Esterification polycondensation reaction, polyurethane chemical reaction

Esterification reaction is a class of organic chemical reactions, which is the reaction of alcohol with carboxylic acid or oxygenated inorganic acid to produce ester and water. It is divided into two categories: carboxylic acid reaction with alcohol and inorganic oxygenated acid reaction with alcohol, and inorganic strong acid reaction with alcohol. The esterification reaction of carboxylic acid and alcohol is reversible, and the reaction is generally very slow, so concentrated sulfuric acid is often used as a catalyst.

Polycarboxylic acids react with alcohols to form a variety of esters. The reaction of inorganic strong acids with alcohols is generally fast. A typical esterification reaction is the reaction of ethanol and acetic acid to produce ethyl acetate with aromatic odor, which is a raw material for the manufacture of dyes and medicines.

Esterification reaction is widely used in organic synthesis and other fields.

Polycondensation reaction is a kind of organic chemical reaction, which is a chemical reaction in which monomers with two or more functional groups react with each other to form polymer compounds, and at the same time produce chemical reactions with simple molecules (such as H2O, Hx, alcohol, etc.). It has the dual meaning of condensing out a low molecule and polymerizing into a high molecule, and the reaction product is called a condensation polymer. The essence of the polycondensation reaction can be seen as a substitution.

According to the reaction conditions, polycondensation reaction can be divided into four types: melt polycondensation reaction, solution polycondensation reaction, interfacial polycondensation reaction and solid-phase polycondensation reaction. Polycondensation reaction is widely used in organic synthesis, chemical fiber and other fields.

A diacid and a diol react under certain conditions to form polymer and water.

The ester condensation reaction is as follows:

Esters containing -hydrogen undergo condensation under the action of alkaline condensation agents such as sodium aloxide, and lose one molecule of alcohol to obtain -ketoate. For example, 2 molecules of ethyl acetate are condensed under the action of metal sodium and a small amount of ethanol to obtain ethyl acetoacetate.

The -hydroacidity of ethyl acetate is very weak, and sodium ethoxide is a relatively weak alkali ethanol PKA, so the negative ions formed by the reaction of ethyl acetate and sodium ethoxide are very few in the equilibrium system.

However, because the final product ethyl acetoacetate is a relatively strong acid, it can react with sodium ethanol to form stable negative ions, so that the equilibrium moves in the direction of the product. Therefore, although the concentration of ethyl acetate anion in the reaction system is very low, once formed, it continues to react, and the result is that the reaction can still be successfully completed.

In addition to sodium ethanol, the commonly used alkaline condensation agents include potassium tert-butoxide, sodium tert-butoxide, potassium hydride, sodium hydride, sodium triphenylmethyl, lithium diisopropylamino LDA and Grignard reagent.

Ester condensation can also occur between two different esters, and theoretically four different products can be obtained, which is called mixed ester condensation, which is not of much significance in preparation. If there is neither -hydrogen atom in one of the ester molecules, and the alkoxycarbonyl group is relatively active, only one condensation product is formed. Such as benzoate, formate, oxalate, carbonate, etc.

When reacted with other esters containing -hydrogen atoms, only one condensation product is formed.

Summary of chemical reactions involved in isocyanates and a brief description of their applications.

Reaction principle. The highly unsaturated structure of the isocyanate group (-NCO) determines that it has high reactivity. According to the electron resonance theory of the isocyanate group of Baken et al., it can be concluded that due to the resonance action of -NCO, its charge distribution is uneven, resulting in nucleophilic centers and electrophilic centers.

Its electron resonance structure is denoted as follows:

The electronegativity order of nitrogen, carbon and oxygen atoms is o n c, so the electron cloud density of nitrogen atom and oxygen atom is larger, which is strongly electronegative and easy to react with electrophiles. On the contrary, due to the action of strong electronegative atoms at both ends, the electron cloud density of carbon atoms decreases, and they exhibit strong electropositivity and become electrophilic centers. As a result, diisocyanates react very easily with compounds containing hydrogen atoms.

1. The basic chemical reactions involved in the synthesis of polyurethane.

1. Reaction of NCO and hydroxyl group.

The reaction of NCO and hydroxyl group is one of the most important reactions in the polyurethane industry, and it can be said to be the foundation of the polyurethane industry. It mainly generates a urethane group, and its reaction mainly occurs in the reaction between isocyanate and polyol and its small molecule alcohol, which is the main reaction of polyurethane synthesis, and its reaction is as follows:

2. Reaction between NCO and water.

The reaction between NCO and water is one of the main reactions of polyurethane foam, and its main application is to use the generated carbon dioxide to foam polyurethane products, and it is also an important reaction in the polyurethane industry, and the reaction mainly generates urea groups. In the synthesis process of ordinary polyurethane products, the moisture content in alcohols, amines and solvents must be strictly controlled, and the reasons are that water reacts with isocyanate as a difunctional reactant to generate urea-based polyurethane, which is a monomer, which affects the continuation of the reaction; Secondly, the relative molecular weight of water is small, and as long as a small amount of water is contained in the reaction system, a large amount of NCO will be consumed, which will affect the accuracy of the formula and adversely affect the performance of the product. In addition, the reaction of isocyanate with water produces carbon dioxide, which leads to adverse effects such as foaming of products that do not need to be foamed. It should be particularly pointed out that even in the production and synthesis process of polyurethane foam, the content of water should be strictly controlled, otherwise the control of the bubble cells will be uncertain, and at the same time, the reaction with water will release a large amount of heat, which may make the product scorch and other undesirable phenomena.

The specific reaction is as follows:

3. Reaction of NCO and amino group.

The reaction between NCO and amino group is also one of the main chemical reactions in the polyurethane synthesis industry, and its main application products are mainly in the fields of CPU, waterborne polyurethane, curing agent and two-component polyurethane. Specific application examples will be shared in the form of special topics in future content. It should be pointed out that in the reaction between NCO and amino group, the reaction temperature needs to be strictly controlled, because the reaction between NCO and amine is fast, and the temperature is too high to produce adverse effects such as condensation and dust.

4. Which of the NCO and carboxyl groups does the slip reaction.

The reaction of NCO and carboxyl group is one of the less widely used reactions in the polyurethane industry, and the main process is the formation of acidic anhydride with poor stability, which is then decomposed into urea and carbon dioxide.

Polyester polycondensation reaction is too late to shoot the scattering process, controlledEsterification reactionThe smooth progress: belongsReversible reactionIn general, the reaction is not carried out completely, according to the principle of reaction equilibrium, in order to increase the yield of esters, it is necessary to separate a component from the product or make one of the reactants excessive, so that the reaction is carried out in the positive direction.

The esterification reaction is a single-line, two-way reaction. It belongs to the substitution reaction.

In order to improve the yield of esters, the water generated by the reaction is removed by azeotropic distillation or water absorbent is often used, and excessive acid or alcohol can also be added during the reaction to move the reaction to the direction of the product.

Product Applications

Polyester is a polymer compound.

Polyethylene terephthalate is produced by polycondensation of terephthalic acid (PTA) and ethylene glycol (EG).

PET), part of which is then pelletized underwater to produce PET. Fiber-grade polyester chips are used to manufacture polyester staple fiber and polyester filament, and he is the raw material for polyester fiber enterprises to process fiber and related products, and polyester as the largest variety of chemical fiber, occupying nearly 80% of the market share in the chemical fiber industry.

Therefore, the market changes and development trends of polyester series are the focus of attention of the chemical fiber industry.

The above content reference: Encyclopedia - polyester.