Sustainable aromatic polyesters with 1,5-disubstituted indole units

This work aims to unravel the impact of disubstitution patterns on the physical properties and processing characteristics of indole-based aromatic polyesters. A series of hydroxyl-carboxylate (AB-type) monomers with 1,5-disubstituted indole and 3–6 methylene units was conveniently synthesized and used in bulk polycondensation to yield the corresponding polyesters with decent molecular weight. These new monomers and polyesters showed enhanced thermal stability compared to the previously reported monomers and polyesters with a 1,3-disubstituted indole structure. According to DSC results, these polyesters showed tunable glass transition temperatures (Tg ∼57–80 °C), depending on the length of the aliphatic methylene units. DSC and WAXD measurements revealed that these polymers did not crystalize from melt, but the ones with 3 or 5 methylene units per repeating unit crystalized from solution. Finally, we demonstrated that the new polyesters with 1,5-disubstituted indole units could be crosslinked using sustainable aromatic aldehyde, which could further enhance their thermal properties.


Introduction
PET is a widely used polyester for many applications (e.g. textiles, packaging, thermoforming for manufacturing, engineering resins), thanks to its desirable thermal and mechanical properties. [1][2][3][4][5] These desirable properties are largely endowed by its rigid aromatic units (i.e. terephthalates) in the backbone, without which the aliphatic polyesters (e.g. polylactide, polycaprolactone, polyhydroxyalkanoates, etc.) usually exhibit inferior thermal and mechanical properties. [6][7][8] Another aromatic unit naphthalene (i.e. naphthalate) that is larger than benzene has also been used for a commercial polyester, polyethylene naphthalate (PEN), which has enhanced thermal (T g $ 120 C), mechanical, and barrier properties compared to PET. 9,10 Unfortunately, both terephthalate and naphthalate are produced in industry from fossil resources, which are non-sustainable. [11][12][13][14][15] Bio-based terephthalate has been produced in laboratory scale, [16][17][18] but its industrial production remains challenging due to the synthetic complexity and high costs. As such, alternative aromatic units from renewable resources have received growing attention. [19][20][21] New sustainable aromatic building blocks for polyester synthesis have been frequently reported using various biomass resources (e.g. sugar, lignin, cinnamic acid, etc.). [22][23][24][25][26][27][28][29][30][31] Among these building blocks, furan-based ones have attracted the most attention. [32][33][34][35][36][37][38][39] Particularly, 2,5-furan-dicarboxylic acid (FDCA) has been conveniently produced from sugar resources and used to synthesize various polyesters. [40][41][42][43][44][45][46] For example, polyethylene furanoate (PEF) is a fully biobased polyester, which can be prepared using FDCA and bio-based ethylene glycol. PEF shows favorable thermal and mechanical properties and superior barrier properties than PET, 47,48 so it is expected to become the sustainable plastic bottle materials for next generation. 49 Enlightened by the rapid development of PEF, we have recently initiated investigation on the use of another sustainable aromatic unit, indole, for polyester backbone structures. 50 Indole is a large aromatic unit that widely exists in nature and wastewater streams, animal feces, and is frequently used in industry. [51][52][53] There are several bio-based production routes for indole, such as thermal conversion and ammonization of furfural, 54 pyrolysis of micro-algae, 55 microbial synthesis from glucose, 56 and direct conversion from bio-based aniline and ethylene glycol. 57 In 2018, we reported our rst indole-based dicarboxylate monomer with aromatic and aliphatic ester groups, which was used to produce a series of polyesters with high glass transition temperatures (T g 55-99 C) and low coloration. 50 To further enhance the thermal stability, indolebased AB and AABB monomers with only aromatic ester groups were developed aerward, which were used to fabricate new indole-based polyesters with enhanced thermal stability and T g (up to 113 C). 58,59 It should be noted that all our previously reported indole-based polyesters contain the same disubstitution pattern (i.e. 1,3-disubstituted indole units). Since di-substitution patterns of benzene or furan units have shown signicant impact on polyester properties, [60][61][62][63] it is expected that different di-substitution patterns of indole will also exert signicant impact on the thermal and processing characteristics of polyesters. This structure feature can be utilized to optimize the molecular design when using indole as a new sustainable aromatic unit for polyesters.
Herein, we report on facile synthesis and polymerization of a series of 1,5-disubstituted indole-based monomers with a carboxylate ester and a hydroxyl group (AB monomers) and 3-6 methylene units. The molecular structures, thermal properties, and processability of the resulting polyesters were characterized and compared to the previously reported AB-type polyesters with isomeric 1,3-disubstituted indole units. As a result, the new polyesters with 1,5-disubstituted indole units showed further enhanced thermal stability. Finally, crosslinking of these new polyesters using a sustainable aldehyde was also investigated.

Polymer synthesis
To a 25 mL two-neck round-bottom ask equipped with mechanical stirrer and a gas inlet (connected to N 2 or vacuum) were added monomer 3a-d (0.50 g), DBTO (5 mg) and mesitylene (5 mL) at room temperature. The reaction mixture was stirred at 180 C for 30 minutes under N 2 . Aerward, the N 2 ow was switched to vacuum, and the reaction was stirred at 180 C for 6 h under vacuum. Then the reaction mixture was cooled to room temperature, dissolved in chloroform (5 mL), precipitated into diethyl ether (200 mL), and dried under vacuum to yield a off-white powder (P3a-d).

Polymer lm casting
A polymer powder (150 mg) was dissolved in 1 mL solvent at room temperature and kept stirring for 1 h to yield a clear, viscous solution. Chloroform was used as the solvent for P3a, P3b and P3d, and a 2 : 1 mixture of chloroform/ hexauoroisopropanol was used as the solvent for P3c. The resulting polymer solution was cast onto a glass Petri dish (diameter of 35 mm) and dried at room temperature for 3 days to yield a polymer lm.

Crosslinking of polymer P3d
To a well-stirred solution of P3d (5.0 mg) in acetonitrile (2 mL) was added 3,4-dimethoxybenzaldehyde (veratraldehyde, 3 mg) and iodine (1 mg) at room temperature. The reaction was stirred at room temperature overnight under N 2 . Aerward, the solvent was evaporated, and the residue was washed with chloroform and dried over vacuum to yield the crosslinked solid P3d (5.8 mg). The swelling ratio (Q) of the crosslinked P3d was evaluated by measuring the mass of the material before and aer being immersed in chloroform for 24 h. The Q value was calculated according to the equation below. 64,65 Q ¼ W s /W d W s is the weight of the swollen polymer and W d is the weight of the crosslinked polymer in dry state.

Analytical methods
Nuclear magnetic resonance (NMR) measurements were carried out on a Bruker DRX 400 spectrometer at the proton frequency of 400.13 MHz and a carbon frequency of 100.61 MHz. Fourier transform infrared (FTIR) spectra were obtained with an attenuated total reection (ATR) setup using a Bruker Alpha FT-IR spectrometer. Twenty-four scans were co-added using a resolution of 4 cm À1 . Gel permeation chromatography (GPC) was carried out with 2xPL-Gel Mix-B LS column and OmniSEC Triple Detectors (refractive index, viscosity, and light scattering). All measurements were carried out at 35 C at a concentration of 3 mg mL À1 using chloroform as the eluent, and at an elution rate of 1 mL min À1 . Calibration was performed with polystyrene standard sample (M n ¼ 96 kg mol À1 from Polymer Laboratories). Differential scanning calorimetry (DSC) measurements were performed using a TA Instruments DSC Q2000. The samples were studied with a heating rate of 10 C min À1 under nitrogen with a purge rate of 50 mL min À1 . The sequence consisted of a heating ramp from 25 C to 200 C, followed by a cooling ramp to 25 C and nally a heating ramp to 200 C, which was employed to determine the glass transition temperature (T g ). Thermogravimetric analysis (TGA) was performed with a thermogravimetric analyser TA Instruments Q500 at a heating rate of 10 C min À1 under nitrogen with a purge rate of 50 mL min À1 . Isothermogravimetric analysis was performed with a thermogravimetric analyser TA Instruments Q500 under nitrogen with a purge rate of 50 mL min À1 . The isothermal temperatures used were 250, 275, 300, 325, 350, 375 and 400 C. High resolution mass spectrometry (HRMS) was performed by direct infusion on a Water Xevo-G2 QTOF mass spectrometer using electrospray ionization. Dynamic mechanical analyses (DMA) were performed from 20 C to 100 C using a TA instrument Q800, in the cantilever bending mode at a heating rate of 3 C min À1 and a frequency of 1 Hz. Samples were molded at 150 C into rectangular bars with dimensions 35 mm (length) Â 5 mm (width) Â 1 mm (thickness) and cooled at room temperature. Wide angle X-ray diffraction (WAXD) measurements were performed using a Stoe STADI MP X-ray powder diffractometer under ambient conditions. Measurements were performed over 2q ranges 10-100 with copper K a (0.15406 nm) radiation. Based on the integrals of sharp WAXD signals, the degrees of crystallinity were quantied according to the equation below: Crystallinity ¼ Area of crystalline peaks Area of all peaks ðcrystalline þ amorphousÞ Â 100% Dynamic rheology measurements were carried out using a TA Instruments Advanced Rheometer AR2000 ETC at 160 and 180 C during 40 min. Measurements were performed using parallel plates with constant strain (2%) and oscillation (1 Hz). The specimens (15 mm diameter and 1 mm thickness) were prepared by hot-pressing the polymer powders.

Synthesis and molecular characterization
Monomers with both a hydroxyl and methyl carboxylate groups (AB monomers 3a-d, Scheme 1) were synthesized by the reaction of commercially available methyl indole-5-carboxylate (1) and u-bromoalkanols (2a-d) under mild basic condition (K 2 CO 3 , room temperature). Three monomers 3a, 3c and 3d were prepared with moderate to high yields (60-98%) and high purity (according to NMR spectra, Fig. 1, 2, and S1-S24, ESI †) by this method. However, for 3b, the reaction proceeded very slowly under such conditions, and aer 24 hours there was only approximately 5% of 3b in the crude reaction mixture according to the 1 H NMR spectrum (Fig. S25, ESI †), this could be attributed to the formation of an undesired cyclic byproduct, tetrahydrofuran (THF), which was evidenced in the 1 H NMR spectrum (Fig. S25, ESI †). The formation of THF was also reported in the literature for the synthesis of poly(butylene terephthalate) and poly(butylene succinate). 66,67 As such, a modied synthetic protocol was used for the synthesis of 3b using tert-butyldimethylsilyl (TBS) protecting group. As shown in Scheme 1, a TBS-protected u-chloroalkanol (2bx) was used to react with 1 under strong basic condition (NaH) to yield a TBS-protected monomer precursor 3bx. The crude 3bx was directly subjected to deprotection using TBAF to yield monomer 3b in 95% yield (from 1) and high purity (according to NMR analyses, Fig. 1, 2 and S7-S9, Scheme 1 Synthesis of indole-based AB monomers (3a-d) and polyesters (P3a-d). ESI †). Melt-polycondensation of monomers 3a-d was carried out according to our previously reported procedure for AB monomers with 1,3-disubstituted indole units. 58 The polymerizations were carried out at 180 C for 6 h under vacuum, followed by a straightforward precipitation from diethyl ether to yield off-white powders as P3a-d in decent yields (66-78%).
P3a-d and their corresponding monomers 3a-d were characterized by 1 H NMR spectroscopy assisted with 2D-NMR spectrum ( Fig. 1 and S1-S24, ESI †). First, it was evident that the 1 H NMR spectra of P3a-d (Fig. 1A, C, E and G) showed broader peaks compared with that of the corresponding monomers 3a-d (Fig. 1B, D, F and H), which indicated the formation of polymers. Furthermore, the 1 H NMR spectra of the monomers showed characteristic signals including CH 2 protons close to indole (a), CH 2 protons close to the OH group (b), protons on the aliphatic CH 2 "bridge" (c-f), and CH 3 protons of ester group (x). Aer the polymerization, the ester signal x for the monomers disappeared in the spectra of all polymers, which conrmed the monomer consumption. Note that the small signal with a slightly different chemical shi (denoted as y) was due to the methyl ester group at the chain end. All the other characteristic signals of the monomers remained. Compared to the corresponding monomer signals, signals b of the polymers showed the most signicant down-eld shis (by 0.68-0.98 ppm) due to the formation of electron-withdrawing ester bonds. The second signicant downeld shis were observed for signal d in the spectra of polymers compared to that of the monomers (by 0.21-0.28 ppm), because this proton is the second closest to the ester bond (next to proton b). The chemical shis of the other signals (a, c, e, f, and aromatic signals) did not change signicantly aer the polymerizations, because they are relatively far away from the reaction site (ester bonds).
The chemical structures of P3a-d were further characterized by 13 C-NMR spectroscopy (Fig. 2). The signals for all the monomers were unambiguously assigned (Fig. 2B, D, F and H). Aer the polymerization, the signal x (ester CH 3 carbon of monomers) disappeared in the 13 C NMR spectra of all polyesters ( Fig. 2A, C, E and G), conrming the success of trans-esterication. All the other characteristic signals remained in the spectra of polyesters, including the signals for carbonyl carbons (n), indole aromatic carbons, and the aliphatic alkylene carbons (a-f).
In addition, P3a-d were characterized by FT-IR spectroscopy (Fig. 3). The characteristic C-H and C]O stretching bands for the polyesters were clearly observed at 2940 and 1710 cm À1 , respectively. In the FT-IR spectra of the corresponding monomers (Fig. S26A, ESI †), an O-H stretching (3150-3650 cm À1 ) band was observed, which completely disappeared in the spectra of all polymers. This further conrmed the consumption of monomers. A close comparison between the FT-IR spectra of monomer 3c and polymer P3c was shown as an example in Fig. S26B. †

Thermal and dynamic mechanical properties
The thermal stability of P3a-d was investigated by TGA measurements in nitrogen. As shown in Fig. 4      Since the corresponding members of the two series of polyesters (P3a-d and P4a-d) showed comparable molecular weights (P3a, P3c showed slightly higher, while P3b and P3d showed slightly lower molecular weights compared to their counterparts in P4ad series), the observed enhanced thermal stability of polyesters P3a-d series was reasonably attributed to the molecular structure of 1,5-disubstituted indole units in the backbones. To further verify this, TGA measurements of 1,5-disubstituted monomers 3a-d and the corresponding 1,3-disubstituted monomers 4a-d were conducted and compared (Fig. S28, ESI †). As a result (Table S1, ESI †), T 95 d and T d values of 3a-d were higher than that of corresponding monomers 4a-d with the same number of CH 2 units. This conrmed that the enhanced thermal stability of P3a-d (compared to the corresponding P4ad with the same number of CH 2 units) could be attributed to the higher thermal stability of the 1,5-disubstituted monomeric units under the TGA conditions.
To gain further insight into the thermal stability of the indole-based polyesters, we performed isothermal TGA measurements on P3d and other two previously synthesized polyesters with 1,3-disubstituted indole units (P4d and P5d, chemical structures shown in Fig. S27, ESI †), 50,58 both all of which are the most thermally stable members of their corresponding series according to their T 95 d values (TGA). The isothermal TGA measurements were performed at seven elevated temperatures (i.e., 250, 275, 300, 325, 350, 375, and 400 C). As shown in Fig. S29 and Table S2 (ESI †), P3d showed a lower weight loss rate compared with P4d and P5d at all the temperatures measured, which indicated its enhanced longterm thermal stability.
The thermal stability of P3d, P4d and P5d under mechanical strain was evaluated by time-sweeping rheology measurements at two mildly high temperatures (160 C and 180 C) at a frequency of 1 Hz using a constant strain of 2%. This ensured that the measurements were performed within the linear viscoelastic region. As shown in Fig. S30 (ESI †), the melt shear storage modulus of P5d at both temperatures showed an increasing trend over time, which indicated crosslinking and gel formation. 68 Such phenomenon was not observed for P3d and P4d at 160 C, indicating the stability of both polymers under such processing conditions. At higher temperature (180 C), both P3d and P4d showed a slight increase of the shear storage moduli, indicating some extent of crosslinking and gelation.
The thermal behavior of polyesters P3a-d was investigated by DSC measurements. Both powders (precipitated from chloroform/ether solution) and lms (cast from chloroform solution, see later section) of P3a-d were measured. The glass transition was clearly observed in the second heating curves of both the lms (Fig. 5B) and the powders (Fig. 5D) and the T g values were consistent ( Table 1). The T g values for P3ad decreased as the increased length of aliphatic alkylene units in the backbone (T g z 80, 75, 64 and 57 C for P3a-d, respectively), which was consistent with the increased backbone exibility. This trend was also observed for the previously reported indole-based polyesters with 1,3-disubstitution pattern (P4a-d, P5a-d), 58 as well as polyesters with other bio-sourced aromatic units (e.g. furan, vanillic acid). [69][70][71][72] DSC results also provided valuable information regarding the crystallinity of P3a-d as solvent-cast lms and solutionprecipitated powders. As shown in Fig. 5A, the powder of P3c showed a melting endotherm in its rst heating curve (peak value at 153 C). However, no melting endotherm was observed during the second heating cycle (Fig. 5B). This indicated that Table 1 Molecular characterization and thermal properties of P3a-d. M n , M w , and PDI were determined by GPC. T 95 d (temperature for 5% weight loss) was measured by TGA. The yield is the isolated polymer yield (in %) a T g was measured by the DSC second heating curves. b T g was taken as the peak values of the loss modulus curves measured by DMA. P3c crystallized from solution but not from melt during the DSC measurements. All the other polymer powders did not crystalize from solution or melt. For solution-cast lms, melting endotherms were observed during the rst heating cycle of P3a and P3c (T m ¼ 138 and 151 C for P3a and P3c respectively, Fig. 5C), which indicated that they crystalized from solution. Like powders, no melting endotherms were observed during the second heating cycle (Fig. 5D), which indicated that P3a and P3c did not crystalize from melts under the DSC conditions. This behavior could be attributed to the higher viscosity of polymer melts compared to their solutions, which caused slower crystallization rate of the polymers in molten state. Similar observation was also reported for other polyesters with rigid cyclic sugar-based units. 73,74 The T g values of P3a-d were independently conrmed by DMA measurements (Fig. 6). The peak values on the loss modulus (E 00 ) curves were taken as the T g values (77, 69, 61 and 51 C for P3a-d, respectively, Fig. 6 and Table 1), which were consistent with the corresponding values measured by DSC (DT g ¼ 3-6 C). The storage moduli (E 0 ) at glassy plateau (25 C) of P3ad showed a decreasing trend as the increased length of alkylene unit (2009,1969,1777 and 1752 MPa, respectively), which was consistent with the trend reported for other indole-based polyesters with 1,3-disubstitution pattern. 50,58 The storage moduli (E 0 ) at glassy plateau (25 C) of P3a-d were higher than the corresponding values for the polyesters with 1,3-disubstitution pattern (P4a-d and P5a-d) under identical measurement conditions, but lower than the corresponding values of bottle-grade PET and PETlike copolyester Akestra™ (from Perstorp AB) under the same measurement conditions ($3000 MPa). 58

Solution-casting lms and WAXD
P3a-d were successfully cast into thin lms according to a solution casting protocol using chloroform (for P3a, P3b and P3d) or chloroform/HFIP mixture (for P3c) as the solvent. 75 UVvis spectra of P3a-d solutions (Fig. S31, ESI †) showed typical indole absorbance band at $280 nm. As shown in Fig. 7, the lms of P3a and P3c were opaque, while the other two lms for P3b and P3d were translucent. This was consistent with the observed crystallinity of P3a and P3c and amorphous nature of P3b and P3d in DSC rst heating cycle discussed earlier.
The crystallinity of P3a-d solution-cast lms and powders were conrmed by WAXD (wide angle X-ray diffraction) analysis (Fig. 8). For P3b and P3d lms, no sharp signal was observed in their WAXD curves (only a broad signal centered at 2q ¼ 19.2 ), which conrmed their amorphous state. In the WAXD pattern of P3a lm (Fig. 8A) Interestingly, it was the polyesters with odd numbers of methylene units per repeating unit that showed higher crystallinity (i.e. P3a, P3c with 3 or 5 CH 2 units per repeating unit, respectively). This was different from the observation for the previously reported polyester series with 1,3-disubstituted indoles (P4a-d), which showed increased crystallinity upon the increased number of methylene units per repeating unit (i.e. P4c and P4d showed degrees of crystallinity as 58 and 77%, respectively, while P4a-b were amorphous). 58 Finally, P3a-d in their powder form (precipitated from chloroform/ether) were also measured by WAXD for comparison. As shown in Fig. 8B, no sharp signal was observed for any of the measured polymer powders, which indicated that no crystallization occurred during the precipitation from chloroform/ether at room temperature or during storage at room temperature. This result also indicated that the small crystallinity of the P3c powder observed in DSC measurements (Fig. 5A) was insignicant.

Crosslinking with biobased aldehyde
1,5-Disubstituted indole unit has no substituent at the 3 position, which is the most electrophilic position of indole. This indicates that the new polyesters P3a-d may be conveniently modied on the indole rings at the 3 position, enabling quick  chemical modication. In this work, we performed a crosslinking reaction of P3d (Scheme 2) with lignin-based 3,4dimethoxybenzaldehyde 76 by a straightforward Friedel-Cras reaction. Aer the crosslinking of P3d in the solution, an insoluble gel was formed, which was dried and subjected to TGA, DSC and FTIR measurements (Fig. 9). According to TGA results, thermal stability of P3d remained almost unchanged aer crosslinking (Fig. 9A). DSC results indicated that the T g value increased from 57 to 75 C aer crosslinking (Fig. 9B), showing enhanced thermal performance. FT-IR spectra of P3d remained almost unchanged aer crosslinking (Fig. S33, ESI †). The swelling ratio of the crosslinked P3d was determined as $6.2, aer the pre-dried sample was immersed in chloroform at room temperature for 24 h. This preliminary investigation indicated the potential for enhancing the performance of polyesters with 1,5-disubstituted indole units in the future.

Conclusions
A new series of polyesters with 1,5-disubstituted indole units was synthesized and compared to our previously reported polyesters with 1,3-disubstitution pattern, aiming to provide fundamental insight into the impact of di-substitution pattern on the physical properties and processing characteristics. According to the DSC and WAXD results and visual inspection on clarity, the solution-cast lms of the obtained polyesters showed crystallinity when odd numbers (3 or 5) of methylene units were present in each backbone repeating units. Those polyesters with even numbers (4 or 6) of methylene units per repeating unit were amorphous. TGA results revealed enhanced thermal stability of the new series of polyesters compared to the previously reported polyesters with 1,3-disubstituted indole units. Melt-rheology measurements conrmed the suitable processability of the new polyesters. Finally, crosslinking at the 3 position of indole units in the new polyester by bio-based aldehyde showed further enhanced thermal properties, which may have the potential to broaden the application range of these new polyesters. Future investigations will be directed toward optimization of the polymerization and processing conditions, which will facilitate investigation on other important material properties, e.g., mechanical and barrier properties.

Author contributions
PW performed the synthesis and characterization of all the polyesters. BZ conceived and advised on the project. PW and BZ wrote the paper.

Conflicts of interest
There are no conicts to declare.