One-pot synthesis of high-molecular-weight aliphatic polycarbonates via melt transesterification of diphenyl carbonate and diols using Zn(OAc)₂ as a catalyst

Abstract

A simple and efficient route for one-pot synthesis of high-molecular-weight aliphatic polycarbonates (APCs) by direct melt transesterification of diphenyl carbonate (DPC) with aliphatic diols at equimolar amounts was developed. Zn(OAc)₂ was found to be the best catalyst for this reaction among the screened transition metal acetates. The effects of reaction conditions on M_w and yield of the poly(1,4-butylene carbonate) (PBC) were investigated, where the highest M_w of 156 200 g mol⁻¹ with a yield of 83% was obtained under suitable reaction conditions. In addition, based on the results of thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR), a plausible reaction mechanism over Zn(OAc)₂ was proposed for APC synthesis.

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1. Introduction

Aliphatic polycarbonates (APCs) as important biodegradable polymers have attracted much attention for their excellent biodegradability, biocompatibility and non-toxicity in recent decades.^1,2 Especially, the high-molecular-weight APCs possess high tensile strength and elongation at break, which are sufficient for the mechanical requirements of biodegradable plastics to solve the problem of white pollution.^3–5 Metal-catalyzed copolymerization of CO₂ with epoxides is a well known approach to obtain such degradable polymers. Among all the catalysts reported, the rare-earth ternary coordination catalysts has been applied in the APCs production in plant since 2004. However, production and application of APCs on a large scale by this method are still limited by high cost and poor molecular structure.^7,8 Additionally, ring-opening polymerization (ROP) of cyclic carbonates has also received significantly attention for synthesis of APCs. Nevertheless, this route was primarily studied for small-scale biomedical applications due to the high cost of feedstocks.⁹

Besides of CO₂/epoxides copolymerisation and ROP discussed above, the transesterification of dialkyl carbonates and aliphatic diols to prepare oligomers in bulk, followed by polycondensation of these oligomers to produce high molecular weight APCs, is a promising route for practical application due to the high efficiency, without any solvent and advantage of synthesizing APCs with diverse structures.¹⁰ Since the pioneering work of Carothers and Natta in 1930,¹¹ catalysts, including enzyme,^12,13 metal oxides,¹⁴ inorganic metal salts¹⁵ and organic base^10,16,17 have been developed. Unfortunately, the molecular weight of these APCs was too low to be used directly as biodegradable plastics. Most recently, Zhu et al.⁵ successfully prepared a PBC with maximal M_w value of 176 000 g mol⁻¹ via condensation polymerization of dimethyl carbonate (DMC) and 1,4-butanediol (BD), catalyzed by a TiO₂/SiO₂-based catalyst. Later, Park and coworkers¹⁸ found CH₃ONa was also highly active in this reaction, producing PBC polymer with M_w of 100 000–200 000 g mol⁻¹, and its yield reached 86%.

However, the drawback of the above systems was two-step route necessary, because the first-step transesterification reaction between DMC and aliphatic diols can not carry out at the same conditions with the second step polycondensation reaction for the low boiling point of DMC. Even so, the reaction temperature for first step is also much higher than its boiling point, and excess DMC was required due to evaporation, generally more than 2. Additionally, the most key factor for producing high-molecular-weight APCs in this strategy is that the obtained oligomers in the first step must bear almost equal numbers of hydroxyl and methyl carbonate end groups. Thus, the loss of DMC make it difficult to control the desired molar ratio of end-groups precisely, especially in large scale production industrially. Moreover, the presence of methanol, coproduct, can form an azeotrope with DMC, which is disadvantageous to the separation of the byproduct and the shift of reaction equilibrium. In fact, DPC rather than DMC was usually used to commercially produce aromatic polycarbonates (PC) on a large scale due to its high boiling point.¹⁹ Yamamoto et al.²⁰ also found that high-molecular-weight PBC with M_w of 119 000 g mol⁻¹ could also be prepared using equal moles of DPC and BD as feedstocks, though ROP was used in this literature. Obviously, using DPC as feedstock has a distinct advantage over DMC not only in atom-economical but also in the separation and cyclic utilization of co-product.

Transition metal acetates with excellent lewis acid property have been thought to be highly effective catalysts in the reaction of transesterification, carbonylation, glycolysis and so on.^21–26 Simultaneously, divalent transition metal ion with d¹⁰ electrons could coordinate with many oxygen containing functional groups to reach the stable construction of 18 electrons. Thus, they are also considered to be good initiators or catalysts for copolymer synthesis. Previous literature had already found transition metal acetates were highly active for polyethylene terephthalate (PET) production via melt transesterification of dimethyl terephthalate (DMT) and glycol.²⁷ Recently, Cu(OAc)₂ and Zn(OAc)₂ were also found to be able to active cyclic ester monomers and lactide in ROP process to synthesis high-molecular-weight polylactones.^28,29 However, there are few reports involving the application of these compounds as catalysts in APCs synthesis. In this work, we present our results on use of transition metal acetates as catalysts for directly one-pot bulk transesterification of DPC and aliphatic diols at equimolar amounts to one-pot produce high-molecular-weight APCs. Here, the co-product, phenol, can be separated easily and reused in the DPC synthesis via transesterification with DMC or oxidative carbonylation.²⁰ The reaction parameters such as catalyst concentration, reaction temperature and time on the M_w, polydispersity index (PDI) and yield of PBC were investigated in detail to obtain the optimum conditions. Furthermore, a plausible reaction mechanism was proposed based on the results of XRD, TG-DTA, SEM and FT-IR.

2. Experimental section

2.1 Materials and reagents

Transition metal acetates obtained from Chengdu Kelong Chemical Reagent Co., Ltd., (in China) were used without treatment, unless otherwise note. Commercial DPC purchased from Guangdong Guanghua Scitech Co., Ltd., China was purification by recrystallization in absolute ethyl alcohol. All the aliphatic diols including 1,3-propanediol (PPD, 98%), 1,4-butanediol (BD, 98%), 1,5-pentanediol (PD, 98%) and 1,6-hexanediol (HD, 98%) were dehydrated by distillation over calcium hydride under dry nitrogen gas. Other reagents were of guaranteed grade and used as received.

2.2 General procedure for the synthesis of APCs

The APCs polymer samples were synthesized by one-pot melt transesterification of DPC and diols, as shown in Scheme 1. A typical procedure for synthesis of PBC polymer via this route was performed as follows. The mixtures of DPC (21.41 g, 0.1 mol), BD (9.01 g, 0.1 mol) and stoichiometric ratio of catalyst under a nitrogen atmosphere was placed in a 150 ml three-necked flask, equipped with a mechanical stirrer, reflux condenser and the thermometer. The reaction mixture was heated at reaction temperature under stirring for a certain time until it became homogeneous. Then a lower pressure (ca. 200 Pa) was applied slowly over a period of ca. 20 min to avoid excessive foaming and to carry out the melt transesterification reaction. And the volatile by-products can be removed through the reflux condenser. The PBC polymer was separated by dissolving in CH₂Cl₂ and precipitating with ethanol, and then dried under vacuum 50 °C for 12 h. Others APCs including poly(trimethylene carbonate) (PTMC), poly(pentamethylene carbonate) (PPMC) and poly(hexamethylene carbonate) (PHC) were also synthesized following the similar procedure using corresponding PPD, PD and HD as reactants.

Scheme 1 Preparation of high-molecular-weight APCs via one-pot melt transesterification of DPC and diols.

2.3 Characterization of PBC

The chemical structures of resulted APCs were identified by ¹H-NMR. ¹H-NMR spectra were acquired in CDCl₃ at 25 °C with a Bruker DRX-300 NMR spectrometer. The M_w and PDI of the obtained APCs were determined by gel permeation chromatography (GPC). The GPC measurements were carried out at 30 °C on Waters 515 HPLC system equipped with a 2690D separation module and a 2410 refractive index deter. Tetrahydrofuran (THF) was used as eluent at a flow rate of 0.5 ml min⁻¹. Polystyrene with a narrow molecular weight distribution were used as standards for calibration.

2.4 Characterization of catalysts

XRD analysis was carried out on a PANalytical X'pert pro diffractometer system, using CoKα radiation (0.1789 nm). TGA experiments were carried out using Q600 SDT thermal analysis machine (TA instrument, USA) under a flow of N₂ in the temperature from 50 °C to 550 °C with a heating rate of 10°C min⁻¹. The morphology of the catalysts was investigated by means of a scanning electron microscope (Inspect F, FEI/Holland). FT-IR spectra were obtained in the range of 400–4000 cm⁻¹ on a Nicolet-38 FT-IR spectrometer by using KBr pellet technique.

3. Results and discussions

3.1 Catalyst screening

Table 1 shows the catalytic activities of different transition metal acetates for the melt transesterification of DPC and BD at given conditions. No fraction and polymer was detected during the reaction for free-catalyst system (entry 1 in Table 1), implying that catalyst is essential for the melt transesterification process. Among the transition metal acetates, the activity of Zn(OAc)₂ is the highest at the given conditions, the M_w of PBC is 93 700 g mol⁻¹ with yield of 85.4%. One also can see that no obvious difference in M_w and yield for Zn(OAc)₂·2H₂O and Zn(OAc)₂, indicating that crystal water has no effect on the PBC synthesis in this process. This is possibly because the temperature of Zn(OAc)₂·2H₂O decompose to Zn(OAc)₂ is much lower than that of the reaction proceeded. Compared the result of entry 3 to entry 4, there is also no obvious difference in M_w and yield between the two PBC samples synthesized via two-step route and one-pot process, which would promote researchers to choose a simpler and more convenient alternative. Fig. S1† shows PBC obtained over Zn(OAc)₂ and Zn(OAc)₂·2H₂O special white, but other samples exhibit more or less dark colour. Additionally, some typical catalysts including CH₃ONa,^3,18 NaOH³⁰ and TiO₂/SiO₂ (PVP)^5,6 also gave rather poor catalytic performance in this system under identical conditions, though they have been reported to be excellent catalysts in condensation polymerization of DMC and diols in two-step process. Zn(OAc)₂ is known to be a green catalyst and has been widely used in fine chemicals synthesis, due to its inexpensive and non-toxic. Therefore, further investigation were carried out using Zn(OAc)₂ as catalyst.

Table 1 The catalytic performance of various transition metal acetates for directly melt transesterification of BD and DPC^a

Entry	Catalysts	Mw (×10^−3 g mol^−1)	Mw/Mn	YPBC^d (%)
a Reaction were carried out at 200 °C and 200 Pa for 120 min and the molar ratio of catalyst to DPC was set at 0.2 mol%.b Reaction time is 15 min (entry 3).c Two-step route was adopted. The mixtures of feedstocks and catalyst in a reactor of 200 °C for 2.0 h under nitrogen atmosphere, while removing phenol at atmosphere pressure, in the first step, then at 200 °C successively under 200 Pa for 2.0 h, in the second step.d Yield expressed as a percentage of the theoretical value which was calculated based on the 100% conversion of DPC to PBC.
1	None	—	—	—
2	Zn(OAc)2	93.7	1.71	85.4
3^b	Zn(OAc)2	9.4	1.53	95.2
4^c	Zn(OAc)2	92.8	1.69	86.8
5	Zn(OAc)2·2H2O	91.8	1.67	84.3
6	Co(OAc)2	39.1	1.77	93.5
7	Cd(OAc)2	36.4	1.63	88.6
8	Mn(OAc)2	56.0	1.76	84.2
9	Cu(OAc)2	33.6	1.61	89.3
10	Pb(OAc)2	42.5	1.73	92.5
11	CH3ONa	50.7	1.56	85.2
12	NaOH	—	—	—
13	TSP-44	41.4	1.45	89.6

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3.2 Characterization of poly(butylene carbonate)

The ¹H-NMR spectrum of the obtained PBC samples in CHCl₃ is displayed in Fig. 1. Two proton signals are observed at 1.74 and 4.12, with equal integration values, which are assigned to middle protons of butylene (–CH₂CH₂O(CO)O–) and protons adjacent (–CH₂O(CO)O–) to carbonate, respectively. For Fig. 1(a), the methylene (–CH₂OH) adjacent to end hydroxyl group signals was observed at 3.64 ppm as triplet, and the phenyl (–C(O)OC₆H₅) signal was split into three peaks in range of 7.33–7.38 ppm. From the relative intensities of these signals, the proton ratio of [–CH₂OH] to [–C(O)OC₆H₅] is approximately 2 : 5, indicating that oligomers bearing almost equal numbers of phenoxy [–OC₆H₅] and hydroxyl [–OH] end-groups were obtained during the reaction. Thus, we infer that the reaction occurred mostly with the elimination of phenol upon the reaction between the [–C(O)OC₆H₅] and [–OH] end groups. As for the ¹HNMR spectrum of the generated PBC copolymer (see Fig. 1(b)), besides of the two proton signals at 1.74 and 4.12, and no end-group signals was detected. Additionally, no signal at 3.4–3.5 ppm was observed for both ¹H-NMR spectra of PBC samples, indicating the absence of any ether linkages (–CH₂–O–CH₂–) in the polymers, implying without decarboxylation during the melt polycondensation under given conditions.⁵ The FT-IR spectrum of PBC is presented in Fig. S2.† The absorption bands appearing 2963 cm⁻¹ and 2875 cm⁻¹ are attributed to asymmetric and symmetric C–H stretching vibration of methylene, respectively. The strong absorption bands at 1744 cm⁻¹ and 1249 cm⁻¹ can be ascribed to the stretching and asymmetric stretching vibrations of C=O and O–C–O of the carbonate backbone, respectively.⁵ This indicated the ester group of the PBC copolymer.

Fig. 1 ¹H-NMR spectra of (a) PBC (entry 3, Table 1) and of (b) PBC (entry 2, Table 1) polymer obtained via one-pot melt transesterification route.

3.3 Optimization of the reaction conditions

In order to optimize reaction conditions, the influence of various reaction parameters was investigated. The impact of the catalyst concentration charged on the PBC synthesis was first examined varying from 0.025 to 0.2 mol% (the molar ratio of Zn(OAc)₂ to DPC). As seen in Fig. 2(a), the M_w of PBC increased with increasing catalyst concentration from 0.025 to 0.1 mol%, then decreased with further increase the amount of catalyst to 0.2 mol%, which suggested that 0.1 mol% catalyst might be sufficient for this process. Additionally, one can see in Fig. 2(b) with increasing catalyst concentration and reaction time the yield of PBC decreased, which at least lies in the following two factors. One reason is elimination of phenol during the polymerization reaction. Generally, the higher the M_w, the more the phenol removed. The other reason is the decomposition of resulting PBC because the same catalyst also can promote the reverse reaction. There would be completion between the effect of polymerization reaction and that of decomposition or depolymerization reaction. Therefore, as long as the former plays a more dominant role at proper conditions, the molecular weight will increase while its yield decrease all the time. As for the correlation of PDI and catalyst concentration was shown in Fig. 2(c), it seems to depend mainly on the M_w of PBC, which gradually increases with increasing M_w. Similar phenomenons on PDI during enzymes-catalyzed synthesis routes were also observed in literature.³¹

Fig. 2 Effect of catalyst concentration on the melt transesterification of DPC and BD (reaction conditions: DPC/BD = 1 : 1, temperature 200 °C and pressure of 200 Pa).

The effect of reaction temperature and time on the catalytic performance were also studied. One can see in Fig. 3(a), after reaction at low temperature of 180 °C for 2.0 h, the M_w of PBC was only 60 500 g mol⁻¹. When the reaction time was prolonged to 5.0, the M_w only increased slightly to 83 300 g mol⁻¹. However, it significantly increased to 113 500 g mol⁻¹ by raising the temperature to 200 °C for 2.0 h, indicating that high reaction temperature can enhance the rate of polycondensation. Further, when the reaction was carried out at 210 °C, the M_w were obviously higher than that at 200 °C, and the maximum M_w of 152 600 g mol⁻¹ was obtained within 3.0 h. Thus, it could be confirmed that the high temperature has a positive effect on promoting this reaction. This can be explained by the fact that higher temperature would reduce the viscosity upon stirring at high shear rates during the synthesis, which might expedite the diffusion-limited polycondensation kinetics,^30,32 consequently achieving higher M_w more easily than in lower temperature. Unfortunately, PBC is unstable at higher temperature due to its depolymerization in the presence of catalyst. Meanwhile, too high temperature over a longer time also can led to the consistently decrease of yield and broadening for PDI, as shown in Fig. 3b and c. This may be reasoned that too high temperature could promote the side reaction and thermal degradation of resultant polymer. Therefore, considering M_w and yield comprehensively, 210 °C and 3.0 h were the proper reaction temperature and time for this process.

Fig. 3 Effect of reaction temperature on the melt transesterification of DPC and BD (reaction conditions: DPC/BD = 1 : 1; catalyst concentration 0.1 mol% and pressure of 200 Pa).

3.4 Substrate scope

A series of diols substrate were also examined for the synthesis of corresponding APCs in the presence of Zn(OAc)₂ with the concentration of 0.1 mol% at 210 °C for 3.0 h via this one-step process. The results are summarized in Table 2. Except for PPD, the Zn(OAc)₂ was found to be application to a variety aliphatic diols, providing the corresponding APCs with high M_w values in high yields. According to literature,³³ relatively low activity of PPD can be attributed to the poor thermal stability and side reaction of corresponding PTMC.

Table 2 One-pot synthesis of APCs via melt transesterification of DPC with various diols using Zn(OAc)₂ as catalyst^a

Entry	Substrate	Product	Mw (×10^−3 g mol^−1)	PDI	Yield (%)
a Reaction were carried out at 210 °C and 200 Pa for 3.0 h and the molar ratio of catalyst to DPC was set at 0.1 mol%.b Removed catalyst was used at the same weight as entry 2.c The molar ratio of BD and HD is 1 : 1.
1	PPD	PTMC	39.5	1.43	34.3
2	BD	PBC	152.6	1.72	82.8
3^b	BD	PBC	142.5	1.71	85.5
4	PD	PPMC	148.8	1.80	89.2
5	HD	PHC	162.4	1.83	92.6
6	BD + HD^c	PBHC	134.6	1.78	86.3

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To further understand the catalytic performance of Zn(OAc)₂ in one-step process, Table 3 gives a comparison with literature data in terms of M_w value and yield under their optimization reaction for PBC synthesis via different methods. Obviously, the M_w and yield of PBC obtained in the present study are much higher than those of PBC synthesized by ROP of large cyclic carbonates. Moreover, some drawbacks also preclude large-scale preparation of PBC by ROP, such as expensive feedstocks and harmful solvents required, though ROP could be carried out at a relatively low temperature. This result is also superior to that of PBC obtained in TiO₂/SiO₂ (PVP), Na(Acac)₂ and BMIM-2-CO₂ catalysis systems. Although the present maximum of 156 200 g mol⁻¹ is evidently inferior to 248 000 g mol⁻¹ that was reported to be the highest date currently, the two-step was necessary in the later reaction. Moreover, this M_w value for the present PBC copolymer is much greater than 70 000 g mol⁻¹, which are sufficient for the M_w requirements of biodegradable plastics. Therefore, one-pot melt transesterification of DPC and BD with Zn(OAc)₂ as catalyst can also considered to be an efficient route for synthesis APCs with high-molecular-weight.

Table 3 Comparison of PBC synthesis in the present study and production of that by other methods

Entry^a	Original materials (molar ratio)	Catalyst (amount)^b	Solvent	Temperature^c/°C	Time^d/h	Mw^e/g mol^−1	PDI	Yield^f/%	Ref.
a Synthetic routes: phosgene route (entry 1), ROP (entry 2–5), two-step process (entry 6–10).b The value of catalyst amounts for entry 5,7 and 8 is mass concentration based on feedstock, entry 10 is also mass concentration based on final polymer, while the rest is molarity.c Copolymerization temperature.d Copolymerization time.e GPC value.f Calculated based on BD.
1	BD + triphosgene (3 : 1)	—	CHCl3	0	12.0	3400	1.70	43	34
2	BD + triphosgene (3 : 1)	TfOEt (0.5%)	CH2Cl2	20	4.0	86 400	1.50	29.7	34
3	BD + triphosgene (3 : 1)	Sec-BuLi (1.0%)	THF	0	1.0	51 400	1.46	26.1	35
4	BD + triphosgene (3 : 1)	HCl + Et2O (4.0%)	CH2Cl2	25	4.0	11 100	1.11	29.4	36
5	BD + DPC (1 : 1)	Lipase (50.0%)	Toluene	120	6.0	119 000	2.2	51.5	20
6	BD + DMC (2.0 : 1)	BMIM-2-CO2 (1.0%)	—	150	1.0	13 900	2.10	—	10
7	DMC/BD (2.5 : 1)	TSP-44 (0.08%)	—	200	4.0	81 300	1.60	85	37
8	DMC/BD (2.5 : 1)	TSP-44 (0.08%)	—	200	4.0	176 000	1.80	85	5
9	DMC/BD (1.6 : 1)	CH3ONa (0.2%)	—	190	8.0	248 000	1.65	86	18
10	DMC/BD (2 : 1)	Na(Acac)2 (0.02%)	—	—	—	139 000	1.76	—	4
11	DPC/BD (1 : 1)	Zn(OAc)2 (0.10%)	—	210	3.0	156 200	1.72	82	This work

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3.5 Results of catalysts characterization

From TG-DTA profiles shown in Fig. 4, one can clearly see the temperature at which phase transformation took place. A dramatic weight loss (ca. 16 wt%) due to dehydration of the Zn(OAc)₂·2H₂O occurred from 50 to 100 °C, accompanied with a strong endothermic peak centred at 95 °C. The second stage ranges from 220 to 330 °C and the weight loss is about 73% with a endothermic peak at 249 °C, which was caused to the decomposition of Zn(OAc)₂ to ZnO. The result is good in accord with previous papers.²¹ From 320 to 450 °C, some obvious exothermic or endothermic peak appeared, whereas no weight loss phenomenon can be observed. It is reported that Zn(OAc)₂ was initially converted to acetic anhydride and ZnO, and then the acetic anhydride could be decomposed into CO₂ and H₂O.³⁸ Therefore, the exothermic peak in higher temperature can be ascribed to the degradation of acetic anhydride. These results indicate that Zn(OAc)₂·2H₂O will decompose into Zn(OAc)₂ during the reaction and Zn(OAc)₂ remains its structure even after reaction at 210 °C.

Fig. 4 TG-DTA curves of Zn(OAc)₂·2H₂O.

The used catalyst was obtained by filtering the CH₂Cl₂ solution of the resulting copolymer, then washed sequentially with CH₂Cl₂ for several times, and dried in vacuo at room temperature for 24 h. Fig. 5 displays the XRD of the fresh and used catalyst. Obviously, a large structural change appeared between the two samples. The fresh catalyst was the typical crystal structure of Zn(OAc)₂ (PDF:33-1464) before the reaction, but the used catalyst can be proved to be Zn₅(OH)₈(OAc)₂·2H₂O by XRD.³⁸ In such a new zinc species, the carboxylate group coordinate to Zn²⁺ as a unidentate ligand,³⁹ which also had excellent catalytic performance to this reaction (see entry 3 in Table 2). In their SEM micrographs (Fig. 6), great difference can be observed between these two samples. It can be seen that the commercial Zn(OAc)₂ presents a more exfoliated morphology, while the used catalyst particles is built in aggregates, the particle size of the later is evidently much smaller than that of former, which may be resulted from the recrystallization and even recombination of Zn²⁺ and CH₃CCOO⁻. Thus, Zn(OAc)₂ appeared to be dissolved in the reactants as a homogeneous catalyst or precursor for this reaction.

Fig. 5 XRD patterns of (a) fresh and (b) used catalyst.

Fig. 6 SEM photographs of (a) fresh and (b) used catalyst.

3.6 Possible reaction mechanism

The interactions of reactants with catalyst were further examined using IR spectra to investigate the plausible reaction mechanism, the results are illustrated in Fig. 7. The FTIR of Zn(OAc)₂ (Fig. 7(a)) has spectral bands at 1456 and 1560 cm⁻¹ that are assigned to carboxyl stretching frequencies of asymmetry (ν_a(COO⁻)) and symmetry (ν_s(COO⁻)) for the bidentate carboxylate group to Zn²⁺,^21–24 as displayed in Scheme 1 (species A). The sample for IR analysis was prepared by mixing of BD and Zn(OAc)₂ at molar ratio of 2 : 1 then pretreatment under the conditions of the reduced pressure of 200 Pa and 200 °C for 2.0 h. As displayed in Fig. 7(c), new strong bands at 1737 cm⁻¹ and 1260 cm⁻¹ were observed, which could be assigned to the appearance of C=O and C–O stretching modes of monodentate carboxylate groups.^21,22 Simultaneously, the ν_a(COO⁻) band for shifts to 1598 cm⁻¹ and that of ν_s(COO⁻) band was found to have a red shift to 1439 cm⁻¹. This difference of ν_s(COO⁻) and ν_a(COO⁻) was 160 cm⁻¹, while the value for Zn(OAc)₂ was only 104 cm⁻¹. This demonstrated that the coordination mode of the Zn²⁺ and acetate groups changes from bidentate coordination to monodentate coordination, leading to the formation of a new Zn-complex.⁴⁰

Fig. 7 FT-IR spectra of (a) Zn(OAc)₂; (b) mixtures of Zn(OAc)₂ and BD at molar ratio of 1 : 2 treatment at 200 °C for 2.0 h in reduced pressure; (c) BD; (d) mixtures of Zn(OAc)₂ and DPC at molar ratio of 1 : 2 treatment at 200 °C for 2.0 h in reduced pressure and (e) DPC.

DPC did not seem to interact with Zn(OAc)₂ directly, no new characteristic bands, which were related to the activation of the carbonyl in DPC, were observed, though the mixtures of DPC and Zn(OAc)₂ were also pretreated according to the same process as the sample of BD and Zn(OAc)₂. This conclusion was also further confirmed by the fact that no change could be observed in Fig. 7(d) compared with the spectrum of DPC and Zn(OAc)₂. Using Zn(OAc)₂ as a catalyst for the reaction of 1,5-naphthalenediamine (NDA) and DMC, Xiao et al. have proposed that what activated the carbonyl of DMC was an intermediate zinc complex rather than Zn(OAc)₂.⁴⁰ The new zinc coordinated complex was the resultant of the interaction between nitrogen atoms of amino in NDA and Zn²⁺ of Zn(OAc)₂. Coordination chemistry revealed that the oxygen atoms of carbonyl and phenoxy in DPC as well as that of hydroxyl in diols molecule all had a chance to coordinate with Zn²⁺, which was usually explained by the fact that the lone electron pair of an oxygen atom can also occupy the empty d-orbital of the zinc atom in the acetate molecule to form a new zinc complex.^41,42 The optimized structures and charge analysis of reactants by Hyperchem 7.5 based on AM1 semiempirical method are displayed in Fig. S3† and their electron densities were −0.335, −0.560, −0.531, and −0.689 of negative charge, respectively. This meant that the oxygen atom of hydroxyl in 1,4-butanediol molecule was more possible than that of DPC molecular to coordinate with Zn²⁺ ion. This theoretical arithmetic results is in a good accord with the experimental result in Fig. 7. Moreover, the similar result has also been found in many literature that the presence of hydroxyl could help transform the bidentate coordinate of Zn(OAc)₂ into monodentate coordination and then facilitated the activation of carbonyl.^21,40 Therefore, based on the published reports and the results in this work, a probable reaction mechanism for one-pot melt transesterification of DPC and aliphatic diols was proposed and depicted in Scheme 2.

Scheme 2 Proposed mechanism for one-pot synthesis of APCs via melt transesterification route.

4. Conclusions

In summary, melt transesterification of DPC and various diols with equal mole quantities using Zn(OAc)₂ as catalyst is a facile and general synthesis strategy for one-pot production of APCs with M_w greater than 100 000 g mol⁻¹. The optimal condition was set as follows: the catalyst concentration of 0.1 mol%, temperature of 210 °C and reaction time of 3.0 h. Compared with using DMC as feedstock in two-step process, using DPC as a raw material is operatively simple and has much higher atomic economy. XRD and FT-IR revealed that BD was activated by Zn²⁺ via the coordination of oxygen atom in hydroxyl group.

Acknowledgements

This work was supported by the National Science and Technology Ministry of China (2013BAC11B05) and Key Research and Innovation Program of Jiangsu Province (Grant No. BE2015055).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18275a

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