Styrene polymerization reaction equation. Block polystyrene (polymerization of styrene in bulk). III. Open circuit

Lab 1

Polymerization of styrene in solution

Theoretical part

There are two options polymerization in solution:

1. polymer and monomer are soluble in solvent;

2. Only the monomer is soluble in the solvent, and the polymer precipitates as it is formed.

Practical part

Exercise.

Write equations for the chemical reactions that occur during the polymerization of styrene in solution. Carry out the polymerization of styrene at 90-95°C for 4 hours according to two recipes (d): a) styrene -20.0; benzoyl peroxide - 0.4; benzene-10.0 g; b) styrene-20.0; benzoyl peroxide-0.4; carbon tetrachloride-10.0 Isolate the polymer and determine its yield (in grams and %) for each formulation Determine the rate of polymerization in different solvents Check the solubility of the resulting polymer in organic solvents, its relationship to heat, the action of acids and bases Carry out depolymerization of polystyrene. Calculate styrene yield

Stage 1 of work. Synthesis of polystyrene in various solvents.

Reagents

Styrene (freshly distilled), 20.0 g

Benzoyl peroxide, 0.4 g

Benzene, 10.0 g

Carbon tetrachloride, 10.0 g

Petroleum ether, 100 ml

Ethanol

Concentrated sulfuric acid

Concentrated nitric acid

Sodium hydroxide, concentrated solution

Devices

Round bottom flask with a ground joint with a capacity of 100 ml - 2 pcs.

Ball reflux cooler – 2 pcs.

Vacuum pump

Chemical beaker, 200 ml

Porcelain evaporation cup – 2 pcs.

Petri dish - 2 pcs

Water bath or heating mantle

Electric stove

Conducting an experiment

Weights of styrene of 10.0 g each are placed in two flasks, 0.2 g of benzoyl peroxide, as well as solvents are added to them: 10.0 g of benzene in one, 10.0 g of carbon tetrachloride in the other. Each flask is connected to a reflux condenser and heated in a water bath or heating mantle at 90-95°C for 4 hours. Then the heating is turned off, the contents of each flask are cooled. Add petroleum ether or ethanol. A polymer precipitate appears. Check the completeness of precipitation. The polymer is washed with a precipitating agent. The precipitate is separated from the liquid, transferred to a weighed porcelain dish (Petri dish) and dried first at room temperature in air, and then in a thermostat at 60-70°C or in a vacuum drying cabinet at a temperature of 30-40°C to constant weight.*

* all operations: synthesis, precipitation and drying of the polymer can be carried out in one flask (pre-weighed). Use the resulting polymer for further experiments.

Present the results in the form of tables.

Table 1

table 2

Calculation example. Polymerization of styrene (molecular weight 104.14 g/mol; density ρ = 0.906 g/ml) was carried out in cyclohexane with the initiator AIBN (molecular weight 164.20 g/mol). Total loading volume 30 ml: 20 ml styrene and 10 ml cyclohexane. Initiator mass 0.6 g. Polymerization time 4 hours. The mass of the resulting polystyrene is 13.2 g.

1. Let's calculate mass and amount of the substance styrene:

mstyrene = 20 0.906 = 18.12 g

nctyrene = 18.12/104.14 = 0.174 mol

2. Calculate the % by weight of the initiator relative to the monomer:

ωDAK = (0.6/18.12) 100 = 3.31% wt (from styrene)

3. Find monomer concentration in solution:

s (styrene) = (18.12/30) 1000 = 604 g/l or 604/104.14 = 5.80 mol/l

4. Find initiator concentration in solution:

s(DAK) = (0.6/30) 1000 = 20 g/l or 20/164.20 = 0.122 mol/l

5. Let's calculate polystyrene yield:

Polystyrene yield = (13.2/18.12) 100 = 72.8%

6. Let's calculate polymerization speed:

υ = 72.8/4 = 18.2%/h or 18.2/60 = 0.303%/min

υ = (5.80 0.728)/(4 3600) = 29.32 10-5mol/l sec

Stage 2 of work. Determination of physical and chemical properties of polystyrene.

Experience 1. Appearance. Strength.

Carefully examine the polystyrene samples, pay attention to the color, test them for fragility.

*Polystyrene is transparent, can be of different colors, and is fragile. When shaken, polystyrene films produce a ringing sound, like a thin metal strip.

Experiment 2. Relation to heating

A thin piece of polystyrene is placed on a heat-resistant mesh and heated slightly. At a temperature of 80-90°C, polystyrene softens, and at >250°C it begins to decompose. A softened piece of polystyrene easily changes its shape under external influence. Threads can be drawn from softened polystyrene. If you connect two softened pieces of polystyrene, they are welded.

*Polystyrene is a thermoplastic (reversible plastic).

Experiment 3. Thermal insulation properties.

To study the thermal insulation properties, polystyrene foam is used. A piece of foam plastic (length 6-7 cm, thickness 4 cm) needs to be placed on an iron rod or wire 10 cm long. Holding the foam with your hand, bring the iron rod into the flame for 1-2 minutes. The heating of the rod and foam (it warms up a little) is set with a thermometer. First, they bring the foam to it, then the rod.

Experiment 4. Effect of solvents.

Small pieces of polystyrene or film are placed in separate test tubes with benzene, acetone, and carbon tetrachloride. Viscous solutions are obtained.

Polystyrene products can be glued with a viscous solution or solvent.

Experiment 5. Combustion of polystyrene

*The experiment is carried out in a fume hood!!

A piece of polystyrene is placed into the flame and held until it ignites.

*Polystyrene burns with a smoky flame, spreading a pungent odor. Outside the flame continues to burn.

Experiment 6. Action of acids and bases

Pieces of polystyrene are placed in concentrated acids: sulfuric (density 1.84 g/ml), nitric (density 1.4 g/ml), and then in a concentrated solution of sodium hydroxide. Observe what happens to polystyrene at room temperature and then when heated.

*Polystyrene at room temperature in concentrated acids and alkalis remains unchanged. When heated, it chars in sulfuric acid, but does not change in alkali and nitric acid.

Experiment 7. Depolymerization of polystyrene

Pieces of polystyrene are placed in a test tube to cover more than 1/5 of its volume. A gas outlet tube with a stopper is attached to the opening of the test tube. The receiver is another test tube placed in cold water and covered with cotton wool on top. The test tube with polystyrene is secured in a stand at an angle (to allow liquid to drain). It is better to make a hole in the rubber stopper closer to the edge to remove the resulting liquid (monomer with impurities). A colorless or yellowish liquid with a specific odor collects in the receiver. Styrene boils at a temperature of 141-146°C.

Among the wide variety of polymer materials, polystyrene occupies a special place. This material is used to produce a huge number of different plastic products for both household and industrial use. Today we will get acquainted with the formula of polystyrene, its properties, methods of production and directions of use.

general characteristics

Polystyrene is a synthetic polymer belonging to the class of thermoplastics. As the name suggests, it is a polymerization product of vinylbenzene (styrene). It is a hard glassy material. The general formula of 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 polystyrene formula is more common.

Chemical and physical properties

The presence of phenolic groups in the formula of the structural unit of polystyrene prevents the ordered arrangement 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, “cross-linking” occurs, resulting in the formation of styrene copolymers with higher structural qualities.

The substance has low moisture absorption and resistance to radioactive radiation. At the same time, it is destroyed under the influence of glacial acetic and concentrated nitric acids. When exposed to ultraviolet radiation, 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 monomer. At the same time, starting from a temperature of 60 ° C, polystyrene loses its shape. At normal temperatures the substance is not toxic.

Basic properties of polystyrene:

1. Density - 1050-1080 kg/m3.
2. The minimum operating temperature is 40 degrees below zero.
3. The maximum operating temperature is 75 degrees Celsius.
4. Heat capacity - 34*10 3 J/kg*K.
5. Thermal conductivity - 0.093-0.140 W/m*K.
6. Thermal expansion coefficient is 6*10 -5 Ohm cm.

In industry, polystyrene is produced using radical polymerization of styrene. Modern technologies make it possible to carry out this process with a minimum amount of unreacted substance. The reaction to produce 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 received widespread industrial application. Emulsion polystyrene is produced by the polymerization of styrene in aqueous solutions of alkalis at a temperature of 85-95 °C. This reaction requires the following substances: water, styrene, an emulsifier and an initiator of the polymerization process. Styrene is first removed from inhibitors (hydroquinone and tributyl-pyrocatechol). The reaction initiators are water-soluble compounds. Typically, this is potassium persulfate or hydrogen dioxide. Alkalies, sulfonic acid salts and fatty acid salts are used as emulsifiers.

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

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

Suspension (PSS)

This method is carried out in a batchwise manner, in a reactor equipped with a stirrer and a heat-removing jacket. To prepare styrene, it is suspended in chemically pure water with the help of 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 primary polystyrene is separated by centrifugation. After this, the substance is washed and dried. This method is also considered obsolete. It is mainly suitable for the synthesis of styrene copolymers. It is used mainly in the production of expanded polystyrene.

Block (PSM)

The production of 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 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 incomplete conversion method, the degree of polymerization reaches 50-60%. The remains of unreacted styrene monomer are removed from the melt by vacuuming, bringing its content to 0.01-0.05%. The polystyrene produced by the block method is characterized by high stability and purity. This technology is the most effective, also because it has virtually no waste.

Application of polystyrene

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

Polystyrene is widely used in construction. Thermal insulation materials are made from it - sandwich panels, slabs, permanent 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. Foamed polystyrene is also used in water purification systems. The food industry uses tons of packaging material made from this polymer.

There is also impact-resistant polystyrene, the formula of which is changed by adding butadiene and butadiene styrene 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, it is possible to obtain mobile solutions in specific concentrations. This determines the use of polystyrene in 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 relationship decreases.

Advantages

White thermoplastic polymer can be an excellent replacement for PVC plastic, and transparent one can be an excellent replacement 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 the glazing of buildings. However, such glazing cannot be placed on the sunny side, since the substance deteriorates under the influence of ultraviolet radiation.

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

Flaws

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

Recycling

Polystyrene itself is not hazardous to the environment, but some products made from it require special handling.

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

1. Disposal of industrial waste that has been heavily contaminated.
2. Processing of technological waste using casting, extrusion and pressing methods.
3. Disposal of worn-out products.
4. Disposal of mixed waste.

Recycling polystyrene allows you to obtain 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 decomposition products formed during thermal destruction or thermal oxidative destruction are toxic. During polymer processing, vapors of benzene, styrene, ethylbenzene, carbon monoxide and toluene can be released through partial destruction.

Burning

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

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

Preparation and polymerization of styrene

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

Another industrial method by which the remaining 15% is obtained is by 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 = C (C 6 H 5) H] n------------> [-CH 2 - C(C 6 H 5)H -]n
styrene polystyrene

Polymerization of styrene 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 macromolecular chains are connected to each other by chemical bonds that form a series of cross bridges (a three-dimensional framework), 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 polymers.

Three-dimensional polymer structures:

Polymer network structure:

Polystyrene

Rice. 1. Linear structure of polystyrene

Polyorganosiloxane

Rice. 2. Three-dimensional structure of polyorganosiloxane

The polymerization reaction involves compounds that contain at least one multiple bond or rings. The reactivity of a monomer depends on its structure, the conjugation of the double bond in the monomer molecule, the number and relative position of substituents, and their polarization effect on the double bond.

Radical polymerization occurs via a chain mechanism and is described by the kinetics of an unbranched chain reaction.

The main stages of the chain reaction:

1. Initiation- formation of active centers;
2. Chain growth- sequential addition of monomers to the active center;
3. Open circuit- death of the active center;
4. Chain transmission- transfer of the active center to another molecule.

I. Chain initiation (nucleation)

This stage is the most energy-intensive. Distinguish physical And chemical initiation.

Physical initiation:

Chemical initiation

This initiation method is used most often. The principle is to use initiating substances(peroxides, azo compounds, red-ox systems), in which the energy of breaking a chemical bond is significantly less than that of monomers. In this case, the process occurs in two stages: first, initiator radicals are generated, which then join the monomer molecule, forming a primary monomer radical.

The initiator is very similar in properties to the catalyst, but its difference is that the initiator is expended during a chemical reaction, but a catalyst does not.

Examples of initiators:

II. Growth of the Chain

Monomers alternately attach to the active center of the primary monomer radical.

III. Open circuit

Chain termination occurs as a result of the death of active centers (kinetic chain termination).

• Break in the kinetic chain- active centers disappear;
• Break in the material chain- when a given chain stops growing, but the active center is transferred to another macromolecule or monomer (chain transfer reaction).

Reactions leading to the death of the kinetic and material chain - reactions recombination And disproportionation.

The type of chain termination reaction (recombination or disproportionation) depends on a number of factors, in particular on the structure of the monomer molecule. If the monomer contains a substituent that is bulky in size or electronegative in chemical nature, then such growing radicals do not collide with each other and chain termination occurs through disproportionation. For example, in the case of methyl methacrylate:

As the radicals grow, the viscosity of the system increases, and due to the mobility of macroradicals, the rate of chain termination by recombination decreases. An increase in the lifetime of macroradicals with an increase in the viscosity of the system leads to an interesting phenomenon - acceleration of polymerization at later stages ( gel effect) due to an increase in the concentration of macroradicals.

IV. Chain transmission

Chain transfer occurs by the detachment of an atom or group of atoms from a molecule by a growing radical. The chain transfer reaction leads to the break of the material chain, and the growth of the kinetic chain continues.

Chain transmissions are distinguished:

Features of radical polymerization:

• High polymerization rate;
• Branching;
• Connections g-g, g-xv, xv-xv are possible;
• Polymolecular polymers.

Kinetics of radical polymerization

Chemical kinetics is a branch of chemistry that studies the mechanism and patterns of a chemical reaction over time, and the dependence of these patterns on external conditions.

To study the kinetics of radical polymerization, it is necessary to consider the dependence of the reaction rate and degree of polymerization on the concentration of starting substances, pressure and temperature.

Designations:

I. The influence of the concentration of starting substances on the reaction rate.

The overall reaction rate depends on the rate of formation of radicals V in (rate of initiation), on the rate of chain growth V r and its termination V o.

We will consider the reaction of free radical polymerization, when initiation is carried out using chemical initiators.

Let's look at each stage:

Consideration of kinetics is greatly facilitated if the reaction occurs under conditions close to stationary mode, at which the rates of appearance and disappearance of free radicals can be considered equal. In this case, the concentration of active centers will be constant.

As can be seen from the curve graph, five sections can be distinguished according to the rates of the main reaction of converting a monomer into a polymer as a result of polymerization:

1 - inhibition site, where the concentration of free radicals is low. And they cannot start the chain polymerization process;

2 - polymerization acceleration section, where the main reaction of converting monomer into polymer begins, and the speed increases;

3 - stationary area, where polymerization of the main amount of monomer occurs at a constant speed (straight-line dependence of conversion on time);

4 - reaction slowdown section, where the reaction rate decreases due to a decrease in the free monomer content;

5 - cessation of the main reaction after exhaustion of the entire amount of monomer. The stationary mode is usually observed at the initial stage of the reaction, when the viscosity of the reaction mass is low and cases of chain nucleation and chain termination are equally likely.

Thus, the rate of the chain growth reaction is:

II. The influence of the concentration of starting substances on the degree of polymerization.

The degree of polymerization depends on the ratio of the rates of growth and chain termination:

Let us take into account the corresponding expressions for speeds

The degree of polymerization is:

III. Effect of temperature on the rate of chain propagation reaction.

Let us substitute the Arrhenius equation into the chain growth rate equation:

Let's take the logarithm of the resulting expression:

The numerator (6+15-4 = 17) is greater than zero, which means that the higher the temperature, the higher the rate of radical polymerization reaction. However, as the temperature increases, the probability of radicals colliding with each other (chain termination by disproportionation or recombination) or with low molecular weight impurities also increases. As a result, the molecular weight of the polymer as a whole decreases, and the proportion of low molecular weight fractions in the polymer increases. The number of side reactions leading to the formation of branched molecules increases. Irregularity in the construction of the polymer chain increases due to an increase in the proportion of head-to-head and tail-to-tail monomer connection types.

Growth activation energy ~ 6 kcal/mol;

Initiation activation energy ~30 kcal/mol;

The termination activation energy is ~8 kcal/mol.

The numerator (6-15-4 = -13) is less than zero, which means that with increasing temperature the degree of polymerization decreases. As a result, the molecular weight of the polymer as a whole decreases, and the proportion of low molecular weight fractions in the polymer increases.

V. Effect of pressure on the polymerization rate

Le Chatelier's principle: If a system is exposed to an external influence, then processes are activated in the system that weaken this influence.

The higher the pressure, the higher the rate of radical polymerization. However, to influence the properties of condensed systems, pressure of several thousand atmospheres must be applied.

A feature of polymerization under pressure is that the increase in speed is not accompanied by a decrease in the molecular weight of the resulting polymer.

Polymerization inhibitors and retarders.

The phenomena of open circuit and transmission are widely used in practice for:

• preventing premature polymerization during storage of monomers;
• to regulate the polymerization process

In the first case, they add to the monomers inhibitors or stabilizers, which cause chain termination and themselves turn into compounds that are unable to initiate polymerization. They also destroy peroxides formed when the monomer reacts with atmospheric oxygen.

Inhibitors: quinones, aromatic amines, nitro compounds, phenols.

Regulators polymerization causes premature termination of the material chain, reducing the molecular weight of the polymer in proportion to the amount of regulator introduced. An example of these are mercaptans.

Thermodynamics of radical polymerization

The chain growth reaction is reversible; along with the addition of the monomer to the active center, its elimination-depolymerization can also occur.

The thermodynamic possibility of polymerization, like any other equilibrium chemical process, can be described using the Gibbs and Helmholtz functions:

However, the Gibbs function is closest to real conditions, so we will use it:

Also, the change in the Gibbs function is related to the equilibrium constant of the reaction by the equation:

The constant of polymerization-depolymerization equilibrium at a sufficiently large molecular weight of the resulting polymer (p>>1) depends only on the equilibrium concentration of the monomer:

Whence it follows that

From equation (a) you can find the temperature at which the polymerization reaction will not occur, and from equation (b) you can find the equilibrium concentration of the monomer, above which polymerization will occur.

Effect of temperature

To determine the effect of temperature on the equilibrium concentration, we present equation (b) as follows:

In the case where ΔH°<0 и ΔS°<0 с ростом температуры увеличивается равновесная концентрация мономера. Верхний предел ограничен концентрацией мономера в массе. Это значит, что есть некоторая верхняя предельная температура - Т в.пр. , выше которой полимеризация невозможна.

In the case when ΔH°>0 and ΔS°>0 an inverse relationship is observed: with decreasing temperature, the equilibrium concentration of the monomer increases. Consequently, for monomers with a negative thermal effect there is a lower limiting temperature T n.a.

There are also known cases when these dependencies do not intersect, but they are not of practical interest.

Thermodynamic probability

Now consider the thermodynamic possibility of a reaction occurring, the condition for which is the equality ΔG<0. Оно определяется как изменением энтальпии так и энтропии, причем вклад энтропийного члена будет изменяться с температурой реакции.

During polymerization along multiple bonds, the entropy of the system always decreases, i.e. the process is unprofitable for entropic reasons. The weak dependence of ∆S° on the nature of the monomer is due to the fact that the main contribution to ∆S° comes from the loss of translational degrees of freedom of the monomer molecules.

But monomers are also known for which an increase in entropy occurs during polymerization. This change in ∆S° is typical for some unstressed cycles. Moreover, since polymerization turns out to be beneficial from an entropic point of view, it can occur even with negative thermal effects (polymerization of the S 8 and Se 8 cycles with the formation of linear polymers)

Calculations and entropy measurements for the polymerization of most vinyl monomers show that ∆S° is about 120 J/K mol.

On the contrary, ∆Н° varies depending on the chemical structure of the monomer over a fairly wide range (∆Q° = −∆Н° varies from several kJ/mol to 100 kJ/mol), which is due to the difference in the nature of the multiple bond and its substituents. Negative values ​​of ∆Н° indicate that polymerization is beneficial from the point of view of the enthalpy factor. At ordinary temperatures of the order of 25°C, polymerization is thermodynamically resolvable for monomers whose thermal effect exceeds 40 kJ/mol. This condition is met for most vinyl monomers. However, during polymerization at the C=O bond, the thermal effects are below 40 kJ/mol. Therefore, the condition ∆G<0 соблюдается только при достаточно низких температурах, когда |TΔS°|<|ΔH°|.

Let us consider the phenomenon of discrepancy between the theoretical and practical enthalpy of polymerization

Less energy is released, where does it go?

1. The coupling effect is destroyed;
2. Steric repulsion (during the synthesis of polystyrene, a helical molecule is formed due to steric repulsion).

The reason for the increase in Q during the polymerization of rings is the thermodynamically unfavorable bond angle between hybridized orbitals and the repulsion of lone electron pairs of the substituent.

1. Cycle opening (ΔS 1° > 0)
2. Chain growth (ΔS 2°< 0)

ΔS° = ΔS 1° + ΔS 2°, ΔS° can be greater or less than zero.

High molecular weight compounds (HMCs) Compounds with a molecular weight greater than 10,000 are called.

Almost all high molecular weight substances are polymers.

Polymers- these are substances whose molecules consist of a huge number of repeating structural units connected to each other by chemical bonds.

Polymers can be produced through reactions that can be divided into two main types: these are polymerization reactions And polycondensation reactions.

Polymerization reactions

Polymerization reactions - These are reactions of polymer formation by combining a huge number of molecules of a low molecular weight substance (monomer).

Number of monomer molecules ( n), combining into one polymer molecule, are called degree of polymerization.

Compounds with multiple bonds in molecules can enter into a polymerization reaction. If the monomer molecules are identical, then the process is called homopolymerization, and if different - copolymerization.

Examples of homopolymerization reactions, in particular, are the reaction of the formation of polyethylene from ethylene:

An example of a copolymerization reaction is the synthesis of styrene-butadiene rubber from 1,3-butadiene and styrene:

Polymers produced by the polymerization reaction and starting monomers

Polycondensation reactions

Polycondensation reactions- these are reactions of the formation of polymers from monomers, during which, in addition to the polymer, a low molecular weight substance (most often water) is also formed as a by-product.

Polycondensation reactions involve compounds whose molecules contain any functional groups. In this case, polycondensation reactions, based on whether one monomer or more is used, similar to polymerization reactions, are divided into reactions homopolycondensation And copolycondensation.

Homopolycondensation reactions include:

* formation (in nature) of polysaccharide molecules (starch, cellulose) from glucose molecules:

* reaction of formation of capron from ε-aminocaproic acid:

Copolycondensation reactions include:

* reaction of formation of phenol-formaldehyde resin:

* reaction of formation of lavsan (polyester fiber):

Polymer-based materials

Plastics

Plastics- materials based on polymers that are capable of being molded under the influence of heat and pressure and maintaining a given shape after cooling.

In addition to the high molecular weight substance, plastics also contain other substances, but the main component is still the polymer. Thanks to its properties, it binds all components into a single whole mass, and therefore it is called a binder.

Depending on their relationship to heat, plastics are divided into thermoplastic polymers (thermoplastics) And thermosets.

Thermoplastics- a type of plastic that can repeatedly melt when heated and solidify when cooled, making it possible to repeatedly change their original shape.

Thermosets- plastics, the molecules of which, when heated, are “stitched” into a single three-dimensional mesh structure, after which it is no longer possible to change their shape.

For example, thermoplastics are plastics based on polyethylene, polypropylene, polyvinyl chloride (PVC), etc.

Thermosets, in particular, are plastics based on phenol-formaldehyde resins.

Rubbers

Rubbers- highly elastic polymers, the carbon skeleton of which can be represented as follows:

As we see, rubber molecules contain double C=C bonds, i.e. Rubbers are unsaturated compounds.

Rubbers are obtained by polymerization of conjugated dienes, i.e. compounds in which two double C=C bonds are separated from each other by one single C-C bond.

1) butadiene:

In general terms (showing only the carbon skeleton), the polymerization of such compounds to form rubbers can be expressed by the following scheme:

Thus, based on the presented diagram, the isoprene polymerization equation will look like this:

A very interesting fact is that it was not the most advanced countries in terms of progress that first became acquainted with rubber, but the Indian tribes, who lacked industry and scientific and technological progress as such. Naturally, the Indians did not obtain rubber artificially, but used what nature gave them: in the area where they lived (South America), the Hevea tree grew, the juice of which contains up to 40-50% isoprene rubber. For this reason, isoprene rubber is also called natural, but it can also be obtained synthetically.

All other types of rubber (chloroprene, butadiene) are not found in nature, so they can all be characterized as synthetic.

However, rubber, despite its advantages, also has a number of disadvantages. For example, due to the fact that rubber consists of long, chemically unrelated molecules, its properties make it suitable for use only in a narrow temperature range. In the heat, rubber becomes sticky, even slightly runny and smells unpleasant, and at low temperatures it is susceptible to hardening and cracking.

The technical characteristics of rubber can be significantly improved by vulcanization. Vulcanization of rubber is the process of heating it with sulfur, as a result of which individual, initially unconnected, rubber molecules are “stitched” together with chains of sulfur atoms (polysulfide “bridges”). The scheme for converting rubbers into rubber using synthetic butadiene rubber as an example can be demonstrated as follows:

Fibers

Fibers are materials based on polymers of a linear structure, suitable for the manufacture of threads, tows, and textile materials.

Classification of fibers according to their origin

Man-made fibers(viscose, acetate fiber) are obtained by chemical treatment of existing natural fibers (cotton and flax).

Synthetic fibers are obtained mainly by polycondensation reactions (lavsan, nylon, nylon).