Resin Transfer Moldable Fluorinated Phenylethynyl-Terminated Imide Oligomers with High Tg: Structure–Melt Stability Relationship

Phenylethynyl-terminated aromatic polyimides meet requirements of resin transfer molding (RTM) and exhibits high glass transition temperature (Tg) were prepared. Moreover, the relationship between the polyimide backbones structure and their melting stability was investigated. The phenylethynyl-terminated polyimides were based on 4,4′-(hexafluorosiopropylidene)-diphthalic anhydride (6FDA) and different diamines of 3,4′-oxydianiline (3,4′-ODA), m-phenylenediamine (m-PDA) and 2,2′-bis(trifluoromethyl)benzidine (TFDB) were prepared. These oligoimides exhibit excellent melting flowability with wide processing temperature window and low minimum melt viscosities (<1 Pa·s). Two of the oligoimides display good melting stability at 280–290 °C, which meet the requirements of resin transfer molding (RTM) process. After thermally cured, all resins show high glass transition temperatures (Tgs, 363–391 °C) and good tensile strength (51–66 MPa). The cure kinetics studied by the differential scanning calorimetry (DSC), 13C nuclear magnetic resonance (13C NMR) characterization and density functional theory (DFT) definitely confirmed that the electron-withdrawing ability of oligoimide backbone can tremendously affect the curing reactivity of terminated phenylethynyl groups. The replacement of 3,4′-ODA units by m-PDA or TFDB units increase the electron-withdrawing ability of the backbone, which increase the curing rate of terminated phenylethynyl groups at processing temperatures, hence results in the worse melting stability.


Introduction
Aromatic polyimides (PIs) are a class of engineering plastics extensively used in electronics, microelectronics and aerospace, due to their high heat resistance, excellent materials properties and electrical properties [1,2]. For the purpose of improving PIs performances and expanding their applications, various polyimide-based composites have been fabricated by doping inorganic or organic species [3,4]. Carbon fiber-reinforced polyimides composites have been widely used for high-temperature applications such as air-engines, aerospace vehicles, and precision machinery for decades [5][6][7].
Traditionally, hand lay-up prepreg/autoclave techniques have been used to fabricate polyimide/carbon fiber composites on large scales [8,9]. However, resin transfer mold-poured into a high-speed laboratory blender (IKA, Guangzhou, China) containing adequate boiling water/ethanol (v/v = 1:1) mixture to precipitate MC-O crude product. The fine powder was isolated by filtration and washed with boiling water/ethanol twice. The solid powder was dried at 200 °C under vacuum for 12 h to obtain a yellow powder (38.12 g, yield: 95.3%). m.p, (DSC): 290 °C. 1  The model compound MC-P was prepared according to the above method except that 3,4′-ODA was replaced by m-PDA to obtain yellow powder (38. The model compound MC-F was prepared according to the method as described above except that 3,4′-ODA was replaced by TFDB to obtain red brown powder (38. The model compound MC-P was prepared according to the above method except that 3,4 -ODA was replaced by m-PDA to obtain yellow powder (38. The model compound MC-F was prepared according to the method as described above except that 3,4 -ODA was replaced by TFDB to obtain red brown powder (38.

Preparation of Oligoimides End-Capped with Phenylethynyl
The oligoimides with designed M n Cs of 1000 g/mol were also prepared by the onepot high-temperature polymerization with the reaction of 6FDA, 4-PEPA and different aromatic diamines (Scheme 2). A typical preparation procedure of PETI-O is illustrated below. To a 250 mL three-neck round bottom flask with a condenser, mechanical stirrer, thermometer, dean stark trap and nitrogen flow was added 6FDA (15.46 g, 34.8 mmol) and NMP (66.0 g). The mixture was stirred at room temperature until the aromatic dianhydrides were completely dissolved. Subsequently, 3,4 -ODA (17.9 g, 89.2 mmol) and about 10 g NMP was added. The mixture was then stirred for 6 h in an ice-water bath. 4-PEPA (27.0 g, 10.9 mmol) and residual NMP were added into the flask, resulting in a reaction mixture with 30% solids (w/w). The mixture was stirred in an ice bath for another 16 h. 13.56 g toluene and a catalytic amount of isoquinoline were added. The mixture was heated to 180 • C for 10 h. For the purpose of elimination of toluene, the mixture was heated to 200 • C. The flask was then cooled by ice-water bath, and the dark brown solution was poured into a high-speed laboratory blender containing adequate boiling water/ethanol (v/v = 1:1). The precipitates were isolated by filtration and washed with boiling water-ethanol twice. The solid was dried at 200 • C under vacuum for 12 h to obtain a yellow powder (53.1 g, yield: 97%).

Preparation of the Thermally-Cured Polyimides
The oligoimide powders were compression moulded in stainless steel (80 × 80 mm) with a hot-press machine (IDM Co, Washington DC, USA). The oligoimide powders were melted at 305 °C. The temperature was gradually raised to 380 °C in 50 min. The pressure

Preparation of the Thermally-Cured Polyimides
The oligoimide powders were compression moulded in stainless steel (80 × 80 mm) with a hot-press machine (IDM Co, Washington DC, USA). The oligoimide powders were melted at 305 • C. The temperature was gradually raised to 380 • C in 50 min. The pressure of 4 MPa was applied, the powders were cured for 2 h. When the cured resins were cooled to about 250 • C, the pressure was released. The PI sheets (80 × 80 × 2 mm) were removed from the mould and stored under ambient temperature.

Measurements
Infrared spectra (IR) were measured on a Perkin-Elmer 782 Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer, Waltham, MA, USA). 1 H and 13 C NMR spectra were obtained on a Bruker AVANCE 400 spectrometer (Bruker, Karlsruhe, Germany) using dimethyl sulfoxide (DMSO-d 6 ) or trifluoroacetic acid (CF 3 COOD) as a solvent. Gel permeation chromatography (GPC) was performed on a Waters system equipped (Waters, Milford, MA, USA) with a model 1515 pump, a 2414 refractive index detector using NMP containing LiBr (J$K Co, Beijing, China, 0.02 mol/L) as the eluant. The elemental analysis was carried out on a Thermo Flash Smart elemental analyzer (Thermo Fisher Scientific, Carlsbad, CA, USA).
Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectra were carried out with an Autoflex IIIMALDI-TOF mass spectrometer (Bruker, Karlsruhe Germany). Wide-angle X-ray diffraction (WXRD) measurements were taken on a Rigaku D/max-2500 X-ray powder diffractometer (Riguku Beijing Co, Beijing, China) with Cu/Kα radiation, operated at 40 kV and 200 mA. The 2θ scan data were collected in the scale of 3 • -60 • . The geometry optimization, Natural bond orbital charge distribution and spin density distribution were achieved by Gaussian09, Revision E.01 program (Gaussian Inc, Wallingford, CT, USA) using the M062X/6-311+G** basis sets.
Differential scanning calorimetry (DSC) analysis was performed on a TA-Q100 analyzer (TA Instruments, New Castle, DE, USA) at a heating rate of 10 • C/min under nitrogen atmosphere. Thermogravimetric analysis (TGA) was accomplished on a TA-Q50 series thermal analysis system (TA Instruments, New Castle, DE, USA) at a heating rate of 20 • C/min under nitrogen or air atmosphere. Complex viscosity (|η*|) was performed on a TA-AR2000 rheometer equipped (TA Instruments, New Castle, DE, USA) with a 25 mm-diameter parallel plate fixture measured at the heating rate of 4 • C/min. The upper plate was oscillated at a fixed strain of 0.1% and a constant angular frequency of 5 rad/s, while the lower one was attached to a transducer that recorded the resultant torque and then converted to the complex viscosity. Dynamic mechanical analyses (DMA) were performed on a TA-Q800 analyzer (TA Instruments, New Castle, DE, USA) with a heating rate of 5 • C/min. A three-point bending mode was employed with the specimen size of 20 × 5 × 2 mm.
The mechanical characteristics of the thermally-cured resins were determined at 25 • C via an Instron 5567 universal testing machine (Instron, Norwood, MA, USA), and the resultant values represented the average of at least five runs per specimen. Tensile strength and modulus of the 5.0 mm-wide sample were acquired at a crosshead speed of 1.0 mm/min following GB/T 1040-2006, and flexural strength and modulus of 3.0 mm-wide one were obtained at a rate of 1.0 mm/min following HG/T 3840-2006, respectively.

Cure Kinetics of Phenylethynyl-Endcapped Oligoimides
The analyses of oligoimides and the measurement of T g at different thermally-heated times and temperatures were accomplished by the DSC method (TA-Q100) under a nitrogen atmosphere at a flow rate of 40 mL/min. Temperatures were calibrated by indium and zinc standards.
In order to calculate the activation energy E a of the phenylethynyl curing reaction, Kissinger's method [36][37][38][39] was used. The uncured imide oligomers were scanned by DSC at different heating rates (2, 5, 10, 15 and 20 • C/min, respectively). The exothermic Polymers 2021, 13, 903 7 of 23 peak temperatures (T p ) increase with the ramping of the heating rate (β). Usually, it was assumed that the reaction rate reaches the maximum at peak temperatures. Hence, Equation (1) can be obtained.
where A is the pre-exponential factor; E a is the activation energy, R is the gas constant, f'(α p ) is the differentiation of the function of convention. The value of E k can be calculated from the slope of the plot of ln β/T 2 p vs. 1/T p . Samples with a mass of about 7 mg in size were heat-treated at 290 • C or 300 • C for different times. The effects on T g by altering of temperature or time were investigated. In order to eliminate the effect of possible crystalline on the T g values, samples were quenched before the measure of T g values. The scan of the samples was firstly carried out from 40 to 300 • C at a rate of 20 • C/min; after being cooled quickly to 40 • C, the samples were heated to 500 • C at the rate of 10 • C/min. The modified DiBenedetto equation was used to calculate the reaction extent (x) by determining the T g for a highly crosslinked network [36,[40][41][42].
where T go and T g∞ represent the glass transition temperatures before cure and after full cure. T g means the glass transition temperature of the sample isothermally cured at each temperature for specific times. λ is equal to the ratio of the complete cured material's isobaric heat capacity of, ∆C p∞ , to that of uncured material, ∆C p0 . Experimentally, λ could be calculated as follows [42].
In addition, even if the selected T g∞ does not correspond to the ideal value of fully cured materials, the calculated x is still valid for kinetic analysis. In this case, x is substituted by x' that refers to x/x M , a relative reaction extent. In this study, T g s of resins cured at 380 • C for 2 h were defined as T g∞ s.

Characterization of Oligoimides End-Capped with Phenylethynyl Groups
A series of phenylethynyl end-capped aromatic oligomers derived from 6FDA was prepared via one-pot high-temperature polymerization (Scheme 2). PETI-O, PETI-F and PETI-P were prepared by reacting 6FDA with three different diamines. PETI-1, PETI-2 and PETI-3 were prepared by reacting 6FDA with a mixture of 3,4 -ODA and m-PDA at different mole ratios. The molecular weights of oligoimides were measured by GPC. As showed in Table 1, all oligoimides showed similar M n of about 1300 g/mol, which were close to the calculated molecular weight of 1000 g/mol. Figure 1 shows the representative FTIR spectra of PETI-O, PETI-F and PETI-P before and after thermally cured at 380 • C for 2 h. The characteristic absorption at 2212 cm −1 attributed to stretching vibrations of ethynyl (-C≡C-) groups and the absorption at 3059 cm −1 attributed to aromatic C-H stretching vibration are observed. The asymmetric and symmetric absorption peaks at 1780 and 1722 cm −1 , as well as the bending vibration peak at 743 cm −1 are attributed to the imide C=O in the oligomer backbone. Meanwhile, the absorption at 1360 cm −1 is assigned to the vibration of C-N stretching. Moreover, the asymmetric and symmetric absorption peaks at 1244 and 1103 cm −1 assigned to C-O-C stretching vibration for PETI-O, and 1124 cm −1 assigned to C-F stretching vibration for PETI-F are also detected. The cured oligoimides show no absorption at 2212 cm −1 , demonstrating that phenylethynyl groups were consumed during the thermal-curing process. tuted by x' that refers to x/xM, a relative reaction extent. In this study, Tgs of resins cured at 380 °C for 2 h were defined as Tg∞s.

Characterization of Oligoimides End-Capped with Phenylethynyl Groups
A series of phenylethynyl end-capped aromatic oligomers derived from 6FDA was prepared via one-pot high-temperature polymerization (Scheme 2). PETI-O, PETI-F and PETI-P were prepared by reacting 6FDA with three different diamines. PETI-1, PETI-2 and PETI-3 were prepared by reacting 6FDA with a mixture of 3,4′-ODA and m-PDA at different mole ratios. The molecular weights of oligoimides were measured by GPC. As showed in Table 1, all oligoimides showed similar Mn of about 1300 g/mol, which were close to the calculated molecular weight of 1000 g/mol. Figure 1 shows the representative FTIR spectra of PETI-O, PETI-F and PETI-P before and after thermally cured at 380 °C for 2 h. The characteristic absorption at 2212 cm −1 attributed to stretching vibrations of ethynyl (-C≡C-) groups and the absorption at 3059 cm −1 attributed to aromatic C-H stretching vibration are observed. The asymmetric and symmetric absorption peaks at 1780 and 1722 cm -1 , as well as the bending vibration peak at 743 cm -1 are attributed to the imide C=O in the oligomer backbone. Meanwhile, the absorption at 1360 cm -1 is assigned to the vibration of C-N stretching. Moreover, the asymmetric and symmetric absorption peaks at 1244 and 1103 cm -1 assigned to C-O-C stretching vibration for PETI-O, and 1124 cm -1 assigned to C-F stretching vibration for PETI-F are also detected. The cured oligoimides show no absorption at 2212 cm -1 , demonstrating that phenylethynyl groups were consumed during the thermal-curing process.  Molecular structures of the representative PEPA end-capped oligoimides of PETI-O, PETI-F and PETI-P were also characterized by MALDI-TOF. Figure 2 shows the result, and the resin species detected are listed in Table S1. All the PETIs mainly consist of 4 different components with different polymerization degrees (n = 0, 1, 2, 3). The molecular weights of the resin species detected were in good accordance with the calculated ones. For instance, PETI-O has a peak located at m/z = 1291 corresponding to the species with n = 1, which shows the highest peak intensity. The main species located at 1532 m/z and 1108 m/z with n = 1 were also detected in the samples of PETI-F and PETI-P, respectively. Clearly, the characterization of MALDI-TOF demonstrated the accuracy of syntheses.  Molecular structures of the representative PEPA end-capped oligoimides of PETI-O, PETI-F and PETI-P were also characterized by MALDI-TOF. Figure 2 shows the result, and the resin species detected are listed in Table S1. All the PETIs mainly consist of 4 different components with different polymerization degrees (n = 0, 1, 2, 3). The molecular weights of the resin species detected were in good accordance with the calculated ones. For instance, PETI-O has a peak located at m/z = 1291 corresponding to the species with n = 1, which shows the highest peak intensity. The main species located at 1532 m/z and 1108 m/z with n = 1 were also detected in the samples of PETI-F and PETI-P, respectively. Clearly, the characterization of MALDI-TOF demonstrated the accuracy of syntheses.   Table 2 summarizes the DSC data of them. It is noteworthy that PETI-F exhibited the highest Tg of 170 °C, which is caused by rigidity and linearity biphenyl moiety substituted by -CF3 groups   Table 2 summarizes the DSC data of them. It is noteworthy that PETI-F exhibited the highest T g of 170 • C, which is caused by rigidity and linearity biphenyl moiety substituted by -CF 3 groups [43,44], while PETI-O and PETI-P show similar T g values. Meanwhile, apparent melting endotherms are observed for PETI-O and PETI-F, while PETI-P shows no melting behavior. The exothermal peaks in the range of 350-450 • C are due to the thermal curing reaction of phenylethynyl. PETI-F shows the lowest curing onset temperature of 350 • C and peak-temperature of 391 • C. PETI-O and PETI-P have similar curing onset temperatures; however, PETI-P shows a lower T exo of 395 • C. It indicates that the curing reactivity of the three PETIs increases in the order: PETI-O < PETI-P < PETI-F. It seems that the m-PDA and TFDB moieties in polymer backbones encourage the curing reaction compared with 3,4 -ODA moiety.  The WXRD patterns of oligomers are shown in Figure 4. Because of the difference chemical structure, PETI-F shows unique crystalline characteristics. PETI-O shows obv ous diffraction peaks, while the WXRD pattern of PETI-P is broad, revealing its amo phous nature. It has been demonstrated that unsymmetrical structures and meta caten tion in polyimides backbones would increase their solubility and melt processability. T bent chains endowed by m-PDA decrease the packing of oligomer chains, hence tend decrease interactions between the oligoimide chains, which lead to the decrease of t crystallinity of oligoimides [45,46]. Other oligoimides copolymerized by 3,4'-ODA and m PDA displayed semi-crystalline behavior to different extent with diffraction peaks b tween 5°-30°. It is distinct that as the increase of m-PDA moieties content in the ma chains, the diffraction peaks gradually become smooth. It implies the crystallinity of PET decrease due to the introduction of m-PDA, which is in consistence with the above DS analysis.  PETI-1, PETI-2 and PETI-3 show T g of 144-148 • C. It was also found that the introduction of m-PDA decreases the crystalline of oligoimides. The melting enthalpy of oligoimides decreased from 38.7 J/g of PETI-O with no m-PDA moieties in its main chains to 6.30 J/g of PETI-3, which was synthesized by 3,4 -ODA and m-PDA at a mole ratio of 50:50. Obviously, the introduction of m-PDA units decreases the crystallinity of PETIs. PETI-1, PETI-2 and PETI-3 have curing onset temperatures of 357-361 • C and peak temperatures of 397-401 • C.

Melt Fluidity of Phenylethynyl-Endcapped Oligoimides
The WXRD patterns of oligomers are shown in Figure 4. Because of the difference in chemical structure, PETI-F shows unique crystalline characteristics. PETI-O shows obvious diffraction peaks, while the WXRD pattern of PETI-P is broad, revealing its amorphous nature. It has been demonstrated that unsymmetrical structures and meta catenation in polyimides backbones would increase their solubility and melt processability. The bent chains endowed by m-PDA decrease the packing of oligomer chains, hence tend to decrease interactions between the oligoimide chains, which lead to the decrease of the crystallinity of oligoimides [45,46]. Other oligoimides copolymerized by 3,4'-ODA and m-PDA displayed semi-crystalline behavior to different extent with diffraction peaks between 5 • -30 • . It is distinct that as the increase of m-PDA moieties content in the main chains, the diffraction peaks gradually become smooth. It implies the crystallinity of PETIs decrease due to the introduction of m-PDA, which is in consistence with the above DSC analysis.  In order to investigate the melting flowing ability of the PEPA end-capped goimides, rheology behaviors of them were characterized, whose detailed results are played in Figure 5 and Table 2. Due to the presence of hexafluoroisopropyliden C(CF3)2-) units with bulky trifluoromethyl in the backbone, which reduce chain pac or lower the stiffness of polymer chain, all oligoimides are provided with low value minimum viscosities (min |η*| < 1 Pa·s) [47,48]. All the rheological curves show sim patterns. For instance, with ramping of temperature, the complex viscosity of PET firstly descends significantly. After temperature values grow higher than 280 °C, the cosity values keep low (< 1 Pa·s) until the temperature values reach 375 °C. Then the cosity value ascends significantly due to the crosslinking of phenylethynyl gro Herein, temperature scale with |η*| < 1 Pa·s (ΔT|η*|≤1 Pa·s) is defined as processing tem ature window for RTM. PETI-O, PETI-F and PETI-P all have low ΔT|η*|≤1 Pa·s of 95 °C, 6 and 65 °C, respectively. With the rigid diamine moieties of TFDB and m-PDA conta by PETI-F and PETI-P, they show worse chain movement ability and higher minim viscosities of 0.31 Pa·s at 331 °C and 0.45 Pa·s at 323 °C. Compared with PETI-P, the lo minimum melt viscosity for PETI-F may be caused by the non-coplanar twist-biph structure of TFDB and increased free volume imparted by -CF3 groups, which disrup chain packing and decrease the interactions between the oligoimide chains for PE [43,49,50]. PETI-1, PETI-2 and PETI-3 copolymerized by 3,4′-ODA and m-PDA show lo crystallinity than PETI-O and PETI-F, having lower melting temperature. Therefore, T also gain wider processing temperature windows with ΔT|η*|≤1 Pa·s of 102 °C, 104 °C 105 °C, respectively. Although PETI-1, PETI-2 and PETI-3 are different from the con of rigid m-PDA moieties in main chains, they have similar minimum viscosities (0.16 ~ 0.18 Pa·s), which are still higher than that of PETI-O (0.15 Pa·s at 309 °C). In order to investigate the melting flowing ability of the PEPA end-capped oligoimides, rheology behaviors of them were characterized, whose detailed results are displayed in Figure 5 and Table 2. Due to the presence of hexafluoroisopropylidene (-C(CF 3 ) 2 -) units with bulky trifluoromethyl in the backbone, which reduce chain packing or lower the stiffness of polymer chain, all oligoimides are provided with low values of minimum viscosities (min |η*| < 1 Pa·s) [47,48]. All the rheological curves show similar patterns. For instance, with ramping of temperature, the complex viscosity of PETI-O firstly descends significantly. After temperature values grow higher than 280 • C, the viscosity values keep low (<1 Pa·s) until the temperature values reach 375 • C. Then the viscosity value ascends significantly due to the crosslinking of phenylethynyl groups. Herein, temperature scale with |η*| < 1 Pa·s (∆T |η*|≤1 Pa·s ) is defined as processing temperature window for RTM. PETI-O, PETI-F and PETI-P all have low ∆T |η*|≤1 Pa·s of 95 • C, 65 • C and 65 • C, respectively. With the rigid diamine moieties of TFDB and m-PDA contained by PETI-F and PETI-P, they show worse chain movement ability and higher minimum viscosities of 0.31 Pa·s at 331 • C and 0.45 Pa·s at 323 • C. Compared with PETI-P, the lower minimum melt viscosity for PETI-F may be caused by the non-coplanar twist-biphenyl structure of TFDB and increased free volume imparted by -CF 3 groups, which disrupt the chain packing and decrease the interactions between the oligoimide chains for PETI-F [43,49,50]. PETI-1, PETI-2 and PETI-3 copolymerized by 3,4 -ODA and m-PDA show lower crystallinity than PETI-O and PETI-F, having lower melting temperature. Therefore, They also gain wider processing temperature windows with ∆T |η*|≤1 Pa·s of 102 • C, 104 • C and 105 • C, respectively. Although PETI-1, PETI-2 and PETI-3 are different from the content of rigid m-PDA moieties in main chains, they have similar minimum viscosities (0.16 Pa·s~0.18 Pa·s), which are still higher than that of PETI-O (0.15 Pa·s at 309 • C). chain packing and decrease the interactions between the oligoimide chains for PE [43,49,50]. PETI-1, PETI-2 and PETI-3 copolymerized by 3,4′-ODA and m-PDA show l crystallinity than PETI-O and PETI-F, having lower melting temperature. Therefore, also gain wider processing temperature windows with ΔT|η*|≤1 Pa·s of 102 °C, 104 °C 105 °C, respectively. Although PETI-1, PETI-2 and PETI-3 are different from the co of rigid m-PDA moieties in main chains, they have similar minimum viscosities (0.16 ~ 0.18 Pa·s), which are still higher than that of PETI-O (0.15 Pa·s at 309 °C).  Table 2.

Melting Stability of the Phenylethynyl Endcapped Oligoimides
The complex viscosity(|η*|) of the oligoimides as a function of time at 280 °C (Fi 6), 290 °C ( Figure 7) and 300 °C (Figure 8) are displayed in Figures 6-8, respectively the detailed data is shown in Table 2.

Melting Stability of the Phenylethynyl Endcapped Oligoimides
The     As mentioned in the introduction section, the variation of viscosity of oligoim was considered to relate to the cure of phenylethynyl groups. As mentioned in Intro tion section, phenylethynyl groups undergo complex linear chain extension, branc cross-linking and/or cyclization (rigidization) during the heating process, which lea the increase of molecular weight. The polymer melt viscosity extremely depends o molecular weight. There is a well-known empirical power law between the zero-shea viscosity η0 and weight average molecular weight Mw [51][52][53]. η0 = KMw α K is a constant, which is related to the property of the polymer. When Mw is l than a critical value (Mc), polymer chain cannot form effective entanglement, whose 1.0. If the polymer has higher Mw, α is 3.4. It is reasonable to believe that the variati viscosity of PETIs is related to their curing rates, melting flow properties and enta ment properties. The different melting stability of the synthesized oligoimides is res from their different chemical structures, which will be discussed in the next sectio combination with more evidence of kinetic experiment.

Investigation of Cure Kinetics of Phenylethynyl Endcapped Oligoimides
In order to explore the effect of chemical structures on melting stability, the cu reactivity of oligoimide with different chemical structures was compared firstly. PE PETI-F and PETI-P were chosen to carry out the kinetic analysis.
Nonisothermal DSC method was employed to roughly estimate the activate en (Ea) of cure reaction. According to Kissinger's method, PETI-O, PETI-F and PETI-P scanned by DSC at different heating rates as described in the experimental section Kissinger plot and the calculated Ea values are shown in Figure 9 and Table 3. PE shows the largest Ea value of 148.3 kJ/mol, meaning that more active energy was ne for PETI-O when its curing reaction occurs. PETI-O should show the least cure react The Ea values of PETI-F and PETI-P are similar, which showing that they may have sim As mentioned in the introduction section, the variation of viscosity of oligoimides was considered to relate to the cure of phenylethynyl groups. As mentioned in Introduction section, phenylethynyl groups undergo complex linear chain extension, branching, crosslinking and/or cyclization (rigidization) during the heating process, which leads to the increase of molecular weight. The polymer melt viscosity extremely depends on the molecular weight. There is a well-known empirical power law between the zero-shear rate viscosity η 0 and weight average molecular weight M w [51][52][53].
K is a constant, which is related to the property of the polymer. When M w is lower than a critical value (M c ), polymer chain cannot form effective entanglement, whose α is 1.0. If the polymer has higher M w , α is 3.4. It is reasonable to believe that the variation of viscosity of PETIs is related to their curing rates, melting flow properties and entanglement properties. The different melting stability of the synthesized oligoimides is resulted from their different chemical structures, which will be discussed in the next section in combination with more evidence of kinetic experiment.

Investigation of Cure Kinetics of Phenylethynyl Endcapped Oligoimides
In order to explore the effect of chemical structures on melting stability, the curing reactivity of oligoimide with different chemical structures was compared firstly. PETI-O, PETI-F and PETI-P were chosen to carry out the kinetic analysis.
Nonisothermal DSC method was employed to roughly estimate the activate energy (E a ) of cure reaction. According to Kissinger's method, PETI-O, PETI-F and PETI-P were scanned by DSC at different heating rates as described in the experimental section. The Kissinger plot and the calculated E a values are shown in Figure 9 and Table 3. PETI-1 shows the largest E a value of 148.3 kJ/mol, meaning that more active energy was needed for PETI-O when its curing reaction occurs. PETI-O should show the least cure reactivity. The E a values of PETI-F and PETI-P are similar, which showing that they may have similar cure reactivity. The other possible reason is that Kissinger method cannot distinguish the difference of their E a value distinctly because of the error of the Kissinger method [38,54].    The T g values' change during the isothermal process of 290 or 300 • C for different times were monitored by DSC. The extent of cure (x) of PETI-O, PETI-F and PETI-P were calculated by modified DiBenedetto Equation (2) as described in the experimental section. The T g s of materials cured at 380 • C measured by DSC at the heating rate of 40 • C/min, which were defined as T g∞ s ( Figure S4). Figure 10 shows the cure extent x as a function of isothermal time. With the time of thermal heating at 290 and 300 • C prolonged, extent of cure increases for all the three samples. The curing rate increase in the order of PETI-O < PETI-P < PETI-F. Furthermore, when 0 < x < 0.9, the cure reaction of phenylethynyl groups meets the First-order rate at various temperatures [40]. Figure 11 and Table 3 display the result of the linear fitting, implying that the thermal cure reactions of oligoimides follow the first-order kinetics model. The rate constants of PETI-F with 0.0018 min −1 at 290 • C and 0.0047 min −1 at 300 • C are larger than them of PETI-O (0.0013 min −1 at 290 • C, 0.0026 min −1 at 300 • C) and PETI-P (0.0017 min −1 at 290 • C, 0.0036 min −1 at 300 • C). It is demonstrated again that the cure reaction reactivity of the three samples increases in the order of PETI-O < PETI-P < PETI-F, which is consistent with the result of the above DSC analysis. Because of the faster cure rates of PETI-F and PETI-P, they show worse melting stability than PETI-O. For PETI-1, PETI-2 and PETI-3, it is also found that the more m-PDA units in the polymer backbone, the worse melting stability. However, although PETI-F has higher cure rates than PETI-P, PETI-F has less variation of viscosity than PETI-P. The phenomenon may be related to their difference in melting flow properties and entanglement properties.

Electronic Effect on Reactivity of Phenylethynyl Groups
According to the previous studies, the thermally induced curing of phenyleth groups was thought to be a free radical reaction [55,56], whose process can be simpl played by Scheme 3.
The initiation of curing reaction occurs via thermally induced generation of free icals, possibly by cleavage of an ethynyl π-bond, affording an excited state with tw paired electrons. Then, the propagation proceeds by the addition of the free radica the oligoimides in their excited state, unpaired electrons can be stabilized by reson stabilization of adjacent phenyl rings [57,58]. The resonance stabilization demands lap of the p orbitals of the one-electron-occupied ethynyl carbons and that of pheny bons that connect to them, which may result in allene-like structure between C1 an (Scheme 3). As we know, the reactive of free radicals is substantially impacted by the tronic effects of the substituent groups [59,60]. For the phenylethynyl-terminated goimides in their excited state, electron-donating units in the backbone increase the tron density across the Cβ/C1 bond, therefore better facilitate the stabilization of the ra at Cβ [35]. In this case, the initiation step will be encouraged, whereas the propagation will be hindered. In contrast to the above situation, electron-withdrawing units/sub ents suppress the initiation step and promote the propagation step. According to the vious research [35], when the reaction temperature is high enough so that different P can gain sufficient energy to overcome the activation energy of the initiation step, th of cure reaction will be strongly dependent on the propagation step, which will be a erated by electron-withdrawing units/substituents on the backbone.

Electronic Effect on Reactivity of Phenylethynyl Groups
According to the previous studies, the thermally induced curing of phenylet groups was thought to be a free radical reaction [55,56], whose process can be simpl played by Scheme 3.
The initiation of curing reaction occurs via thermally induced generation of fre icals, possibly by cleavage of an ethynyl π-bond, affording an excited state with tw paired electrons. Then, the propagation proceeds by the addition of the free radica the oligoimides in their excited state, unpaired electrons can be stabilized by reso stabilization of adjacent phenyl rings [57,58]. The resonance stabilization demands lap of the p orbitals of the one-electron-occupied ethynyl carbons and that of pheny bons that connect to them, which may result in allene-like structure between C1 a (Scheme 3). As we know, the reactive of free radicals is substantially impacted by the tronic effects of the substituent groups [59,60]. For the phenylethynyl-terminate goimides in their excited state, electron-donating units in the backbone increase the tron density across the Cβ/C1 bond, therefore better facilitate the stabilization of the r at Cβ [35]. In this case, the initiation step will be encouraged, whereas the propagatio will be hindered. In contrast to the above situation, electron-withdrawing units/sub ents suppress the initiation step and promote the propagation step. According to th vious research [35], when the reaction temperature is high enough so that different can gain sufficient energy to overcome the activation energy of the initiation step, th of cure reaction will be strongly dependent on the propagation step, which will be erated by electron-withdrawing units/substituents on the backbone.

Electronic Effect on Reactivity of Phenylethynyl Groups
According to the previous studies, the thermally induced curing of phenylethynyl groups was thought to be a free radical reaction [55,56], whose process can be simply displayed by Scheme 3.
The initiation of curing reaction occurs via thermally induced generation of free radicals, possibly by cleavage of an ethynyl π-bond, affording an excited state with two unpaired electrons. Then, the propagation proceeds by the addition of the free radical. For the oligoimides in their excited state, unpaired electrons can be stabilized by resonance stabilization of adjacent phenyl rings [57,58]. The resonance stabilization demands overlap of the p orbitals of the one-electron-occupied ethynyl carbons and that of phenyl-carbons that connect to them, which may result in allene-like structure between C 1 and C 2 (Scheme 3). As we know, the reactive of free radicals is substantially impacted by the electronic effects of the substituent groups [59,60] . For the phenylethynyl-terminated oligoimides in their excited state, electron-donating units in the backbone increase the electron density across the C β /C 1 bond, therefore better facilitate the stabilization of the radical at C β [35]. In this case, the initiation step will be encouraged, whereas the propagation step will be hindered. In contrast to the above situation, electron-withdrawing units/substituents suppress the initiation step and promote the propagation step. According to the previous research [35], when the reaction temperature is high enough so that different PETIs can gain sufficient energy to overcome the activation energy of the initiation step, the rate of cure reaction will be strongly dependent on the propagation step, which will be accelerated by electron-withdrawing units/substituents on the backbone. As to PETI-O, PETI-F and PETI-P, imide moieties impart electron-withdrawing effect to the phenylethyl groups [20,35]. For PETI-F, the -CF3 pendant increases the electronwithdrawing ability of backbone, resulting in the highest cure reactivity of PETI-F. For PETI-O, the -O-linkage of electron-donating ability decrease the electron-withdrawing ability of the backbone, which results in the least cure reactivity of PETI-O. There is no extra electron effect for PETI-P, which impart the medium cure reactivity between PETI-O and PETI-F to PETI-P. This explanation is in good accordance with the study of cure kinetics. 13 C NMR was used to determine the electron density of Cα and Cβ, which indicate the electronic effect of backbone on the terminated phenylethynyl groups. Figure 12 shows the chemical shifts of ethynyl carbon atoms (Cα and Cβ) belonging to PETI-O, PETI-F and PETI-P, respectively, and the detailed data are listed in Table 4. As mentioned above, imide units with electron-withdrawing ability shift electron density from terminal phenyl ring (attached to Cα), onto the C2-Cα bond while at the same time shift electron density away from the C1-Cβ bond [20,35]. The increased electron-withdrawing ability will intensify the effect, embedding Cβ with increased deshielding and larger chemical shift while embedding Cα with increased shielding and smaller chemical shift. Hence, the difference in chemical shift between the two ethynyl carbons also grows larger. The chemical shifts of Cα increase in the order: PETI-F ≈ PETI-P < PETI-O; The chemical shifts of Cβ increase in the order of PETI-F > PETI-P > PETI-O; The difference of chemical shifts of two ethynyl carbons (Cβ-Cα) increase in the order of PETI-F > PETI-P > PETI-O. It can be concluded that the electron-withdrawing ability of the backbone of the three PETIs increase in the order: PETI-F > PETI-P > PETI-O, which demonstrate the above explanation. As to PETI-O, PETI-F and PETI-P, imide moieties impart electron-withdrawing effect to the phenylethyl groups [20,35]. For PETI-F, the -CF 3 pendant increases the electronwithdrawing ability of backbone, resulting in the highest cure reactivity of PETI-F. For PETI-O, the -O-linkage of electron-donating ability decrease the electron-withdrawing ability of the backbone, which results in the least cure reactivity of PETI-O. There is no extra electron effect for PETI-P, which impart the medium cure reactivity between PETI-O and PETI-F to PETI-P. This explanation is in good accordance with the study of cure kinetics. 13 C NMR was used to determine the electron density of C α and C β , which indicate the electronic effect of backbone on the terminated phenylethynyl groups. Figure 12 shows the chemical shifts of ethynyl carbon atoms (C α and C β ) belonging to PETI-O, PETI-F and PETI-P, respectively, and the detailed data are listed in Table 4. As mentioned above, imide units with electron-withdrawing ability shift electron density from terminal phenyl ring (attached to C α ), onto the C 2 -C α bond while at the same time shift electron density away from the C 1 -C β bond [20,35]. The increased electron-withdrawing ability will intensify the effect, embedding C β with increased deshielding and larger chemical shift while embedding C α with increased shielding and smaller chemical shift. Hence, the difference in chemical shift between the two ethynyl carbons also grows larger. The chemical shifts of C α increase in the order: PETI-F ≈ PETI-P < PETI-O; The chemical shifts of C β increase in the order of PETI-F > PETI-P > PETI-O; The difference of chemical shifts of two ethynyl carbons (C β -C α ) increase in the order of PETI-F > PETI-P > PETI-O. It can be concluded that the electron-withdrawing ability of the backbone of the three PETIs increase in the order: PETI-F > PETI-P > PETI-O, which demonstrate the above explanation.

DFT Study Based on Model Compounds
Density functional theory (DFT) study has widely been used in the research of polyimide [61,62]. Model compounds, MC-O, MC-F and MC-P were prepared in good yield and characterized (Scheme 1). The thermal behavior of the model compounds was investigated by DSC to explore the effect of chemical structures on their thermal curing behavior. Figure 13 shows the DSC curves of the model compounds, and Table 5 lists the DSC data. Both MC-O and MC-F exhibit sharp endothermal peaks at 290 °C, which are assigned as the melt points. However, MC-P shows two endothermal peaks at 238 °C and 279 °C. According to the research of Zhou et al. [47], MC-P experiences a melting-recrystallization-melting process with the ramping of temperature. This multiple melting phenomenon is caused by crystal transition during the heating procedure. The wide exothermic peaks in the range of 332-398 °C are attributed to the thermal crosslinking reactions of phenylethynyl groups. MC-F has curing onset, peak and end temperature of 332, 361 and 382 °C, whose curing reaction occurs at the lowest temperature range. As is expected, the temperature ranges where cure reaction proceeds increase in the order: MC-F < MC-P < MC-O, which is also the order of cure reactivity. The result is in consistence with the analysis of the cure reactivity of PETIs.

DFT Study Based on Model Compounds
Density functional theory (DFT) study has widely been used in the research of polyimide [61,62]. Model compounds, MC-O, MC-F and MC-P were prepared in good yield and characterized (Scheme 1). The thermal behavior of the model compounds was investigated by DSC to explore the effect of chemical structures on their thermal curing behavior. Figure 13 shows the DSC curves of the model compounds, and Table 5 lists the DSC data. Both MC-O and MC-F exhibit sharp endothermal peaks at 290 • C, which are assigned as the melt points. However, MC-P shows two endothermal peaks at 238 • C and 279 • C. According to the research of Zhou et al. [47], MC-P experiences a melting-recrystallization-melting process with the ramping of temperature. This multiple melting phenomenon is caused by crystal transition during the heating procedure. The wide exothermic peaks in the range of 332-398 • C are attributed to the thermal crosslinking reactions of phenylethynyl groups. MC-F has curing onset, peak and end temperature of 332, 361 and 382 • C, whose curing reaction occurs at the lowest temperature range. As is expected, the temperature ranges where cure reaction proceeds increase in the order: MC-F < MC-P < MC-O, which is also the order of cure reactivity. The result is in consistence with the analysis of the cure reactivity of PETIs.
In order to further investigate the different reactivity of three model compounds, the electronic structures of model compounds at the ground state and the spin density distribution of the model compounds at the excited state were predicted by the Gaussian09 program. It should be paid attention that the structures of the three model compounds were simplified reasonably for the convenience of calculation. Figure 14 displays the energy minimized structures of the three simplified compounds after geometry optimization. Natural bond orbital (NBO) charge distribution of the model compounds was predicted, which reveals the electron density of each atom or unit [63,64]. The NBO charge values of ethynyl indicate the local electron density of ethynyl units, which are also shown in Figure 14. As is estimated, the values of NBO charge increase in the order: MC-O < MC-P < MC-F, which is caused by the increasing ability of electron-withdrawing of diamine moieties. This result is also consistent with the above 13 C NMR characterization of PETIs.  In order to further investigate the different reactivity of three model compounds, the electronic structures of model compounds at the ground state and the spin density distribution of the model compounds at the excited state were predicted by the Gaussian09 program. It should be paid attention that the structures of the three model compounds were simplified reasonably for the convenience of calculation. Figure 14 displays the energy minimized structures of the three simplified compounds after geometry optimization. Natural bond orbital (NBO) charge distribution of the model compounds was predicted, which reveals the electron density of each atom or unit [63,64]. The NBO charge values of ethynyl indicate the local electron density of ethynyl units, which are also shown in Figure 14. As is estimated, the values of NBO charge increase in the order: MC-O < MC-P < MC-F, which is caused by the increasing ability of electron-withdrawing of diamine moieties. This result is also consistent with the above 13 C NMR characterization of PETIs.
Spin density distribution was usually used to study the delocalization of free radicals and to predict their reactivity [65][66][67][68]. For the purpose of characterization of the resonance stabilization of radicals at Cα and Cβ, spin density populations of the simplified model compounds at excited state were calculated by Gaussian 09 software ( Figure 15); the detailed data of Cα and Cβ are shown in Table 6. The spin density of Cα and C β are all far lower than unity, meaning that the radicals are delocalized to the conjugated phenyl rings, imide groups and simplified diamine units. Compared with MC-P and MC-F, ethynyl carbon atoms of MC-O have extremely low spin density with α spin density of 0.067 at Cα and β spin density of − 0.039 at Cβ. It is worthy of noting that the oxygen atom of the simplified diamine unit also delocalizes the spin density (Figure 15a). MC-O shows the best resonance stabilization and displays the lowest reactivity. While the radicals of the excited MC-F and MC-P are more local with larger spin density values in Cα and Cβ. It is noteworthy that the spin density value of Cα of excited MC-F (0.403) is larger than that of excited MC-P (0.397), which reveals the worse resonance stabilization and higher reactivity for the exciting MC-F. It is obvious that the increased electron-withdrawing ability of  diamine moieties of the excited model compounds hinders the stabilization of free radicals at Cα and Cβ.
In conclusion, the prediction of DFT method indicates that the increased electronwithdrawing ability of oligoimide backbone decreases the electron density of ethynyl units and suppresses the resonance stabilization of free radicals at ethynyl carbon atoms.   Spin density distribution was usually used to study the delocalization of free radicals and to predict their reactivity [65][66][67][68]. For the purpose of characterization of the resonance stabilization of radicals at C α and C β , spin density populations of the simplified model compounds at excited state were calculated by Gaussian 09 software ( Figure 15); the detailed data of C α and C β are shown in Table 6. The spin density of C α and C β are all far lower than unity, meaning that the radicals are delocalized to the conjugated phenyl rings, imide groups and simplified diamine units. Compared with MC-P and MC-F, ethynyl carbon atoms of MC-O have extremely low spin density with α spin density of 0.067 at C α and β spin density of −0.039 at C β . It is worthy of noting that the oxygen atom of the simplified diamine unit also delocalizes the spin density (Figure 15a). MC-O shows the best resonance stabilization and displays the lowest reactivity. While the radicals of the excited MC-F and MC-P are more local with larger spin density values in C α and C β . It is noteworthy that the spin density value of C α of excited MC-F (0.403) is larger than that of excited MC-P (0.397), which reveals the worse resonance stabilization and higher reactivity for the exciting MC-F. It is obvious that the increased electron-withdrawing ability of diamine moieties of the excited model compounds hinders the stabilization of free radicals at C α and C β .

Thermal Properties of the Cured Resins
The synthesized oligoimides were cured at 380 °C for 2 h. The thermogravim measurement of the cured resins in N2 and under air atmosphere is presented in F 16 and Figure S5. The temperature of 5% weight loss (Td5) and the char yields at 70 are summarized in Table 7. Very small weight loss was detected below 500 °C both and N2 atmosphere. The thermally cured resins provided with Td5 of 558-564 °C und and char yields in the range of 63.1-72.0%, which display excellent thermal stability Figure 17 exhibits the curves of storage modulus and tanδ of the cured resins the detailed data are listed in Table 7. In this study, the temperature corresponding t onset of the decline of storage modulus was defined as Tg of cured resins. Because of diamine moieties contained in backbone, cured PETI-F and PETI-P have higher Tg va compared with cured PETI-O (363 °C). It is worth noting that cured PETI-F has the hi Tg value of 438 °C, whose DMA curve shows two relaxation processes. TFDB moiety provides polyimides with high Tg value because of their rigid rod-like structure. The  In conclusion, the prediction of DFT method indicates that the increased electronwithdrawing ability of oligoimide backbone decreases the electron density of ethynyl units and suppresses the resonance stabilization of free radicals at ethynyl carbon atoms.

Thermal Properties of the Cured Resins
The synthesized oligoimides were cured at 380 • C for 2 h. The thermogravimetry measurement of the cured resins in N 2 and under air atmosphere is presented in Figure 16 and Figure S5. The temperature of 5% weight loss (T d5 ) and the char yields at 700 • C are summarized in Table 7. Very small weight loss was detected below 500 • C both in air and N 2 atmosphere. The thermally cured resins provided with T d5 of 558-564 • C under N 2 and char yields in the range of 63.1-72.0%, which display excellent thermal stability.
Polymers 2021, 13, 903 2 relaxation process is related to the motion of TFDB moieties in the main chains. The temperature relaxation process is defined as the sub-glass transition process, and the temperature one is defined as glass transition process [43,69]. By copolymerization of 3,4′-ODA and m-PDA, cured PETI-1, PETI-2 and PE show higher Tg values than cured PETI-O. Furthermore, cured resins show higher T ues with the increased content of rigid m-PDA moiety. For instance, cured PETI-3 sy sized by 3,4′-ODA and m-PDA content at a mole ratio of 50:50 has the Tg value of 39 9 °C higher than that of cured PETI-2 (382 °C), and 11 °C higher than that of cured P 1 (380 °C). While synthesized by homopolymerization of m-PDA, cured PETI-P h value of 398 °C, 7 °C higher than that of cured PETI-3.   Figure 17 exhibits the curves of storage modulus and tanδ of the cured resins, and the detailed data are listed in Table 7. In this study, the temperature corresponding to the onset of the decline of storage modulus was defined as T g of cured resins. Because of rigid diamine moieties contained in backbone, cured PETI-F and PETI-P have higher T g values, compared with cured PETI-O (363 • C). It is worth noting that cured PETI-F has the highest T g value of 438 • C, whose DMA curve shows two relaxation processes. TFDB moiety often provides polyimides with high T g value because of their rigid rod-like structure. The two relaxation process is related to the motion of TFDB moieties in the main chains. The low-temperature relaxation process is defined as the sub-glass transition process, and the high-temperature one is defined as glass transition process [43,69].
Polymers 2021, 13, 903 20 relaxation process is related to the motion of TFDB moieties in the main chains. The temperature relaxation process is defined as the sub-glass transition process, and the h temperature one is defined as glass transition process [43,69]. By copolymerization of 3,4′-ODA and m-PDA, cured PETI-1, PETI-2 and PE show higher Tg values than cured PETI-O. Furthermore, cured resins show higher T ues with the increased content of rigid m-PDA moiety. For instance, cured PETI-3 syn sized by 3,4′-ODA and m-PDA content at a mole ratio of 50:50 has the Tg value of 39 9 °C higher than that of cured PETI-2 (382 °C), and 11 °C higher than that of cured P 1 (380 °C). While synthesized by homopolymerization of m-PDA, cured PETI-P h value of 398 °C, 7 °C higher than that of cured PETI-3.   By copolymerization of 3,4 -ODA and m-PDA, cured PETI-1, PETI-2 and PETI-3 show higher T g values than cured PETI-O. Furthermore, cured resins show higher T g values with the increased content of rigid m-PDA moiety. For instance, cured PETI-3 synthesized by 3,4 -ODA and m-PDA content at a mole ratio of 50:50 has the T g value of 391 • C, 9 • C higher than that of cured PETI-2 (382 • C), and 11 • C higher than that of cured PETI-1 (380 • C). While synthesized by homopolymerization of m-PDA, cured PETI-P has T g value of 398 • C, 7 • C higher than that of cured PETI-3. show low strength and toughness because of their high crosslink density. The thermalcured PETI-1, PETI-2 and PETI-3 do not show apparent difference in mechanical compared with PETI-O and PETI-P, revealing that copolymerization did not exert obvious impacts on mechanical properties of thermally cured resins.

Conclusions
A series of phenylethynyl-endcapped polyimides with a calculated molecular weight of 1000 g/mol based on 6FDA was prepared, whose thermal property, rheology behavior and mechanical property were characterized. This study has found that PETIs copolymerization by 3,4 -ODA and m-PDA gain excellent balance between processability, mechanical property and heat-resistance properties. The rigid m-PDA units endow oligoimides' backbone with meta catenation, resulting in lower crystalline, lower melting temperature, wider processing temperature window and higher cured T g values. Meanwhile, copolymerization does not exert negative effect on mechanical property of the cured resins. PETI-O and PETI-1 are all amenable to RTM. In particular, PETI-1 has low processing temperature (280 • C) and high T g of 380 • C. Its neat-cured resin has good mechanical properties with tensile strength of 62.2 MPa and elongation at breakage of 3.4%.
The curing reaction of phenylethynyl groups proceeds during isothermal heating, which leads to the variation of melt viscosity. The melting stabilities of oligoimides are substantially related to their curing rates, melting flow properties and the entanglement properties. Herein, the electronic effect of backbone on the curing rates was investigated. By the method of DSC, 13 C NMR and DFT, it was confirmed that the increased electronwithdrawing ability of oligoimides backbone results in higher reactivity of terminated phenylethynyl groups and worse melting stability. The trifluoromethyl (−CF 3 ) groups attached to backbones increase the electron-withdrawing ability of oligoimide backbone; Ether linage (-O-) on the backbones, decrease the electron-withdrawing ability of oligoimide backbone. The curing reactivity of terminated phenylethynyl groups increases in the order of PETI-O < PETI-P < PETI-F. PETI-F and PETI-P also gain worse melting stability. The electron-withdrawing ability of oligoimide backbones plays an important role in the melting stability of phenylethynyl end-capped oligoimides.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.