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

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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., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01421 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Sustainable Chemistry & Engineering

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†,||

†

State Key Laboratory of Pollution Control and Resource Reuse, and Shanghai Institute of Pollution Control and Ecological Security, College of Environmental 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 || Key Laboratory of Advanced Civil Engineering Materials, College of Materials Science and Engineering, Tongji University, 1239 SiPing Road, Shanghai 200092, China

Full mailing address of all the authors: State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 SiPing Road, Shanghai 200092, China

Corresponding Authors E-mail: [email protected] (X.G.L.), [email protected] (M.R.H.)

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ACS Paragon Plus Environment

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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 non-toxic Zr-Mg catalyst that is designed and synthesized in our laboratory, avoiding undesirable eco-hazardous cocatalyst and by-products 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

1

H-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 60600 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 by-products. 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 catalyse 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 crosslinker 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. Poly(ester-co-ether) (PEEs) have now become one of the most important thermoplastic elastomers as the third-generation rubbers without toxic discharge,1 in which semi-crystalline polyesters act as hard segments while amorphous polyethers 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 low-temperature flexibility, much better recyclability, better environmental friendliness, and higher resistances to heat, oil, weather, and UV than most of 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 equipments.8,9 The transesterification among aromatic diester, aliphatic diols and polyether, especially among dimethyl terephthalate (DMT), 1,4-butandiol (BDO) and polyether were 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 by-product as well as complicated three-feeding and multi-step 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 by-product, 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, 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 compared with terephthalic acid (TPA). 3



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 synthesis of polyesters.17 In fact, the direct esterification of TPA for synthesizing PEEs produces two by-products including major water and minor tetrahydrofuran (THF). They can be easily separated by fractional distillation and recycled as chemical products. But transesterification route of DMT produces by-products like methanol and THF that have very close boiling points of 65.4°C and 66.0°C, respectively, and are an azeotropic mixture. Due to 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 energy-consuming to some extent.18,19 Moreover, methanol is a highly inflammable, toxic, and volatile liquid that rises explosion and dangerous level for manufacture. In a word, TPA is more eco-benign for the preparation of the PEEs than 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 synthesis of PEEs through TPA are found till 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 catalysed 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 anticipant 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 supporting 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 by-product water was distilled out at 230 °C, then additional catalyst solution containing Zr, Ti, and Ti ions of 185 µmol respectively was added into the reactor again. 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 Figures 2A and 2B. Characterization methods have been provided in the Supporting information.

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a)

C4H9O

OC4H9 Zr

C4H9O

OC4H9

HO C O O H2 C C + H2C OH C HO C O OH

b)

Zr-Mg catalyst 6

2

5

1 4 3

HO OH

- Butanol 80oC, 30min, yield 87%

O H3C

CO Mg O C

CH3 O

3

Reflectance

O O C O H2 C C H2C O C HO C O Zr OC4H9 O

CH COO-Mg-OOCCH

3

Citric acid

90oC, 2h, yield 93%

Zr(OC4H9)4 in n-HOC4H9

1,4-Butandiol O O C β O H2 C C β H2C α O C CH3 HO C O Zr O Mg O C O O Zr-Mg catalyst

c)

β α

3.67 3.66

1.92 1.91

3500

Zr-Mg catalyst in D2O

3000

2500 2000 1500 Wavenumber (cm-1)

1000

d)

Zr-Mg catalyst

CH3COO-Mg-OOCCH3 in D2O

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Citric acid in D2O

CH COO-Mg-OOCCH 3

3

Citric acid Zr-(OC4H9)4 in n-HOC4H9 in CDCl3

4.0

3.5 3.0 2.5 2.0 1.5 Chemical shift (ppm)

1.0

5

10

15

20 25 30 2θ (degree)

35

40

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

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A

Successful synthesis O HO C

O COH

B

Unsuccessful synthesis O O HO C C OH

HOCH2CH2CH2CH2OH

HOCH2CH2CH2CH2 OCH2CH2CH2CH2 OCH2CH2CH2CH2OH d* e e f f e e f 12 f e e d*

HO (CH2 )4 OH

Dehydration esterification 0.0088 mol% Zr-Mg catalyst 230oC at 1.01 bar for 2 h

Dehydration esterification

Eco-safe water byproduct

H O(CH2)4 14OH 0.0088mol% TMCorTBT 230oC at 1.01 bar for 2 h

Oligomers

Oligomers 0.0088 mol% Zr-Mg catalyst 250oC at 0.5 mbar for 2 h

Transesterification polycondensation

a a a a O O O O HOCH2CH2CH2CH2O C C OCH2CH2CH2CH2O C C m a a b c c b i h c b a a

O HO (CH2 )4 O C

Transesterification polycondensation

H

n

OCH2CH2CH2CH2 OCH2CH2CH2CH2 OCH2 CHCH 2 2CH2O f e e f 12 f e g d d g e f

High-molecular-weight poly(ester-co-ether)s

C

a

d

250oC at 0.5 mbar for 2 h

O C O(CH2)4 O m

O(CH2 )4

O O C 14

O C n

High-molecular-weight poly(ester-co-ether)s

D

e

f

d*

H

x 0.0088mol% TMCorTBT

d

d*

a f

b

c

b

c

e

d d* PEE40

f

e

PEE30

PEE20

d PEE20-TBT

PEE10

PEE20-TMC

d*

PEE20b-ZMC

d*

PTMG PBT

PEE20a-ZMC

6 4 2 0 Chemical shift (ppm) Figure 2. A) Successful synthesis of poly(ester-co-ether) prepared by ZMC with the assignments of 1H-NMR 8

6 4 2 Chemical shift (ppm)

8

0

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 and 40 wt% PTMG (Mn1000).

RESULTS AND DISCUSSION Characterization of the Zr-Mg catalyst. The Zr-Mg catalyst has been characterized by IR, 1H-NMR and X-ray diffraction. Compared with the three raw materials, Figures 1b-d suggest 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 7



resonance peak (β) of -CH2- at low field, a new strong and a broad weak diffraction peaks because of the complexation of Zr between citric acid and magnesium acetate. These results significantly signify that a new Zr-Mg catalyst has been obtained. Much improved transesterification between ester and ether segments by Zr-Mg catalyst. TBT as a traditional efficient catalyst for successful synthesis of PBT and PEE by DMT route,8-9 has an unfavourable 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 feed by direct esterification between TPA and BDO and then more important 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 with 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:

TBT 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 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 so high catalyzing efficiency only by Zr-Mg catalyst at 18 mmol% concentration that is 2.8∼9.4 times lower concentration of other catalysts at 50∼170 mmol%.12,13,39,49-55 Indeed, the unique synergistic catalytic 15



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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.

Scheme 1. Proposed catalytic mechanism of Zr-Mg catalyst to the direct esterification of TPA with BDO and PTMG

O H2C C

H2C

O C

CH3

HO

+

Complexation

HO

O

C

C O

C

14

+H2O

Catalyst regeneration

O C H2C C

H2C C

HO

C O

CH3

O Zr O Mg O C O

O

H O O C

Comonomers BDO & PTMG (2) O H OH / HO k2 HO 14 ~230oC at 101.3kPa for 1h

O

O

C

O H

O C

OH

O

O

CH3

O Zr O Mg O C O

H O

O

HO C


O

H2C C

O

Ester-ether oligomer

O H2C

CH3

O Zr O Mg O C O

Regenerated Zr-Mg catalyst

~220oC at 101.3kPa for 1h

O C

O

C

C O

TPA monomer O O C OH HO C

(1)

O

H2C C

H2C

O Zr O Mg O C O

O

k1

O

C

C

HO

O

Zr-Mg catalyst O

O C

O H C O+ O H O 14H

Intermediate

Catalytic action O

O

O C H2C HO

H2C C C

C O

H O O C

O C

O

O

H2C

CH3

HO

O Zr O Mg O C O

H2C C O

C

C O

O C

C O O H O 14 H

Intermediate

16

CH3

O Zr O Mg O C O

H O

O H

O


O H C O O H O 14H

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Comprehensive Comparison of the PEEs Based on Three Ester Sources by Four Catalysts. To demonstrate the cost-efficiency 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, i.e., TPC- and DMT-based three-step routes. These two methods have been proved to be 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 ester source, resulting in the formation of cyclic oligomer of 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 ester source in order to synthesize random and block copolymers through (1) melt prepolymerization of DMT with BDO and (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 relatively complicated three-step polymerization procedure but also formed undesirable hazardous by-products, 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 by-products because water was the by-products accompanying with 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 accompanying with the formation of large amount of corrosive hydrochloric acid/chlorane and hazardous methanol as by-products, 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 high-molecular-weight and tough PEEs if directly using TPA as ester source, because only brittle PEE with 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 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 to 91.7 %. That is why no reports on the synthesis of high-performance PEEs directly from TPA were found till 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, 17



successfully resulting in the cost-effective and eco-benign obtainment of the high-molecular-weight, melt processible, 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 accompanying with the formation of eco-safe water as major by-products. 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, high resistances to many factors (fat, voltage, low temperature, chemical solvents, industrial chemicals, wear, crack, creep, skid, mould aging, flexural fatigue, and impact) like PU and rubbers. Compared with fluorine and silicon rubbers, the PEEs-based thermoplastic elastomers have cost advantage. In particular, the PEEs exhibit simpler and better processability, better processing diversity(like extrusion, injection, blow, rotation, melt casting, compression mouldings, 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, higher resistances to heat/oil/weather/UV than most of 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, 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 other TPA-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, CH3OH solvents and/or by unsafe high-pressure methods. Moreover, PU, most rubbers, EVA copolymer, and TPC- and other TPA-based PEEs will discharge harmful or even toxic substances including HCN, HCl, DABCO, pyridine, CH2Cl2, toluene, CH3OH, and monomers during their 18


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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 trace level is also one of the non-essential 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 TPC- and 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

Table 3. Cursory comparison of economic costs of ZMC, TBT and the PEEs obtained based on three ester sources by two catalysts at the same industrial grade Catalyst cost Ester source

Catalyst

Raw material cost (USD/ton)

Unit cost (USD/kmol metal)

Raw Dosage

Total cost

comonomer

PEE cost

(metal/polymer)

(USD/ton

cost

(USD/ton)

(mol%)

polymer)

(USD/ton)

DABCO

50000

5600

1.67

141.7

TBT

5000

1700

0.07

1.80

DMT

TBT

5000

1700

0.05

TPA

ZMC

15000

6000

0.02

TPC

19


2300

2443.5

1.30

1200

1201.3

1.80

990

991.8


a COCl

H2N

SO 2

NCO

NH2

F

b

F

NCO CH3

N COCl

H CN H Cl Pb O

DM F D M Ac D A BCO C H 3O H

C H 2C l2

CH2=C-CH=CH2

N=CH-C3H7

Cl

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) Non-toxic discharge of the environ-benign TPA-based PEE elastomers synthesized by direct dehydration polymerization among TPA, BDO and PTMG by the Zr-Mg catalyst.

Relative environ-benignity

High

10 Raw materials Manufacturing Usage

8

6

4

2 Low

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0

Rubber

Polyurethane Polyolefin

Traditional TPA-based DMT-/TPCPEEs in based PEEs this work

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 20


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Zr-Mg catalyst do not discharge anything toxic or harmful, resulting in excellent environ-benignity from raw materials to manufacturing and usage stages.

CONCLUSIONS A series of random poly(ester-co-ether)s with Mw up to 60600 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 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 multi-step synthetic routes from TPC and DMT. The ester segments as well as the ether segments were randomly distributed along the chains since 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 cost-effective 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 two-step route to synthesize random PEEs by the direct polycondensation between TPA and diol/relatively inactive polyethers. As compared with TBT, only a trace amount of eco-benign and highly active Zr-Mg catalyst can achieve cost-efficient 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.

ASSOCIATED CONTENT Supporting Information Experimental details, Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION 21



Corresponding Authors *Phone: +86-21-65983869. E-mail: [email protected]；[email protected] (X.G.L.), [email protected] (M.R.H.) ORCID Xin-Gui Li: 0000-0001-7750-7158 Mei-Rong Huang: 0000-0002-8563-7910 Author Contributions X.-G.L. and G.S. contributed equally. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS 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 Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Japan for their NMR and WAXD measurements of raw materials and Zr-Mg catalyst.

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TOC graphic

O One-step HO C comonomer feeding HO (CH2 )4 OH

O C OH H O(CH2)4 14OH

Direct esterification

0.009mol%Zr-Mg catalyst 230oC at 1.01 bar for 2 h


Oligomers 0.009mol%Zr-Mg catalyst 250oC at 0.5 mbar for 2 h

Transesterification polycondensation O HO (CH2 )4 O C H Highly sustainable elastomer

O CO(CH2)4 O m

O O(CH2 )4 14 O C

O C n

High-molecular-weight copoly(ester-ether)

High costefficiency

Synopsis Green poly(ester-co-ether) elastomers were cost-efficiently, scalably, and sustainably synthesized via direct esterification of terephthalic acid, 1,4-butandiol, and poly(tetramethylene glycol) with clean and active Zr-Mg catalysts.

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

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