obtained by polymerization. §xiii.1

Polymerization and polycondensation

Synthetic polymers are obtained as a result of polymerization and polycondensation reactions. The preparation of polymers by polymerization and polycondensation reactions are the main routes for the synthesis of IUDs today.

Polymerization is the process of joining together a large number monomer molecules due to multiple bonds (C = C, C = O, etc.) or opening of cycles containing heteroatoms (O, N, S). During polymerization, the formation of low molecular weight by-products usually does not occur, as a result of which the polymer and monomer have the same elemental composition.

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

The polymerization of monomers with multiple bonds proceeds according to the laws of chain reactions as a result of the breaking of unsaturated bonds. A macromolecule during chain polymerization is formed very quickly and immediately acquires finite dimensions, that is, it does not increase with an increase in the duration of the process.

Polymerization of monomers of a cyclic structure occurs due to ring opening and in some cases bakes not according to a chain, but according to a stepwise mechanism.

A macromolecule during stepwise polymerization is formed gradually, i.e., first a dimer is formed, then a trimer, etc., therefore molecular mass polymer grows with time.

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

Reactions in polymer chains

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 compounds when polymers are obtained by the polymerization and polycondensation reaction. The synthesis of such polymers is carried out on the basis of macromolecular compounds, the macromolecules of which contain reactive functional groups. For these groups, the polymers enter into the same reactions as low-molecular compounds containing such groups.

Reactions in polymer chains can occur without a significant change in the molecular weight of the polymer (the so-called polymer-analogous transformations), with an increase in the molecular weight of the polymer (synthesis of graft and block copolymers), or with a decrease in molecular weight (destruction of macromolecules).

Literature

1. Encyclopedia of polymers.. M., Soviet Encyclopedia. Vol. 1, 1972, Vol. 2, 1974, Vol. 3, 1977.
2. E. A. Brantskhin and E. S. Shul’gina, Tekhnologiya plasticheskih masses. M., Chemistry, 1974

Announcements for the purchase and sale of equipment can be viewed at

You can discuss the advantages of polymer grades and their properties at

Lecture 20-21

Polymers. Getting polymers. Reactions of polymerization and polycondensation. Classification of polymers. Types of polymers used in engineering.

Polymers- high-molecular compounds with a regularly alternating large number of identical or unequal atomic groups connected in a chain by chemical bonds; they may have side branches or be spatial grids. Polymer molecules, also called macromolecules. Due to the large molecular weight of macromolecules, polymers acquire some specific properties. Therefore, they are allocated to a special group of chemical compounds.

Depending on the composition of the main chain, polymers are divided into organic (― S—S―), inorganic (― Si—Si―), elementorganic (― S―Me―S―).

The ability of chemical compounds to form polymers is determined the functionality of their molecules. Functionality characterizes the number functional groups in a molecule:

Where M- molecular mass chemical compound;

is the equivalent molar mass;

M FG is the molar mass of the functional group;

With FG is the concentration of functional groups (in % by weight).

We will consider organic synthetic polymers.

Examplesfunctional groups:

hydroxyl; carboxyl; amino group amino group sulfo group carbonyl primary secondary

The functionality of a substance can also be determined by the presence of double or triple bonds in its molecule or by the presence of mobile hydrogen atoms.

Preparation of polymers

The main methods for the synthesis of macromolecular compounds:

1. Polymerization;

2. Polycondensation;

POLYMERIZATION – chain process. This is the synthesis of a polymer by sequentially attaching molecules of a low molecular weight substance (monomer) to an active center located at the end of a growing chain. Compounds containing multiple bonds ( С≡С, C=C, C=O, С≡N etc.), or capable of opening cyclic groups (oxidation of olefins, lactam, etc.).

, , .

During polymerization, multiple bonds are broken or cycles are opened in monomers and chemical bonds between groups to form macromolecules, for example:

nCH 2 \u003d CH 2 (-CH 2 -CH 2 -) n

ethylene polyethylene

styrene polystyrene"

Depending on the type of monomers involved in polymerization, there are homopolymerization (one type of monomer) and copolymerization (two or more types of monomers).

Polymerization is a spontaneous exothermic process (<0), так как разрыв двойных связей или циклов с образованием ординарных 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 influences. Most often, polymerization initiators are peroxides, azo compounds (having the functional group -N=N-) and other compounds with weakened bonds. Initially, radicals are formed, for example:

(C 6 H 5 COO) 2 2C 6 H 5 COO ● (R ●)

benzoyl peroxide.

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

R ● + CH 2 \u003d CHCI RCH 2 -CHCl ●

RCH 2 -CHCl ● + CH 2 \u003d CHCI RCH 2 -CHC1-CH 2 -CHCl ● etc.

b) Chain growth occurs due to attachment to radicals
the resulting monomers to give new radicals.

c) The transfer of the chain consists in the transfer of the active center
to another molecule (monomer, polymer, solute molecules)
body):

R-(-CH 2 -CHS1-) n -CH 2 -CHS ● + CH 2 = CHS1

R- (-CH 2 -CHS1-) n -CH 2 -CH 2 C1 + CH \u003d CHCl ●

As a result, chain growth stops, 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.

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

R-(-CH 2 -CHC1-) 2 -CH 2 -CHCl ● + R-(-CH 2 -CHC1-) n -CH 2 -CHCl

R-(-CH 2 -CHCI-) n -CH 2 -CHS1-CH 2 -CHCl-(-CH 2 -CHCI-) n -R

Chain termination can also occur when low-active radicals are formed that are not able 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 with the help of initiators, inhibitors, and other substances. Nevertheless, chain transfer and chain termination can occur at different stages of chain growth; therefore, macromolecules have different molecular weights, i.e., they are 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-CHS1-] n, polyvinyl acetate [-CH 2 -CH (OCOCH3) -] P, polystyrene [-CH 2 -CH (C 6 H 6) -] n, polyacrylate, [-CH 2 -C (CH 3) (COOR) -] n, polyethylene [-CH 2 -CH 2 -] p, polydienes [ -CH 2 -C(R)=CH-CH 2 -] n and various copolymers.

Ionic polymerization also goes through the stage of formation of active sites, growth and chain termination. The role of active centers in this case is played by anions and cations. Accordingly, they distinguish anionic And cationic polymerization. The initiators of cationic polymerization are electron-withdrawing compounds, including protic acids, such as H 2 SO 4 and HC1; inorganic aprotic acids (SnCl 4 , TiCl 4 , AlCl 3 , etc.), organometallic compounds A1 (C 2 H 5) 3, etc. As initiators of anionic polymerization, electron-donating substances and compounds are used, including alkali and alkaline earth metals, alcoholates alkali metals, etc. Often several polymerization initiators are used simultaneously.

Chain growth can be written by the reaction equations for cationic polymerization

and anionic polymerization.

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

Bulk (block) polymerization is the polymerization of a liquid monomer(s) in an undiluted state. In this case, a sufficiently pure polymer is obtained. The main difficulty of the process is associated with the removal of heat. In solution polymerization, the monomer is dissolved in the solvent. With this method of polymerization, it is easier to remove heat and control the composition and structure of polymers, however, the problem of removing the solvent arises.

Emulsion polymerization (emulsion polymerization) consists in 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 large 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 are formed ranging in size from 10 ~6 to 10 ~3 m. The disadvantage of the method is the need to stabilize the suspension and wash the polymers from stabilizers.

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 (H 2 O, NH3, HC1, CH 3 O, etc.), is called polycondensation. The polycondensation of bifunctional compounds is called linear, for example,

2NH 2 - (CH 2) 5 -COOH

aminocaproic acid

NH 2 -(CH 2) 5 -CO-NH-(CH 2) 5 -COOH + H 2 O

NH 2 - (CH 2) 5 -CO-NH- (CH 2) 5 -COOH-NH 2 - (CH 2) 5 -COOH NH 2 - (CH 2) 5 -CO-NH- (CH 2) 6 - CO-NH- (CH 2) 5 -COOH + H 2 O, etc.

The final product will be poly--caproamide (nylon)

[-CO-NH-(CH 2) 5 -] n.

The 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:

NH 2 -CO-NH 2 + CH 2 O NH 2 -CO-NH-CH 2 OH

NH 2 -CO-NH-CH 2 OH + CH 2 O CH 2 OH-NH-CO-NH-CH 2 OH

2CH 2 OH-NH-CO-NH-CH 2 OH

H 2 O + CH 2 OH-NH-CO-NH-CH 2 -O-CH 2 -NH-CO-NH-CH 2 OH

The first step is to synthesize oligomer linear structure:

[-CH 2 -NH-CO-NH-CH 2 -O-] n

At the second stage, when heated in an acidic medium, further polycondensation of the oligomer occurs with the release of CH 2 O and the appearance of a network structure

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

Since, in the process of polycondensation, along with high molecular weight products, low molecular weight products are formed, the elemental compositions of polymers and initial substances do not coincide. In this respect, polycondensation differs from polymerization. The polycondensation proceeds according to a stepwise mechanism, while the intermediate products are stable, i.e., the polycondensation can stop at any stage. The resulting low molecular weight reaction products (H 2 0, NH3, HC1, CH 2 O, etc.) can interact with the intermediate products of polycondensation, causing their splitting (hydrolysis, aminolysis, acidolysis, etc.), for example

NH-CO-(CH 2) 5 -NH-CO-(CH 2) 5 - + H 2 O

- NH-CO- (CH 2) 5 -NH 2 -HO-CO- (CH 2) 5

Therefore, low molecular weight products have to be removed from the reaction medium.

Monofunctional compounds present in the reaction medium react with intermediates to form non-reactive compounds. This results in chain termination, so the starting monomers must be purified from monofunctional compounds. Monofunctional compounds may be formed during the reaction due to thermal or oxidative degradation of intermediates. This leads to a stop of the polycondensation reaction and a decrease in the molecular weight of the polymer.

Polycondensation is carried out either in the melt, or in solution, or at the interface.

Polycondensation in the melt is carried out without solvents by heating the monomers at a temperature 10-20°C higher than the melting (softening) temperature of the polymers (usually 200-400°C). The process begins in an inert gas environment and ends in a vacuum.

In solution polycondensation, a solvent is used, which can also serve as an absorbent for a low molecular weight product.

Interfacial polycondensation occurs at the interface between gas-solution phases or two immiscible liquids and ensures the production of high molecular weight polymers.

About a quarter of the produced polymers are obtained by the polycondensation method, for example, polycaproamide (nylon), polyhexamethylene adipamide (nylon) [-NH (CH 2) 6 NHCO (CH 2) 4 CO-] n, polyesters (polyethylene terephthalate [- (-OC) C 6 H 4 (CO) OCH 2 CH 2 -] n), polyurethanes [-OROCONHR "NHCO-] n, polysiloxanes [-SiR 2 -O-] n, polyacetals [-OROCHR "-] n, urea-formaldehyde resins, phenol-formaldehyde resins

Chemical properties of polymers depend on their composition, molecular weight and structure. Polymers are characterized by cross-linking reactions of macromolecules, interaction of functional groups with each other and with low molecular weight substances, and destruction. The presence of double bonds and functional groups in macromolecules causes an increase in the reactivity of polymers (Tables 1, 2).

Polymers can undergo degradation, i.e., destruction under the action of oxygen, light, heat, and radiation. Often, destruction is caused by the simultaneous action of several factors. As a result of degradation, the molecular weight of macromolecules decreases, the chemical and physical properties of polymers change, and, in the end, the polymers become unsuitable for further use. The process of deterioration of the properties of polymers over time as a result of the destruction of macromolecules is called the aging of polymers. . To slow down the degradation, stabilizers, most often antioxidants, i.e., inhibitors of the oxidation reaction (phosphites, phenols, aromatic amines) are introduced into the composition of polymers. Stabilization is usually due to chain termination during the interaction of antioxidants with free radicals formed during the oxidation reaction.

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

Synthetic polymers are obtained as a result of polymerization and polycondensation reactions.

Polymerization is the process of connecting a large number of monomer molecules with each other due to multiple bonds (C = C, C = O, etc.) or opening cycles containing heteroatoms (O, N, S). During polymerization, the formation of low molecular weight by-products usually does not occur, as a result of which the polymer and monomer have the same elemental composition,

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

Polymerization of monomers of a cyclic structure occurs due to ring opening and in some cases bakes not according to a chain, but according to a stepwise mechanism. A macromolecule during stepwise polymerization 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 and stepwise and polycondensation is that at different stages of the process the reaction mixture always consists of a monomer and a polymer and does not contain di-, tri-, tetramers. As the reaction time increases, only the number of polymer macromolecules increases, while the monomer is consumed gradually. The molecular weight of the polymer does not depend on the degree of completion of the reaction or, what is the same, on the conversion of the monomer, which determines only the yield of the polymer.

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

The advantages of polymeric materials are sufficiently high strength and wear resistance, good antifriction properties and chemical resistance. Repair of parts using polymeric materials does not require sophisticated equipment, is low labor intensive, is accompanied by low heating of the part (250–320 °C), allows high wear (1–1.2 mm), and in some cases does not require subsequent machining. It is used for sealing cracks, dents, holes, shells, spalls, for restoring the dimensions of worn parts, for the manufacture of wear parts or their individual parts, for anti-corrosion protection. Due to their valuable properties, polymers are used in mechanical engineering, the textile industry, agriculture and medicine, automotive and shipbuilding, aircraft manufacturing, and in everyday life (textiles and leather products, dishes, glue and varnishes, jewelry and other items). Based on macromolecular compounds, rubber, fibers, plastics, films and paint coatings are produced. All tissues of living organisms are macromolecular compounds.

Traditionally, products made of polymers are distinguished by reliability and high quality.

The use of polymeric materials in the household has been one of the first challenges of the polymer industry since the beginning. There were many prerequisites for this. They are easy to paint in any color, and thanks to this they can decorate our everyday life.

Washable foam wallpaper provides both comfort and festive atmosphere in the room.

Modern reliable floor coverings made of polymeric materials also facilitate cleaning of premises. It should be especially noted that polymer processing wastes can be used for their manufacture.

86. Finishing polymeric materials.

Raw materials for the production of polymeric materials. Basic technological operations in the production of polymers. Nomenclature of polymeric materials. Wall-paper moisture resistant and paper.

Polymer- an organic substance, the long molecules of which are built from the same repeatedly repeating units - monomers.

Polymer types:

1. Natural ; are formed as a result of the vital activity of plants and animals and are found in wood, wool, and leather. These are protein, cellulose, starch, lignin, latex.

For example: leather, furs, wool, silk, cotton, cement, lime, clay.

2. synthetic; Most often, synthetic, artificial polymers are used in construction. Otherwise they are called resins. obtained by synthesis from low molecular weight substances that have no analogues in nature. Synthetic polymers include plastics, etc. The structural units (monomers) of plastics are low molecular weight (small) molecules that are isolated from oil, coal or natural gas.

For example: based on them, films, paints and varnishes, batteries, TVs, sockets, nylon, nylon, foam rubber are produced ...

Types of polymers:

Solid (plastics);

Plastic-viscous (mastics);

Liquid-viscous (varnishes, paints) consist of polymer solutions, pigments and inert additives. Various: acrylic, enamel…

Plastic properties:

1. Easy reshaping;

2. Light weight;

3. Easy cutting;

4. Low cost;

5. Casting of any shape.

Basic polymers:

· PVC (poly-vinyl-chloride) maintains big loadings;

· Polystyrene

· Polyethylene;

polypropylene

· Rubber, artificial rubber.

Finishing materials made of polymers:

1. Structural and finishing materials ;

For example: fiberglass, all chipboard, etc.. ! have an expiration date

2. Materials for finishing walls and ceilings;

For example: rolled materials, tiles, panels, linoleum, stretch ceilings, self-leveling floors, carpet synths, linoleum ... when burned, it smolders, releasing harmful gases.

3. Profile-molded products;

For example: skirting boards, moldings, linings, etc.

4. Porcelain tile.

Types of wallpaper based on:

1. Paper;

2. Fabric;

3. Non-woven;

4. Liquid. (not polymers)

5. Vinyl.

paper wallpaper- allow air to pass through and allow the walls to "breathe". The main disadvantages are that they tear and stretch, burn out and turn yellow and therefore have a short service life.

True, high quality paper wallpapers have recently appeared,

among them there are even moisture-resistant, impregnated with a water-repellent composition.

Among paper wallpapers are very popular wallpaper duplex consisting of two or

more layers of paper. Often these wallpapers are covered with special compounds,

which improve their light and moisture resistance. The leader among duplex wallpapers

embossed wallpapers are considered.

Vinyl wallpapers- the basis of which is interlining or paper, and the top layer is polyvinyl chloride;

Vinyl wallpapers are washable, waterproof and durable. As a rule, vinyl is resistant to sunlight, so paints do not change their quality on the surface for 20 years.

Vinyl wallpapers are two-layer. The top layer of PVC is a protective layer against moisture, light and various mechanical influences. Vinyl wallpapers, as a rule, are impregnated with various compounds that protect against the formation of fungi and mold.

Textile- rolled material for interior decoration, consisting of several layers, the front side is fabric;

The basis of textile wallpaper is a simple paper canvas. Textiles are threads made of natural and mixed fibers or artificial fabrics glued to the base. As a rule, textile wallpapers have increased sound and heat insulation and are resistant to

fading in the sun. Such wallpaper can be pasted over both walls and ceilings in

residential areas and offices.

Textile wallpapers come in several types:

velor, linen, silk, felt, jute And synthetic wallpaper .

non-woven- rolled material for pasting walls, consists of two layers - interlining (mineral fiber with cellulose) and a polymer coating;

Non-woven wallpaper can be glued with ordinary wallpaper glue.

Liquid wallpaper in its original form they are a dry mix, finished wallpapers are obtained by mixing with water;

Special types of wallpapers:

- photo wallpaper ;

- embossed wallpaper - with a protruding texture;

- sound-absorbing - on a paper basis with a nap surface; Sound-absorbing wallpaper vacuums.

- glass wall paper - a special type of rolled wall covering, made by weaving from glass fibers of various densities and thicknesses, followed by impregnation with a special composition to give stability to the woven fabric.

Features: fire resistance, durability, environmental friendliness, strength.

For example: for evacuation routes.

Metal wallpaper.(satin) –

with a pattern of paints, which include aluminum or bronze powder.

The metal surface of the wallpaper-foil is wear-resistant and easy to clean, but it is vapor-tight.

Wallpaper types:

Regular: unprimed (the pattern is printed directly on white or colored paper); primed (the pattern is applied to a pre-painted paper surface); background (without a pattern, monophonic matte color), embossed (with a relief protruding pattern);

Moisture-resistant wallpaper: printed, made on paints with the addition of waterproof polymers; printed with a protective film on the front side, formed by polymer emulsions and varnishes; obtained by applying a thin colored polymer film on a paper base, followed by embossing; in the form of a baseless polymeric opaque film with a printed pattern;

Moisture-resistant wallpapers are characterized by increased resistance to abrasion and moisture, they can be washed with warm water and detergents.

§ Sound-absorbing wallpaper

The quality of wallpaper is determined by the main characteristics: moisture resistance,

density, light fastness, vapor permeability and price.

moisture resistance– the ability to wash the wallpaper with water. Such wallpapers are covered with emulsion

synthetic resin paint. Moisture resistant wallpaper used in the kitchen,

in the bathrooms.

Density. Wallpapers are divided into light and dense.

Lightfastness- susceptibility of wallpaper to fading in the sun.

Vapor permeability– the possibility of the surface to dry properly,

affects the microclimate of the room

Types of wallpaper by type of gluing:

overlap;

Seamless.

rapport- the basic element of the ornament, repeatedly repeating part of the pattern.

Polymerization

This is a reaction of the connection of monomer molecules, which proceeds without changing the elemental composition and is not accompanied by the release of by-products.

There are chain and step polymerization. Chain polymerization consists of three stages:

The active center may be a radical. The appearance of a radical requires energy - thermal, light, ionizing radiation. The radical can be introduced from outside (initiator). The catalyst can accelerate any kind of polymerization.

As an initiator, substances are used that, when heated, decompose with the formation of radicals. For example, benzoyl peroxide:

The chain growth reaction determines the rate of the polymerization process, the molecular weight of the polymer and the structure of the polymer chain. This is determined by processes such as:

The active site may be a cation or an anion. In the cationic version, the center is the carbonium ion:

The reaction takes place on catalysts, such as:

AlCl3,SnCl4,TiCl4

In the anionic version, the center is a carbanion:

Catalysts - alkali metals, their alkyls, etc.

With the joint polymerization of two or more monomers, it is possible to change the properties of the copolymers in a wide range. An important example used in the preparation of ion-exchange membranes and ion exchangers is the copolymerization of styrene and divinylbenzene, when a three-dimensional copolymer is formed.

Rice. 1.
Scheme of the copolymerization reaction of styrene and divinylbenzene.

Polymerization is carried out in the gas phase, in the mass of monomers, in solution (two options: when the solvent dissolves both the monomer and the polymer - the varnish method, the polymer is then precipitated; when the solvent dissolves only the monomer, and the polymer precipitates); in an emulsion (the dispersed phase is water, and the monomer is in drops; emulsifiers are added - surfactants that stabilize the emulsion; the product of such polymerization is called latex, they are used directly or coagulated with electrolytes); in the solid phase (near the melting point).

.Polycondensation

Polycondensation is the synthesis of polymers by the interaction of bifunctional and polyfunctional monomers, accompanied by the release of a low molecular weight product (water, alcohol, NH3, salts, etc.). It can be linear if there are two functional groups on the monomer molecule and three-dimensional if there are more than two groups.

Usually, polycondensation is an equilibrium process, i.e. by-product is released. As a result, the resulting polymers have a lower molecular weight.

In the process of polycondensation, monomers with amino, carboxy, and hydroxy groups are used, and polymers are easily formed from various monomers, which is very important for obtaining polymers with desired properties. Examples in Fig.2.

Fig.2. Examples of polycondensation reactions

The choice of the method for carrying out polycondensation is determined by the physicochemical properties of the initial substances and the resulting polymers. Polycondensation can be carried out in melt, solution, emulsion and solid phase. The melt and solid phase require high temperatures.

It is polycondensation that underlies the formation of cellulose, starch. It is widely used in industry for the synthesis of polyamides, polycarbonates, phenol-formaldehyde resins, polysulfones, and organosilicon compounds.

Synthesis of polymers with inorganic atoms in chains

The same mechanisms of polymerization and polycondensation are possible in these reactions. For example, polyorganosiloxanes are formed as follows:

Rice. 3. Scheme for the synthesis of polyorganosiloxanes

The Si-O-Si bond is called siloxane. Side substituents may be any alkyl or aryl radicals. There may be groups here - O - R. Inorganic atoms can be in the side chains and in the main chain. Sometimes compounds are classified on this basis. On fig. 4 . various groups of heterochain compounds are presented.

Rice. 4. Groups of heterochain compounds.

Functional group reactions

Many polymers cannot be made by polymerization from monomers, either because the original monomers do not exist or they do not polymerize. The way out is in the synthesis of such polymers from other polymers. Reactions of such modification should not lead to destruction. They are called polymer-analogous transformations and take place on functional groups:

This is how cellulose acetates and its other esters are obtained by replacing hydroxyl groups with:

Polyvinyl alcohol is obtained by saponification of polyvinyl acetate (see Fig. 5.)

Rice. 5. Scheme of PVA synthesis

Synthesis of graft copolymers and block copolymers

They are obtained from homopolymers, or from a homopolymer and a monomer. Their formation is possible by the chain transfer mechanism, when an active atom appears in the middle of the chain. Their synthesis is possible by the method of activation of the polymer molecule, by the method of introducing functional groups into the polymer, which decompose when heated to form radicals.

Polymer degradation reactions

These are polymer backbone scission reactions. The reasons can be physical (thermal, mechanical, photochemical and radiation), as well as chemical (oxidative reactions, hydrolysis, and others).

Thermal degradation

Its intensity depends on the value of the binding energy between the atoms. The C-C bond is very stable, but the presence of hydrogen atoms greatly reduces its stability:

The strength of the C-C bond is affected by the degree of branching of the polymers and the presence of substituents in the molecule. The most weakened place is the connection of the side and main chain. Polyethylene is more heat resistant than polypropylene and polyisobutylene:

Some substituents increase heat resistance. For example, fluorine:

Oxygen anywhere in the chain greatly reduces the thermal stability.

Mechanical destruction

Mechanical effects include grinding, rolling, mixing, forcing through holes, etc. Destruction is due to the localization of mechanical energy, the occurrence of internal stresses commensurate with the chemical bond energy.

Radiation destruction

The degree of destruction depends on the energy of the particles and the intensity of irradiation (in total, on the received dose). Destruction goes with the formation of radicals, unsaturated bonds, with the release of gases. Sometimes irradiation contributes to an increase in molecular weight (crosslinking). This is noticeable on PAK, PVA, PVP, polyacrylamide.

Oxidative degradation

The stability of the polymer depends on the presence of oxidizable groups and bonds in the macromolecule. O2, Cl2, O3 are known among oxidizers. If Cl and F are present in the molecule as substituents, the stability of the polymer increases. It falls when heated, in the light.

Hydrolysis

This is the reaction of the addition of water molecules at the site of breaking chemical bonds. Compounds with acetal, amide and ester bonds are most susceptible to hydrolysis:

The hydrolyzing agent is of great importance. Cellulose is very weakly hydrolyzed by alkali and strongly dilute acids at the acetal bond. The most powerful hydrolyzing agents are sulfuric, hydrochloric, and hydrofluoric acids, which saponify cellulose to glucose.

Polyamides are hydrolyzed in concentrated sulfuric, hydrochloric and formic acids:

acidolysis

This is destruction under the action of carboxylic acids with the formation of lower molecular weight products (see Fig. 2.12). The degree of degradation is proportional to the amount of dicarboxylic acid.

Aminolysis

Destruction proceeding under the action of amines. Example: interaction of polyamides with hexamethylenediamine:

Rice. 6. Examples of acidolysis and aminolysis reactions

Crosslinking reactions

These are reactions of the formation of cross chemical bonds between macromolecules with the formation of a spatial network. In the rubber industry, these reactions are called vulcanization, in the industry - curing.

With a small amount of cross-links (sparse network), soft elastic products are obtained, i.e. if the degree of crosslinking of the polymer is low, it retains its solubility. A large number of crosslinks leads to the formation of a very rigid structure.

Cross-links can be formed between carbon atoms without the addition of any substances or with the help of vulcanizers or hardeners. Sulfur in rubbers - rubber and ebonite (from 3 to 32% wt.).

Since an insoluble three-dimensional network structure is formed at a high density of cross-linking bonds, such highly cross-linked materials are obtained as a result of heat treatment and are called thermoset or thermoset. The products are infusible and insoluble.

Polymers that do not form crosslinks when heated and retain their solubility and meltability are called thermoplastic.

Currently, there are 4 main methods for the synthesis of IUDs:

1) polymerization

2) polycondensation

3) stepwise polymerization

4) transformation reactions

Polymerization is a chain reaction for obtaining IUDs, during which monomer molecules are sequentially attached to the active center located at the end of the growing chain. The polymerization reaction is typical for compounds with double bonds, the number and nature of which in the monomer molecule can be different. The polymerization of olefins and their derivatives by opening double bonds is the simplest example. Monomers containing two or more double bonds (polyenes), triple bonds (acetylene derivatives) can also be polymerized.

During the polymerization reaction, there is always a decrease in the number of double bonds in the reactants, a decrease in the total number of molecules in the system, and an increase in their average molecular weight.

As a result of the polymerization of unsaturated hydrocarbons, carbon chain polymers are formed.

Polymerization is not accompanied by the release of by-products and proceeds without changing the elemental composition of the reactants. The polymerization process consists of three main stages:

1) the formation of an active center associated with the initiation of monomer molecules, i.e., their transition to the active state: A à A * .

2) chain growth, characterized by the growth of macromolecules and the transition of the active center to some other particle.

3) chain termination associated with the death of the active center as a result of a reaction with another active center or some other substance.

Active centers in polymerization reactions can be either a free radical or an ion. Depending on this, radical and ionic polymerization are distinguished.

In radical polymerization, free radicals are active centers - electrically neutral particles having one or two unpaired electrons, due to which free radicals easily react with various monomers. The formation of free radicals can be associated with the conversion of a monomer into a primary radical under the influence of external factors (thermal energy, light, ionizing radiation), as well as due to the introduction of free radicals into the system from outside or substances that readily decompose into free radicals (initiators).

During ionic polymerization, active centers are positively and negatively charged particles - ions formed in the presence of catalysts, which are metal compounds (aluminum, titanium), which easily donate or accept electrons. Depending on the charge of the forming ion, cationic and anionic polymerization are distinguished. During cationic polymerization, the growing chain has a positive charge, while during anionic polymerization, it has a negative charge. Unlike radical polymerization initiators, catalysts that activate the ionic polymerization process are not consumed in the course of ongoing reactions and are not part of the polymer.

Polycondensation is a reaction of the formation of IUDs from several molecules of monomers of the same or different structure, proceeding according to the mechanism of substitution of functional groups. Polycondensation reactions proceed with the release of low molecular weight products (water, alcohol, ammonia, etc.), as a result of which the elemental composition of the forming polymer differs from the elemental composition of the monomers. An indispensable condition for the reaction to proceed is the content in the monomers of at least two functional groups (-OH, -COOH, -NH 2, etc.). The functionality of the starting materials affects the structure and properties of the resulting products.

During the polycondensation of bifunctional compounds, linear or cyclic IUDs are formed. If tri- or tetrafunctional monomers are used as the monomer, their polycondensation reaction leads to the formation of spatially cross-linked IUDs.

Methods for carrying out synthesis reactions.

1. Synthesis in block or mass

2. When synthesized in

3. Synthesis at the interface (interfacial).

4. Melt synthesis

5. Synthesis in the solid phase.

6. Synthesis in the gas phase

There are a number of methods of synthesis through the use of chemical reagents for reactions that cause the appearance of new substances. Through chemical transformations, various atoms (fluorine, chlorine, amine groups, etc.) can be introduced into the material, which allow you to control the length of macromolecules, as well as subject them to crosslinking. Very often, these methods are used when it is impossible to obtain an IUD in another way due to its instability in any environment.

1) The reactions of intramolecular rearrangements consist in rearrangements of atoms in the polymer chain.

2) Cross-linking (structuring) reaction - the reaction of the formation of cross chemical bonds between the macromolecule and the formation of systems of a network structure.

3) Destruction reaction - a reaction that proceeds with the breaking of a chemical bond in the main chain of a macromolecule. The reaction leads to a decrease in the molecular weight of the polymer. It is characterized by the concept of the degree of destruction - the ratio of the number of broken valence bonds in the main chain to their total number.