Write an equation for the polymerization of polystyrene. Emulsion polymerization of styrene

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Topic: Polymerization of styrene in emulsion

The purpose of the work: to carry out the polymerization of styrene by the emulsion method, to plot the dependence of the yield of polystyrene on time, to determine the molecular weight of the polymer by the viscometric method.

Theoretical part

Polymerization is the process of formation of macromolecular compounds as a result of the combination of a large number of monomer molecules into one macromolecule. In this case, the monomer and polymer molecules have the same elemental composition. In general, the polymerization reaction can be represented as follows:

polymerization styrene emulsion method

where X is a substitute. It does not emit any by-products.

Compounds containing double or triple bonds, as well as carbo- and heterocycles, can enter into the polymerization reaction.

Most polymerization processes have a chain character and proceed through the stages of chain initiation, growth, and chain termination.

Chain initiation occurs by attaching the active center to the monomer molecule, resulting in a hemolytic or heterolytic cleavage of its reactive bonds. The newly formed active center is an active radical or an ion:

Depending on the type of active centers that initiate the chain process, there are radical and ionic polymerization.

Chain growth is a multiple repetition of the acts of attachment of monomer molecules to the active center at the end of the chain, resulting in the formation of an active polymer.

Chain termination usually occurs either as a result of the interaction of two growing chains (recombination) or as a result of the interaction of a growing macromolecule with impurity or solvent molecules (chain transfer).

Radical polymerization

In radical polymerization, the active site is a free radical. Depending on the method of formation of radicals (initiation), one can distinguish thermal polymerization, photochemical, radiation (under the action of gamma rays, X-rays, accelerated electrons), as well as chemically initiated polymerization occurring in the presence of chemical initiators - compounds that easily decompose under reaction conditions with the formation of free radicals.

Chemically initiated polymerization is one of the most common radical polymerization methods. Peroxides, hydroperoxides, azo and diazo compounds, redox systems, etc. are used as initiators. For example, the decomposition of benzene peroxide proceeds with the formation of two radicals:

Azobisisobutyric acid dinitrile decomposes with the release of nitrogen, and also forms two radicals:

The activation energy for the decay of most initiators is over 120 kJ/mol.

Redox initiation is often used in polymerization. A feature of this initiation is a low activation energy, which will allow the process to be carried out at low temperatures. An example of such initiation is the interaction of hydrogen peroxide with ferrous salts, resulting in the formation of free radicals:

The activation energy in redox systems averages about 40 kJ/mol.

The polymerization reaction begins with the addition of free radicals to monomer molecules, which leads to the formation of a reaction chain:

The resulting compound is also a free radical and then reacts with a large number of monomer molecules, i.e. chain grows:

Thus, the stage of chain growth consists of a successive series of acts of interaction of a free radical with monomer molecules. The rate of radical polymerization is determined by the equation

where k p is the growth rate constant; k is the initiation rate constant; k o - rate constant of chain termination; [I] -concentration of the initiator; [M] - monomer concentration.

Chain termination or chain termination is usually the result of the interaction of two radicals and occurs either by recombination of macroradicals or by disproportionation. During the recombination of macroradicals, one polymer molecule is formed, which is not able to participate in further growth:

During disproportionation, the number of macromolecules does not change.

Chain termination can also occur as a result of a chain transfer reaction. Chain transfer is carried out by the interaction of growing macroradicals with monomer, polymer molecules, as well as with impurities or solvents:

The resulting active radical R, reacting with monomer molecules, gives rise to a new chain:

In the case of the formation of an inactive radical that is not able to continue the reaction chain, polymerization stops.

Ionic polymerization

The active centers of ionic polymerization are ions that form ion pairs in nonpolar solvents. In polar solvents, solvate-separated ion pairs and free ions appear.

Depending on the nature of the catalysts and the charge of the resulting ions, cationic and anionic polymerization are distinguished.

Cationic polymerization

Cationic polymerization proceeds under the action of acids and Fidel-Crafts catalysts (AlCl 3 , BF 3 , SnCl 4 , FeCl 3 , etc.), i.e. electron accepting substances. In the presence of water, acids, esters and other substances that play the role of a co-catalyst, an active catalytic complex is formed that initiates the reaction:

When this complex interacts with a monomer molecule, an active carbonenium center:

The growth reaction consists in the attachment of monomer molecules to the active carbenium center with the regeneration of this active center at the end of the chain:

The growth rate is described by the equation

where [C] is the catalyst concentration.

Cationic polymerization proceeds, as a rule, at a very high rate, which allows the process to be carried out at low temperatures. For example, the polymerization of isobutylene is carried out at t= -100°C in liquid ethylene.

Chain termination occurs as a molecular reaction with the elimination of a proton from the carbon atom adjacent to the carbenium ion and the dissociation of the catalytic complex:

Anionic polymerization

Anionic polymerization proceeds in the presence of alkali metals, organometallic compounds, sodium amide, alkali metal alcoholates, and other electron-supporting compounds. Of greatest practical importance is the polymerization proceeding under the action of alkali metals or their alkynes.

The polymerization of acrylonitrile under the action of potassium amide in liquid ammonia is caused by free ions, due to the dissociation of the amide:

The formation of a carbon anion occurs when an amide ion interacts with a monomer molecule:

Chain growth occurs as a result of the interaction of the resulting carbanion with the monomer molecule to form a new anion. Chain termination occurs by the interaction of the carbanion with the ammonia molecule with the regeneration of the amide ion, i.e. a chain transfer reaction takes place.

Ion-coordination polymerization

Ion-coordination polymerization is caused by Ziegler-Natta complex catalysts. Most often, organometallic aluminum compounds and titanium chlorides are used as catalysts.

Active centers in ion-coordination polymerization are organometallic compounds of the transition metal. They arise in the presence of a cocatalyst or in the interaction of the starting monomers with metal hydride sites on the catalyst surface.

The formation of an active metallographic compound occurs as follows:

The growth of the polymer chain is carried out by introducing a monomer molecule through a bond into the transition metal-carbon:

The stage of introduction of the monomer molecule is preceded by its coordination on the metal with the formation of an unstable p-component. Therefore, complex catalysts are called ionically -coordinating. Chain termination occurs as a result of the migration of a hydrogen atom from a carbon atom to a metal with the formation of a transition metal hydride and a polymer molecule.

The use of complex organometallic catalysts for polymerization leads to the formation stereoregular polymers. These catalysts are highly stereospecificity.

2. POLYMERIZATION METHODS

In industry, polymerization is carried out in the following main ways: in the gas phase, block (mass), solution, emulsion and suspension.

2.1 Gas polymerization

Gaseous monomers (ethylene, propylene) undergo gas-phase polymerization. The process is initiated by oxygen, which is added to the monomer in small amounts (0.002 x 0.008% vol.) and carried out under high pressure.

When ethylene reacts with oxygen, peroxide or hydroperoxyl compounds of ethylene are formed:

The unstable peroxide bond - O - O under the influence of heat is broken with the formation of bi- and monoradicals: OCH 2 -CH 2 O · and CH 2 = CHO ·. Free radicals initiate the polymerization of ethylene.

2.2 Bulk polymerization

Bulk polymerization or bulk polymerization is carried out in the condensed phase in the absence of solvent. As a result of polymerization, a concentrated solution (or melt) of the polymer in the monomer or a monolithic solid mass (block) is formed.

Usually block polymerization is carried out in the presence of initiators or thermal initiation. As the degree of polymerization of the monomer increases, the molecular weight of the medium and its viscosity increase, which makes it difficult to remove heat from the reaction zone. As a result, local overheating of the reaction mass may occur, as a result of which the polymer is obtained inhomogeneous in molecular weight. Therefore, block polymerization is carried out at a low rate.

2.3 Solution polymerization

There are two ways to carry out the polymerization in solution. The first method uses a solvent that dissolves both the monomer and the polymer. The resulting polymer solution (lacquer) is used as such or the polymer is isolated. The second method uses a solvent that dissolves the monomer but does not dissolve the polymer. The resulting polymer precipitates out.

During solution polymerization, the removal of heat generated during the reaction is significantly improved, but as a result of chain transfer reactions through the solvent, the resulting polymers have a lower molecular weight.

2.4 Emulsion polymerization

In emulsion polymerization, water is usually used as the dispersion medium. Various emulsifiers (oleates, palmitates, and other salts of fatty acids) are used to stabilize the emulsion. Emulsion polymerization is carried out in the presence of water-soluble initiators (potassium persulfate, bicarbonate pyrophosphates). Mercaptans are added to reduce chain branching.

To create a thin emulsion, the reaction mixture is vigorously stirred, as a result of which the monomer is broken into small droplets coated with an emulsifier layer.

Polymerization proceeds in the adsorption layers of the emulsifier on the surface of polymer-monomer particles. The growing macromolecule is the center around which the latex particle is formed. The resulting latex is coagulated by introducing an electrolyte solution into the system, and the precipitated polymer is separated. As a result of emulsion polymerization, a polymer with a high molecular weight and a low degree of polydispersity is obtained.

The possibility of using the emulsion method in some cases limits the formation of a large amount of wastewater that requires purification from toxic monomers, as well as the complexity of the stage of drying the fine polymer. In addition, the disadvantage of this method is the contamination of the polymer with residues of the emulsifier and other additives, which worsens its electrical properties.

2.5 Suspension polymerization

Suspension polymerization is also carried out in water. To increase the stability of the resulting coarser emulsion, weak emulsifiers are used - polyvinyl alcohol, water-soluble cellulose ethers, gelatin, clay, aluminum oxide, etc. The initiators used are soluble in the monomer.

Polymerization occurs in droplets, which are, in essence, small blocks, so this polymerization is sometimes called drop (granular) polymerization.

In contrast to emulsion polymerization, in this case there is no need for coagulation, since the resulting polymer granules are freely released from the aqueous phase.

Work order

The polymerization of styrene by the emulsion method is carried out on a laboratory plant, the scheme of which is shown in Figure 1.

The polymerization of styrene is carried out according to the recipe below (in parts by weight):

Styrene 50 g.

Distilled water 90 ml

Ammonium persulfate 0.35 g

Potassium stearate 2.3 g

A solution of the emulsifier in water is prepared in a reaction flask at 70°C. Styrene is added dropwise with good stirring, and after 10-15 minutes, the initiator dissolved in 10 ml of water is introduced. After 30, 60 and 90 minutes after the introduction of the initiator, samples of the reaction mass are taken with a pipette exactly 10 ml each. The emulsion in the samples is destroyed by adding 10 - 15 ml of NaCl solution and 2 drops of 1N nitric acid.

The polymer precipitate formed during the breaking of the emulsion is filtered off on a pre-weighed filter and washed with water. The polymer is dried in air to constant weight.

1 - heating mantle; 2 - three-necked flask; 3 - reverse refrigerator; 4 - water seal; 5 - mixer; 6 - thermometer; 7 - LATR

Figure 1 - Diagram of the laboratory setup

Processing of experimental data

The polymer yield in each sample is determined by the equation

where G n is the mass of the polymer in the sample;

G M is the mass of the monomer in the sample before the start of the experiment.

Table 1 - Time dependence of mass and yield of polymer

Based on the data obtained, we build the dependence of the polymer yield on time

Figure 2 - Graph of polymer yield versus time

Determination of polymer molecular weight

The molecular weight of the obtained polystyrene is determined by the viscometric method. To do this, three weighed portions of the polymer weighing 0.1 are taken from the dried third sample; 0.2 and 0.3 g and each dissolved in 20 ml of toluene.

To determine the molecular weight, a glass viscometer with two risks is used. Sequentially determine the time of expiration of 20 ml of pure toluene and polymer solutions, in order of increasing polymer concentration, between the upper and lower marks.

The determination of the expiration time is repeated three times for each sample and the average value of the time is determined.

Table 2 - polymer flow time and pure toluene.

The obtained values of the expiration time of pure toluene and three solutions are used in the calculations. Determine the relative viscosity of each solution by the formula:

where t is the expiration time of the polymer solution;

t o - time of expiration of pure solvent.

Specific viscosity:

Reduced viscosity:

where C is the concentration of the polymer in solution (g/100 ml of solvent).

Let's find concentrations:

Substituting into the equation, we get:

Having determined the reduced viscosity for each solution, the dependence of the reduced viscosity on the polymer concentration is plotted. Extrapolating the obtained dependence to zero polymer concentration, one obtains Xaracteristic viscosity.

An example of plotting the graphical dependence of the reduced viscosity on the concentration of the polymer and determining the intrinsic viscosity is shown in Figure 3.

Table 3 - Viscosities for three samples

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Figure 3 - Determination of intrinsic viscosity

To determine the molecular weight of a polymer, the Mar-ka-Hooving equation is used:

Based on the equation of direct dependence of the viscosity of the polymer solution on the concentration, we see, = 1.2767, and for the polystyrene-toluene system at a temperature of 25 ° C, the constants have the following values: a = 0.69, K = 1.7· 10 -4. Substituting, we get:

M = 413875.3 g/mol

In the course of this work, styrene polymerization was carried out by the emulsion method, the dependence of the polystyrene yield on time was plotted, and the molecular weight of the polymer was determined by the viscometric method: M = 413875.3 g/mol.

As a recommendation for the process, it can be taken into account that a change in the design of the mixing element is required to form a finer emulsion, which will lead to better production of styrene polymerization reaction products.

It is necessary to use a more advanced heater for precise control of the process temperature and the best output of the process to the regime.

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general characteristics

Polystyrene is a synthetic polymer belonging to the class of thermoplastics. As the name implies, it is a polymerization product of vinylbenzene (styrene). It is a hard glassy material. The general formula for polystyrene is as follows: [CH 2 CH (C 6 H 5)] n. In an abbreviated version, it looks like this: (C 8 H 8) n . The abbreviated formula of polystyrene is more common.

Chemical and physical properties

The presence of phenolic groups in the formula of the structural unit of polystyrene prevents the ordered placement of macromolecules and the formation of crystalline structures. In this regard, the material is rigid but brittle. It is an amorphous polymer with low mechanical strength and high light transmission. It is produced in the form of transparent cylindrical granules, from which the necessary products are obtained by extrusion.

Polystyrene is a good dielectric. It is soluble in aromatic hydrocarbons, acetone, esters, and its own monomer. Polystyrene is insoluble in lower alcohols, phenols, aliphatic hydrocarbons, and ethers. When the substance is mixed with other polymers, "crosslinking" occurs, as a result of which styrene copolymers are formed, which have higher structural qualities.

The substance has low moisture absorption and resistance to radioactive irradiation. However, it is destroyed by the action of glacial acetic and concentrated nitric acids. When exposed to ultraviolet, polystyrene deteriorates - microcracks and yellowness form on the surface, and its fragility increases. When a substance is heated to 200 °C, it begins to decompose with the release of a monomer. In this case, starting from a temperature of 60 ° C, polystyrene loses its shape. At normal temperatures, the substance is not toxic.

The main properties of polystyrene:

Density - 1050-1080 kg / m 3.
The minimum operating temperature is 40 degrees below zero.
The maximum operating temperature is 75 degrees Celsius.
Heat capacity - 34*10 3 J/kg*K.
Thermal conductivity - 0.093-0.140 W / m * K.
Thermal expansion coefficient - 6 * 10 -5 Ohm cm.

In industry, polystyrene is obtained by radical polymerization of styrene. Modern technologies allow this process to be carried out with a minimum amount of unreacted substance. The reaction of obtaining polystyrene from styrene is carried out in three ways. Let's consider each of them separately.

Emulsion (PSE)

This is the oldest synthesis method, which has never been widely used industrially. Emulsion polystyrene is obtained in the process of polymerization of styrene in aqueous solutions of alkalis at a temperature of 85-95 °C. For this reaction, the following substances are needed: water, styrene, an emulsifier and an initiator of the polymerization process. Styrene is preliminarily removed from inhibitors (hydroquinone and tributyl pyrocatechol). The initiators of the reaction are water-soluble compounds. As a rule, it is potassium persulfate or hydrogen dioxide. Alkalis, salts of sulfonic acids and salts of fatty acids are used as emulsifiers.

The process is as follows. An aqueous solution of castor oil is poured into the reactor, and styrene is added with thorough mixing along with polymerization initiators. The resulting mixture is heated to 85-95 degrees. The monomer dissolved in soap micelles, coming from the drops of the emulsion, begins to polymerize. This is how polymer-monomer particles are obtained. During 20% of the reaction time micellar soap goes to the formation of adsorption layers. Further, the process goes inside the polymer particles. The reaction is completed when the content of styrene in the mixture is approximately 0.5%.

Next, the emulsion enters the precipitation stage, which makes it possible to reduce the content of residual monomer. For this purpose, it is coagulated with a salt solution (table) and dried. The result is a powdery mass with a particle size of up to 0.1 mm. The remainder of the alkali affects the quality of the resulting material. It is impossible to completely eliminate impurities, and their presence causes a yellowish tint of the polymer. This method allows to obtain the polymerization product of styrene with the highest molecular weight. The substance obtained in this way has the designation PSE, which can be periodically found in technical documents and old textbooks on polymers.

Suspension (PSS)

This method is carried out according to a periodic scheme, in a reactor equipped with a stirrer and a heat-removing jacket. To prepare styrene, it is suspended in chemically pure water using emulsion stabilizers (polyvinyl alcohol, sodium polymethacrylate, magnesium hydroxide), as well as polymerization initiators. The polymerization process takes place under pressure, with a constant increase in temperature, up to 130 ° C. The result is a suspension from which virgin polystyrene is separated by centrifugation. After that, the substance is washed and dried. This method is also deprecated. It is suitable mainly for the synthesis of styrene copolymers. It is mainly used in the production of expanded polystyrene.

Block (PSM)

Obtaining general purpose polystyrene within the framework of this method can be carried out according to two schemes: complete and incomplete conversion. Thermal polymerization according to a continuous scheme is carried out on a system consisting of 2-3 series-connected column apparatus-reactors, each of which is equipped with a stirrer. The reaction is carried out in stages, increasing the temperature from 80 to 220 °C. When the degree of styrene conversion reaches 80-90%, the process stops. With the method of incomplete conversion, the degree of polymerization reaches 50-60%. The remains of unreacted styrene monomer are removed from the melt by vacuum, bringing its content to 0.01-0.05%. The polystyrene obtained by the block method is characterized by high stability and purity. This technology is the most efficient, also because it has practically no waste.

Application of polystyrene

The polymer is produced in the form of transparent cylindrical granules. They are turned into final products by extrusion or casting, at a temperature of 190-230 °C. A large number of plastics are produced from polystyrene. It gained distribution due to its simplicity, low price and a wide range of brands. A lot of items are obtained from the substance, which have become an integral part of our daily life (children's toys, packaging, disposable tableware, and so on).

Polystyrene is widely used in construction. Heat-insulating materials are made from it - sandwich panels, slabs, fixed formwork, etc. In addition, finishing decorative materials are produced from this substance - ceiling baguettes and decorative tiles. In medicine, the polymer is used to produce disposable instruments and some parts in blood transfusion systems. Expanded polystyrene is also used in water treatment systems. The food industry uses tons of packaging material made from this polymer.

There is also high-impact polystyrene, the formula of which is changed by adding butadiene and styrene-butadiene rubber. This type of polymer accounts for more than 60% of the total production of polystyrene plastic.

Due to the extremely low viscosity of the substance in benzene, mobile solutions can be obtained at marginal concentrations. This causes the use of polystyrene as part of one of the types of napalm. It plays the role of a thickener, in which, as the molecular weight of polystyrene increases, the viscosity-temperature dependence decreases.

Advantages

White thermoplastic polymer can be an excellent replacement for PVC plastic, and transparent - for plexiglass. The substance gained popularity mainly due to its flexibility and ease of processing. It is perfectly formed and processed, prevents heat loss and, importantly, has a low cost. Due to the fact that polystyrene can transmit light well, it is even used in building glazing. However, it is impossible to place such glazing on the sunny side, since the substance deteriorates under the influence of ultraviolet radiation.

Polystyrene has long been used to make foams and related materials. The heat-insulating properties of polystyrene in the foamed state make it possible to use it for insulation of walls, floors, roofs and ceilings, in buildings for various purposes. It is thanks to the abundance of insulation materials, headed by polystyrene foam, that ordinary people know about the substance we are considering. These materials are easy to use, resistant to decay and aggressive environments, as well as excellent thermal insulation properties.

Flaws

Like any other material, polystyrene has disadvantages. First of all, it is environmental insecurity (we are talking about the lack of safe disposal methods), fragility and fire hazard.

Recycling

Polystyrene itself does not pose a danger to the environment, however, some products derived from it require special handling.

Waste material and its copolymers accumulate in the form of obsolete products and industrial waste. Recycling of polystyrene plastics is done in several ways:

Disposal of industrial waste that has been heavily contaminated.
Processing of technological waste by casting, extrusion and pressing.
Disposal of worn out products.
Disposal of mixed waste.

The secondary use of polystyrene allows you to get new high-quality products from old raw materials without polluting the environment. One of the promising areas of polymer processing is the production of polystyrene concrete, which is used in the construction of low-rise buildings.

Polymer degradation products formed during thermal degradation or thermal-oxidative degradation are toxic. During the processing of the polymer, vapors of benzene, styrene, ethylbenzene, carbon monoxide, and toluene can be released by partial degradation.

Burning

When the polymer is burned, carbon dioxide, carbon monoxide and soot are released. In general, the reaction equation for the combustion of polystyrene looks like this: (C 8 H 8) n + O 2 \u003d CO 2 + H 2 O. The combustion of a polymer containing additives (components that increase strength, dyes, etc.) leads to the release of a series other harmful substances.

Block polystyrene is produced by bulk polymerization. The polymerization of styrene in the mass (block) is now widely used. It can be carried out in the presence and in the absence of the initiator.

polymerization initiators but are usually benzoyl peroxide, azobisisobutyric acid dinitrile, etc. The decomposition products of the initiators are part of polystyrene macromolecules, as a result of which it is not possible to obtain polystyrene with high dielectric values by this method.

In industry, to obtain high-purity polystyrene, polymerization is carried out without an initiator (thermal polymerization).

The kinetics of radical polymerization of styrene up to deep conversions has been studied much more completely than the kinetics of polymerization of other monomers. This makes it possible to very accurately calculate the temperature regime of polymerization in order to obtain polystyrene with desired properties.

Thermal polymerization of styrene to complete conversion monomer continuous way in column-type apparatuses without stirring (the principle of "ideal" displacement) is not currently used, since this process has a number of serious drawbacks. The main disadvantages of the technological process of polymerization of styrene in bulk with complete conversion of the monomer are its long duration, the need to carry out the process at high temperatures. (200-230 °С) at the final stages to achieve high conversion (99%), as well as obtaining a polymer with a low molecular weight (Figure 1) and a wide molecular weight distribution. In addition, with the depth of conversion, the viscosity of the reaction mass increases greatly, reaching by the end of the process 1 10 3 – 1 10 4 Pa s. Carrying out thermal polymerization of styrene up to incomplete monomer conversion (80-95%) in a cascade of apparatus with stirring (the principle of "ideal" mixing) and the removal of residual monomer allows the reaction to be carried out at lower temperatures (140-160 °С) and receive polystyrene from narrower molecular weight distribution. This provides a significant intensification of the process and the production of higher quality polystyrene.

Industrial processes for the polymerization of styrene to incomplete conversion of the monomer were developed using mathematical modeling methods.

The first step in modeling the process is a mathematical description (model) of the reaction of thermal polymerization of styrene. To calculate industrial processes, not a complete kinetic model can be used, but the dependence of the overall reaction rate on conversion.

For polystyrene in the range of working temperatures 110-150 °С the molecular weight of the polymer depends only on temperature and does not depend on the degree of monomer conversion:

The second stage of process modeling is the mathematical description of reactors for carrying out polymerization processes. It contains a description of the properties of the reaction medium and the heat exchange conditions in the reactor.

The properties of the reaction medium include:

viscosity,
thermal conductivity,
heat capacity,
vapor pressure over the polymer solution.

A feature of the polymerization of styrene is high viscosity of the reaction medium, which fluctuates in reactors from 1 before 1 10 3 Pa s.

To ensure a given heat transfer in reactors, mixers of a certain type are used and power costs for mixing are calculated. When converting to 40% and viscosity of the reaction medium up to 10 Pa s apply sheet agitators(in the first reactor), at higher viscosities become advantageous spiral (ribbon) mixers.

One of the main issues in polymerization in an isothermal reactor is heat dissipation. A high intensity of the styrene polymerization process can be achieved with heat removal by evaporation and return of the monomer to polymerization. In addition, partial heat removal is carried out through the jacket of the apparatus. The required temperature difference between the reaction mass and the coolant in the reactor jacket is determined from the heat balance equation

Q E + Q N - Q BX -Q X \u003d 0

where Q e- heat of exothermic reaction; Qn- heat released during the operation of the mixer; QBX- heat spent on heating the input stream of the reaction medium; Qx- heat removal through the reactor wall.

To ensure a stable regime in the reactor, the following condition must be observed: the change in heat removal depending on temperature must occur faster than the change in heat release.

After determining the conditions for stable operation of the reactors, the question of the possibility of controlling them and the choice of appropriate means of automatic control is decided.

Currently block polymerization of styrene until incomplete conversion of the monomer into a polymer is carried out in a cascade of stirred reactors according to two options:

in the absence of solvents;
using solvents.

Production general purpose block polystyrene carried out in the presence of ethylbenzene (15-20%), the presence of which in the process facilitates heat removal, operation of equipment, especially pumps, due to a decrease in the viscosity of the reaction mass, as well as process control as a whole.

Below are descriptions of technological processes for obtaining general-purpose block polystyrene.

Production of block general purpose polystyrene up to partial conversion of the monomer in a cascade of stirred reactors

The technological scheme for the production of block general-purpose polystyrene in a cascade of two stirred reactors has become the most widely used. The process includes stages:

preparation of initial styrene,
styrene polymerization in reactors of the 1st and 2nd stages,
removal and rectificationunreacted monomer,
polystyrene melt dyeing,
polystyrene granulation,
packaging and packaging of polystyrene granules.

The scheme for producing block polystyrene in a cascade of stirred reactors is shown in Figure 1.

From containers 1 styrene is continuously fed by a dosing pump into 1st stage reactor, which is a vertical cylindrical apparatus with a conical bottom with a capacity of 16 m 3 . The reactor is equipped with a sheet agitator with a speed of 30-90 rpm. Polymerization in reactor 1st stage 2 running at a temperature 110-130 °С before conversions 32-45% depending on the brand of the product. Removal of excess heat of reaction occurs due to the evaporation of part of the styrene from the reaction mass.

Reactor 2nd stage 3 similar in design and dimensions to the 1st stage reactor, but equipped with a ribbon stirrer with a speed of 2- 8 rpm. This ensures efficient mixing of highly viscous reaction media. Polymerization in the 2nd stage reactor proceeds up to 75- 88% conversion rates at temperature 135-160 °С depending on the grade of the resulting polymer.

Solution of polystyrene in styrene from the 2nd stage reactor unloading pump 5 served in vacuum chamber 6 through a pipe heated with steam at a pressure of at least 2.25 MPa. At the same time, it happens prepolymerization styrene up to 90% conversion rate.

The polystyrene melt enters vacuum chamber 6 with temperature 180-200 °С. In the tube of the superheater of the vacuum chamber, the polystyrene melt is heated up to 240 °C and enters a hollow chamber with a volume of 10 m 3 with a residual pressure of 2.0-2.6 kN/m 2 . When this occurs, the evaporation of styrene from the melt and the content of residual monomer is reduced to 0.1-0.3%. Styrene vapors are sent for regeneration and then returned to capacity 1.

Polystyrene melt from vacuum chambers 6 goes to extruder 7 and for granulation.

Upon receipt of general purpose polystyrene in the presence of ethylbenzene, the latter is in a closed cycle mixed with styrene. The volume of excess heat of reaction in the devices is carried out by evaporation under vacuum of a part of styrene and ethylbenzene. The evaporated mixture condenses and returns to the reaction zone. To maintain the normal operation of the stirrers in the polymerizers, the viscosity of the reaction mass is continuously monitored. The specified viscosity is maintained automatically by changing the supply of a mixture of styrene and ethylbenzene.

Both polymerizers operate under vacuum, the process temperature fluctuates at 115-135 °С and 140-160 °С respectively. Polymer content in 1st stage reactor reaches 30-40% , in reactor of the 2nd stage - 65-70%. The solution contains 15-20% ethylbenzene. From the reactor of the 2nd stage, the polymer solution enters the evaporator, in which a vacuum is maintained (residual pressure of about 2.6 kPa). Vapors of styrene and ethylbenzene are removed, and the polymer melt is collected in the lower part of the evaporator, from where 200-230 °С sent for staining and granulation.

Vapors of styrene and ethylbenzene from the evaporator enter the scrubber for cleaning, then condense and return to the original container of styrene and ethylbenzene.

Thus, the technological scheme for the production of block general purpose polystyrene using ethylbenzene in the process differs from the technological scheme shown in Figure 1, only scrubber and styrene and ethylbenzene vapor condenser.

Comparative Evaluation of Block Polymerization of Styrene with Complete and Incomplete Monomer Conversion

The method of block polymerization of styrene with incomplete monomer conversion has a number of advantages over the method of block polymerization with complete conversion of styrene:

1) the productivity of the polymerization unit is increased by more than 2 times due to the reduction in the duration of polymerization, which leads to a decrease in capital investments and energy costs;

2) hardware design allows you to adjust the technological parameters of the process and obtain products of different quality depending on the requirements of the consumer;

3) polystyrene leaving the vacuum chamber contains less residual monomer (up to 0.2%) than the product leaving the column with complete monomer conversion (0.5%).

However, when carrying out the process with incomplete conversion of the monomer, waste products are inevitable - styrene stripping condensates. When implementing large-scale production, it becomes necessary to use stripping condensates. With a total production capacity of 100-120 thousand tons/year of polystyrene, about 10-12 thousand tons/year of stripping condensates are obtained.

Utilization of stripping condensates is carried out in two directions:

1) purification of stripping condensates to obtain styrene of standard purity (rectification);

2) polymerization of distilled condensates to obtain polystyrene of somewhat poorer quality, but which can be used for the production of less critical products. Both directions are developing in the industry.

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Task 449 (sh)
How is styrene produced in industry? Give the scheme of its polymerization. Draw linear and three-dimensional structures of polymers using diagrams.
Solution:

Obtaining and polymerization of styrene

Most styrene(about 85%) in the industry receive dehydrogenation m ethylbenzene at a temperature of 600-650°C, atmospheric pressure and diluted with superheated water vapor by 3-10 times. Oxide iron-chromium catalysts with the addition of potassium carbonate are used.

The other industrial method by which the remaining 15% is obtained is through dehydration. methylphenylcarbinol, formed during the production of propylene oxide from ethylbenzene hydroperoxide. Ethylbenzene hydroperoxide is obtained from ethylbenzene by non-catalytic oxidation of air.

Scheme of anionoid polymerization of styrene:

Polystyrene- thermoplastic amorphous polymer with the formula:

[CH 2 \u003d C (C 6 H 5) H] n------------> [-CH 2 - C (C 6 H 5) H -] n
styrene polystyrene

Styrene polymerization occurs under the action of sodium or potassium amides in liquid ammonia.

Polymer structures:

Peculiarity linear and branched polymers- absence of primary (chemical) bonds between macromolecular chains; special secondary intermolecular forces act between them.

Linear polymer molecules:

Branched linear molecules:

If a macromolecular chains are interconnected by chemical bonds that form a series of transverse bridges (three-dimensional frame), then the structure of such a complex macromolecule is called spatial. Valence bonds in spatial polymers diverge randomly in all directions. Among them are polymers with a rare arrangement of cross-links. These polymers are called network.

Three-dimensional structures of polymers:

Network structure of the polymer:

Polystyrene

Rice. 1. Linear structure of polystyrene

Polyorganosiloxane

Rice. 2. Three-dimensional structure of polyorganosiloxane

Synthetic polymers

In the twentieth century, the emergence of synthetic high-molecular compounds - polymers - was a technical revolution. Polymers are widely used in various practical fields. Based on them, materials were created with new, in many respects, unusual properties, significantly superior to previously known materials.

Polymers are compounds whose molecules consist of repeating units - monomers.

known natural polymers . These include polypeptides and proteins, polysaccharides, nucleic acids.

Synthetic polymers obtained by polymerization and polycondensation (see below) of low molecular weight monomers.

Structural classification of polymers

a) linear polymers

They have a linear chain structure. Their names are derived from the name of the monomer with the addition of the prefix poly-:

b) network polymers:

c) networked three-dimensional polymers:

Copolymerization of various monomers gives copolymers . For example:

The physicochemical properties of polymers are determined by the degree of polymerization (value n) and the spatial structure of the polymer. They can be liquids, gums or solids.

Solid polymers behave differently when heated.

Thermoplastic polymers- when heated, they melt and after cooling they take any given shape. This can be repeated an unlimited number of times.

Thermoset polymers- These are liquid or plastic substances that, when heated, solidify in a given form and do not melt when heated further.

Polymer formation reactions polymerization

Polymerization is the sequential attachment of monomer molecules to the end of a growing chain. In this case, all monomer atoms are part of the chain, and nothing is released during the reaction.

To start the polymerization reaction, it is necessary to activate the monomer molecules with the help of an initiator. Depending on the type of initiator, there are

radical

cationic and

anionic polymerization.

Radical polymerization

Substances capable of forming free radicals during thermolysis or photolysis are used as initiators of radical polymerization, most often these are organic peroxides or azo compounds, for example:

When heated or illuminated with UV light, these compounds form radicals:

The polymerization reaction includes three stages:

initiation,

chain growth,

Chain break.

An example is the polymerization of styrene:

reaction mechanism

a) initiation:

b) chain growth:

c) open circuit:

Radical polymerization proceeds most easily with those monomers in which the resulting radicals are stabilized by the influence of substituents at the double bond. In the example given, a benzyl-type radical is formed.

Radical polymerization produces polyethylene, polyvinyl chloride, polymethyl methacrylate, polystyrene and their copolymers.

Cationic polymerization

In this case, the activation of monomeric alkenes is carried out by protic acids or Lewis acids (BF 3 , AlCl 3 , FeCl 3) in the presence of water. The reaction proceeds as an electrophilic addition to the double bond.

For example, the polymerization of isobutylene:

reaction mechanism

a) initiation:

b) chain growth:

c) open circuit:

Cationic polymerization is typical for vinyl compounds with electron-donating substituents: isobutylene, butylvinyl ether, α-methylstyrene.

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Polymer structures:

Linear polymer molecules:

Branched linear molecules:

Three-dimensional structures of polymers:

Network structure of the polymer:

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Synthetic polymers

Structural classification of polymers

Polymer formation reactions polymerization

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