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Synthesis and Characteristics of a Biobased High‑Tg Terpolyester of Isosorbide, Ethylene Glycol, and 1,4-Cyclohexane Dimethanol: Effect of Ethylene Glycol as a Chain Linker on Polymerization Won Jae Yoon,† Sung Yeon Hwang,† Jun Mo Koo,† Yoo Jin Lee,‡ Sang Uck Lee,§ and Seung Soon Im*,† †

Department of Organic and Nano Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, 133-791, Korea ‡ Department of Chemicals R&D Center, SK Chemicals Co., Ltd., 686 Sampeoung-dong, Bundang-gu, Seongnam-si, Gyeonggi-do, 463-400, Korea § Department of Chemistry, University of Ulsan, 93 Daehak-ro, Ulsan, 680-749, Korea S Supporting Information *

ABSTRACT: A solution for overcoming the low reactivity of terephthalic acid and isosorbide (ISB) is proposed that uses 1,4cyclohexane dimethanol and ethylene glycol. Using the different reactivities, volatilities, and degree of steric hindrances among the three diols, a highly heat-resistive biobased terpolyester (PEICT; glass transition temperature = 93−143 °C) was synthesized with a high degree of polymerization (weight-average molecular weight 65 400; number-average molecular weight 25 400). After esterification, most of the oligomer end groups were found to consist of ISB, which decreases the overall reactivity of transesterification due to its characteristics. However, this end group changed gradually into ethylene units, which accelerated the transesterification and chain growth in the polycondensation process via chain scission at the carbonyl carbon adjacent to the ethylene unit. To substantiate this mechanism, the Fukui function was used to calculate the reactivity difference between monomers. The sequence distribution was analyzed using 13C-nuclear magnetic resonance to elucidate the function of each diol unit in transesterification. Finally, a polycondensation process for the PEICT terpolyester is proposed.

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INTRODUCTION Global weather is changing dramatically due to the rapid increase in the atmospheric carbon dioxide concentration, increased temperatures, and unexpected patterns of precipitation.1,2 The reserves of petroleum, the energy resource that led to the industrialization of civilization, have diminished drastically due to indiscriminate extraction.3 As these have become international issues, many scientists are now focusing on eco-friendly materials to conserve limited resources and prevent pollution.4−12 Biobased polymers are eco-friendly materials that utilize renewable resources by replacing a part of the petroleum monomer, hopefully delaying the depletion of petroleum and energy sources. The trade-off is that the decreased biomass content positively affects some of the polymer properties, impacting the ability to use biobased polymers in specific applications. When replacing general engineering plastics, it is necessary to consider issues related to the health of humans who use these plastics routinely. For example, bisphenol A (BPA), a key monomer in the production of polycarbonate plastics,13 has significant effects14−17 on adipogenesis,18 inflammatory cytokines,18,19 and increased oxidative stress20−22 at below the “no observed adverse effect level” (NOAEL). © 2013 American Chemical Society

Therefore, highly thermal-resistant biobased polymers that can be used to make dishwashable ware, baby bottles, and electric outlet covers should not contain BPA or other endocrinedisrupting chemicals. The renewable monomer isosorbide (ISB) is a material that can satisfy all of these issues. ISB, a biomass monomer derived from glucose, has a bifunctional hydroxyl group that can be used in condensation polymerization or addition polymerization.23 Because of its rigid molecular structure and chirality, which enhance the glass transition temperature (Tg) and the transparency of the resulting polymers,24 numerous studies have examined the use of ISB in polyesters, 5,10,25−31 epoxies,32,33 polyurethane,34,35 and polycarbonates.36−39 However, a high degree of polymerization using ISB alone has not been achieved. This study focuses on a terephthalic acid (TPA) based copolyester that has a high Tg and good mechanical properties, even with a low ISB content as compared to aliphatic copolyesters. In this copolyester, the low reactivity of ISB Received: July 17, 2013 Revised: August 21, 2013 Published: September 12, 2013 7219

dx.doi.org/10.1021/ma4015092 | Macromolecules 2013, 46, 7219−7231

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added to this system to improve the rate of transesterification by using its linear structure and lower boiling point (BP) to facilitate its removal from the system in the polycondensation process. The resulting products had an Mn from 19 000 to 25 000 and Tg from 90 to 143 °C. The results show that the effect of EG on the transesterification rate of PEICT terpolyester exceeded expectations. To investigate this effect in more detail, the 1H-nuclear magnetic resonance (NMR) spectrum of PEICT was used to analyze the end hydroxyl group composition, and the reactivities of the three diols and TPA were calculated using Fukui functions (FFs). The sequence distribution of terpolyester was analyzed using 13C NMR spectra to compare the degree of transesterification related to the three different reactive diol units. Consequently, a polycondensation mechanism for PEICT terpolyester is proposed and the effects of EG as a chain linker in this polymer as a result of a topological selective chain scission process are examined.

prevents the achievement of high-molecular-weight polyesters.40 Storbeck et al. synthesized the copolyester by reacting terephthalic acid dichloride and ISB in toluene for 2 days at 90−100 °C because of the low reactivity of TPA with ISB.41 Bersot et al. performed melt polymerization of TPA, ISB, and ethylene glycol (EG) with various catalysts, reporting that with the ISB content fixed at 20 mol %, the highest number-average molecular weight (Mn) of 9600 g mol−1 was obtained when using catalysts of antimony(III) oxide and aluminum ethoxide.42 Quintana et al. studied terpolyesters containing ISB and reported that the samples with the highest heat resistance had a Tg of 105 °C and Mn of 11 800.43 To overcome these difficulties, 1,4-cyclohexane dimethanol (CHDM) was incorporated to assist the reaction between TPA and ISB, despite the biomass content, with a slight reduction in the Tg of the resulting polymer. The isomer structure of CHDM is 70% trans and 30% cis. As shown in Scheme 1A, at

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Scheme 1. (a) CHDM Conformation with an a,a/e,e Transition due to Flipping of the Cyclohexylene Ring. (b) Stress Relaxation Scheme via the Longer-Range Translational Motion of TPA along with the Cyclohexylene Ring

EXPERIMENTAL SECTION

Materials. TPA (99.9%), EG (99.8%), CHDM (99.8%) with 70 mol % trans-isomer, and isosorbide (99.8%, Roquette Fre′res) were supplied by SK Chemicals (Korea). The monomers were high-purity commercial products and used as received. The solvents and chemical reagents used for acetylation and characterization, including acetic anhydride, chloroform, and o-chlorophenol, were all high-purity grade and used as received. Synthesis of Polyesters. PEICT terpolyester was synthesized in a 5-L bench reactor made of stainless steel with an anchor, agitator, condenser, and trap. For the esterification reaction, the reactor was purged with N2 to remove any residual oxygen. TPA (1937 g, 11.7 mol), ISB (852 g, 5.8 mol), CHDM (757 g, 5.2 mol), EG (181 g, 2.9 mols), germanium dioxide (GeO2, 150 ppm relative to the total weight of monomers), and dibutyl tin oxide (DBTO; 150 ppm relative to the total weight of monomers) were weighed into the reactor at a molar ratio of COOH/OH = 1/1.2 for PEICT-I32 (Table 1). The reaction mixture was heated to 255 °C at a rate of 100 °C h−1 and held at this temperature for 2 h to reach the end point of esterification. At this stage, most of the H2O generated during the esterification had been distilled out. The temperature was then increased slowly to 270 °C, while stirring at 98 rpm. The stirring speed was decreased slowly to 30 rpm as the degree of polymerization increased and the vacuum was reduced gradually to 0.4 mbar. Polycondensation was performed under isothermal conditions until the torque reached 30 N m. Finally, the pressure was returned to atmospheric pressure using N2 to prevent oxidative degradation. The resulting polyester was removed from the reactor, quenched in a water bath, and analyzed.

room temperature, the e,e-trans cyclohexylene ring flips to form an a,a-trans cyclohexylene ring via the twist-boat structure.44−47 This ring flip increases the mobility of the chain, resulting in a higher impact strength45 and better stress relaxation,44,47 as seen in Scheme 1B. Similar to ISB, however, CHDM has a high boiling point and bulk structure, which make it unfavorable for a polycondensation reaction in that it should discharge the CHDM and ISB generated as a byproduct during the transesterification reaction in a vacuum. Therefore, EG was

Table 1. Characteristics of PEICT Terpolyesters Synthesized with Various Amounts of ISB sample codea

feed ratio (ISB/CHDM /EG/AAb)

composition ratio (NMR)c (ISB/ CHDM/EG)

PC timed (min)

I.V.e (°C)

Mnf (g/mol)

Mwg (g/mol)

Mw/Mn

Tgf (°C)

PEICT-I8 PEICT-I23 PEICT-I32 PEICT-I32N PEICTI32a2 PEICT-I42 PEICT-I50 PEICT-I55

9/45/46/0 22/45/33/0 30/45/25/0 30/70/0/0 30/45/25/2

7.5/48.5/42.9 22.7/47.2/29.8 32.2/45.0/22.2 28.8/71.2/0 33.8/47.4/17.8

90 165 200 325 200

0.73 0.61 0.61 0.57 0.58

25 400 22 400 20 100 18 300 20 300

65 400 51 200 46 600 38 300 43 300

2.6 2.3 2.3 2.1 2.1

92 113 120 116 120

40/45/15/0 48/30/22/0 54/30/16/0

41.9/46.0/12.1 49.6/32.3/17.9 54.6/30.5/15.1

290 310 330

0.60 0.60 0.60

19 700 19 400 19 100

46 900 48 000 48 400

2.4 2.5 2.5

131 137 143

a

Sample codes denote the ISB ratio based on 1H NMR. bAcetic anhydride. cThe composition ratio was measured using the relative integrated area of the oxymethylene proton of each of the three diols using 1H NMR spectra and does not include the ratio of diethylene glycol (DEG). dThe polycondensation time taken until the torque reached 30 N m. eIntrinsic viscosity (dL g−1) measured in o-chlorophenol at 30 °C. fNumber- (Mn) and weight-average (Mw) molecular weights determined by GPC. gGlass transition temperatures measured by DSC. 7220


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Scheme 2. Polymerization of PEICT: (A) General Polymerization Method; (B) in Situ Acetylation Polymerization

Characterization. The intrinsic viscosity of the PEICT polymers was measured in o-chlorophenol using an automated Ubbelohde viscometer thermostated at 25 ± 0.1 °C. The Mn and sample distribution were determined by gel permeation chromatography using o-chlorophenol/chloroform (1/3) as the mobile phase at a velocity of 0.7 mL min−1. Separation was performed with two Shodex LF804 columns at 40 °C (mixed bed, maximum pore size 3000 Å) equipped with a Malvern TDA 305 refractive index detector. Then, 30 mg of sample were predissolved in 3 mL of o-chlorophenol at 150 °C for 15 min, and 9 mL of chloroform were added at room temperature. The sample solutions were filtered through polytetrafluoroethylene microporous membranes (Merck Millipore) with 0.45-μm pores before injection. Mn and weight-average molecular weight (Mw)/Mn were calculated after universal calibration with polystyrene standards.1H NMR spectra were obtained on a JEOL Oxford 600 spectrometer operating at 600 MHz. Deuterated chloroform was used as the solvent. Tetramethylsilane (TMS) was used as an internal standard and as a

reference for chemical shifts. Sixteen scans with 16 K data points each were acquired for each 1H NMR spectrum. The relaxation delay was 5 s. 13C NMR spectra were recorded at 150 MHz using the same NMR spectrometer and solvent mixture. Each 13C NMR spectrum comprised 8,192 scans with 64 K data points each. The relaxation delay was 2 s. Differential scanning calorimetry (DSC) was performed using a Mettler-Toledo DSC 820 calorimeter. The samples were melted at 290 °C for 5 min, and then quenched to 40 °C at a rate of 200 °C min−1. Then, the temperature was raised from 40 to 280 °C at 10 °C min−1 in a N2 atmosphere. The Tg was estimated as the onset point in heat capacity associated with a transition. Indium and zinc were used as standard materials for temperature and enthalpy calibration. The decomposition temperature was determined using a Perkin-Elmer TGA 7 with vertical balance instruments from 30 to 700 °C at a scan rate of 20 °C min−1 under N2 gas subtracting a blank curve to compensate for the reduced buoyancy effects of the 7221


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surrounding gas resulting from the decrease in density on heating and the thermal behavior.

was kept in the range 30−45 mol % since the amorphous polyester with 30−50 mol % CHDM exhibits enhanced toughness.49−51 The remainder was EG. Quintana et al.43 fixed the COOH:diol ratio at 1:1.2 and added the desired amounts of ISB and CHDM to the system with excess EG. In this study, less reactive ISB was suggested to be employed as an excess diol in PEICT synthesis. TPA preferentially reacts with the more reactive primary diols, and then the ISB reacts with the remaining carboxylic end groups. To achieve a desirable Tg, ISB should be in excess after fixing the desired proportions of CHDM and EG. Consequently, it can be inferred that most of the end group in the oligomers of PEICT copolyester consisted of a less reactive ISB unit. PEICT-I32N was used as a reference sample to compare the polycondensation time with PEICT-I32 to identify the effect of EG on terpolyester synthesis. The polycondensation time increased gradually with the ISB content, confirming that EG enhances the rate of polycondensation. PEICT-I32a2 was synthesized to compare the effectiveness of the in situ acetylation method.12 To decrease the steric hindrance of ISB, 2 mol % acetic anhydride (AA) was used, as illustrated in Scheme 2B. This amount of AA had a significant effect on the polymerization of poly(isosorbide cis/trans-1,4cyclohexanedicarboxylate), PICD in a previous study. It was postulated that acetylated ISB (Ac-ISB) would accelerate the reaction with TPA by decreasing steric hindrance, as depicted in Figure 1S in the Supporting Information. However, the polycondensation times and molecular weights of PEICT-I32 and PEICT-I32a2 did not differ, demonstrating that in situ acetylation in PEICT has no effect on polymerization. The advantage of in situ acetylation is that it utilizes the acetic acid (AcOH) generated in the reaction between TPA and Ac-ISB as another acetylation reagent for free ISB, which named the “infinite-loop mechanism” in author’s previous publication.12 Unlike PICD, PEICT contains three different reactive diols. The reaction between AA and the primary diols (CHDM and EG) has priority over that between AA and ISB. The acetic acid released from the acetylation reaction also preferentially acetylates CHDM and EG, implying that the in situ acetylation of PEICT terpolyester was less effective in this polymerization. The reactivity of the acetylation between AA and the three diols, as well as that between the acetylated diols (Ac-diols) and TPA, is described in the

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RESULTS AND DISCUSSION Synthesis. A terpolyester (PEICT) was synthesized via the esterification of TPA, ISB, CHDM, and EG at 255 °C, followed by polycondensation at 270 °C using GeO2 and DBTO, as depicted in Scheme 2A. These catalysts gave excellent catalytic results in a previous paper.12 To compensate for the low reactivity of ISB, the primary diol CHDM was incorporated in the melt phase, considering the better toughness of PEICT terpolyester via the a,a/e,e transition of the cyclohexylene ring. Both ISB and CHDM have bulky ring structures and boiling points exceeding the reaction temperature, which results in some problems removing the released ISB and CHDM monomers during the polycondensation process under vacuum.48 This emission of byproducts becomes more problematic as the degree of polymerization and melt viscosity both increase. Raising the reaction temperature to facilitate emission leads to thermal degradation. Consequently, EG was introduced to resolve this problem owing to its flexibility and lower BP. The various compositions and characteristics of the PEICT samples are presented in Table 1. The ISB content was directly related to the increased Tg of the polymer because of its two rigid fused rings, as shown in Figure 1. The CHDM content

Figure 1. Molecular structure of ISB with two rings fused at 120° to each other.

Scheme 3. Proposed Mechanism of the Acidolysis of the PEICT Esterification Reaction Using in Situ Acetylation (ISB Part)

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Figure 2. (A) 13C NMR and (B) 1H NMR spectra of the PEICT polymer.

Figure 3. Correlation spectrum (COSY) of PEICT-I32. 7223


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Figure 4. Heteronuclear single-quantum correlation spectrum (HSQC) of PEICT-I32.

ISB moiety at δ 4.31, δ 5.51, δ 5.10, δ 4.71, δ 5.45, δ 4.20, and δ 4.10 ppm were assigned to hydrogen atoms 5, 6, 7, 8, 9, 10a, and 10b, respectively. Peaks 5 and 10a overlapped hydrogen atom 11 cis (11(c)) and 11 trans (11(t)) of the CHDM moiety, respectively. In the 1H NMR spectrum of the CHDM moiety, δ 4.20, δ 4.31, δ 1.83, δ 2.08, δ 1.95, δ 1.68, δ 1.17, and δ 1.59 ppm were assigned to hydrogen atoms 11(t), 11(c), 12 trans (12(t)), 12 cis (12(c)), 13a trans (13a(t)), 13a cis (13a(c)), 13b trans (13b(t)), and 31b cis (13b(c)), respectively. The ratio of trans- to cis-isomers in CHDM was determined by integrating the peaks corresponding to H12 (t) and H12 (c). The trans/cis ratio estimated from all of the copolymers was similar to that of the CHDM monomer. Additionally, the copolymer compositions of ISB, CHDM, and EG were calculated from the relative peak intensities of the oxymethylene protons of each diol. Figure 3 shows the COSY spectrum of PEICT-I32, identifying the presence of each diol residue bonded at both sides to TPA. The existence of ISB fragments was confirmed by cross peaks 5/6, 7/8, 7/9, and 9/10b in the COSY spectrum. Correlation signals 6/7 and 9/10a were not detected in the COSY spectrum because these two pairs of hydrogen molecules were located in the opposite plane of two five-membered rings of ISB, as shown in Figure 1. However, cross peak 7/9 was observed in the spectrum, as it was in the same plane. The correlation signals 11(c)/12(c), 11(t)/12(t), 12(c)/ 13b(c), 12(t)/13b(t), and 13b(t)/13a(t) confirmed the presence of CHDM residues. The overlapping peak (H5, 11(c)) at δ 4.31 ppm verified the observation of correlation signals 5/6 and 11(c)/12(c). The fact that H10a and H11(t) overlapped was verified by comparison with the integration values between δ 4.20 and δ 4.10 ppm. However, this

reactivity section of the Supporting Information, with detailed explanations based on FFs.52−54 Scheme 3 shows the proposed mechanism of TPA reacting with Ac-ISB. First, a hydroxyl group of TPA (1) attacks the carbonyl carbon atom of Ac-ISB (2), generating a tetrahedral intermediate (3).12,55 In the esterification reaction, the hydroxyl group of ISB with a monoacetoxy end group (4) can attack two different carbonyl carbon atoms in Ac-TPA (5). However, the nucleophilic acyl substitution reaction occurs predominantly at the carbonyl carbon atom of the nearby benzene ring. This occurs for two reasons: first, the electrophilic character of the carbonyl carbon atom adjacent to the benzene ring is greater than that of the other carbonyl carbon due to the resonance effect in the benzene ring, and second, the AcOH byproduct of esterification with this carbonyl carbon atom is constantly removed from the system by being acetylated by other free diols and evaporation. Other proposed mechanisms between TPA and other Ac-diols are illustrated in A and B in Scheme 2S (see the Supporting Information). The chemical structure and composition of newly synthesized PEICT copolyester samples were evaluated using 1H NMR and 13C NMR spectra, as shown in Figure 2. The peak assignments were confirmed by two-dimensional (2D) NMR spectroscopy, which provided detailed information about the structure of PEICT and the sequences of numerous dyads. Various NMR techniques, including correlation spectroscopy (COSY) and heteronuclear single-quantum correlation spectroscopy (HSQC), were used to determine the sequence of specific protons and carbon atoms in PEICT-I32. In Figure 2, the peak in the 1H NMR spectrum of the EG moiety at δ 4.71 ppm was assigned to hydrogen atom 1. This peak overlapped that of hydrogen atom 8 of the ISB moiety. The peaks of the 7224


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Scheme 2SA (see Supporting Information). The calculated reactivity of the exo hydroxyl group of ISB is 1.55 times higher than that of the endo hydroxyl group. Collectively, these results imply that the transesterification reaction involving the endo hydroxyl group proceeds with great difficulty in the polycondensation process. The byproduct, bulky ISB with a high BP, also decreases the reaction rate. We found a critical clue that some EG is eliminated from the reactor by comparing the feed and composition ratios (Table 1), although EG end groups are not detected in the 1H NMR spectrum of the PEICT oligomer. Most of the byproducts are generated in the diols located in the end group of the polymer chain, unless unreacted free diols remain in the reactor. It can be expected with this result that some ISB end groups change to EG end groups via chain scission in the polycondensation, which accelerates the rate of transesterification and the degree of randomness of the PEICT terpolyester, as supported by the analysis of sequence distribution that follows. Sequence Distribution. The microstructure of PEICT terpolyester is very important for property analysis because it contains three diols in the polymer chain with different reactivities and steric hindrances. The two quaternary carbons in the benzene ring of TPA with a chemical shift at around 134 ppm in the 13C NMR spectrum were used to clarify the dyad sequence distribution because this shift is more sensitive to sequence effects than any other aromatic carbon due to the occurrence of through-space and through-bond interactions between neighboring residues.58 The quaternary carbons of TPA are split into six peaks corresponding to CTI, ETC, CTC, ETI, ETE, and ITI as illustrated in Table 2, where T, C, I, and E represent TPA, CHDM, ISB, and EG units, respectively. The chemical shifts of these peaks are detected at 134.8−134.5 and 133.4−133.1 ppm, 134.5−134.4 and 133.6−133.5 ppm, 134.3−134.1, 134.0− 133.9, and 133.5−133.4 ppm, and 133.9−133.7 and 133.7− 133.6 ppm, respectively, as depicted in Figure 6.

overlapping peak is explained more clearly by the HSQC spectra. On the basis of the above results, the proton signals were then assigned to the corresponding 13C signals in the HSQC spectrum shown in Figure 4. The peaks in the 13C NMR spectra of PEICT-I32 at δ 63.1, δ 165.7, δ 134.0, δ 129.7, δ 73.4, δ 78.9, δ 81.2, δ 86.2, δ 75.0, δ 70.9, δ 70.2, δ 68.0, δ 37.2, δ 34.6, δ 29.0, and δ 25.4 ppm were assigned to carbon atoms 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11(t), 11(c), 12(t), 12(c), 13(t), and 13(c), respectively. These assignments were confirmed by the cross peaks H1/C1, H4/C4, H5/C5, H6/C6, H7/C7, H8/C8, H9/C9, H10a/C10, H10b/C10, H11(c)/C11(c), H11(t)/ C11(t), H12(c)/C12(c), H13a(c)/C13(c), H13a(t)/C13(t), H13b(c)/C13(c), and H13b(t)/C13(t), respectively, in the HSQC spectrum. The three overlapping peaks observed at δ 4.71, δ 4.31, and δ 4.20 ppm in Figure 2 confirmed the correlation signals (H1/C1 and H8/C8), (H5/C5 and H11(c)/ C11(c)), and (H10b/C10 and H11(t)/C11(t)), respectively, in the HSQC spectra. A sample of PEICT-I32 oligomer was removed from the reactor after esterification and its 1H NMR spectrum examined (Figure 5A). Here, 10x, 10n, 11(t)-end, and 11(c)-end indicate

Table 2. Various Dyads of PEICT Terpolyestera Figure 5. 1H NMR spectra of PEICT oligomer and polymer: Expanded 1H NMR spectra of PEICT-I32 (A) oligomer and (B) polymer.

the oxymethylene protons of the exo/endo end group of ISB and the methylene protons of the trans/cis end group of CHDM, respectively.12,56 No conspicuous peak of an EG end group was detected in the 1H NMR spectrum of the PEICTI32 oligomer. The number of CHDM end groups (in the red dotted circle) was negligible compared with the ISB end groups. The reaction between TPA and the primary diols (EG and CHDM) can be easily imagined to precede the reaction of TPA and ISB. This investigation confirmed this assumption as most of the end groups of the oligomer samples in the PEICT copolyester were ISB, as discussed earlier. To ascertain the difference in reactivity between the endo/exo hydroxyl end groups of ISB,57 the ratio of each end group was measured by integrating 10n-a + 10n-b and 10x-a + 10x-b, corresponding to the endo and exo hydroxyl end groups, respectively. The resulting ratio was 6/4, implying that the reactivity of the exo hydroxyl group of ISB is 1.5 times greater than that of the second hydroxyl group. This concurs with the reactivities of the endo/exo hydroxyl groups of ISB calculated with FFs in

a

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T, C, I, and E represent TPA, CHDM, ISB, and EG, respectively. dx.doi.org/10.1021/ma4015092 | Macromolecules 2013, 46, 7219−7231

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134.7 and δ 133.3 ppm, the remaining dyads (CTI and ITI) could be determined. Finally, the ETI dyad was assigned using PEICT-I32 samples because all of the other dyad signals had been determined. An attempt was made to assign these signals using heteronuclear multiple-bond correlation (HMBC) spectra, but observing the cross peaks of the four-bond correlation between the quaternary carbons and oxymethylene/oxymethine protons was very difficult due to the extremely low 4JH−C coupling constant. The expanded 13C NMR spectra in the quaternary carbon region of various PEICT terpolyesters are shown in Figure 2S (see the Supporting Information). The molar fractions were calculated by integrating the relative peak areas. The numberaverage sequence length and degree of randomness (R) of these dyads were calculated using the following equations:43,58 LE =

Figure 6. Extended 13C NMR spectra of various polyesters containing at least one diol from among EG, CHDM, and ISB.

To assign these various dyads peaks, the location of each peak was compared among the homopolyesters and copolyester PET, PCT, and PETG. PET and PCT are TPA-based homopolyesters containing EG and CHDM, respectively, and PETG is synthesized with TPA, EG, and CHDM. These polyester samples were selected because they have intrinsic viscosity (IV) values similar to the newly synthesized PEICT copolyester and were analyzed as received from SK Chemicals. The PET, PCT, and PETG are called SKYPET HR7055 (IV = 0.70 dL g−1), Puratan0302 (IV = 0.65 dL g−1), and SKYGREEN PN100 (IV = 0.72 dL g−1), respectively. Consequently, the peaks of the ETE and CTC dyads in Figure 6 were readily assigned using the 13C NMR spectra of PET (δ 133.9−133.7 ppm) and PCT (δ 134.3−134.1 ppm), respectively. Using the expanded 13C NMR spectrum of the quaternary carbons of PETG, the ETC dyads can be determined easily at around both δ 134.5 and δ 133.5 ppm. The 13C NMR spectrum of PEICT-I32N shows three dyad signals: CTI, CTC, and ITI. The CTC dyad peak was already identified from the spectrum of PCT. Furthermore, because of the similar intensity of the two peaks positioned at around δ

( (

fITI +

LI =

(

fETC + fETI 2

fETC + fETI

(

2

2

2

fETI + fCTI

)

)

fETI + fCTI

2

(1)

fETC + fCTI

fETC + fCTI

(

)

)

2

fCTC +

LC =

R=

(

fETE +

(2)

)

)

(3)

1 1 1 + + −1 LE LC LI

(4)

Here LE, LC, and LI represent the number-average sequence length of the EG, CHDM, and ISB units, respectively, and fETE, fCTC, fITI, fETC, fETI, and fCTI correspond to the proportion of integrated intensities of the ETE, CTC, ITI, ETC, ETI, and CTI dyads, respectively. Table 3 shows the number-average sequence length and degree of randomness of PEICT copolyesters with the theoretical values for an ideal random distribution terpolyester

Table 3. Characteristics of PEICT Terpolyesters Synthesized with Various Amounts of ISB dyad fractiond sample code PEICT-I8 PEICT-I23 PEICT-I32 PEICT-I42 PEICT-I50 PEICT-I55

type a

expt calcdb expta calcdb expta calcdb expta calcdb expta calcdb expta calcdb

block lengthd

composition ratioc (ISB/CHDM/EG)

f CTI

f ETC

f ETI

f CTC

f ETE

f ITI

LI

LC

LE

Re

7.5/48.5/42.9

3.5 3.6 10.0 10.7 13.5 14.5 17.4 19.3 15.4 16.0 16.0 16.7

22.9 20.8 18.6 14.1 13.5 10.0 9.9 5.6 8.5 5.8 9.9 4.6

2.0 3.2 3.8 6.8 6.2 7.1 4.8 5.1 8.4 8.9 7.6 8.2

23.5 23.5 22.3 22.3 20.4 20.3 23.0 21.2 11.8 10.4 9.8 9.3

19.0 18.4 9.1 8.9 5.8 4.9 1.7 1.5 3.6 3.2 3.1 2.3

0.5 0.6 4.0 5.2 6.6 10.4 11.1 17.6 15.6 24.6 20.2 29.8

1.1 1.1 1.3 1.3 1.3 1.5 1.5 1.7 1.7 2.0 1.9 2.2

1.9 2.0 1.8 1.9 1.8 1.8 1.8 1.9 1.5 1.5 1.4 1.4

1.8 1.8 1.4 1.4 1.3 1.3 1.1 1.1 1.2 1.2 1.2 1.2

1.0 1.0 1.1 1.0 1.1

22.7/47.2/29.8 32.2/45.0/22.2 41.9/46.0/12.1 49.6/32.3/17.9 54.6/30.5/15.1

1.1 1.0 1.1 1.0 1.1 1.0

a

Experimental values. bTheoretical values. cThe composition ratio was measured using the relative integrated area of each oxymethylene proton in the three diols from 1H NMR spectra. dThe dyad fraction was estimated experimentally by deconvoluting the 13C NMR spectra and calculated theoretically using the composition ratio by 1H NMR based on a perfect random distribution. Block length was calculated using the equation from ref 54. eDegree of randomness. 7226


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based on the composition ratio by integrating the 1H NMR spectra. All block lengths obtained from the dyad sequences estimated by 13C NMR agreed well with those calculated from the dyad sequences using 1H NMR, supporting the dyad peak assignments in Figure 6. Consequently, the degree of randomness of all samples was almost unity, confirming that the structural unit distribution obeys Bernoullian statistics. If R < 1, the units tend to form blocks of each unit, while R = 0 in a homopolymer mixture, and if R > 1 the sequence length becomes shorter and R = 2 indicates an alternating copolymer.58 Figure 7 compares the number-average sequences obtained theoretically (calcd; solid lines) and experimentally (expt; solid

esterification reaction in the polycondensation process and H2O is removed from the reactor. Three transesterification reactions take place in oligomer type 3. In part 3A of Scheme 4, the two oligomers are connected by nucleophilic acyl substitution between the hydroxyl group of ISB and a carbonyl carbon, with assistance of the catalyst, strengthening the positive charge at the carbonyl carbon. Without a catalyst, this transesterification might be much lower due to the reactivity of the ISB hydroxyl group, comprising predominantly endo hydroxyl end groups. Chain scission (part 3B of Scheme 4) occurs more frequently at carbonyl carbons alongside ethylene units, which have less steric hindrance than CHDM or ISB subunits. This chain scission process has two advantages in the PEICT terpolyester. First, it increases the degree of randomness in the polymer chain. The oligomer has primary diols (EG and CHDM) in the middle of the oligomer chain and the secondary diol (ISB) in the outer part due to the reactivity difference explained in Figure 5 and Scheme 2SA in the Supporting Information. If the EG content is quite low or zero, this chain scission process occurs infrequently, decreasing the degree of randomness leading to anisotropic properties in the chain. Second, this process increases the number of EG end groups in the oligomer or polymer chain. The increased EG end groups accelerate the transesterification reaction, as seen in 3C in Scheme 4, because EG has greater reactivity and a lower BP for better emission. This corresponds to the polycondensation times of PEICT-I32 and PEICT-I32N in Table 1. The other primary diol, CHDM, is not eliminated from the system, as indicated in Table 1, confirming that this chain scission is sensitive to the structure of the diol and its reactivity. Collectively, these factors indicate that the chain growth of the PEICT terpolyester is accelerated via topological selective chain scission by EG in the polycondensation process. Additionally, the minimum quantity of EG (MQEG) needed to accelerate transesterification by removing itself from the system is calculated and summarized in the MQEG section of Supporting Information. Thermal Properties. Figure 3S in the Supporting Information shows the second heating scans of all of the samples, carried out at 10 °C min−1 after the first heating scan with a 5-min isotherm to remove the thermal history. At these scan rates, none of the PEICT samples showed the ability to crystallize, so they have the features of a completely amorphous polymer. The commercial CHDM used as one of the comonomers of PEICT terpolyester has a 70−72% trans ratio, which is slightly below the thermodynamic equilibrium value of a 76% trans ratio.60 The trans configuration of the CHDM chain is a more stretched form and has greater symmetry with better packing ability, whereas the cis isomer introduces a kink in the polymer chain, interrupting the formation of stable crystals. Therefore, PETG containing 30 mol % CHDM (trans ratio =72%), which replaces EG units, has perfectly amorphous characteristics.51 The structure of ISB is also bulky and twisted due to the endo/ exo hydroxyl group, as depicted in Figure 1, which assists in hindering the crystallization of the polymer chain.12 Furthermore, the rigid structure and less flexible nature of ISB dramatically enhanced the positive effect of incorporating ISB on Tg, as shown in Figure 8. In this figure, the Tg of the PEICT samples is directly proportional to the ISB content, regardless of the quantity of EG and CHDM subunits in the polymer chain, and the estimated maximum T g of the ISB homopolyester when the ISB content is 100% is about 195

Figure 7. Number-average sequence length of several PEICT terpolyesters determined experimentally (expt) and theoretically (calcd) versus the composition ratio of each monomer obtained using 1H NMR spectra.

circles). The block length of EG units in the polymer chain is close to the theoretical value, while that of the other units deviates slightly from the theoretical sequence length. The difference between the theoretical and experimental values increases in the order EG, CHDM, and ISB. The variation in the block length of each unit is closely related to the change in chemical structure, especially the intermolecular chain reaction, i.e., transesterification of the three diols.59 Therefore, it can be inferred that the transesterification reaction takes place actively in ethylene units in this polymer chain, implying that EG acts as a chain linker in the polycondensation of PEICT terpolyester, as discussed earlier. Scheme 4 explains the expected polycondensation mechanism of PEICT terpolyester based on the results of the end group analysis in Figure 5, investigation of sequence distribution in Table 3, and calculated reactivities of the diols using FFs in the Supporting Information. After esterification, the oligomers can be classified into three types, which are predicted to be composed mainly of carboxylic and hydroxyl end groups from TPA and ISB based on the results shown in Figure 3. Oligomer types 1 and 2, which have at least one carboxylic end groups, increase their chain length via the 7227


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Scheme 4. Polycondensation of PEICT Terpolyester Enhanced by Chain Scission and EG End Groups

°C. Generally, 1,4-naphthalenedicarboxylic acid (NDA) is replaced by TPA to increase the heat resistance and mechanical properties of PET. However, the T g of polyethylene naphthalate (PEN) synthesized using 100 mol % NDA instead of TPA is just 120 °C, corresponding to the value of PEICT terpolyester containing about 30 mol % ISB, as seen in Table 1.61 In addition, the high price of NDA is an obstacle to increase the PEN applications. Figure 9 compares the effect of the number-average sequence length of each of the three diols on Tg with experimental (circles and solid line) and theoretical (rectangles and dashed line) values. Tg increased with the number-average sequence length of ISB and decreased with an increase in flexible EG units, whereas the block length of CHDM did not affect Tg directly. It is thought that CHDM is a thermally stable, rigid diol that has a positive effect on Tg based on a comparison of the Tg of PET and PCT used as the reference sample for

assigning the splitting signal for sequence distribution; the respective values are 78 and 88 °C. Thermogravimetric analysis was conducted in an inert atmosphere using TGA and the thermal behaviors are depicted in Figure 4S (Supporting Information). The samples were carefully predried at 80 °C for 24 h in a vacuum before analysis to allow an exact evaluation of the temperatures of initial decomposition (T0d), the maximum rate of decomposition 1/2 0 (Tmax d ), and of half decomposition (Td ). Td is obtained from the onset temperature at the end of the initial plateau region, and T1/2 d is the temperature at which the loss of weight during pyrolysis reaches 50% of its final value. As shown in Table 5, the decomposition of the PEICT samples takes place in one stage, with the maximum rate occurring at 432−450 °C, and just 2−6% of the residual weight remains. Unlike the results for Tg, the values of T0d, T1/2 d , and for these samples showed no relationship to the contents Tmax d of the three diols, as presented in Table 5, indicating that ISB 7228


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and CHDM do not increase Td, unlike their effects on Tg, as already explained in Figure 9. However, these values appear to be interrelated as follows: Td1/2 Td0

= 1.06 ± 0.01

Tdmax ≈ Td1/2

Amari et al.56 reported that thermal decomposition is associated with the release of unsaturated compounds following cis-β-elimination and found that CT units were less stable with heating than ET units because the ester link between TPA and CHDM is weaker thermally than that between TPA and EG. The ether linkage of the furan ring of ISB seems to be the source of the structural weakness.12 Using electrospray ionization (ESI) mass spectroscopy, the authors previously confirmed that the decomposition of the homopolyester containing ISB takes place predominantly via McLafferty rearrangement considering the three kinds of cleavage of a polymer chain, i.e., McLafferty rearrangement, β-elimination, and i-cleavage.62 Therefore, the T d values of PEICT terpolyester samples show no interrelationship with content of monomers. A Tmax of 432−450 °C is sufficiently high to d enable its general application in the plastics industry. Furthermore, the fact that Tmax does not change noticeably d on increasing the Tg of PEICT terpolyesters could be useful from the perspective of recycling and reusing the polymer waste to prevent or reduce environmental pollution.

Figure 8. Relationship between the ISB content and glass transition temperature (Tg) of the PEICT terpolyester. The blue rectangle is the Tg of PETG used as a reference sample for peak assignment. The red rectangle is the expected max Tg when the ISB content is 100%.

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CONCLUSIONS A biobased copolyester with a very high thermal resistance was synthesized using two-step melt polymerization. This copolyester consists of three different reactive diols, ISB, CHDM, and EG, which made this reaction more complicated. This study investigated whether excess ISB monomer is required because of its much lower reactivity compared to the other primary diols to achieve a polymer with a desirable Tg. By analyzing end hydroxyl groups using 1H NMR spectra, investigating the sequence distribution, and comparing the reactivity of each diol using FFs, it was found that EG improved the polycondensation rate via chain scission at the carbonyl carbon next to the EG unit. Completely biobased polymers or biodegradable materials have some limits of mechanical and thermal properties when replacing general engineering plastics. In order to meet the essential properties required in general plastics’ applications, it is necessary to utilize additional monomers such as TPA, EG and CHDM even with decreased contents of biomass in resulting polymer. Consequently, the structure and BP of the comonomer are key factors when considering the use of another monomer to improve the properties of a biobased polymer.

Figure 9. Glass transition temperature (Tg) as a function of the number-average sequence length of each diol unit measured experimentally (expt) and theoretically (calcd). The dotted and solid lines were obtained by polynomial fitting.

Table 5. Comparison of Tg and Various Decomposition Temperatures of PEICT Terpolyester sample codea Tg (oC)b Td0 (oC)c Tdmax (oC)d Td1/2 (oC)e PEICT-I8 PEICT-I23 PEICT-I32 PEICT-I42 PEICT-I50 PEICT-I55

92 113 120 131 137 143

406 415 424 416 420 413

432 444 450 437 445 436

435 443 448 439 444 439

WRes (%)f 1.6 3.0 5.5 2.8 6.8 4.1

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ASSOCIATED CONTENT

* Supporting Information S

a

Sample codes denote the composition ratio of ISB based on 1H NMR. bGlass transition temperatures measured using DSC. cInitial decomposition temperature obtained from the onset temperature at the end of the initial plateau region using TGA. dTemperature of the maximum rate of decomposition estimated from the derivative line of the TGA thermogram. eTemperature of half decomposition at which the loss of weight during pyrolysis is 50% of its final value. fResidual weight%

Scheme of the reaction between TPA and ISB by acetylation using Ac2O, expanded 13C NMR spectra of various PEICT terpolyesters, proposed mechanism of acidolysis of PEICT esterification reaction by in situ acetylation, DSC thermogram and TGA trace of PEICT samples, calculated results of reactivity of various reaction among TPA, CHDM, ISB, EG, Ac-CHDM, Ac-ISB, Ac-EG, and acetic anhydride using Fukui functions, and calculation method of the minimum quantity of 7229


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EG needed to accelerate transesterification by removing itself out of the system. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (S.S.I.) [email protected]. Notes

The authors declare no competing financial interest.

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