What Is Radical Polymerization?

Free radical polymerization is a polymerization reaction initiated by free radicals that causes the chain to grow (chain growth). Also called free radical polymerization. In addition polymerization, most of them use ethylenic monomers containing unsaturated double bonds as raw materials. By opening the double bonds in the monomer molecules, repeated addition reactions are repeated between molecules to connect many monomers. Get up and form macromolecules. It is mainly used in addition polymerization of olefins. The most common methods for generating free radicals are thermal decomposition of the initiator or redox decomposition of the two-component initiator. Free radicals can also be generated by heating, ultraviolet irradiation, high-energy radiation, electrolysis, and plasma initiation.

Radical Polymerization

Free radical polymerization plays an extremely important role in polymer chemistry. It is the earliest human development and the most thorough research on a kind of polymerization process. More than 60% of polymers are obtained through free radical polymerization, such as low density polyethylene,

Free radical polymerization is the most widely used chemical reaction in the polymer synthesis industry. Most of the olefinic monomers are polymerized or copolymerized using free radical polymerization. The resulting polymers are all linear polymer compounds.

Comparison of the characteristics of the four polymerization methods

There are five methods for implementing radical polymerization according to the physical state of the reaction system: bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, and supercritical carbon dioxide polymerization. Their characteristics are different, and the form and use of the products obtained are also different.

Free radical polymerization

Bulk polymerization is the polymerization of monomers under the initiation of initiator, heat, light, radiation, etc. without any other medium. Sometimes it is necessary to add a small amount of colorants, plasticizers, lubricants, molecular weight regulators and other additives. Therefore, the main characteristics of bulk polymerization are purity of the product, simple process and equipment, and it is suitable for preparing products such as transparent plates and profiles with good electrical properties. The disadvantages are the large viscosity of the reaction system, the significant automatic acceleration, the difficulty in deriving the polymerization reaction heat, the difficulty in controlling the temperature, the local overheating and the uneven molecular weight distribution.

Gaseous, liquid, and solid monomers can be used for bulk polymerization. The bulk polymerization of liquid monomers is the most important.

Ontology aggregation process

Bulk polymerization process

Aiming at the problem that the polymerization heat of the bulk polymerization method is difficult to radiate, a two-stage polymerization process is mostly used in industrial production. The first stage is pre-polymerization, which can be carried out at a lower temperature, and the conversion rate is controlled between 10% and 30%. Generally before self-acceleration, the system has a low viscosity and easy heat dissipation. The polymerization can be carried out in a large kettle. . The polymerization is continued in the second stage, in a thin-layer or plate-shaped reactor, or using staged polymerization to gradually increase the temperature and increase the conversion rate. Because the reaction temperature in the bulk polymerization process is difficult to control and constant, the molecular weight distribution of the product is relatively wide.

The post-treatment of bulk polymerization is mainly to exclude monomers remaining in the polymer. The commonly used method is to remove the monomers and volatiles from the molten polymer in vacuum. The equipment used is a screw or a vacuum degasser. It is also useful to use a foam degassing method. The polymer is heated under pressure to melt it, and then the pressure is suddenly reduced to make the polymer foamy, which is conducive to the escape of the monomer.

Bulk Polymerization Reactor

In order to solve the problem of polymerization reaction heat in industry, the heat transfer area should be considered when designing the shape and size of the reactor.

There are several types of reactors used in radical polymerization.

The model reactor is mainly suitable for body casting polymerization to prepare plates, tubes, rods, etc. The shape and size of the model are determined according to the requirements of the product, and the heat transfer during polymerization must be considered.

Kettle reactor polymerization kettle with stirring device. Because the later materials are high viscosity fluids, most of them use spiral ribbon (such as single spiral ribbon or double spiral ribbon) stirred tank. The operation mode can be batch or continuous. There are also continuous operation methods that use several polymerization tanks in series and segment polymerization according to the change in viscosity during the polymerization process.

There are two types of continuous polymerization reactors: tubular and tower reactors. The general tube reactor is an empty tube, and the material flows in a laminar flow in the tube reactor. Some tube reactors are equipped with a fixed mixer in the tube.

The tower reactor is equivalent to an enlarged tube reactor without a stirring device. The material flows in a plunger shape in the tower.

Free radical polymerization solution polymerization

A polymerization method in which a monomer and an initiator are dissolved in a suitable solvent is called a solution polymerization method. The polymer produced by the solution polymerization is dissolved in the solvent used for homogeneous polymerization. If the polymer is insoluble in the solvent used and precipitates, it is heterogeneous polymerization, also called precipitation polymerization.

The use of solvents in the solution polymerization process reduces the viscosity of the system, so it is easier to mix and transfer heat, the temperature is easy to control, the gel effect is less, and local overheating can be avoided.

In addition, due to the use of solvents in the solution polymerization process, the monomer concentration of the system is low, the polymerization rate is slow, and the production capacity and utilization of the equipment are reduced. Such as the production of solid products, after-treatment must be carried out, the cost of solvent recovery is high, increasing production costs. Therefore, industrial solution polymerization is mostly used in applications where polymer solutions are used directly, such as coatings, adhesives, impregnants, dispersants, thickeners, and the like. If a solid polymer is required, other solvents that are miscible with the solvent and insoluble with the polymer can be added to the solution to precipitate the polymer, and then separated and dried to obtain the solid polymer.

Choice of solvent

The solvent used in the solution polymerization is mainly an organic solvent or water. The choice of solvent is important in solution polymerization. Pay attention when selecting solvents in free radical solution polymerization:

Solvent-induced decomposition of initiators and chain transfer reactions to chain radicals.

The solvent can be selected by referring to the CS value according to the molecular weight requirements of the polymerized product.

The appropriate solvent is selected according to the solubility of the solvent in the polymer and the use of the polymer product. Common organic solvents are alcohols, esters, ketones, benzene, toluene, and the like.

When selecting a solvent for ionic polymerization, the solvation ability of the solvent must be considered first, and then the chain transfer reaction must be considered.

Solution polymerization process

When an organic solvent is used for solution polymerization, the initiator is a peroxide or an azo compound that is soluble in the organic solvent. Select an appropriate initiator based on the reaction temperature and the half-life of the initiator.

When water is used as a solvent, water-soluble initiators such as persulfate and its oxidation-reduction system are used.

The solution polymerization reaction temperature is performed at the reflux temperature of the solvent, so a low boiling point solvent is mostly used. To facilitate the control of the polymerization temperature, solution polymerization is usually operated semi-continuously in a kettle reactor. For the directly used polymer solution, the monomer content should be minimized before the reaction is ended, or the residual monomers should be removed by chemical or distillation methods. To obtain solid materials, post-treatment is required, that is, evaporation, degassing extrusion, drying, etc. are used to remove the solvent and unreacted monomers to obtain a powdery polymer.

Change the amount of initiator, the ratio of monomer to solvent, and add molecular weight regulators to control the molecular weight of the product.

Free radical polymerization

A method in which an initiator-dissolved monomer is suspended in water in the form of droplets to perform radical polymerization is called a suspension polymerization method. As a whole, water is a continuous phase, and monomer is a dispersed phase. Polymerization is carried out in each droplet, and the reaction mechanism is the same as that of bulk polymerization, which can be regarded as the bulk polymerization of beads. Similarly, the polymer can be classified into homogeneous and heterogeneous polymerization according to the solubility of the polymer in the monomer. For example, the method of suspending an aqueous solution of a water-soluble monomer as a dispersed phase in an oil continuous phase and polymerizing under the action of an initiator is called an inverse suspension polymerization method.

Suspension polymerization system generally consists of four basic components: monomer, initiator, water, and dispersant. The water-insoluble monomer is pulverized and dispersed into small droplets under strong stirring. It is unstable. With the progress of the reaction, the dispersed droplets may condense into agglomerates. To prevent adhesion, the Add dispersant.

The particle diameter of the suspension polymerization product is generally 0.05-0.2 mm. Its shape and size depend on the strength of the stirring and the nature of the dispersant.

The suspension polymerization method uses water as a medium, the system has low viscosity, good heat transfer, and easy temperature control. The molecular weight of the product and its distribution are relatively stable. The product is solid particles, and the post-treatment is simple. It only needs to be centrifuged and dried, so the cost is low. However, there is also an automatic acceleration effect, which makes the polymerization speed difficult to control; the dispersant in the product cannot be completely removed, which affects the purity of the product. Suspension polymerization is widely used in industrial production.

Granulation mechanism and dispersion

Granulation mechanism

Suspension-polymerized monomers styrene, vinyl chloride, etc. have very low solubility in water, and are basically incompatible with water, but only float on the water surface in two layers. Under the strong shearing action of the agitator, the monomer liquid layer was dispersed into droplets.

In the suspension polymerization system with a dispersant, when the conversion rate reaches 20 to 70%, the droplets enter the tacky stage. If they are not stirred, there is still the risk of sticking to the block. Important factor.

Dispersant and its role

There are two main types of dispersants for industrial production: protective gum dispersants and inorganic powder dispersants.

Protective glue dispersants are all water-soluble polymer compounds. Synthesis of natural polymer compounds such as gelatin, protein, starch, cellulose derivatives, sodium alginate, partially hydrolyzed polyvinyl alcohol, polyacrylic acid and its salts, sulfonated polystyrene, maleic anhydride-styrene copolymer, etc. Polymer compounds. The mechanism of this type of dispersant is to adsorb on the surface of the droplets, forming a protective film, which plays the role of protecting colloid.

The action mechanism of inorganic powdery dispersants such as carbonate, phosphate, talc, and kaolin is that the fine powder adsorbs on the surface of the droplet and plays a mechanical isolation role. Inorganic dispersants are more suitable for high temperature polymerization. In addition, after the suspension polymerization reaction is completed, the inorganic powdery dispersant is easily eluted with a dilute acid, so the obtained polymer contains fewer impurities.

The choice of dispersant and the determination of the amount depend on the type of polymer and particle requirements. Sometimes a small amount of co-dispersant is added to the suspension polymerization system, such as sodium lauryl sulfate, polyether and the like. The amount of dispersant is about 0.1% of the amount of monomer, and the amount of dispersant is 0.01 to 0.03%.

Suspension polymerization process

Suspension polymerization process

The typical production process of the suspension polymerization method is to add monomers, water, initiators, dispersants, etc. into the reaction kettle, heat them, and take appropriate measures to keep them at a certain temperature for the polymerization reaction. Monomer, dehydrated by centrifugation, and dried to obtain the product.

The monomer or monomer mixture used in suspension polymerization should be liquid, and the monomer purity is required to be> 99.98%.

In industrial production, the initiator and molecular weight regulator are added to the reaction kettle. The amount of initiator is 0.1% to 1% of the amount of monomer.

Deionized water, dispersant, co-dispersant, pH adjuster, etc. make up the aqueous phase. The ratio of water phase to monomer is generally in the range of 75:25 to 50:50.

The suspension polymerization process of various monomers is operated by a batch method.

Free radical polymerization emulsion polymerization

Emulsion polymerization is a unique method that can be used for certain free-radical polymerizations, and it involves the polymerization of monomers in the form of an emulsion. It refers to the polymerization reaction of monomers dispersed in a dispersion medium under the action of emulsifiers and mechanical stirring. The composition of the emulsion polymerization system is relatively complicated, and generally consists of four components: a monomer, a dispersion medium, an initiator, and an emulsifier. The monomer of classical emulsion polymerization is oil-soluble, the dispersion medium is usually water, and a water-soluble initiator is used. When a water-soluble monomer is selected, the dispersion medium is an organic solvent, and the initiator is oil-soluble. Such an emulsion system is called inverse emulsion polymerization.

Emulsion polymerization is widely used in industrial production. Many synthetic resins and synthetic rubbers are synthesized by emulsion polymerization. Therefore, the emulsion polymerization method is of great significance in the polymer synthesis industry.

The biggest feature of the emulsion polymerization method is that it can increase the polymerization rate and molecular weight at the same time, and also has the following advantages:

It is cheap and safe with water as the dispersion medium. The system has low viscosity, easy heat transfer, and easy control of reaction temperature.

Fast polymerization rate, high molecular weight, can be polymerized at lower temperature.

It is suitable for the occasions where latex is used directly, such as the production of latex paint, adhesives, etc.

In addition, there are also shortcomings in emulsion polymerization, that is, substances such as emulsifiers are left in the product, which affects the electrical properties of the product. When a solid product is required, the emulsion needs to undergo processes such as coagulation (demulsification), washing, dehydration, and drying, and the production cost is relatively high.

Emulsion polymerization mechanism

Emulsification

Emulsion polymerization firstly disperses monomers into an emulsion state with the aid of an emulsifier. This two-phases, which are incompatible with each other, namely oil (monomer) -water, are transformed into a relatively stable emulsion that is difficult to separate. This process is called emulsification. The emulsifier can emulsify in the emulsion system because the emulsifier molecule is composed of hydrophilic polar groups and lipophilic non-polar groups.

When the emulsifier is dissolved in water, when the concentration of the emulsifier is very low, the emulsifier is dissolved in water in a molecular state. When the concentration reaches a certain value, the emulsifier molecules form micelles, and the concentration at which the emulsifier starts to form micelles The critical micelle concentration, referred to as CMC, is about 0.01 to 0.03% at this time. In most emulsion polymerizations, the concentration of the emulsifier (about 2 to 3%) always exceeds the CMC value by 1 to 3 orders of magnitude, so most emulsifiers are in a micellar state. The number and size of the micelles depends on the amount of emulsifier. In a typical emulsion polymerization, the concentration of micelles is 1017 to 1018 particles / cm3.

The role of the emulsifier is to reduce the surface tension and disperse the monomers into fine droplets; a protective layer is formed on the surface of the droplets to prevent aggregation and keep the emulsion stable; solubilizing effect makes part of the monomers dissolved in the micelles. Taken together, these three aspects are emulsification.

There are four types of emulsifiers: cationic, anionic, amphoteric and non-ionic. Anionic and non-ionic emulsifiers are mostly used in emulsion polymerization.

Anionic emulsifiers are polar groups such as sodium lauryl sulfate (C12H25SO4Na), rosin soap, and the like. It is relatively stable in alkaline solutions.

Non-ionic emulsifiers cannot dissociate into positive and negative ions in water, and their typical representatives are ethylene oxide polymers, or block copolymers of ethylene oxide and propylene oxide, and polyvinyl alcohol, which are non-ionic emulsifiers. Agent. It cannot be used alone in the emulsion polymerization process. It is often used as a co-emulsifier. Adding a small amount can improve emulsion stability, particle size and particle size distribution of latex particles.

Polymerization mechanism

The emulsion polymerization mechanism also undergoes chain initiation, chain growth, and chain termination reactions. In the emulsion polymerization process, the basic components of the polymerization system exist in different states, and the changes are:

1) Phase change during polymerization

Before the polymerization reaction started, the monomers and emulsifiers were in the following three phases: a. In the water phase, a very small amount of monomers and a small amount of emulsifiers, most of the initiator; b. Monomer droplets, most of Emulsifier molecules are adsorbed on the surface to form a stable emulsion; c. Micelles, most of the emulsifier molecules are aggregated. Generally, each micelle is composed of 50-100 emulsifier molecules, some of which are It is a solubilized beam containing a monomer.

Free radicals generated by the decomposition of the initiator in the water phase diffuse into the micelles, causing the monomers dissolved in the micelles to polymerize. As the polymerization progresses, the monomers in the water phase continue to enter the micelles to replenish the consumed monomers, and the monomers in the monomer droplets are dissolved into the water phase to form a dynamic equilibrium. It can be seen that micelles are the reaction site for emulsion polymerization, and monomer droplets are the warehouse for providing monomers.

At the beginning of the polymerization reaction, there are three kinds of particles in the reaction system, namely monomer droplets, micelles that undergo polymerization reactions-called latex particles and micelles that have not reacted. As the reaction proceeds, the number of micelles decreases until it disappears, and the number of latex particles gradually increases to stabilize. The reaction enters the middle stage of polymerization, the number of latex particles is stable, and the number of monomer droplets is reduced. By the end of the reaction, all the monomer droplets disappeared, and the latex particles continued to increase. Only polymer latex particles were in the system. This is the change in system composition during the polymerization process.

2) Nucleation mechanism

The process of polymer micelles forming polymer latex particles is also called nucleation.

The mechanism of nucleation of the emulsion polymerized particles proceeds in two simultaneous processes. First, free radicals (including primary free radicals generated by the decomposition of initiators and solution-polymerized short-chain free radicals) diffuse into the micelles from the aqueous phase and initiate growth. This process is nucleation of micelles. Another process is that the short-chain free radicals generated by solution polymerization precipitate out in the aqueous phase. The precipitated particles become stable from the aqueous phase and monomer droplets by adsorbing emulsifier molecules, and then diffuse into the monomer to form and micelles. The process of nucleating the same particles is called homogeneous nucleation. The relative degree of micellar nucleation and homogeneous nucleation will vary with the monomer's water solubility and surfactant concentration. Higher monomer water solubility and low surfactant concentration are conducive to homogeneous nucleation; monomers with low water solubility and high surfactant concentration are conducive to micelle nucleation. For vinyl acetate, which is water-soluble, homogeneous nucleation is the main mechanism of particle formation, while for lipophilic styrene, it is mainly the mechanism of micelle nucleation.

3) polymerization process

Polymerization process

Typical emulsion polymerization can be divided into three stages according to the number of latex particles, namely the polymer latex particle formation stage; the polymer latex particles coexist with the monomer droplets, the monomer droplets disappear, and the monomer in the polymer latex particles Aggregation stage. The three stages of change are shown schematically.

Phase I-The latex particle formation phase, that is, the nucleation phase. From the beginning of the initiation until the disappearance of the micelles, the polymerization rate increases throughout the stage. Conversion rate can reach 2 ~ 15%. .

Phase II-constant speed phase. From the disappearance of micelles to the disappearance of monomer droplets. The micelles disappear, the number of latex particles is constant, the volume increases, the monomer droplets disappear, and the emulsion polymerization rate is constant. Conversion rate reaches 50%.

Phase III-the speed reduction period. After the monomer droplets disappear, the initiation, growth, and termination in the latex particles continue until the monomers in the latex particles are completely converted. The number of latex particles does not change, the volume increases, and the final particle size can reach 500 2000 & Aring ;.

The polymerization rate decreased with the decrease of the monomer concentration in the latex particles.

Emulsion polymerization production process

Emulsion polymerization process

Emulsion polymerization is one of the important production methods in the polymer synthesis industry. It mainly produces synthetic rubber, synthetic resin, adhesives, and latex for coatings. The products produced industrially by the emulsion polymerization method are solid blocks, solid powders and fluid latexes. Such as styrene-butadiene rubber, neoprene, polyvinyl chloride resin and acrylic latex.

The simple production process of emulsion polymerization is shown in the figure.

In addition to the above components in the emulsion polymerization process formula, additives such as buffering agents, molecular weight regulators, dielectrics, chain terminators, antioxidants and the like are also added.

In the emulsion polymerization process, water, monomers, emulsifiers, and other auxiliaries are added into the polymerization kettle according to the formula, and then the reaction is heated up. According to the way of feeding the polymerization kettle, it can be divided into batch operation, semi-continuous operation and continuous operation, and the reaction is performed in the polymerization kettle with a stirring device. Batch operation and semi-continuous operation carry out the polymerization reaction in a single kettle, while continuous operation uses multiple kettles to perform the polymerization reaction in series. Usually a group of 4 to 12 kettles form a production line.

The latex can be used directly as coatings, adhesives, etc. without further treatment; if necessary, the solid content must be adjusted, and dilution or concentration methods can be used. To obtain powdered latex, spray drying can be used. The latex is continuously sent to a spray drying tower, and hot air is sprayed into contact with the atomized latex, and dried into powder particles.

In addition, post-treatment can be performed by agglomeration. The simple method is to add a demulsifier to the latex to separate the polymer, and then wash and dry to obtain a dispersible latex powder. The coagulation method can remove most of the emulsifiers, and the purity of the product is higher than that of the spray drying method.

Development of emulsion polymerization

In recent years, the typical emulsion polymerization method has made great progress in theory. The emulsion polymerization mechanism and kinetic model have been established by Harkins, Smith, and Ewart, and many scholars have also proposed shell or core-shell models. Some reference theories. At the same time, the emulsion polymerization technology is also constantly developing and innovating, and many new emulsion polymerization methods have appeared, such as inverse emulsion polymerization of water-soluble monomers, core-shell emulsion polymerization, soap-free emulsion polymerization, emulsion-oriented polymerization, and emulsion radiation polymerization. Emulsion graft copolymerization and seed emulsion polymerization provide a wealth of new content in the field of emulsion polymerization technology, making emulsion polymerization more widely used in polymer synthesis industry. The soap-free emulsion polymerization method can be used to obtain regular monodisperse polymers. The core-shell emulsion polymerization method can adjust the chemical composition, molecular weight and glass transition temperature of the core and shell to achieve the required performance requirements of the product. These provide more channels for developing new products with superior performance.