• Domestic science and medicine in the 19th - early 20th centuries Chemistry of the 19th century in medicine
• What are peptides? Types and types of peptides. All about peptides and anti-aging cosmetics with peptides Additional peptides are not formed
• Toxic effect on humans of hazardous chemicals
• Radioautography Method of autoradiography in cytology
• Identification of bacteria by antigenic structure
• Organic matter in wastewater What are organic compounds in water
• Hydrogen index (pH factor)
• What is the pH of water and why is it important to know it?
• Redox potential Redox systems
• Reversible redox system Reversible redox systems

Lecture 5. Cationic and anionic polymerization.

Differences from radical polymerization:

the growing chain is not a free radical, but a cation or anion; the catalyst is not consumed in the polymerization process and is not part of the polymer.

Depending on the macroion sign distinguish between cationic and anionic polymerization. At cationic polymerization:

· at the end of the growing chain there is + a charge that arises in the process of initiation and disappears when the chain breaks or is transferred.

At anionic polymerization :

the charge of the growing macroion is (negative).

Since instead of initiators in ionic polymerization, ionic initiators are used - catalysts, ionic polymerization is called catalytic .

Cationic polymerization

1877 carried out the polymerization of isobutylene in the presence of sulfuric acid.

Catalytic polymerization proceeds in the presence of acids (HCl, H3PO4, H2SO4) and Friedel-Crafts catalysts (AlCl3 , BF3, TiCl4, SnCl4, etc.). These substances are electron-withdrawing (electrophilic) and, by adding a monomer, they form a carbonium ion.

Schematically, the process can be depicted as follows:

The subsequent interaction of the carbonium ion with monomer molecules is a chain growth reaction, with growing chain herself is cation with increasing molecular weight during the reaction. Reaction chain growth is accompanied by a transfer of charge along the circuit.

chain break associated with proton splitting.

High molecular weight polymers can be obtained.

Of great importance are:

the nature of the catalyst

the nucleophilicity of the monomer.

Example: the polymerization of isobutylene in the presence of BF3 proceeds at low temperatures almost instantly and with an explosion; in the presence of Al F3 - within a few minutes; in the presence of TiCl3 - for several hours.

Features that distinguish cationic polymerization from radical:

· The molecular weight of the polymer decreases in the presence of small additions of water and other ionizable substances in the reaction medium and often does not depend on the concentration of the monomer.

· Polymerization is significantly accelerated when small additions of water, acids and other proton donors (co-catalysts) are used along with the catalyst. The maximum speed is achieved at a certain catalyst:cocatalyst ratio. The acceleration effect increases with the acidity of the cocatalyst. The addition of a cocatalyst in an amount not exceeding the stoichiometric ratio with the catalyst increases the rate of polymerization and reduces the molecular weight of the polymer. An increase in the content of the cocatalyst above the stoichiometric level does not affect the rate of polymerization, since only those molecules that are associated with the catalyst participate in the reaction. The role of the cocatalyst depends on the nature of the medium. In a polar solvent, HCl accelerates the polymerization process, since the resulting complex with the catalyst dissociates with the release of H+ ions, which excite polymerization. In a non-polar solvent, such as carbon tetrachloride (dipole moment is 0). The dissociation of the complex is low and HCl only binds the catalyst, reducing the rate of polymerization.

The reaction is significantly affected by the dielectric constant of the medium. The rate of catalytic polymerization depends on the polarity of the medium. With an increase in polarity, the rate of polymerization increases and the molecular weight of the polymer increases.

Example. Influence of the dielectric constant of the solvent on the rate of polymerization of methylstyrene and the molecular weight of polystyrene.

· The activation energy of cationic polymerization is always less than 63 kJ/mol. In the case of radical polymerization, it exceeds this value. Due to this, the cationic polymerization of the proteaket, as a rule, at a very high rate.

When a cocatalyst interacts with a catalyst molecule, a complex is formed:

which protonates the monomer with the formation of an active center - a carbonium ion:

chain growth consists in the attachment of monomer molecules to the crbonium ion with its subsequent regeneration.

Some solvents, as well as tert-alkyl chloride, can play the role of co-catalysts.

Example: Styrene does not polymerize in an aqueous medium in the presence of SnCl4. The addition of tert-butyl chloride leads to rapid polymerization:

When tert-butyl chloride reacts with stannous chloride, a complex is formed which, when reacted with the monomer, yields a carbonium ion.

Molecular chain termination may happen:

As a result of chain transfer to the monomer:

The kinetic chain continues.

during the regeneration of the catalytic complex:

It has been experimentally shown that polymerization rate (e.g. styrene in the presence of stannous chloride) is directly proportional to the catalyst concentration, and the average degree of polymerization (n) does not depend on the catalyst concentration and is directly proportional to the monomer concentration.

Average degree of polymerization:

i.e. average the degree of polymerization does not depend on the catalyst concentration and is directly proportional to the monomer concentration.

The overall polymerization rate can be determined from the equation:

Provided that[m] = const , i.e. e. the overall rate of cationic polymerization is directly proportional to the catalyst concentration.

Ionic polymerization is very sensitive to changes in reaction conditions and the nature of the medium. The influence of impurities. Therefore, the reaction is often more complicated. as shown in the diagrams.

anionic polymerization.

During anionic polymerization, the appearance of an active center is associated with the formation of a carbanion. Conventionally, it is subdivided into anion and anion-coordination. The latter includes polymerization in the presence of organometallic compounds.

The tendency to anionic polymerization is most pronounced in monomers with electron-withdrawing substituents, which cause polarization of the double bond, enhancing its electrophilicity and stabilizing the resulting anions.

Catalysts are substances that donate electrons. (bases. Alkali metals, their hydrides and amides, organometallic compounds)

More electrophilic monomers require less basic catalysts with lower electron donating power to initiate.

An example of an anionic polymerization mechanism:

Polymerization of unsaturated compounds in the presence of potassium amide in liquid ammonia.

It has been established that during the polymerization of styrene in the presence of potassium amide in liquid ammonia, each resulting polymer macromolecule contains an NH2 group. In this case, the molecular weight of the polymer does not depend on the catalyst concentration and is directly proportional to the monomer concentration. As the temperature rises, the molecular weight of the polymer decreases.

The rate of polymerization is proportional to the square of the monomer concentration and the square root of the catalyst concentration.

chain break during anionic polymerization occurs:

By attaching H+ or another positive particle;

by transferring the chain to the solvent.

The catalyst is not consumed by the reaction.

Polymerize with amides: acrylonitrile, methyl methacrylate, methacrylonitrile.

Otherwise, polymerization occurs in the presence of organometallic catalysts R-Me (butyllithium, ethylsodium, triphenylmethylsodium).

Me in the complex is bound to the monomer by a coordination bond - polymerization is therefore called anion-coordination. A feature of this polymerization is the bifunctional addition of a monomer (during catalysis with metal amides, a bifunctional monomer is added at one function).

The more polar the metal-carbon bond in the catalyst, the more the polymerization mechanism approaches a purely ionic one. The lowest polarity of the Li–C bond.

a) polymerization of butadiene in the presence of organic compounds of sodium, potassium (1,2 structures predominate)

b) in the presence of lithium-organic compounds (solvent-hydrocarbon), structures 1,4 predominate by 90%. get stereoregular cis-1,4-polybutadiene

In an environment of polar solvents, the influence of the catalyst is weakened, since a solvent-catalyst complex is formed, and not a catalyst-monomer. And if we add, for example, alcohol, phenol, then in the process of polymerization we get polybutadiene with a predominance of the structure 1,2.

chain break in the absence of impurities that are proton donors and capable of chain termination, in many cases there may be none !!!

The reaction proceeds until the monomer is depleted. As a result, macromolecules containing active sites and capable of initiating polymerization are formed. They are called "living" polymers. When a new portion of the monomer is added to such a polymer, its molecular weight increases. If another monomer is added, a block copolymer is formed.

When polymerized with organometallic compounds and alkali metals, in the absence of impurities capable of causing chain termination, polymers with a very high molecular weight can be obtained. Ideally, the molecular weight under these conditions is determined by the monomer:catalyst ratio

Conclusions:

Since spontaneous chain termination does not occur during anionic polymerization, polymers that are monodisperse in molecular weight can be obtained. Basic conditions for this:

complete absence of impurities: good mixing (the rate of formation of active centers is high).

2. Various compounds can be introduced into the “living” polymer to terminate the chain and oligomers with various end groups can be obtained.

If the act of introduction of the monomer into the growing polymer chain is preceded by the act of its coordination on the active site, then such a process is called ion-coordination polymerization. Monomer coordination can take place in both anionic and cationic polymerization, but it is more characteristic of anionic polymerization.

In 1954, A.A. Korotkov obtained stereoregular rubber from isoprene, using organolithium compounds as a catalyst. Upon polymerization on lithium or organolithium compounds, stereoregular cis-1,4-polyisoprene is formed only in hydrocarbon media. This is explained by the coordination of the monomer on the polar, but undissociated active site, as a result of which the monomer unit takes on a configuration corresponding to cis-1,4-structure:

The addition of only a few percent of an electron donor compound (ether, tetrahydrofuran, alkylamine) dramatically changes the microstructure of polyisoprene, becoming predominant trance-1,4- (80-90%) and 3,4-structure (10-20%). An electron-donating compound polarizes the bond before splitting into ions:



In this case, the coordination of the Li + ion with the terminal unit of the macroion, which has an allyl structure, occurs. In the allyl structure, the -electrons are delocalized. With this in mind, the coordination of the Li + ion with the macroion can be represented by a cyclic structure:

that leads to trance-1,4- and 3,4-structure.

In 1955, the German chemist Karl Ziegler (for the production of polyethylene under mild conditions of 50-80°C and 1 MPa) and the Italian chemist Giulio Natta (for the production of crystallizing polypropylene and polystyrene) proposed a catalytic system of triethylaluminum and titanium chloride.

In particular, Natta and collaborators in Milan investigated polymers obtained from propylene by X-ray diffraction and found that some of the studied polymers obtained by polymerization of the monomer under the action of the reaction products of trialkylaluminum with titanium chloride (Ziegler catalyst) or under the action of chromium trioxide supported on aluminum oxide , have a much more regular structure than other polypropylene polymers. The stereoregularity of a polymer greatly affects its physical properties. For example, conventional atactic polypropylene is a soft rubbery material, while isotactic modification is a fibrous material that can be spun and woven. Therefore, it is not surprising that Natta and Ziegler received the Nobel Prize in 1963 for the discovery of stereoregular polymers and the catalysts needed to obtain them.

Typical catalysts for ion-coordination polymerization are compounds of transition metals of d-groups (groups IV-VIII - Ti, V, Cr), which form upon interaction with aluminum alkyl (or other organic compounds of non-transition metals of groups I-III) -bond (I) Me -C, but retaining the ability to form a -bond (II) - the so-called group of Ziegler-Natta catalysts:

In the act of coordination, the monomer acts as a donor of -electrons, and the transition metal of the catalyst Ti, due to the presence of vacant d-orbitals, is an acceptor (II). The formation of the monomer-Ti -complex leads to a weakening of the Ti-C bond, the introduction of the monomer through this bond is facilitated (III).

The growth of the chain is carried out by the introduction of the monomer according to the “head to tail” type, which is associated with overcoming relatively lower activation barriers than with attachment by other types. The act of coordination leads to a certain orientation monomer molecules, providing double bond opening and selection strictly defined conformation monomer when introduced into the polymer chain. In this case, the polymer chain will have stereoregular isotactic structure. If the monomer conformations selected in the act of insertion are opposite and alternate regularly, then stereoregular syndiotactic subsequence. Heterogeneous and homogeneous Ziegler-Natta catalysts are known, the former mainly produce isotactic polymers, while the latter can also produce syndiotactic polymers.

The stereospecificity of catalytic systems of the Ziegler-Natta type is due to the influence of the ligand environment in the coordination sphere of the transition metal, while in the reactions of radical and ionic polymerization stereoregulation is carried out by the terminal link of the growing macrochain. Thus, the initiation of stereospecific polymerization proceeds by a three-stage mechanism - coordination, orientation and implementation.

Ziegler-Natta catalysts are widely used for the polymerization of ethylene, propylene, dienes, and some polar and heterocyclic monomers. In these processes, the intensity of chain growth restriction reactions depends on temperature. Chain termination occurs as a result of the same reactions as in anionic polymerization, in particular, the reaction of transferring a hydride ion to a monomer or counterion. In addition, the molecular weight of the resulting polymer can also be controlled by introducing chain transfer agents into the reaction medium - hydrogen and aluminum hydride.

The chain growth rate can be expressed by a kinetic equation similar to that for anionic polymerization:

where [I] and [M] are the concentrations of the initiator and monomer, and  and  are the reaction orders with respect to the initiator and monomer.

In industry, the method of anion-coordination polymerization is obtained stereoregular rubbers and polyolefins.

12. Copolymerization. Derivation of the kinetic regularities of the reaction of radical copolymerization of two different monomers. The value of copolymerization constants and their corresponding dependence mol. the proportion of monomer in the resulting polymer.

Copolymerization refers to the joint polymerization of two or more monomers. It is widely used in practice because it is a simple and very effective method for modifying the properties of polymers. Copolymerization can be either radical or ionic.

The most common and studied binary copolymerization. For this case, one can derive kinetically (or statistically) without specifying the mechanism and nature of active centers copolymer composition equation - dependence between the compositions of the copolymer and the initial mixture of monomers (since, as a rule, they are not equal).

Assumptions:

Constant rate of initiation;

The reactivity of the active site is constant;

All stages are irreversible;

The monomer is consumed only for chain growth;

There is a stationary state;

Homophasic polymerization;

The copolymer is obtained with M n  10 4 .

Monomer conversion rate< 5%, когда состав мономерной смеси мало отличается от исходной.

In this case, four chain propagation reactions can be written:

The rates of exhaustion of monomers during copolymerization are:

Dividing these expressions, we obtain the ratios of the concentrations of monomer units in the resulting copolymer:

In the stationary state, stationary concentrations of active centers of each type are established. The condition for this type of stationarity is:

.

Substituting this value into the expression for the ratios of the concentrations of monomer units, after simplification, we obtain differential equation for the composition of a Mayo-Lewis copolymer::

,

where i- copolymerization constants, or relative activities of monomers, equal to the ratio of the rate constants of attachment to the growth radicals of one's own and "foreign" monomers. The parameters for any pair of monomers are determined only by the nature of these monomers and temperature and do not depend on the solvent, initiator and chain transfer agent.

This equation relates the instantaneous (current) concentrations of monomers in the copolymer and monomer mixture through the values ​​of the relative activities of the monomers.

According to the equation, you can find the constants r 1 and r 2 during copolymerization up to 5-7% of the degree of conversion of the monomers. Under this condition, the ratio / can be considered equal to the specified one, and the instantaneous composition is equal to the average composition of the copolymer formed at the initial stage, i.e.

.

Thus, the chemical composition of the copolymer (at low degrees of conversion) depends on the concentrations of the monomers and their copolymerization constants.

Instead of absolute molar concentrations, it is more convenient to use relative mole fractions.

The composition equation can be solved graphically or analytically. There are a number of methods for solving the equation.

The graphical expression of the equation of composition are copolymer composition curves, the form of which is determined by the constants r 1 and r 2:

Statistical analysis of the alternation of links in the copolymer chain indicates three cases:

when r 1 r 2 =1 links of both types are placed in the polymer chain according to the law of chance;

for r 1 r 2< 1 вероятность чередования звеньев увеличивается;

at r 1 r 2  0, a regularly alternating copolymer is possible in the limit.

The point of intersection of curve 3 (or curve 8) with straight line 1 corresponds to azeotropic copolymerization (when the composition of the copolymer is equal to the composition of the monomer mixture).

13. Technological methods for the implementation of polymer synthesis processes. Bulk polymerization, solution polymerization, emulsion polymerization and suspension polymerization. Solid state polymerization, gas phase polymerization. Advantages and disadvantages of these methods.

RADICAL POLYMERIZATION METHODS

Radical polymerization is carried out mainly in the block (mass), solution, emulsion, suspension and gas phase. In this case, the process can proceed under homogeneous or heterogeneous conditions. In addition, the phase state of the initial reaction mixture may also change during polymerization.

Polymerization in block (in bulk) is carried out in the absence of a solvent, so that no contamination of the polymer occurs. However, the process is difficult to control due to the high exothermicity polymerization. As polymerization increases, the viscosity of the medium increases and heat removal becomes more difficult, as a result of which local overheating occurs, leading to the destruction of the polymer and its inhomogeneity in molecular weight. The advantage of bulk polymerization is the possibility of obtaining a polymer in the form of a vessel in which the process is carried out without any additional processing.

Solution polymerization devoid of many of the disadvantages of block polymerization. When it is carried out, the possibility of local overheating is eliminated, since the heat of reaction is easily removed by the solvent, which also acts as a diluent, the viscosity of the reaction system decreases, which facilitates its mixing. However, this polymerization method also has disadvantages. When polymerization is carried out in a number of solvents, the proportion of chain transfer reactions increases, which leads to a decrease in the molecular weight of the polymer. In addition, the polymer may be contaminated with solvent residues, which are not always easily removed from the polymer.

Solution polymerization is carried out in two ways. In the first method, a solvent is used for polymerization, in which both the monomer and the polymer are dissolved. The resulting solution is used as such or the polymer is isolated by precipitation or evaporation of the solvent. According to the second method, solution polymerization is carried out in a liquid in which the monomer is dissolved, but the polymer is not. The polymer precipitates out in solid form as it forms and can be separated by filtration.

Suspension polymerization(beaded or granular) is widely used for the synthesis of various polymers. In this case, the monomer is dispersed in water in the form of small droplets. The stability of the dispersion is achieved by mechanical mixing and the introduction of special additives - stabilizers into the reaction system. In suspension polymerization, monomer-soluble initiators are used. The polymerization process is carried out in monomer droplets, which can be considered as microreactors block polymerization. The advantage of this method is good heat removal, and the disadvantage is the possibility of contamination of the polymer with stabilizer residues.

emulsion polymerization(latex polymerization, latex is an aqueous colloidal dispersion of polymer particles 10 -4 -10 -5 cm in size) is also a widespread method for obtaining polymers. In emulsion polymerization, water is usually used as a dispersion medium, and various soaps are used as an emulsifier. To initiate the process, water-soluble initiators, redox systems are most often used. Polymerization can take place in a molecular solution of the monomer in water, on the interface between a drop of monomer and water, on the surface or inside soap micelles, on the surface or inside the resulting polymer particles swollen in the monomer.

The advantage of emulsion polymerization is the possibility of carrying out the process at high speeds with the formation of a polymer of high molecular weight, as well as the ease of heat removal; The disadvantages of emulsion polymerization are the need to remove emulsifier residues and a large amount of wastewater that requires special treatment.

At gas-phase polymerization the monomer (eg ethylene) is in the gaseous state. Oxygen and peroxides can be used as initiators. The process takes place at high pressure.

Solid state polymerization - it is the polymerization of monomers that are in a crystalline or glassy state. In this case, the monomer molecules are rigidly fixed in space and their mobility is extremely limited, which determines the features of the process kinetics and the structure of the resulting macromolecules. To initiate polymerization, accelerated electrons or  radiation are used.

Exists two extreme cases transition of a monomeric crystal into a polymer (many intermediate cases are possible):

The structure of a monomeric crystal significantly determines the structure of the polymer, the so-called. topotactic process(e.g. polymerization of conjugated diacetylenes or trioxane)

(Another example of topotactic polymerization is the radiation-chemical polymerization of 2,3-dimethylbutadiene-1,3 in hexagonal urea crystals, in which channels are formed filled with linear sequences of monomers, and the polymer turns out to be stereoregular.);

The polymer appears as an independent phase in extended defects of the monomer crystal lattice, which leads to further breakage of the monomer crystal; the resulting polymer phase amorphous(for example, polymerization acrylamide).

14. Polycondensation and polymer analogous transformations of polymers. General characteristics of these processes. The main chemical reactions for the implementation of these methods for obtaining IUDs. Kinetics of processes. The main types of polymers obtained in industry by these two methods.

polycondensation- this is the synthesis of polymers by the interaction of bi- or polyfunctional monomers or oligomers, usually accompanied by the release of a low molecular weight product (water, alcohol, ammonia, hydrogen halide, corresponding salts, etc.). For example, obtaining polyesters:

Functional groups can be considered reaction (active) centers during polycondensation.

Polycondensation processes play an important role in nature and technology. Polycondensation underlies the formation of all natural VIS: proteins, cellulose, starch, nucleic acids, etc. The first industrial production of a synthetic polymer, phenol-formaldehyde resin (L. Baekeland, 1909), is based on polymerization reactions. A great contribution to the development of knowledge about the processes of polycondensation was made by domestic scientists: V.V. Korshak, G.S. Petrov, K.D. Andrianov, American scientists W. Carothers, P. Flory, P. Morgan. Currently, more than 30% of the total production of polymers is obtained by the polycondensation method.

In anionic polymerization, the appearance of an active center is associated with the formation of a carbanion. Anionic polymerization is often divided into anionic and anionic-coordination ones. The latter includes polymerization in the presence of organometallic compounds, proceeding through the stage of formation of an intermediate complex of catalysts - a monomer, in which the catalyst is linked to the monomer by coordination bonds. Depending on the polarity of the medium and other reaction conditions, the polymerization mechanism can change from purely ionic to ionic-coordination and vice versa.

During the polymerization of styrene in the presence of potassium amide in liquid ammonia, each resulting polystyrene macromolecule contains an NH 2 group. In this case, the molecular weight of the polymer does not depend on the catalyst concentration and is directly proportional to the monomer concentration. As the polymerization temperature increases, the molecular weight of the polymer decreases. Chain termination occurs when a carbanion interacts with ammonia as a result of the addition of an ammonia proton with the regeneration of an amide ion.

Acrylic acid derivatives - methyl methacrylate, acrylonitrile, methacrylonitrile - also polymerize with alkali metal amides. These monomers contain electronegative substituents, i.e. are electron acceptors and therefore very active in anionic polymerization.

A feature of this polymerization is the bifunctional addition of the monomer. BM joins one function at a time. The chain growth reaction during polymerization involves 2 catalyst centers - metal and alkyl (two-center polymerization mechanism).

The mechanism is not fully understood and is very complex. It is assumed that the formation of a complex with a catalyst precedes the connection of a monomer molecule.

In such complexes, the metal is bound to the monomer by a coordination bond; therefore, polymerization proceeding with the formation of such complexes is called anionic-coordination polymerization.

In anionic polymerization, chain growth is carried out with the participation of a carbonion or an ion pair; while the terminal group of the growing macromolecule, having high activity at the same time, is quite stable. Therefore, anionic polymerization in the absence of impurities can lead to chain termination, in many cases it can proceed without chain termination until the monomer is completely exhausted. As a result of such polymerization, polymers are formed, the macromolecules of which contain active centers and are capable of initiating polymerization. These polymers are called "living" polymers. When a new portion of the monomer is added to such a polymer, its molecular weight increases.

Feature of "living" polymers:

• - when another monomer is added to “living” polymers or oligomers, block copolymers can be obtained (method for determining “living” macromolecules);
• - A "living" polymer for chain termination can be introduced with various compounds and polymers with various terminal functional groups can be obtained, which opens up great opportunities in the synthesis of block copolymers with heterochain oligomers.

In recent years, anion-coordination polymerization in the presence of complex Ziegler-Natta catalysts has become widespread. (This method is used in the industrial synthesis of stereoregular polymers.) Ziegler-Natta catalysts include organometallic compounds of groups I-III and chlorides of groups IV-VII with transitional valence. The most commonly used are organometallic compounds of aluminum and titanium chlorides, which easily form coordination bonds. Such complex catalysts are insoluble and their structure has not been established, but it is assumed that they represent a bimetallic complex with coordination bonds.

The dependence of the polymerization rate on the conformation of the molecular chain of synthetic polymers was first shown by the example of the polymerization of N-carboxyl anhydrides of amino acids with the formation of polypeptides. In this case, the reaction proceeds in 2 stages, which differ in speed. Stage 1 proceeds relatively slowly until an oligomer capable of coiling into a coil is formed, then the reaction proceeds at a high rate with the formation of a high molecular weight polypeptide. The presence of isomeric amino acids in the reaction mixture sharply reduces the rate of polymerization.

Then, ideas about the guiding role of the conformation of the resulting molecular chain during polymerization were transferred to vinyl monomers. From this point of view, the effect of the nature of the solvent and temperature on the stereospecificity of the polymerization of vinyl compounds is considered. Thus, it was shown that the polymerization of styrene in the presence of triphenylmethylpotassium in benzene leads to the formation of atactic polystyrene, and with the same catalyst in hexane a stereoregular polymer is obtained. From the standpoint of the so-called helical polymerization, this is explained by the high stability of the helical conformation of the growing polystyrene macromolecules during polymerization in a poor solvent compared to benzene - hexane. The formation of stereoregular polystyrene during polymerization in the presence of butyllithium at -30°C in a hydrocarbon environment and the lack of stereospecificity in the polymerization of styrene with this catalyst at a higher temperature are explained in a similar way. Such a new direction in the study of the mechanism of stereospecific polymerization is extremely interesting, although there is still little experimental data to create a harmonious concept.

Polymers can be obtained not only by chain radical polymerization reactions, but also by chain reactions in which the growing chain is not a free macroradical, but a macroion. This method of obtaining polymers is called ionic polymerization, and substances that dissociate into ions and excite the polymerization of monomers by the ionic mechanism are called catalysts.

Depending on the sign of the charge of the growing macroion, cationic and anionic polymerization. In cationic polymerization, the carbon atom at the end of the growing chain (carbocation) has a positive charge. The charge arises at the stage of initiation and disappears when the circuit is broken or transferred. During anionic polymerization, the charge of the growing macroion (carbanion) is negative.

In ionic polymerization, the same elementary stages can be distinguished as in radical polymerization: initiation, growth, chain termination, and chain transfer. Polymerization under the influence of ionic catalysts usually occurs at higher rates than with radical ones and leads to the production of a polymer of a higher molecular weight. The reaction system in the case of ionic polymerization is often heterogeneous (inorganic or organometallic solid catalyst and liquid organic monomer).

Ionic also includes polymerization that occurs by coordinating the monomer on the surface of a solid catalyst (coordination-ionic polymerization). The surface of the catalyst in this case plays a special role as a matrix, which specifies the order in which the monomer enters the growing chain with an ordered spatial arrangement of monomer units. All stereoregular polymers are obtained by coordination ionic polymerization.

Catalysts cationic polymerization are strong electron-withdrawing compounds. Typical catalysts are protic acids (H 2 S0 4, HC10 4, H 3 P0 4, etc.) and aprotic acids (BF 3, ZnCl 2, A1C1 3, TiCl 4, etc.). The latter are active in the presence of small amounts of water or other substances - proton donors, called cocatalysts.

Monomers of the vinyl and divinyl series easily enter into cationic polymerization, containing electron-donating substituents at the double bond, for example, propylene, β-methylstyrene, esters of acrylic and methacrylic acids, etc. Some heterocyclic monomers are also active in cationic polymerization: olefin oxides, lactones, a series of carbonyl-containing compounds such as formaldehyde.

Cationic polymerization begins with the fact that the catalyst, interacting with the co-catalyst, forms a complex compound, which is a strong acid. In the reaction medium, it dissociates, for example:

The emerging proton attaches to the monomer molecule, resulting in the formation of an ion pair consisting of a carbonium ion and a complex counterion:

These two reactions constitute the step of initiating the cationic polymerization.

Chain growth consists in the sequential attachment of monomer molecules to a carbonium ion, while a positive charge is always retained at the end of the chain:

The carbonium ion polarizes the monomer molecule; therefore, oia enters the chain in a certain way, and the resulting macromolecules always have a regular structure.

Chain termination by recombination or disproportionation is impossible in this case due to the repulsion of like-charged ions. It occurs by rearranging the ion pair, in which a neutral polymer molecule is formed with a double C=C bond at the end and the initial catalytic complex is generated:

In cationic polymerization, as in radical polymerization, chain transfer to the monomer and solvent is observed:

Since cationic polymerization is associated with the formation and dissociation of an ion pair, the rate of the process is affected by the dielectric constant of the medium. An increase in the permittivity significantly accelerates the process, but has little effect on the molecular weight of the polymer. Compared to radical polymerization, cationic polymerization is characterized by a low activation energy (60 kJ/mol), so it proceeds at a high rate, which decreases with increasing temperature.

Catalysts anionic polymerization are substances that are electron donors: alkali metals, alkalis, hydrides and amides of alkali metals, organometallic compounds. In anionic polymerization reactions, vinyl monomers with electron-withdrawing substituents are the most active, for example, styrene CH 2 =CH-C 6 H 5 acrylonitrile CH 2 =CH-C=N. In anionic polymerization, a carbanion acts as an active center - a compound with a trivalent carbon bearing a negative charge, and the growing chain itself is a macroanion.

The mechanism of anionic polymerization in the presence of amides of alkali metals and organometallic compounds is described by the same schemes. Thus, the polymerization of styrene in liquid ammonia, catalyzed by sodium amide, proceeds as follows.

Chain Growth:

those. a monomer molecule is introduced between the ions of an ion pair.

Chain termination, as in cationic polymerization, is impossible by combining growing macroanions because they have the same charge. It most often occurs as a result of chain transfer reactions to a solvent or monomer:

If alkali metals (Li, Na) are catalysts for anionic polymerization, then at the initiation stage, radical ions monomer, which, when combined, turn into a two-center organometallic compound - bianion. The chain growth is carried out by the introduction of a monomer between the ions of the ion pair at both centers of the resulting bianion, i.e. the chain grows simultaneously in two directions. In this way, the polymerization of butadiene under the action of metallic sodium is carried out:

Initiation

chain growth (at both ends of the bianion)

This type of polymerization, associated with the formation of radical ions, is interesting in that it makes it possible to obtain “living” polymer chains, i.e. the growing macrobianion is able to initiate polymerization for a long time when new portions of the monomer are added. Chain termination, even by methods of transfer to a solvent or monomer, is completely excluded. Polymerization stops only after the exhaustion of the entire monomer. The polymers obtained by this method are characterized by high molecular weight and low polydispersity.

Anionic polymerization is effective at low temperatures in carefully deaerated (deaerated) and dried basic solvents.

Coordination-ionic polymerization is carried out under the action of complex catalysts with high selectivity. Such catalysts are complexes formed by the interaction of alkyls of metals of groups I-III of the D.I. Mendeleev with halides of transition metals of IV-VIII groups. A typical catalyst is a complex of triethylaluminum and titanium trichloride:

At the stage of initiation, the titanium atom of the catalyst complex coordinates the monomer in a certain way. With such coordination, the bonds of the monomer are loosened and the bonds are redistributed in the catalyst complex. A p-complex is formed between the monomer and the catalyst. Thus, the initiation of the stereospecific polymerization of propylene can be represented as follows:

p-complex rearranges into a six-membered ring, in the structure of which the monomer is introduced:

Next, a catalyst complex of the initial structure is generated, in the field of attraction of which is the first monomer unit. The introduction of each next monomeric unit occurs through the stage of formation of a rearranged p-complex, and the growing polymer chain, as it were, moves away from the catalyst:

This does not occur in radical, cationic, or anionic polymerization.

During coordination-ionic polymerization, the resulting macromolecules are characterized not only by a chemically regular connection of monomer units (which is generally inherent in ionic polymerization), but also by a strict alternation in space of substituents at carbon atoms of the main polymer chain. The stereospecificity of macromolecules of polymers synthesized by coordination-ionic polymerization is provided by nature complex catalyst. Aluminum and titanium compounds of a similar structure, but taken separately, are not stereospecific catalysts.

16. Types and principles of polycondensation reactions

chemical hydrocarbon polycondensation polymer

Polycondensation, along with polymerization, is one of the main methods for obtaining polymers. Polycondensation called the stepwise process of the formation of polymers from two- or polyfunctional compounds, accompanied in most cases by the release of a low molecular weight substance (water, alcohols, hydrogen halides, etc.). A necessary condition for polycondensation is the participation in the reaction of molecules, each of which contains two or more functional groups that can interact with each other. In general, the polycondensation process can be represented as follows:

where A and B are the remnants of the reacting molecules; a and b - functional groups; ab - low molecular weight product.

The above scheme shows the stepwise formation of a polymer during polycondensation: first, monomer molecules interact with each other to form dimers, then dimers turn into trimers, trimers into tetramers, etc., i.e. in oligomers. Due to the presence of functional groups, oligomers can interact with each other and with monomers. This interaction determines the growth of the polymer chain. If the molecules of the initial monomers contain two functional groups, the growth of the polymer chain occurs in one direction and linear macromolecules are formed. The presence of more than two functional groups in the molecules of the initial monomers leads to the formation of branched macromolecules or cross-linked (three-dimensional) structures. Bifunctional substances may have functional groups of the same or different structures. As a result of each act of interaction, a product is formed with terminal functional groups capable of further interaction. For example, polyamides can be obtained from diamines and dicarboxylic acids or from amino acids. At the first stage of the reaction, dimers are formed, which then turn into higher molecular weight products:

Tri- and tetrafunctional substances, as well as their mixtures with bifunctional compounds, form branched or three-dimensional products during polycondensation. For example, the condensation of glycerol with phthalic acid proceeds according to the following scheme:

1. Dimer formation:

2. Formation of branched products:

3. Formation of three-dimensional structures from branched products:

There are several differences between polycondensation and polymerization.

1. Polymerization is a chain process that follows the attachment mechanism; polycondensation is a stepwise process that follows the substitution mechanism. Intermediates at individual stages of the polycondensation process can be isolated and characterized.

2. Polymerization is not accompanied by the release of low molecular weight products; in polycondensation, this occurs in most cases.

3. The isolation of a low molecular weight product, in turn, leads to two features: first, the chemical structure of the repeating unit of the polymer molecular chain obtained by polycondensation does not correspond to the composition of the initial monomers; secondly, the released low molecular weight reaction product can interact with the emerging polymer molecule to form the initial substances. This means a violation of the established equilibrium of the reaction. It can be shifted in the direction of polymer formation by removing a low molecular weight product from the reaction sphere.

4. During polymerization, the molecular weight of the polymer, as a rule, does not depend on the duration of the reaction; in polycondensation, it increases as the reaction proceeds.

Depending on the nature of the functional groups of the starting substances, polycondensation is divided into homofunctional and heterofunational. The process that occurs as a result of the interaction of functional groups of the same chemical nature is homopolycondensation. Homopolycondensation produces, for example, polyesters from glycols:

Heteropolycondensation is a process of interaction of functional groups of different chemical nature. An example of heteropolycondensation is the interaction of diamines with dichlorides:

Depending on the structure of the starting materials, polycondensation can be represented by various types of chemical processes: esterification, amination, amidation, cyclization, etc. Polycondensation is the main method for obtaining heterochain polymers.

In polycondensation, it is of great importance to maintain a stoichiometric ratio between monomers, which is the main prerequisite for obtaining polymers of high molecular weight. If the ratio of monomers in the mixture is equimolecular, i.e. the functional groups of both types of monomers are contained in equal amounts, the polycondensation process proceeds to the end, until the complete exhaustion of both monomers. If the reaction mixture contains one of the monomers in excess, the polycondensation process proceeds until the monomer present in a smaller amount is consumed. In this case, at the end of the reaction, the macromolecules of the resulting polymer will contain the same functional groups of the component present in excess in the reaction medium at both ends. This will stop the polycondensation process and, consequently, reduce the molecular weight of the polymer. A similar result is observed if, for example, a monofunctional compound is introduced into the initial equimolecular mixture of two bifunctional compounds. A monofunctional substance blocks functional groups of a different type, as a result of which the polycondensation process stops. This technique is used in practice when additives of monocarboxylic acids are introduced into the reaction mixture of diamines and dicarboxylic acids in the synthesis of polyamides.

The stoichiometry of the ratio of the initial substances during the process can be violated if these substances have different volatility, and also if the nature of the functional groups changes during the reaction.

An increase in temperature (up to certain limits) accelerates the polycondensation reaction, facilitates the removal of a low molecular weight product, which, in the case of equilibrium polycondensation, leads to a shift in the equilibrium towards the formation of higher molecular weight polymers. In some cases, an increase in temperature changes the course of the reaction and the nature of the resulting product.

Mechanism of anionic polymerization

Anionic polymerization is characteristic of vinyl compounds with electron-withdrawing substituents: acrylonitrile, alkyl acrylates, styrene, etc.

Main stages:

According to the anionic mechanism, cyclic monomers are also polymerized, for example, obtaining ethylene oxide:

Anionic polymerization kinetics:

I. Influence of the concentration of starting substances on the rate of the chain propagation reaction

According to the principle of stationarity:

Let us express the macroion concentration from here:

Let us substitute the macroion concentration equation into the chain growth reaction rate equation:

II. Influence of the concentration of initial substances on the degree of polymerization.

The degree of polymerization is equal to the ratio of the growth and chain termination rates:

Substitute the velocity equations:

It should be noted that the degree of polymerization does not depend on the catalyst concentration.

Features of anionic polymerization:

The termination reaction has the highest activation energy, which means that at low temperatures there will be no termination

A macroanion will exist in the system, which is called a "living" chain, because this macro anion can initiate the polymerization reaction of another monomer. This is how I get block copolymers.

Anionic polymerization is used to obtain polymers with a narrow molecular weight distribution (chains of the same length)

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