What Is Solution Polymerization?

The monomer is dissolved in a suitable solvent and an initiator (or catalyst) is added to perform the polymerization reaction in a solution state. Solution polymerization is an important synthetic method in polymer synthesis. Generally carried out at the reflux temperature of the solvent, the reaction temperature can be effectively controlled, and at the same time, the heat released by the exothermic reaction can be dissipated by the evaporation of the solvent. If the resulting polymer can also be dissolved in a solvent, the product is a solution, called homogeneous solution polymerization, such as the polymerization of acrylonitrile in dimethylformamide, and poured into some liquid that does not dissolve the polymer. That is, the polymer can be obtained by precipitating and distilling off the solvent from the solution. If the polymer produced cannot be dissolved in the solvent, the polymer produced will precipitate continuously as the reaction progresses. This polymerization is called heterogeneous (or heterogeneous) solution polymerization, also known as precipitation polymerization. Such as acrylonitrile solution polymerization. The advantages of solution polymerization are: compared with bulk polymerization, the solvent can be used as a heat transfer medium to make the system easier to transfer heat, and the temperature is easy to control; the system has a lower viscosity, reduces gel effect, and can avoid local overheating; it is easy to adjust the molecular weight The molecular weight distribution. The disadvantages of solution polymerization are: lower monomer concentration, slower polymerization rate, lower equipment production capacity and utilization; low monomer concentration and transfer to the solvent chain result in lower polymer molecular weight; increase when using organic solvents Cost and environmental pollution; the cost of solvent separation and recovery is high, and it is difficult to remove the residual solvent in the polymer. In the industry, solution polymerization is suitable for applications where polymer solutions are used directly, such as coatings, adhesives, and synthetic fiber spinning solutions.

Chinese name: Solution polymerization
Foreign name: solution polymerization

Missing point: Slow dilution polymerization rate consumes solvent environmental pollution
Composition: VA061 initiator azodicyanovaleric acid

Solution polymerization definition

Solution polymerization is a process in which monomers and initiators (catalysts) are dissolved in a suitable solvent for polymerization. The solvent is generally an organic solvent, or it can be water, depending on the nature of the monomer, initiator (or catalyst), and polymer produced. If the polymer formed is soluble in the solvent, the polymerization reaction is a homogeneous reaction, which is a typical solution polymerization; if the polymer formed is insoluble in the solvent, the polymerization reaction is a heterogeneous reaction, called precipitation polymerization, or called Slurry polymerization.

Solution polymerization composition

Monomer + (oil-soluble) initiator + solvent, or monomer + water-soluble initiator + water. Oil-soluble initiators are mainly azo initiators and peroxy initiators. Azo initiators are azobisisobutyronitrile, azobisisoheptonitrile, azobisisovaleronitrile, and azodicyclohexylmethyl. Nitrile, azobisisobutyrate initiator, etc. Water-soluble initiators include persulfate, redox initiation system, azobisisobutyrate hydrochloride (V-50 initiator), azobis Isobutimidazoline hydrochloride (VA-044 initiator), azobisisobutimidazoline (VA061 initiator), azodicyanovaleric acid initiator, etc.

Solution polymerization characteristics

The viscosity of the polymerization system is lower than that of the bulk polymerization, the mixing and heat dissipation are relatively easy, the production operation and temperature are easy to control, and the evaporation of the solvent can be used to eliminate the polymerization heat. In the case of free radical polymerization, an automatic acceleration effect may not occur when the monomer concentration is low, thereby avoiding explosive polymerization and simplifying the design of the polymerization reactor. The disadvantage is that for radical polymerization, the yield is often low, and the degree of polymerization is smaller than other methods. The use and recovery of a large number of expensive, flammable, and even toxic solvents not only increase production costs and equipment investment, reduce equipment production capacity, but also cause the environment. Pollution. If solid polymer is to be prepared, it is necessary to configure separation equipment and increase the steps of washing, solvent recovery and purification. Therefore, in the industry, solution polymerization is used only when it is difficult to use other polymerization methods or directly use a polymer solution.

advantage:

The polymerization heat is easy to diffuse and the polymerization reaction temperature is easy to control; it can be directly finished in solution; the materials are easy to be transported after the reaction; the low molecular substances are easy to remove; the automatic acceleration phenomenon can be eliminated. Aqueous solution polymerization uses water as a solvent, which is very beneficial to environmental protection.

Disadvantages:

The monomer is diluted by the solvent, the polymerization rate is slow, and the molecular weight of the product is low; the solvent is consumed, the solvent is recycled, the equipment utilization rate is low, and the cost is increased; the use of the solvent causes environmental pollution problems

Solution polymerization solvent selection

The solvent used in the solution polymerization is mainly an organic solvent or water. The proper solvent should be selected according to the solubility properties of the monomers and the solution application of the polymer produced. Commonly used organic solvents are alcohols, esters, ketones, and aromatic hydrocarbons (benzene, toluene), etc. In addition, aliphatic hydrocarbons, halogenated hydrocarbons, naphthenes, etc. are also used. When selecting a solvent for solution polymerization, pay attention to the following issues:

Effect of Solvent on Polymerization Activity. Solvents are often not absolutely inert, they can induce decomposition of initiators, and chain radicals have chain transfer reactions to solvents. Both effects may affect the polymerization rate and molecular weight. In ionic polymerization, the influence of the solvent is greater. The polarity of the solvent has a significant effect on the existence and activity of the active ion pair, the polymerization reaction rate, the degree of polymerization, the molecular weight and its distribution, and the chain microstructure. For copolymerization reactions, especially ionic copolymerization, the polarity of the solvent will affect the reactivity ratio of the monomers, and then affect the copolymerization behavior, such as copolymerization composition, sequence distribution, and so on. So be very careful when choosing a solvent. The effects of various solvents on the decomposition rate of peroxy initiators (increase in order) are as follows: aromatics, alkanes, alcohols, ethers, and amines. Azobisisobutyronitrile has the same first-order decomposition rate in many solvents and has less induced decomposition. As a result of the transfer to the solvent chain, the molecular weight will decrease. The chain transfer constants of various solvents vary greatly, water is zero, benzene is smaller, and halogenated hydrocarbons are larger.

The effect of rhenium solvents on the solubility and gelation of polymers. When a good solvent is selected, it is homogeneous polymerization. If the monomer concentration is not high, the gel effect may not occur and the normal free radical polymerization kinetics rules are followed. When a precipitating agent is selected, it becomes precipitation polymerization, and the gel effect is significant. The effects of poor solvents are somewhere in between, and the depth of the effects depends on the quality and concentration of the solvent. When there is a gel effect, the reaction accelerates automatically and the molecular weight increases. When chain transfer and gel effect occur at the same time, the molecular weight distribution will be determined by the depth of these two opposite factors. To ensure that the polymerization system is homogeneous during the reaction, the solvent chosen should have good solubility for the initiator or catalyst, monomer and polymer. This is beneficial to reduce the viscosity, slow down the gel effect, and lead to the heat of polymerization reaction. If necessary, mixed solvents can be used. For polymerization systems where an ideal solvent cannot be found, starting from the needs of the polymerization reaction, a solvent with good solubility for certain components (generally monomers and initiators) is selected. Such as the coordination polymerization of ethylene, hydrogenated gasoline as a solvent, although the solubility of the initiator system and polymer is not good, but the monomer ethylene has good solubility. Of course, from another perspective, it is also desirable to easily separate the solvent from the polymer after the polymerization is completed.

Other aspects: such as good economy, easy to recycle, easy to refining, non-toxic, easy to obtain, cheap, easy to transport and store.

Solution polymerization industrialized varieties

Polyacrylonitrile one-step method; two-step method

PVAc emulsion white latex

Solution alcoholysis PVA acetalized vinylon

Polyacrylamide

Polyvinylpyrrolidone

Acrylic coatings, adhesives, etc.

Solution polymerization industrial application

Industrial applications In industry, the polymerization method is selected according to the nature of the polymerization system, the use of the product, and whether it is easy to increase in size (see table). For ionic polymerization or coordination polymerization, since organometallic compounds and Lewis acids are often used as catalysts in the polymerization, the catalyst is easily destroyed by water. Therefore, suspension polymerization or emulsion polymerization methods using water as the medium cannot be used. They can only be used in aprotic organic compounds. Solution polymerization or bulk polymerization is performed in a solvent. For gradual polymerization or polycondensation reactions with large equilibrium constants, because it is easy to reach equilibrium and obtain high molecular weight products, solution polymerization is also often used, such as the early synthesis of polyamide, the synthesis of polysulfone, polyphenylene ether, etc .; some monomers The melting point is very high or the melting point is higher than the decomposition temperature. Solution polymerization is also used. Where polymer solutions are used directly, such as coatings, impregnants, spinning solutions, adhesives, or to continue polymer conversion, solution polymerization is used.

Solution polymerization in industry can use continuous and batch methods, and continuous methods are commonly used in large-scale production. Polymerization reactors are generally stirred tanks, and some tops are equipped with a condenser for the solvent to condense the reflux of the solvent; heat exchangers such as internal cooling pipes are usually not installed in the tanks to prevent sticking walls.

Solution polymerization

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.

Aggregation process

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

Polyacrylamide polymerization

The production of polyacrylamide is based on the acrylamide aqueous solution as a raw material, and the polymerization reaction is performed under the action of the initiator. After the reaction is completed, the polyacrylamide gel block is cut, granulated, dried, and pulverized to finally obtain polypropylene Amide products. The key process is the polymerization reaction. In the subsequent processing, attention should be paid to mechanical cooling, thermal degradation and cross-linking to ensure the relative molecular mass and water solubility of polyacrylamide.

Acrylamide + water (initiator / polymerization) polyacrylamide gel block granulation drying crushing polyacrylamide product

China's polyacrylamide production technology has probably gone through 3 stages:

The first stage is the earliest use of disc polymerization, that is, the mixed polymerization reaction solution is placed in stainless steel pans, and then these stainless steel pans are pushed into the insulation drying room. After a few hours of polymerization, they are pushed out of the drying room and the polymerization is performed with a trowel. The acrylamide is cut into strips, granulated in a meat grinder, dried in a drying room, and pulverized to obtain a finished product. This craft is completely hand-worked.

The second stage is the use of a kneader, that is, the mixed polymerization reaction solution is heated in the kneader. After the polymerization starts, the kneader is started and kneaded while polymerizing. After the polymerization, the granulation is basically completed, and the discharged material is dried. 2, crushed to obtain the finished product.

The third stage was the development of the conical kettle polymerization process in the late 1980s, which was successfully commissioned by the Jiangsu Jiangdu Chemical Plant of the 5th Ministry of Nuclear Industry. In this process, a rotary knife for making materials is provided at the lower part of the conical kettle, and the polymer is granulated while being extruded, dried by a drum dryer, and pulverized to obtain a product.

In order to prevent the polyacrylamide gel from sticking to the wall of the polymerization kettle, some technologies use a polymer compound of fluorine or silicon to coat the inner wall of the polymerization kettle, but this coating layer is easily peeled off during the production process and pollutes the polymer. Acrylamide products.

The production technology of polyacrylamide at home and abroad is basically similar to the third stage described above, but there are some differences in the equipment: the size and type of the polymerization kettle (there are fixed cone kettles and rotatable cone kettles. After the polymerization reaction is completed, The polymerization kettle is inverted to pour out the polyacrylamide gel block, the granulation method (including mechanical granulation, cutting granulation, and wet granulation, that is, granulation in the dispersion), and the drying method (through-flow rotary drying is also useful. Vibration fluidized bed drying) and pulverization method. Some of these differences are differences in equipment quality, and some are differences in the specific methods of oil used, but in general, the polymerization technology tends to fixed cone kettle polymerization, vibrating fluidized bed drying technology.

In addition to the above-mentioned unit operations, polyacrylamide production technology has obvious differences in process formulas. For example, the production process for producing ultra-high molecular weight polyacrylamide is also initiated at low temperature, and there is a pre-alkali co-hydrolysis process. There are advantages and disadvantages of the two methods after the addition of alkali and post-hydrolysis. The process of co-hydrolysis before the addition of alkali is simple, but there are problems such as hydrolysis and heat transfer that are prone to cross-linking and loss of relative molecular weight. The process is increased, but the hydrolysis is not easy to produce cross-linking, and the relative molecular mass loss of the product is not large.

At present, the initiators for the polymerization of polyacrylamide in China include inorganic initiators, organic initiators and inorganic-organic mixed systems 3. (1) Peroxide

Peroxides are broadly divided into inorganic peroxides and organic peroxides. Inorganic peroxides such as potassium peroxy acid, ammonium persulfate, sodium perbromide and hydrogen peroxide. Organic peroxides such as benzoyl peroxide, lauroyl peroxide, and t-butylhydroxyperoxide. The reducing agents they use are ferrous sulfate, ferrous chloride, sodium metabisulfite, and sodium thiosulfate.

(2) Azo compounds

Such as azobisisobutyronitrile, azobisdimethylvaleronitrile, sodium azobiscyanovalerate and the azosulfonium salt series developed in the 1980s, such as A class of competitively-developed products, which are added at a concentration of 0.005-1 to 10,000, has a high catalytic efficiency, which helps to produce products with high relative molecular mass, and is soluble in water and easy to use.