1,6-diol (2.00 g, 16.69 mmol), and TMG (0.05 g, 0.43 mmol) was set at 110 °C, and the produced methanol was removed by a Dean–Stark apparatus after 2 h. Consumption of hexane-1,6-diol was monitored by 1H NMR and 2.00 g of hexane-1,6-diol was employed. After repeatedly adding hexane-1,6-diol for the fifth time, the liquid in the round-bottom solidified and the magnet could no longer stir. The produced poly(hexane-1,6-diol carbonate) was purified through extraction [H2O/dichloromethane (100 mL/100 mL) × 3] as a white wax-like solid (yield = 90% with MnGPC = 7000 and Đ = 1.8) and confirmed by 1H and 13C NMR (Fig. S17). When a mixture of hexane-1,6-diol and dimethyl carbonate was heated at 110 °C without TMG for one day, hexane-1,6-diyl dimethyl bis(carbonate) was not formed. Furthermore, combining hexane-1,6-diyl dimethyl bis(carbonate) with hexane-1,6-diol in the absence of TMG and heating at 110 °C for one day did not result in any polymerization. Compared to reaction conditions employing metal salt catalysts reported in the literature,42b,43 this method using TMG as a catalyst not only operates at a lower polymerization temperature but also requires a shorter polymerization time.
 |
| Fig. 4 Synthesis of poly(hexane-1,6-diol carbonate). | |
Depolymerization of polylactide, poly-ε-caprolactone, polyethylene terephthalate, and poly(bisphenol A carbonate) using TMG as a catalyst
TMG was systematically evaluated for its efficacy in the alcoholysis of PLA, PCL, PVL, PET, and PBAC in a sealed tube, and the alcoholysis of these polymers is illustrated in Fig. 5. The product of the alcoholysis of PLA in methanol is methyl lactate, and the product of the hydrolysis of PLA is lactic acid (Fig. 5A). The products of the alcoholysis of PCL and PVL in methanol are methyl 6-hydroxyhexanoate and methyl 5-hydroxypentanoate, respectively, and the products of the hydrolysis of PCL and PVL are 6-hydroxyhexanoic acid and 5-hydroxypentanoic acid (Fig. 5B). The alcoholysis of PET using methanol and ethylene glycol yields dimethyl terephthalate and bis(2-hydroxyethyl) terephthalate, respectively (Fig. 5C). Since the hydrolysis of PET produces terephthalic acid, which is insoluble in organic solvents and thus difficult to purify, it was not studied further. The products of the alcoholysis of PBAC in methanol are dimethyl carbonate and bisphenol A (BPA). The alcoholysis of PBAC using ethylene glycol yields ethylene carbonate and BPA. The products of the hydrolysis of PBAC are carbonic acid and BPA. The results revealed that TMG can depolymerize five polymers in methanol. The degradation rates of these five polymers are as follows: PBAC > PLA > PET > PCL > PVL.
 |
| Fig. 5 Alcoholysis and hydrolysis of PLA, PCL, PVL, PET, and PBAC. | |
The comparative results of depolymerization using various organocatalysts were investigated as summarized in Table 5. The results revealed that three commercially available organocatalysts have their depolymerization advantages for different polymers. The results of PLA depolymerization (entry 1 of Table 4) show that DBU has an absolute advantage with only 3 minutes to complete PLA depolymerization compared to TBD (18 h) and TMG (80 min). Similar results also occurred in PVL depolymerization (entry 2 of Table 4, 30 min for DBU, 12 h for TBD, and 10 h for TMG). However, TMG exhibited its absolute advantage over PCL, PET, and PBAC depolymerization in that PCL and PET can only be degraded by TMG. Furthermore, the short depolymerization time of PBAC only needed 30 min using TMG as a catalyst compared to that of DBU (165 min) and TBD (inactive). The depolymerization results revealed that O−TMG+ (Fig. 3) was effective in activating the carbonyl group of various polymers.
Table 4 Depolymerization of PLA, PCL, PVL, PET, and PBAC using TMG as a catalysta
Entry |
Polymer |
Reaction condition |
Timeg (min) |
Reactions were performed in a closed 20 mL sealed tube with TMG (0.1 mmol) at 60 °C. Depolymerization conversion was identified through 1H NMR analysis. Time refers to the complete depolymerization and makes the polymer unobservable. 2 mL of solvents were used for the alcoholysis of methanol, ethanol, water, and ethylene glycol. 0.72 g of PLA was used. 0.57 g of PCL was used. 0.5 g of PVL was used. 0.90 g of PET was used. 1.3 g of PBAC was used. Depolymerization time with 100% conversion. |
1b |
PLA (Mn = 11 000 kg mol−1) |
Alcoholysis with MeOH |
80 |
2b |
|
Hydrolysis |
420 |
3c |
PCL (Mn = 4000 kg mol−1) |
Alcoholysis with MeOH |
540 (91%) |
4d |
PVL (Mn = 24 900 kg mol−1) |
Alcoholysis with MeOH |
600 |
5e |
PET (Mn = 24 000 kg mol−1) |
Alcoholysis with MeOH |
180 |
6e |
|
Alcoholysis with ethylene glycol |
270 |
7f |
PBAC (Mn = 17 500 kg mol−1) |
Alcoholysis with MeOH |
30 |
8f |
|
Alcoholysis with ethylene glycol |
60 |
9f |
|
Hydrolysis |
180 |
Table 5 Depolymerization of PLA, PCL, PVL, PET, and PBAC using various organocatalystsa
Entry |
Polymer |
Depolymerization time with 100% conversion |
DBU |
TBD |
TMG |
Reaction conditions: The polymers (5 mmol), solvent (50 mmol), and catalyst (0.1 mmol) were taken in a sealed tube and heated at 60 °C. Depolymerization was not obtained after 2 d. |
1 |
PLA |
3 min |
18 h |
80 min |
2 |
PCL |
—b |
—b |
9 h (91%) |
3 |
PVL |
30 min |
12 h |
10 h |
4 |
PET |
—b |
—b |
3 h |
5 |
PBAC |
185 min |
—b |
30 min |
Conclusions
Commercially available TMG exhibited great catalytic activity for highly controlled LA ROP in toluene at 25 °C and 110 °C under neat conditions. In addition, various alcohols, such as PEG, could be used as initiators for LA ROP. Poly(hexane-1,6-diol carbonate) also could be produced from hexane-1,6-diyl dimethyl bis(carbonate) and hexane-1,6-diol by using TMG as a catalyst. Moreover, TMG could depolymerize PLA, PCL, PVL, PET, and PBAC in alcohol and water to generate alcoholysis and hydrolysis products. Compared to DBU and TBD, TMG exhibited an absolute advantage over PCL, PET and PBAC depolymerization. Faced with society's current need for many polyesters and polycarbonates, TMG may satisfy roles as a catalyst to promote polymerization and as an agent to effectively degrade these polymers.
Author contributions
R. K., T.-Y. L., F. H., and M. R. T. performed the experiments; T. B. H. performed additional edits and proofreading; H.-C. T and S. D. performed NMR measurements; and H.-Y. C. conceptualized the project, wrote and edited the manuscript. All authors have read and agreed to the published version of the manuscript.
Conflicts of interest
There are no conflicts to declare.
Data availability
Supplementary information: Kinetic and experimental data supporting this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5su00245a.
Acknowledgements
This study was supported by the National Science and Technology Council of Taiwan (Grant NSTC 112-2113-M-037-023, 111-2314-B-037-094-MY3, and 110-2113-M-037-017) and Kaohsiung Medical University “NSYSU-KMU JOINT RESEARCH PROJECT” (NSYSU-KMU-113-P24, KMU-DK109004, and KMU-TB114009). We thank the Center for Research Resources and Development at Kaohsiung Medical University for instrumentation, equipment support, and providing storage resources.
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