Lisa Feuillerat
email: lisa.feuillerat@mines-albi.fr

Olivier De Almeida

Jean-Charles Fontanier

Carole Aubry

Pascal Rumeau

Fabrice Schmidt

Effect of melt spinning on the integrity of poly(ether ether ketone) for commingled yarn based composite preforms

Keywords:

PEEK yarns are used for commingled yarn based preforms in order to manufacture high performance thermoplastic composites. In a previous work, characterizations of PEEK commingled yarns revealed degradation, causing poor consolidation of final composites. The sizing applied onto the yarn surface in the melt spinning was identified as a degradation factor. Thus, the aim of this work is to assess the effect of different melt spinning parameters: extrusion, sizing and draw ratio, on PEEK integrity and its degradation process. Yarns at different stages of melt spinning were characterized through various techniques. For all yarns, degradation was identified during further transformation, in the molten state, inducing a decrease of the crystallization temperature and an increase of viscosity due to crosslinking. Extrusion initiates degradation earlier without changing the PEEK decomposition process. Sizing is responsible for an acceleration and modifications of degradation steps.

Introduction

Poly(ether ether ketone) (PEEK) is increasingly used as matrix for high performance composites because of its thermo-mechanical stability and its chemical resistance [START_REF] Díez-Pascual | Poly(phenylene sulphide) and poly(ether ether ketone) composites reinforced with single-walled carbon nanotube buckypaper: I -Structure, thermal stability and crystallization behaviour[END_REF] . PEEK can be used up to a peak service temperature of 200 • C thanks to its glass transition and melting temperatures respectively of 143 • C and 343 • C .

However, the processing of such heat-stable matrix composites is a major difficulty due to the high molten viscosity of PEEK [START_REF] Sala | Heated chamber winding of thermoplastic powderimpregnated composites: part 1. Technology and basic thermochemical aspects[END_REF] . To overcome this difficulty, manufacturers have developed preforms to reduce the distance the matrix has to travel to impregnate the reinforcements. The main solutions proposed are preimpregnated tapes, powdered fabrics, film stacking, cowoven fabrics and commingled yarns, which are generally consolidated by thermocompression.

Preimpregnated tapes consist of reinforcements previously embedded in a thermoplastic matrix, such as the commercial product AS4/APC-2, a PEEK tape reinforced with unidirectional carbon fiber. This solution provides good interfacial bonding, but the manufacturing of complex shapes is difficult due to its stiffness [START_REF] Xu | Study on mechanical properties of unidirectional continuous carbon fiber-reinforced PEEK composites fabricated by the wrapped yarn method[END_REF] . Powdered fabrics, as the name suggests are fabrics covered by a PEEK powder melted to wet reinforcements. Homogeneous dispersion and sufficient adhesion of PEEK with reinforcements are the main difficulties of powdered fabrics. Another solution is to stack polymer films with reinforcement fabrics, but the difficulty is to force the polymer flow through reinforcements made of thousands of filaments. Furthermore, this solution does not allow the production of complex geometries [START_REF] Goud | Influence of various forms of polypropylene matrix (fiber, powder and film states) on the flexural strength of carbon-polypropylene composites[END_REF] . The matrix can also be cowoven into a fabric, where reinforcements and matrix yarns are combined as warp and weft yarns, but this leads to a heterogeneous distribution. Finally, the commingled yarns are the most interesting solution with an intimate contact and therefore an excellent dispersion between the reinforcement and the matrix [START_REF] Bernet | Commingled yam composites for rapid processing of complex shapes[END_REF] .

However, as shown in a previous publication, unacceptable void contents of about 10% are obtained when consolidating commingled yarn based products involving a PEEK matrix [START_REF] Feuillerat | Effect of poly(ether ether ketone) degradation on commingled fabrics consolidation[END_REF] . The main reason of such behavior is a rapid thermal degradation of PEEK, in the range of processing temperature beyond 380 • C . As widely studied in the literature, in the molten state, PEEK is subjected to degradation [START_REF] Hay | Thermal decomposition of poly(aryl ether ketones)[END_REF][START_REF] Day | Thermal degradation of poly(aryl-ether-ether-ketone) (PEEK): a differential scanning calorimetry study[END_REF][START_REF] Day | The thermal stability of poly(aryl-ether etherketone) as assessed by thermogravimetry[END_REF][START_REF] Jonas | Thermal stability and crystallization of poly(aryl ether ether ketone)[END_REF][START_REF] Cole | Fourier transform infra-red spectroscopic study of thermal degradation in poly(ether ether ketone)-carbon composites[END_REF][START_REF] Mylläri | Characterization of thermally aged polyetheretherketone fibres -mechanical, thermal, rheological and chemical property changes[END_REF][START_REF] Deignan | Insights into wide variations in carbon fibre/polyether-ether-ketone rheology data under automated tape placement processing conditions[END_REF][START_REF] Martín | On the determination of thermal degradation effects and detection techniques for thermoplastic composites obtained by automatic lamination[END_REF][START_REF] Pascual | On the stability of PEEK for short processing cycles at high temperatures and oxygen-containing atmosphere[END_REF] . Firstly chain scission occurs at ether or ketone bonds, then oxidation takes place and leads to the formation of hydroperoxides and other degradation products, which finally leads to crosslinking [START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF][START_REF] Courvoisier | Multi-scale and multi-technique analysis of the thermal degradation of poly(ether ether ketone)[END_REF] . This degradation process induces significant changes of the thermal, rheological and mechanical properties of PEEK, such as a decrease of the crystallization and melting temperatures, an increase of its viscosity, glass transition temperature, and molecular weight. The degradation of the PEEK matrix in the molten state has been characterized at different scales and the rapid increase of its viscosity makes impossible the impregnation and consolidation of such architecture made of commingled yarns.

The previous study has shown that the initial state of the PEEK yarns is responsible for the acceleration of PEEK degradation and thus the impossibility to consolidate this commingled yarn based preform [START_REF] Feuillerat | Effect of poly(ether ether ketone) degradation on commingled fabrics consolidation[END_REF] . Indeed, a first processing step is achieved in order to prepare commingled yarn based preforms: the melt spinning operation to obtain PEEK yarns. During this process, PEEK is melted and extruded through a spinneret leading to a multifilament yarn. In order to prevent breakage, friction and electrostatic effect, a specific solution, called sizing is applied onto the surface of yarns before drawing and winding. Sizing has been identified as a factor responsible for degradation by accelerating it during the further processing of the commingled yarn based preform, in the molten state. Therefore, the manufacturing of PEEK yarns is a crucial processing step that can be critical for the consolidation of composites, as it is carried out at high temperature, under high mechanical stress and requires additives that induce an initial degradation of the PEEK matrix.

However, although yarn sizing has been identified to have an effect on PEEK degradation, the extrusion and drawing may also have a critical effect on the PEEK stability. Thus, this paper aims to determine, through various experimental analyses, the influence of different melt spinning parameters on the stability, thermal and rheological properties of poly(ether ether ketone) and its degradation process. In order to evaluate the consequences of melt spinning on the integrity of PEEK, yarns were collected at different stages of the melt spinning process, characterized and compared to the properties of raw PEEK.

Experimental

Materials

The raw PEEK polymer was processed by melt spinning at 390 • C. This process is composed of different equipments which are schematically shown in Fig. 1 . The raw material in pellets form is melted through the extruder and the molten polymer flows through a spinneret. The spun filaments are then drawn in the molten state while cooled in the cooling chamber, and a sizing is sprayed onto the surface of yarns. The filaments are then guided over godets heated above the glass transition temperature and finally wound up. A drawing stage takes place between the first and the last godet. This draw ratio ( DR ) is defined as the ratio between the speed of rotation of the last godet ( V f ) and that of the first godet ( V 1 ), as described in the Eq. [START_REF] Díez-Pascual | Poly(phenylene sulphide) and poly(ether ether ketone) composites reinforced with single-walled carbon nanotube buckypaper: I -Structure, thermal stability and crystallization behaviour[END_REF] .

DR = V f V 1 (1)
In this study, the spinneret was composed of 48 holes of 0.4 mm and the sizing used was made up mainly of fatty acid ester and poly(ethylene-glycol) ester. The resulting sizing ratio was 1.5% in mass. The same extrusion flow rate was applied in order to obtain filaments of the same diameter for all yarns. Then, various yarn samples were collected at different stages of the process, as summarized in Table 1 . The first yarn, referred to as extruded yarn, was collected right after extrusion in the cooling chamber before being sized and drawn, the other yarns were collected at the end of the melt spinning process after being impregnated with sizing. The latter are referred to as drawn yarns DR = 1 , DR = 2 , DR = 3 . 75 . The drawn yarns DR = 1 correspond to the yarns obtained with all godets at the same speed of rotation. The drawn yarns DR = 2 and DR = 3 . 75 correspond to yarns that were drawn with a faster rotational speed of the last godet equal to 2 and 3.75 times that of the first godet respectively.

In order to avoid morphological effects during characterizations, the filaments were ground using a Retsch ultra centrifugal mill ZM 100 with a 0.5 mm sieve. The mill was cooled with liquid nitrogen to avoid overheating. The powders obtained were then characterized through different analyses.

Thermogravimetric analysis

The thermal degradation was studied by means of thermogravimetric analysis (TGA) with a Setsys SETARAM TGA. About 20 mg of each sample was first held at 100 • C for 15 min, and then heated to 800 • C at a heating rate of 10 • C min -1 , under argon. In order to check the repeatability, the test was reproduced 3 times. The temperature related to a 5% weight loss ( T 5% ) was taken to compare the samples.

Differential scanning calorimetry

Differential scanning calorimetry (DSC) was achieved in order to evaluate the evolution of thermal properties in the molten state with a power-compensated Perkin Elmer 8500 DSC. For all experiments, aluminum sealed pans filled with 3.7 ± 0.02 mg of sample were used. The samples were subjected to four heating and cooling cycles in a nitrogen atmosphere between 50 and 400 • C . The thermal treatment consisted in heating the sample at 20 • C min -1 until 400 • C , then in applying an isotherm at this temperature for 20 min, and finally in cooling the sample with a constant rate of 10 • C min -1 to induce PEEK crystallization in identical conditions. The first one was applied to erase the thermal history and the isotherm at 400 • C was only 5 min. As explained by Jonas et al. [START_REF] Jonas | Thermal stability and crystallization of poly(aryl ether ether ketone)[END_REF] , above 385 • C , self-nucleation is eliminated and starting from 400 • C , thus ensured starting crystallization from a freenuclei melt. During the last stage of the thermal treatment, the crystallization temperature ( T c ) was measured. For convenience, the T c was taken at the maximum heat flow of the crystallization exothermic peak. Identical trends on the T c were obtained by taking the peak onset as a reference for T c . Nevertheless, the results were much more scattered. In order to check the repeatability, the test was reproduced 3 times.

Rheometry

The evolution of the melt viscosity was also studied in order to assess the effect of the melt spinning transformation on the degradation kinetics of PEEK. The samples were prepared with the same method to avoid morphological effects. The powders were compacted with a force of 29 kN during 1 min using an Instron press 5567 and a cylindrical die with a diameter of 20 mm. The resulting tablets were then consolidated in a press using a pilot plant for thermo-compression molding process equipped with the 3iTechë technology from Roctool. The applied thermo-compression cycle consisted of a heating phase at 42 • C min -1 , a 3 min isothermal step at 370 • under a force of 125 kN, and finally in a cooling stage at 50

• C min -1 .
Rheological measurements were performed on these consolidated samples with a Mars Haake rheometer using a plate-plate geometry in oscillatory mode with 20 mm diameter disposable aluminum plates. In order to check the repeatability, the test was reproduced 3 times. The heating conditions were optimized so as to get the fastest heating rate without a temperature overshoot higher than 1 • C . This strategy was selected for limiting the exposure time of PEEK to an oxidant atmosphere once melted, which in turn may generate temperature differences within the sample during the several tens of seconds. During the isothermal experiments at 400 • , constant shear and shear rates of 0.01 and 6.28 S -1 (Hz) respectively were applied. In order to guarantee the linear viscoelastic range, strain sweep tests were carried out beforehand so that the complex viscosity could be determined simply from the elastic and viscous moduli G S and G L .

Thermogravimetric analysis coupled to Fourier transform infrared spectroscopy

Thermogravimetric analysis coupled to Fourier Transform Infrared Spectroscopy (TGA-FTIR) was performed using a NETZSCH 209F1 Iris instrument. The samples were first held at • C for 15 min, and then heated to 800 • C at a heating rate of 10 • C min -1 under nitrogen. The emitted gases were analyzed with a BRUKER Tensor 27 FTIR. The spectra were collected with an acquisition frequency of 6 spectra per minute and the measurements were performed with a resolution of 4 cm -1 . In order to check the repeatability, the test was reproduced 3 times.

Pyrolysis-gas chromatography-mass spectroscopy/flame ionization detector

Analytical pyrolysis was carried out in a micropyrolysis unit (Multi-shot pyrolyzer Model EGA/PY 3030D). This device was coupled to a gas chromatograph (GC, Agilent 8890 Gas Chromatograph) equipped with a mass spectrometry (Agilent 5977 Series MSD System) and a flame ionization detector (FID). Approximately 0.2 mg of the sample was placed in a steel cup and weighted with a micro-balance. The cup was loaded into the multi-shot sampler of the micro-pyrolysis unit. A heating rate of 56 • C min -1 was applied until reaching 600 • C under a helium flow of 120 mL min -1 . Volatile compounds were separated and analyzed with a GC-MS/FID system using a HP-5MS column (length 30 m; internal diameter 0.25 mm; film thickness 0.25 μm, Agilent Technologies, USA). The injector temperature was kept at 300 • C. The initial GC oven temperature was 50 • C, heated to 200

• C at 2 • C min -1
and then to 320 • C at 3 • C min -1 . Peak identification of chromatograms was achieved using the NIST (2017) mass spectral library.

Results and discussion

Thermal stability of raw material and yarns

The thermogravimetric results obtained in non-isothermal conditions ( 10 • C min -1 ) under argon atmosphere are presented in Fig. 2 . The temperature related to a weight loss of 5% ( T 5% ) are also summarized in Table 2 . Three groups of data can be distinguished: the sizing, the sized drawn yarns and the extruded yarn with the raw material.

The sizing has a lower thermal stability compared with the PEEK samples, with a first intense weight loss below 100 • C. This first decrease of weight correspond to a weight loss of 5% at around 88 • C. It is due to the evaporation of the sizing solvent. The second intense weight loss around 300 • C can be related to the degradation of fatty acid ester and poly(ethylene-glycol) ester, which are the sizing compounds.

In the melt spinning process, sizing is applied to the PEEK yarn before the yarns being drawn at a temperature of 170 • C. Thus, the volatile compounds of the sizing are evaporated and only the fatty phase remains on the yarn. This fatty phase, corresponding to the dry sizing, remains stable until 300 • C. In this sense, the stability of sizing must be evaluated with regard to the process, i.e. in the dry state. Thus, the temperature related to a mass loss of 5% for the dry sizing is about 316 • C.

An intense degradation phenomenon is observed for the raw material with a maximum intensity at around 574 • C. This degradation is directly linked to random chain scissions between aromatic rings and ether or ketone bonds [START_REF] Cunha Vasconcelos | Evaluation of decomposition kinetics of poly (ether-ether-ketone) by thermogravimetric analysis[END_REF] . The same behavior can

Table 2

Temperature related to a weight loss of 5% ( T 5% ) and uncertainty.

Sample be noticed for the extruded yarn. This suggests that the extrusion seems to have no significant effect on the thermal stability of PEEK.

T 5% ( • C )
On the contrary, the drawn yarns have a different behavior with degradation starting at around 300 • C. The relative weight decreases gradually until an intense degradation at about 539 • C, 535 • C and 542 • C respectively for the drawn yarns DR = 1 , DR = 2 and DR = 3 . 75 . A decrease of about 35 • C is observed for the T 5% compared with the raw material.

Extrusion does not seem to be the parameter responsible for this acceleration of degradation because this phenomenon is not observed on the extruded yarn. At this stage, either the sizing or the draw ratio above the glass transition could be responsible for this phenomenon. When comparing the different drawn yarns, it appears that the draw ratio ( DR ) seems to have no significant effect on the thermal stability of yarns. The last parameter is the presence of sizing agents on the surface of yarns. In this way, it seems that the sizing has a more significant effect on the thermal stability of PEEK. The gradually weight loss of the drawn yarns from 300 • C may be related to the degradation of the sizing.

Thermal and rheological properties of raw material and yarns

In order to assess the influence of the melt spinning transformation on the thermal and rheological properties of the different samples, DSC and rheometry analyses were carried out.

The DSC analysis consisted in applying four heat treatments at 400 • C during 20 min in order to induce progressive degradation. An example of the evolution of crystallization peaks over the heat treatment cycles is presented in Fig. 3 . These exothermic peaks were recorded after melting the polymer, when cooling the sample at 10 • C min -1 . For both samples, the sharp peak in the first cycle broadens over the heat treatment cycles, shifting the peak maximum to lower temperatures. This effect is related to crosslinking, which induces a lower molecular mobility of chains, therefore a decrease in the crystallization ability of the polymer and the existence of a large amount of degraded polymer that crystallizes at a slower rate [START_REF] Day | Thermal degradation of poly(aryl-ether-ether-ketone) (PEEK): a differential scanning calorimetry study[END_REF][START_REF] Jonas | Thermal stability and crystallization of poly(aryl ether ether ketone)[END_REF] .

The crystallization peak of the raw material is slightly shifted towards lower temperatures as the number of heat treatment cycles increases, from about 304 • C at the first cycle to about 302 • C at the last cycle. For the drawn yarns, this shift is more pronounced, from about 300 • C in the first cycle to about 287 • C in the last cycle.

The evolution of T c over heat treatment cycles for all samples is presented in Fig. 4 . The crystallization temperature ( T c ) was mea- sured at the maximum heat flow of the exothermic peak during the cooling ramp at 10 • C min -1 . On one hand, it can be noticed that for all samples, the T c decreases with the number of cycles. On the other hand, this phenomenon is increased after transformation of PEEK into yarns.

After extrusion, a 2 • C decrease in T c compared to the raw material is observed from the first cycle. Then the gap between the raw material and the extruded yarn increases with the number of cycles.

In the case of the drawn yarns, the T c is 4 • C lower than that of the raw material in the first cycle, then the crystallization temperature drops over the heat treatment cycles. The final T c is about 16 • C lower than that of the raw material.

Thus, it appears that the melt spinning transformation induces an acceleration of PEEK degradation in the molten state, on one hand due to the extrusion and on the other hand due to the sizing. These two factors therefore affect the crosslinking kinetics that cause the evolution of the crystallization behavior of PEEK. Extrusion is sufficient to accelerate the degradation. In this study, the level of degradation resulting from extrusion corresponds to a residence time in the extruder of about 10 min and an extrusion temperature of 390 • C. The effect of sizing on the crystallization temperature is even more pronounced and seems to be additional to that of extrusion.

The classification of drawn materials according to T c suggests that a low draw ratio favors the degradation of PEEK: the higher the draw ratio, the lower the degradation. However, the sizing rate measured by Soxhlet extraction (see [START_REF] Feuillerat | Effect of poly(ether ether ketone) degradation on commingled fabrics consolidation[END_REF] for details) also appears to be affected by the draw ratio. It appears that the lowest sizing rate is measured on yarns of lower degradation. As the sizing conditions of the yarns are identical, this difference in sizing rate may be due to the mechanical and chemical interactions of the sized yarns at the rubbery state on the drawing godets. This effect of sizing rate on degradation kinetics has been observed in a previous study on the effect of additives on PEEK degradation [START_REF] Almeida | Influence of composite preform fabrication on the integrity of poly-aryl-ether-ketone matrices[END_REF] . Therefore, although variations in the draw ratio may affect degradation, the differences in crystallization temperature are likely to be the consequence of a difference in sizing ratio.

The evolution of viscosity with time at 400 • C under air atmosphere is presented in Fig. 5 . Viscosity rapidly increases in an exponential way due to degradation for all materials. In addition to reducing the ability of polymer to crystallize, crosslinking also results in a significant increase in viscosity, similarly to that of a thermoset during curing [START_REF] Mylläri | Characterization of thermally aged polyetheretherketone fibres -mechanical, thermal, rheological and chemical property changes[END_REF][START_REF] Martín | On the determination of thermal degradation effects and detection techniques for thermoplastic composites obtained by automatic lamination[END_REF][START_REF] Phillips | Thermal stability of PEEK/carbon fiber in air and its influence on consolidation[END_REF] . The extruded yarn has the same initial viscosity as the raw material but the viscosity increases more rapidly. In 20 min, the viscosity of the raw material doubles whereas it almost triples for the extruded yarn.

The drawn yarns have a lower initial viscosity than the raw material and the extruded yarn. It seems that the sizing has a lubricating effect. This could be a beneficial factor in the processing of composites, as a lower facilitates the impregnation of fibrous reinforcements. However, this effect is very short lasting and the sudden increase in viscosity that follows can be disastrous for further processing of the composite like welding or hot stamping.

Finally, the same conclusion is obtained as in the DSC results, the melt spinning transformation has a huge effect on the degradation kinetics of PEEK. The time at the cross point between the storage and the loss moduli confirms also the acceleration of the degradation kinetics ( Fig. 6 ). The loss modulus ( G L ) is higher than the storage modulus ( G S ) at the early stage of the experiments but with degradation, a faster increase of G S compared to G L occurs, leading to a crossover point that can be considered as a gel point. The crossover time between G S and G L has been analyzed by Chan et al. [START_REF] Chan | Crosslinking of poly(arylene ether ketone)s 1. Rheological behavior of the melt and mechanical properties of cured resin[END_REF] . They actually noticed that the lower is the crossover time, the higher is the rate of crosslinking and therefore the gel content in the polymer. The crossover times of the different samples are summarized in Table 3 . It appears that the drawn yarns, regardless of their draw ratio, have the same crossover time, which is considerably less than that of the extruded yarn and the raw material. In other words, the gel point is reached more quickly for the drawn yarns than the extruded yarns and the raw material.

The DSC and rheological results show that extrusion and sizing are the main factors that are responsible for an acceleration of the PEEK degradation in the molten state. Since extruded (non-sized) yarns degrade less quickly than drawn (and sized) yarns, the effect of sizing seems to be predominant over that of extrusion process. In addition, the draw ratio induces different sizing ratio leading to different degradation kinetics. This effect was identified in DSC but not by rheological analysis.

These results are in good agreement with the recent literature on poly(aryl ether ketone) (PAEK) degradation [START_REF] Pascual | On the stability of PEEK for short processing cycles at high temperatures and oxygen-containing atmosphere[END_REF][START_REF] Almeida | Influence of composite preform fabrication on the integrity of poly-aryl-ether-ketone matrices[END_REF][START_REF] Choupin | Macromolecular modifications of poly(etherketoneketone) (PEKK) copolymer at the melting state[END_REF] . However, the physico-chemical process that results in an acceleration of the degradation kinetics has never been analyzed and was therefore further investigated in this study. In particular, the way in which the different melt spinning parameters affect the degradation process of PEEK has been analyzed.

Thermal degradation process of raw material and yarns

Thermogravimetric analysis coupled to Fourier Transform Infrared Spectroscopy (TGA-FTIR) and pyrolysis-Gas Chromatography coupled to Mass Spectroscopy and Flame Ionization Detector (py-GC-MS/FID) were used to probe the effect of extrusion, sizing and draw ratio on the PEEK degradation process.

FTIR analysis were carried out upon heating in TGA, in order to characterize the gases formed during the degradation of the different samples. FTIR spectra of the emitted gases at different temperatures during the heating stage are presented in Fig. 7 . The main molecules identified and their related absorption bands are summarized in Table 4 .

For the raw material, the evolution of the FTIR spectra with temperature shows the appearance of intense absorption bands attributed to phenol, carbon dioxide, carbon monoxide and aromatic ethers. These molecules are characteristic of the PEEK degradation [START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF][START_REF] Courvoisier | Multi-scale and multi-technique analysis of the thermal degradation of poly(ether ether ketone)[END_REF] .

The degradation by-products of the extruded and drawn yarns, whatever the draw ratio, have the same chemical nature as the raw material. However, the absorption bands characteristic of sizing degradation products (carboxylic acid esters and anhydrides) are not detected. The low proportion of sizing on the extruded yarns can explain the absence of sizing by-products, but it can also be the consequence of some interaction of those chemicals with PEEK material. These observations are confirmed by py-GC-MS/FID analysis. As presented in Fig. 8 , the chromatograms of the extruded and drawn yarns are perfectly superimposed. The same degradation products are formed as the same retention times in the column are observed between the extruded and drawn yarns. The separated species were then analyzed by Mass Spectroscopy and Flame Ionization Detector (MS/FID) in order to identify the related molecules, as summarized in Table 5 . These degradation products detected are consistent with those identified in the literature [START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF] . No additional peaks were observed, thus no new species were identified. These chromatograms were also compared to the sizing chromatogram and no common peaks were identified. Although all yarns have the same chemical nature, with the same emitted gases during the degradation, they do not have the same degradation kinetics. Indeed, as illustrated in Fig. 7 the absorption bands of the by-products appear at a lower temperature for the drawn yarns (about 540 • C), compared with the raw material and the extruded yarn (about 580 • C).

In order to highlight this acceleration of degradation, Fig. 9 shows the absorbance profiles of specific bands corresponding to the main degradation products of PEEK (phenol, carbon monoxide and carbon dioxide). This allows monitoring the evolution of the degradation process.

The comparison of the different absorbance peaks for PEEKbased materials indicates that phenol and carbon dioxide have the highest intensity and are therefore the main degradation products, which is in agreement with the literature [START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF][START_REF] Ramgobin | A case study of polyether ether ketone (I): investigating the thermal and fire behavior of a high-performance material[END_REF] . Furthermore, all these products appear in the same range of temperature. For the absorption band related to phenols, a unique peak is observed contrary to that of carbon monoxide and carbon dioxide, which exhibit a double peak. This confirms that several steps are involved in the thermal decomposition mechanism of PEEK, as described in the literature [START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF] .

It can be noticed that for the extruded yarn, the peak maximum is not shifted but the peak onset for all degradation products is 10 to 20 • C lower than that of the raw material. In addition, whatever the by-product, the peaks for raw and extruded PEEK have similar shapes. The absorbance peaks that correspond to the degradation steps have for both materials the same maximum temperature regardless of the by-product. This indicates that the extrusion step of the melt spinning process caused slightly earlier degradation without changing its chemical process. Extrusion is therefore likely to have initiated formation of radicals by chain breakage.

For the drawn yarns, regardless of their draw ratio, the same changes can be observed on the absorbance profiles, which indicates that sizing is responsible for modifications of the degradation process. The onset of absorbance peaks is significantly shifted to temperatures 25 to 45 • C lower than those of the raw material, leading to broader absorbance peaks. For phenols and carbon dioxide, this onset shift induces a distinct shift in the maximum of the absorbance peaks. For carbon monoxide, the maximum absorbance occurs at the same temperature as for the raw and extruded material, but the double peak shape indicates that the chemical modifications occur over a broader temperature range, suggesting a more gradual degradation process.

However, for carbon dioxide, the peak shape of the drawn yarns is significantly modified compared to the raw material. Instead of a double peak, the absorbance profile exhibits a triple peak, suggesting a change in decomposition mechanisms compared to the raw material. This particular peak shape that appears is relatively close to that of the sizing. It is therefore likely that the degrada-tion by-products of sizing are involved in PEEK degradation mechanisms. In particular, it appears that carbonization that occurs above 750 • C is affected, with an intense carbon dioxide absorbance peak at about 780 • C [START_REF] Ramgobin | A case study of polyether ether ketone (I): investigating the thermal and fire behavior of a high-performance material[END_REF][START_REF] Tsai | A study of thermal degradation of poly(arylether-ether-ketone) using stepwise pyrolysis/gas chromatography/mass spectrometry[END_REF] .

These results show that, although the same decomposition products are identified, thermal degradation of PEEK is modified due to melt spinning processing. Extrusion appears to induce only a slightly earlier degradation process, while sizing applied to the surface of the drawn yarns appears to modify the degradation mechanisms with modifications occurring at lower temperatures. Nevertheless, no significant effect of the draw ratio on the degradation mechanisms could be detected with these techniques.

In this study, the products formed during sizing decomposition and the products resulting from their interaction with PEEK could not be identified due to the low quantity of sizing on the yarn surface (1.5% in mass). Further analyses with larger amount of additives will be required in order to identify the chemical process of PEEK decomposition in presence of additives. The nature of the sizing also should be investigated to better understand the interaction of PEEK with its environment.

Conclusion

The influence of different melt spinning parameters on the stability, thermal and rheological properties of poly(ether ether ketone) and its degradation process was analyzed using various experimental techniques. To do so, yarns collected at different stages of melt spinning were characterized and compared with raw PEEK: extruded yarns collected in the cooling chamber before being sized and drawn, and drawn yarns corresponding to yarns that were extruded, sized and drawn at different draw ratios.

PEEK degradation has been identified on the different yarns. As widely described in the literature, this degradation induces a decrease of the crystallization temperature and an increase of the viscosity in the molten state. This degradation is irreversible and can be a limiting factor in the processability of thermoplastic composite preforms.

The results show that the melt spinning operation has a significant effect on the PEEK properties. First, the extrusion affects the crystallization temperature and viscosity. It shifts slightly the beginning of degradation at lower temperatures without changing the decomposition process.

On the contrary, the drawn yarns have higher degradation kinetics than that of the raw PEEK and the extruded yarn. In addition, the TGA coupled with infrared spectroscopy suggests that sizing modifies PEEK degradation mechanisms due to an interaction of sizing by-products with PEEK. In addition, the draw ratio seems to induce different sizing ratio causing different degradation kinetics.

Further work should be carried out to investigate the effect of the chemical nature and stability of sizing on PEEK degradation.

CRediT author statement

Lisa Feuillerat and Carole Aubry performed the experiments.

Fig. 1 .

 1 Fig. 1. Schematic of the melt spinning process.

Fig. 2 .

 2 Fig. 2. TGA of all samples, performed in neutral atmosphere with a heating rate of 10 • C min -1 .

Fig. 3 .

 3 Fig. 3. Evolution of the crystallization peak during cycles at 400 • C during 20 min, under neutral atmosphere, for all samples.

Fig. 4 .

 4 Fig. 4. Temperature of crystallization ( T c ) against the number of cycles at 400 • C during 20 min, under neutral atmosphere, for all samples. Error bars are representing standard deviations.

Fig. 5 .

 5 Fig. 5. Viscosity against time at 400 • C under air atmosphere for all samples. DY: Drawn yarn. Color halos are representing minimum and maximum values.

Fig. 6 .

 6 Fig. 6. Storage and loss moduli ( G S and G L ) against time during an isotherm at 400 • C under air atmosphere for all samples. DY: Drawn yarn. For clarity, the drawn yarns DR = 2 and DR = 3 . 75 are not shown in this figure because they are superimposed on the drawn yarns DR = 1 .

Fig. 7 .

 7 Fig. 7. FTIR spectra of emitted gases during TGA at 10 • C min -1 , performed in neutral atmosphere. (a) Raw material and extruded yarn, (b) Sizing (c) Drawn yarns.

Fig. 8 .

 8 Fig. 8. Chromatograms of extruded and drawn yarns.

Fig. 9 .

 9 Fig. 9. Peak intensity profiles of the main degradation products of PEEK (a) Phenol (b) Carbon monoxide and (c) Carbon dioxide, during thermogravimetric analysis at 10 • C min -1 .

Table 1

 1 Name and sampling zone for the different yarns (DR: Draw ratio).

	Sampling zone	Name	Sizing
	B	Extruded yarn	-
	D	Drawn yarn DR = 1	Yes
	D	Drawn yarn DR = 2	Yes
	D	Drawn yarn DR = 3 . 75	Yes

Table 3

 3 Crossover time ( t cr ) between G S and G L , during rheometry tests at 400 • C, under air atmosphere and uncertainty.

	Sample	t cr (min)
	Raw material	33 ± 0.9
	Extruded yarn	26 ± 1
	Drawn yarns	7 ± 0.6

Table 4

 4 Molecules associated with the absorption bands of FTIR spectra.

	Molecules	Absorption bands ( cm -1 )
	Carbon dioxide	2358 + 2310
	Carbon monoxide	2179 + 2113
	Phenol	3648, 1601, 1490, 1339, 1249, 1185 and 748
	Aromatic ethers	1600, 1493 and 1233
	Carboxylic acid esters	2930, 2861, 1745, 1461, 1367, 1237, 1135,
		965 and 720
	Carboxylic acid anhydrides	1770

Table 5

 5 Molecules identified by MS/FID with the related retention time of chromatogram peaks.

	Molecules	Retention time (min)
	Carbon dioxide	3
	Benzene	3.5
	Toluene	5.2
	Phenol	14.9
	Diphenyl ether	41.9
	Dibenzofuran	48.5
	2-phenylphenol	48.7
	Fluorene	52.3
	Benzophenone	55.3
	4-phenoxyphenol	60.9

All authors interpreted the results. Lisa Feuillerat and Olivier De Almeida wrote the manuscript with the support of Jean-Charles Fontanier, Fabrice Schmidt, Carole Aubry and Pascal Rumeau. All authors provided critical comments and helped to shape the research, analysis and manuscript.

Acknowledgments

The authors thank the Region Occitanie for their financial support in this project. The authors wish to thank the IFTH for their support in the melt spinning and TGA-FTIR characterizations. The authors wish also to thank Marion Carrier and Justine Arocas for their support on py-GC-MS/FID experiments, which were supported by the ANR funding of the PYROKINE project (ANR-18-MPGA-0013).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


A Biocis

Guangkuan Zhao

Christelle Roudaut

Vincent Gandon
email: vincent.gandon@u-psud.fr

Mouad Alami
email: mouad.alami@u-psud.fr

Olivier Provot
email: olivier.provot@u-psud.fr

Synthesis of 2-Substituted Indoles through Cyclization and Demethylation of 2-Alkynyldimethylanilines by Ethanol

Herein, we demonstrated that 2-alkynyldimethylamines easily cyclize in EtOH according to a 5-endo-dig annulation into 2substituted indoles without the aid of any additive or any metal catalyst to activate the triple bond. Thus, a variety of functionalized 2-styrylindoles, 2-arylindoles, 2-alkynylindoles, and 2-alkylindoles were prepared in high to excellent yields according to an environmentally friendly protocol. The mechanism has been explored to better understand this eco-friendly access to 2substituted indoles and DFT computations rationalized the role of the solvent in this N-annulation/dealkylation process.

Introduction

Indoles are important heterocycles in biologically active natural products 1 and are also frequently found in various pharmaceutical products. 2,3 Consequently, this important structural motif has led to the development of a very large number of synthetic methodologies. Among them, the 5-endodig hetero-annulation of alkynyldimethylanilines in the presence of electrophiles (ICl, I 2 , ArSCl…) developed by Larock 4 and others, 5 as well as transition-metal catalyzed transformations, 6 played a major role in this field. In this challenging context, we recently reported the one-pot synthesis of 2-styrylindoles from ortho-substituted chloroenynes. 7 In a hot toluene/MeOH mixture, using a Pd(0) catalyst, boronic acids and K 2 CO 3 , 2-alkynyldimethylanilines 1 were transformed into 2-styrylindoles after successive Suzuki arylation,

8

N-cyclization and N-demethylation reactions (Scheme 1).

Scheme 1. Our approaches for the synthesis of 2-substituted indoles

The success of this N,N-dimethyl-cyclization/N-demethylation process prompted us to examine the role of each reagent in this transformation. To this end, we have removed one by one all of the reagents used in our previous protocol to transform 1a 8 into 2a (Scheme 2).

Scheme 2. Synthesis of (E)-2-(2-chlorovinyl)-1-methyl-1H-indole 2a from cyclization of 1a followed by a N-demethylation reaction

In the presence of only hot MeOH, we were delighted to observe the transformation of diarylenyne 1a into indole 2a with an excellent yield (89%). Even if the N-cyclization of orthoalkynylanilines into indole derivatives is well-known, to our knowledge, this result show that the formation of a 2-substituted-N-methylindole can be carried out in the absence of any activating agent since a simple heating in MeOH is sufficient to promote the N-cyclization and the N-demethylation reactions. For comparison, Ribecai reported in 2009 the cyclization of 2-alkynylanilines into 2substituted-1H-indoles in water at 200 °C under microwave heating in the absence 9 or in the presence of additives 10 (e.g. KCl, NaHCO 3 , pyrrolidine,…), necessary to increase yields and the scope of the cyclization reactions.

In this paper, we wish to present our results concerning the development of this "green" annulation/demethylation process, its application to a wide range of functionalized substrates and also a possible reaction mechanism which is supported by DFT calculations.

Results and discussion

Before studying the scope of this process using EtOH as only reagent, we first examined the influence of other solvents on the cyclization and demethylation steps of (E)-chloroenyne 1b, a poorly cooperative substrate in which the chlorine atom, useful for further transformations, 11 disfavoured the 5-endo-dig annulation reaction. 12 The results of this solvent and experimental screening are reported in Table 1. Under the conditions used in Scheme 2, 1b was transformed into indole 2b after 48 h of reaction in a poor yield of 40% (entry 1).

Compound 1b was then heated in MeOH for 2 h at a higher temperature of 120 °C under microwave irradiation (MWI). Gratifyingly, after 2 h, 2b was isolated in 70% yield (entry 2). We next replaced MeOH by EtOH, i-PrOH, hexafluoroisopropanol (HFIP), toluene and H 2 O to evaluate the influence of other solvents (entries 3-7) and found that EtOH and MeOH gave nearly similar yields (70-75%, entries 2,3). A more congested alcohol as i-PrOH allowed the expected transformation but with a poor performance (30%, entry 4), while the reaction run in HFIP or toluene was inefficient (entries 5,6). Moreover, as it could be observed in entries 7-9, water and its association with EtOH or acetone did not provide indole 2b with satisfactory yields. The scope of this metal-and additive-free process was then evaluated with a variety of N,Ndimethylanilines 1 bearing various alkyne substrates at the orthoposition using EtOH as only reagent at 120 °C under MWI. The results of this study are reported in Table 2. As it can be seen, under our optimal conditions, alkynes 1a, 1c-t were transformed into indoles 2a, 2c-t with excellent yields. In details, diarylenynes 1a and 1c, (di)chloroenynes 1d-f and conjugated ethyl acrylate 1g reacted well in hot EtOH or MeOH (to avoid esterification, entry 7) under MWI to provide the expected indoles 2a, 2c-h in good yields (entries 1-7). Having in mind a deeper exploration of the structure-activity relationships of isoCombretastatin A4 13 derivatives by replacing its B-ring with substituted-indoles, 7,14 2-chlorovinyl-indoles 2d and 2e in which the chlorine atom will be useful for further transformations, were obtained in acceptable yields regardless of the configuration of the chloroenyne double bond (entries 3, 4). Interestingly, diarylalkynes 1h-j, arylheteroarylalkynes 1k,l bearing a thiophene or a pyridine moiety, and diaryldiyne 1m were transformed into the expected 2-substituted indoles 2i-n with excellent yields (entries 8-13). Alkyl groups on the alkyne triple bond were also well tolerated in this process as indole derivatives 2o-q and azaindole 2r were obtained in good yields (entries 14-17).

A terminal alkyne such as 1r was well tolerated to give 1-methyl-1Hindole 2s in a good 89 % yield (entry 18) In entries 19 and 20, we noted that silylated alkynes 1s,t were also transformed into indole derivatives 2s,t with good yields, but the TMS-group did not survive under these conditions. To summarize this study, this protocol is found to be practical for a variety of alkyne substrates in which a great number of functionalized group are well tolerated. Intrigued by the large scope of this transformation, we then checked that there were no traces of metals (Pd and Cu) in our samples since alkynes 1 were prepared according to Sonogashira and Suzuki reactions.

We thus purified diarylenyne 1i one (sample 1, S1), two (S2) and three times (S3) on silica gel. These three samples were then reacted in hot EtOH 15 under MWI, using new magnetic stir bars 16 and new microwave tubes in view of their transformation into the expected 2-arylindole 2i (Fig. 1). The results indicated that these samples (S1, S2 and S3) gave similar yields in 2i (88, 90 and 92% respectively). The quantity of residual traces of Pd and Cu in these samples was quantified by atomic absorption after mineralization of the samples. The results, obtained with internal standards, indicated that no traces of Cu in the three samples (below 5 ppb if any) and that Pd quantities were also negligible (7 (S1), 3 (S2) and 1 ppb (S3), respectively).

Encouraged by these results, we turned our attention to examining primary and secondary anilines 3 and 4 (Scheme 3).

Scheme 3. Reactions of anilines 3 and 4 in hot EtOH under MWI.

As it can be seen, heating N-methylaniline 4 at a higher temperature of 180 °C furnished mostly indole 2o (80%) accompanied with a small quantity of NH-indole 5 (9%) which is consistent with our proposition of mechanism involving an ammonium intermediate and where an ethanolate anion, in this case, acted as a base rather than a nucleophile. This process is less suitable for primary anilines as 3 since indole 5 and ketone 6, resulting from the hydration of the triple bond, were obtained in equivalent quantities.

Next, we were interested to examine the behaviour of tertiary anilines 7 and 8 (Scheme 4) having respectively on their nitrogen atom, two different groups (Me and Ph for 7, Me and i-Pr for 8).

4. attempts of anilines 7 and 8 under MWI in EtOH After heating 7 in EtOH for 10 h at 120 °C under MWI irradiation, we did not observed any transformation of the poorly nucleophilic and encumbered diphenylmethylaniline 7, which was totally recovered unchanged. On the contrary, under similar conditions, a careful examination of the crude mixture by 1 H NMR revealed that aniline 8 was successfully transformed into N-methylindole 2o (91 %) with no trace of 2-butyl-1-isopropylindole. In this case, it is reasonable to think that the ethanolate anion formed during this process acted as base to eliminate 1-propene rather than a nucleophile (see Scheme 6).

To gain mechanistic insight into cyclization/demethylation process observed with dimethylanilines, a series of experiments was then conducted. First, o-hexynyldimethylaniline 1n was heated in EtOH for 2 hours at 120 °C under MWI in the presence of 1 equiv of 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO), a well-established radical scavenger. [START_REF] Keana | [END_REF] The presence of TEMPO did actually significantly alter the yield in indole 2o (84% vs 95% without TEMPO), which does not support a radical pathway.

Scheme 5. Synthesis of (D)-2i

Our second interrogation concerned the activation of the alkyne triple bond as EtOH or MeOH were the only reagents used in this protocol. Therefore, we have heated diarylalkyne 1h in MeOD at 120 °C under MWI (Scheme 5). In the crude mixture, after 10 h of reaction, we observed the unique formation of (D)-2i indicating that MeOD may activate the alkyne triple bond of 1h. Next, to demonstrate the fundamental role of EtOH or MeOH as being also the nucleophilic reagents responsible for the N-demethylation step, we performed other experiments on ortho-substituted anilines 1u and 1v in which the nitrogen atom is included into a ring (Scheme 6). Scheme 6. Synthesis of indoles 2u,v and possible mechanism.

When 1u,v were heated in EtOH or MeOH under MWI, we were very pleased to observe the formation of 2u and 2v demonstrating the fundamental role of MeOH or EtOH in the last alkylation/dealkylation steps (demethylation in the case of dimethylanilines). A possible reaction mechanism is proposed in Scheme 6. First, the nitrogen atom of the pyrrolidine reacts with the triple bond (possibly activated by EtOH) according to a 5-endo-dig cyclization to give intermediate I. Then, EtOH as solvent or possibly an ethanolate species generated during the cyclization process reacts on one of the methylene groups close to the ammonium center by SN 2 to open the pyrrolidinium intermediate to promote 2u,v.

Density functional theory (DFT) calculations were carried out to understand the origin of the activation of the substrates under the reaction conditions. The above experiments having put aside the intervention of radical species, the main issue to address was the feasibility of an ionic mechanism triggered by just the solvent. The Gaussian 09 software package was used for the calculations. [START_REF] Frisch | Gaussian 09, revision D.01[END_REF] Gas phase geometry optimization was carried out at the M06-2X 19 /6-31G (d,p) 20 level of theory with the grid=ultrafine keyword. Correction to the Gibbs free energy was computed at 393.15 K. The values presented include solvent correction obtained with the PCM 21 solvation model for methanol. This solvent was chosen rather than EtOH to save computer time, as it provides similar experimental results. The model substrate is presented in Scheme 7. Its cyclization was studied in the presence of H-bonded methanol clusters (MeOH) n of various sizes (n = 1-6). 22 Linear clusters were privileged to be able to bridge the reactive sites. We found three steps starting from the molecular system An comprising the substrate and (MeOH) n optimized together: i) a 5-endo-dig cyclization yielding the vinyl anion ammonium species Bn, ii) a proton transfer from the (MeOH) n cluster to the vinyl anion center to give the H-bonded methylate/ammonium species Cn, and iii) SN 2 reaction to give the final dealkylated product Dn.

The relative free energies of the computed intermediates and transition states are collected in Table 3 as a function of n, taking An as a reference. With only one MeOH molecule (n = 1), the cyclization of A1 into B1 requires a high free energy of activation of 29.9 kcal/mol and is markedly endergonic by 27.2 kcal/mol. No other steps could be computed. With n = 2, the first barrier is 26.4 kcal/mol, which is significantly lower than with n = 1. The cyclization is endergonic by 23.4 kcal/mol. The deprotonation step could not be computed. The SN 2 step was achieved through a transition state lying at 26.9 kcal/mol above A2. Product D2 is obtained in a strongly exergonic fashion, as it lies 38.7 kcal/mol below A2. With n = 3-6, the full pathway could be calculated. It can be seen that the ratedetermining step is the first one in all cases. The lowest barrier for this first step is 24.4 kcal/mol. It corresponds to a cluster of 4 Clearly, the a network of H-bonded molecules has a positive impact on the the effect is not linear as the barrier increases with n > 4. on n = 4, the cyclization step is endergonic by 18.5 kcal/mol (Scheme 8). The next step, the proton transfer connecting B4 to C4, is achieved through a transition state that has a lower energy than B4. This unusual situation is typical of what is referred to as a collapse mechanism and means that B4 is a very unstable intermediate, which, if it exists, is immediately transformed into C4. This species is less stable than A4 by 3.6 kcal/mol. What funnels the reaction towards the final product is the dealkylation step. The corresponding transition state TS C4D4 , lies 20.2 kcal/mol above A4. This step is strongly exergonic, D4 being located at -35.2 kcal/mol on the free energy surface. Overall, the calculated free energies of activation are realistic with regard to the experimental conditions and the reaction is a highly favorable process. The free energy profile and the geometry of the intermediates and transition states with n = 4 are displayed in Scheme 8. The MeOH molecule that donates its proton to the vinyl anion does not become the methanolate of the cluster. It is simultaneously reprotonated by the nearby MeOH molecule, which is in turn reprotonated by the third one. The proton transfer stops here, leaving the third molecule of the cluster as a doubly H-bonded methanolate. The SN 2 step could not be achieved from this methanolate but from the less hindered tail MeOH, which is simultaneously deprotonated by the methanolate. Looking at higher clusters in Table 3 (n = 5, 6), similar characteristics can be drawn, notably regarding the instability of Bn and its straightforward collapse to Cn. However, the n = 4 case appears to be the best compromise in terms of free energy for the entire process. This does not mean that only linear (MeOH) 4 clusters are active species in solution for this reaction, but the feasibility of an ionic mechanism based on the simple activation of the substrate by methanol aggregates is validated by these computations.

Conclusions

In summary, a variety of 2-substituted indoles 2 were prepared in high yields from the annulation/demethylation process of 2alkynyldimethylanilines. A simple heating of these compounds readily accessible in EtOH allowed rapid and efficient access to various N-methylindoles with high yields. This approach, which does not require metal catalysts or additives, and which is applicable to different types of alkynes bearing various functional groups, is in perfect harmony with the requirements of modern environmentally friendly chemistry.

Fig 1 .

 1 Fig 1. Synthesis of 2i from metal-free samples of 1i

Scheme 7 .

 7 Scheme 7. Computed Steps.

Table 1 .

 1 EffectsReaction achieved at 90 °C. d 30W Microwave irradiation. e Complex mixture.

	of solvents and experimental conditions on the
	transformation of (E)-chloroenyne 1b into 2-(2-chlorovinyl)-1-methyl-1H-indole 2b. a

b Isolated yield.

c

Table 2 .

 2 Synthesis

		Indoles 2 c-s	t (h)	Yield b (%)
	1	1a	2a 2	90
	2	1c	2c 2	90
	3	1d	2d 4	56
	4	1e	2e 6	42 c

of 2-substituted indoles 2 from 2alkynyldimethylanilines 1. a o-Alkynylanilines 1c-t a Reaction conditions are identical to those described in footnote a of Table 1. b Isolated yield. c Reaction achieved at 140 °C. d Reaction achieved in MeOH. e Reaction achieved at 160 °C.

Acknowledgments

Conflicts of interest