Compared to block polymerization, the solution polymerization reaction proceeds at a lower rate (provided there are no initiators) and the resulting polymer has a lower molecular weight. Average molecular weight polystyrene depends on the polymerization conditions and the type of solvent. The molecular weight values can be adjusted by selecting the type and amount of solvent and the reaction temperature.

Basic Principles of Solvent Chain Transfer were formulated by Flory, but Mayo expanded them and introduced the concept of “transfer constant,” which he considered as the quotient of the rate constants for chain transfer by a solvent and the rate of chain growth. The growth rate constants are close to each other in different solvents, but the chain transfer constants, and therefore the degree of polymerization, differ noticeably.

When polymerizing styrene in benzene, cyclohexane, tert-butylbenzene and toluene, it is possible to obtain polymers with a higher molecular weight than when polymerizing in other solvents, since the chain transfer constants have the lowest value (Table 1).

Obtaining polymers in solution is convenient for making varnishes. For other purposes, the polymer is precipitated from solution by adding a precipitant in which the monomer dissolves, but the polystyrene does not dissolve. As such solvents - precipitants use petroleum hydrocarbons, methanol and ethanol.

Other methods for isolating polystyrene from solution involve distilling off the solvent under reduced pressure or distilling it with steam. With any of these methods, complete removal of the solvent requires prolonged drying of the polymer in vacuum.

In industry the process of polymerization of styrene in solution can be carried out as periodic, so continuous methods.

Periodic method includes three stages of production:

1) polymerization in a reactor;

2) isolation of the polymer from solution;

3) crushing and coloring of the polymer.

Continuous method consists of the same stages, but differs in that, starting from the supply of styrene and solvent and ending with the unloading of powdered polymer from the collector, it proceeds continuously (Fig. 1).

Fig.1. Continuous solution polymerization method of styrene

Styrene from meter 1 and solvent from meter 2 are mixed in a certain ratio in pump 3 and supplied to polymerization columns 4, 5 and 6, operating in series. All columns are equipped with stirrers and jackets for heating and cooling. During the polymerization process, a large amount of heat is released and the viscosity of the solution increases significantly. In each column, the temperature in three zones is controlled and automatically adjusted in accordance with the specified mode. At the beginning of the process (top of column 4), it is necessary to heat the mixture of styrene in the solvent to the polymerization temperature, and in the remaining two zones of column 4 and three zones of column 5, the heat of reaction must be removed. In column 6, polymerization proceeds slowly, so external heat is required.

A viscous solution of polystyrene in a solvent from the column enters evaporator 7. Before entering this apparatus, the solution flow is distributed into separate jets (up to 20). At 225 o C, the evaporator removes the solvent and unreacted monomer, which, after condensation and appropriate purification, are returned to production. The figure shows the return of solvent to measuring cup 2.

After removing the liquid components of the solution, polystyrene in the form of a softened mass is sent to the extrusion machine 8. For each jet, both a separate extrusion machine and all subsequent equipment are provided. At the exit of the extrusion machine, polystyrene strips are cooled with water in bath 9, then crushed using a crusher 10. The crushed polymer is supplied to the lubricator 11 using pneumatic transport, then poured into the collection 13. Then the powdered polymer is poured into bags and weighed.

The quality of the finished product is controlled by the viscosity of a 10% solution in toluene, the softening temperature and the content of volatile compounds in it.

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

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

Monomer		The resulting polymer
Structural formula	Name options	Structural formula	Name options
	ethylene, ethene		polyethylene
	propylene, propene		polypropylene
	styrene, vinylbenzene		polystyrene, polyvinylbenzene
	vinyl chloride, vinyl chloride, chlorethylene, chloroethene		polyvinyl chloride (PVC)
	tetrafluoroethylene (perfluoroethylene)		teflon, polytetrafluoroethylene
	isoprene (2-methylbutadiene-1,3)		isoprene rubber (natural)
	butadiene-1,3 (divinyl)		butadiene rubber, polybutadiene-1,3
	chloroprene(2-chlorobutadiene-1,3)		chloroprene rubber
	butadiene-1,3 (divinyl) styrene (vinylbenzene)		styrene butadiene rubber

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).

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 arrangement 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:

Initiation- formation of active centers;
Chain growth- sequential addition of monomers to the active center;
Open circuit- death of the active center;
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

The 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 growth and chain termination rates:

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 us 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. The 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?

The coupling effect is destroyed;
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.

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

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

Synthetic polymers

In the twentieth century, the emergence of synthetic high-molecular compounds - polymers - was a technical revolution. Polymers are very widely used in a wide variety of practical fields. Based on them, materials were created with new and, in many ways, 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, and nucleic acids.

Synthetic polymers are 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) network three-dimensional polymers:

By joint polymerization of various monomers one obtains copolymers . For example:

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

Solid polymers behave differently when heated.

Thermoplastic polymers– melt when heated and, after cooling, 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 shape and do not melt upon further heating.

Reactions of polymer formation polymerization

Polymerization - This is the sequential addition of monomer molecules to the end of the growing chain. In this case, all monomer atoms are included in the chain, and nothing is released during the reaction.

To start the polymerization reaction, it is necessary to activate the monomer molecules using 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

Circuit break.

Example - polymerization of styrene:

Reaction mechanism

a) initiation:

b) chain growth:

c) open circuit:

Radical polymerization occurs 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 occurs as an electrophilic addition at a double bond.

For example, polymerization of isobutylene: