Synthesis of Aliphatic Polycarbonates from Diphenyl Carbonate and Diols over Zinc (II) Acetylacetonate

APCs (aliphatic polycarbonates) are one of the most important types of biodegradable polymers and widely used in the fields of solid electrolyte, biological medicine and biodegradable plastics. Zinc-based catalysts have the advantages of being low cost, being non-toxic, having high activity, and having excellent environmental and biological compatibility. Zinc (II) acetylacetonate (Zn(Acac)2) was first reported as a highly effective catalyst for the melt transesterification of biphenyl carbonate with 1,4-butanediol to synthesize poly(1,4-butylene carbonate)(PBC). It was found that the weight-average molecular weight of PBC derived from Zn(Acac)2 could achieve 143,500 g/mol with a yield of 85.6% under suitable reaction conditions. The Lewis acidity and steric hindrance of Zn2+ could obviously affect the catalytic performance of Zn-based catalysts for this reaction. The main reasons for the Zn(Acac)2 catalyst displaying a higher yield and Mw than other zinc-based catalysts should be ascribed to the presence of the interaction between acetylacetone ligand and Zn2+, which can provide this melt transesterification reaction with the appropriate Lewis acidity as well as the steric hindrance.


Introduction
As an environment-friendly polymer, aliphatic polycarbonates (APCs) have attracted much attention in recent years for their widespread application in the fields of waterborne polyurethane, solid electrolytes and biological medicine; with a number-average molecular weight (M w ) greater than 70 Kg/mol, they can also partly replace polyethylene as biodegradable plastics to solve the problem of white pollution [1][2][3][4]. Therefore, the effective synthesis of APCs with a high molecular weight has become more and more important. The current reported routes for APC synthesis mainly include copolymerization of CO 2 with epoxides or diols [5,6], ring-opening polymerization of cyclic carbonates (ROP) [7] and condensation polymerization of dialkyl carbonate with aliphatic diols [8]. Moreover, both of the latter processes can also be considered as indirect utilization of CO 2 . Particularly, melt transesterification of dialkyl carbonate with aliphatic diols has established a bridge from CO 2 to APCs, as dimethyl carbonate (DMC) and diphenyl carbonate (DPC) can be produced from CO 2 and methanol or phenol [9].
Recently, the melt transesterification of dialkyl carbonate with diols to prepare highmolecular-weight polycarbonates has received increasing attention for synthesizing polymers with diverse structure and few catalysts. It is well known that the activity of a catalyst for this process is closely related to its acidity and chelating ability of metal specie. Therefore, numerous transition metal compounds were found to be highly active catalysts for this process, such as organotin oxides [10], titanium compounds [3,11] and even simple metal salts [12,13]. As highly effective polymerization catalysts, many zinc salts have been extensively used in the synthesis of various polymers due to their characteristics of being low cost, Molecules 2022, 27, 8958 2 of 8 nontoxic and biofriendly [14][15][16]. In a previous paper, the Zn 2+ cation was found to be able to activate the alcoholic hydroxyl groups by coordinating the oxygen atom in the hydroxyl group and inducing the transesterification reaction [13]. Pastusiak et al. [17] found that Zinc(II) acetylacetonate (Zn(Acac) 2 ) could initiate the ring-opening polymerization of cyclic trimethylene carbonate resulting in high-molecular-weight poly(trimethylenecarbonate). Zn(Acac) 2 was also found to be an excellent catalyst for polylactic acid synthesis [18].
Inspired by these referenced works, Zn(Acac) 2 was first employed as a catalyst for the one-pot melt transesterification of DPC and aliphatic diols to synthesize high-molecularweight APCs. Using ZnCl 2 as a contrast, the reaction conditions were explored in detail with Zn(Acac) 2 as a catalyst for its excellent catalytic performance. In addition, XPS was used to characterize the catalyst structure to understand the influencing factors of zinc salt in the melt transesterification reaction of DPC with diols.

Selection of Catalysts
The catalytic performance of various zinc compounds for the melt transesterification of DPC with BD to synthesize PBC was evaluated at 180 • C and 200 Pa with a reaction time of 90 min; the M w , Ð and corresponding yield are listed in Table 1. As reported in a previous paper [13], this reaction scarcely occurred in the absence of a catalyst due to the relatively low nucleophilicity of the hydroxyl group in BD, and no reaction fraction could be observed in the blank experiment. One can also see in Table 1 that the M w of PBC obtained over Zn(Acac) 2 is 69,400 g/mol at the given conditions, with a Ð and yield of 1.64 and 93.7%, respectively. As discussed in the literature [3,19,20], the presence of by-products, including tetrahydrofuran, cyclic carbonate and other volatile oligomers, would occur during the melt transesterification of dialkyl carbonates with BD, resulting in a decrease in the PBC yield. This reaction was also performed using other zinc compounds as catalysts; ZnO, ZnCO 3 and Zn(OAc) 2 were all found to demonstrate relatively less activity, producing PBCs with a M w of 8300, 28,200 and 53,200 g/mol at the same conditions, respectively. Though the M w of PBC derived with ZnCl 2 possessed the maximum value of 82,300 g/mol, the PBC yield was the lowest among all the tested catalysts. Additionally, not all the zinc compounds were active in this reaction; for example, ZnSO 4 was almost inert with no fraction and polymer detected under the same conditions. Additionally, Ð seems to only depend on the M w of PBC, and the higher the M w value, the wider the Ð value. One can also see from Table 1 that increasing the molar ratio of Zn(Acac) 2 to DPC from 0.025% to 0.1% increases the M w of PBC from 9600 g/mol to 69,400 g/mol; continuing to increase the Zn(Acac) 2 concentration to 0.2% led to an obvious decrease in the M w and yield of PBC to 62,500 g/mol and 88.4%, respectively. Obviously, Molecules 2022, 27, 8958 3 of 8 a molar ratio of Zn 2+ to DPC of 0.1 mol% should be the suitable catalyst amount for this reaction considering the M w and yield comprehensively. Therefore, Zn(Acac) 2 was selected as the model catalyst for further research.

Characterization of Poly (Butylene Carbonate)
The resultant polymers obtained over Zn(Acac) 2 and ZnCl 2 were analyzed by FT-IR and 1 H NMR, and the results are shown in Figure 1. As observed in Figure 1a, the FT-IR spectra of the two samples are very close to each other. The absorption bands appearing at 2963 cm −1 and 2875 cm −1 are attributed to the 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 [3,13]. One can also see that all the synthesized PBC samples described herein have identical 1 H NMR spectra, and only two strong signals appearing at 4.12 and 1.73 ppm can be observed in Figure 1b for both samples, which are attributed to a and b protons from BD units. No remarkable feature for the end-group was detected with their chemical shift at 3.64 or 7.32-7.38 ppm, suggesting the resultant polymers bear rather high molecular weight. Additionally, no peak at 3.4-3.5 ppm can be observed in the 1 H-NMR spectrum of the PBC polymer, indicating there is no ether linage (-CH 2 -O-CH 2 -) in the PBC polymer, which is not hydrolysable and decreases the mechanical properties of the polymer [3]. Obviously, all the peaks for the two polymers are well concordant with the standard spectrum of PBC, and it is consistent with what is expected for a PBC structure [3,[11][12][13].

Characterization of Poly (Butylene Carbonate)
The resultant polymers obtained over Zn(Acac)2 and ZnCl2 were analyzed by FT-IR and 1 H NMR, and the results are shown in Figure 1. As observed in Figure 1a, the FT-IR spectra of the two samples are very close to each other. The absorption bands appearing at 2963 cm −1 and 2875 cm −1 are attributed to the 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 [3,13]. One can also see that all the synthesized PBC samples described herein have identical 1 H NMR spectra, and only two strong signals appearing at 4.12 and 1.73 ppm can be observed in Figure 1b for both samples, which are attributed to a and b protons from BD units. No remarkable feature for the endgroup was detected with their chemical shift at 3.64 or 7.32-7.38 ppm, suggesting the resultant polymers bear rather high molecular weight. Additionally, no peak at 3.4-3.5 ppm can be observed in the 1 H-NMR spectrum of the PBC polymer, indicating there is no ether linage (-CH2-O-CH2-) in the PBC polymer, which is not hydrolysable and decreases the mechanical properties of the polymer [3]. Obviously, all the peaks for the two polymers are well concordant with the standard spectrum of PBC, and it is consistent with what is expected for a PBC structure [3,[11][12][13].

XPS of Catalyst
The electronic property of Zn 2+ in ZnCl2 and Zn(Acac)2 was also further examined using XPS to understand the relationship between catalytic performance and the nature of the catalyst; the results are illustrated in Figure 2. One can see that the binding energy of Zn 2p3/2 in the Zn(Acac)2 catalyst appeared at 1021.7 eV, ascribed to the presence of bivalence of Zn(II) [21,22]. The binding energy of ZnCl2 appeared at 1022.8 eV, which was

XPS of Catalyst
The electronic property of Zn 2+ in ZnCl 2 and Zn(Acac) 2 was also further examined using XPS to understand the relationship between catalytic performance and the nature of the catalyst; the results are illustrated in Figure 2. One can see that the binding energy of Zn 2p 3/2 in the Zn(Acac) 2 catalyst appeared at 1021.7 eV, ascribed to the presence of bivalence of Zn(II) [21,22]. The binding energy of ZnCl 2 appeared at 1022.8 eV, which was higher than that of Zn(Acac) 2 . That is to say, the Lewis acid strength of ZnCl 2 is stronger than that of Zn(Acac) 2 in view of the concept for Lewis acid. higher than that of Zn(Acac)2. That is to say, the Lewis acid strength of ZnCl2 is stronger than that of Zn(Acac)2 in view of the concept for Lewis acid.

Effect of Reaction Conditions
In order to obtain the optimum conditions and further understand the relationship between the structure and catalytic performance of the catalyst, the melt transesterification of DPC and BD was performed under various reaction parameters with the ZnCl2 catalytic system as the control experiment. The effect of polymerization temperature was first examined in the range of 160-210 °C. The results, shown in Figure 3a, indicate that the Mw values of PBC over Zn(Acac)2 and ZnCl2 sharply increased when raising the polymerization temperature from 160 to 190 °C, which can often be ascribed to the acceleration of the diffusion-limited polycondensation kinetics due to the decrease in polymer viscosity at a higher temperature [23]. Then, the Mw gradually decreased as the temperature continuously increased. As for the ZnCl2 catalyst, the optimum temperature for the highest Mw values of 102,400 g/mol can be observed at 190 °C, while that for Zn(Acac)2 is 200 °C. Clearly, the Mw for ZnCl2 at a lower temperature is much higher than that of Zn(Acac)2, indicating that strong Lewis acidity seems to be a positive factor for the improvement of polymerization rate. Simultaneously, it is well known that there would be a completion effect between polymerization and decomposition during the whole process because the melt transesterification of DPC and BD is a typical reverse reaction. Therefore, pure ZnCl2 more easily expedites the decomposition and depolymerization of the obtained PBC polymer, which often can be explained by the fact that strong Lewis acidity is prone to attack the carbonyl oxygen atoms in APCs and impede the growing of the polymer chain to lower the Mw and yield [24,25]. This is also in accordance with the results shown in Figure 3b, in which the yield for the ZnCl2 catalyst sharply decreased with the rise of temperature. The excellent catalytic performance of Zn(Acac)2 may be explained by the fact that the coordination bond formed between the acetylacetone ligand and Zn 2+ not only can decrease the Lewis acidity of the central Zn 2+ but can also make Zn 2+ attack carbonyl oxygen atoms and hinder undesirable side reactions [19]. Hence, the connection of -OH and the -OC(O)OC6H5 end-group while removing the generated phenol at reduced pressure to increase the polymer molecular chain proceeded smoothly. Obviously, 200 °C should be selected as the suitable polymerization temperature for the Zn(Acac)2 catalyst considering its Mw and yield comprehensively.

Effect of Reaction Conditions
In order to obtain the optimum conditions and further understand the relationship between the structure and catalytic performance of the catalyst, the melt transesterification of DPC and BD was performed under various reaction parameters with the ZnCl 2 catalytic system as the control experiment. The effect of polymerization temperature was first examined in the range of 160-210 • C. The results, shown in Figure 3a, indicate that the M w values of PBC over Zn(Acac) 2 and ZnCl 2 sharply increased when raising the polymerization temperature from 160 to 190 • C, which can often be ascribed to the acceleration of the diffusion-limited polycondensation kinetics due to the decrease in polymer viscosity at a higher temperature [23]. Then, the M w gradually decreased as the temperature continuously increased. As for the ZnCl 2 catalyst, the optimum temperature for the highest M w values of 102,400 g/mol can be observed at 190 • C, while that for Zn(Acac) 2 is 200 • C. Clearly, the M w for ZnCl 2 at a lower temperature is much higher than that of Zn(Acac) 2 , indicating that strong Lewis acidity seems to be a positive factor for the improvement of polymerization rate. Simultaneously, it is well known that there would be a completion effect between polymerization and decomposition during the whole process because the melt transesterification of DPC and BD is a typical reverse reaction. Therefore, pure ZnCl 2 more easily expedites the decomposition and depolymerization of the obtained PBC polymer, which often can be explained by the fact that strong Lewis acidity is prone to attack the carbonyl oxygen atoms in APCs and impede the growing of the polymer chain to lower the M w and yield [24]. This is also in accordance with the results shown in Figure 3b, in which the yield for the ZnCl 2 catalyst sharply decreased with the rise of temperature. The excellent catalytic performance of Zn(Acac) 2 may be explained by the fact that the coordination bond formed between the acetylacetone ligand and Zn 2+ not only can decrease the Lewis acidity of the central Zn 2+ but can also make Zn 2+ attack carbonyl oxygen atoms and hinder undesirable side reactions [19]. Hence, the connection of -OH and the -OC(O)OC 6 H 5 end-group while removing the generated phenol at reduced pressure to increase the polymer molecular chain proceeded smoothly. Obviously, 200 • C should be selected as the suitable polymerization temperature for the Zn(Acac) 2 catalyst considering its M w and yield comprehensively.   Figure 4a, under a short reaction time of 30 min, the Mw was only 67,400 g/mol over Zn(Acac)2. As the reaction proceeded, the Mw of PBC rapidly increased to 143,500 g/mol with increasing reaction time to 120 min. However, when the time was beyond 120 min, the value for Mw showed no significant improvement. Likewise, the Mw values for ZnCl2 also increased as the reaction time was prolonged, and the Mw reached the maximum value of 122,500 g/mol at 60 min; then, the Mw of PBC would rapidly decline to 12,000 g/mol with further increases of time to 150 min. The reason for this phenomenon might be that this polymerization process very easily proceeds at the beginning, but with an increase in molecular weight, the viscosity of the reaction system becomes high, which causes a negative influence to further polymerization [13,19]. Therefore, excessive reaction time could enhance its reverse reaction and lead to the decrease in Mw. Meanwhile, one can also see in Figure 4b that the yield of PBC continuously decreases with the prolongation of reaction time, which can be reasoned by the presence of a side reaction and sublimation of oligomer as reported in previous works [20]. Considering the above results, a temperature of 200 °C and a reaction time of 120 min were selected as the optimum reaction conditions for realizing the highest Mw with satisfactory yield for Zn(Acac)2.     Figure 4a, under a short reaction time of 30 min, the M w was only 67,400 g/mol over Zn(Acac) 2 . As the reaction proceeded, the M w of PBC rapidly increased to 143,500 g/mol with increasing reaction time to 120 min. However, when the time was beyond 120 min, the value for M w showed no significant improvement. Likewise, the M w values for ZnCl 2 also increased as the reaction time was prolonged, and the M w reached the maximum value of 122,500 g/mol at 60 min; then, the M w of PBC would rapidly decline to 12,000 g/mol with further increases of time to 150 min. The reason for this phenomenon might be that this polymerization process very easily proceeds at the beginning, but with an increase in molecular weight, the viscosity of the reaction system becomes high, which causes a negative influence to further polymerization [13,19]. Therefore, excessive reaction time could enhance its reverse reaction and lead to the decrease in M w . Meanwhile, one can also see in Figure 4b that the yield of PBC continuously decreases with the prolongation of reaction time, which can be reasoned by the presence of a side reaction and sublimation of oligomer as reported in previous works [20]. Considering the above results, a temperature of 200 • C and a reaction time of 120 min were selected as the optimum reaction conditions for realizing the highest M w with satisfactory yield for Zn(Acac) 2 .  Figure 4 shows the dependence of the Mw and yield of PBC versus reaction time over different catalysts. As shown in Figure 4a, under a short reaction time of 30 min, the Mw was only 67,400 g/mol over Zn(Acac)2. As the reaction proceeded, the Mw of PBC rapidly increased to 143,500 g/mol with increasing reaction time to 120 min. However, when the time was beyond 120 min, the value for Mw showed no significant improvement. Likewise, the Mw values for ZnCl2 also increased as the reaction time was prolonged, and the Mw reached the maximum value of 122,500 g/mol at 60 min; then, the Mw of PBC would rapidly decline to 12,000 g/mol with further increases of time to 150 min. The reason for this phenomenon might be that this polymerization process very easily proceeds at the beginning, but with an increase in molecular weight, the viscosity of the reaction system becomes high, which causes a negative influence to further polymerization [13,19]. Therefore, excessive reaction time could enhance its reverse reaction and lead to the decrease in Mw. Meanwhile, one can also see in Figure 4b that the yield of PBC continuously decreases with the prolongation of reaction time, which can be reasoned by the presence of a side reaction and sublimation of oligomer as reported in previous works [20]. Considering the above results, a temperature of 200 °C and a reaction time of 120 min were selected as the optimum reaction conditions for realizing the highest Mw with satisfactory yield for Zn(Acac)2.

Catalytic Activity towards Other Diols
To evaluate the potential and general application range of the Zn(Acac) 2 catalyst, the catalytic melt transesterification of DPC with a verity of aliphatic diols, including 1,3propanediol (PPD), 1,5-pentanediol (PD) and 1,6-hexanediol (HD) to synthesize corresponding aliphatic polycarbonates, poly(trimethylene carbonate) (PTMC), poly(pentamethylene carbonate)(PPMC), poly(hexamethylene carbonate) (PHC) and poly(hexamethylene)-copoly(butylene carbonate) (PHBC) were investigated. As shown in Table 2, other common aliphatic diols, including PD, HD and their mixtures can also undergo efficient transesterification with DPC to high-molecular-weight APCs with high yields under the optimized reaction conditions. The inferior catalytic performance of PPD was thought to be related to its low boiling point or the poor thermal stability of the corresponding PTMC polymer [13].

Materials
Commercial DPC, purchased from Guanghua Scitech Co., Ltd., Shenzhen, China, was purified by recrystallization in absolute ethyl alcohol. 1,4-butanediol (BD, 98%) was dehydrated by distillation over calcium hydride under dry nitrogen gas. ZnCl 2 , Zn(OAc) 2 , Zn(NO 3 ) 2 and ZnSO 4 were obtained from Chengdu Kelong Chemical Reagent Co., Chengdu, China, and were dehydrated as described in the literature [11] before use. Other chemicals and catalysts were used without any further purification and treatment.

Synthesis of APCs
PBC polymer was synthesized by a one-pot melt polymerization method [13]. Typically, DPC (21.41 g, 0.1 mol), BD (9.01 g, 0.1 mol) and a certain amount of catalyst were charged into a 150 mL three-necked flask equipped with a mechanical stirrer, reflux condenser and thermometer. The reaction mixture was heated to 120 • C under stirring for a certain time until it became homogeneous under a nitrogen atmosphere. Then, a lower pressure (ca 200 Pa) was applied slowly over a period of ca 20 min to carry out the melt transesterification reaction at the given temperature. After a certain time, the pressure was returned to atmospheric pressure, and the volatile by-products could be removed through the reflux condenser. The resulting PBC polymer was separated by dissolving in CH 2 Cl 2 and precipitating with ethanol, and then dried under vacuum at 50 • C for 12 h.

Characterization
The chemical structures of the resulting PBCs were identified by 1 H-NMR. The 1 H-NMR spectra were acquired in CDCl 3 at 25 • C with a Bruker DRX-300 NMR (Brucker, Romanshorn, Switzerland) spectrometer. The weight-average molecular weight (M w ) and dispersity (Ð) of the obtained APCs were determined by gel permeation chromatography (GPC). The GPC measurements were carried out at 30 • C on a Waters 515 HPLC system (Waters, Milford, MA, USA) equipped with a 2690D separation module and a 2410 refractive index detector. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 0.5 mL/min. Polystyrene with a narrow molecular weight distribution was used as the standard for calibration. All the given data in this study were collected by averaging the scores on at least three tests.
Fourier transform infrared spectroscopy (FT-IR) was carried out on a Nicolet-38 FT-IR spectrometer (Thermo Electron, Boston, MA, USA) in the range of 400-4000 cm −1 using the KBr pellet technique. The TGA experiments were carried out using a Q600 SDT thermal analysis machine (TA instrument, Waltham, MA, USA) under a flow of N 2 in a temperature range from 50 • C to 550 • C with a heating rate of 10 • C/min. The binding energy values and the atomic surface concentration of the corresponding elements of the samples were analyzed by X-ray photoelectron spectroscopy and performed on an ESCA LAB 250 photoelectron spectroscope at 3.0 × 10 −10 mbar with a hemispherical analyzer and monochromatic Mg Kα radiation (E = 1253.6 eV). All the binding energies were referenced to the C1s peak at 284.5 eV of the surface adventitious carbon.

Conclusions
The catalytic properties of zinc (II) acetylacetonate in melt transesterification of diphenyl carbonate with aliphatic diols were investigated. The M w , yield and Ð of the obtained PBCs are influenced by the catalyst used, reaction temperature and time. Lewis acidity is found to be dominant for the polymerization rate at lower temperature, and increasing steric hindrance seems to give a positive effect on the improvement of yield. Therefore, the rise in M w and yield for Zn(Acac) 2 compared with ZnCl 2 is mainly due to the decrease in Lewis acidity and the increase in steric hindrance.