Methods for obtaining polymers and their transformations. Basic methods for obtaining polymers Methods for synthesizing polymers
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.
Polymerization call the process of sequential addition of free radicals or monomer ions to a growing chain of a polymer macromolecule (Fig. 12.1). Active sites during polymerization are formed as a result of breaking multiple or cyclic bonds. If free bonds are formed due to the elimination of functional groups (active terminal atoms or their combinations) from the initial monomers and low molecular weight by-products are released, then the process is called polycondensation(Fig. 12.2).
Some polymers (polyurethanes, epoxy resins) are obtained as a result of stepwise polymerization (polyaddition). In this case, the monomer molecules initially form short molecular chains ( prepolymers), which then combine into long macromolecules. Polymer formation reactions proceed in three main stages:
1. Initiation reactions (formation of an active center). The polymerization reaction does not start on its own. It is necessary to expend energy to break a multiple or cyclic bond, resulting in the formation of active centers - free radicals or ions. The formation of active centers occurs under the influence of heat, light, radiation and in the presence of initiators - substances containing in their molecules unstable chemical bonds (O - O, N - N, S - S, O - N, etc.), which are broken much more easily than bonds in a monomer molecule. The amount of introduced initiator is usually small (0.1-1%).
In contrast to polymerization, polycondensation occurs spontaneously during the interaction of functional groups.
2. chain growth. During polymerization, monomers are sequentially added to the growing polymer chain according to the scheme [- A -] p + -BUT- -*? [ - BUT - ] n + v In this case, the macromolecule must remain a free macroradical (macroion).
During polycondensation, independent from each other acts of combining monomeric radicals and the chains formed from them occur according to the scheme [-A - x + [- A - y ^[-A - x + y. The polyaddition reaction proceeds according to the same scheme, however, despite the similarity to polycondensation, this reaction is a polymerization reaction, since the formation of active centers occurs as a result of bond breaking.
3. Chain break. The end of polymerization is associated with the disappearance of the free bond at the last link of the macromolecule. This happens in three ways: 1) as a result of the connection between two macroradicals (recombination reaction) according to the scheme: x- [- BUT - + + [- BUT -]-x->x-[- BUT -] + - x; 2) as a result chain transfer reactions, when the active center passes to any other molecule (solvent or impurity) which, turning into a radical, gives rise to a new macromolecule: x-[- BUT - ] n+ RH->x-[- A -] p - H + + R - ; 3) upon introduction inhibitors - substances that, when inter-
Rice. 12.1.
Rice. 12.2. The polycondensation reaction, using the example of obtaining a phenol-formaldehyde resin, interacts with free radicals to form low-active particles that are unable to initiate the polymerization process.
The polycondensation process can stop for several reasons: due to a violation of the equivalent ratio of functional groups, an increase in the viscosity of the reaction medium and the associated decrease in the mobility of macromolecules, a steady equilibrium state, when both the formation of longer chains and their decay (destruction) occur simultaneously. The reversibility of the reaction is a characteristic feature of the polycondensation process. To avoid degradation, it is necessary to remove the resulting by-products. The molecular weight of the resulting polymer can be limited by introducing monofunctional compounds that block the functional groups of one of the monomers and stop the growth of the polymer chain.
Polymers are obtained by polymerization or polycondensation methods.
Polymerization (polyaddition). This is a 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. macromolecules does not differ from the composition of monomer molecules. As monomers, compounds with multiple bonds are used: 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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In the process of polymerization, multiple bonds are broken or cycles are opened in monomers and chemical bonds appear between groups with the formation of macromolecules, for example:
According to the number of types of monomers involved, homopolymerization (one type of monomer) and copolymerization (two or more types of monomers) are distinguished.
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 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 a functional group - N = N -) and other compounds with weakened bonds. 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 \u003d CHCl ® RCH2 - CHCl - CH2 - CHCl *, etc .;
b) the growth of the chain occurs due to the addition of the resulting monomers to the radicals to obtain new radicals;
c) chain transfer consists in the transfer of 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 \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-(-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 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. 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 formation of active sites, growth 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, and others. Often several polymerization initiators are used simultaneously.
Chain growth can be written by the reaction equations:
in cationic polymerization and
Mn+ + M ® M+n+1
in anionic polymerization
Mn- + M ® M-n+1
Let us consider as an example the cationic polymerization of isobutylene with AlCl3 and H2O initiators. The latter form a complex
A1Cl3 + H2O " H + [AlONClz] -
Denoting this complex by the formula H + X - the process of polymerization initiation can be represented as
H2C=C+ +H+X-®H3C-C+ X-
The resulting complex cation, together with the X- counterion, forms a macroion, which ensures chain growth:
CH3 CH3 CH3 CH3
H3C - C + X- + H2C \u003d 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 having a regular structure (stereoregular polymers). For example, such a complex initiator may be a complex of titanium tetrachloride and trialkylaluminum AIR3.
The method of ionic polymerization is used in the production of poly-isobutylene [-CH2-C (CH3) 2-] p, polyformaldehyde [-CH2 O-] n, polyamides, for example, poly-e-caproamide (nylon) [-NH-(CH2) 5- CO-]n, synthetic rubbers, for example butadiene rubber [-CH2-CH=CH-CH2-]n.
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 applied to obtain polypropylene and other polymers.
Polycondensation. The reaction of polymer synthesis from compounds having two or more functional groups, accompanied by the formation of low molecular weight products (Н2О,NH3, HCl, CH2O, etc.) is called polycondensation. A significant contribution to the study of polycondensation processes was made by Russian scientists V. Korshak, G. Petrov and others, from foreign scientists - W. Carothers, P. Flory, P. Morgan and others. The polycondensation of bifunctional compounds was called linear, for example:
2NH2-(CH2)5-COOH®
amiocaproic acid
®NH2-(CH2)5-CO-NH-(CH2)5-COOH + Н2О®
NH2-(CH2)5-CO-NH-(CH2)5-COOH + NH2-(CH2)5-COOH ®
® NH2-(CH2)5-CO-NH-(CH2)5-CO-NH-(CH2)5-COOH+ H2O etc.
The final product will be poly-e-caproamide [-CO-NH-(CH2)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:
NH2-CO-NH2 + CH2O ® NH2-CO-NH-CH2OH
NH2-CO-NH-CH2OH + CH2O ® CH2OH-NH-CO-NH-CH2OH
2 CH2OH-NH-CO-NH-CH2OH ®
® Н2О + CH2OH-NH-CO-NH-CH2-O-CH2- NH-CO-NH-CH2OH
At the first stage, an oligomer with a linear structure is synthesized:
[-CH2-NH-CO-NH-CH2-O]n
At the second stage, when heated in an acidic medium, further polycondensation of the oligomer occurs with the release of 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 given in Table. 14.1.
Table 14.1. Chemical bonds between the functional groups of some monomers arising from their polycondensation
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Polymers |
Polymer examples |
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Polyamides Polyesters Polyurethanes Polyureas Silicones |
¾O ¾ C¾ NH ¾ ¾NH ¾ C ¾ NH ¾ ¾ Si ¾ O ¾ Si ¾ |
Nylon, capron Polyethylene terephthalate, terylene Vyrin, lycra Polynonamethylene urea, uralon Dimethylsiloxane rubber |
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. polycondensation can stop at any stage. The resulting low molecular weight reaction products (H2O, NH3, HCl, CH2O, etc.) can interact with the intermediate products of polycondensation, causing their splitting (hydrolysis, aminolysis, acidolysis, etc.), for example.
General information about macromolecular compounds
Topic 11. Technology of macromolecular compounds
Security 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. Due to what is the necessary selectivity of the process achieved in the synthesis of methanol from synthesis gas?
4. What technological schemes are used in methanol production?
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 ethylene hydration 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 catalyzes each stage?
11. What compounds are classified as higher synthetic fatty acids (HFA) and alcohols (HFA)?
12. Specify the main industrial methods for the production of VZhK and VZhS.
13. What is common in the chemistry of obtaining HFA and HFA by the oxidation of alkanes?
14. How do HFAs interrupt the oxidation process in the production, preventing the destruction of the alkane molecule?
15. What are synthetic detergents and what is their connection with HFA, HFA?
| Plastics, rubbers, chemical fibers and polymer composite materials as the main types of polymer materials. The share of polymeric materials in the gross chemical production of 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 the production of PE (low and high density). Polycondensation processes and their technological design. Phenolic-formaldehyde and urea-aldehyde, pillowcase and resole resins. Silicone polymers. Polyurethanes. Basic properties and areas of their application. Chemical fibers: man-made cellulose-based and synthetic. Basic techniques for the formation of fibers from solutions and melts. Properties and applications. Manufacture of synthetic rubbers. Rubbers for special purposes. Processing rubber into rubber. Ecological aspects of the production of polymeric materials and products based on them. |
All the animate and inanimate nature around us is built from molecules, which in turn consist of atoms. Atoms, connecting with each other in various ratios, form molecules that differ from each other in size, structure, chemical composition and properties.
Substances built from a small number of atoms are called low molecular weight. Their molecular weight does not exceed several hundred units. Low molecular weight substances are salts, acids, alkalis, alcohols and other compounds.
At the same time, many substances consist of giant molecules, which include thousands, tens and hundreds of thousands of atoms. Such molecules are called macromolecules; their molecular weight reaches hundreds and even thousands of units. For example, the molecular weight of the molecules that make up natural rubber is 136,000-340,000.
Compounds built from macromolecules are called macromolecular or polymers.
Polymers by origin are divided into natural and synthetic.
Natural, i.e. natural, polymers include cellulose, which is part of wood, cotton and other plants; proteins that are part of living organisms; natural rubber, etc.
Synthetic polymers are produced artificially, by chemical synthesis; they are part of plastics, synthetic rubbers, chemical fibers, varnishes, etc.
Composition and properties of polymers. Polymer molecules are long chains in which identical links alternate. If these links are denoted by the letter A, then the polymer molecule can be represented as follows:
In synthetic polymers, these units are the remnants of the molecules of the original compounds, consisting of only a few atoms. These parent 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 the carbon atoms is opened, and due to the free carbon valences formed, a large number of units obtained from the monomer are connected to each other. Schematically, this can be represented as follows:
The diagram shows only three links in the composition of the polymer, in fact, their number in polyethylene is from 1000 to 10,000, and the molecular weight of such a polymer ranges from 28,000 to 280,000.
It can be seen from the above diagram that both in the monomer and in 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, a change in the properties of the resulting polymers occurs. Thus, as the molecular weight increases, polyethylene becomes more viscous, then pasty, and finally solid. The properties of polymers also depend on the chemical composition of the monomers, the shape of the chains of molecules and their structure (polymer structure).
In a macromolecule of a linear structure, the elementary units form a filamentous molecule, i.e., each unit is connected only with two neighboring units (Fig. a). Filamentous (linear) macromolecules can be arranged parallel to each other in the polymer (Fig. b) or intertwine without chemical bonding of individual macromolecules (Fig. in). They can be curved, rolled into a ball (Fig. d, e) etc. Macromolecules of a linear structure are characteristic of polyethylene, polypropylene, cellulose, polyesters, polyamides and many other high-molecular compounds widely used to obtain fibers, films, plastics, and rubber. These polymeric materials are generally strong, elastic, and capable of dissolving and melting when heated.
Branched macromolecules have side branches from the main chain (Fig. e). Polymers with a branched molecular structure are more difficult to dissolve and melt than linear ones.
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 the figure. Typically, such a structure of polymer molecules is depicted as interconnected linearly constructed large molecules (Fig. and). In this case, it is always meant that the linear molecules are chemically bonded to the molecules located above and behind the plane of the paper. Such a 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 and the properties of a solid body are largely manifested in it.
The chain structure of polymer molecules can be different. In some cases, polymeric molecules are formed, in which the elementary units have a different spatial arrangement of the side groups, in others - a strictly regular spatial arrangement. Polymers with a strictly regular molecular structure are called isotactic. This type of polymers have 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 as follows:
Such high molecular weight compounds are called copolymers. They combine the characteristic properties of polymers obtained from each component separately.
Thus, it is possible to impart some specific properties to polymers, for example, to obtain rubbers with increased gasoline and oil resistance, chemical resistance, etc.
Of interest are the so-called graft copolymers. The chains of their molecules are built according to the following scheme:
Such a polymer can be compared to a fruit tree to which another variety of fruit tree is grafted. As a result of such “grafting”, fruits are obtained that combine the most valuable qualities of both varieties. In a graft copolymer, one polymer is grafted onto a "stem" 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 relatively low molecular weight "blocks" derived from various monomers. The scheme of such a block copolymer has the form:
Block copolymers also combine the properties of the original polymers.
Until now, the elementary units in a macromolecule have been conventionally denoted as A and B. It can be seen that organic polymers are based on 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. 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 then solidify on subsequent cooling. Such polymers are called thermoplastic. Polymers whose molecules have a spatial structure do not melt when heated: they are called thermosets.
The transition temperature of a polymer from a solid to an elastic state (or vice versa) is called the glass transition temperature, the transition temperature to a fluid state is called the pour point.
The 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 fluid polymers.
Thus, by changing the size of the resulting macromolecule, its molecular weight and shape, by composing a macromolecule from various initial monomers, by grafting a polymer chain from units formed by another monomer to one macromolecule, it is possible to change the physical and chemical properties of polymers to a wide extent, to obtain 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 are easily processed and processed into products. The valuable properties of polymers have led to their widespread use in various sectors of the national economy: in mechanical engineering, construction, automotive, aviation, nuclear, space and other branches of technology, for the manufacture of fabrics, artificial leather, household items, medicine, etc.
The production of polymeric materials in our country is developing at a very rapid pace, exceeding the growth rates of the entire industry and other branches of the chemical industry.
Polymers can be obtained by polymerization and polycondensation methods.
Polymerization. The polymerization method consists in the fact that monomer molecules under the influence of heating, catalysts, γ-rays, light, initiators combine with each other into large molecules. In this case, macromolecules of a linear, branched, network structure, molecules of copolymers, graft copolymers are formed.
The rate of polymerization and the 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 takes place in an apparatus (autoclave),
where the initial 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 heating is stopped, since polymerization is accompanied by heat release. To maintain a certain temperature in the apparatus during polymerization, sometimes they resort to cooling the reacting mass. At the end of the polymerization, a solid mass is removed from the apparatus, the polymer in the form of a block. The polymerization process can be either batch or continuous.
In bulk polymerization, it is difficult to ensure the same temperature throughout the reacting mass, so the resulting polymer consists of macromolecules with different degrees of polymerization. This method produces polystyrene, polymers of methacrylic acid, butadiene rubber, etc.
The emulsion polymerization method consists in the fact that the monomer is mixed with an initiator and an emulsifier and converted with the help of agitators into tiny droplets suspended in another liquid, most often in water. (Emulsifiers are substances that prevent liquid droplets from coalescing.) The resulting emulsions are heated to a temperature at which the monomer polymerizes. In this case, the heat released during polymerization is easily removed and the resulting polymer is more homogeneous than that obtained by the block method. The disadvantage of this method lies in the difficulty of separating the emulsifier from the polymer. This method produces copolymers of butadiene, vinyl acetate, acrylonitrile, etc.
Solution polymerization is carried out in a solvent that mixes with the monomer and dissolves the resulting polymer. From the resulting solution, the polymer is isolated by solvent evaporation 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. By this method, polyvinyl acetate, polybutyl acrylate, etc. are obtained.
The suspension method involves grinding (dispersing) the monomer in the form of droplets in a poorly soluble medium, usually water. Polymerization proceeds in each drop of monomer. The resulting polymer in the form of solid particles that do not dissolve in water is precipitated and separated from the liquid by filtration.
Polycondensation. The method lies in the fact that the connection between the molecules of monomers occurs during the reaction between them, which proceeds with the release of by-products. For example, let's designate the molecule of one of the reacting substances as a-A-a, and the second as b-B-b. The reaction scheme between them can be represented as follows:
A molecule of substance a-a-b-b was formed from the reacting molecules, and at the same time substance a-b was released. A molecule of substance 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 the polymer molecules somewhat differs from the initial monomers.
In the process of polycondensation, polymers are obtained that have a linear as well as a network structure.
The polycondensation process is exothermic, and therefore, based on the Le Chatelier principle, in order to shift the equilibrium from left to right, it is necessary to carry out the process at a low temperature. However, to increase the rate of the process, it is necessary to increase the temperature. Therefore, to increase the rate of polycondensation, the process is first carried out at an elevated temperature, and then it is gradually reduced to shift the equilibrium of the reaction and thereby obtain a product with a higher molecular weight.
The 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. Thus, a high speed of the process and the removal of low molecular weight products are ensured.
During polycondensation in solution, the monomers are dissolved in the solvent - the reaction proceeds at a low rate, and the removal of low molecular weight products is not ensured. This method is not used in industry.
Polycondensation at the phase boundary is that there are two immiscible liquids, in each of which the initial monomers are dissolved. The polycondensation reaction instantly proceeds at the phase boundary with the formation of a polymer film. Thus, the reaction products are removed from the reaction sphere, which promotes the reaction to proceed at a high rate. When the film is removed, the interface is released and the reaction continues.
Polymers are obtained by polymerization or polycondensation methods.
Polymerization (polyaddition). This is a 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. macromolecules does not differ from the composition of monomer molecules. As monomers, compounds with multiple bonds are used: C C, C N, C=C, C=O, C=C=O, C=C=C, C=N, or compounds with cyclic groups capable of opening.
Polymerization is a spontaneous exothermic process (), since the breaking of double bonds leads to a decrease in the energy of the system. However, without external influences (initiators, catalysts, etc.), polymerization usually proceeds slowly. Polymerization is a chain reaction. Depending on the nature of the active particles, radical and ionic polymerizations are distinguished.
At 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.
Ionic polymerization also occurs through the stage of formation of active centers, 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, H2SO4 and HCI, inorganic aprotic acids (SnCI4, TiCI4, AICI3, etc.), organometallic compounds AI (C2H5)3, etc. Electron-donor substances are used as initiators of anionic polymerization and compounds, including alkali and alkaline earth metals, alkali metal alcoholates, and others. Often several polymerization initiators are used simultaneously.
Chain growth can be written by the reaction equations:
in cationic polymerization and
in anionic polymerization
Bulk polymerization (in block ) 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.
At 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 ranging in size from to m are formed. The disadvantage of the method is the need to stabilize the suspension and wash the polymers from stabilizers.
At 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 ( H2O, NH3, HCI, CH2O, etc.), 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. The polycondensation of bifunctional compounds was called linear, for example:
2NH2 (CH2) 5 - COOH
aminocaproic 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-β-caproamide 2)5 n.
Such a polymer cannot be converted to its original state, it does not have thermoplastic properties and is called thermoset 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, heating the monomers at a temperature 10–20 higher than the melting (softening) temperature of polymers (usually 200–400). 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, poly--caproamide (kapron), polyhexamethylene adipamide (nylon) -NH (CH2) 6NHCO (CH2) 4CO- n, polyesters (polyethylene terephthalate - (-OC) C6H4 (CO) OCH2CH2- n ), polyurethanes -OROCONHR NHCO- n, polysiloxanes -SiR2-O- n, polyacetals -OROCHR- t, phenol-formaldehyde resins
