Scalable Synthesis of Poly(ester-co-ether) Elastomers via Direct

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 9074−9085

pubs.acs.org/journal/ascecg

Scalable Synthesis of Poly(ester-co-ether) Elastomers via Direct Catalytic Esterification of Terephthalic Acid with Highly Active Zr− Mg Catalyst Xin-Gui Li,*,†,‡,∥ Ge Song,†,‡,§ Mei-Rong Huang,*,†,‡ and Yun-Bin Xie†,‡

ACS Sustainable Chem. Eng. 2018.6:9074-9085. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/21/19. For personal use only.

†

State Key Laboratory of Pollution Control and Resource Reuse, and Shanghai Institute of Pollution Control and Ecological Security, College of Environmental Science and Engineering, and ‡Key Laboratory of Advanced Civil Engineering Materials, College of Materials Science and Engineering, Tongji University, 1239 SiPing Road, Shanghai 200092, China § Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China ∥ Key Laboratory of Theory & Technology for Micro-Nano Optoelectronic Information System of Ministry of Industry & Information Technology, College of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Guangdong 518055, China S Supporting Information *

ABSTRACT: The poly(ester-co-ether)s (PEEs) comprising poly(butylene terephthalate) and poly(tetramethylene glycol) (PTMG) segments were cost-efficiently synthesized by direct esterification of terephthalic acid (TPA) and 1,4-butandiol (BDO)/PTMG with Mn of 1000 in one feeding step by a unique nontoxic Zr−Mg catalyst that is designed and synthesized in our laboratory, avoiding undesirable eco-hazardous cocatalyst and byproducts that must be produced when terephthaloyl chloride (TPC) and dimethyl terephthalate (DMT) were chosen. The structure of the Zr− Mg catalyst and PEEs was systematically analyzed by high-resolution 1H NMR, ATR-FTIR spectroscopies, and X-ray diffraction. The size-exclusion chromatography suggested that the weight-average molecular weight of the PEEs reaches up to 60 600 g/mol. In particular, the ether-segment retention, molecular weight, melt processability, and mechanical properties of the PEEs are significantly higher by Zr−Mg catalyst than by Ti−Mg and traditional tetrabutyl titanate (TBT) catalysts. DSC and DMA analyses revealed that the PEE copolymers obtained by the Zr− Mg catalyst have random segment distribution, glass transition temperature down to −34 °C, and good elasticity and strong toughness at ambient temperature. TBT is an efficient catalyst for the polycondensation between TPC/BDO or DMT/BDO prepolymers and PTMG for synthesizing excellent PEEs, unfortunately accompanying with the usage of toxic and expensive bases and the formation of hazardous HCl or methanol byproducts. This study confirmed that TBT is not powerful enough to achieve high-molecular-weight and thus tough PEEs via the environ-benign direct polycondensation among TPA, BDO, and PTMG anymore, while the Zr−Mg catalyst developed here is active enough to catalyze the direct polycondensation without compromise or toxic discharge. Furthermore, the TPA-based PEE elastomers demonstrate potential to replace current eco-unsafe elastomers like rubbers, polyurethane, polyolefin, and TPC-/DMT-based PEEs. KEYWORDS: Green copolymer, Sustainable elastomer, Terephthalic acid, Direct polycondensation, Clean Zr−Mg catalyst, Cost-efficient production, Thermoplastic rubber, Eco-safe poly(ether-co-ester)

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INTRODUCTION Rubber and polyurethane (PU) have been widely used in various fields. Unfortunately, their processing is one of the most polluting industrial activities in the world because of their high greenhouse gas emissions and enormous amounts of toxic wastes and effluent. Most rubbers, PU, and polyolefin elastomers would persistently discharge toxic substances including HCN, toluene, monomers, catalysts, and cross-linker during usage and after discard especially at elevated temperature. The reduction of the pollution from rubber, PU, and polyolefin elastomers has become critical issues. Therefore, it still remains an urgent challenge: how to develop eco-safe elastomers. © 2018 American Chemical Society

Poly(ester-co-ether) (PEEs) have now become one of the most important thermoplastic elastomers as the thirdgeneration rubbers without toxic discharge,1 in which semicrystalline polyesters act as hard segments while amorphous polyethers act as soft segments. They combine good processability and vital recyclability of polyesters with the elasticity of elastomers.2,3 PEEs have already demonstrated much better processability, stronger toughness, higher lowtemperature flexibility, much better recyclability, better environReceived: March 29, 2018 Revised: May 13, 2018 Published: June 4, 2018 9074

DOI: 10.1021/acssuschemeng.8b01421 ACS Sustainable Chem. Eng. 2018, 6, 9074−9085

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) Synthesis procedure, (b) ATR-IR spectra, (c) 1H NMR spectra at 400 MHz (JNM-ECZ400S/L1 spectrometer), and (d) wide-angle Xray diffraction curves of the Zr−Mg catalyst and three raw materials (CP).

polymerization route that obviously limited the large-scale industrialization in the pursuit of environmentally friendly objectives. Similarly, if terephthaloyl chloride (TPC) was used, an even more sophisticated three-step polymerization route, another undesirable eco-hazardous catalyst, solvent, and chlorane byproduct, and high expense will be inevitable during (1) the solution prepolymerization between TPC and BDO using toxic and expensive bases like 1,4-diazabicyclo[2,2,2]octane (DABCO), pyrrolidine, N(C2H5)3, and pyridine in harmful dichloromethane and (2) melt polycondensation between the prepolymers and polyether. TPC itself is highly toxic, highly flammable, strongly corrosive, and heat- and moisture-sensitive. Moreover, the production of TPC and DMT is usually based on toxic SOCl2 and methanol, respectively, as raw chemicals, which are environmentally harmful as compared to terephthalic acid (TPA). TPA is a widely used monomer in polyester industry. Direct esterification of TPA is simpler than esterification and transesterification from DMT and TPC for the sustainable

mental friendliness, and higher resistances to heat, oil, weather, and UV than most of the conventional rubbers, PU, and polyolefin elastomers. In fact, PEEs have been important industrial materials with wide applicability as automotive parts, pipes, packaging, bearing, advanced gears, cable clothing, driving belt, phone parts, medical tube and bag, sustainable materials,4,5 biomaterials,6,7 consumer goods, spring, appliance and power tools, sporting goods, furniture, elevator slipway, and transportation/chemical equipment.8,9 The transesterification among aromatic diester, aliphatic diols, and polyether, especially among dimethyl terephthalate (DMT), 1,4-butandiol (BDO), and polyether, was the most frequently applied method for PEE synthesis.10−13 DMT has also been used to synthesize other PEEs like poly(trimethylene terephthalate)-poly(ethylene oxide),14 poly(butylene 2,6-naphthalate)-poly(tetramethylene glycol) (PTMG),15 and poly(ethylene-co-butylene terephthalate)-PTMG.16 Nevertheless, the DMT-based synthetic route has tough problems including inevitable toxic methanol byproduct as well as a complicated three-feeding and multistep 9075


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Figure 2. (A) Successful synthesis of poly(ester-co-ether) prepared by ZMC with the assignments of 1H NMR spectra. (B) Unsuccessful synthesis of poly(ester-co-ether) prepared by TMC and TBT. (C) 1H NMR of the PEEs with the same PTMG feeding content of 20 wt % catalyzed by ZMC, TMC, and TBT. (D) 1H NMR of poly(butylene terephthalate) (PBT), PTMG, and PEEs with 10, 20, 30, and 40 wt % PTMG (Mn 1000).

synthesis of polyesters.17 In fact, the direct esterification of TPA for synthesizing PEEs produces two byproducts including major water and minor tetrahydrofuran (THF). They can be easily separated by fractional distillation and recycled as chemical products. Yet the transesterification route of DMT produces byproducts like methanol and THF that have very close boiling points of 65.4 and 66.0 °C, respectively, and are an azeotropic mixture. Because of the azeotropic nature, the methanol−THF mixture cannot be separated by simple conventional distillation, leading to more waste discharge to the environment. Certainly, the methanol−THF mixture could be separated by heteroazeotropic distillation and pervaporation,18 but this process may be sophisticated and energyconsuming to some extent.18,19 Moreover, methanol is a highly inflammable, toxic, and volatile liquid that raises explosion to a dangerous level for manufacture. In a word, TPA is more ecobenign for the preparation of the PEEs than are DMT and TPC. Therefore, the situation would be much better if replacing DMT and TPC with TPA that is much cheaper and has more industrial capacity. However, little attention has been paid to the synthesis of PEEs from TPA to the best of our knowledge.20 One major reason is that the transesterification between ester and ether segments is insufficient if using TPA and traditional catalysts. Some investigations have been focused on new catalysts for polyester synthesis through either

DMT21,22 or TPA.23,24 On the contrary, no reports on the design of specific powerful catalysts for the synthesis of PEEs through TPA are found until today to the best of our knowledge. Here, we design Zr−Mg bimetals for cost-efficient synthesis of PEEs through TPA via a simple route just involving one feeding of three starting materials and two-step polymerization. The proposed Zr−Mg bimetals are so active that relatively inert TPA monomer can be efficiently catalyzed to achieve high molecular weight PEE, and thus a sustainable copolycondensation process could be realized. More importantly, the Zr−Mg bimetals effectively promote the esterification between inert PTMG macromonomer and TPA, and therefore the resulting PEEs have greatly improved ether segment content that is high enough to enable good elasticity even at the relatively low temperature under extreme outdoor conditions. Furthermore, the PEEs have been proved to have a good processability and outstanding mechanical properties. It is anticipated that the sustainable PEE elastomers would possibly replace eco-unsafe rubbers, PU, and polyolefin elastomers, and traditional TPC-/DMT-based three-step polymerized PEEs.

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EXPERIMENTAL SECTION

Synthesis of Zr−Mg Catalyst. A new Zr−Mg catalyst (ZMC) soluble in water and BDO was prepared using a solution route according to a detailed procedure summarized in the Supporting

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ACS Sustainable Chemistry & Engineering Table 1. Molecular Structure and Intrinsic Viscosity of Five PEEs Catalyzed by ZMC, TMC, and TBT PTMG content (wt %)

molecular weight of PEEs

poly(ester-co-ether)s

catalysts

diols/TPA molar ratio

feed

observed in the copolymer

Mn (g/mol)

Mw (g/mol)

PDI

intrinsic viscosity [η] (dL/g)

PEE20 PEE20a PEE20b PEE20c PEE20d

ZMC ZMC ZMC TMC25 TBT

1.4 1.6 1.8 1.6 1.6

20 20 20 20 20

19.3 20.3 21.5 18.8 15.2

15530 12590 9170 11530 7800

36110 26310 15640 24790 17450

2.32 2.09 1.71 2.15 2.24

0.93 0.92 1.08 1.04 1.14

Table 2. Ether Segment Content, Molecular Weight, CIE Color, and Shore D Hardness of the Poly(ester-co-ether)s with Feed PTMG (Mn 1000) Contents Ranging between 0 and 40 wt % by ZMC CIE colora of PEE chips

molecular weight of PEEs poly(ester-coether)s

PTMG content in PEEs (wt %)

Mn (g/mol)

Mw (g/mol)

PDI

intrinsic viscosity (dL/g)

L

a

b

Shore D hardness

PBT PEE10 PEE20 PEE30 PEE40 PEE3018,19

0 8.9 19.3 27.1 39.1 26.0−28.0

22040 17190 15530 18110 21330 17442

41530 40290 36110 49120 60600 NA

1.88 2.34 2.32 2.71 2.84 NA

1.03 1.01 0.93 1.44 1.55 0.85

93.45 87.5 88.3 84.4 83.8 NA

−0.41 0.06 0.35 0.90 1.56 NA

2.76 7.41 6.25 7.48 7.87 NA

66 60 58 51 47 NA

a L means lightness ranging between 0 and 100, and 0 signifies all black and 100 signifies all bright. a means red, ranging between −30 and +30. More negative means greener and more positive means redder. b means yellow, ranging between −30 and +30. More negative means bluer and more positive means more yellow.

Information.25 A nominal synthetic procedure and structure characterization of Zr−Mg catalyst are shown in Figure 1. Synthesis of Poly(ester-co-ether)s. After TPA, BDO, and PTMG were added into a 2 L reactor, ZMC, Ti−Mg catalyst (TMC), and tetrabutyl titanate (TBT) were used, respectively, as catalysts at the same Zr, Ti, and Ti ion content of 185 μmol. Enough byproduct water was distilled out at 230 °C, and then additional catalyst solution containing Zr, Ti, and Ti ions of 185 μmol, respectively, was added into the reactor again. A higher temperature to 250 °C and high vacuum down to 0.5 mbar was applied for transesterification of 2 h. A nominal polycondensation has been illustrated in Figure 2A and B. Characterization methods have been provided in the Supporting Information.

feed by direct esterification between TPA and BDO and then, more importantly, transesterification between ester segment and PTMG segment. The PEEs prepared by ZMC and TMC kept more ether segments on polymer backbones, higher molecular weights, and narrower molecular-weight distribution but lower intrinsic viscosity than by TBT. In particular, the PEEs by ZMC have the highest ether segment content on polymer backbones, highest molecular weights, and narrowest molecular-weight distribution but the lowest intrinsic viscosity among three catalysts. Apparently, the ZMC invariably provides the highest catalytic efficiency in transesterification between ester and PTMG segments. Moreover, the content of ether segments in polymer backbones gradually increases from 19.3 to 21.5 wt % with increasing feed diols/TPA molar ratio from 1.4 to 1.8, whereas the molecular weight and its polydispersity index both decrease obviously. Note that relatively more ether segments in the PEE backbones synthesized by ZMC and TMC result in lower chain rigidity and finally lower intrinsic viscosity regardless of their higher molecular weight. Figure 2C reveals the 1H NMR spectra of the PEEs prepared by ZMC together with TMC and TBT for a careful comparison. Methylene oxide hydrogen (peak f at δ 3.85) and methylene hydrogen (peak e at δ 1.87) were compared to aromatic hydrogen (peak a at δ 8.21) to determine the percentage of ether segment in the final products. Methylene oxide hydrogen (peak d at δ 4.62) at polymer backbone and methylene oxide hydrogen (peak d* at δ 4.07) at an independent polyether were used to calculate ether segment contents in copolymers listed in Table 1.26,27 In summary, the catalytic activity of the three catalysts for the synthesis of the PEEs with high-molecular weight and excellent comprehensive performance through the direct polycondensation of TPA, BDO, and PTMG ranks in an increasing order:

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RESULTS AND DISCUSSION Characterization of the Zr−Mg Catalyst. The Zr−Mg catalyst has been characterized by IR, 1H NMR, and X-ray diffraction. As compared to the three raw materials, Figure 1b− d suggests that the Zr−Mg catalyst demonstrates totally different molecular and supramolecular structure characteristics including a broad, strong, and shifted CO absorbance (3) and a very broad O−H absorbance (1), an obviously shifted resonance peak (β) of −CH2− at low field, and a new strong and a broad weak diffraction peak because of the complexation of Zr between citric acid and magnesium acetate. These results signify that a new Zr−Mg catalyst whose molecular structure is shown at the bottom of Figure 1a has been obtained. Much Improved Transesterification between Ester and Ether Segments by Zr−Mg Catalyst. TBT as a traditional efficient catalyst for the successful synthesis of PBT and PEE by DMT route8,9 has an unfavorable effect on the direct polymerization of TPA, BDO, and PTMG for the synthesis of PEEs with both high PTMG content and high molecular weight, as shown in Figure 2B. As summarized in Table 1, the PEE20d obtained by TBT contains the least ether segment in the polymer chains of all five samples. Novel Zr− Mg catalyst (ZMC) facilely results in the efficient synthesis of unique PEE with PTMG (Mn = 1000) content of 20 wt % in

TBT < < TMC < ZMC

PEEs Prepared by ZMC with Various Feeding PTMG. Higher Molecular Weight of the PEEs Containing Much Less 9077


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Figure 3. (a) Relationship between molecular weight/intrinsic viscosity and PTMG content and (b,c) ATR-FTIR spectra of poly(butylene terephthalate) and poly(ester-co-ether)s with PTMG (Mn 1000) contents of 10, 20, 30, and 40 wt %.

Figure 4. (a) Heating and cooling curves at a heating rate of 10 °C/min and cooling rate of −10 °C/min in nitrogen flow of 50 mL/min of PBT and PEEs with PTMG (Mn 1000) percentage of 10, 20, 30, and 40 wt %. (b) The storage modulus, loss modulus, and tan δ as a function of temperature for PBT and PEEs with PTMG (Mn 1000) content of 10, 20, 30, and 40 wt % at a heating rate of 3 °C/min. (c) α relaxation temperature, melting temperature, and crystallization temperature from the melts of PBT and PEEs with PTMG (Mn 1000) feed content between 10 and 40 wt %.

Unreactive PTMG. Previous studies on the PEEs synthesized by transesterification among DMT, BDO, and PTMG demonstrated that less ether segments remained on the PEE backbone than feed ones.16,28,29 The preparation of PEEs through direct esterification was essentially difficult because an excessive amount of BDO was usually used to maintain vapor−liquid equilibrium. The activation energy for hydroxyl groups on

PTMG macromonomers is higher than that for hydroxyl groups on BDO, unfortunately leading to less effective collision between TPA and PTMG. However, the creative usage of the ZMC has significantly tackled these problems and accomplished the cost-effective synthesis of the PEEs with both high PTMG esterification rate and high molecular weight. Table 2 summarized ether segment contents, molecular weight, CIE 9078


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Figure 5. (a) Stress−strain curves of injection molded specimens of PBT and PEEs with PTMG (Mn 1000) weight contents of 10, 20, 30, 40% and PEE20d prepared by TBT (☆). (b) Pictures of broken specimens of (1) PBT, (2) PEE10, (3) PEE20, (4) PEE30, (5) PEE40, and (6) PEE20d after tensile tests. (c) Systematical variation of five typical parameters of mechanical properties of PBT and the PEEs with PTMG feed content catalyzed by ZMC and TBT (☆).

peak at 1709 cm−1 is due to the stretching vibration of ester groups,30 and the weak absorbance at 874 cm−1 is due to the surface bending vibration of phenylene ring.31,32 After normalization on the basis of the peak intensity of ester groups at 1709 cm−1, semiquantitative analysis has been performed on the weak double peak to the stretching vibration of methylene groups in Figure 3c. The vibration of methylene groups corresponds to a main peak at 2960 cm−1 and a sub peak in hard segment PBT, and double peaks at 2940 and 2855 cm−1 in soft segment PTMG. With increasing PTMG ether segment content, the absorbance peak at 2960 cm−1 shifts to lower wavenumber, and the intensity of the absorbance peak at 2855 cm−1 becomes stronger. This further proves a successful chemical incorporation of PTMG segment into the PEE main chains. Excellent Elasticity in the Temperature Range from −34 to 158 °C. To systematically explore the thermal transition for the PEEs, DSC (Figure 4a) was applied to analyze the glass transition, melting, and crystallization temperatures from polymer melt. The heating curves of PEEs and PBT indicate that a single melting transition temperature from crystalline phase to liquid melt, Tm, shifted to a lower value with increasing PTMG ether segment contents. Moreover, the melting peak changed remarkably from a sharp and strong peak of PBT to a broad and weak peak of PEE40. The glass transition (Tg) or cold crystallization (Tcc) between amorphous phase and crystalline phase were not observed on the heating curves. The cooling curves of PBT and PEEs indicate that the transition temperature from melting to crystalline phase (Tmc) decreased with increasing the ether segment contents. A gradual broadening tendency of the Tmc peaks also occurred with increasing the PTMG ether segment content, but the broadening degree was weaker than the melting peaks. The glass transition temperatures of the PEEs were further examined by DMA (Figure 4b). The loss modulus (G′′) peaks from −49 to 30 °C corresponded to the α relaxation temperature (Tα). The tan δ peak from −34 to 42 °C related

color, and Shore D hardness of the PEEs prepared by the ZMC with feeding PTMG content ranging between 0 and 40 wt %. The ether segment contents in the copolymers demonstrated that the vast majority of ether segments have been connected to the polymer backbones by direct esterification from TPA, BDO, and PTMG. 1H NMR spectra in Figure 2D are very useful to confirm the successful esterification and transesterification polycondensation between TPA and BDO/ PTMG because the characteristic resonance peak d* at δ 4.07 corresponding to the methylene oxide hydrogen at independent PTMG end groups has completely disappeared. The retention rate of ether segments in the PEEs obtained from TPA by ZMC was comparable to the transesterification route based on DMT.12,13 When ether segment contents in the copolymers increased from 0 to 40 wt %, the number- and weight-average molecular weights and intrinsic viscosity all decrease first and then increase, demonstrating the minimum values at 20 wt % PTMG, as shown in Figure 3a. It is of interest that the weightaverage molecular weight dramatically rises while the numberaverage molecular weight slightly rises with increasing PTMG content from 20 to 40 wt %. The appearance of the minimal molecular weight and intrinsic viscosity in Figure 3a implies the chemical interaction (i.e., polycondensation) between TPA/ BDO ester segments and PTMG blocks. Only a monotonically linear dependency of the molecular weight and intrinsic viscosity on the PTMG content would be observed if there is no chemical interaction (i.e., polycondensation) between TPA/ BDO ester segments and PTMG blocks. Note that the PDI and CIE color a basically increase with increasing PTMG content, but CIE color L and Shore D hardness basically decrease. These strongly signify that the PTMG ether segments have been connected to the copolymer backbones by esterification. To further investigate the influence of ether segment on the molecular structure of the PEEs, the ATR-FTIR spectra of poly(butylene terephthalate) and PEEs with PTMG (Mn 1000) content of 10−40 wt % have been semiquantitatively analyzed. As shown in Figure 3b, the strongest and sharpest absorbance 9079


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segments were covalently connected on the polymer backbone. It should be noticed that the Young’s modulus of PEE20d prepared by TBT was between those of PEE10 and PEE20, which is consistent with its ether segment content on polymer backbone determined by 1H NMR in Table 1. The residual free polyether component in PEE20d makes the specimen brittle. Also, it caused an intensively yellow color of the specimen. Yield strain and elongation at break generally increased with increasing ether segment contents, while the yield stress decreased. Notice that the tensile stress exhibits nonmonotonical variation in a relatively small range. This phenomenon can be caused by its semirigid structure. Hard segment of PBT crystal made a good combination with soft segment of polyether, and its deformation pattern after 300% elongation was quite similar to the elastic deformation with strong strain hardening behavior or high toughness. Catalyzing Mechanism of Zr−Mg Catalyst toward Direct Esterification of TPA. Zirconium-based catalysts have long been applied to catalyze the esterification of carboxylic acids with alcohols.36,37 However, their application in esterification/ transesterification polymerization has not yet received enough attention. Zr ion has strong acidity,23 which establishes the basis for esterification and transesterification reactions. Note that the strongly acidic Zr ions tend to react with water to produce a stable hydrolysis precipitate,38 unfortunately losing catalytic activity. Because the hydrolysis rate of zirconate is much higher than that of titanate, zirconium compounds with chelate structures are more suitable for polyester polymerization than is zirconate.23,39 The acidity of the metal ions in the catalyst is not completely linearly related to the catalyzing activity, while there is an optimal acidity range for particular polyester species.40,41 In case of semiaromatic polyesters, the diol monomers with longer chains usually required the catalyst with higher acidity.40,41 To achieve two-step direct esterification/transesterification polycondensation between TPA and BDO/PTMG, not only the esterification must be greatly catalyzed by using novel catalysts, but also the efficiency of the transesterification reaction must be largely enhanced. In this case, the residual free PTMG can still be incorporated into the PEE chains by powerful transesterification. It was confirmed that magnesium compounds can obviously promote the transesterification reaction,39,42,43 but are not suitable alone as a catalyst for esterification/transesterification polycondensation because of the lower acidity (relatively slight basicity) of Mg ions. Zr−Mg catalyst carefully designed in this work is a zirconia and magnesia composite catalyst with proper chelating structure, which would make up for the disadvantage of zirconate catalyst that is easy to hydrolyze, while the Zr−Mg activity center with moderate acidity between Zr and Mg ions is suitable for two-step direct esterification/transesterification polycondensation between TPA and BDO/PTMG for the efficient synthesis of PEEs because of the proper interaction between oxygen in −OH or >CO in the oligomers/ unpolymerized PTMG and Zr in the Zr−Mg complex catalyst, as shown in Scheme 1.23 Al−Mg, Zn−Mg, Zr−Mg, and Zr− Al−Zn catalysts have been proved to be efficient in transesterification reaction of soybean oil/methanol, tributyrin/ methanol, and jatropha oil/methanol.44−47 The basicity of Mg ions could accelerate transesterification between hydroxy end compound by inductive effects shown in red arrows.48 The Zr− Mg catalytic mechanism could be proposed: the carbonyl oxygen of the carboxylic group combines with Zr to form an activated complex, which is attacked by the nucleophile alcohol

to loss modulus peak is contributable to the glass transition temperature of the PEEs.31,33 Worth mentioning is that the magnitude of maximum relaxation in the tan δ curve reflects the relative amount of amorphous materials in the copolymers.26 As the hard segment content in the PEEs increased, its peak position shifted to higher temperature and the peak intensity decreased. The weak peaks at 100−160 °C on tan δ curves may be attributed to the relaxation and reorientation of the carboxyl groups in the amorphous phases. The relationship between the thermal transition temperatures and PTMG content of the PEEs is summarized in Figure 4c. The melting temperatures, Tm and Tmc, slowly and monotonically decreased with increasing PTMG content from 0 to 40% because the increasing amount of soft segment to the polymer backbone can lower the barrier of the movement of the copolymer chains in the crystalline region. Thus, the melt enthalpy for the copolymer decreased, leading to weaker peak intensity. Particularly, the Tα rapidly and monotonically decreased with increasing PTMG content; that is, the Tα is more sensitive to the PTMG content than is Tm. These thermal transition results suggest that the PEEs obtained are random copolymers rather than block copolymers with double Tg16,34 or a polymer mixture with double Tm.35 The PEEs containing 30 to 40% PTMG can be used as thermoplastic elastomers in a wide temperature range from around −18 to 158 °C for PEE30 and −34 to 132 °C for PEE40. Excellent Melt Processable and Mechanical Properties. The PEEs synthesized in this work have exhibited excellent melt processability at 250 °C that is a little higher than their melt temperature ranging between 193 and 213 °C. Several type 5B dumbbell-shaped samples for tensile test have been successfully prepared by a melt injection-molding technology according to ISO standard 527-2:2012. Tensile stress−strain curves in Figure 5a signify that the PBT behaves like a rigid plastic, while the PEEs with relatively low ether segment contents between 10 and 20 wt % behave like semirigid plastics. Particularly, both PEE30 and PEE40 demonstrate a very steadily elongational strengthening behavior in a wide elongation from 30% to 470%, which is characteristic of tough chain-like polymers materials. Except for PBT, their moduli of elasticity are all below 700 MPa and decreased steadily with increasing PTMG content from 10 to 40 wt %. The breaking patterns for the PEEs start with elastic deformation and then reach the yield point, followed by yielding and plastic deformation combined with strain hardening, and at last the specimens were broken down. The two PEEs with the highest PTMG segment contents of 30−40 wt % in feed behave like soft rubberlike materials. They show elastic deformation until breaking; the elongation at break reaches up to 485% for PEE40. As shown in Figure 5b, both specimens (PEE30 and PEE40) shrink after breaking. Note that the final broken samples exhibit a gradually decreased length with increasing PTMG content from 20 to 40 wt % because of the gradually increased elastic shrinkage regardless of the highest elongation at break of PEE40. Particular attention should be paid to the stress−strain curve of PEE20d prepared by TBT. The PEE20d behaved as a brittle material and broke at low stress and very small elongation because of its lowest Mn and the highest chain rigidity (i.e., the highest intrinsic viscosity) in Table 1. Figure 5c summarized the tensile mechanical properties of PBT and PEEs. Young’s modulus decreased monotonically with increasing PTMG segment content, indicative that the soft 9080


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polyester that was further connected with ether segment via ROP for the synthesis of multiblock copolymer.17,18,40,41,55,56 Another three-step method has used DMT as the ester source to synthesize random and block copolymers through (1) melt prepolymerization of DMT with BDO, (2) low-vacuum, and (3) high-vacuum melt polycondensation of the DMT/BDO prepolymers with PTMG.13,35,54,55 It is pitiful that both of the methods not only used a relatively complicated three-step polymerization procedure but also formed undesirable hazardous byproducts, consequently leading to relatively high production cost and environmentally unfriendliness. Fortunately, relatively inexpensive and environmentally friendly TPA has been directly used to cost-effectively two-step synthesize random PEEs with similarly low residue of free ether without hazardous byproducts because water was the byproduct accompanying the polycondensation by applying ZMC catalyst in this work. As listed in Table S1, TBT has been served as an efficient catalyst for the synthesis of the high-molecular-weight and tough PEEs from TPC and DMT as ester sources, unfortunately accompanied by the formation of a large amount of corrosive hydrochloric acid/chlorane and hazardous methanol as byproducts, respectively. Furthermore, note that the toxic chemicals such as SOCl2 and methanol are ordinally necessary to produce TPC and DMT, respectively, which are obviously environmentally harmful. Anyhow, TBT may not be an efficient catalyst any more for the synthesis of the highmolecular-weight and tough PEEs if directly using TPA as the ester source, because only brittle PEE with a much lower molecular weight has been obtained when TBT is used as catalyst for the polymerization between TPA and BDO/ PTMG. Besides, the retention rate for PTMG ether segment in the PEEs obtained by TBT is low down to 76%. Zr(OC3H7)4, Zn(OAc)2/Sb2O3, TBT, and TBT/Mg(OAc)2 are inefficient for the synthesis of the PEEs by the one-step feeding route of DMT, BDO, and PTMG either because of their low retention rate for PTMG ether segment in the PEEs down to 68.3− 91.7%. That is why no reports on the synthesis of highperformance PEEs directly from TPA were found until today to the best of our knowledge, because of the absence of powerful catalysts for the direct one-stage polymerization between TPA and BDO/PTMG so far. Fortunately, the Zr−Mg complex catalyst designed and prepared here can catalyze and activate the polycondensation between TPA and BDO/PTMG, successfully resulting in the cost-effective and eco-benign obtainment of the high-molecular-weight, melt processable, and tough PEE elastomers. Therefore, it could be concluded that the Zr−Mg complex catalyst developed in this investigation for the first time is indeed a powerful catalyst for the cost-effective and eco-benign synthesis of the advanced PEEs directly from TPA, BDO, and PTMG accompanied by the formation of eco-safe water as major byproducts. Sustainable and Versatile Applicability. PEEs have become versatile materials with extremely extensive applicability. It seems that the PEEs are almost everywhere in the modern world, because the PEEs as typical thermoplastic elastomers demonstrate excellent performance including electrical insulation, wide range of service temperatures, excellent transient performance at elevated temperature, good resilience, and high resistances to many factors (fat, voltage, low temperature, chemical solvents, industrial chemicals, wear, crack, creep, skid, mold aging, flexural fatigue, and impact) like PU and rubbers. As compared to fluorine and silicon rubbers, the PEEs-based thermoplastic elastomers have a cost advantage. In particular,

Scheme 1. Proposed Catalytic Mechanism of Zr−Mg Catalyst to the Direct Esterification of TPA with BDO and PTMG

to form an adduct intermediate. The esterification is accomplished, and at the same time the Zr−Mg catalyst is regenerated once the intermediate eliminates water. This may be used to explain such a high catalyzing efficiency only by Zr− Mg catalyst at 18 mmol % concentration that is of 2.8−9.4 times lower concentration than other catalysts at 50−170 mmol %.12,13,39,49−55 Indeed, the unique synergistic catalytic effect between Zr and Mg atoms would effectively activate the esterification between PTMG and the oligoesters (formed when TPA mostly esterifies with BDO) with the end groups of TPA instead of DMT or TPC, successfully reducing the residue of unreacted PTMG in the resulted PEEs. Comprehensive Comparison of the PEEs Based on Three Ester Sources by Four Catalysts. To demonstrate the costefficiency of synthesizing random PEEs from two-step direct esterification of TPA with BDO and PTMG, Table S1 has been used to systematically compare the PEEs synthesized in this study with those by two other typical methods developed so far, that is, TPC- and DMT-based three-step routes. These two methods have been proved to be of efficient synthetic technology of the PEEs with low residue of free ether in the resulting polymers. One three-step approach consisting of solution prepolymerization of TPC with BDO by using toxic and expensive base catalyst and melt transesterification polycondensation of the TPC/BDO prepolymer with PTMG has applied TPC as the ester source, resulting in the formation of cyclic oligomer of 9081


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Table 3. Cursory Comparison of Economic Costs of ZMC, TBT, and the PEEs Obtained On the Basis of Three Ester Sources by Two Catalysts at the Same Industrial Grade catalyst cost ester source TPC DMT TPA

catalyst

raw material cost (USD/ton)

unit cost (USD/kmol metal)

dosage (metal/ polymer) (mol %)

total cost (USD/ton polymer)

raw comonomer cost (USD/ton)

PEE cost (USD/ton)

DABCO TBT TBT ZMC

50000 5000 5000 15000

5600 1700 1700 6000

1.67 0.07 0.05 0.02

141.7 1.80 1.30 1.80

2300

2443.5

1200 990

1201.3 991.8

Figure 6. (a) Possible toxic discharge of PU, vulcanized rubber, chloroprene rubber, fluororubber, EVA copolymer, and other polyolefin elastomers, and traditional TPC- and DMT-based PEE elastomers. (b) Nontoxic discharge of the environ-benign TPA-based PEE elastomers synthesized by direct dehydration polymerization among TPA, BDO, and PTMG by the Zr−Mg catalyst.

CH3OH solvents, and/or by unsafe high-pressure methods. Moreover, PU, most rubbers, EVA copolymer, and TPC- and DMT-based PEEs will discharge harmful or even toxic substances including HCN, HCl, DABCO, pyridine, CH2Cl2, toluene, CH3OH, and monomers during their production and usage especially at elevated temperature. Fortunately, the PEEs developed here do not have toxic discharge or other adverse environmental impacts. Zirconium element that could be found in food at a trace level is also one of the nonessential harmless elements in human teeth, bones, liver, and muscle.23 Magnesium is an essential element in the maintenance of human life. Therefore, the Zr−Mg-catalyzed TPA-based PEEs as the versatile and totally sustainable elastomers have an enormous potential to replace eco-unsafe PU, most rubbers, EVA copolymer, and other polyolefin elastomers, and TPCand TPA-based traditional PEEs extensively used in our daily life. In a word, the sustainable raw materials’ production and application of the environ-benign PEEs catalyzed by Zr−Mg catalyst developed in this study are more beneficial to the ecosystem maintenance of the living earth to some extent than rubber, PU, polyolefin, and traditional PEES, as shown in Figures 6 and 7.57

the PEEs exhibit simpler and better processability, better processing diversity (like extrusion, injection, blow, rotation, melt casting, compression moldings, and hot welding due to their better melt fluidity, more stable molten state, and lower shrinkage ratio), stronger toughness, higher low-temperature flexibility, more stable and higher transparency, higher dimensional stability, higher energy conservation, higher production efficiency, stronger adhesion to common polymer materials (ABS, PC, PVC) and metals, more comfortable feel, more exquisite appearance, and higher resistances to heat/oil/ weather/UV than most of the common rubbers and PU.56,57 It is predicted that the annual demand for thermoplastic PEE elastomers worldwide will be as high as 5 600 000 tons and further increase at an average annual growth rate of 6.3%.57 Such a versatile applicability and huge demand make a sustainability analysis very essential for the sustainable development of the PEE elastomers in the current era of advocating environmental friendliness. More importantly, the PEEs synthesized by direct dehydration polymerization among TPA, BDO, and PTMG by eco-safe Zr−Mg catalyst are more energy-saving, of lower economic cost (Table 3), vulcanization-free, environmentally friendlier, much better recyclable, and eco-safer elastomers than most rubbers, PU, and ethylene-vinyl acetate (EVA) copolymer and other polyolefin elastomers, and TPC- and DMT-based PEEs that are synthesized from toxic monomers including toluene diisocyanate, aromatic diamines, chloroprene, vinylidene fluoride, TPC, and harmful vulcanizater (such as butylaldehyde-aniline condensate as well as PbO) and toxic catalysts like DABCO in harmful DMF, DMAc, CH2Cl2,

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CONCLUSIONS A series of random poly(ester-co-ether)s with Mw up to 60 600 g/mol and the same elasticity as typical PEE have been successfully prepared by a facile direct polycondensation route from TPA. Novel Zr−Mg catalyst that was designed and synthesized for the first time is the key to cost-efficient and environmentally benign catalysis of the direct polycondensation 9082



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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01421. Experimental details, Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. *E-mail: [email protected]. ORCID

Xin-Gui Li: 0000-0001-7750-7158 Mei-Rong Huang: 0000-0002-8563-7910 Figure 7. Qualitative environ-benignity rating of elastomers based on raw materials, manufacturing processes, and usage. The relative environ-benignity of rubber is low because of its toxic raw materials, and toxic discharge during manufacturing processes and usage. The environ-benignity of polyurethane, polyolefin, and traditional DMT-/ TPC-based PEEs is also low if based on their toxic raw materials and toxic discharge during manufacturing processes, but high if based on their little toxic discharge during usage. The TPA-based PEEs catalyzed by the Zr−Mg catalyst do not discharge anything toxic or harmful, resulting in excellent environ-benignity from raw materials to manufacturing and usage stages.

Author Contributions ⊥

X.-G.L. and G.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Shenzhen Fundamental Research Project (JCYJ20160318095112976), the JSPS Invitational Fellowship for Research in Japan, and the National Natural Science Foundation of China (51273148). We are grateful to Professor Hiroshi Imahori, Professor Tomokazu Umeyama, Ms. Naoko Nishiyama, Mr. Tomoya Ohara, and Mr. Kensho Igarashi at the Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Japan, for their NMR and WAXD measurements of raw materials and Zr−Mg catalyst.

between TPA and BDO/PTMG, achieving the productive synthesis of the high-molecular-weight PEEs exhibiting satisfactory comprehensive performance. The esterification degree of PTMG ether segment in the PEEs was comparable to conventional multistep synthetic routes from TPC and DMT. The ester segments as well as the ether segments were randomly distributed along the chains because a single glass transition temperature at −34 °C was observed. Good elastic recovery was revealed for the copolymers with high ether segment contents. This is practically relevant as a consumable thermoplastic-elastomer can be large-scale manufactured by a cleaner two-step route and used in cruel environment. Simply, the Zr−Mg catalyst developed in this work is a powerful catalyst for the cost-effective and eco-benign synthesis of the high-molecular-weight high-performance PEEs directly based on TPA, BDO, and PTMG, significantly avoiding toxic catalyst like DABCO and a large amount of environmentally hazardous hydrochloric acid or methanol as byproducts. It could be predicted that the Zr−Mg catalyst will be extended to the costeffective and eco-benign synthesis of other PEE elastomers based on TPA with PTMG or other relatively unreactive polyethers like poly(ethylene glycol) or poly(propylene glycol) and other diol monomers like EG, PDO, and 1,4-cyclohexanedimethanol. The development of the powerful Zr−Mg catalyst would open a new cost-effective and eco-benign twostep route to synthesize random PEEs by the direct polycondensation between TPA and diol/relatively inactive polyethers. As compared to TBT, only a trace amount of ecobenign and highly active Zr−Mg catalyst can achieve costefficient and sustainable synthesis of high-performance PEEs as one of the completely new third-generation rubbers, and also almost all other similar PEEs without toxic discharge or other poisonous environmental impacts, which have an intense potential to replace eco-unsafe rubbers, PU, polyolefin elastomers, and TPC-/TPA-based PEEs that have been applied daily.

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