Synthesis,structural characterization,and thermal degradation kinetics of s -triazine-based homopolyesters

Abstract

A series of s-triazine-based homopolyesters was synthesized via polycondensation of a 4,6-bis-(N-(4-(benzoylchloride)amino))-2-(N-benzyl-piperazin-1-yl)-1,3,5-triazine (monomer) with various aliphatic and aromatic diols. The structures were confirmed using FT-IR and ¹H NMR spectroscopy. Physicochemical characterization revealed that polyesters containing aromatic moieties exhibited higher density, intrinsic viscosity, and improved thermal stability compared to those derived from aliphatic diols. Solubility studies indicated enhanced dissolution behaviour in polar aprotic solvents at elevated temperatures. Thermogravimetric analysis demonstrated significant thermal resistance, particularly for bisphenol-based systems. The thermal degradation kinetics were evaluated using Coats–Redfern, Horowitz–Metzger, Broido, and Chan methods, showing consistent trends with variations attributed to model assumptions. The overall degradation trends remained consistent. The combined physicochemical and thermal characteristics suggest that these s-triazine-based homopolyesters may be promising candidates for advanced materials requiring thermal resistance and structural stability.

Graphical Abstract

Keywords

polycondensation thermal degradation kinetics activation energy high-performance polymers

Introduction

Polymers incorporating heterocyclic moieties have attracted considerable attention owing to their superior thermal stability, chemical resistance, and tunable functional properties.^1–3 Among these, the 1,3,5-triazine (s-triazine) ring represents a highly versatile structural unit due to its electron-deficient aromatic framework and symmetric geometry. These features facilitate diverse chemical transformations, particularly nucleophilic substitution reactions, enabling the rational design of structurally complex and functionally advanced polymeric systems.^2,3 Consequently, s-triazine-based materials have emerged as promising candidates for high-performance applications.^4,5

The integration of s-triazine units into polyester backbones provides an effective strategy for enhancing the intrinsic limitations of conventional polyesters, such as moderate thermal and oxidative stability. While traditional polyesters are valued for their processability, mechanical strength, and biodegradability, their performance under elevated temperatures remains restricted.^6,7 Incorporation of rigid, aromatic s-triazine segments into the polymer chain introduces improved thermal resistance, dimensional stability, and flame retardancy, while also offering reactive sites for further chemical modification.^4,6 Such hybrid architectures therefore, hold significant potential for advanced applications in coatings, electronics, and high-temperature engineering materials.^7–9

In recent years, extensive efforts have been devoted to modifying the s-triazine core to tailor polymer properties. A variety of substituents—including morpholine, piperidine, N-methylpiperazine, N-ethylaniline, carbazole, and naphthylamine derivatives—have been incorporated to modulate electronic properties, solubility, and intermolecular interactions.^10–13 In parallel, the incorporation of functional diacid or diol components such as salicylic acid, hydroxy-naphthoic acids, hydroxybenzoic acid, and aminobenzoic acid into polymer backbones has enabled fine control over rigidity, polarity, and thermal behavior.^11–15 Despite these advancements, systematic investigations correlating structural variation with physicochemical properties and thermal degradation kinetics remain relatively limited.^14,15

Despite the extensive development of s-triazine-containing polymers, most reported systems have primarily focused on cyanate esters, polyamides, epoxy networks, or triazine-functionalized thermosets, whereas systematic investigations on s-triazine-based homopolyesters incorporating structurally diverse aromatic and aliphatic diols remain comparatively limited.^16–20 Previously reported triazine polymers largely emphasize thermal stability and mechanical performance but often lack detailed correlations between polymer architecture, solution behavior, and degradation kinetics. Moreover, comparative kinetic analyses using multiple non-isothermal models to understand the degradation mechanism of triazine-containing polyesters are rarely reported. In the present study, a benzylpiperazine-functionalized s-triazine acid chloride monomer was employed to synthesize a new series of homopolyesters through high-temperature polycondensation with different diols, enabling systematic modulation of chain rigidity and polarity. Unlike earlier studies, the present work integrates physicochemical characterization (solubility, density, and intrinsic viscosity), detailed structural confirmation (FT-IR and ¹H NMR), thermogravimetric evaluation, and comparative activation energy analysis using Coats–Redfern, Horowitz–Metzger, Broido, and Chan methods (Table 1). The structure–property relationship established herein provides new insights into the role of diol structure in governing processability and thermal performance of s-triazine-based high-performance polyesters.

Table 1.

Comparison of reported s-triazine-based polymers with the present study.

Polymer system	Synthesis route	ηinh/Molecular feature	Thermal stability/Char yield	Kinetic method	Ref.
s-Triazine cyanate ester polymers	Thermal curing	Crosslinked network	High Td, moderate char	Kissinger	6–8
Piperidine-substituted triazine polyamides	Polycondensation	Moderate intrinsic viscosity	Good thermal resistance	TGA only	9–12
N-Methylpiperazine triazine polymers	Step-growth polymerization	Enhanced solubility	Improved decomposition behavior	Limited kinetic study	13–15
Carbazole/naphthylamine triazine polymers	Condensation	Rigid aromatic backbone	High char residue	Coats–Redfern	16–19
Present work: Benzylpiperazine s-triazine homopolyesters	High-temperature polycondensation with aromatic/aliphatic diols	ηinh = 0.49–0.82 dL g ^-1	Residual weight up to 6.8% at 800°C	Coats–Redfern, Horowitz–Metzger, Broido, Chan et al.	This work

Experimental

Materials

The laboratory grades general chemicals, such as DMF, acetone, carbon tetrachloride, methanol, DMSO, DMAc were used. 2,4,6-Trichloro-1,3,5-triazine was refined by recrystallization from C₆H₆, which was received from Fluka. 1-benzylpiperazine was used as given by Enzal Chemicals Ltd, Ankleshwar. 4-Aminobenzoic acid was used as received from Atul Ltd, Valsad. Different diols like catechol, bisphenol-A, bisphenol-S, hydroquinone, resorcinol, 1,4-dihydroxy anthraquinone, phenolphthalein, 1,8-dihydroxy anthraquinone, diethylene glycol, ethylene glycol etc. received nearly 99% purity from Atul Ltd, Valsad.

Preparation of 2-(4-benzylpiperazin-1-yl)-4,6-dichloro-1,3,5-triazine [I]

2,4,6-trichloro-1,3,5-triazine (18.44 g, 0.1 mol) was dissolved in 60 mL of acetone with continuous stirring at a temperature maintained between 0 and 5°C in a 250 mL reaction flask. Separately, a mixture of 1-benzylpiperazine (17.60 mL, 0.1 mol) and acetone (10 mL) was dropwise added to a chilled s-triazine solution with constant agitation, while carefully controlling the pH at 7.0 throughout the addition. This led to the formation of a reaction slurry. The reaction mass was agitated continuously for 3 h at 0–5°C. The resulting white solid was filtered and rinsed carefully with cold water to remove impurities, purified by recrystallization using ethanol, and dried in a vacuum desiccator. The isolated product had a yield of 71% and a melting point of 133–136°C.

Preparation of 4,6-bis(N-(4-carboxyphenyl)amino)-2-(N-benzyl-piperazin-1-yl)-1,3,5-triazine

A solution of 2-(4-benzylpiperazin-1-yl)-4,6-dichloro-1,3,5-triazine (3.24 g, 0.01 mol) in 40 mL of acetone was taken in a 250 mL RBF. In this solution, a mixture of p-aminobenzoic acid (2.74 g, 0.02 mol) and sodium carbonate (4.20 g, 0.04 mol) in water (80 mL) was gradually added at 25°C with constant agitation for 2 h. Followed by refluxing it at 70–80°C for an additional 2 h. Upon completion, an off-white precipitate formed, was separated by filtration, thoroughly rinsed with hot water, and dried at 100°C under vacuum. The precipitate was refined by recrystallization from acetone. The final yield was 76%, with a melting point of 249–251°C.

Preparation of 4,6-bis(N-(4-(benzoylchloride)amino))-2-(N-benzyl-piperazin-1-yl)-1,3,5-triazine [II]

Thionyl chloride (11.9 mL, 0.1 mol) was added drop by drop to a dry round-bottom flask containing 4,6-bis(N-(4-carboxyphenyl)amino)-2-(N-benzylpiperazin-1-yl)-1,3,5-triazine (5.25 g, 0.01 mol). The reacting mass was then refluxed for 2 h at 78°C. The surplus thionyl chloride was eliminated by distillation under vacuum. The resulting off-white solid was collected and dried. The yield was 64%, and the melting point was 199–201°C.

Preparation of polyesters [III] (reaction scheme)

A solution of 4,6-bis(N-(4-(benzoyl chloride)amino))-2-(N-benzylpiperazin-1-yl)-1,3,5-triazine (5.62 g, 0.01 mol) was prepared in a minimum amount of anhydrous dimethylformamide (DMF, ∼10 mL) and heated to approximately 150°C under continuous stirring. DMF was employed as a high-boiling polar aprotic medium to facilitate effective dissolution of the monomer and promote uniform polycondensation throughout the reaction system.

Subsequently, the corresponding diol (0.01 mol) was introduced into the reaction mixture along with cetrimide (0.25 g), which served as a dispersing and phase-transfer additive to improve monomer compatibility and ensure homogeneous reaction conditions. A catalytic quantity of triethylamine (TEA) was then added dropwise to neutralize the hydrogen chloride generated during esterification, thereby promoting chain propagation and minimizing undesirable side reactions (Figure 1).

Figure 1.

Reaction scheme.

The resulting reaction mixture was maintained at 150–165°C for 8 h under constant stirring. Considering that the boiling point of DMF is approximately 153°C, the polymerization was carried out under controlled elevated-temperature conditions rather than conventional reflux. Upon completion of the reaction, the viscous mixture was allowed to cool to ambient temperature and was gradually poured into crushed ice with vigorous stirring to induce polymer precipitation.

The precipitated polymer was isolated by filtration and washed thoroughly with hot distilled water to remove inorganic salts and residual triethylammonium chloride. Additional washing with ethanol and acetone was performed to eliminate traces of unreacted monomer, solvent, and catalyst residues. Purification was continued until a neutral filtrate was obtained.

Finally, the purified polymers were dried in a vacuum oven at 80–100°C for 24 h to remove residual solvent and absorbed moisture before physicochemical characterization. This purification protocol ensured that subsequent analyses, including viscosity, density, solubility, and thermal measurements, accurately represented the inherent properties of the synthesized polyesters.

Results and discussion

Solubility behaviour

The solubility profiles of the synthesized homopolyesters PEBPA, PEBPS, PE14AQ, PE18AQ, PEPh, PER, PEC, PEHQ, PEEG, and PEDEG in different organic solvents are summarized in Table 2. A distinct dependence of solubility on polymer structure is evident.

Table 2.

Solubility of polyesters in various solvents.

Solvent	I	II	III	IV	V	VI	VII	VIII	IX	X
PEBPA	− −	− −	− −	− −	− −	+ +	+ +	− +	− +	− −
PEBPS	− ±	− −	− −	− −	− −	± +	+ +	− +	− +	− −
PE14AQ	− −	− −	− −	− −	− −	+ +	+ +	± +	− +	− ±
PE18AQ	− −	− −	− −	− −	− −	+ +	+ +	+ +	± +	− −
PEPh	− ±	− −	− −	− −	− −	±+	+ +	± +	− ±	− ±
PER	− −	− −	− −	− −	− −	+ +	+ +	+ +	+ +	− −
PEC	− −	− −	− −	− −	− −	+ +	+ +	± +	− +	− −
PEHQ	− ±	− −	− −	− −	− −	± +	+ +	± +	± +	− −
PEEG	− ±	− −	− −	− −	− −	± +	+ +	± +	− ±	− ±
PEDEG	− −	− −	− −	− −	− −	+ +	+ +	+ +	± +	− −

(+ +): Soluble at both room temperature and 50°C.

(− +): Soluble only at elevated temperature.

(− ±): Partially soluble at elevated temperature.

(− −): Insoluble under both conditions.

Solvent: (I) Acetone (II) Benzene (III) Chlorobenzene (IV) Chloroform (V) Carbon tetrachloride (VI) Dimethylacetamide (VII) Dimethyl formamide (VIII) Ethyl acetate (IX) 1,4-Dioxane (X) Methanol.

The majority of the polyesters remain insoluble in non-polar and chlorinated solvents such as benzene, chlorobenzene, chloroform, and carbon tetrachloride, indicating strong intermolecular cohesion and limited interaction with low-polarity media. In contrast, appreciable solubility is observed in polar aprotic solvents, particularly dimethylacetamide (DMAc) and dimethylformamide (DMF), with solubility further improving at elevated temperatures.

This behaviour can be rationalized by the presence of polar functionalities such as ester groups, amine linkages, and, in some cases, sulfone moieties, which enhance interactions with polar solvents. Additionally, incorporation of bulky aromatic units disrupts efficient chain packing, thereby increasing free volume and facilitating solvent penetration. The increase in solubility with temperature suggests that intermolecular forces, including hydrogen bonding and π–π interactions, are weakened upon heating.

Overall, the solubility characteristics indicate that these materials are readily processable in polar aprotic solvents, which is advantageous for solution-based fabrication techniques.

Density characteristics

The densities of the synthesized polyesters, determined by the pyknometric method, are listed in Table 3 and fall within the range of 1.825–1.887 g cm⁻³. The observed variation in density is closely related to the structural features of the repeating units.

Table 3.

Density and yield of polyesters.

Polyesters	Density (g/cm³)	% yield
PEBPA	1.883	78
PEBPS	1.887	65
PE14AQ	1.873	80
PE18AQ	1.870	67
PEPh	1.879	63
PER	1.863	76
PEC	1.857	69
PEHQ	1.844	71
PEEG	1.825	64
PEDEG	1.830	62

Polyesters derived from aromatic diols exhibit relatively higher densities compared to those prepared from aliphatic diols. In particular, PEBPS shows the highest density, which can be attributed to the presence of the highly polar sulfone group that promotes tighter chain packing and reduces free volume. Similarly, rigid aromatic frameworks such as anthraquinone and bisphenol units contribute to enhanced packing efficiency and higher mass density.

In contrast, polyesters containing flexible aliphatic segments (e.g., PEEG and PEDEG) display comparatively lower densities due to increased segmental mobility and less efficient molecular packing. These findings suggest that polymer density is governed by a combination of backbone rigidity, polarity, and intermolecular interactions, and can serve as an indirect measure of structural compactness.

Viscosity behaviour and molecular characteristics

The viscosity measurements for PEBPS solutions in DMF at varying concentrations (Table 4) demonstrate a consistent increase in relative and specific viscosity with increasing concentration, confirming typical polymer solution behavior. The linearity observed in Huggins and Kraemer plots supports the reliability of the viscosity data shown in Figure 2.²¹

Table 4.

Solution viscosity of polyester PEBPS.

Conc. g/dl	Flow time (t second)	η _rel = t/t_o	η_sp = η _rel-1	η _red = η _sp/C	(ln η _rel)/C
0.2	167	1.169	0.169	0.845	0.780
0.4	192	1.346	0.346	0.867	0.742
0.6	219	1.533	0.533	0.8893	0.712
0.8	246	1.721	0.721	0.9017	0.678
1.0	274	1.923	0.923	0.923	0.653

Solvent: DMF, t_o = 143 s.

Figure 2.

Huggins & Kraemer plots for intrinsic viscosity.

Intrinsic viscosity values ([η]) calculated for the polyester series (Table 5) range from 0.49 to 0.82 dL g⁻¹. Among the synthesized polymers, PEBPS exhibited the highest intrinsic viscosity, indicating comparatively higher hydrodynamic volume, greater chain rigidity and stronger intermolecular interactions, which may be attributed to the rigid aromatic bisphenol-S moiety and the presence of the sulfone group in the polymer backbone. In contrast, PEEG exhibited the lowest intrinsic viscosity, likely due to increased chain flexibility imparted by the aliphatic ethylene glycol segment, which reduced its hydrodynamic volume in solution. Thus, the observed variation in intrinsic viscosity reflects differences in polymer chain architecture and rigidity rather than direct molecular-weight determination.^22,23 These findings suggest that intrinsic viscosity is significantly influenced by polymer architecture and can provide indirect information regarding chain dimensions, conformational behavior, and relative molecular characteristics in solution.

Table 5.

Huggins and Kraemer coefficient for polyesters.

Polyester code	[η]	[η_sp/C] (dl/g)	[(ln η_rel)/C] (dL/g)	Huggins slope	Kraemer slope	Huggins constant K_H	Kraemer constant K_K
PEBPA	0.72	0.802	0.587	0.142	−0.1285	0.8064	0.8287
PEBPS	0.82	0.923	0.653	0.115	−0.0835	0.6908	0.7099
PE14AQ	0.68	0.781	0.576	0.109	−0.092	0.6676	0.6894
PE18AQ	0.70	0.721	0.542	0.0414	−0.0646	0.7239	0.7387
PEPh	0.62	0.756	0.563	0.0855	−0.0985	0.6171	0.6305
PER	0.56	0.664	0.609	0.983	−0.097	0.5517	0.5716
PEC	0.66	0.662	0.506	0.092	−0.042	0.6506	0.6716
PEHQ	0.58	0.591	0.540	0.1335	−0.0775	0.5713	0.5881
PEEG	0.49	0.603	0.470	0.1209	−0.1148	0.4718	0.5166
PEDEG	0.52	0.597	0.468	0.0975	−0.055	0.4997	0.5192

Concentration of all polyester solution is 1 g/dL.

The calculated Huggins and Kraemer constants fall within acceptable ranges, indicating favorable polymer–solvent interactions and good solvation in DMF. The negative values of the Kraemer constant further support efficient chain expansion in solution.

Structure–property relationships

A comparative evaluation of solubility, density, and viscosity highlights clear relationships between chemical structure and physicochemical properties of the synthesized polyesters. All density and viscosity measurements were performed in triplicate, and the reported values represent average measurements. Minor differences among polymers were interpreted conservatively to avoid overestimating structure–property relationships.

Polymers incorporating rigid aromatic units, such as bisphenol, anthraquinone, and hydroquinone derivatives, exhibit higher densities and intrinsic viscosities, reflecting strong intermolecular interactions and limited chain flexibility. These materials also show good solubility in polar solvents due to the presence of polar functional groups, despite their rigid backbones.

Polyesters based on simpler phenolic units display intermediate behaviour, balancing rigidity and processability. In contrast, those derived from aliphatic diols demonstrate lower density and intrinsic viscosity, indicating increased flexibility and reduced intermolecular cohesion.

The incorporation of the s-triazine ring contributes significantly to the overall behaviour by enhancing polarity and facilitating intermolecular interactions, while also maintaining structural integrity. Functional groups such as sulfone further modulate packing efficiency and molecular weight characteristics.

FTIR and ¹H NMR spectral characterization

The successful formation of the synthesized homopolyesters was confirmed through FTIR and ¹H NMR spectroscopy, which provided clear evidence for ester bond formation and incorporation of the designed structural fragments. To avoid repetition, only the principal diagnostic spectral features are discussed in the main text, while detailed peak assignments for all polymers are summarized in Table 6.

Table 6.

Characteristic FTIR and ¹H NMR spectral assignments of synthesized Homopolyesters.

Polymer	FTIR characteristic peaks (cm⁻¹)	¹H NMR (δ, ppm)
PEBPA	805 (s-triazine), 1178 (ester), 1365 (C–N), 1604 (piperazine), 3428 (N–H)	1.5 (CH₃), 2.6–3.1 (piperazine CH₂), 3.6 (N–CH₂), 6.8–8.1 (Ar–H), 8.9 (NH)
PEBPS	812 (s-triazine), 1181 (ester), 1286/1111 (SO₂), 1601, 3433	2.6–3.1, 3.6, 6.9–8.4, 8.9
PE14AQ	802, 1173, 1704 (quinone C = O), 1607, 3435	2.5–3.2, 3.5, 7.2–8.3, 9.0
PE18AQ	810, 1179, 1684 (quinone C = O), 1611, 3430	2.4–3.5, 3.7, 7.2–8.1, 9.1
PEPh	798, 1170, 1745 (lactone C = O), 1598, 3436	2.7–3.2, 3.4, 6.8–8.1, 8.8
PER	805, 1175, 1368, 1608, 3430	2.7–3.2, 3.6, 6.7–8.0, 8.9
PEC	807, 1172, 1365, 1607, 3430	2.6–3.1, 3.7, 7.0–8.0, 9.0
PEHQ	811, 1179, 1363, 1603, 3434	2.6–3.1, 3.6, 7.1–8.2, 8.9
PEEG	798, 1030 (C–O), 1183, 1607, 3446	2.7–3.4, 3.5, 3.8–4.4, 7.2–8.0, 9.2
PEDEG	802, 1083 (C–O–C), 1185, 1605, 3443	2.6–3.3, 3.5, 3.5–4.3, 7.2–8.0, 9.0

FTIR analysis

The FTIR spectra of all synthesized polyesters confirmed successful polycondensation through the disappearance of the characteristic acid chloride stretching vibration of the precursor monomer (–COCl, typically observed near 1780–1810 cm⁻¹) and the emergence of absorption bands corresponding to ester functionalities. A strong absorption band in the range of 1169–1185 cm⁻¹, attributed to ester C (=O)–O stretching, confirmed the formation of polyester linkages between the acid chloride monomer and corresponding diols.

Characteristic absorption bands corresponding to the s-triazine ring were consistently observed in the 797–812 cm⁻¹ region due to out-of-plane ring deformation, whereas bands between 1355 and 1373 cm⁻¹ were assigned to C–N stretching vibrations of the triazine nucleus. Additional absorptions in the 1598–1611 cm⁻¹ range indicated the presence of the piperazine moiety, while broad bands appearing near 3427–3446 cm⁻¹ were attributed to N–H stretching of secondary amine groups, confirming retention of the amino linkage after polymerization.

Distinct structural features originating from individual diol segments were also observed. For instance, PEBPS displayed characteristic sulfone (–SO₂–) stretching bands at 1285.72 and 1110.83 cm⁻¹, confirming incorporation of the bisphenol-S moiety. Similarly, PE14AQ and PE18AQ exhibited additional anthraquinone carbonyl absorptions at 1703.94 and 1683.94 cm⁻¹, respectively. In PEPh, a pronounced band at 1745.36 cm⁻¹ confirmed the presence of the lactone carbonyl group associated with the phenolphthalein segment. The presence of ether C–O–C stretching vibrations in PEDEG (1082.83 cm⁻¹) and C–O stretching in PEEG (1030 cm⁻¹) further supported incorporation of aliphatic glycol units into the polymer backbone.

¹H NMR analysis

The molecular structures of the synthesized homopolyesters were further verified by ¹H NMR spectroscopy, recorded in DMSO-d₆ using tetramethylsilane (TMS) as the internal reference standard at a concentration of approximately 10–15 mg mL⁻¹.

All polymers exhibited characteristic resonances corresponding to the piperazine methylene protons (–CH₂–) within the δ 2.4–3.5 ppm region, confirming successful incorporation of the benzylpiperazine-substituted triazine unit. The benzyl methylene (>N–CH₂) proton consistently appeared as a singlet around δ 3.4–3.7 ppm. Aromatic protons associated with triazine-linked phenyl rings and aromatic diol units resonated within the δ 6.7–8.4 ppm range depending on the polymer structure. A distinct singlet at δ 8.8–9.2 ppm, assigned to the secondary amine (–NH–) proton, further substantiated the proposed polymer framework.

Additional resonances specific to the diol structures were also evident. For example, PEBPA showed a singlet at δ 1.5 ppm, corresponding to the methyl groups of the bisphenol-A unit. In contrast, PEEG and PEDEG displayed additional multiplets in the δ 3.5–4.4 ppm region, attributable to methylene protons of ethylene glycol and diethylene glycol segments, respectively. These observations collectively confirm successful incorporation of both aromatic and aliphatic diols into the polyester backbone.

Thermal behavior and degradation kinetics

Thermogravimetric analysis

The thermal stability of the synthesized PEBPA, PEBPS, PEHQ, and PEEG homopolyesters was investigated by thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10°C min⁻¹ from room temperature to 800°C. These polymers were deliberately selected to represent structurally distinct classes, including bulky aromatic (PEBPA), sulfone-containing aromatic (PEBPS), rigid aromatic (PEHQ), and flexible aliphatic (PEEG) diol segments. This selection enabled meaningful comparison of structure–thermal property relationships while maintaining experimental feasibility. Representative TGA thermograms for PEBPA, PEBPS, PEHQ, and PEEG are presented in Figure 3, while the derived thermal degradation parameters are summarized in Table 7.

Figure 3.

Thermograph of (I) PEBPA (II) PEBPS (III) PEHQ and (IV) PEEG.

Table 7.

Thermal degradation parameters.

Polymer	T_max/DTG peak (°C)	Residue at 800°C (%)
PEBPA	472	5.0
PEBPS	477	5.0
PEHQ	601	6.8
PEEG	603	1.0

All investigated polyesters exhibit a single major degradation step, indicating relatively uniform thermal decomposition behavior. Polyesters derived from aromatic diols PEBPA, PEBPS, and PEHQ show significantly higher thermal resistance and char yield compared to the aliphatic polyester PEEG (Table 7). This enhanced stability can be attributed to the presence of rigid aromatic segments and the s-triazine ring, which promote resonance stabilization and hinder chain mobility during thermal degradation. In contrast, the lower residue observed for PEEG indicates extensive chain scission and volatilization, characteristic of flexible aliphatic systems.

The higher char formation in aromatic polyesters suggests a condensed-phase degradation mechanism, whereas aliphatic polymers predominantly undergo volatilization, leading to minimal residue. These observed char formations confirm that the incorporation of aromatic moieties effectively improves thermal stability and suggest potential relevance for high-temperature applications.

Determination of activation energy

Activation energies were estimated using linearized forms of the Coats–Redfern, Horowitz–Metzger, Broido, and Chan et al. methods are summarized in Table 8. Linear regression analysis was carried out over the principal degradation region, and the activation energy was calculated from the slope of the corresponding linear plot. Correlation coefficients (R²) were employed only as indicators of fitting quality and not as sole evidence of mechanistic validity. Since kinetic parameters were obtained from single-heating-rate TGA experiments, the calculated activation energies should be regarded as approximate descriptors of thermal degradation behavior rather than absolute mechanistic constants. Therefore, conclusions regarding degradation mechanisms have been interpreted cautiously.

Table 8.

Activation energy (E_a) calculated by Coats Redfern, Horowitz & Metzger, Broido, and Chan et al. model.

Model	Results	PEBPA	PEBPS	PEHQ	PEEG
Coats Redfern	R²	0.9967	0.9979	0.9827	0.9711
	Slope	3651.1	2858.5	2825.5	4500.2
	E_a (kCal/mol)	7.24	5.67	5.60	8.93
Horowitz & Metzger	R²	0.9923	0.9953	0.9632	0.8563
	Slope	−0.0077	−0.0053	−0.0052	−0.0065
	T_max ⁰C	472	477	601	603
	E_a (kCal/mol)	3.41	2.39	3.72	4.69
Broido	R²	0.9934	0.995	0.9559	0.9462
	Slope	2509.4	1716.8	1683.7	3358.5
	E_a (kCal/mol)	4.98	3.40	3.34	6.66
Chan et al.	R²	0.9886	0.9942	0.968	0.8852
	K	2497.3	1369.5	3140.2	2152.9
	E_a (kCal/mol)	4.96	2.72	6.23	4.27

The thermal conversion parameter (α) was determined from thermogravimetric data using the relation

α = \frac{w_{0} - w_{t}}{w_{0} - w_{f}}

where W₀, W_t, and W_f correspond to the initial, intermediate, and final masses, respectively. Kinetic fitting was performed within the principal degradation interval associated with the main decomposition stage observed in the TGA/DTG profile.

Coats- Redfern method

The Coats-Redfern technique²⁴ is another method for assessing the activation energy (E_a). It uses the equation:

\log [\frac{- \ln (1 - α)}{T^{2}}] = \log \frac{A R}{α E} [1 - \frac{2 R T}{E}] - \frac{E_{a}}{2.303 R T}

A straight-line graph is obtained by plotting $\log [\frac{{(1 - α)}^{- 1}}{T^{2}}] v s \frac{1}{T}$ against $\frac{1}{T}$ . The slope of this plot provides the value of E_a for each homopolyester analyzed.

Horowitz and Metzger method

In this method²⁵ estimates E_a through the following formula:

\ln [- \ln (1 - α)] = \frac{E_{a} θ}{R {T_{\max}}^{2}}

Here, $θ = T - T_{\max}$ , Where $T_{\max}$ is the temperature corresponding to the peak decomposition rate and T is the temperature at a given degree of weight loss. Plotting $\ln [- \ln (1 - α)]$ against $θ$ yields a linear relationship, which is then used to determine E_a.

Broido method

The Broido method²⁶ is utilized to compute the E_a of thermal degradation using the following expression:

\ln [- \ln (1 - α)] = - \frac{E_{a}}{R T} + K

In this equation, T denotes the absolute temperature, α is the fraction of material that has undergone decomposition, E_a is the activation energy, and R is the gas constant. A plot of $\ln [- \ln (1 - α)]$ against $1 / T$ produces a straight line, from which the E_a can be deduced from the slope.

Chan et al. method

The model proposed by Chan and co-workers²⁷ assumes a first-order kinetic mechanism for thermal decomposition. The activation energy is derived from an Arrhenius-type equation:

S l o p e o f \ln \frac{k}{W} v s \frac{1}{T} = - \frac{E_{a}}{R}

In this relation, k represents the decomposition rate, and W is the remaining mass percentage. The slope of the linear plot allows for direct calculation of the activation energy.

The high R² values (>0.95 in most cases) indicate good linear fitting and validate the applicability of these models for the present systems. However, variation in Ea values among different methods is evident, which is expected due to differences in mathematical approximations and kinetic assumptions inherent to each model.

According to the Coats–Redfern method, the activation energy follows the order:

PEEG (8.93 kcal {mol}^{- 1}) > PEBPA (7.24 kcal {mol}^{- 1}) > PEBPS (5.67 kcal {mol}^{- 1}) \approx PEHQ (5.60 kcal {mol}^{- 1})

A similar trend is broadly observed with the Broido method, where PEEG again exhibits the highest activation energy. The relatively higher Ea for PEEG, despite its lower thermal stability, suggests that initial bond cleavage in aliphatic chains requires higher energy, but once initiated, degradation proceeds rapidly through chain scission mechanisms.

In contrast, aromatic polyesters show comparatively lower Ea values but higher thermal stability, indicating that their degradation involves progressive and controlled bond cleavage processes stabilized by aromatic resonance. This apparent discrepancy highlights that activation energy alone does not fully describe thermal stability and must be interpreted in conjunction with degradation pathways and char formation.

Horowitz–Metzger and DTG-based interpretation

The Horowitz–Metzger method, which is based on the temperature corresponding to the maximum degradation rate (T_max), provides additional insight into degradation kinetics. The T_max values for the studied polyesters range from 472°C to 603°C, with PEHQ and PEEG exhibiting higher peak temperatures.

The relatively higher T_max values for aromatic systems indicate delayed degradation onset and greater resistance to thermal decomposition. The lower Ea values obtained from this method are typical, as it relies on peak-based approximations rather than the entire degradation profile. The correlation between T_max and thermal stability suggests that aromatic polyesters undergo degradation at higher temperatures with more gradual mass loss, whereas aliphatic polyesters decompose more abruptly.

Comparative evaluation of kinetic models

The Broido and Chan methods yield intermediate Ea values and further support the observed degradation trends. The consistency in relative ordering across different models reinforces the reliability of the kinetic analysis. The differences in absolute Ea values among the methods can be attributed to variation in assumed reaction order, differences in linearization procedures and sensitivity to specific regions of the TGA curve. Thus, the use of multiple kinetic models provides a more comprehensive understanding of the degradation behaviour rather than relying on a single method.

Structure–kinetics relationship

The polyesters obtained from aromatic diols PEBPA, PEBPS, and PEHQ exhibit higher thermal stability, higher char yield, and relatively consistent degradation behaviour. The presence of aromatic rings and s-triazine units enhances intermolecular interactions and stabilizes the polymer backbone. In contrast to the polyester obtained from aliphatic diol polyester PEEG shows lower thermal stability and minimal char formation but higher apparent activation energy, indicating a degradation process dominated by chain scission and volatilization. The presence of functional groups such as –SO₂– in PEBPS further influences degradation behaviour by introducing polar interactions and localized structural rigidity.

The combined thermogravimetric and kinetic analyses indicate that the degradation of the synthesized polyesters is governed by both structural rigidity and chemical composition. While model-fitting methods provide useful average activation energy values, the overall thermal stability is more strongly influenced by the ability of the polymer to form stable intermediate structures and char during decomposition.

The results demonstrate that incorporation of aromatic diols and s-triazine units significantly enhances thermal resistance, while aliphatic segments promote rapid degradation through chain scission mechanisms.

Conclusion

A series of s-triazine-based homopolyesters incorporating different aliphatic and aromatic diols was successfully synthesized and systematically evaluated to understand their structure–property relationships. The results clearly demonstrate that the nature of the diol component plays a decisive role in governing the physicochemical and thermal characteristics of the resulting polymers.

Solubility behaviour revealed that the presence of polar functional groups within the polymer backbone promotes dissolution in polar aprotic solvents, while the overall rigidity and intermolecular interactions restrict solubility in nonpolar and chlorinated media. Density measurements indicated that polymers containing rigid and highly polar groups exhibit more efficient chain packing, leading to higher density values. In parallel, intrinsic viscosity data confirmed that aromatic-based polyesters possess higher molecular weights and chain rigidity compared to those derived from flexible aliphatic diols.

Thermal analysis showed that polyesters containing aromatic units display enhanced resistance to thermal degradation, as reflected by higher residual char and improved stability at elevated temperatures. In contrast, aliphatic polyesters undergo more rapid decomposition with minimal residue formation. The kinetic parameters obtained from different model-fitting methods provided consistent trends, although variations in activation energy values were observed due to differences in underlying assumptions. These findings emphasize that thermal stability is influenced not only by activation energy but also by degradation pathways and structural features.

In summary, the incorporation of s-triazine units combined with appropriate diol selection offers an effective strategy for tailoring polymer properties. The synthesized materials exhibit a balanced combination of processability, thermal stability, and structural integrity, making them suitable for potential applications in high-performance coatings, films, and advanced functional materials. The relatively enhanced thermal stability of PEBPA and PEBPS suggests their potential applicability in moderate-to-high temperature polymer systems; however, additional thermal characterization such as DSC and long-term thermal aging studies would further establish their application scope.

Supplemental material

Supplemental material - Synthesis, structural characterization, and thermal degradation kinetics of s-triazine-based homopolyesters

Supplemental Material for Synthesis, structural characterization, and thermal degradation kinetics of s-triazine-based homopolyesters by R. B. Tailor, P. S. Patel in High Performance Polymers.

Footnotes

Acknowledgements

Dr Rahul B. Tailor acknowledges the UGC Cell, Veer Narmad South Gujarat University, Surat–395007, Gujarat, India, for financial support through Minor Research Project F. No.: UGC/27254/2023.

ORCID iD

R. B. Tailor

Consent to participate

No test, measurements or experiments were performed on humans as a part of this work.

Consent for publication

The authors have agreed to submit it in its current form for consideration for publication.

Author contributions

R. B. Tailor had conducted the experimental work and prepared the manuscript, while P. S. Patel served as the supervisor.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work has been funded by UGC Cell, Veer Narmad South Gujarat University, Surat-395007, Gujarat, India, under the Minor Research Project Grant F. No. UGC/27254/2023.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Data supporting the findings of this study are available from the corresponding author via email upon reasonable request.*

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Synthesis,structural characterization,and thermal degradation kinetics of s -triazine-based homopolyesters

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of 2-(4-benzylpiperazin-1-yl)-4,6-dichloro-1,3,5-triazine [I]

Preparation of 4,6-bis(N-(4-carboxyphenyl)amino)-2-(N-benzyl-piperazin-1-yl)-1,3,5-triazine

Preparation of 4,6-bis(N-(4-(benzoylchloride)amino))-2-(N-benzyl-piperazin-1-yl)-1,3,5-triazine [II]

Preparation of polyesters [III] (reaction scheme)

Results and discussion

Solubility behaviour

Density characteristics

Viscosity behaviour and molecular characteristics

Structure–property relationships

FTIR and 1H NMR spectral characterization

FTIR analysis

1H NMR analysis

Thermal behavior and degradation kinetics

Thermogravimetric analysis

Determination of activation energy

Coats- Redfern method

Horowitz and Metzger method

Broido method

Chan et al. method

Horowitz–Metzger and DTG-based interpretation

Comparative evaluation of kinetic models

Structure–kinetics relationship

Conclusion

Supplemental material

Supplemental material - Synthesis, structural characterization, and thermal degradation kinetics of s-triazine-based homopolyesters

Footnotes

Acknowledgements

ORCID iD

Consent to participate

Consent for publication

Author contributions

Funding

Declaration of conflicting interests

Data Availability Statement

Supplemental material

References

Supplementary Material

FTIR and ¹H NMR spectral characterization

¹H NMR analysis