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. 2025 May 2;10(18):18802–18811. doi: 10.1021/acsomega.5c00500

Improving the Thermal and Mechanical Properties of Polycarbonate via the Copolymerization of Tetramethylbisphenol A with Bisphenol A

Baohe Wang †, Lu Liu †, Jing Zhu †,*, Zhongfeng Geng †,‡, Jing Ma ‡, Bohan Liu †,‡
PMCID: PMC12079278  PMID: 40385206

Abstract

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Polycarbonate is a widely used engineering plastic. However, synthesis procedures using phosgene produce toxic gases that cause environmental pollution. It is important to improve the performance of polycarbonate by using a green and safe process. Here, we synthesized tetramethylbisphenol A and bisphenol A copolymers by a green, nonphotogas melt-transesterification process using bisphenol A, diphenyl carbonate, and tetramethylbisphenol A as reaction materials. The catalysts required for the synthesis were screened. The chemical structures of the polymerization products were confirmed by an infrared spectrometer and a nuclear magnetic resonance spectrometer. The thermal and mechanical properties of polycarbonate materials were measured through differential scanning calorimetry, thermogravimetry, and an electronic Universal Testing Machine. The results showed that tetramethylbisphenol A copolycarbonate was successfully synthesized by a melt-transesterification process. Moreover, the addition of tetramethylbisphenol A significantly improved the thermal and mechanical properties of polycarbonate. The effects of catalyst dosage, diphenyl carbonate/diphenol molar ratio, polycondensation reaction temperature, and time on the molecular weight of tetramethylbisphenol A and bisphenol A copolymer were investigated, and the optimal conditions were obtained.

1. Introduction

Bisphenol A polycarbonate (APC), which has high transparency, good heat resistance, and excellent mechanical properties, has become one of the engineering plastics with high market demand and paves the way for the development of new sustainable polymerized materials that could replace the traditional petroleum products that dominate today’s society.1 APC has a wide range of applications in home appliances, electrical and electronic devices, medical equipment, optical devices, etc.2,3

Considering the application of APC in special fields such as the military industry, it is of great significance to conduct research on the modification of polycarbonate to improve its high-temperature resistance, mechanical properties, etc., to adapt to different application scenarios.4 The modification of APC mainly includes physical modification and chemical modification. Physical modification is the physical mixing of polycarbonate with other materials to obtain new materials, such as glass fiber,5 silica,6 polybutylene terephthalate,7 graphene,8 polystyrene (PS),9 polyketone,10 and so on. Chemical modification is a method of changing the physical and chemical properties of polymers through chemical reactions with other monomers. Monomers containing cyclic, aliphatic rings or large polar groups are often chosen, such as bisphenol fluorene,11 isosorbide,12 4,4′-methylenebiscyclohexanol,13 9,9′-bis(4-hydroxyphenyl) fluoride,14 and so on. Han et al.15 synthesized polycarbonate (BPZ-PC) by the melt-transesterification process using bisphenol Z (BPZ) and diphenyl carbonate (DPC) as raw materials, and the BPZ-PC product showed a low dielectric constant, thermal stability, and good mechanical properties. Jang et al.16 used 3,3′-dibenzoyl-4,4′-dihydroxybiphenyl (DBHP) to synthesize the side-benzoyl polycarbonate copolymer (DBHP-PC) by interfacial polycondensation, and the DBHP-PC product exhibited thermal stability and scratch resistance. Yi Zheng et al.17 used tetrabromobisphenol A to synthesize brominate polycarbonate (B50PC) by interfacial polycondensation, and the B50PC product exhibited flame retardancy and transparency.

Polycarbonate materials are used in a wide range of applications in the electrical and electronic, automotive, and aerospace fields. Therefore, it is necessary to improve the thermal stability and mechanical properties of polycarbonate materials. Tetramethylbisphenol A (TMBPA) is a bisphenol analogue commonly used as a flame retardant and an alternative to bisphenol A (BPA).18,19 M.B. Moe et al.20 found that tetramethylbisphenol A polycarbonate (TMPC) and tetramethylbisphenol A polysulfone were significantly more permeable than PC and polysulfone and also had higher glass-transition temperatures (Tg) due to the presence of bulky methyl groups; TMPC can also be blended to improve polymer properties. For example, PS-tetramethylene bisphenol A polycarbonate blends have a higher Tg than PS.21−24 Also, the properties of copolycarbonates synthesized with TMBPA as the third monomer were improved. In the study of Lei Li et al.,25 the copolymer BaTb-PF prepared from BPA, TMBPA, dichloromethane (DCM), and 1-methylpyrrole showed improved Tg compared to the copolymer BPA-PF. Maurizio Penco et al.26 synthesized copolycarbonates containing different BPA/TMBPA molar ratios (P-TBPA/BPA) by phosgene synthesis. It was found that the Tg of P-TBPA/BPA increased on increasing the TMBPA content. In related articles above, P-TBPA/BPA was synthesized by the solution phosgene method or the interfacial phosgene method.26−30 Synthesis processes using phosgene produce toxic gases and cause environmental pollution. It makes sense to obtain a green and safe process to synthesize P-TBPA/BPA with better performance.

In this paper, TMBPA, BPA, and DPC were used as monomers to synthesize P-TBPA/BPA through a melt-transesterification process, and the catalysts required for the synthesis were screened. The molecular structure of the polymer was confirmed by FT-IR and 1H NMR. The Mη of the copolycarbonates synthesized was measured by using a Wool viscometer. The mechanical and thermal properties of copolycarbonates prepared were measured, and the results showed that the addition of TMBPA could improve not only the thermal properties of the copolycarbonate but also the mechanical properties significantly. Comparisons revealed that P-TBPA/BPA with a viscosity-average molecular weight of around 26,000 performed better. The effects of catalyst dosage, DPC/diphenol molar ratios, polycondensation reaction temperature, and time on the molecular weight of P-TBPA/BPA were studied.

2. Materials and Methods

2.1. Materials

2,2-Bis(4-hydroxyphenyl) propane (BPA, 99.9% purity), DPC (99.6% purity), 2,2-bis(4-hydroxy-3,5-dimethylphenyl) propane (TMBPA, 99.9% purity), potassium hydroxide (KOH, 99% purity, Hines), cesium carbonate (Cs2CO3, 99.99% purity), tetramethylammonium hydroxide (TMAH, 25 wt %), and methylene chloride (DCM, analytically pure) were used.

2.2. General Procedure for the Synthesis of P-TBPA/BPA

BPA (30.0268 g, 0.1314 mol), TMBPA (37.4062 g, 0.1314 mol), and DPC (59.3622 g, 0.2760 mol) were added to a 250 mL three-neck flask. A certain amount of catalyst was added, and the system was replaced with nitrogen more than 3 times. The material was then melted upon heating, and stirring was initiated. The temperature was raised to 140 °C, and the reaction was carried out at normal pressure for 30 min. The pressure was adjusted from normal pressure to 100 mmHg (absolute pressure, the same below), and the temperature was gradually increased to 230 °C. The pressure gradually decreased to 1 mmHg, and the temperature gradually increased to the polycondensation reaction temperature. The pressure was maintained below 1 mmHg, and the polycondensation reaction was carried out for a period of time. After the end of the reaction, it was cooled to room temperature, and samples were taken for analysis. The reaction equation is shown in Scheme 1.

Scheme 1. Preparation of P-TBPA/BPA via Melt Transesterification of BPA, TMBPA, and DPC.

Scheme 1

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2.3. Preparation of the P-TBPA/BPA Films

Polycarbonate (P-TBPA/BPA, 2 g) was dissolved in DCM (20 g) under magnetic stirring at 25 °C. The completely dissolved polycarbonate was cast onto a clean, smooth glass plate, and then a film of the same thickness was prepared using a film scraper. The thickness of the films was about 20–40 μm. The prepared polycarbonate film was dried in an oven at 80 °C to completely remove the solvent. Finally, the oven was gradually cooled to room temperature to obtain a polycarbonate film.

2.4. Characterization

2.4.1. Viscosity-Average Molecular Weight of the Polycarbonate

Mη of the polycarbonate is measured using a Ubbelohde-type viscometer (ball volume C 2.0 ± 0.1 mL, capillary inner diameter d 0.037 ± 0.001 cm, 20.00 ± 0.10 °C), and DCM was used as a blank solvent.

The Mη of polycarbonate can be calculated by eqs 1–3:

2.4.1. 1
2.4.1. 2
2.4.1. 3

where ηsp is the specific viscosity, t is the average value of the three measurements of the sample, t0 is the average of the three measurements of the blank solvent, ηr is the relative viscosity, η is the intrinsic viscosity, and C is the sample concentration (g/mL).

The Mη of polycarbonate can be calculated by eq 4:

2.4.1. 4

where K is the proportionality constant, Mη is the viscosity-average molecular weight, and α is the expansion factor, which can be found in the polymer manual.

2.4.2. FT-IR

The FT-IR spectra of reaction materials and prepared polycarbonate films were measured with a VERTEX 80v full-vacuum high-end research-grade infrared spectrometer within the wavelength range of 4000–400 cm–1.

2.4.3. 1H NMR

1H NMR testing was performed on an AVANCE NEO 500 liquid NMR spectrometer at room temperature, and deuterated chloroform (CDCl3, deuterium content 99.8%) was used as the solvent.

2.4.4. DSC

The Tg values of polycarbonate films were analyzed using differential thermal scanning calorimetry (DSC) under a N2 atmosphere, and the detailed process is based on the following steps. First, the samples about 5–10 mg were heated from 25 to 200 °C at a heating rate of 10 °C/min, and the temperature was maintained at 200 °C for 3 min to eliminate thermal history; subsequently, the samples were quenched to 25 °C with a cooling rate of 10 °C/min. Finally, the cooled samples were reheated from 25 to 200 °C at the same heating rate as the first heating process.

2.4.5. TGA

The thermal stability of the prepared polycarbonate films was evaluated on a thermogravimetric analyzer (TGA), and the samples were heated from 30 to 800 °C at the rate of 10 °C/min under a flowing N2 atmosphere (50 mL/min).

2.4.6. GPC

The molecular weights of the prepared polycarbonate films were obtained by Waters 1515 gel permeation chromatograph testing at 30 °C, and the Waters 2414 refractive index detector and the Waters 1515 isocratic HPLC pump were used for the test. Tetrahydrofuran was used as the mobile phase, with a flow rate of 0.6 mL/min.

The mechanical properties of polycarbonate films, which were cut into rectangles (10 × 50 mm2, 0.02–0.04 mm thick), were measured on a CMT6103 electronic universal testing machine using Metersi Industrial Systems in China, and the tensile test was performed at room temperature at a tensile rate of 10 mm/min.

3. Results and Discussion

3.1. Chemical Structures

The infrared spectra of the reactants and products are shown in Figure 1. For the FT-IR of BPA, 3334 cm–1 is the tensile vibrational absorption peak of phenolic hydroxy–OH. For the FT-IR of TMBPA, 3402 cm–1 is the tensile vibrational absorption peak of phenolic hydroxyl–OH. 2915 cm–1 and 2854 cm–1 are the C–H extensional vibrational absorption peaks of −CH3 on the alkane backbone and −CH3 on the benzene ring, respectively. For the FT-IR of the DPC, 1775 cm–1 is the tensile vibration absorption peak of C=O. For the FT-IR of the synthesized P-TBPA/BPA, the synthesized P-TBPA/BPA had two C–H group absorption peaks around 2932 and 2854 cm–1 (saturated C–H tensile vibration absorption peaks of the −CH3 group) and 1768 cm–1 for the C=O tensile vibration absorption peak.

Figure 1.

Figure 1

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FT-IR spectra of BPA, TMBPA, DPC, TMPC, P-TBPA/BPA 50/50 (TMBPA/BPA feed ratio(mol/mol) is 50/50), and P-TBPA/BPA 100/0 (TMBPA/BPA feed ratio(mol/mol) is 100/0).

Compared with the FT-IR of BPA, TMBPA, and DPC, in the FT-IR of P-TBPA/BPA, the −OH tensile vibration peak of BPA at 3334 cm–1 and the −OH tensile vibration peak of TMBPA at 3402 cm–1 disappeared, and the C=O tensile vibration absorption peak appeared at 1768 cm–1, indicating the occurrence of the melt-transesterification reaction.

To further confirm the structure of synthesized P-TBPA/BPA 50/50, the 1H NMR measurement was carried out, and the obtained spectra are displayed in Figure 2. The peaks around 7.12–7.19 ppm are derived from the protons of the benzene ring (He, He′, He″, He‴). Peaks around 7.03–7.19 ppm are attributed to protons (Hb, Hb′, Hb″, Hb″, Hb‴, Hd″, Hd″, Hd″, Hd‴). Peaks around 2.10–2.34 ppm are attributed to protons (Ha, Ha′, Ha″, Ha‴ on the methyl group). The peak at 1.60–1.73 ppm is derived from protons (Hf, Hf′) on the methyl group. The peak at 1.41–1.59 ppm is derived from the protons (Hc, Hc′) on the methyl group. The −OH proton peak signal of TMBPA disappeared at 9.11 ppm, and the integrated area of the chemical shift is consistent with the number of protons in the structure of the target product.

Figure 2.

Figure 2

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1H NMR spectrum of synthesized P-TBPA/BPA 50/50.

3.2. Reaction Mechanism

Synthesis of polycarbonate by melt ester exchange can be divided into two stages, ester exchange and polycondensation, depending on the reaction conditions and the reaction process. The reaction formulas are shown in eqs 5 and 6) (R = H, CH3).

3.2. 5
3.2. 6

The DPC molecule contains a carbonyl center connected with two phenolic groups, and the carbon atom in the carbonyl group imparts a small amount of positive charge. BPA and TMBPA are both phenolic compounds that are weakly acidic and can dissociate into the BPA phenol anion (BPA–) and the TMBPA phenol anion (TMBPA–) and hydrogen ions.

Analogous to the melt-transesterification reaction mechanism of BPA and DPC, the reaction mechanism without a catalyst is shown in Figure 3. The prepolymer with a phenol group as the end group and a degree of polymerization of y (Py) dissociates into the phenol-negative ion as the end group and a chain segment with a degree of polymerization of y (Py–). Py–, as a nucleophilic reagent, attacks the central carbon atom of the prepolymer (Px) with phenyl carbonate as the end group and a degree of polymerization of x and undergoes a nucleophilic substitution reaction to obtain a polymer with a degree of polymerization of x + y (Px+y) and releases a small molecule of phenol (PhOH). However, the nucleophilic reaction is hindered by the benzene ring site-blocking effect of DPC, and the molecular weight of the polymer obtained is very small.

Figure 3.

Figure 3

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Melt-transesterification reaction mechanism of BPA, TMBPA, and DPC in the absence of catalysts (R = H, CH3).

A nucleophilic reagent (Nu) is usually used as a catalyst, and the reaction mechanism is shown in Figure 4. Nu attacks the carbonyl center carbon atom of Px to form the intermediate Px*. Px* releases the phenoxy group to form the intermediate Px+. Px+ reacts with Py– to form the polymer Px+y, and the molecular chains grow. During the catalytic transesterification reaction of BPA and DPC, the catalyst provides electrons to the carbonyl center to form a tetrahedral intermediate, which in turn reacts with the chain segments where the phenol group is the end group and promotes chain growth. It has been demonstrated that alkaline nucleophilic reagents as catalysts can not only directly carry out nucleophilic substitution reactions with carbonyl carbon atoms but also promote the ionization of bisphenols in the initial materials and thus promote the nucleophilic substitution reactions of phenol anions attacking the carbonyl carbon.31,32

Figure 4.

Figure 4

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Melt-transesterification reaction mechanism of BPA, TMBPA, and DPC in the presence of catalysts (R = H, CH3).

3.3. Catalyst Screening

In the process of melt-transesterification of BPA and DPC, hydroxides, carbonates, and complexes of alkali metals and alkaline earth metals are widely used as catalysts due to the high catalytic activity.33,34 In addition, organic strong alkali such as TMAH, tetraethylammonium hydroxide, and other quaternary ammonium salts also have good catalytic effects. Cs2CO3, KOH, and TMAH were selected as catalysts to catalyze the reaction. Table 1 summarizes the results of these transesterification catalysts. Molecular weight reflects the degree of polymerization of the polymer. The molecular weight of the polymer can be used as the basis for selecting a catalyst. In the absence of a catalyst, the melt transesterification and polycondensation of DPC and bisphenol are slow and the molecular weight of the polycarbonate is small. Under the action of a catalyst, the reaction speed is accelerated comparatively, the phenol distillation temperature decreases, and the Mη of P-TBPA/BPA increases.

Table 1. Catalytic Performance of Various Catalysts for the Melt-Transesterification Synthesis of P-TBPA/BPA.

catalyst TMBPA/BPA feed ratio (mol/mol) nCat:ndiphenol phenol distillation temperature (°C) Mη×104 PDI
no 50/50   220 0.4587  
Cs2CO3 50/50 3 × 10–6 192 2.1897 2.651
KOH 50/50 3 × 10–6 154 3.2266 4.200
TMAH 50/50 5 × 10–4 156 2.7804 3.868
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As a catalyst, the carbonate ion (CO32–) ionized by Cs2CO3 acts as a strong base that can deprotonate bisphenol to generate phenoxy ions, which, in turn, trigger the transesterification reaction. CO32– has a large spatial site resistance and is not strongly nucleophilic.35 The hydroxide radical (OH–) ionized by KOH acts as a strong base, deprotonates BPA to generate phenoxy ions, attacks the carbonyl carbon atom of DPC, and triggers the ester exchange reaction. With strong nucleophilicity, OH– can also directly attack the carbonyl carbon of dpc and promote chain growth. Compared to Cs2CO3, KOH catalyzes the transesterification of BPA and TMBPA with DPC more efficiently with higher molecular weight polymers. At the same time, due to the high alkalinity of KOH, the reaction rate is fast, more side reactions, such as depolymerization and ring opening, occur during the polycondensation stage, and the molecular weight distribution of the polymers is more dispersed. The alkalinity of TMAH is close to that of KOH, but due to its poor thermal stability, it will decompose in the high-temperature environment during the polycondensation stage, which has a certain attenuating effect on the side reactions. Therefore, the molecular weight distribution of TMAH-catalyzed polymers is more concentrated than that of KOH.

3.4. Thermal Properties of Synthesized P-TBPA/BPA

Molecular weight has a direct impact on the properties of the polymer.13 DSC and TGA tests and comparisons of P-TBPA/BPA with similar molecular weights but different compositions and P-TBPA/BPA with the same composition but different molecular weights were performed, and the results are shown in Figures 5, 6, and Table 2.

Figure 5.

Figure 5

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DSC results of APC (a) and P-TBPA/BPA synthesized (b).

Figure 6.

Figure 6

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TGA (a,b) and DTG (c,d) results of P-TBPA/BPA synthesized.

Table 2. Thermal Properties of P-TBPA/BPA Synthesized.

polycarbonate Mη×104 PDI Tg (°C) Td5% (°C) Tmax (°C)
APC 2.6000 3.010 143.30 467.72 514.83
P-TBPA/BPA 50/50-1 1.9829 4.138 142.93 437.83 506.83
P-TBPA/BPA 50/50-2 2.6583 3.756 150.20 441.17 489.33
P-TBPA/BPA 50/50-3 3.1070 3.568 155.70 441.50 490.50
P-TBPA/BPA 100/0 2.5810 3.861 154.26 436.50 490.17
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As shown in Figure 5a, the Tg of P-TBPA/BPA was higher than that of APC, which may be due to the following reasons. TMBPA has four methyl groups on the benzene ring, which increases the rigidity of the molecular chain and restricts the movement of the chain segments. In addition, tetramethyl bisphenol A has higher symmetry and regularity, and the molecular chains are more tightly arranged, which reduces the free volume between the molecular chains and makes the movement of the chain segments more difficult. Therefore, the Tg of P-TBPA/BPA was higher than APC.11 As shown in Figure 5b, the Tg of P-TBPA/BPA with the same composition increases with an increasing molecular weight. This is due to the higher molecular weight polymers having longer molecular chains and stronger entanglements and interactions between the molecular chains, which limit the movement of the chain segments. In addition, as the molecular weight increases, the proportion of chain ends decreases, and the influence of chain ends on the overall molecular chain motion is weakened. Therefore, within a certain range, the higher the molecular weight of a polymer, the higher is its Tg.36

Figure 6 shows the results of TGA and DTG. The molecular weights, Tg, 5% weight loss temperatures (Td50%), and maximum weight loss temperatures (Tmax) are listed in Table 2. As shown in Figure 6a,c and Table 2, the Td50% of P-TBPA/BPA is slightly lower than that of APC and the residual carbon content of P-TBPA/BPA is also lower. The reason may be that the carbon–hydrogen bond (C–H) of the methyl group is weaker than the carbon–carbon bond (C–C) or the carbon–oxygen bond (C–O) on the benzene ring and is more likely to break at high temperatures. Tetramethyl bisphenol A polycarbonates may initiate additional thermal decomposition pathways (e.g., oxidation or fracture of the methyl group) due to the presence of methyl groups, which have lower activation energies, resulting in thermal decomposition being more likely to occur. Moreover, the four methyl groups on the benzene ring of TMBPA increase the spatial site resistance of the molecular chain, leading to an increase in localized stress in the molecular chain. At elevated temperatures, this stress exacerbates the breakage of the molecular chain, making the polymer more susceptible to thermal decomposition.37

As shown in Figure 6b,d and Table 2, the Td5% and residual carbon content of P-TBPA/BPA 50/50 increased slightly with increasing molecular weight. This may be due to the fact that when the molecular weight is low, the proportion of polymer chain ends is high, the movement of chain ends is strong, the thermal stability is poor, and, therefore, the weight loss temperature is low. As the molecular weight increases, the proportion of chain ends decreases, and thermal decomposition depends mainly on the breakage of the main chain; the chemical bonds of the main chain (e.g., carbonate bonds) are usually more stable than those of the chain ends, leading to thermal stability increasing and the temperature of the weight loss rising. The difference in Td5% between P-TBPA/BPA 50/50-2 and P-TBPA/BPA 50/50-3 is not significant because, when the molecular weight is increased to a certain level, the influence of chain-end effect becomes negligible, and the interactions and entanglements between molecular chains reach equilibrium. At this time, the weightlessness temperature no longer changes significantly with the increase of molecular weight.38,39

3.5. Mechanical Properties of P-TBPA/BPA Films

The mechanical properties of synthesized P-TBPA/BPA with similar molecular weights but different compositions and the same compositions but different molecular weights were determined at room temperature according to GB 13022-1991. The results of the mechanical properties of the polymers are shown in Table 3.

Table 3. Mechanical Properties of Prepared P-TBPA/BPA Films.

polycarbonate Mη×104 PDI elastic modulus (MPa) elongation at break (%) tensile stress at break (MPa) tensile strength (MPa) maximum force (N)
APC 2.6000 3.010 436.75 5.72 15.18 34.69 13.87
P-TBPA/BPA 50/50-1 1.9829 4.138 661.43 6.62 19.98 30.81 12.32
P-TBPA/BPA 50/50-2 2.6583 3.756 1309 6.08 35.58 46.23 18.49
P-TBPA/BPA 50/50-3 3.1070 3.568 1606.62 3.81 36.37 44.93 17.97
P-TBPA/BPA 100/0 2.5810 3.861 1749.26 3.89 44.94 48.93 19.57
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As shown in Figure 7, the mechanical properties change significantly with the change in TMBPA content. The test results showed that the addition of TMBPA increased the elastic modulus, tensile stress at break, tensile strength, and maximum force of the polycarbonate films. The methyl substituents on the benzene ring of TMBPA increased the rigidity and spatial site resistance of the molecular chain of P-TMBPA/BPA, which made the material exhibit a higher tensile strength and elastic modulus when subjected to force.36 Besides, the higher the molecular weight of P-TBPA/BPA 50/50, the higher the modulus of elasticity and tensile stress at break and the smaller the elongation at break. This is due to the fact that higher molecular weight polymers have stronger entanglement and forces between the molecular chains and a smaller proportion of the ends of the molecular chains, which makes the material require greater external forces during stretching.20,40−43

Figure 7.

Figure 7

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Stress–strain curves of the prepared P-TBPA/BPA films.

3.6. Effect of Synthesis Conditions on the Molecular Weight of P/TBPA/BPA 50/50

The above comparison of the properties of the synthesized P-TBPA/BPA materials revealed that P-TBPA/BPA 50/50 has better thermal and mechanical properties, and the synthesis conditions of P-TBPA/BPA 50/50 were investigated.

Theoretically, 1 mol of BPA needs to react completely with 1 mol of DPC. However, in the actual reaction, DPC may be lost due to volatilization or decomposition, resulting in insufficient ester groups in the reaction system. Therefore, the DPC/diphenol feed ratio should be increased to compensate for the loss of DPC due to volatilization or decomposition during the reaction process to ensure that there are enough ester groups in the reaction system to react with the hydroxyl group of BPA. In addition, the initial molar ratio of the reactants affects the concentration of the bifacial groups of the prepolymer, which in turn affects the molecular weight of the polymer in the melt-esterification reaction.

As shown in Figure 8, the molecular weight of the synthesized P-TBPA/BPA 50/50 was maximum when the initial molar ratio of DPC/diphenol was 1.05. When the initial molar ratio of DPC/diphenol is less than 1.05, there are not enough ester groups in the reaction system to fully react with the hydroxyl group (−OH) of bisphenol, resulting in an incomplete reaction and limited chain growth. When the initial molar ratio of DPC/diphenol is too high, the content of DPC is higher than that of BPA and TMBPA, and the excess DPC may act as a chain terminator, reacting with the hydroxyl group at the end of the polymer chain and preventing further chain growth, resulting in a lower molecular weight.44,45

Figure 8.

Figure 8

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Effect of DPC/diphenol molar ratios on the melt-transesterification synthesis of P-TBPA/BPA 50/50 (nTMAH:ndiphenol = 1 × 10–3; polycondensation reaction stage: 280 °C, 30 min).

The catalyst dosage has an effect on the extent and rate of the reaction. The experimental results in Figure. 9 show that the highest molecular weight of the polymer was achieved at a catalyst dosage of nTMAH:ndiphenol = 1 × 10–3. When the dosage of TMAH was too small, there were insufficient catalytically active centers, which led to a significant decrease in the reaction rate, thus resulting in a higher temperature of the phenol distillate and a lower molecular weight of the polymer. When the amount of TMAH is too high, the molecular weight of the polymer decreases instead of increasing, which may be due to the fast reaction rate leading to localized overheating, triggering polymer degradation or side reactions.46

Figure 9.

Figure 9

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Effect of catalyst dosage on the melt-transesterification synthesis of P-TBPA/BPA 50/50 (initial material molar ratio: bisphenol: DPC = 1:1.05; polycondensation reaction stage: 280 °C, 30 min).

As shown in Figure 10a, the molecular weight of P-TBPA/BPA 50/50 increased significantly with increasing temperature of the polycondensation reaction. This is because increasing the temperature is favorable to reducing the viscosity of the reactants, enhancing the mobility of the reactant molecules, promoting chain growth, and increasing the molecular weight of the polymer. At the same time, the polycondensation reaction is a heat-absorbing reaction, and increasing the temperature is conducive to the positive shift of the reaction equilibrium.47 The thermal and mechanical properties of P-TBPA/BPA 50/50 with molecular weight around 26000 are superior, and the polymerization reaction temperature that is too high will lead to polymer degradation and an increase in side reactions; hence, 260 °C is selected as the optimal polycondensation reaction temperature.

Figure 10.

Figure 10

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Effect of polycondensation temperature and time on the melt-transesterification synthesis of P-TBPA/BPA 50/50 (initial material molar ratio: bisphenol: DPC = 1:1.05; nTMAH:ndiphenol = 1 × 10–3; polycondensation reaction stage (a): 30 min; polycondensation reaction stage (b): 260 °C).

As shown in Figure 10b, the extended polycondensation reaction time is conducive to increasing the molecular weight of P-TBPA/BPA 50/50. This is because the longer polycondensation reaction time allows the oligomer to fully react with the high-molecular-weight polymer. The insignificant increase in molecular weight of the polymer obtained after 60 min of polycondensation reaction may be due to the fact that the sustained high temperature may lead to the occurrence of side reactions such as cross-linking or thermal degradation, which is not conducive to the increase in molecular weight of the polymer.15 Therefore, 45 min of polycondensation reaction is considered the optimum time for the polycondensation reaction.

4. Conclusions

In this paper, P-TBPA/BPA was synthesized using BPA, DPC, and TMBPA as reaction materials by a green and environmentally friendly nonphotogas melting esterification process. FT-IR spectroscopy and 1H NMR characterization confirmed the successful synthesis of P-TBPA/BPA. The synthesis of P-TBPA/BPA was catalyzed by Cs2CO3, KOH, and TMAH, separately; the molecular weight and molecular weight distribution of the polymers were compared, and it was found that the catalytic effect of TMAH was better.

The mechanical and thermal properties of the synthesized P-TBPA/BPA were determined, and the results showed that the addition of TMBPA improved the thermal and mechanical properties of the polycarbonate. When the molecular weight of the polymers was similar, the higher the TMBPA content, the higher the Tg of P-TBPA/BPA. The Tg values of APC, P-TBPA/BPA 50/50, and P-TBPA/BPA 100/50 were 143.30, 150.20, and 154.26 °C, respectively. Also, the higher the molecular weight of P-TBPA/BPA 50/50, the higher the Tg of the polymer. Addition of TMBPA increases the modulus of elasticity and tensile strength of the polymer. Meanwhile, the elastic modulus and tensile strength of P-TBPA/BPA 50/50 increased with the increase in molecular weight. It was found that P-TBPA/BPA 50/50 with molecular weight around 26,000 had better performance. The optimum conditions for the synthesis of P-TBPA/BPA 50/50 were investigated as follows: catalyst TMAH of 0.10% mol (relative to bisphenol), molar ratio of DPC to bisphenol of 1.05:1, and polycondensation at 260 °C for 45 min.

Acknowledgments

The authors acknowledge the funding received from the R&D Center for Petrochemical Technology, Tianjin University.

The authors declare no competing financial interest.

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