Basic methods for producing polymers. Methods for producing polymers Methods for synthesizing polymers

Polymers are obtained by polymerization or polycondensation methods.

Polymerization (polyaddition). This is the reaction of the formation of polymers by the sequential addition of molecules of low molecular weight substances (monomers). Domestic scientists S.V. Lebedev, S.S. Medvedev and others and foreign researchers G. Staudinger, G. Mark, K. Ziegler and others have made a great contribution to the study of polymerization processes. macromolecules do not differ from the composition of monomer molecules. Compounds with multiple bonds are used as monomers: C \u003d C, C \u003d N, C \u003d C, C \u003d O, C \u003d C \u003d O, C \u003d C \u003d C, C \u003d N, or compounds with cyclic groups capable of opening, eg:

In the course of polymerization, multiple bonds are broken or rings open in monomers and chemical bonds arise between groups with the formation of macromolecules, for example:

According to the number of types of participating monomers, homopolymerization (one type of monomer) and copolymerization (two or more types of monomers) are distinguished.

Polymerization - spontaneous exothermic process (DG<0, DH<0), так как разрыв двойных связей ведет к уменьшению энергии системы. Однако без внешних воздействий (инициаторов, катализаторов и т.д.) полимеризация протекает обычно медленно. Полимеризация является цепной реакцией. В зависимости от характера активных частиц различают радикальную и ионную полимеризации.

In radical polymerization, the process is initiated by free radicals. The reaction goes through several stages: a) initiation; b) chain growth; c) transmission or open circuit:

a) initiation - the formation of active centers - radicals and macroradicals - occurs as a result of thermal, photochemical, chemical, radiation or other types of effects. The most common initiators of polymerization are peroxides, azo compounds (having a functional group - N \u003d N -) and other compounds with weakened bonds. Radicals are formed initially, for example:

(С6Н5СОО) 22C6H5COO * (R *)

benzoyl peroxide

Then macroradicals are formed, for example, during the polymerization of vinyl chloride:

R * + CH2 \u003d CHCl ® RCH2 - CHCl *

RCH2 - CHCl * + CH2 \u003d CHCl® RCH2 - CHCl - CH2 - CHCl *, etc .;

b) chain growth occurs due to the addition of the resulting monomers to the radicals to obtain new radicals;

c) chain transfer consists in transferring the active center to another molecule (monomer, polymer, solvent molecule):

R - (- CH2-CHCl-) n-CH2-CHCl * + CH2 \u003d CHCl ®

®R - (- CH2 -CHCl-) n -CH2 -CH2Cl + CH \u003d CHCl *

As a result, the chain stops growing, and the transmitter molecule, in this case, the monomer molecule, initiates a new reaction chain. If the transmitter is a polymer, chain branching can occur.

At the stage of chain termination, radicals interact with the formation of valence-saturated molecules:

R - (- CH2 - CHCl-) n- CH2- CHCl * + R - (- CH2- CHCl-) n- CH2- CHCl * ® R- (-CH2- CHCl-) n- CH2- CHCl - CH2- CHCl - (-CH2-CHCl) n- R

Chain termination can also occur with the formation of low-activity radicals that are unable to initiate the reaction. Such substances are called inhibitors.

Thus, the regulation of the length and, accordingly, the molecular weight of macromolecules can be carried out using initiators, inhibitors and other substances. Nevertheless, chain transfer and termination can occur at different stages of chain growth; therefore, macromolecules have different molecular weights, i.e. polydisperse. Polydispersity is a distinctive feature of polymers.

Radical polymerization serves as an industrial method for the synthesis of many important polymers such as polyvinyl chloride [-CH-CHCl-] n, polyvinyl acetate [-CH2-CH (OCOCH3) -] n, polystyrene [-CH2-CH (C6H5) -] n, polyacrylate [ -CH2-C (CH3) (COOR) -] n, polyethylene [-CH2-CH2-] n, polydienes [-CH2-C (R) \u003d CH-CH2-] n, and various copolymers.

Ionic polymerization also occurs through the stage of active center formation, growth, and chain termination. In this case, anions and cations play the role of active centers. Accordingly, anionic and cationic polymerization is distinguished. The initiators of cationic polymerization are electron-withdrawing compounds, including protic acids, for example, H2SO4 and HCl, inorganic aprotic acids (SnCl4, TiCl4, A1Cl3, etc.), organometallic compounds A1 (C2H5) 3, etc. Electron-donating substances are used as initiators of anionic polymerization and compounds, including alkali and alkaline earth metals, alkali metal alcoholates, etc. Often, several polymerization initiators are used simultaneously.

Chain growth can be written by the reaction equations:

with cationic polymerization and

Mn + + M ® M + n + 1

with anionic polymerization

Mn- + M ® M-n + 1

Let us consider, as an example, the cationic polymerization of iso-butylene with initiators AlCl3 and H2O. The latter form a complex

А1Сl3 + Н2О «Н + [АlONSlз] -

Denoting this complex by the formula H + X-, the polymerization initiation process can be represented as

H2C \u003d C + + H + X-®H3C-C + X-

The resulting complex cation, together with the counterion X-, forms a macroion that provides chain growth:

CH3 CH3 CH3 CH3

Н3С - С + X- + Н2С \u003d С ®Н3С ¾ С - СН2 - С + X-, etc.
CH3 CH3 CH3 CH3

With the help of some complex initiators, it is possible to obtain polymers having a regular structure (stereoregular polymers). For example, such a complex initiator can be a complex of titanium tetrachloride and trialkylaluminum AIR3.

The method of ionic polymerization is used in the production of poly-isobutylene [-CH2-C (CH3) 2-] p, polyformaldehyde [-CH2O-] n, polyamides, for example, poly-e-caproamide (nylon) [-NH- (CH2) 5- CO-] n, synthetic rubbers, for example butadiene rubber [-CH2-CH \u003d CH-CH2-] n.

3/4 of the total volume of produced polymers is obtained by the polymerization method. Polymerization is carried out in bulk, solution, emulsion, suspension or gas phase.

Bulk (block) polymerization is the undiluted polymerization of liquid monomer (s). This gives a fairly pure polymer. The main difficulty in carrying out the process is associated with heat removal. In solution polymerization, the monomer is dissolved in a solvent. With this method of polymerization, it is easier to remove heat and control the composition and structure of polymers, but the problem arises of removing the solvent.

Emulsion polymerization (emulsion polymerization) is the polymerization of a monomer dispersed in water. To stabilize the emulsion, surfactants are introduced into the medium. The advantage of the method is the ease of heat removal, the possibility of obtaining polymers with a high molecular weight and a high reaction rate, the disadvantage is the need to wash the polymer from the emulsifier. The method is widely used in industry for the production of rubbers, polystyrene, polyvinyl chloride, polyvinyl acetate, polymethyl acrylate, etc.

In suspension polymerization (suspension polymerization), the monomer is in the form of droplets dispersed in water or other liquid. As a result of the reaction, polymer granules with a size of 10-6 to 10-3 m are formed. The disadvantage of the method is the need to stabilize the suspension and remove the stabilizers from the polymers.

In gas polymerization, the monomer is in the gas phase, and the polymer products are in the liquid or solid state. The method is used to obtain polypropylene and other polymers.

Polycondensation. The reaction of polymer synthesis from compounds having two or more functional groups, accompanied by the formation of low molecular weight products (Н2О,NH3, HCl, CH2O, etc.) is called polycondensation. A significant contribution to the study of polycondensation processes was made by Russian scientists V. Korshak, G. Petrov and others, from foreign scientists - U. Karozers, P. Flori, P. Morgan and others. Polycondensation of bifunctional compounds was called linear, for example:

2NH2- (CH2) 5-COOH ®

amiocaproic acid

®NH2- (CH2) 5-CO-NH- (CH2) 5-COOH + Н2О®

NH2- (CH2) 5-CO-NH- (CH2) 5-COOH + NH2- (CH2) 5-COOH ®

® NH2- (CH2) 5-CO-NH- (CH2) 5-CO-NH- (CH2) 5-COOH + H2O etc.

The end product is poly-e-caproamide [-CO-NH- (CH2) 5-] n. Polycondensation of compounds with three or more functional groups is called three-dimensional. An example of three-dimensional polycondensation is the interaction of urea and formaldehyde:

NH2-CO-NH2 + CH2O ® NH2-CO-NH-CH2OH

NH2-CO-NH-CH2OH + CH2O ® CH2OH-NH-CO-NH-CH2OH

2 CH2OH-NH-CO-NH-CH2OH ®

® Н2О + CH2OH-NH-CO-NH-CH2-O-CH2- NH-CO-NH-CH2OH

At the first stage, an oligomer with a linear structure is synthesized:

[-CH2-NH-CO-NH-CH2-O] n

At the second stage, when heated in an acidic medium, further polycondensation of the oligomer occurs with the release of СН2О and the appearance of a network structure:

N-CH2-N - CH2 -N - CH2 -N -CH2-N -CH2 -

N -CH2¾N -CH2 -N -CH2 -N -CH2 -N -CH2 -

Such a polymer cannot be converted to its original state, it does not have thermoplastic properties and is called a thermosetting polymer.

In addition to the considered chemical bond between monomers during polycondensation, chemical bonds arise between other groups of monomers, some of them are given in table. 14.1.

Table 14.1. Chemical bonds between functional groups of some monomers arising from their polycondensation

Polymers		Examples of polymers
Polyamides Polyesters Polyurethanes Polyurea Silicones	¾О ¾ С¾ NH ¾ ¾NH ¾ C ¾ NH ¾ ¾ Si ¾ O ¾ Si ¾	Nylon, nylon Polyethylene terephthalate, terylene Vyrin, lycra Polynonemethylene urea, uralone Dimethylsiloxane rubber

Since in the process of polycondensation, along with high-molecular products, low-molecular-weight products are formed, the elemental compositions of the polymers and the starting materials do not coincide. This is how polycondensation differs from polymerization. The polycondensation proceeds according to a stepwise mechanism, while the intermediate products are stable, i.e. polycondensation can stop at any stage. The resulting low-molecular reaction products (H2O, NH3, HCl, CH2O, etc.) can interact with intermediate polycondensation products, causing their cleavage (hydrolysis, aminolysis, acidolysis, etc.), for example.

abstract

Polymers: classification, application

Completed:

5th year student of 4th group

Svensky S.I.

Oryol, 2012

Introduction ……………………………………………………………………………… ..3

1. Historical background ………………………………………………………………… 3

2. Classification of polymers ………………………………………………………… 4

3. The main methods of producing polymers ............................................................. 7

4. Application of polymeric materials …………………………………………… ... 8

Conclusion …………………………………………………………………………… ... 13

Literature ……………………………………………………………………………… 14

Introduction.

Polymers (Greek πολύ- - a lot; μέρος - a part) - inorganic and organic, amorphous and crystalline substances, consisting of "monomer units" connected into long macromolecules by chemical or coordination bonds. A polymer is a high molecular weight compound: the number of monomer units in the polymer (degree of polymerization) must be large enough. In many cases, the number of units can be considered sufficient to classify a molecule as a polymer if the addition of the next monomer unit does not change the molecular properties. Typically, polymers are substances with molecular weights from several thousand to several million. In the structure of the polymer, a monomeric unit can be distinguished - a repeating structural fragment containing several atoms. Polymers consist of a large number of repeating groups (units) of the same structure, for example, polyvinyl chloride (-CH 2 -CHCl-) n, natural rubber, etc. High-molecular compounds, the molecules of which contain several types of repeating groups, are called copolymers or heteropolymers.

A polymer is formed from monomers by polymerization or polycondensation reactions. Polymers include numerous natural compounds: proteins, nucleic acids, polysaccharides, rubber and other organic substances. In most cases, the concept refers to organic compounds, but there are many inorganic polymers. A large number of polymers are synthetically obtained on the basis of the simplest compounds of elements of natural origin through polymerization reactions, polycondensation and chemical transformations. Polymer names are derived from the monomer name with the prefix poly-: polyethylene, polypropylene, polyvinyl acetate, etc.

Historical reference

The term "polymerization" was introduced into science by I. Berzelius in 1833 to designate a special type of isomerism, in which substances (polymers) having the same composition have different molecular weights, for example, ethylene and butylene, oxygen and ozone. Thus, the content of the term did not correspond to modern concepts of polymers. The "true" synthetic polymers were not yet known by that time.

A number of polymers were apparently obtained as early as the first half of the 19th century. However, chemists then usually tried to suppress polymerization and polycondensation, which led to the "resinification" of the products of the main chemical reaction, ie, actually, to the formation of polymers (until now they were often called "resins"). The first mentions of synthetic polymers date back to 1838 (polyvinylidene chloride) and 1839 (polystyrene).

The chemistry of polymers arose only in connection with the creation of the theory of chemical structure by A.M. Butlerov (the beginning of the 1860's). AM Butlerov studied the relationship between the structure and the relative stability of molecules, manifested in polymerization reactions. Further development (until the end of the 1920's) the science of polymers received mainly due to the intensive search for methods of rubber synthesis, in which the leading scientists of many countries (G. Bouchard, W. Tilden, the German scientist C. Harries , I. L. Kondakov, S. V. Lebedev and others). In the 30s. the existence of free radical (G. Staudinger and others) and ionic (American scientist F. Whitmore and others) mechanisms of polymerization was proved. The works of W. Carothers played an important role in the development of the concept of polycondensation.

Since the beginning of the 20s. 20th century theoretical concepts of the structure of polymers are also being developed. Initially, it was assumed that biopolymers such as cellulose, starch, rubber, proteins, as well as some synthetic ones with similar properties (for example, polyisoprene), consist of small molecules that have an unusual ability to associate in solution into colloidal complexes due to non-covalent bonds (theory of "small blocks"). G. Staudinger was the author of a fundamentally new concept of polymers as substances consisting of macromolecules, particles of an unusually large molecular weight. The victory of the ideas of this scientist (by the beginning of the 40s of the 20th century) forced to consider polymers as a qualitatively new object of research in chemistry and physics.

Classification of polymers.

A huge number of polymers can be divided into three main classes that underlie the currently accepted classification.

TO first class includes a wide group of carbo-chain polymers, the macromolecules of which have a skeleton built of carbon atoms. Typical representatives of this class of polymers include polyethylene, polypropylene, polyisobutylene, polymethyl methacrylate, polyvinyl alcohol, and many others. A fragment of the macromolecule of the first of them has the following structure:

[—CH 2 —CH 2 -] n.

To second class There is a no less extensive group of heterochain polymers, the macromolecules of which in the main chain contain heteroatoms in addition to carbon atoms (for example, oxygen, nitrogen, sulfur, etc.). Polymers of this class include numerous simple and complex polyesters, polyamides, polyurethanes, natural proteins, etc., as well as a large group of organoelement polymers: polyethylene oxide (polyether); polyethylene terephthalate (polyester) polyamide; polydimethylsiloxane.

Third class polymers - high molecular weight compounds with a conjugated system of bonds. These include various polyacetylenes, polyphenylenes, polyoxadiazoles, and many other compounds. Examples of such polymers include: polyacetylene; polyphenylene; polyoxadiazole.

An interesting group of chelated polymers, which include various elements capable of forming coordination bonds, belongs to this class. The elementary unit of such polymers often has a complex structure.

Among the numerous polymeric materials, materials based on representatives of the first class of polymers - high-molecular-weight carbo-chain compounds - are still finding the greatest practical application. The most valuable materials can be obtained from carbo-chain polymers - synthetic rubbers, plastics, fibers, films, etc., and historically these polymers have found the first practical application (obtaining phenol-formaldehyde resins, synthetic rubber, organic glass, etc.). Many of the carbon chain polymers later became classical objects for research and the creation of a theory of the mechanical behavior of polymer bodies (for example, polyisobutylene, polymethyl methacrylate, polypropylene, phenol formaldehyde resin, etc.).

According to their recyclability, polymers are classified into thermoplastics and thermosets... Reactoplastics, when heated during processing, pass into an infusible and insoluble state. The processing of thermosetting plastics is irreversible - they do not soften again. Let's consider the first in more detail. Thermoplastic materials or thermoplastics include polymers that, when heated during processing, pass from a solid state of aggregation to a liquid state: highly elastic or viscous flow (injection molded thermoplastics pass into a viscous state). When the material is cooled, a reverse transition to a solid state occurs. The behavior on heating distinguishes thermoplastics from thermosetting materials or thermosets, which cure during processing and are not able to further transform into a liquid aggregate state.

Physical states of thermoplastics

Depending on the accepted phase states, thermoplastic materials are divided into amorphous and crystalline (more precisely, crystallizing). Crystallizing injection molded plastics always retain some fraction of non-crystallized (amorphous) material, therefore these materials are sometimes called partially crystalline. Some materials, in principle capable of crystallization, do not crystallize during injection molding, remaining amorphous. There are materials that can be amorphous or crystallize depending on the casting conditions. Others - very much change the degree of crystallinity and properties when changing the technological regime. The ability to crystallize is a very important property of materials, which determines their behavior during processing, and which must be taken into account when designing products and molds and choosing the technological mode of casting. Crystallizing materials have a high level of shrinkage and shrinkage anisotropy (the difference between longitudinal and transverse shrinkage). Pigments and other additives, acting as nucleators (crystallization nucleators), can significantly alter the structure and properties of crystallizing materials.

Depending on the temperature, amorphous thermoplastics have 3 physical states: glassy, \u200b\u200bhighly elastic and viscous.

The glassy state is characterized by small elastic deformations. The transition from a highly elastic state to a glassy state occurs in a certain temperature range, the center of which is called the glass transition temperature Tc. The glass transition temperature can vary significantly depending on the determination method. As the glass transition temperature rises, the operating temperature of the amorphous material rises.

A polymer in a highly elastic state is capable of large reversible deformations, reaching hundreds and more%. As the temperature rises, the molded thermoplastic material changes from a highly elastic state to a viscous flow. The temperature of this transition is called the pour point Tm. Above the pour point, irreversible deformation of the viscous flow appears in the polymer. When an amorphous material is heated, a nonphase transition is usually observed visually, resembling the melting process for crystallizing thermoplastics. The temperature of this transition is conventionally called the melting point of the amorphous material.

In crystallizing thermoplastics, the amorphous phase can acquire the physical states described above. When heated, the crystalline phase melts. The temperature of this phase transition is called the melting point Tm. The properties of crystallizing polymers depend on the content of the crystalline phase and on the physical state (glassy or highly elastic) of the amorphous phase at the operating temperature.

Classification of thermoplastics by performance

Injection molded thermoplastic materials are divided into several groups depending on the level of performance properties. These properties primarily include the temperature of long-term operation.

Plastics are rather conventionally divided into groups (different publications provide different classification criteria):

General purpose or general technical materials;

Construction plastics or plastics for engineering purposes;

Super structural or high temperature resistant polymers.

Among thermoplastics, a special group of thermoplastic elastomers or thermoplastic elastomers (TPE) is distinguished, which are conventional thermoplastics in terms of technological properties, and are similar to rubbers and rubbers in terms of performance, i.e. capable of large reversible deformations. Depending on the temperature of long-term operation, thermoplastic elastomers are also subdivided into general-purpose and engineering-technical materials.

Classification of thermoplastics by chemical structure

In terms of chemical structure, numerous injection molded thermoplastic materials are usually divided into several groups (classes). Modern industry produces a large number of types of polyolefins, the most important of which are the groups of polyethylene and polypropylene. Numerous types of materials are presented in the groups of styrene plastics, polyamides, polyesters.

Traditionally, groups of polymers based on cellulose, fluoropolymers, or fluoroplastics are distinguished. Manufacturers of acrylic polymers or acrylates often indicate only the belonging of the material to this group and do not indicate the type of material.

Classification of thermoplastics by production volume

Often in the literature, a group of large-capacity materials is distinguished, which include polyethylene and polypropylene, basic styrene plastics, and especially ABS, acrylates, PVC and bottle PET.

Homopolymers. Copolymers. Stereoisomers

Polymers built of the same monomers are called homopolymers, from different ones - copolymers.

For some types of materials (polypropylene, polystyrene, etc.), in addition to the chemical formula, stereoisomerism is of great importance - the type of spatial configuration of the side groups of atoms relative to the polymer chain. The most important types of stereoisomers are:

Isotactic - side groups are located on one side of the polymer chain;

Syndiotactic - side groups alternate sequentially on one and the other side of the polymer chain;

Atactic - an irregular arrangement of side groups on one and the other side of the polymer chain.

The development of technology for the synthesis of polymers using metallocene catalysts has made it possible in recent years to establish industrial production of various stereoisomers.

As an example of the effect of stereoisomerism on the performance properties of a material, we can cite syndiotactic polystyrene, which is a crystallizable material in contrast to conventional amorphous atactic polystyrene.

By structure, copolymers are divided into several types:

Block copolymer - regular alternation of sequences (blocks) of units in the main chain;

Statistical copolymer - an irregular alternation of sequences of units;

Graft copolymer - has a backbone in the form of a homopolymer or copolymer, to which side chains are attached;

An alternate or alternate copolymer is a regular alternation of units in the main chain.

Recently, interpolymers, copolymers that form a homogeneous structure (components do not precipitate into separate phases), have received great development.

In addition to double copolymers constructed from two types of monomer units, ternary copolymers, consisting of three types of units, as well as copolymers with four or more types of units are available. Ternary copolymers are ABS plastics, ACA copolymers, etc.

Classification of thermoplastics by type of filler

Fillers can significantly change the operational and technological properties of thermoplastics.

Thermoplastics containing fiberglass and other types of glass fillers are traditionally called fiberglass. In recent years, materials filled with long glass fibers, which require special processing conditions, have become widespread.

Carbon fiber materials are materials that contain carbon fiber.

Sometimes a group of "special" thermoplastics is distinguished. These include materials containing fire retardants (materials with increased resistance to combustion), electrically conductive additives (antistatic, electrically conductive, EMI-shielding materials), antifriction additives (materials with a reduced coefficient of friction), additives that impart wear resistance, etc.

The main methods for producing polymers.

Natural polymers are formed during biosynthesis in the cells of living organisms. With the help of extraction, fractional precipitation, and other methods, they can be isolated from plant and animal raw materials.

Synthetic polymers are produced by polymerization and polycondensation reactions.

Polymerization is the process of combining a large number of monomer molecules with each other through multiple bonds (C \u003d C, C \u003d O, etc.) or opening of rings containing heteroatoms (O, N, S). During polymerization, there is usually no formation of low molecular weight by-products, as a result of which the polymer and monomer have the same elemental composition,

Polycondensation is the process of combining with each other molecules of one or several monomers containing two or more functional groups (OH, CO, COC, NHS, etc.) capable of chemical interaction, in which low molecular weight products are cleaved. Polymers obtained by the polycondensation method do not correspond to the initial monomers in terms of their elemental composition.

Polymerization of monomers with multiple bonds proceeds according to the laws of chain reactions as a result of the breaking of unsaturated bonds. During chain polymerization, a macromolecule is formed very quickly and immediately acquires a finite size, i.e., it does not increase with an increase in the duration of the process.

Polymerization of monomers with a cyclic structure occurs due to the opening of the ring and in some cases it is baked not by a chain, but by a stepwise mechanism. During stepwise polymerization, a macromolecule is formed gradually, i.e., first a dimer is formed, then a trimer, etc., so the molecular weight of the polymer increases with time.

The fundamental difference between valuable polymerization from stepwise and from polycondensation is that at different stages of the process, the reaction mixture always consists of monomer and polymer and does not contain di-, tri-, tetramers. With an increase in the reaction time, only the number of polymer macromolecules increases, and the monomer is consumed gradually. The molecular weight of the polymer does not depend on the degree of completion of the reaction or, which is the same, on the monomer conversion, which only determines the polymer yield.

Many polymers cannot be obtained either by polymerization or polycondensation, since either the starting monomers are unknown, or the monomers do not form high molecular weight compounds, the synthesis of such polymers is carried out starting from high molecular weight compounds, the macromolecules of which contain reactive functional groups. For these groups, polymers undergo the same reactions as low-molecular-weight compounds containing such groups.

Reactions in polymer chains can occur without significant changes in the molecular weight of the polymer (also called polymer-like transformations), with an increase in the molecular weight of the polymer (synthesis of grafted and block copolymers) or with a decrease in molecular weight (destruction of macromolecules).

Similar information.

Stepwise synthesis (polycondensation and stepwise polymerization) proceeds along the terminal functional groups of the monomers, the growing chain after each act of addition remains a stable compound, the process of polymer formation proceeds in steps at a low rate. In this case, MM grows gradually, and the molecular weight distribution changes continuously (Fig. 10). Polymers with a narrow molecular weight distribution are formed at the initial stages of the reaction, and with an increase in the conversion of monomers, it becomes wider. Therefore, it takes longer to complete chain growth than chain processes.

Fig. 10. Dependence of the degree of polymerization p (a)

and molecular weight distribution (b) on the degree of conversion of p functional groups: Мх - molecular weight of the fraction; Wх - mass fraction of the fraction.

Polycondensation is the process of formation of polymers from bi- or polyfunctional monomers with the release of low molecular weight by-products (water, alcohols, etc.), therefore, the elemental composition of their units does not correspond to the composition of monomers:

p (A-R-A) + p (B-R1-B) A - [- R-R1-] n-B + (2n1) AB,

where A-R-A and B-R1-B are the starting monomers; A and B functional groups; AB-side low molecular weight compound. First, dimers are formed, then trimers, tetramers, and then oligomers, reacting with each other to a polymer that is formed at the stage of high completion of the reaction (more than 98%). The yield and MW of the polymer depend on the reaction time. Due to the stability of the molecules, oligomers can be isolated and used in further condensation reactions with each other or with other monomers to synthesize new polymers. The reactions of homogeneous molecules with different or with the same functional groups are called homopolycondensation:

nH2N (CH2) 6COOH [-NH (CH2) 6CO-] n + (n-1) H2O,

nHO- (CH2) x-CO-OH H [-O- (CH2) x-CO-] nOH + (n-1) H2O.

nHO (CH2) 2OH [-CH2-O-] n + (n-1) H2O.

Heteropolycondensation involves dissimilar molecules with different functional groups, for example, in the synthesis of polyamides:

nH2N (CH2) 6NH2 + nНООС (СН2) 4СООН

H [-NH (CH2) 6NHCO (CH2) 4CO-] n-OH + (2n-1) H2O.

The process of obtaining high-molecular compounds, in which two or more monomers participate, each of which is capable of forming its own polymer, is called copolycondensation:

2nH2N (CH2) 6NH2 + nHOOC (CH2) 4COOH + nHOOC (CH2) 8COOH

[-NH (CH2) 6NHCO (CH2) 4CONH (CH2) 6NHCO (CH2) 8CO-] n + 4nH2O.

Bifunctional monomers are classified into three main classes:

· With various functional groups interacting with each other: amino acids (H2N-R-COOH), hydroxy acids (HO-R-COOH), etc., for polycondensation, one monomer of this class can be used;

· With the same functional groups that do not interact with each other: diamines (H2N-R-NH2), dicarboxylic acids and their derivatives, therefore, two monomers are required for polycondensation;

· Monomers with the same functional groups capable of interacting with each other, for example, glycols (HO-R-OH); in this case, the synthesis is carried out using one monomer with the same functional groups. An example is the reaction for the synthesis of ethers from glycol. Bifunctional monomers form linear macromolecules (linear polycondensation), and monomers with three or more functional groups (for example, phenol-formaldehyde resins) form branched and networked structures.

Stepwise or migration polymerization (polyaddition) is similar to polycondensation according to the laws of the process. Monomer molecules attach to the growing chain, which is a stable particle, without the release of low-molecular-weight products by the movement (migration) of hydrogen. Polyaddition is similar to polymerization according to the correspondence between the compositions of units and monomers, but it consists of separate independent stages and obeys the basic laws of equilibrium polycondensation. The reaction involves two bi- or polyfunctional monomers, one of which contains a mobile hydrogen atom (amines, phenols, alcohols, acids), and the second group capable of attaching it. The reaction of diepoxides with dicarboxylic acids, polyamines, bisphenols and polyalcohols proceeds according to the stepwise polymerization mechanism. Epichlorohydrin with dihydroxydiphenylpropane in an alkaline medium form a variety of products from viscous liquid to solid consistency:

Along with the terminal epoxy groups, the macromolecule contains secondary hydroxyl groups, which are also capable of entering into further reactions with bifunctional compounds to form three-dimensional polymers. For curing epoxy oligomers, in addition to those listed above, anhydrides of dicarboxylic acids, diisocyanates, and various oligomers containing functional groups (polyamides, polysulfides) that interact with secondary hydroxyls are used. Reacting with epoxy end groups, they increase the chain length and strength of the polymers.

When diamines interact with terminal epoxy groups, secondary hydroxyl groups arise, which can also react with diisocyanates or dianhydrides to form "crosslinked" structures:

2 ~ CH-CH2 + H2N-R-NH2 ~ CH-CH2-HN-R-NH-CH2-CH ~.

When dicarboxylic acids react with terminal epoxy groups, oligomers are obtained that also contain ester groups. The synthesis of polyurethanes from isocyanates and glycols is similar:

To obtain a polymer, it is necessary that the starting monomers contain at least two functional groups. If the glycol is replaced with a polyhydric alcohol (glycerin, pentaerythritol, etc.) or a diisocyanate with a triisocyanate, then spatially crosslinked polymers are obtained, similar to the products of a three-dimensional polycondensation reaction. Aromatic isocyanates and fatty alcohols are more reactive than aliphatic isocyanates and bisphenols. The MM of polyurethanes increases with an increase in the duration of the polyaddition reaction. The ratio of diisocyanate to glycol and the temperature of synthesis have a significant effect on MM.

Ring-opening polymerization of monomeric molecules (ethylene and propylene oxides, trioxane, e-caprolactam, cyclopentene) also often proceeds as a stepwise reaction. Caprolactam is activated by water, acid or base, which are attached only to the first monomer molecule, and then the mechanism of migration polymerization is realized:

The stepwise synthesis of polymers includes equilibrium (reversible) and nonequilibrium (irreversible) processes. A feature of equilibrium processes, for example, the synthesis of polyamides by heating dicarboxylic acids with diamines, is the occurrence of reverse reactions with a low molecular weight product, leading to the decomposition of polymer chains. The released low-molecular-weight product (water from diamine) can react with amide groups, and as a result of hydrolysis, initial structures are formed or low-molecular fragments are released from macromolecules. The synthesis of phenol-formaldehyde resins with a network structure is an example of a nonequilibrium reaction. The liberated water and formaldehyde cannot react again with ether bonds or methylene groups between phenolic nuclei, respectively, and the reaction equilibrium is almost completely shifted towards the formation of a reticulated polymer. In addition, the very network structure of the polymer contributes to the shift of the reaction to the right, since the system becomes insoluble and infusible. Therefore, its functional groups, even in those cases when they can react with low molecular weight components, are inaccessible to them, and the reverse reaction practically does not occur.

A low molecular weight product of linear polycondensation of dihalogenated hydrocarbons and sodium polysulfide, which is incapable of reacting with functional groups in macromolecules of the polysulfide elastomer, is also the reason for the non-equilibrium reaction, even if the system does not lose its solubility and fusibility: nCl-R-Cl + nNa2Sx- ( -R-Sx-) n- + 2nNaCl. Isolation of the low-molecular-weight component in the gaseous state at the interface between the phases of monomers that do not mix with each other also ensures the nonequilibrium of the reaction. In the synthesis of polyamides from acid dichloroanhydrides and diamines, the polymer formation reaction also takes place in a narrow region - at the interface between two immiscible monomer solutions. The polyamide formed in the form of a thin film can be continuously removed mechanically, which makes it possible to carry out the reaction under practically nonequilibrium conditions until the monomers are completely depleted.

Reversible and irreversible reactions of polymer synthesis by a stepwise mechanism are quantified by the equilibrium constant - the ratio of the rate constants of the forward and reverse reactions: Kp \u003d Kpryam / Kobratn. The reaction of polymer synthesis is considered to be equilibrium at Кр no more than 102 and nonequilibrium at Кр more than 103. At intermediate values \u200b\u200bof Кр, equilibrium is assessed by the reaction conditions: for reversible reactions - low rates and high activation energy (80-170 kJ / mol), and irreversible ones - high rates and low activation energy (8-42 kJ / mol).

According to the regularities of the course, stepwise reactions differ significantly from chain reactions. Two factors determine the size and structure of polymer macromolecules: stoichiometry, if the number of components is more than one, and the degree of completion of the reaction by the consumption of functional groups of the reacting components. If the functional groups are contained in the initial composition in equimolar ratios, then their stepwise reactions with each other continue until they are completely exhausted, and free functional groups are always present at the ends of macromolecules. If the system has an excess of functional groups of the same nature, then the functional groups of the opposite nature are quickly consumed in the reactions. Excessive terminal functional groups of the same nature cannot react with each other, and the growth of macromolecules will stop. The greater the excess of some groups in relation to others, the earlier the growth of macromolecules will stop and the lower the value of the average MM of the final product will be. Thus, an excess of functional groups of one of the monomers plays the role of a stopper of the polymer formation reaction and interrupts this reaction at the stage of low molecular weight products or oligomers. The main differences between stepwise synthesis processes from radical chain and ionic reactions:

· Gradual increase in MM over time, and in chain reactions - the rapid formation of macromolecules, the size of which changes little over time;

· The initial monomers are quickly consumed into low-molecular and oligomeric products that react with each other to form a polymer, and in chain reactions they are gradually consumed and are present at any stage up to complete conversion;

· Intermediate reaction products - stable molecules, in contrast to unstable free radicals or ions with a short lifetime;

· High-molecular-weight products are present in the reaction system in appreciable amounts only at high degrees of conversion of functional groups, i.e. with a long reaction time, and in chain reactions, they are present at any conversion of monomers;

· Initial, intermediate and final products are quantitatively determined at any stage of the reaction, since they are stable and their sizes are continuously changing, and there are no intermediate products in chain processes, and only initial and final products are present at any stage of the reaction.

It follows from the foregoing that, in terms of a number of indicators, stepwise processes are inferior to chain reactions of polymer synthesis. To this it should be added that the starting monomers for chain processes are generally more available and cheaper than monomers with functional groups for stepwise synthesis. For these reasons, chain synthesis processes are used more in the production of general-purpose high-tonnage polymers. However, the nature of the monomers and the raw material sources of their production for both types of processes are significantly different. A number of the most important industrial polymers (polyamides, polyurethanes, various polyesters, including polyarylenes and polythioethers, as well as phenol-formaldehyde and other resins) can be obtained only as a result of stepwise synthesis processes. The choice of these processes is determined not only by the availability and cost of raw materials, but also by the requirements that technology places on the properties of polymers, as well as the possibilities of their satisfaction due to the structure of the corresponding polymers.

General information about high molecular weight compounds

Topic 11. Technology of high molecular weight compounds

Test questions for topic X

"OO and HX synthesis technology"

1. List the main industrial syntheses based on synthesis gas and carbon monoxide (II).

2. What properties does methanol have?

3. How is the required process selectivity achieved in the synthesis of methanol from synthesis gas?

4. What technological schemes are used in the production of methanol?

5. List the most important uses of methanol.

6. From what types of raw materials can ethanol be produced on an industrial scale?

7. Explain the advantages of the direct ethylene hydration method over the sulfuric acid hydration method in the production of synthetic ethanol.

8. What catalysts are used in the production of ethanol by direct hydration of ethylene in the vapor phase?

9. What is hydrolysis production? Why is it low-waste?

10. What stages does the hydrolysis of ethanol production consist of and how each stage is catalyzed?

11. What compounds are classified as higher synthetic fatty acids (HFA) and alcohols (HFA)?

12. Indicate the main industrial methods for the production of HFA and HFA.

13. What is common in the chemistry of obtaining high-fatty acids and high-fatty acids by alkane oxidation?

14. How is the oxidation process interrupted in the production of HFL, preventing the destruction of the alkane molecule?

15. What are synthetic detergents and what is their relationship with VZhS, VZhK?

Plastics, rubbers, chemical fibers and polymer composite materials as the main types of polymer materials. The share of polymeric materials in the gross chemical production of industrially developed countries. Methods for carrying out polymerization reactions in the gas phase, in solution, in suspension, in emulsion and bulk polymerization. The advantages and disadvantages of these methods. Industrial production of polyethylene, polypropylene, polystyrene, polyvinyl chloride, as well as copolymers based on them. Comparison of various technological schemes for obtaining PE (low and high density). Polycondensation processes and their technological design. Phenolic-formaldehyde and urea-aldehyde, pillow and resole resins. Organosilicon polymers. Polyurethanes. Basic properties and areas of their application. Man-made fibers: artificial, cellulose-based and synthetic. The main methods of forming fibers from solutions and melts. Properties and fields of application. Production of synthetic rubbers. Rubbers for special purposes. Processing of rubber into rubber. Environmental aspects of the production of polymer materials and products based on them.

All living and inanimate nature around us is built of molecules, which in turn are composed of atoms. Atoms, connecting with each other in various ratios, form molecules that differ from each other in size, structure, chemical composition and properties.

Substances built from a small number of atoms are called low molecular weight. Their molecular weight does not exceed several hundred units. Low molecular weight substances are salts, acids, alkalis, alcohols and other compounds.

At the same time, many substances are composed of giant molecules, which include thousands, tens and hundreds of thousands of atoms. Such molecules are called macromolecules; their molecular weight reaches hundreds and even thousands of units. For example, the molecular weight of the molecules that make up natural rubber is 136,000-340,000.

Compounds built from macromolecules are called high molecular weight or polymers.

Polymers are subdivided into natural and synthetic by origin.

Natural, that is, natural, polymers include cellulose, which is part of wood, cotton and other plants; proteins that make up living organisms; natural rubber, etc.

Synthetic polymers are produced artificially by chemical synthesis; they are part of plastics, synthetic rubbers, chemical fibers, varnishes, etc.

Composition and properties of polymers. Polymer molecules are long chains in which identical links alternate. If we designate these links with the letter A, then the polymer molecule can be represented as follows:

In synthetic polymers, these units are the remnants of the molecules of the parent compounds, consisting of only a few atoms. These starting compounds are called monomers. For example, ethylene CH 2 CH 2 is a monomer for producing a high molecular weight compound called polyethylene. During the formation of a polymer, a double bond between ethylene molecules opens up, and due to the resulting free valences of carbon, a large number of units obtained from the monomer are connected to each other. This can be schematically represented as follows:

The diagram shows only three links in the composition of the polymer, in fact, their number in polyethylene is from 1000 to 10,000, and the molecular weight of such a polymer ranges from 28,000 to 280,000.

It can be seen from the above scheme that both in the monomer and in the polymer there are two hydrogen atoms per carbon atom, i.e., the elementary composition of the resulting polymer is the same as the monomer.

With a change in the number of monomer molecules bound to each other, the properties of the resulting polymers change. Thus, polyethylene, as its molecular weight increases, becomes more viscous, then pasty, and finally solid. The properties of polymers also depend on the chemical composition of monomers, the shape of molecular chains and their structure (polymer structure).

In a macromolecule with a linear structure, the elementary units form a filamentous molecule, i.e., each unit is connected only with two neighboring units (Fig. and). Filamentary (linear) macromolecules can be located parallel to each other in the polymer (Fig. b) or intertwine without a chemical bond between individual macromolecules (Fig. in). They can be curved, rolled into a ball (Fig. d, e) etc. Macromolecules of linear structure are characteristic of polyethylene, polypropylene, cellulose, polyesters, polyamides and many other high molecular weight compounds widely used to obtain fibers, films, plastics, and rubber. These polymeric materials are generally strong, resilient, dissolve and melt when heated.

Branched macromolecules have side branches from the main chain (Fig. e). Branched polymers dissolve and melt more difficult than linear polymers.

Macromolecules with a network structure are constructed as follows: long chains of molecules are linked to each other by short chains in three dimensions, which is difficult to depict in the figure. Usually, such a structure of polymer molecules is depicted in the form of linearly constructed large molecules connected to each other (Fig. f). It always means that linear molecules are chemically bonded to molecules located above and behind the plane of the paper. This molecular structure is also called spatial or three-dimensional. The greater the number of "bridges" in such a macromolecule, the less elastic the polymer is and the properties of a solid are largely manifested in it.

The chain structure of polymer molecules can be different. In some cases, polymer molecules are formed, in which the elementary units have different spatial arrangement of side groups, in others - a strictly regular spatial arrangement. Polymers with a strictly regular molecular structure are called isotactic. Polymers of this type have high hardness and heat resistance.

Polymer molecules may not be composed of the same units. They can be obtained from different monomers, for example A and B. Then the macromolecule can be depicted as follows:

Such high molecular weight compounds are called copolymers. They combine the characteristic properties of polymers obtained from each component separately.

Thus, it is possible to impart some specific properties to polymers, for example, to obtain rubbers with increased benzo and oil resistance, chemical resistance, etc.

Of interest are the so-called graft copolymers. The chains of their molecules are built according to the following scheme:

Such a polymer can be compared to a fruit tree to which another fruit tree variety is grafted. As a result of such "grafting", fruits are obtained that combine the most valuable qualities of both varieties. In a grafted copolymer, one polymer is grafted to the "stem" of another polymer. The resulting "hybrid" has the properties of the starting materials. Thus, it is possible to obtain polymers that combine, for example, high electrical insulating properties with fire resistance and resistance to gasoline and oils.

Macromolecules can be built from relatively low molecular weight “blocks” derived from various monomers. The scheme of such a block copolymer is:

Block copolymers also combine the properties of the parent polymers.

Until now, the elementary links in the macromolecule were conventionally designated A and B. It can be seen that the organic polymers are based on carbon, the atoms of which have joined together, forming the "skeleton" of the molecule, framed by hydrogen atoms. Instead of hydrogen atoms, there can be groups of atoms, in which atoms of other elements can be present along with carbon atoms.

If the skeleton of polymer molecules is built of carbon atoms, they are called carbon chain. There are molecules in the skeleton of which carbon atoms periodically alternate with atoms of other elements, for example:

Such polymers are called heterochain.

The behavior of polymers when heated depends on the structure of the molecules. Linear and branched polymers soften when heated, with subsequent cooling they pass into a solid state. These polymers are called thermoplastic. Polymers, the molecules of which have a spatial structure, do not melt when heated: they are called thermosetting.

The temperature of transition of a polymer from a solid to an elastic state (or vice versa) is called the glass transition temperature, the transition temperature to a fluid state is called the pour point.

Polymers can be either completely amorphous substances - amorphous polymers, or substances containing crystalline and amorphous regions - crystalline polymers. According to the types of deformations that arise in polymers under the influence of external conditions at room temperature, they are subdivided into solid polymers, elastic polymers, or elastomers, and flowable polymers.

Thus, by changing the size of the resulting macromolecule, its molecular weight and shape, making up a macromolecule from various initial monomers, grafting a polymer chain from units formed by another monomer to one macromolecule, it is possible to broadly change the physical and chemical properties of polymers, to obtain them with predetermined properties, change their physical state, make them liquid, solid, plastic and elastic.

Polymers have a low density (the lightest plastics are 800 times lighter than steel), high mechanical strength (exceeds the strength of wood, glass, ceramics), high thermal, sound and electrical insulating properties, high chemical resistance, excellent optical properties, they are able to absorb and to damp vibrations, form extremely thin films and fibers, they are easily processed and processed into products. The valuable properties of polymers have led to their widespread use in various sectors of the national economy: in mechanical engineering, construction, automotive, aviation, nuclear, space and other branches of technology, for the manufacture of fabrics, artificial leather, household items, in medicine, etc.

The production of polymer materials in our country is developing very rapidly, exceeding the growth rates of the entire industry and other branches of the chemical industry.

Polymers can be obtained by polymerization and polycondensation methods.

Polymerization. The method of polymerization consists in the fact that the molecules of monomers under the influence of heating, catalysts, γ-rays, light, initiators combine with each other into molecules of large sizes. In this case, macromolecules of a linear, branched, network structure, copolymer molecules, graft copolymers are formed.

The polymerization rate and molecular weight of the polymer depends on temperature, pressure, catalyst activity, etc.

There are the following polymerization methods: in bulk (block method), in emulsions, in solution and the so-called suspension polymerization.

Bulk polymerization takes place in an apparatus (autoclave),
where the starting monomer is fed with a catalyst or initiator - a substance that reacts with the monomer and accelerates polymerization. At the beginning of the polymerization, the reacting mass is heated, then the heating is stopped, since the polymerization is accompanied by the release of heat. To maintain a certain temperature in the apparatus during the polymerization process, sometimes they resort to cooling the reacting mass. At the end of polymerization, a solid mass of polymer in the form of a block is removed from the apparatus. The polymerization process can be either batch or continuous.
When polymerizing in bulk, it is difficult to ensure the same temperature throughout the reacting mass, therefore, the resulting polymer consists of macromolecules with different degrees of polymerization. This method is used to obtain polystyrene, methacrylic acid polymers, butadiene rubber, etc.

The emulsion polymerization method consists in the fact that the monomer is mixed with an initiator and an emulsifier and is converted by means of stirrers into tiny droplets suspended in another liquid, most often in water. (Emulsifiers are substances that prevent liquid droplets from merging.) The resulting emulsions are heated to a temperature at which the monomer polymerizes. At the same time, the heat released during the polymerization process is easily removed and the resulting polymer is more uniform than that obtained by the block method. The disadvantage of this method is the difficulty in separating the emulsifier from the polymer. This method is used to obtain copolymers of butadiene, vinyl acetate, acrylonitrile, etc.

Solution polymerization is carried out in a solvent that mixes with the monomer and dissolves the resulting polymer. The polymer is isolated from the resulting solution by solvent evaporation or precipitation. The polymerization is also carried out in a solvent that dissolves the monomer but does not dissolve the polymer. In this case, the polymer precipitates, which is filtered off. This method is used to obtain polyvinyl acetate, polybutyl acrylate, etc.

The suspension method involves grinding (dispersing) the monomer in the form of drops in a poorly dissolving medium, usually water. Polymerization takes place in each drop of monomer. The resulting polymer in the form of solid particles that do not dissolve in water is precipitated and separated from the liquid by filtration.

Polycondensation. The method consists in the fact that the connection of monomer molecules with each other occurs during the reaction between them, which proceeds with the release of by-products. For example, let's designate the molecule of one of the reacting substances through a-A-a, and the second b-B-b. The reaction scheme between them can be represented as follows:

From the reacting molecules, a molecule of substance a-A-B-b was formed, and at the same time substance a-b was released. The molecule of substance a-A-B-b can continue to react with monomers. Due to the addition of new monomer molecules, the polymer chain grows. In this case, the addition of each new molecule is accompanied by the release of substance a-b.

As a result, the chemical composition of polymer molecules is somewhat different from the original monomers.

In the process of polycondensation, polymers are obtained that have a linear as well as a network structure.

The polycondensation process is exothermic, and therefore, based on the Le Chatelier principle, to shift the equilibrium from left to right, it is necessary to carry out the process at a low temperature. However, to increase the speed of the process, it is necessary to increase the temperature. Therefore, to increase the rate of polycondensation, the process is first carried out at an elevated temperature, and then it is gradually reduced to shift the equilibrium of the reaction, and thus a product with a higher molecular weight is obtained.

The polycondensation is carried out both in the presence of a catalyst and without it. It is carried out in melt, solution and at the interface between two phases.

Melt polycondensation is carried out at a high temperature (220-280 ° C) in a reactor in an inert gas atmosphere. Thus, they provide a high speed of the process and the removal of low molecular weight products.

During polycondensation in solution, the monomers are dissolved in the solvent - the reaction proceeds at a low rate, the removal of low molecular weight products is not ensured. This method is not used in industry.

Polycondensation at the interface consists in the fact that there are two immiscible liquids, in each of which the starting monomers are dissolved. The polycondensation reaction occurs instantly at the interface with the formation of a polymer film. Thus, the reaction products are removed from the reaction sphere, which facilitates the reaction to proceed at a high rate. When the film is removed, the interface is released and the reaction continues.

Polymerization is called the process of sequential addition of free radicals or monomer ions to the growing chain of the polymer macromolecule (Fig. 12.1). During polymerization, active centers are formed as a result of breaking multiple or cyclic bonds. If free bonds are formed due to the cleavage of functional groups (active terminal atoms or their combinations) from the initial monomers and the release of low molecular weight by-products occurs, then the process is called polycondensation (fig.12.2).

Some polymers (polyurethanes, epoxies) result from step polymerization (polyconnection). In this case, monomer molecules initially form short molecular chains ( prepolymers), which are then combined into long macromolecules. Polymer formation reactions proceed in three main stages:

1. Initiation reactions (formation of an active center). The polymerization reaction does not start by itself. It is necessary to expend energy to break a multiple or cyclic bond, as a result of which active centers are formed - free radicals or ions. The formation of active centers occurs under the influence of heat, light, radiation and in the presence of initiators - substances containing unstable chemical bonds in their molecules (O - O, N - N, S - S, O - N, etc.), which break much more easily than bonds in a monomer molecule. The amount of initiator introduced is usually small (0.1-1%).

In contrast to polymerization, polycondensation occurs spontaneously when functional groups interact.

2. Chain growth. During polymerization, monomers are sequentially added to the growing polymer chain according to the scheme [- A -] n + -AND- -*? [ - AND - ] n + v In this case, the macromolecule must remain a free macroradical (macroion).

During polycondensation, independent from each other acts of unification of monomeric radicals and chains formed from them occur according to the scheme [-A - x + [- A - y ^ [- A - x + y. The polyaddition reaction proceeds according to the same scheme; however, despite the similarity to polycondensation, this reaction is polymerization, since the formation of active centers occurs as a result of bond breaking.

3. Open circuit. The end of polymerization is associated with the disappearance of the free bond at the last link of the macromolecule. This happens in three ways: 1) as a result of the connection between two macroradicals (recombination reaction) according to the scheme: x- [- AND - + + [- AND - ] - x-\u003e x- [- AND - ] + - x; 2) as a result chain transfer reactions, when the active center passes to any other molecule (solvent or impurity) which, turning into a radical, gives rise to a new macromolecule: x- [- AND - ] n + RH-\u003e x- [- A -] n - H + + R -; 3) when introducing inhibitors - substances that, when inter-

Figure: 12.1.

Figure: 12.2. The polycondensation reaction on the example of obtaining a phenol-formaldehyde resin and reacting with free radicals form inactive particles that are unable to initiate the polymerization process.

The polycondensation process can stop for several reasons: due to a violation of the equivalent ratio of functional groups, an increase in the viscosity of the reaction medium and the associated decrease in the mobility of macromolecules, an established equilibrium state, when both the formation of longer chains and their decay (destruction) occur simultaneously. The reversibility of the reaction is a characteristic feature of the polycondensation process. To avoid destruction, it is necessary to remove the formed by-products. The molecular weight of the resulting polymer can be limited by introducing monofunctional compounds that block the functional groups of one of the monomers and stop the growth of the polymer chain.