Comparative Catalytic Efficiency and Polycondensation Kinetics of Succinic Acid and 1,3-Propanediol: A Kinetic Study of p-Toluene sulfonic Acid (p-TSA) versus AlCl3

In recent decades, challenges related to waste management and environmental contamination from bio-resistant synthetic plastics have intensified. Many countries now prioritize the reuse and recycling of plastic waste. The development of biodegradable polymers offers an alternative solution to these issues. Currently, biodegradable polymers are used across various industries, including packaging, agriculture, sanitation, and biomedical applications. Among these materials, the aliphatic polyester family is considered particularly promising due to its susceptibility to biological degradation [1]. Consequently, additional research has focused on aliphatic polyesters synthesized through the polycondensation of dicarboxylic acids and diols.

In 1931, Carothers [2] synthesized and classified several aliphatic polyesters derived from diacids and diols, noting the challenges of producing high-molecular-weight polyesters at the time. From the 1930s to the 1990s, research on aliphatic polyesters was limited due to their inferior mechanical properties and higher costs compared to conventional plastics, rendering them unsuitable for engineering applications. Following the 1990s, increased environmental concerns stimulated renewed interest in aliphatic polyesters. Advances in chain extenders, high-vacuum technologies, and efficient transesterification catalysts have since enabled the synthesis of high-molecular-weight aliphatic polyesters with improved mechanical properties.

In 1990, Showa Highpolymer Co., Ltd. used polyfunctional isocyanate as a chain extender and organic compounds containing phosphorus and metals as reactive catalysts to commercialize specific aliphatic polyesters with molecular weights ranging from several tens of thousands to several hundred thousand [3]–[6]. These products are marketed under the BIONOLLE trademark. Among these, only poly(ethylene succinate) (PES) and poly(butylene succinate) (PBS) display melt temperatures above 100 °C (Tm approximately 108 °C and 114 °C, respectively) [7]. Consequently, many research groups have focused on PES [8]–[14] and PBS to improve their physical properties while maintaining biodegradability. One method of changing the characteristics of biodegradable PES and PBS was to create copolymers by adding various diacids or diols [15]–[29]. Additionally, biodegradability and its connection to polyesters' molecular weight and chemical structure were examined [28]–[31].

Previously, 1,3-propanediol's use was limited by low yield and high cost. Recent advances in production methods are expected to address these challenges [32]–[34]. Aliphatic copolyesters with odd-numbered carbon atoms in their backbone were seldom studied due to poor mechanical and thermal properties and a lack of crystallinity. However, Bikiaris' lab recently synthesized PES, PBS, and poly(trimethylene succinate) (PTS) with average molecular weights of 6800–6900 g/mol using polycondensation [12]–[14]. Investigating the morphology, odd-even effects, thermal properties, and crystallinity of these copolyesters is of significant interest.

Aliphatic polyesters represent the most extensively researched and produced category among biodegradable materials. Based on chemical structure, these polyesters are classified into two groups: those derived from hydroxy acids and those synthesized from alkanediols and aliphatic dicarboxylic acids. The former group is typically prepared by ring-opening polymerization, whereas the latter is typically synthesized by polycondensation. Three primary polycondensation approaches have been investigated: transesterification of aliphatic diols with dimethyl esters of dicarboxylic acids, esterification of aliphatic diols with dicarboxylic acids, and polycondensation of alkanediols with dichlorides of dicarboxylic acids in solution. Although the final method incurs higher costs during scale-up, it offers greater adaptability.

In this study, the polycondensation of succinic acid with 1,3-propanediol in equimolar ratios has been investigated in the absence and presence of p-toluene acid (p-TS) & AlCl₃ as catalysts. The kinetic studies of the uncatalyzed and catalyzed reactions were analyzed using integration and graphical methods at various temperatures. The degree of polymerization (DPn) in the uncatalyzed, (p-TS) catalyst, and AlCl₃ catalyst reaction was also investigated and compared. The activation energy has also been examined accordingly. Finally, find out the suitable catalyst and conditions in the polycondensation of succinic acid and 1,3-propanediol.

The following scheme 1 represents the condensation polymerization reaction between succinic acid and 1,3-propanediol.

¹H Nuclear Magnetic Resonance Spectroscopy: The materials' ¹H nuclear magnetic resonance (NMR) spectra were captured using a Bruker AVANCE-III 500 MHz spectrometer (Billerica, MA, USA). The materials were dissolved in room-temperature Deuterated Chloroform (CDCl3) solvent in 5 mm NMR tubes. Figure 1 shows the structural configuration of the polymer. The polymer structure was confirmed by ¹H NMR spectroscopy below.

¹H NMR spectrum (CDCl3, 500 MHz, 25 0C ): δ 5.12 (m, 1H); 4.17-4.14 (dd, J = 3.8, 11.5 Hz, 1H); 4.08-4.05 (dd, J = 6.3, 11.5 Hz, 1H); 2.61 (bm, 4H); 1.22 (d, J = 6.3, 3H)

δ5.12 (m, 1H): A multiplet integrating to one proton. The downfield shift suggests a proton attached to a carbon bearing an electronegative group, likely an oxygen (methine proton of an ester or alcohol). δ4.17-4.14 (dd, J = 3.8, 11.5 Hz, 1H) and δ 4.08-4.05 (dd, J = 6.3, 11.5 Hz, 1H): These two signals represent diastereotopic protons of a methylene group (-CH2-), typically adjacent to a chiral center. The large coupling constant (J = 11.5 Hz) is characteristic of geminal coupling (2JHH). The smaller couplings (3.8 Hz and 6.3 Hz) represent vicinal coupling (3JHH) to the proton at δ 5.12.

δ 2.61 (bm, 4H): A broad multiplet integrating to four protons. This pattern is often seen in ethylene groups (-CH2CH2-) within a ring or adjacent to carbonyl groups (e.g., in a succinate or similar moiety).

δ1.22 (d, J = 6.3 Hz, 3H): A doublet integrating to three protons, characteristic of a methyl group (-CH3) coupled to a single methine proton. The coupling constant (6.3 Hz) matches one of the vicinal couplings seen in the downfield signals, confirming its proximity to the proton at δ 5.12.

The data is highly consistent with a 1,2-propanediol derivative, such as a cyclic ester or a substituted glycerol fragment. Specifically, the spin system CH3-CH(O-)-CH2O- is clearly defined by the couplings between the signals at δ 1.22, 5.12, and the pair at 4.15/4.06.

Fourier Transform Infrared Spectroscopy: Using a PerkinElmer Spectrum One FT-IR spectrometer (ATR testing model) in the transmission mode in the range of 4000−650 cm⁻¹ with a resolution of 4 cm⁻¹, 16 scans were used to describe the chemical structure of the synthesized Poly(trimethylene succinate) (PTSu).

The absorption bands associated with both the succinic acid and trimethylene segments shift slightly depending on the polymer's phase (amorphous or crystalline). The primary functional groups and their corresponding approximate absorption ranges are listed in Table 1. The Absence of a broad –OH band near 3300 cm⁻¹ indicates successful condensation.

Degree of polymerization: Poly(propylene succinate), formed by condensation polymerization of succinic acid and 1,3-propanediol, typically achieves degrees of polymerization corresponding to the monomer starting from 1.757, depending on reaction conditions like temperature, catalyst, and time. The higher DPn of up to 9,277 has been reported for the final polyester in this study. Condensation polymerization occurs via melt polycondensation, often in bulk at 150–200°C, removing water to drive the equilibrium toward higher chains. The polycondensation of succinic acid with 1,3-propanediol in equimolar ratios has been investigated in bulk at 120-1600C in the absence and presence of p-toluene acid (p-TSA) & AlCl₃ as catalysts.

The kinetic studies were investigated. The rate constants for the uncatalyzed and catalyzed reactions were analyzed using integration and graphical methods at various temperatures. The activation energy has also been determined. From the investigation results, it was observed that the degree of polymerization (DPn) in the uncatalyzed reaction is the square root of that obtained in the p-TSA-catalyzed reaction and the DPn obtained in the AlCl₃-catalyzed reaction. But the degree of polymerization obtained in the AlCl₃-catalyzed reaction is almost similar to that of p-TSA-catalyzed polymerization, as shown in Figure 2. Catalysts like p-TSA (p-toluene sulfonic acid) and AlCl₃ significantly enhance the rate of step-growth polycondensation reactions, such as polyesterification, compared to uncatalyzed processes, as shown in Figure 3.

The p-TSA, a Brønsted acid, initiates polycondensation (e.g., polyesterification) by protonating the carbonyl oxygen of carboxylic acid groups, enhancing electrophilicity for nucleophilic attack by alcohol hydroxyls to form esters and water. The AlCl₃, a Lewis acid, primarily initiates cationic vinyl polymerizations by coordinating to monomer double bonds or impurities (e.g., water), generating carbocations that propagate chain growth. In shared contexts like polyesterification, both are catalysed via acid activation, but differ in the strength of their mechanism and coordination mode [37]. This acceleration allows the reaction to reach higher conversions (p closer to 1), directly increasing the degree of polymerization (DPn) via Carothers' equation:

The reaction rates for the non-catalysed, p-TSA-catalysed, and AlCl₃-catalyzed polycondensations between succinic acid and 1,3-propanediol at 160 0C were investigated. The graphical representations of these are shown in Figure 3. It has been seen that the rate of reaction of p-TSA-catalysed is much faster than that of the non-catalysed one. But the rate of reaction with AlCl₃-catalyzed polycondensation is almost similar to that of p-TSA catalyst. The AlCl₃ and p-TSA catalysed polycondensation through distinct mechanisms, the Lewis acid coordination versus Brønsted acid protonation, but achieve comparable rates by similarly activating carbonyl groups in monomers like diacids and diols [38]. The p-TSA protonates the carbonyl oxygen of the carboxylic acid, forming a highly electrophilic protonated species that facilitates nucleophilic attack by the alcohol hydroxyl group. Proton transfer to the departing water molecule regenerates neutral p-TSA, completing the cycle; this process repeats efficiently in high-temperature melts with vacuum-driven water removal. The AlCl₃ binds directly to the carbonyl oxygen via dative bonding, polarizing the C=O bond and creating a Lewis acid-activated complex akin to protonation in electrophilicity. Nucleophilic addition forms a tetrahedral intermediate, followed by elimination of water and dissociation of AlCl₃, which recycles by coordinating fresh monomer; Chloride ligands may modulate activity but do not hinder turnover [40]. The comparison of p-TSA and AlCl₃ catalysts in the cycle at different aspect are summarized in Table 2.

In uncatalyzed polycondensation, such as between a diacid and diol, the reaction requires simultaneous collision of three species: two monomers and a water molecule displaced in the reverse direction, or effectively ternary encounters in the rate-determining step. This leads to third-order kinetics as shown in Figure 4, typically expressed as rate = k [monomer]² [chain end] or similar, where chain growth depends on bifunctional interactions without facilitation [43].

The p-TSA (p-toluenesulfonic acid) and AlCl₃ act as strong acids or Lewis acids, protonating or coordinating the carbonyl oxygen of succinic acid to form a highly electrophilic intermediate. This activated species then undergoes nucleophilic attack by the alcohol from 1,3-propanediol in a bimolecular step, reducing the order to second (rate = k [activated acid] [alcohol]) as shown in Figure 5 and Figure 6. Catalysts lower the activation energy and decouple the process from ternary collisions [44]. Uncatalyzed relies on direct monomer-monomer interactions needing three-body collisions; catalysis creates a steady-state activated monomer, making it binary.

Activation energies for the condensation polymerization of succinic acid and 1,3-propanediol vary by catalyst and are typically determined via integral methods analyzing reaction rates at different temperatures. No direct Arrhenius plots exist in literature for the exact condensation polymerization of succinic acid and 1,3-propanediol (PPSu) using the integration method across non-catalyzed, p-TSA, and AlCl₃ cases. The activation energies were calculated from lnk vs. 1/T plots as shown in Figure 7. Activation energy Ea follows the Arrhenius equation:

where the slope of the linear plot lnk vs. 1/T (in K-1) equals −Ea / R (R = 8.314 J/mol·K). The integration method converts time-conversion data to rate constants k at isothermal conditions. Activation energies for the polycondensation of succinic acid and 1,3-propanediol decrease with catalysts like p-TSA or AlCl₃ compared to the uncatalyzed reaction, as catalysts lower the energy barrier for esterification and chain growth steps. Activation energies for the polycondensation of succinic acid and 1,3-propanediol vary by catalyst, with catalysis generally lowering the barrier compared to uncatalyzed reactions. Specific values for p-TSA and AlCl₃ are scarce in the literature, but data for the uncatalyzed first step (esterification) and similar systems provide context for comparison. Lower activation energies indicate faster reaction rates under catalysis.

The graphical method for determining activation energy in condensation polymerization, such as between succinic acid and 1,3-propanediol, typically involves plotting the natural logarithm of the rate constant (ln k) versus the inverse of absolute temperature (1/T), known as the Arrhenius plot. The activation energy (Ea) is calculated from the slope of the linear fit, where slope = -Ea/R, and R is the gas constant (8.314 J/mol·K). This method applies to non-catalyzed, p-TSA-catalyzed (p-toluenesulfonic acid, a Brønsted acid catalyst), and AlCl₃-catalyzed (a Lewis acid catalyst) conditions. In these reactions, catalysts typically lower the activation energy (Ea) by stabilizing the transition state during esterification and polycondensation, as illustrated in Figure 8.

In condensation polymerization, rate constants are derived from experimental data on water evolution, monomer conversion, or molecular weight growth at varying temperatures. For non-catalyzed reactions, Ea is higher due to reliance on uncatalyzed proton transfers; p-TSA reduces Ea by providing protons for nucleophilic attack on carbonyls; AlCl₃ further lowers it via coordination to oxygen, enhancing electrophilicity.

Effects of Non-catalyst on Ea reflect high energy barriers for diacid-diol esterification. The effects of p-TSA catalyst on Ea drops as sulfonic acid accelerates carboxyl activation. The effects of AlCl3 catalyst- Lowest Ea from strong Lewis acid coordination, reducing nucleophilic attack barriers. AlCl₃ and p-TSA pathways in melt polycondensation exhibit comparable activation energies due to equivalent carbonyl activation strengths, despite mechanistic differences, leading to similar observed reaction rates [41]. The studies show that Lewis acid (AlCl₃) coordination to the carbonyl oxygen yields activation barriers closely matching those of Brønsted acid (p-TSA) protonation, as both polarize the C=O bond similarly (e.g., for esterification steps). Experimental kinetics in polyester melt reactions confirm this parity, with AlCl₃ pathways sometimes slightly lower due to tighter coordination without proton transfer entropy losses [38].

The p-TSA protonation involves higher entropic penalties due to solvent-like proton mobility in melts, which are partially offset by rapid turnover. In contrast, the AlCl₃ rigid adduct lowers enthalpy barriers but risks compromising adduct stability. Overall, effective Ea aligns (e.g., reported for both in polycondensation), explaining the rate similarity with the uncatalyzed process.

Table 3 summarizes the implications for the rates of uncatalyzed, p-TSA-catalyzed, and AlCl₃-catalyzed reactions, as shown in Figure 7 and Figure 8.

The mechanism involves the protonation of the carboxylic acid carbonyl oxygen (fast, low Ea ), followed by nucleophilic addition by the alcohol (slow, Ea controlling the overall rate), and deprotonation/elimination of water (fast). This addition step governs kinetics due to steric and electronic barriers in the melt phase [39].

The analyses confirm that the transition state for C-O bond formation post-protonation has the highest barrier, as protonation alone lowers but does not eliminate repulsion between approaching groups. Vacuum removal of water accelerates subsequent steps, isolating this addition as rate-limiting, consistent with observed orders in polyesterification [38].

In this article, we compared the non-catalyst, p-TSA catalyst, and AlCl₃ catalyst performance, and kinetic studies were also compared in the polycondensation reaction between succinic acid and 1,3-propanediol to synthesize aliphatic biodegradable polyester. The NMR and FTIR analyses were used to confirm that the polymerization was successful. The degree of polymerization, rate of reaction, reaction order, esterification rate, and activation energy were investigated during polycondensation using three different catalysts: non-catalyst, p-TSA, and AlCl₃. Polycondensation generally proceeds via step-growth mechanisms with the removal of water to form ester linkages. Catalysts such as Lewis acids (e.g., AlCl₃) and Brønsted acids (e.g., p-toluene sulfonic acid) can significantly influence reaction kinetics, degree of polymerization, and lower activation energy. AlCl₃ functions by coordinating to carbonyl oxygens, enhancing electrophilicity, whereas p-TSA protonates carboxyls, accelerating ester formation. The investigated results reveal that the p-TSA catalyst and AlCl₃ have almost similar activities on the polycondensation reaction, but it is better than that of the non-catalyst one. These catalysts can be selected based on desired polymer properties for specific applications. The Poly(trimethylene succinate) (PTSu) is a biodegradable aliphatic polyester suitable for compostable films, controlled-release matrices, and potentially biomedical applications.

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