Mechanistic investigations of antimony‐catalyzed polycondensation in the synthesis of poly(ethylene terephthalate)

The chemical aspects of poly(ethylene terephthalate) synthesis via the antimony‐catalyzed polycondensation of hydroxy ethylene terephthalate end groups were studied to elucidate its mechanism. A polycondensation mechanism was proposed in which activation occurs by the formation of a chelate ligand on antimony composed of the hydroxyl end group and alcoholic oxygen of the ester of the same end group. The rate‐determining step of the polycondensation reaction was concluded to be the coordination of a second chain end to antimony. The low activity of antimony at a high concentration of hydroxyl end groups was proposed to result from the competition between hydroxyl end groups and the chelate structure leading to the transition state. The high selectivity of antimony is probably due to its relatively low Lewis acidity. Moreover, antimony was found to stabilize hydroxyl end groups against degradation by preventing their complexation to carbonyl functionalities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1049‐1059, 2006     

Optimization of simultaneous thermal analysis for fast screening of polycondensation catalysts

Dynamic simultaneous thermal analysis was optimized to screen activity of different catalysts for polycondensation of bis-hydroxy ethylene terephthalate (BHET) to polyethylene terephthalate. Reactions were performed by heating BHET to 300 °C at a linear heating rate in 50 μl thermal analysis crucibles under inert gas purging. A sensitive and reproducible screening method was obtained after overcoming of critical problems such as monomer evaporation, catalytic activity of crucible material, and optimization of gas purging, monomer amount in the crucible and heating rate. Under the applied conditions mass transport limitations were absent and the reaction was controlled solely by chemistry. The temperature at which maximum reaction rate occurs was used as an index of catalytic activity. It was obtained from maximum differential scanning calorimetry signal together with the maximum derivative of thermogravimetry signal. Temperature at which the reaction starts was also applied as an activity index. It was obtained from the onset of mass loss. The value of these three indices was smaller for more active catalysts.The optimized method was applied to study the activity of a new polycondensation heterogeneous catalyst based on hydrotalcite. This new catalyst was shown to be much more active than the conventional antimony catalyst under the applied conditions.     

Synthesis,Characterization and Catalytic Behaviour of Silica Supported Zinc Salicylaldimine Complex

Zinc salicylaldimine complex immobilized on silica gel was used as a promising catalyst for the transesterification reaction of dimethyl terephthalate (DMT) and ethylene glycol (EG).The catalyst was characterized by Fourier transform infra‐red spectroscopy (FT‐IR), thermogravimetric analysis (TGA) and atomic absorption spectroscopy (AAS). The product bis‐(2‐hydroxyethyl)terephthalate (BHET)was confirmed by mass and ¹H‐NMR studies. In comparison to zinc acetate i.e., homogeneous catalyst, a polymer supported catalyst showed better stability, catalytic activity and ease of separation from the reaction product. The catalyst can be reutilized during successive catalytic cycles.     

反相高效液相色谱法分析超临界甲醇解聚聚对苯二甲酸乙二醇酯的产物

配合超临界甲醇解聚聚对苯二甲酸乙二醇酯的工艺开发,采用高效液相色谱法对聚对苯二甲酸二乙酯的超临界甲醇解聚固体产物进行了分离、定性和定量分析。采用反相色谱体系,色谱柱为Zorbax-C8柱,流动相为甲醇－水（70／30,V／V）,紫外检测器。该法具有高色谱分辨率、简便、准确、重复性好等特点。     

Glycolysis of poly(ethylene terephthalate) (PET) using basic ionic liquids as catalysts

The glycolysis of poly(ethylene terephthalate) (PET) was studied using several ionic liquids and basic ionic liquids as catalysts. The basic ionic liquid, 1-butyl-3-methylimidazolium hydroxyl ([Bmim]OH), exhibits higher catalytic activity for the glycolysis of PET, compared with 1-butyl-3-methylimidazolium bicarbonate ([Bmim]HCO₃), 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and 1-butyl-3-methylimidazolium bromide ([Bmim]Br). FT-IR, ¹H NMR and DSC were used to confirm the main product of glycolysis was bis(2-hydroxyethyl) terephthalate (BHET) monomer. The influences of experimental parameters, such as the amount of catalyst, glycolysis time, reaction temperature, and dosages of ethylene glycol on the conversion of PET, yield of BHET were investigated. The results showed a strong influence of the mixture evolution of temperature and reaction time on depolymerization of PET. Under the optimum conditions of m(PET):m(EG): 1:10, dosage of [Bmim]OH at 0.1 g (5 wt%), reaction temperature 190 °C and time 2 h, the conversion of PET and the yield of BHET were 100% and 71.2% respectively. Balance between the polymerization of BHET and depolymerization of PET could be changed when the reaction time was more than 2 h and contents of catalyst and EG were changed.     

聚酯催化缩聚与热降解过程的半经验量子化学研究

以苯甲酸β-羟乙酯(BAHET)为模型反应物,采用半经验量子化学方法(PM3),分析了生成聚酯的催化缩聚与热降解机理。计算结果表明,端羟基氧与酯羰基氧是反应物与金属催化剂作用的活性位。金属催化剂与两者作用后,可以使酯羰基碳的正电性与端羟基氧的负电性增加。由于静电作用力增加,促使缩聚反应得以进行。研究结果表明,在钛系催化体系中,端羟基氧和酯醚键氧能与钛原子作用形成热力学稳定的五元环结构,它的存在将影响钛系催化体系中聚酯的热降解行为。     

Transesterification of Ethylene Carbonate with Dimethyl Terephthalate over Various Metal Acetate Catalysts

IntroductionDimethyl carbonate(DMC) is known to be a novelbuilding block in organic synthesis. As an environmen-tally benign compound and a unique intermediate,DMC has attracted much attention[1,2]. Among the va-rious methods for synthesizing DMC, the tra…     

The formation of polyethylene terephthalate in the presence of dicarboxylic acids

The process of polyethylene terephthalate (PET) formation in the presence of dicarboxylic acids has been studied. Certain amounts of terephthalic acid (TPA) have two- to threefold accelerating efficiency in the polycondensation process. To elucidate the causes of the acceleration the main reactions leading to PET formation in the presence of dicarboxylic acids have been investigated by the use of models. The evaluation of kinetic and equilibrium parameters obtained for model reactions made it possible to conclude that the influence of carboxyl-containing additives on the apparent rate of polycondensation manifests itself in accelerating direct reactions and facilitating the liberation of the eliminated by-product; that is, ethylene glycol (EG) from the polymer melt. Carboxylic acid acts as a catalyst on the ester interchange of 2-hydroxyethyl ester end groups and thus increases the rate of polymer formation in this reaction 10–40 times. The parallel interaction between the 2-hydroxyethyl ester end group and the carboxyl group of the added acid is also catalyzed by the acid and its rate constant is four times larger than that of the catalytic polycondensation of 2-hydroxyethyl ester end groups. Unlike EG, the reaction water formed in the process is more readily removed from the reaction system and thus promotes the intensification of the process. In addition, the carboxyl groups react with the eliminated EG to decrease its amount and shift the equilibrium toward polymer formation. The investigation of the consequent parallel reactions on models made it possible to draw a conclusion about the higher reactivity of 2-hydroxyethyl esters in the esterification processes. This fact has been explained by strengthening the nucleophilicity of the oxygen atom in the hydroxyl of a 2-hydroxyethyl ester group compared with that of EG; for example, by the formation of an intramolecular cycle involving a hydrogen bond. Simultaneously, it has been found that in the system simulating PET polycondensation in the presence of dicarboxylic acids the reaction mechanism involves the catalysis by a proton formed during the carboxyl group dissociation and accepted by the 2-hydroxyethyl ester group.     

Ultrafast 99% Polyethylene terephthalate depolymerization into value added monomers using sequential glycolysis-hydrolysis under microwave irradiation

Effective and efficient hybrid depolymerisation technologies are emerging as high potential sustainable routes with considerable benefits over conventional recycling methods for the achievement of circular economies for plastics. Herein, combined green and fast glycolysis-hydrolysis depolymerization of polyethylene terephthalate (PET) was carried out under microwave irradiation (MW) with excellent efficiencies. In MW assisted glycolysis of PET, the catalytic activity of two deep eutectic solvents (DES) based on (choline chloride-urea (DES 1)) and (choline chloride-thiourea (DES 2)) was evaluated and compared. Optimised glycolysis conditions were determined using Box Behnken Design (BBD) to attain maximum weight loss of PET, low crystallinity and increased carbonyl index of residual PET. DES volume of 4 mL, 5.5–6 mL of ethylene glycol, and 0.5 min MW irradiation time resulted in a prominent rise in PET weight loss and carbonyl index of residual PET. DES 2 showed an improved catalytic activity than that of DES 1 which is associated to its stronger interaction with EG and PET polymer chains during the course of the reaction. Residual PET obtained post glycolysis reaction was further depolymerized using MW assisted hydrolysis in the presence of weakly basic Na₂CO₃ and EG. Within 3-minute, the proposed sequential depolymerization technologies facilitated ≈99% conversion of PET to terephthalic acid (TPA), monohydroxyethyl terephthalate (MHET), and bis (2-hydroxyethyl) terephthalate (BHET) monomers produced at a yield of 62.79–80.66%, 17.22–34.79% and 0.54–0.59% respectively. Application on post-consumer PET sample also revealed very satisfactory results with 96.77–98.25% PET conversion and 60.98–78.10% yield of TPA.     

Formation of diethylene glycol as a side reaction during production of polyethylene terephthalate

Polymers of poly(ethylene terephthalate) (PET) always contain a certain amount of incorporated diethylene glycol (DEG), substituting the incorporated glycol. DEG is formed in a side reaction during the ester interchange of dimethyl terephthalate (DMT) with ethylene glycol or during direct esterification of terephthalic acid with ethylene glycol, and to a smaller extent during the polycondensation of the low-molecular material. DEG is formed via an unusual type of reaction: ester + alcohol → ether + acid. Some evidence of this type of reaction is given by the formation of dioxane in low molecular PET and of methyl Cellosolve and methyl carbitol during the ester interchange of DMT with ethylene glycol and diethylene glycol, respectively. The strongest support for this type of reaction, however, was obtained from kinetic data. Polyesters of low molecular weight with OH group contents ranging from 3 to 0.5 mole/kg were heated at 270°C in sealed tubes for 1–7 hr. The kinetic equation for the proposed reaction is: d[DEG]/dt = k[OH] [ester]. With the aid of one rate constant the formation of DEG in all esters could be described.     

Room-temperature polycondensation of β-amino acid derivatives. V. Synthesis of hydrophilic polyamide by the polycondensation of β-amino acid derivatives

N-(Hydroxyalkyl) β-alanine ester which was obtained from amino alcohol and acrylate yielded polyamide at room temperature in the presence of a basic catalyst. Alkali and alkali earth metal alkoxides had a strong catalytic effect on the room-temperature polycondensation of N-(hydroxyethyl)-β-alanine esters. The catalytic activity of metal alkoxides decreased in the order: Li > Na > K > Cs and Ca > Zn > Mg. Aluminum and titanium alkoxide had a weak catalytic effect, while boron (III), tin (IV), antimony (V), and tellurium (VI) alkoxides did not show any catalytic activity for the polycondensation. It was also found that solvent had an effect on the course of the polycondensation of N-(hydroxyethyl)-β-alanine esters, and the highest molecular weight polymer was formed only in methanol solution. The solid-phase polycondensation of the low molecular weight prepolymer resulted in a high molecular weight polymer with an inherent viscosity of 1.0 in the presence of a catalytic amount of phosphoric acid. The polymer obtained is hydrophilic and its moisture absorption is more than twice that of nylon 6.     

Catalysts and temperature driven melt polycondensation reaction for helical poly(ester‐urethane)s based on natural L‐amino acids

Catalyst and temperature driven melt polycondensation reaction was developed for natural L‐amino acid monomers to produce new classes of poly(ester‐urethane)s. Wide ranges of catalysts from alkali, alkali earth metal, transition metal and lanthanides were developed for the condensation of amino acid monomers with diols to yield poly(ester‐urethane)s. A‐B Diblock and A‐B‐A triblock species were obtained by carefully choosing mono‐ or diols in model reactions. More than two dozens of transition metal and lanthanide catalysts were identified for the polycondensation to yield high molecular weight poly(ester‐urethane)s. Theoretical studies revealed that the carbonyl carbon in ester possessed low electron density compared to the carbonyl carbon in urethane which driven the thermo‐selective polymerization process. Optical purity of the L‐amino acid residues in the melt polycondensation process was investigated using D‐ and L‐isomers and the resultant products were analyzed by chiral‐HPLC and CD spectroscopy. CD analysis revealed that the amino acid based polymers were self‐assembled as β‐sheet and polyproline type II secondary structures. Electron and atomic force microscopic analysis confirmed the formation of helical nano‐fibrous morphology in poly(ester‐urethane)s. The newly developed melt polycondensation process is very efficient and optimized for wide range of catalysts to produce diverse polymer structures from natural L‐amino acids. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1065–1077     

A High-Valent Ru-PCP Pincer Catalyst for Hydrogenation of Carbonyl and Carboxyl Compounds under Molecular Hydrogen

Low-valent metals traditionally dominate the domain of catalytic hydrogenation. However, metal-ligand cooperating (MLC) catalytic systems, operating through heterolytic H−H bond splitting by a Lewis acidic metal and a basic ligand site, do not require an electron-rich metal. On the contrary, high-valent metals that induce weaker back donation facilitate heterolytic bond activation. Here we report, for the first time, the efficient hydrogenation of carbonyl and carboxyl compounds under molecular hydrogen catalyzed by a structurally well-defined Ru^IV catalyst bearing a bifunctional PCP pincer ligand. The catalyst exhibits reactivity toward molecular hydrogen superior to that of the low-valent analog and allows hydrogen activation even at room temperature.     

Nucleophilic substitution reactions of alcohols with use of montmorillonite catalysts as solid Brønsted acids

We have developed an environmentally benign synthetic approach to nucleophilic substitution reactions of alcohols that minimizes or eliminates the formation of byproducts, resulting in a highly atom-efficient chemical process. Proton- and metal-exchanged montmorillonites (H- and Mn+-mont) were prepared easily by treating Na+-mont with an aqueous solution of hydrogen chloride or metal salt, respectively. The H-mont possessed outstanding catalytic activity for nucleophilic substitution reactions of a variety of alcohols with anilines, because the unique acidity of the H-mont catalyst effectively prevents the neutralization by the basic anilines. In addition, amides, indoles, 1,3-dicarbonyl compounds, and allylsilane act as nucleophiles for the H-mont-catalyzed substitutions of alcohols, which allowed efficient formation of various C-N and C-C bonds. The solid H-mont was reusable without any appreciable loss in its catalytic activity and selectivity. Especially, an Al3+-mont showed high catalytic activity for the alpha-benzylation of 1,3-dicarbonyl compounds with primary alcohols due to cooperative catalysis between a protonic acid site and a Lewis acidic Al3+ species in its interlayer spaces.     

聚酯酰胺的合成及表征

 用两种方法合成了聚酯酰胺（ＰＥＡ）共聚物．一种是两步法，即先合成对苯二甲酸乙醇酰胺（ＢＡＥＴ）单体，然后与对苯二甲酸乙二酯（ＢＨＥＴ）共缩聚；另一种是一步法．即在酯交换反应中直接添加乙醇胺（ＥＡ）．两种方法制得的聚酯酰胺（ＰＥＡ）共聚物测试证明了为产物，并分析了合成中的化学反应．     

聚酯酰胺的合成及表征

用两种方法合成了聚酯酰胺（ＰＥＡ）共聚物．一种是两步法，即先合成对苯二甲酸乙醇酰胺（ＢＡＥＴ）单体，然后与对苯二甲酸乙二酯（ＢＨＥＴ）共缩聚；另一种是一步法．即在酯交换反应中直接添加乙醇胺（ＥＡ）．两种方法制得的聚酯酰胺（ＰＥＡ）共聚物测试证明了为产物，并分析了合成中的化学反应．     

Modulating the acidity: highly acidic Brønsted acids in asymmetric catalysis

Recently, chiral highly acidic Brønsted acids have emerged as powerful catalysts for enantioselective C C and C X bond‐forming reactions. Their strong acidity renders them valuable tools for the activation of imines, carbonyl compounds, and other weakly basic substrates. As a result, new perspectives are opened and highly stereoselective transformations based on the concept of chiral contact‐ion‐pair catalysis can be realized. This Minireview gives an overview of the design and application of these new organocatalysts and presents recent results in this rapidly growing field.     

Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface-bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid-base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O^2? ions; ion-pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid-base pair sites such as [Mg^2⊕O^2?]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface-bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.     

A rotating disk electrode study on catalytic activity of iron(II) phthalocyanine-modified electrodes for oxygen reduction in acidic media

Journal of Solid State Electrochemistry - Catalytic activity of monometric metal macrocycles for oxygen reduction reaction (ORR) was investigated using rotating disk electrode voltammetry in acidic...     

Preparation and characterization of poly(butylene terephthalate)/poly(ethylene terephthalate) copolymers via solid‐state and melt polymerization

To increase the T_g in combination with a retained crystallization rate, bis(2‐hydroxyethyl)terephthalate (BHET) was incorporated into poly(butylene terephthalate) (PBT) via solid‐state copolymerization (SSP). The incorporated BHET fraction depends on the miscibility of BHET in the amorphous phase of PBT prior to SSP. DSC measurements showed that BHET is only partially miscible. During SSP, the miscible BHET fraction reacts via transesterification reactions with the mobile amorphous PBT segments. The immiscible BHET fraction reacts by self‐condensation, resulting in the formation of poly(ethylene terephthalate) (PET) homopolymer. ¹H‐NMR sequence distribution analysis showed that self‐condensation of BHET proceeded faster than the transesterification with PBT. SAXS measurements showed an increase in the long period with increasing fraction BHET present in the mixtures used for SSP followed by a decrease due to the formation of small PET crystals. DSC confirmed the presence of separate PET crystals. Furthermore, the incorporation of BHET via SSP resulted in PBT‐PET copolymers with an increased T_g compared to PBT. However, these copolymers showed a poorer crystallization behavior. The modified copolymer chain segments are apparently fully miscible with the unmodified PBT chains in the molten state. Consequently, the crystal growth process is retarded resulting in a decreased crystallization rate and crystallinity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 882–899, 2007.