Polymerization
This is a reaction of combining monomer molecules, which occurs 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. A radical can be introduced from outside (initiator). The catalyst can accelerate any type of polymerization.
Substances that decompose when heated to form radicals are used as an initiator. 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 such processes as:
The active site can be a cation or an anion. In the cationic version, the center is the carbonium ion:
The reaction occurs on catalysts, such as:
AlCl3,SnCl4,TiCl4
In the anionic version, the center is a carbanion:
Catalysts - alkali metals, their alkyls, etc.
When two or more monomers are co-polymerized, the properties of the copolymers can be changed over a wide range. An important example used in the production of ion exchange membranes and ion exchangers is the copolymerization of styrene and divinylbenzene, when a spatial copolymer is formed.
Rice. 1.
Scheme of the copolymerization reaction of styrene and divinylbenzene.
Polymerization is carried out in the gas phase, in a 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.
Typically, polycondensation is an equilibrium process, i.e. a by-product is released. In this regard, the resulting polymers have a lower molecular weight.
In the polycondensation process, 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 method for carrying out polycondensation is determined by the physicochemical properties of the starting substances and the resulting polymers. Polycondensation can be carried out in a melt, in solution, in emulsion and in the solid phase. The melt and solid phase require high temperatures.
It is polycondensation that underlies the formation of cellulose and 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
In these reactions, the same mechanisms of polymerization and polycondensation are possible. For example, polyorganosiloxanes are formed as follows:
Rice. 3. Scheme for the synthesis of polyorganosiloxanes
The Si-O-Si bond is called a siloxane bond. The side substituents can 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 according to this criterion. In Fig. 4 . various groups of heterochain compounds are presented.
Rice. 4. Groups of heterochain compounds.
Reactions of functional groups
Many polymers cannot be made by polymerization from monomers, either because the starting monomers do not exist or they do not polymerize. The solution is to synthesize 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 other cellulose esters are obtained by replacing hydroxyl groups with:
Polyvinyl alcohol is obtained by saponification of polyvinyl acetate (see Fig. 5.)
Rice. 5. PVA synthesis scheme
Synthesis of graft copolymers and block copolymers
They are obtained from homopolymers, or from a homopolymer and a monomer. They can be formed by the chain transfer mechanism, when the 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.
Reactions of polymer destruction
These are reactions that break the polymer backbone. The reasons can be physical (thermal, mechanical, photochemical and radiation), as well as chemical (oxidative reactions, hydrolysis and others).
Thermal destruction
Its intensity depends on the magnitude of the binding energy between 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 influenced by the degree of branching of the polymers and the presence of substituents in the molecule. The connection between the side and main chains is most weakened. Polyethylene is more heat-resistant than polypropylene and polyisobutylene:
Some substituents increase heat resistance. For example, fluorine:
Oxygen anywhere in the chain greatly reduces heat resistance.
Mechanical destruction
Mechanical influences include grinding, rolling, mixing, pressing through holes, etc. Destruction is caused by the localization of mechanical energy and the occurrence of internal stresses commensurate with the energy of the chemical bond.
Radiation destruction
The degree of destruction depends on the energy of the particles and the intensity of irradiation (total of the dose received). Destruction occurs with the formation of radicals, unsaturated bonds, and the release of gases. Sometimes irradiation increases molecular weight (cross-linking). This is noticeable on PAA, PVA, PVP, and polyacrylamide.
Oxidative destruction
The stability of the polymer depends on the presence of oxidizable groups and bonds in the macromolecule. Among the oxidizing agents O2, Cl2, O3 are known. 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 water molecules joining at the site where chemical bonds are broken. 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 highly 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 destruction is proportional to the amount of dicarboxylic acid.
Aminolysis
Destruction occurring under the influence of amines. Example: interaction of polyamides with hexamethylenediamine:
Rice. 6. Examples of acidolysis and aminolysis reactions
Cross-linking reactions
These are reactions of the formation of cross-sectional chemical bonds between macromolecules with the formation of a spatial network. In the rubber industry, these reactions are called vulcanization, and in industry, curing.
With a small number of cross-links (sparse mesh), soft elastic products are obtained, i.e. if the degree of cross-linking of the polymer is low, it retains its solubility. A large number of cross-links leads to the formation of a very rigid structure.
Cross-links can be formed between carbon atoms without adding any substances or with the help of vulcanizers or hardeners. Sulfur in rubbers - rubber and ebonite (from 3 to 32% wt.).
Since a high density of cross-linking links forms an insoluble three-dimensional network structure, such highly cross-linked materials are obtained as a result of heat treatment and they are called thermosetting or thermosetting. Products are infusible and insoluble.
Polymers that do not form cross-links when heated and retain their solubility and ability to melt are called thermoplastic.
General information about high molecular weight compounds
Topic 11. Technology of high-molecular compounds
Test questions for topic X
"Technology of OO and NC synthesis"
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 selectivity of the process achieved during 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 production of ethanol consist of and what is each stage catalyzed by?
11. What compounds are classified as higher synthetic fatty acids (HSFA) and alcohols (HSA)?
12. Indicate the main industrial methods for the production of VZHK and VZhS.
13. What is common in the chemistry of producing VLC and VLC by the oxidation of alkanes?
14. How is the oxidation process interrupted in the production of VLS, preventing the destruction of the alkane molecule?
15. What are synthetic detergents and what is their connection with VZhS, VZHK?
| Plastics, rubbers, chemical fibers and polymer composite materials are the main types of polymer materials. Share of polymer materials in the gross chemical production of industrialized countries. Methods for carrying out polymerization reactions in the gas phase, in solution, in suspension, in emulsion and block polymerization. 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 producing PE (low and high density). Polycondensation processes and their technological design. Phenol-formaldehyde and urea-aldehyde, pillowcase and resol resins. Organosilicon polymers. Polyurethanes. Basic properties and areas of their application. Chemical fibers: cellulose-based artificial and synthetic. Basic techniques for forming fibers from solutions and melts. Properties and applications. Production of synthetic rubbers. Special purpose rubbers. Processing rubber into rubber. Environmental aspects of the production of polymeric materials and products based on them. |
All living and inanimate nature around us is built from molecules, which in turn consist of atoms. Atoms, combining with each other in different proportions, 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 consist of giant molecules, which contain 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.
Based on their origin, polymers are divided into natural and synthetic.
Natural, i.e. 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 units alternate. If we designate these units by the letter A, then the polymer molecule can be represented as follows:
In synthetic polymers, these units are remnants of molecules of the original 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. When a polymer is formed in ethylene molecules, the double bond between carbon atoms opens, and due to the free valences of carbon formed, a large number of units resulting from the monomer are connected to each other. Schematically, this can be represented as follows:
The diagram shows only three units in 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.
From the above diagram it is clear that in both the monomer and the polymer there are two hydrogen atoms per carbon atom, i.e., the elemental composition of the resulting polymer is the same as the monomer.
With a change in the number of interconnected monomer molecules, the properties of the resulting polymers change. Thus, as the molecular weight increases, polyethylene becomes more viscous, then paste-like and, finally, solid. The properties of polymers also depend on the chemical composition of the monomers, the shape of the molecular chains and their structure (polymer structure).
In a macromolecule with a linear structure, the elementary units form a thread-like molecule, i.e., each unit is connected to only two neighboring units (Fig. A). Thread-like (linear) macromolecules can be located parallel to each other in the polymer (Fig. b) or intertwine without chemical bonding of individual macromolecules (Fig. V). 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 compounds widely used to produce fibers, films, plastics, rubber. These polymeric materials are usually strong, elastic, and capable of dissolving and melting when heated.
Macromolecules with a branched structure have side branches from the main chain (Fig. e). Polymers with a branched molecular structure dissolve and melt more difficult than linear polymers.
Macromolecules with a network structure are built as follows: long chains of molecules are connected to each other by short chains in three dimensions, which is difficult to depict in a picture. Typically, this structure of polymer molecules is depicted in the form of large linear molecules connected to each other (Fig. and). It always means that linear molecules are chemically bonded to molecules located above and behind the plane of the paper. This structure of molecules is also called spatial or three-dimensional. The greater the number of “bridges” in such a macromolecule, the less elastic the polymer is and it largely exhibits the properties of a solid.
The structure of the chains of polymer molecules can be different. In some cases, polymer molecules are formed in which the elementary units have a different spatial arrangement of side groups, in others - a strictly regular spatial arrangement. Polymers with a strictly regular molecular structure are called isotactic. This type of polymer has high hardness and heat resistance.
Polymer molecules may not consist of identical units. They can be obtained from different monomers, for example A and B. Then the macromolecule can be depicted like this:
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 gasoline and oil resistance, chemical resistance, etc.
The so-called graft copolymers are of interest. The chains of their molecules are built according to the following scheme:
Such a polymer can be compared to a fruit tree onto which another variety of fruit tree has been grafted. As a result of this “grafting,” fruits are obtained that combine the most valuable qualities of both varieties. In a graft copolymer, one polymer is grafted onto the “trunk” of another polymer. The resulting “hybrid” has the properties of the original substances. 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 "blocks" of relatively low molecular weight, obtained from various monomers. The scheme of such a block copolymer looks like this:
Block copolymers also combine the properties of the original polymers.
Until now, the elementary units in a macromolecule have been conventionally designated A and B. It can be seen that the basis of organic polymers is carbon, the atoms of which are connected to each other, forming a “skeleton” of the molecule, framed by hydrogen atoms. Instead of hydrogen atoms, there may be groups of atoms in which, along with carbon atoms, atoms of other elements may be present.
If the skeleton of polymer molecules is built from carbon atoms, they are called carbon chain molecules. 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, and upon subsequent cooling they turn into a solid state. Such polymers are called thermoplastic. Polymers whose molecules have a spatial structure do not melt when heated: they are called thermosetting.
The temperature at which a polymer transitions from a solid to an elastic state (or vice versa) is called the glass transition temperature, and the temperature at which it transitions to a fluid state is called the flow temperature.
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 occur in polymers under the influence of external conditions at room temperature, they are divided into solid polymers, elastic polymers, or elastomers, and flowing polymers.
Thus, by changing the size of the resulting macromolecule, its molecular weight and shape, composing a macromolecule from various initial monomers, grafting onto one macromolecule a chain of polymer from units formed by another monomer, it is possible to widely change the physical and chemical properties of polymers, obtaining them with predetermined properties, change their physical state, make them liquid, solid, plastic and elastic.
Polymers have 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 dampen vibrations, form extremely thin films and fibers, they can be 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 industries, for the manufacture of fabrics, artificial leather, household items, medicine, etc.
The production of polymer materials in our country is developing at a very fast pace, exceeding the growth rate of the entire industry and other branches of the chemical industry.
Polymers can be obtained by polymerization and polycondensation methods.
Polymerization. The polymerization method consists in the fact that monomer molecules, under the influence of heat, catalysts, γ-rays, light, and initiators, are combined with each other into large molecules. In this case, macromolecules of a linear, branched, network structure, molecules of copolymers, and graft copolymers are formed.
The polymerization rate and molecular weight of the polymer depend 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 occurs in an apparatus (autoclave),
where the starting monomer is supplied with a catalyst or initiator - a substance that reacts with the monomer and accelerates polymerization. At the beginning of polymerization, the reacting mass is heated, then the heating is stopped, since polymerization is accompanied by the release of heat. To maintain a certain temperature in the apparatus during the polymerization process, they sometimes 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 periodic or continuous.
When polymerizing in bulk, it is difficult to ensure the same temperature throughout the entire reacting mass, so the resulting polymer consists of macromolecules having different degrees of polymerization. This method produces polystyrene, methacrylic acid polymers, butadiene rubber, etc.
The emulsion polymerization method involves mixing the monomer with an initiator and an emulsifier and using stirrers to turn it into tiny droplets suspended in another liquid, most often water. (Emulsifiers are substances that prevent liquid droplets from coalescing.) The resulting emulsions are heated to a temperature at which polymerization of the monomer occurs. In this case, the heat generated during the polymerization process is easily removed and the resulting polymer is more homogeneous than that obtained by the block method. The disadvantage of this method is the difficulty of separating the emulsifier from the polymer. This method produces copolymers of butadiene, vinyl acetate, acrylonitrile, etc.
Solution polymerization occurs in a solvent that mixes with the monomer and dissolves the resulting polymer. The polymer is isolated from the resulting solution by evaporation of the solvent or precipitation. 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. Using this method, polyvinyl acetate, polybutyl acrylate, etc. are obtained.
The suspension method involves grinding (dispersing) the monomer in the form of drops in a poorly dissolving medium, usually water. Polymerization occurs in each drop of monomer. The resulting polymer in the form of solid particles that are insoluble 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 a reaction between them, which occurs with the release of by-products. For example, let’s denote the molecule of one of the reacting substances as a-A-a, and the second as 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 substance molecule a-a-b-b can further 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 differs somewhat from the original monomers.
The polycondensation process produces polymers that have a linear as well as a network structure.
The polycondensation process is exothermic, and therefore, based on Le Chatelier's principle, in order to shift the equilibrium from left to right, it is necessary to carry out the process at 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 reaction equilibrium and thereby obtain a product with a higher molecular weight.
Polycondensation is carried out both in the presence of a catalyst and without it. It is carried out in a melt, solution and at the interface between two phases.
Polycondensation in the melt is carried out at high temperature (220-280° C) in a reactor in an inert gas atmosphere. This ensures high process speed and removal of low molecular weight products.
During polycondensation in solution, monomers are dissolved in a solvent - the reaction proceeds at a low speed, and the removal of low molecular weight products is not ensured. This method is not used in industry.
Polycondensation at the interface consists of two immiscible liquids, in each of which the original monomers are dissolved. The polycondensation reaction occurs instantly at the interface with the formation of a film of polymers. Thus, the reaction products are removed from the reaction sphere, which helps the reaction proceed at a high rate. When the film is removed, the phase interface is released and the reaction continues.
Lecture 20-21
Polymers. Preparation of polymers. Polymerization and polycondensation reactions. Classification of polymers. Types of polymers used in technology.
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 classified as a special group of chemical compounds.
Depending on the composition of the main chain, polymers are divided into organic (― S―S―), inorganic (― Si-Si―), organic element (― S―Me―S―).
The ability of chemical compounds to form polymers is determined functionality of their molecules. Functionality characterizes the number of functional groups in a molecule:
Where M– molecular weight of the chemical compound;
– equivalent molar mass;
M FG– molar mass of the functional group;
With FG– 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 the presence of mobile hydrogen atoms.
Preparation of polymers
The main methods for the synthesis of high molecular weight compounds:
1. Polymerization;
2. Polycondensation;
POLYMERIZATION – chain process. This is the synthesis of a polymer by sequential addition of molecules of a low molecular weight substance (monomer) to the active center located at the end of the growing chain. Compounds containing multiple bonds ( С≡С, C=C, C=O, С≡N etc.), or cyclic groups capable of opening (oxidation of olefins, lactam, etc.).
, , 
.
During the polymerization process, multiple bonds are broken or rings open in monomers and chemical bonds arise between groups with the formation of macromolecules, for example:
nCH 2 =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 circuit break.
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, peroxides, azo compounds (having the functional group -N=N-) and other compounds with weakened bonds serve as polymerization initiators. 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 =CHCI RCH 2 -CHCl ●
RCH 2 -CHCl ● + CH 2 =CHCI RCH 2 -CHC1-CH 2 -CHCl ● etc.
b) Chain growth occurs due to addition to radicals
the resulting monomers to produce new radicals.
c) Chain transfer involves the transfer of the active center
to another molecule (monomer, polymer, solution molecules
body):
R-(-CH 2 -SNS1-) n -CH 2 -SNS ● + CH 2 =SNS1
R- (-CH 2 -CHC1-) n -CH 2 -CH 2 C1 + CH=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, then chain branching may occur.
At the chain termination stage, radicals interact to form 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 -CHC1-CH 2 -CHCl-(-CH 2 -CHCI-) n -R
Chain termination can also occur with the formation of low-active radicals that are unable to initiate a reaction. Such substances are called inhibitors. Thus, regulation of the length and, accordingly, the molecular weight of macromolecules can be carried out using initiators, inhibitors and other substances. However, chain transfer and termination can occur at different stages of chain growth, so 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-CHC1-] n, polyvinyl acetate [-CH 2 -CH(OCOCH3)-] P,polystyrene [-CH 2 -CH(C 6 H 6)-] n, polyacrylate, [-CH2-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, for example 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. Electron-donating substances and compounds are used as initiators of anionic polymerization, including alkali and alkaline earth metals, alcoholates alkali metals, etc. Often several polymerization initiators are used simultaneously.
Chain growth can be written using the reaction equations for cationic polymerization 
and anionic polymerization. 
The polymerization method produces 3/4 of the total volume of produced polymers. Polymerization is carried out in mass, solution, emulsion, suspension or gas phase.
Bulk polymerization (in block) is the polymerization of liquid monomer(s) in an undiluted state. In this case, a fairly pure polymer is obtained. The main difficulty of the process is related to heat removal. In solution polymerization, the monomer is dissolved in a solvent. With this polymerization method, it is easier to remove heat and control the composition and structure of the polymers, but the problem of solvent removal arises.
Emulsion polymerization (polymerization in emulsion) involves 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 high molecular weight and 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 (polymerization in suspension), the monomer is in the form of droplets dispersed in water or other liquid. As a result of the reaction, polymer granules with sizes ranging from 10 ~6 to 10 ~3 m are formed. 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 produce 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. 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 (kapron)
[-СО-NH-(CH 2) 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:
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
At the first stage, it is synthesized oligomer linear structure:
[-CH 2 -NH-CO-NH-CH 2 -O-] n
At the second stage, when heated in an acidic environment, 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, low molecular weight products are formed along with high molecular weight products, the elemental compositions of polymers and starting substances do not coincide. This is how polycondensation differs from polymerization. Polycondensation proceeds by a stepwise mechanism, with the intermediate products being stable, i.e., 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 intermediate polycondensation products, 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 intermediate products, forming non-reactive compounds. This leads to chain termination, so the starting monomers must be purified from monofunctional compounds. Monofunctional compounds can be formed during the reaction due to thermal or oxidative destruction of intermediate compounds. This leads to the stop of the polycondensation reaction and a decrease in the molecular weight of the polymer.
Polycondensation is carried out either in a melt, or in a solution, or at an interface.
Polycondensation in the melt is carried out without solvents, heating the monomers at a temperature 10-20°C above the melting (softening) temperature of the polymers (usually 200-400°C). The process begins in an inert gas environment and ends in a vacuum.
During polycondensation, a solvent is used in solution, which can also serve as an absorbent for a low-molecular-weight product.
Interfacial polycondensation occurs at the interface between a gas and a solution or two immiscible liquids and provides polymers with high molecular weight.
Approximately a quarter of the produced polymers are produced by the polycondensation method, for example, polycaproamide (kapron), polyhexamethylene adipinamide (nylon) [-NH(CH 2) 6 NHCO(CH 2) 4 CO-] n, polyesters (polyethylene terephthalate [-(-OC)C 6 H 4 (CO)OSH 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 reactions of joining macromolecules with cross-links, interaction of functional groups with each other and low-molecular 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 be subject to degradation, that is, destruction under the influence of oxygen, light, heat and radiation. Often destruction is caused by the simultaneous influence of several factors. As a result of destruction, the molecular weight of macromolecules decreases, the chemical and physical properties of polymers change, and ultimately, 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 polymer aging . To slow down the destruction, stabilizers are introduced into the polymers, most often antioxidants, i.e. inhibitors of the oxidation reaction (phosphites, phenols, aromatic amines). Stabilization is usually caused by chain termination when antioxidants interact with free radicals formed during the oxidation reaction.
Polymers are produced by polymerization or polycondensation methods.
Polymerization (polyaddition). This is the reaction of the formation of polymers by sequential addition of molecules of a low molecular weight substance (monomer). A great contribution to the study of polymerization processes was made by domestic scientists S.V. Lebedev, S.S. Medvedev and others and foreign researchers G. Staudinger, G. Mark, K. Ziegler and others. During polymerization, no by-products are formed and, accordingly, the elemental composition macromolecules does not differ from the composition of monomer molecules. Compounds with multiple bonds are used as monomers: C=C, C=N, C=C, C=O, C=C=O, C=C=C, C=N, or compounds with cyclic groups capable of opening, For example:
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During the polymerization process, multiple bonds are broken or rings open in monomers and chemical bonds arise between groups with the formation of macromolecules, for example:
Based on the number of types of monomers involved, a distinction is made between homopolymerization (one type of monomer) and copolymerization (two or more types of monomers).
Polymerization is a 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 circuit break:
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, peroxides, azo compounds (having the functional group - N = N -) and other compounds with weakened bonds serve as initiators of polymerization. Initially, radicals are formed, for example:
(C6H5COO)22C6H5COO*(R*)
benzoyl peroxide
Then macroradicals are formed, for example during the polymerization of vinyl chloride:
R* +CH2 = CHCl ® RCH2 – CHCl*
RCH2 - CHCl* + CH2 = CHCl ® RCH2 - CHCl - CH2 - CHCl*, etc.;
b) chain growth occurs due to the addition of the resulting monomers to the radicals to produce new radicals;
c) chain transfer consists of transferring the active center to another molecule (monomer, polymer, solvent molecules):
R-(-CH2-CHCl-)n-CH2-CHCl* + CH2=CHCl ®
®R-(-CH2 -CHCl-)n -CH2 -CH2Cl + CH = 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, then chain branching may occur.
At the chain termination stage, radicals interact to form 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-active radicals that are unable to initiate a reaction. Such substances are called inhibitors.
Thus, regulation of the length and, accordingly, the molecular weight of macromolecules can be carried out using initiators, inhibitors and other substances. However, chain transfer and termination can occur at different stages of chain growth, so 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)=CH-CH2-]n, and various copolymers.
Ionic polymerization also occurs through the stage of active site formation, chain propagation, and chain termination. The role of active centers in this case is played by anions and cations. Accordingly, anionic and cationic polymerization are 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-donor substances are used as initiators of anionic polymerization and compounds, including alkali and alkaline earth metals, alkali metal alcoholates, etc. Several polymerization initiators are often used simultaneously.
The chain growth can be written by reaction equations:
during cationic polymerization and
Mn+ + M ® M+n+1
during anionic polymerization
Mn- + M ® M-n+1
Let us consider, as an example, the cationic polymerization of iso-butylene with the initiators AlCl3 and H2O. The latter form a complex
А1Сl3 + Н2О « Н+ [АlONСlз]-
Denoting this complex with the formula H+X-, the process of initiating polymerization can be represented as
H2C=C+ +H+X-®H3C-C+ X-
The resulting complex cation, together with the counterion X-, forms a macroion, which ensures chain growth:
CH3 CH3 CH3 CH3
H3C - C+ X-+H2C = C ®H3C ¾ C - CH2 - C+ X-etc
CH3 CH3 CH3 CH3
With the help of some complex initiators it is possible to obtain polymers with a regular structure (stereoregular polymers). For example, such a complex initiator can be a complex of titanium tetrachloride and trialkylaluminum AIR3.
The ionic polymerization method is used in the production of poly-isobutylene [-CH2-C(CH3)2-]p, polyformaldehyde [-CH2O-]n, polyamides, for example poly-e-caproamide (kapron) [-NH-(CH2)5- CO-]n, synthetic rubbers, for example butadiene rubber [-CH2-CH=CH-CH2-]n.
The polymerization method produces 3/4 of the total volume of produced polymers. Polymerization is carried out in mass, solution, emulsion, suspension or gas phase.
Bulk polymerization (in block) is the polymerization of liquid monomer(s) in an undiluted state. In this case, a fairly pure polymer is obtained. The main difficulty of the process is related to heat removal. In solution polymerization, the monomer is dissolved in a solvent. With this polymerization method, it is easier to remove heat and control the composition and structure of the polymers, but the problem of solvent removal arises.
Emulsion polymerization (polymerization in emulsion) involves 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 high molecular weight and 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 (polymerization in suspension), the monomer is in the form of droplets dispersed in water or other liquid. As a result of the reaction, polymer granules with sizes ranging from 10-6 to 10-3 m are formed. 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 produce 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 (H2O,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 - W. Carothers, P. Flory, P. Morgan and others. Polycondensation of bifunctional compounds is called linear, for example:
2NH2-(CH2)5-COOH ®
amiocaproic acid
®NH2-(CH2)5-CO-NH-(CH2)5-COOH + H2O®
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 final product will be 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 environment, further polycondensation of the oligomer occurs with the release of CH2O 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 listed in Table. 14.1.
Table 14.1. Chemical bonds between the functional groups of some monomers that arise during their polycondensation
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Polymers |
Examples of polymers |
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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 Vayrin, lycra Polynonamethylene urea, uralon Dimethylsiloxane rubber |
Since in the process of polycondensation, low molecular weight products are formed along with high molecular weight products, the elemental compositions of polymers and starting substances do not coincide. This is how polycondensation differs from polymerization. Polycondensation proceeds by a stepwise mechanism, with intermediate products being stable, i.e. polycondensation can stop at any stage. The resulting low molecular weight reaction products (H2O, NH3, HCl, CH2O, etc.) can interact with intermediate polycondensation products, causing their splitting (hydrolysis, aminolysis, acidolysis, etc.), for example.
The vast majority of high-molecular compounds are obtained as a result of polymerization and polycondensation reactions.
Polymerization
Polymerization is a process for producing polymers in which the construction of macromolecules occurs by sequential addition of molecules of a low molecular weight substance (monomer) to the active center located at the end of the growing chain. For polymerization, the stages of initiation and chain growth are mandatory.
Initiation - This is the transformation of a small fraction of monomer molecules M into active centers AM*, capable of attaching new monomer molecules. For this purpose, pathogens are introduced into the system ( initiators I or catalysts) polymerization. The initiation of polymerization can be represented as follows:
If one monomer participates in polymerization, then we get homopolymers, if two or more then copolymers. Depending on the nature of the active center, there are radical And ionic polymerization And copolymerization.
Radical polymerization
Radical polymerization always occurs via a chain mechanism. The functions of active intermediates in radical polymerization are performed by freeradicals. Common monomers that undergo radical polymerization include vinyl monomers: ethylene, vinyl chloride, vinyl acetate, vinylidene chloride, tetrafluoroethylene, acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, styrene, and diene monomers (butadiene, isoprene, chloroprenide).
Radical polymerization is characterized by all the signs of chain reactions known in the chemistry of low-molecular compounds (for example, the interaction of chlorine and hydrogen in light). Such signs are: the sharp influence of a small amount of impurities on the speed of the process, the presence of an induction period and the course of the process through a sequence of three stages dependent on each other - the formation of an active center (free radical), chain growth and chain termination. The fundamental difference between polymerization and simple chain reactions is that at the growth stage the kinetic chain is embodied in the material chain of a growing macroradical, and this chain grows until the formation of a polymer macromolecule.
The initiation of radical polymerization comes down to the creation of free radicals in the reaction medium that are capable of starting reaction chains. The initiation stage includes two reactions: the appearance of primary free radicals of the initiator R* (1a) and the interaction of the free radical with the monomer molecule (16) with the formation of radical M*:
Reaction (1b) proceeds many times faster than reaction (1a). Therefore, the rate of initiation of polymerization is determined by reaction (1a), as a result of which free radicals R* are generated. Free radicals, which are particles with an unpaired electron, can be formed from molecules under the influence of physical influence - heat, light, penetrating radiation, when they accumulate energy sufficient to break the π bond. Depending on the type physical impact per monomer upon initiation (formation of the primary radical M*), radical polymerization is divided into thermal, radiation and photopolymerization. In addition, initiation can be carried out due to the decomposition into radicals of substances specially introduced into the system - initiators. This method is called material initiation.
Thermal initiation is self-initiation at high temperatures of polymerization of pure monomers without introducing special initiators into the reaction medium. In this case, the formation of a radical occurs, as a rule, due to the decomposition of small amounts of peroxide impurities, which can arise during the interaction of the monomer with atmospheric oxygen. In practice, so-called block polystyrene is obtained in this way. However, the method of thermal initiation of polymerization has not found widespread use, since it requires large amounts of thermal energy, and the rate of polymerization in most cases is low. It can be increased by increasing the temperature, but this reduces the molecular weight of the resulting polymer.
Photoinitiation polymerization occurs when the monomer is illuminated with the light of a mercury lamp, in which the monomer molecule absorbs a quantum of light and goes into an excited energy state. Colliding with another monomer molecule, it is deactivated, transferring part of its energy to the latter, and both molecules turn into free radicals. The rate of photopolymerization increases with increasing irradiation intensity and, unlike thermal polymerization, does not depend on temperature.
Radiation initiation polymerization is in principle similar to photochemical. Radiation initiation consists of exposure of monomers to high energy radiation (γ-rays, fast electrons, α - particles, neutrons, etc.). The advantage of photo- and radiation-chemical initiation methods is the ability to instantly “turn on and off” radiation, as well as polymerization at low temperatures.
However, all these methods are technologically complex and may be accompanied by undesirable side reactions, such as destruction, in the resulting polymers. Therefore, in practice, chemical (material) initiation of polymerization is most often used.
Chemical initiation is carried out by introducing into the monomer medium low-molecular unstable substances containing low-energy bonds - initiators that easily decompose into free radicals under the influence of heat or light. The most common initiators of radical polymerization are peroxides and hydroperoxides (hydrogen peroxide, benzoyl peroxide, hydroperoxides mpem-butyl and isopropylbenzene, etc.), azo and diazo compounds (azobisisobutyric acid dinitrile, diazoaminobenzene, etc.), potassium and ammonium persulfates. Below are the decomposition reactions of some initiators.
Peroxide tert-butyl(alkyl peroxide):
The activity and possibility of using radical polymerization initiators is determined by the rate of their decomposition, which depends on temperature. The choice of a specific initiator is determined by the temperature required to carry out the synthesis of the polymer. Thus, dinitrile of azobiisobutyric acid is used at 50-70 ° C, benzoyl peroxide - at 80-95 ° C, and peroxide tert- butyl - at 120-140°C.
Effective initiators that allow the radical polymerization process to be carried out at room and low temperatures are redox systems. Peroxides, hydroperoxides, persulfates, etc. are usually used as oxidizing agents. Reducing agents are metal salts of variable valency (Fe, Co, Cu) in the lowest oxidation state, sulfites, amines, etc.
The oxidation-reduction reaction takes place in a medium containing the monomer, producing free radicals that initiate polymerization. You can select oxidizing-reducing pairs that are soluble in water (for example, hydrogen peroxide - iron (II) sulfate) or in organic solvents (for example, benzoyl peroxide - dimethylaniline). Accordingly, radical polymerization can be initiated in both aqueous and organic media. For example, the decomposition of hydrogen peroxide in the presence of iron (II) salts can be represented by the following equations:
Radicals HO* and HOO*, joining the monomer molecule, initiate radical polymerization.
Chain growth is carried out by the sequential addition of monomer molecules to radicals (2) formed during reaction (1b), for example:
In the chain process of radical polymerization, the growth of the kinetic chain occurs almost instantly with the formation of a material chain of a macroradical and ends with its termination.
Open circuit represents the process of stopping the growth of kinetic and material chains. It leads to the disappearance of active radicals in the system or to their replacement by low-active radicals that are unable to attach monomer molecules. At the termination stage, a polymer macromolecule is formed. Circuit breakage can occur through two mechanisms:
two growing macroradicals, colliding, connect with each other into a single chain, that is, they recombine (Za);
macroradicals, colliding, turn into two macromolecules, and one of them, donating a proton, turns into a macromolecule with a double C=C bond at the end, and the other, accepting a proton, forms a macromolecule with a simple terminal C-C bond; such a mechanism is called disproportionation (3b):
When chains are terminated by recombination, initiator residues are located at both ends of the macromolecule; when chains are broken by disproportionation - at one end.
As the chains of macroradicals grow, the viscosity of the system increases and their mobility decreases, as a result of which chain termination becomes more difficult and the overall rate of polymerization increases. This phenomenon is known as gel effect. The gel effect causes increased polydispersity of polymers, which usually leads to a deterioration in their mechanical properties. Limitation of material chains during radical polymerization can also occur through the addition of a macroradical to the primary radical (termination at the initiator) and as a result of chain transfer reactions.
Chain transmission consists in the separation of a mobile atom from a molecule of any substance by a growing macroradical - a solvent, monomer, polymer, impurity. These substances are called circuit transmitters. As a result, the macroradical transforms into a valence-saturated macromolecule and a new radical is formed, capable of continuing the kinetic chain. Thus, during transfer reactions the material chain breaks, but the kinetic chain does not.
The chain transfer reaction to a solvent (for example, carbon tetrachloride) can be represented as follows:
Free radicals formed from solvent molecules can attach monomer molecules, that is, continue the kinetic chain:
If their activity differs from the activity of primary radicals, then the And speed polymerization.
When the chain is transferred to the polymer, branched macromolecules are formed:
The probability of chain transfer to a polymer increases at high monomer conversion, when the concentration of macromolecules in the system is high.
In some cases, the monomer itself can play the role of a chain transfer agent if its molecules contain a mobile hydrogen atom. In this case, the growing radical does not attach a new monomer molecule to itself via a double bond, but abstracts a mobile hydrogen atom from it, saturating its free valency and simultaneously converting the monomer molecule into a monomer radical. This occurs during the polymerization of vinyl acetate:
Chain transfer reactions to solvent underlie the preparation telomeres. If the polymerization of a monomer is carried out at high concentrations of a solvent, the molecules of which contain mobile hydrogen or halogen atoms, then the reaction product will be substances with a low molecular weight, consisting of several monomer units containing fragments of solvent molecules at the ends. These substances are called telomeres, and the reaction to produce them is telomerization.
Chain transfer reactions can be used to control the molecular weight of polymers and even prevent their formation. This is widely used in practice, often using chain transfer regulators during polymerization, and inhibitors when storing monomers.
Circuit regulators- these are substances that, breaking off the growing polymer chains, practically do not affect the overall speed of the process. Typical chain regulators are mercaptans containing a mobile hydrogen atom in the mercapto group. The transmission of the chain on them can be represented as follows:
Polymers synthesized in the presence of chain regulators have an optimal average molecular weight and MWD for processing.
Inhibitors- these are substances that break off growing polymer chains, turning into compounds that are not capable of initiating polymerization. Substances whose chain transfer leads to the formation of inactive (stable) radicals are usually used as inhibitors. In practice, hydroquinone, benzoquinone, aromatic amines, and nitrobenzene are often used to inhibit radical polymerization and store monomers.
Kinetics of radical polymerization
Theoretical and practical information about the influence of various factors on radical polymerization, namely the conversion of the monomer and, accordingly, the yield of the polymer, its molecular parameters (molecular weight, polydispersity and MWD) can be obtained by studying the patterns of development of this process over time, that is, its kinetics . Of the three main elementary stages - initiation, growth and chain termination - the slowest and most energy-intensive is initiation. To start it, an activation energy of 84-126 kJ/mol is required, which is 3-4 times higher than the activation energy of the chain growth stage and almost 10 times the activation energy of the chain termination stage.
The initiator is characterized by efficiency. Let us consider in more detail the stage of decomposition of the initiator into radicals.
The initiator breaks down into two radicals, which can give rise to two kinetic chains. However, the radical pair is surrounded by environmental molecules, which create a dense environment called a cage. The density of the medium prevents the rapid diffusion separation of the radical pair, so some of the radicals die by recombination without entering the volume.
The initiation efficiency (probability of chain initiation) is expressed by the following equation:
To determine δ, the inhibitory method is used. It is especially important to take into account δ in environments with low molecular mobility, where the release of radicals from the cell is low. This is well illustrated by the following example. When moving from liquid ethylbenzene with high molecular mobility to polystyrene with extremely low molecular mobility, the initiation efficiency decreases by a factor of 20: from 0.6 to 0.03.
Overall rate of radical polymerization V is equal to the rate of consumption of monomer M when it interacts with a growing radical.
Based on the law of mass action, the rate of each elementary reaction v The polymerization process can be represented by the following equations:
where v and and kAnd, v p and kp,v 0 And ko - rate and rate constant of initiation, justification and chain termination reactions, respectively; [I], [M*], [R], [M] are the concentrations of the initiator, radicals, growing radicals and monomer, respectively.
Since the number of monomer molecules involved in the reaction with the primary radical upon initiation is very small compared to the number of monomer molecules involved in chain growth (the initiator is usually introduced in an amount of up to 1% of the monomer mass), the monomer concentration can be considered constant, and then
During radical polymerization, a few seconds after the start of the reaction, a stationary process mode is established: radicals appear upon initiation and disappear upon termination at the same rate, that is vu = v o a d/dt = 0. Then [M*] = (k and /k o) 1/2 [I] 1/2 and the equation for the overall polymerization rate takes the form:
Equation (9) is valid in the initial stage of polymerization, when the monomer conversion and polymer yield are low (10-15%).
The molecular weight of the polymer is the same as the degree of polymerization P, determined by the length of the kinetic chain, which depends on the ratio of the rates of chain termination and growth reactions
The more vp compared with vo, the more monomer molecules manage to join the growing radical before the chain breaks, the longer the chain length. Taking into account equation (9) and the condition of stationarity of the process, we obtain
The physical meaning of equations (9) and (11) is as follows. The molecular weight of the polymer and the rate of radical polymerization are directly dependent on the concentration of the monomer, an increase in which causes an acceleration of the process and an increase in the length of chain molecules. Likewise, the rate and molecular weight of the polymer are affected by an increase in pressure, as compression brings the reacting molecules closer together, facilitating the polymerization process.
With increasing initiator concentration in the system, the number radicals. These radicals react with a large number of monomer molecules, thereby increasing the rate of their conversion into macroradicals, that is, the rate of polymerization. But an increase in the concentration of radicals increases the probability of their collision, that is, an increase in the rate of termination of the polymerization chain. This leads to a decrease in the molecular weight of the polymer.
Similarly, the kinetics of radical polymerization is affected by temperature. Typically, the polymerization rate increases 2-3 times with an increase in temperature by 10°C. An increase in temperature facilitates the decomposition of the initiator into radicals; at the same time, the mobility of all particles of the system - molecules and radicals - increases, therefore, the probability of particle collisions increases. This leads to an increase in the rates of chain growth and chain termination reactions. Thus, with increasing temperature, the overall rate of polymerization always increases, and the molecular weight of the polymer decreases, and the proportion of low molecular weight fractions increases. An increase in temperature simultaneously promotes the formation of branched macromolecules and disruption of the chemical regularity of the polymer chain, since the probability of monomers entering the chain according to the “head-to-head” or “tail-to-tail” principle increases.
On the polymerization rate and molecular The mass of the polymer is significantly influenced by various impurities and atmospheric oxygen, and oxygen, depending on the nature of the monomer and the polymerization conditions, can accelerate or slow down the polymerization. Oxygen slows down the photopolymerization of vinyl acetate, but accelerates the photopolymerization of styrene, inhibits the polymerization of vinyl chloride initiated by benzoyl peroxide, which occurs in a nitrogen or argon atmosphere with a good polymer yield and high molecular weight. Therefore, to obtain polymers, high-purity monomers (~ 99%) are used and the technological process is carried out in an inert gas atmosphere.
To this day, most modern synthetic polymers are produced by radical polymerization. Despite the obvious advantages of this method over ionic polymerization (mild synthesis conditions, a wide range of monomers, etc.), its significant drawback is that it does not allow the production of narrowly dispersed homo- and copolymers with a given molecular weight and structure.
Intensive research around the world over the past decade has shown that these problems can be solved using unconventional radical processes, collectively called “pseudo-living radical polymerization.” In these processes, macromolecules arising from the target monomer interact with specially introduced stable additives - reversible chain transfer agents. The resulting macromolecules are able to “revive” and regenerate growth radicals, which can again participate in the reaction of chain growth until the next act of limiting it by breaking or transferring. In such processes, the reaction of quadratic termination of macroradicals, characteristic of classical radical polymerization, ceases to play a significant role. The repeatedly repeated stages of restriction (breaking) and “revival” of chains ensure the consistent growth of macromolecules during polymerization and a decrease in the width of the MWD. The most common reversible chain transfer (RCT) agents are sulfur-containing compounds of the general formula
where Z is a stabilizing group, Y is a leaving group.
They make it possible to carry out controlled synthesis of polymers and copolymers in practice now. At the same time, the scientific theoretical interpretation of the mechanism of RAFT during polymerization requires comprehension.
Ionic polymerization
Polymers can be obtained not only by chain radical polymerization reactions, but also by chain reactions in which the growing chain is not a free macroradical, but a macroionic one. This method of producing polymers is called ion polymerization, and substances that dissociate into ions and excite the polymerization of monomers by an ionic mechanism are called catalysts.
Depending on the sign of the charge of the growing macroradical, there are cationic And anionic polymerization. During cationic polymerization, the carbon atom at the end of the growing chain (carbocation) has a positive charge K(M) n M + . The charge arises at the initiation stage and disappears when the circuit is broken or transferred. During anionic polymerization, the charge of the growing macroion (carbanion) is negative A(M)nM ¯ .
In ionic polymerization, the same elementary stages can be distinguished, kinetically interconnected as in radical polymerization: initiation, growth, termination and chain transfer. Polymerization under the influence of ionic catalysts usually occurs at higher rates than with radical ones and leads to the production of a polymer of higher molecular weight. The reaction system in the case of ionic polymerization is often heterogeneous (inorganic or organometallic solid catalyst and liquid organic monomer). The kinetics of ionic polymerization, unlike radical polymerization, cannot be described by a single equation.
Ionic polymerization also includes polymerization that occurs by coordinating the monomer on the surface of a solid catalyst. (coordination ion polymerization). In this case, the surface of the catalyst plays a special role as a matrix, which sets a certain order of entry of the monomer into the growing chain with an ordered spatial arrangement of monomer units. All stereoregular polymers are obtained by coordination-ionic polymerization.
Cationic polymerization
Catalysts cationic polymerization are strong electron-withdrawing compounds. Typical catalysts are protic acids (H 2 SO 4, HC1O 4, H 3 PO 4, etc.) and aprotic acids (BF 3, ZnCl 2, AlCl 3, TiCl 4, etc.). The latter are active in the presence of small amounts of water or other substances - proton donors, called cocatalysts.
Monomers of the vinyl and divinyl series, containing electron-donating substituents at the double C=C bond, for example, propylene, α-methylstyrene, esters of acrylic and methacrylic acids, etc., easily enter into cationic polymerization. Some heterocyclic monomers are also active in cationic polymerization: olefin oxides, lactones, a number of carbopyl-containing compounds, for example, formaldehyde.
Cationic polymerization begins with the catalyst reacting with a cocatalyst to form a complex compound that is a strong acid. In the reaction medium, its dissociation occurs, for example:
The resulting proton attaches to the monomer molecule, resulting in the formation of an ion pair consisting of a carbonium ion and a complex counterion:
These two reactions constitute the initiation step of cationic polymerization. The nature of the reaction center—an ion pair, or solvated ions separated in space—has a major influence on the kinetics of cationic polymerization.
Chain growth consists of the sequential addition of monomer molecules to a carbonium ion, while a positive charge is always retained at the end of the chain:
The carbonium ion polarizes the monomer molecule, so it enters the chain in a certain way and the resulting macromolecules always have a regular “head-tail” structure.
Chain termination by recombination or disproportionation is impossible in this case due to the repulsion of like-charged ions. It occurs by rearrangement of the ion pair, in which a neutral polymer molecule with a double C=C bond at the end is formed and the initial catalytic complex is generated:
In cationic polymerization, as in radical polymerization, chain transfer to the monomer and solvent is observed:
Since cationic polymerization is associated with the formation and dissociation of an ion pair, the rate of the process is influenced by the dielectric constant of the medium. Increasing the dielectric constant significantly speeds up the process, but has little effect on the molecular weight of the polymer. Compared to radical polymerization, cationic polymerization is characterized by a low activation energy (60 kJ/mol), so it proceeds at a high rate, which decreases with increasing temperature.
Anionic polymerization
Catalysts anionic polymerization are substances that are electron donors: alkali metals, alkalis, alkali metal hydrides and amides, organometallic compounds. In anionic polymerization reactions, the most active are vinyl monomers with electron-withdrawing substituents, for example, styrene CH 2 =CH-C 6 H5, acrylonitrile CH 2 =CH-C=N. In anionic polymerization, the active center is a carbanion, a compound with a trivalent carbon that carries a negative charge, and the growing chain itself is a macroanion.
The mechanism of anionic polymerization in the presence of amides of alkali metals and organometallic compounds is described by the same schemes. Thus, the polymerization of styrene in liquid ammonia, catalyzed by sodium amide, proceeds as follows:
that is, a monomer molecule is inserted between the ions of an ion pair.
Open circuit by recombination of growing macroanions, as in cationic polymerization, is impossible due to the presence of the same charge. It most often occurs as a result of chain transfer reactions to a solvent or monomer:
If the catalysts for anionic polymerization are alkali metals (Li, Na), then at the initiation stage they are formed radical ions monomer, which, when combined, turn into a two-center organometallic compound - bianion. The chain grows by introducing a monomer between the ions of the ion pair at both centers of the resulting bionion, that is, the chain grows simultaneously in two directions. In this way, butadiene is polymerized under the action of sodium metal:
This type of polymerization, associated with the appearance of radical ions, is interesting because it makes it possible to obtain “living” polymer chains, that is, the growing macrobianion is able to excite polymerization for a long time when new portions of the monomer are added. Chain breakage is completely excluded even by transfer methods to a solvent or monomer. Polymerization stops only after all the monomer has been exhausted. Polymers obtained by this method are characterized by high molecular weight and low polydispersity.
Anionic polymerization is effective at low temperatures in thoroughly deaerated and dried basic solvents.
Coordination-ionic (stereospecific) polymerization
Coordination ion polymerization is carried out under the action of complex catalysts with high selectivity. Such catalysts are complexes formed by the interaction of metal alkyls of groups I-III of the periodic system D.I. Mendeleev with halides of transition metals of groups IV-VIII. A typical catalyst is a complex of triethylaluminum and titanium trichloride
At the initiation stage, the titanium atom of the catalyst complex coordinates the monomer in a certain way. With such coordination, the bonds of the monomer are loosened and bonds are redistributed in the catalyst complex. A π-complex arises between the monomer and the catalyst. Thus, the initiation of stereospecific polymerization of propylene can be represented as follows:
Next, a catalyst complex of the initial structure is generated, in the attractive field of which the first monomer unit is located. The introduction of each subsequent monomer unit occurs through the stage of formation of a rearranged π-complex, and the growing polymer chain seems to move away from the catalyst:
This does not occur during radical, cationic, or anionic polymerization.
In coordination-ionic polymerization, the resulting macromolecules are characterized not only by a chemically regular connection of monomer units of the H-X type (which is generally inherent in ionic polymerization), but also by a strict alternation in space of substituents at the carbon atoms of the main chain, that is, stereoregularity. The stereospecificity of polymer macromolecules synthesized by coordination ion polymerization is ensured by nature complex catalyst. Aluminum and titanium compounds of similar structure, but taken separately, are not stereospecific catalysts.
Step polymerization
Reaction step polymerization can be expressed by the same equation as the chain polymerization reaction:
P M → M P,
that is, there is no difference in the composition of the monomer and polymer macromolecules.
However, unlike chain polymerization, stepwise polymerization proceeds through the formation of intermediate, completely stable compounds - di-, tri-, tetra-, penta-, hexamers, etc., and therefore the molecular weight of the polymer increases over time. Not only homogeneous, but also dissimilar monomer molecules can participate in the stepwise polymerization reaction. An example is the polymerization of e-caprolactam in the presence of ε-aminocaproic acid, which is formed as a result of the catalytic action of water on ε-caprolactam:
According to the type of stepwise polymerization of unequal monomer molecules, the interaction of diisocyanates with dioxy compounds occurs with the formation of polyurethanes, diisocyanates with diamines with the formation of polyureas, etc. These reactions are based on the movement of a mobile hydrogen atom from one molecule to another containing a double bond, followed by the addition of the entire residue the first molecule. For example, the formation of polyurethanes from glycols and diisocyanates occurs as follows:
Stepwise polymerization reveals similarities with polycondensation in the nature of the reactions through the sequential formation of dimers, trimers, tetramers, etc. The yield and molecular weight of the polymer depend on the duration of the reaction. A high molecular weight polymer is formed only at a very high (~98%) degree of reaction completion (or monomer conversion). However, polycondensation differs from stepwise polymerization in the release of low molecular weight products during the reaction.
LECTURE 4. Copolymerization. Technical methods for carrying out homo- and copolymerization. Polycondensation. The influence of various factors on the rate of polycondensation and molecular weight. Co-polycondensation. Technical methods for carrying out polycondensation.