Federal Agency for Education of the Russian Federation
Bryansk State Engineering and Technology Academy
Test No. 2
By discipline: “Concepts of modern natural science”
On the topic: “Polymers, their production, properties and applications”
Performed: Bazanova Elena Ilyinichna
Code: 05-2.254
Faculty: economics
Group FC 103
Address: Klintsy
st. Mira no. 113 apt. 122
Checked: Evtyukhov K.N.
Bryansk 2006
Types of polymers, their general properties and methods of obtaining.
Natural IUDs or biopolymers. Properties, application, production.
Chemical IUDs. Properties, application, production.
Bibliography.
Types of polymers, their general properties and methods of production.
Polymers or high molecular weight compounds (HMCs) are complex substances with large molecular weights, the molecules of which (macromolecules) consist of a large number of regularly or irregularly repeating structural units (units) of one or more types. The molecular weights of polymers can range from several thousand to millions.
Based on their origin, polymers are divided into:
Natural, biopolymers (polysaccharides, proteins, nucleic acids, rubber, gutta-percha).
Chemical:
Artificial - obtained from natural fibers through chemical transformations (celluloid, acetate, copper-ammonium, viscose fibers).
Synthetic – obtained from monomers (synthetic rubbers, fibers /nylon, lavsan/, plastics).
By composition:
Organic.
Organoelement - divided into three groups: the main chain is inorganic, and the branches are organic; the main chain contains carbon and other elements, and the branches are organic; The main chain is organic and the branches are inorganic.
Inorganic - have main inorganic chains and do not contain organic side branches (elements of the upper rows of groups III - VI).
According to the structure of the macromolecule:
Linear (highly elastic).
Branched.
Mesh (low elasticity).
By chemical composition:
Homopolymers (contain the same monomer units).
Heteropolymers or copolymers (contain different monomer units)
According to the composition of the main chain:
Homochain (the main chain includes atoms of one element).
Heterochain (the main chain includes different atoms)
By spatial structure:
Stereoregular - macromolecules are built from units of the same or different spatial configurations, alternating in a chain with a certain periodicity.
Non-stereoregular (atactic) - with arbitrary alternation of links of different spatial configurations.
According to physical properties:
Crystalline (have long stereoregular macromolecules)
Amorphous
By method of receipt:
Polymerization.
Polycondensation.
By properties and application:
Plastics.
Elastomers.
General properties of polymers (characteristic of most IUDs).
IUDs do not have a specific melting point; they melt over a wide range of temperatures; some decompose below the melting point.
They are not subject to distillation, because they decompose when heated.
Insoluble in water or difficult to dissolve.
They have high strength.
Inert in chemical environments, resistant to environmental influences.
Preparation of polymers.
Three processes lead to the formation of an IUD:
Polymerization reaction – the process by which molecules of a low molecular weight compound (monomers) are linked to each other using covalent bonds, forming a polymer. This reaction is typical for compounds with multiple bonds.
Polycondensation reaction – the process of formation of a polymer from low-molecular compounds containing 2 or several functional groups, accompanied by the release, due to these groups, of substances such as water, ammonia, hydrogen halide, etc. (Capron, nylon, phenol-formaldehyde resins).
Copolymerization reaction – the process of forming polymers from two or more different monomers. (Preparation of styrene butadiene rubber).
Now let’s look at polymers, combining two characteristics: by origin – natural and chemical, and by properties and application – proteins, polysaccharides, nucleic acids, plastics, elastomers, fibers.
Natural polymers. Properties, application, production.
Natural polymers are IUDs of plant or animal origin. These include:
Polysaccharides.
Elastomers (natural rubber).
Nucleic acids.
Now let's look at each point in more detail.
Squirrels.
Proteins are natural organic, nitrogen-containing BMCs (biopolymers), the structural basis of which is made up of polypeptide chains built from amino acid residues. They come in 2 types:
Proteins (simple proteins) - consist only of amino acids, and proteids (complex proteins) - contain not only amino acids, but other groups of atoms.
The structure of protein structures.
There are 4 levels of structural organization of protein molecules.
Properties of proteins.
The properties of proteins are varied. Some dissolve in water, forming colloidal solutions, others in salt solutions, and others are insoluble. Proteins undergo oxidation-reduction reactions, esterification, alkylation, nitration; they are amphoteric. Proteins are also capable of reversibly changing their structure.
Functions and application.
Plastic function - proteins serve as the building material of the cell.
Transport function - transport various substances.
Protective function – neutralize foreign substances.
Energy function – supply organisms with energy.
Catalytic function - accelerate the occurrence of chemical reactions in the body.
Contractile function - perform all types of body movements.
Regulatory function - regulate metabolic processes in organisms.
Signal (receptor) – communicate with the environment.
Proteins are an essential part of human food, the lack of which can lead to serious diseases. Proteins are also used in almost all areas of human activity: medicine, food industry, chemical industry and much more.
Polysaccharides.
Polysaccharides are high-molecular non-sugar-like carbohydrates containing from 10 to 100 thousand monosaccharide residues linked by glycosidic bonds. Starch and cellulose are the most important natural representatives. General empirical formula (C H O)n. The monomer is glucose.
Starch, its properties, application and production.
Amorphous white powder, tasteless and odorless, poorly soluble in water, in hot water swells to form a colloidal solution. Starch consists of 2 fractions: amylose (20-30%) and amylopectin (70-80%).
Starch is formed as a result of photosynthesis and is deposited “in reserve” in tubers, rhizomes, and grains. It is obtained by processing them.
Starch undergoes hydrolysis, which results in the release of glucose. In the technique, it is boiled for several hours with dilute sulfuric acid, then chalk is added to it, filtered and evaporated. The result is a thick sweet mass - starch syrup, which is used for confectionery and technical purposes. To obtain pure glucose, the solution is boiled longer, the glucose is concentrated and crystallized.
Heating dry starch produces a mixture called dextrin, which is used in light industry and to make glue. Starch is also a raw material for the production of ethyl and n-butyl alcohols, acetone, citric acid, and glycerin. It is also used in medicine. The biological role of starch is great. It is the main nutrient reserve of plants.
Cellulose, or fiber, its properties, application, production.
Cellulose, or cellulose, is a fibrous substance, the main component plant cell, synthesized in plants. Pure cellulose is a white, odorless, tasteless fibrous substance, insoluble in water, diethyl ether and ethyl alcohol. Does not decompose under the influence of dilute acids, is resistant to alkalis and weak oxidizing agents. When treated in the cold with concentrated sulfuric acid, it dissolves, forming a viscous solution. It is hydrolyzed by enzymes, the end product of which is glucose. Forms esters, is on.
Receipt: The most common industrial method for extracting cellulose from wood is to treat crushed wood at elevated temperature and pressure with a solution of calcium hydrogen sulfate. The wood is destroyed, the lignin goes into solution, but the cellulose remains unchanged. It is separated from the solution, washed, dried and sent for further processing.
Application: As a component of wood, cellulose is used in construction, carpentry and as fuel. Wood is used to produce paper, cardboard and ethyl alcohol. In the form of fibrous materials (cotton, flax, jute), cellulose is used to prepare fabrics and threads. Cellulose ethers are used in the production of nitro-varnishes, films, smokeless powder, plastics, artificial fibers, and medical collodions.
Elastomers.
Elastomers are natural or synthetic IUDs with highly elastic properties. The most important representatives of natural elastomers are rubber and gutta-percha. Macromolecules of elastomers are chains twisted into balls that can be stretched under the influence of external force, and after its removal they twist again.
Natural rubber and gutta-percha.
Natural rubber is a high-molecular unsaturated hydrocarbon, the molecules of which contain a large number of double bonds; its composition can be expressed by the formula (CH)n - where n is from 1000 to 3000). It is a polymer of isoprene.
Natural rubber is contained in the milky sap of rubber-bearing plants, mainly tropical ones (Hevea tree). It is obtained from their juice.
Another natural product is gutta-percha. It is also a polymer of isoprene, but with a different molecular configuration.
The most important physical properties of rubbers are:
Elasticity is the ability to restore shape.
Impermeable to water and gases.
Raw rubber is sticky, fragile, and becomes brittle when the temperature drops slightly. To give products made from rubber the necessary strength and elasticity, the rubber is subjected to vulcanization - sulfur is introduced and heated. Vulcanized rubber is rubber.
Unfortunately, we do not have the ability to produce natural rubber.
Nucleic acids.
Nucleic acids are natural BMCs (biopolymers), the macromolecules of which consist of mononucleotides. Mainly nucleic acids are polynucleotides. They were discovered in 1868 by the Swiss chemist F. Miescher in the cell nucleus.
Polynucleotide formation scheme:
Nucleic acids are of 2 types:
Deoxyribonucleic acids - store and transmit genetic information.
Ribonucleic acids - copy genetic information, transfer it to the site of protein synthesis, and participate in protein synthesis itself.
Fibers.
Fibers are naval forces of natural synthetic origin, processed into threads. They are characterized by high molecular order (linear polymers).
Natural fibers come in 2 types:
animal origin - protein. They are obtained from animals (wool, silk).
plant origin - cellulose. They are produced from vegetation (cotton, flax, jute).
Used in light industry for clothing and other accessories. Also for making ropes, ropes, etc.
Chemical IUDs, properties, application, preparation.
Chemical polymers are BMCs that are obtained either by processing natural BMCs (artificial) or by synthesis from low molecular weight substances (synthetic).
Chemical IUDs are divided into:
plastics.
elastomers.
Plastics.
Plastics are materials based on natural and synthetic IUDs (plastics often contain other components) that can, under the influence of high temperature and pressure, take on any given shape and retain it after cooling (plasticity). If a polymer goes from a highly elastic state to a glassy state at temperatures below room temperature, it is classified as an elastomer; at higher temperatures, it is classified as a plastic.
Plastics are divided into two types: thermoplastic and thermoset.
Thermoplastic – plastics that harden and soften reversibly.
Properties:
Their structure is linear.
They lack strong connections between individual chains.
They melt easily and are used for re-melting.
Thermosetting – plastics that, when heated, lose their ability to transform into a viscous-flow state due to the formation of a network structure.
Properties:
Mesh structure.
There are strong connections between individual circuits.
They melt with difficulty and cannot be remelted.
Let's take a closer look at the types of plastics.
Polymerization resins.
Polymerization resins are polymers obtained by the polymerization of predominantly ethylene hydrocarbons or their derivatives.
Representatives of polymerization resins.
Polyethylene – a polymer formed by the polymerization of ethylene. When it is compressed to 150-250 mPa and at a temperature of 150-250 degrees, high-density polyethylene is obtained. With catalysts (CH) Al and TiCL (triethylaluminum and titanium chloride IV) - low-density polyethylene. At 10 MPa and chromium oxides - medium pressure polyethylene.
Polyethylene is a colorless, translucent in thin layers and white in thick layers, waxy but hard material with a melting point of 110-125. It has high chemical resistance and water resistance, low gas permeability. The properties of polyethylene depend on the production method. High-density polyethylene has a lower density than low-density polyethylene. For food products, only high-density polyethylene is used.
Polyethylene is used as an electrical insulating material, in the manufacture of films, as a packaging material, in the manufacture of lightweight unbreakable tableware, hoses and pipelines for the chemical industry.
2. Polypropylene – propylene polymer:
Obtained by polymerization in the presence of a catalyst. Depending on the polymerization conditions, polypropylene differs in its properties. This is a rubber-like mass, more or less hard and elastic, waterproof. Polypropylene is used for electrical insulation, the manufacture of protective films, hose pipes, gears, instrument parts, high-strength and chemically resistant fiber (ropes, fishing nets). Food in polypropylene packaging can be sterilized and even boiled.
3. Polystyrene – formed during the polymerization of styrene:
It can be obtained in the form of a transparent glass mass. It is used as organic glass, for the manufacture of industrial goods (buttons), and as an electrical insulating material.
4. Polyvinyl chloride (polyvinyl chloride) – obtained by polymerization of vinyl chloride:
This is an elastic mass that is resistant to acids, alkalis, and water. Widely used for lining pipes and vessels in the chemical industry, electrical insulator, for the manufacture of artificial leather, linoleum, and waterproof raincoats. By chlorinating it, perchlorovinyl resin is obtained, from which chemically resistant chlorin fiber is obtained.
5. Polytetrafluoroethylene – tetrafluoroethylene polymer:
It comes in the form of a plastic called Teflon. Resistant to alkalis and concentrated acids, superior to gold and platinum. Non-flammable, has high dielectric properties. Used in chemical engineering and electrical engineering.
6. Polyacrylates and polyacrylonitrile. The most important representatives are methyl acrylate and polymethyl methacrylate - solid, transparent, resistant to heat and light, and transmit ultraviolet radiation. I use them to make sheets of organic glass. Nitron synthetic fiber is obtained from polyacrylonitrile, which is used for the production of knitwear and fabrics.
Condensation resins.
Condensation resins - these include polymers obtained by polycondensation reactions.
Representatives of polycondensation resins.
1. Phenol-formaldehyde resins – VMCs formed as a result of the interaction of phenol (CH OH) with formaldehyde (CH = O) in the presence of catalysts. These resins have remarkable properties: when heated, they first soften, and with further heating (with a catalyst) they harden. Plastics - phenolics - are prepared from them: the resins are mixed with various fillers (ancient flour, asbestos, etc.), dyes, plasticizers, and various products are made from the resulting mass by hot pressing. They are also used in construction and foundry.
2. Polyester resins. An example is the product of condensation of a dibasic acid with the same alcohol - polyethylene terephthalate - a polymer in the molecules of which the ester group is repeated many times. This resin is produced under the name LAVSAN. It is used to make a fiber that resembles wool, but is stronger than it and the fabric does not wrinkle. Lavsan has high heat, moisture and light resistance. Resistant to alkalis, acids and oxidizing agents.
3. Polyamide resins – synthetic analogues of proteins (amide bonds). Fibers are obtained from them - CAPRON, ENANT, ANID. They are superior to natural silk in some properties. More details in the fiber section.
Elastomers.
We talked about elastomers above. We examined them using the example of natural rubber and gutta-percha. Now let's look at synthetic elastomers. Since in our country it is not possible to obtain natural rubber, our chemists were the first to develop and implement a method for producing synthetic rubber (1928-1930). According to Lebedev, the starting material was butadiene, which was obtained from ethyl alcohol. Now it has been developed to obtain it from Buran, associated petroleum gas. The chemical industry now produces a lot various types synthetic rubbers that are superior to natural rubber in some properties. In addition to polybutadiene rubber (SBR), copolymer rubbers are widely used - products of co-polymerization of butadiene with other unsaturated compounds, for example with styrene (SKS) or acrylonitrile (SKN).
They also produce synthetic polyisoprene rubber (SRI), which is similar in properties to natural rubber.
Synthetic rubber, like natural rubber, is vulcanized to produce rubber and ebonite.
In technology, rubber is used to make tires for transport, used as an electrical insulating material, in the production of industrial goods and medical devices, in light, construction and other fields.
Fibers.
We also considered fibers, but natural ones. Now let's look at the chemical ones. There are 2 types of chemical fibers:
1. artificial – products of processing of natural polymers (viscose, acetate, copper-ammonia).
2. synthetic – polymers formed from low molecular weight substances (polyesters, polyamides).
Artificial fibers.
The production of artificial fiber from cellulose is carried out in 3 ways: viscose, acetate and copper-ammonia.
1. Viscose. Cellulose is treated with caustic soda and then with carbon disulfide. The resulting orange mass (xanthate) is dissolved in a weak solution of sodium hydroxide to obtain viscose. It is pressed through special caps - dies into a precipitation bath with a solution of sulfuric acid. Shiny threads are formed, the composition of cellulose is slightly changed - viscose fiber.
2. Acetate. A solution of cellulose acetate in acetone is forced through dies towards warm air. Streams of solution turn into the thinnest threads - acetate fiber.
3. Copper-ammonia. His method is less common. Cellulose is again isolated from an ammonia solution of copper oxide in which cellulose is dissolved under the action of acids. The threads make up the fiber.
Synthetic fibers.
Synthetic fibers are obtained from polyester and polyamide resins. We considered polyester resins in plastics (lavsan). Now let's look at polyamide resins and the fibers that are made from them.
Capron – aminocaproic acid polycondensate containing a chain of 6 carbon atoms.
Enant – aminoethanoic acid polycondensate (7 carbon atoms).
Anid (nylon) – polycondensate of dibasic adipic acid and hexamethyldioamne. Structural link:
List of used literature.
General chemistry: Tutorial for universities. – 23rd edition, revised / Edited by V. A. Rabinovich. – L.: Chemistry, 704 pp.
Chemistry. Manual - tutor for those entering universities // 7th edition. – Rostov-on-Don: Phoenix Publishing House, 2003. – 768 pages
Organic chemistry. / Ed. N. E. Kuzmenko. – M.: LLC “Publishing House “ONICS 21st Century”: LLC “Publishing House “Peace and Education”, 2002 - 640 pp.
General chemistry. Textbook for technical schools. I.G. Khomchenko. – M.: Novaya Volna Publishing House LLC. –2003 - 464 pp.
Syntheses of polymers are usually carried out on the basis of two types of reactions: polymerization and polycondensation. In addition, some types of polymers are obtained using the polymer-analogous transformation method, which is based on chemical transformations of finished polymer compounds.
Polymerization. Polymerization is the chemical reaction of the formation of high molecular weight organic compounds from low molecular weight ones (monomers), and the resulting polymers have the same elemental composition as the original monomers. Polymerization can be chain or stepwise.
The mechanism of chain polymerization is similar to the mechanism of reactions receiving common name chain, the theory of which was developed by the Soviet scientist academician N. N. Semenov.
Chain polymerization, which results in the formation of long polymer macromolecules, consists of three main stages: 1) the beginning of chain growth (the emergence of active centers); 2) chain growth; 3) open circuit.
In order for chain growth to begin, it is necessary to activate the monomer molecules. For this purpose, initiators or catalysts are used.
Some monomers (for example, styrene) have the ability to polymerize when exposed to elevated temperatures. In this case, initiation apparently occurs as a result of the thermal decomposition of the monomer into radicals. The rate of thermal initiation is significantly less than the rate of initiation in the presence of an initiator.
Relatively unstable materials are used as polymerization initiators. chemical substances(mainly of a census nature), capable of decomposing with the formation of free radicals. For example, benzoyl peroxide, often used as a polymerization initiator, decomposes to form free radicals with an unpaired electron:
The resulting free radical interacts with the unsaturated monomer; this breaks the double bond and
a new free radical with an unpaired electron is formed;
With each addition, one electron of the double bond forms a pair with the electron of the free radical (covalent bond), and the second electron remains unpaired (free) and can again join the double bond of the monomer molecule. Thus, many monomer molecules are added to the growing chain within a short period of time, resulting in the formation of a macroradical.
When such a macroradical collides with another free radical or with a solvent molecule, the reaction chain breaks:
The resulting polymer macromolecule loses its ability to participate in further reactions.
The initiator residue is included in the polymer as a terminal group of the chain.
From the above diagram it can be seen that the initiator is entirely consumed for the formation of a polymer macromolecule.
If the polymerization reaction is carried out in the presence of catalysts (for example, such as aluminum chloride, boron trifluoride, etc.), then the formation of active centers occurs by adding the catalyst to the unsaturated monomer; this results in an unstable complex ion. Such a complex ion (like a free radical) attaches monomer molecules to form a macroion. In contrast
from radical polymerization, the termination of the reaction chain occurs with the elimination of the catalyst, which is therefore not consumed for the formation of a polymer macromolecule.
Chain polymerization proceeds at high speed, and it is not possible to isolate intermediate reaction products.
Polymerization under the influence of ionic catalysts is called ionic catalytic polymerization, in contrast to radical polymerization, which occurs under the influence of free radicals.
The rate of polymerization depends on temperature, pressure, amount of initiator (and in the case of ionic polymerization, on chemical nature catalyst).
The molecular weight of a polymer, both during radical and ionic polymerization, depends on the ratio of the rates of chain growth and chain termination reactions. The higher the growth rate of depi and the lower the rate of its termination, the longer the chain of the resulting macromolecule and the greater the molecular weight of the resulting polymer.
Relatively recently, a new reaction was discovered - the telomerization reaction. Its essence lies in the radical polymerization of unsaturated compounds in the presence of saturated halogenated hydrocarbons or other saturated compounds (telogens) capable of breaking the reaction chain by joining the polymer macromolecule at its two ends.
If we denote a saturated compound (telogen) by then, the telomerization reaction can be depicted by the diagram:
As a result of the telomerization reaction, relatively low molecular weight compounds are formed.
The significance of the telomerization reaction lies in the fact that with its help it is possible, starting from the simplest raw materials, to quite easily obtain various higher bifunctional compounds (glycols, dicarboxylic acids, amino acids, hydroxy acids, etc.), the production of which in other ways is usually associated with great difficulties.
An example of the practical application of the telomerization reaction is the method developed in the Soviet Union for obtaining -aminoenanthic acid - the starting material for the production of polyamide fiber - enanth (p. 349).
First, ethylene is telomerized in the presence of carbon tetrachloride at
For example, benzoyl peroxide is used as a reaction initiator.
Then, from the resulting mixture of tetrachloroalkanes, -tetrachloroheptane is isolated by rectification and -chlorrenanthic acid is obtained from it, which is converted into -aminoenanthic acid by the action of ammonia:
Stepwise polymerization occurs with a gradual (stepwise) increase in molecular weight. During stepwise polymerization, the addition of each subsequent monomer molecule occurs to form intermediate compounds that can be isolated.
The mechanism of the reaction of step polymerization of olefins is the movement of a hydrogen atom and the formation of intermediate compounds with a double bond at the end of the growing chain, for example:
Step polymerization also includes the polymerization of cycles, for example, the polymerization of amino acid lactams. Activators of polymerization of cycles are water, some organic acids, sodium metal, etc.
For example, when water acts as an activator on caprolactam (p. 199), the amino acid is first formed
The resulting linear structure addition product again interacts with a new caprolactam molecule:
The reaction is carried out at elevated temperature and pressure.
Polycondensation. Polycondensation reactions are also used to produce broad polymers. It differs significantly in mechanism from the polymerization reaction. During polymerization, the transformation of a monomer into a polymer occurs without the release of any other chemical compounds. The polycondensation reaction consists of the interaction of functional groups of monomers and is accompanied by the release of a substance, for example water, ammonia, hydrogen chloride. The polycondensation reaction is stepwise: the chain grows gradually. First, two molecules of the original substance react with each other, then the resulting compound interacts with the third molecule of the original substance, with the fourth, etc.
All intermediate reaction products formed as a result of the gradual addition of new monomer molecules are quite stable and can be isolated. They retain their reactivity, which is determined by the presence of unreacted functional groups.
Polymers can only form if the reacting molecules have at least two functional groups. Compounds with three or more functional groups can form spatial polymers.
The polycondensation reaction is often divided into homopolycondensation and heteropolycondensation.
Homopolycondensation is a polycondensation reaction in which homogeneous molecules participate, for example the polycondensation of -aminoenanthic acid:
Heteropolycondensation is a polycondensation involving two or more dissimilar compounds, the molecules of which have two or more identical functional groups, for example, the polycondensation of diamines with dicarboxylic acids:
The polycondensation reaction is carried out in the presence of ionic catalysts (acids or bases).
Polymer-analogous transformations. The production of polymers by polymer-analogous transformations is based on chemical reactions functional groups in polymer macromolecules. Functional groups in polymer compounds have the same reactivity as the corresponding functional groups in low molecular weight compounds.
Polymer-analogous transformations are resorted to in the absence of the corresponding starting monomers or in the case of the impossibility of synthesizing the polymer using available methods. In this way, for example, polyvinyl alcohol is obtained, which cannot be obtained from monomeric vinyl alcohol due to its instability and rapid isomerization into acetaldehyde:
Polyvinyl alcohol is obtained by hydrolysis of polymeric vinyl esters, for example polyvinyl acetate:
Mechanochemical method. Graft and block copolymers can be obtained not only chemically, but also mechanically. For example, when two different rubbers are rolled (ground between rollers) in an oxygen-free environment, the molecules of the rubbers taken break apart to form high molecular weight free radicals. Such macromolecule residues can attach a residue or molecule of another rubber. If the remainder
or the molecular chain of one rubber forms a site in the main chain of the molecule of the second rubber, then block copolymers are obtained; if it forms side chains, the result is graft copolymers.
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.
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 circuit break.
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-donating substances are used as initiators of anionic polymerization and compounds, including alkaline and alkaline earth metals, alkali metal alcoholates, etc. Often several polymerization initiators are used simultaneously.
The chain growth can be written by reaction equations:
during cationic polymerization and
during anionic polymerization
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 (emulsion polymerization) 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 speed reactions, 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 up 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 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, 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, and foreign scientists by W. Carothers, P. Flory, P. Morgan and others. Polycondensation of bifunctional compounds is 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-caproamide 2)5 n.
Such a polymer cannot be converted to its original state; it does not have thermoplastic properties and is called thermosetting 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 above the melting (softening) temperature of the polymers (usually 200 - 400 ). 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, poly--caproamide (capron), polyhexamethylene adipinamide (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
In 1833, J. Berzelius coined the term “polymerism,” which he used to name one of the types of isomerism. Such substances (polymers) had to have the same composition, but different molecular weights, such as ethylene and butylene. TO modern understanding J. Berzelius’s conclusion does not correspond to the term “polymer”, because true (synthetic) polymers were not yet known at that time. The first mentions of synthetic polymers date back to 1838 (polyvinylidene chloride) and 1839 (polystyrene).
Polymer chemistry arose only after A. M. Butlerov created the theory chemical structure organic compounds and received further development thanks to intensive searches for methods for synthesizing rubber (G. Bushard, W. Tilden, K. Harries, I. L. Kondakov, S. V. Lebedev). Since the beginning of the 20s of the 20th century, they began to develop theoretical ideas about the structure of polymers.
DEFINITION
Polymers — chemical compounds with a high molecular weight (from several thousand to many millions), the molecules of which (macromolecules) consist of a large number of repeating groups (monomeric units).
Classification of polymers
The classification of polymers is based on three characteristics: their origin, chemical nature and differences in the main chain.
From the point of view of origin, all polymers are divided into natural (natural), which include nucleic acids, proteins, cellulose, natural rubber, amber; synthetic (obtained in the laboratory by synthesis and having no natural analogues), which include polyurethane, polyvinylidene fluoride, phenol-formaldehyde resins, etc.; artificial (obtained in the laboratory by synthesis, but based on natural polymers) - nitrocellulose, etc.
Based on their chemical nature, polymers are divided into organic polymers (based on monomer - organic matter- all synthetic polymers), inorganic (based on Si, Ge, S and other inorganic elements - polysilanes, polysilicic acids) and organoelement (a mixture of organic and inorganic polymers - polyloxanes) nature.
There are homochain and heterochain polymers. In the first case, the main chain consists of carbon or silicon atoms (polysilanes, polystyrene), in the second - a skeleton of various atoms (polyamides, proteins).
Physical properties of polymers
Polymers are characterized by two state of aggregation– crystalline and amorphous and special properties – elasticity (reversible deformations under small loads - rubber), low fragility (plastics), orientation under the action of a directed mechanical field, high viscosity, and the dissolution of the polymer occurs through its swelling.
Preparation of polymers
Polymerization reactions – chain reactions, which are the sequential addition of molecules of unsaturated compounds to each other with the formation of a high-molecular product - a polymer (Fig. 1).
Rice. 1. General scheme for polymer production
For example, polyethylene is produced by polymerization of ethylene. Molecular mass molecules reaches 1 million.
n CH 2 =CH 2 = -(-CH 2 -CH 2 -)-
Chemical properties of polymers
First of all, polymers will be characterized by reactions characteristic of the functional group present in the polymer. For example, if the polymer contains a hydroxo group characteristic of the class of alcohols, therefore, the polymer will participate in reactions like alcohols.
Secondly, interaction with low molecular weight compounds, interaction of polymers with each other with the formation of network or branched polymers, reactions between functional groups that are part of the same polymer, as well as the decomposition of the polymer into monomers (destruction of the chain).
Application of polymers
The production of polymers has found wide application in various areas life of mankind - the chemical industry (plastic production), machine and aircraft construction, at oil refining enterprises, in medicine and pharmacology, in agriculture(production of herbicides, insecticides, pesticides), construction industry (sound and heat insulation), production of toys, windows, pipes, household items.
Examples of problem solving
EXAMPLE 1
EXAMPLE 1
| Exercise | Polystyrene is highly soluble in non-polar organic solvents: benzene, toluene, xylene, carbon tetrachloride. Calculate mass fraction(%) polystyrene in a solution obtained by dissolving 25 g of polystyrene in benzene weighing 85 g. (22.73%). |
| Solution | We write down the formula for finding the mass fraction:
Let's find the mass of benzene solution: m solution (C 6 H 6) = m (C 6 H 6)/(/100%) |
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: CºC, CºN, C=C, CO, C=C=O, C=C=C, C=N, or compounds with cyclic groups capable of opening, for example:
During the polymerization process, multiple bonds are broken or rings open in monomers and the formation of chemical bonds between groups to form 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 (DC<0, DН<0), так как разрыв двойных связей ведет к уменьшению энергии системы. Однако без внешних воздействий (инициаторов, катализаторов и т.д.) полимеризация протекает обычно медленно. Политизация 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, nitrogen compounds (having the functional group - N = N -) and other compounds with weakened bonds serve as initiators of polymerization. Initially, radicals are formed, for example:
Then macroradicals are formed, for example during the polymerization of vinyl chloride:
R + CH 2 = SHC1 → YASN 2 - CHCl ·
RCH 2 - CHCl + CH 2 = CHCl → RCH 2 - CHCl - CH 2 - CH1 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-(-CH 2 -CHC1-) n -CH 2 -CHCl + CH 2 =CHC1→
→ 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-) n -CH 2 -CHCl · + R-(-CH 2 -CHC1-) n -CH 2 -CHCl · →
→ R- (-CH 2 - CHN1-) n - CH 2 - CHN1 - CH 2 - CHN1- (-CH 2 -CHN1) 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; 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-CHC1-] n, polyvinyl acetate [-CH 2 -CH(OSOCN 3)-] n, polystyrene n, polyacrylate [-CH 2 -C(CH 3) (SOOC)-] n, polyethylene [-CH 2 -CH 2 -] n, polydienes [-CH 2 -C(R)=CH-CH 2 -] 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 H 2 SO 4 and HC1, organic protic acids (SnCl 4, TiCl 4, AlCl 3, etc.), organometallic compounds Al (C 2 H 5)3, etc. Electron-donor substances and compounds are used as initiators of anionic polymerization, including alkali and alkaline earth metals, alkali metal alcoholates, etc. Often several polymerization initiators are used simultaneously.
The chain growth can be written by reaction equations:
during cationic polymerization and
M n + + M → M n +1 +
during anionic polymerization
M n - + M → M n +1 -
Let us consider, as an example, the cationic polymerization of isobutylene with the initiators A1C1 3 and H 2 O. The latter form a complex
A1C1 3 + H 2 O ↔ H + [A1OHC1 3 ] –
Designating this complex with the formula H + X –, the process of initiating polymerization can be represented as
The resulting complex cation, together with the counterion X – forms a macroion, which ensures chain growth:
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 A1R 3 .
The ionic polymerization method is used in the production of polyisobutylene [-CH 2 -C(CH 3) 2 -] n, polyformaldehyde [-CH 2 O-] n, polyamides, for example poly-ε-caproamide (kaprone) [-NH-(CH 2) 5 -CO-] n, synthetic rubbers, for example butadiene rubber [-CH 2 -CH=CH-CH 2 -] 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 (H 2 O, NH3, HC1, CH 2 O, 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:
2NH 2 -(CH 2) 5 -COOH →
aminocaproic acid
→ NH 2 -(CH 2) 5 -CO-NH-(CH 2) 5 -COOH+H 2 O→
NH 2 -(CH2) 5 -CO-NH-(CH 2) 5 -COOH+NH 2 -(CH 2) 5 -COOH→
→NH 2 -(CH 2) 5 -CO-NH-(CH 2) 5 -CO-NH-(CH 2) 5 -COOH+H 2 O, etc.
The final product will be poly-ε-caproamide [-CO-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
2 CH 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, an oligomer with a linear structure is synthesized:
[-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 and 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 shown in the table:
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 (H 2 O, NH 3, HC1, CH 2 O, etc.) can interact with intermediate polycondensation products, causing their splitting (hydrolysis, aminolysis, acidolysis, etc.), for example:
NH-CO-(CH2)5-NH-CO-(CH2)5- + H 2 O →
→ - NH-CO-(CH2) 5 -NH 2 -HO-CO-(CH2) 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 a reaction due to thermal or oxidative degradation of intermediates. 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, poly-ε-caproamide (capron), polyhexamethylene adipinamide (nylon) [–NH(CH 2) 6 NHCO(CH 2) 4 CO-] n,
polyesters (polyethylene terephthalate [-(-OS)C 6 H4(CO)OSH 2 CH 2 -] n),
polyurethanes [-OROCONНR’NНСО-] n, polysiloxanes [-SiR 2 -O-] n,
polyacetals [- OROCHR "-] n, phenol-formaldehyde resins
urea-formaldehyde resins, etc.
Thus, polymers are obtained by polymerization and polycondensation methods. Polymerization occurs via a chain mechanism. Phi polycondensation produces both polymers and low molecular weight products.
Questions for self-control
1. Write the structural formula of vinyl acetate. Give a scheme for the polishing of this compound.
2. Give a scheme for the copolymerization of acrylonitrile, CH 3 CH 2 CN and vinyl acetate.
3. Give a scheme for the polycondensation of terephthalic acid C 6 H 4 (COOH) and ethylene glycol.
STRUCTURE OF POLYMERS
Shape and structure of polymer macromolecules. Macromolecules of polymers can be linear, branched and network. Linear polymers are formed during the polymerization of monomers or linear polycondensation. Branched polymers can be formed both during polymerization and polycondensation. Branching of polymers during polymerization can be caused by chain transfer to a macromolecule, growth of side chains due to copolymerization, and other reasons. Branched polymers are formed during the polycondensation of multifunctional compounds, as well as as a result of grafting side chains onto macromolecules. Grafting is carried out either by reacting polymers with oligomers or monomers, or by physically influencing (for example, γ-irradiation) a mixture of polymer and monomers. Network polymers are formed as a result of cross-linking of chains during vulcanization, the formation of thermosetting resins, etc. The shape of macromolecules affects the structure and properties of polymers.
Linear and branched macromolecules, due to the ability of atoms and groups to rotate around single bonds, constantly change their spatial shape, or, in other words, have many conformational structures. This property ensures the flexibility of macromolecules, which can bend, twist, and straighten. Therefore, linear and branched polymers are characterized by a highly elastic state, i.e. the ability to undergo reversible deformation under the influence of relatively small external forces. They also have thermoplastic properties, i.e. capable of softening when heated and hardening when cooling without chemical transformations. When polymers branch, the elastic and thermoplastic properties become less pronounced. When a network structure is formed, thermoplasticity is lost. As the length of the chains in the mesh cells decreases, the partiality of the polymers is also lost, for example, during the transition from rubber to ebonite.
Linear macromolecules can have a regular and irregular structure. In polymers with a regular structure, individual chain links are repeated in space in a certain order. Polymers with a regular structure are called stereoregular. Polymers in which individual units are randomly arranged in space have an irregular structure. As an example, we give polypropylene of irregular (a) and regular (b) structure:
Stereoregular polymers are usually prepared by ion polarization using complex catalysts. Natural rubber, as well as some synthetic polymers, such as polyisobutylene, polyethylene, and polypropylene, have a stereoregular structure. The stereoregularity of the structure changes the thermal and mechanical properties of polymers.
Crystalline state of polymers. Most polymers are usually in an amorphous state. However, some polymers under certain conditions may have a crystalline structure. Only stereoregular polymers have the ability to crystallize. Due to their regular structure and flexibility, macromolecules can approach each other at a sufficiently close distance so that effective intermolecular interactions and even hydrogen bonds arise between them, which lead to ordering of the structure. The polymer crystallization process occurs through several stages. At the first stage, packs appear - associates of ordered molecules. Fibrils and spherulites are formed from the packs. Fibrils are aggregates of oblong-shaped packs, and spherulites are needle-shaped formations radiating from one center. Finally, single crystals are formed from fibrils and spherulites. Crystalline polymers consist of a large number of crystals, between which there are regions with a disordered structure (amorphous regions). Therefore, such polymers are characterized by a certain degree of crystallinity. For example, the degree of crystallinity of polyethylene can reach 80%. The ability to form crystals is most pronounced in polyolefins, polyamides and polyesters. The polymer carbyne has a crystalline structure. The properties of crystalline and amorphous polymers differ significantly. Thus, amorphous polymers are characterized by a range of softening temperatures, i.e. the region of gradual transition from solid to liquid, and crystalline polymers - the melting point. Some polymers form liquid crystals.
Physical states of amorphous polymers. Amorphous polymers can be in glassy, highly elastic and viscous states. To determine the temperature limits for the existence of these states, the dependence of the polymer deformation on temperature is studied, on the basis of which a thermomechanical curve is constructed:
At low temperatures, the polymer is in a glassy state (region 1), in which the polymer behaves like another solid. In this state, there is no movement, either in molecules or individual units, and only vibrations of atoms around the equilibrium position appear. As the temperature increases, the polymer transforms into a highly elastic state, characteristic only of high-molecular compounds (region 2); a substance in a highly elastic state is capable of significant reversible deformations, which is due to the mobility of the links and, accordingly, the flexibility of macromolecules.
The movement of links does not occur instantly, therefore the deformations of polymers in a highly elastic state are of a relaxation nature, i.e. characterized by the time it takes to establish equilibrium. The highly elastic state of polymers manifests itself in the range from the glass transition temperature (Tst) to the flow temperature (Tm) (region 2). If the temperature range Tst -Tt is wide enough and covers normal temperatures, then such polymers are called elastics, or elastomers or rubbers. Polymers with a narrow temperature range T st -T t, shifted to the region of elevated temperatures, are called plastics or plastomers. At normal temperatures, plastics are in a glassy state. At a temperature above the flow temperature Tt (region 3), the polymer goes into a viscous flow state. An increase in temperature above T p leads to destruction and destruction of the polymer. A substance in a viscous flow state under the influence of shear stress flows as a viscous liquid, and the deformation of the polymer is irreversible (plastic). The viscous flow state is characterized by the mobility of both individual units and the entire macromolecule. As the polymer flows, the macromolecules straighten and come closer together, leading to increased intermolecular interaction, as a result of which the polymer becomes rigid and its flow stops. This phenomenon, characteristic only of amorphous polymers, is called mechanical glass transition. It is used in the formation of fibers and films. The polymer can also be converted into a viscous flow state by adding solvents or plasticizers, for example esters of phosphoric and phthalic acids.
So, polymers can have linear, branched and network structures and be in an amorphous, and some polymers are in a crystalline state.
Questions for self-control
4. How do polymers with irregular and regular structures differ in structure and properties?
5. What are the differences in properties between amorphous and crystalline polymers?
PROPERTIES OF POLYMERS
Chemical properties of polymers. Chemical properties depend on the composition, molecular weight and structure of the polymers. They are characterized by reactions of connecting 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. For the same reason, individual macromolecules can be cross-linked. Examples of cross-linking include vulcanization and the transfer of linear macromolecules of thermosetting polymers into network structures. During vulcanization, rubber interacts with a vulcanizing agent, usually sulfur, to form rubber (0.5 - 5% sulfur) or hard rubber (20% or more sulfur), for example:
Reactions of interaction of functional groups with low molecular weight substances include halogenation of polyolefins, hydrolysis of polyacrylates, etc.
Polymers can be subject to destruction, i.e. 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 destruction, stabilizers are introduced into the polymers, most often antioxidants, i.e. oxidation reaction inhibitors (phosphites, phenols, aromatic amines). Stabilization is usually caused by chain termination when antioxidants interact with free radicals formed during the oxidation reaction.
Mechanical properties of polymers. Mechanical properties are determined by the elemental composition, molecular weight, structure and physical state of macromolecules.
Polymers are characterized by certain features, such as a highly elastic state under certain conditions, mechanical glass transition, and the ability of thermosetting macromolecules to form rigid network structures. The mechanical strength of polymers increases with increasing molecular weight, during the transition from linear to branched and then network structures. Stereoregular structures have higher strength than polymers with a disordered structure. A further increase in the mechanical strength of polymers is observed during their transition to the crystalline state. For example, the tensile strength of crystalline polyethylene is 1.5-2.0 orders of magnitude higher than the strength of amorphous polyethylene. The specific strength per unit cross-sectional area of crystalline polymers is comparable, and per unit mass is an order of magnitude higher than the strength of alloy steels.
The mechanical strength of polymers can also be increased by adding fillers, such as carbon black and chalk, and reinforcing fibers, such as glass fiber.
Electrical properties of polymers. All substances are divided into dielectrics, semiconductors and conductors.
Dielectrics have very low conductivity (σ<10 –8 Ом –1 . см –1), которая увеличивается с повышением температуры. Под Действием внешнего электрического поля происходит поляризация Диэлектриков, т.е. определенная ориентация молекул. Вследствие Поляризации внутри диэлектрика возникает собственное электрическое поле, которое ослабляет воздействие внешнего поля. Количественной характеристикой ослабления воздействия внешнего поля слупит диэлектрическая проницаемость, показывающая, во сколько раз сила взаимодействия двух зарядов в диэлектрике меньше, чем в вакууме. Вследствие поляризации в диэлектрике возникают диэлектрические потери, т.е. превращение электрической энергии в тепловую, при некотором высоком напряжении внешнего электрического поля Диэлектрик теряет свои электроизоляционные свойства. Это напряжение получило название напряжения пробоя, а отношение напряжения пробоя к толщине диэлектрика - электрической прочности.
Most polymers are dielectrics. However, their dielectric properties vary widely and depend on the composition and structure of macromolecules. Dielectric properties are largely determined by the presence, nature and concentration of polar groups in macromolecules. The presence of halogen, hydroxide, carboxy and other polar groups in macromolecules worsens the dielectric properties of polymers. For example, the dielectric constant of polyvinyl chloride is 1.5 times higher, the electrical resistivity and electrical strength are an order of magnitude lower, and the dielectric losses are two orders of magnitude higher than those of polyethylene. Therefore, polymers that do not have polar groups, such as fluoroplastic, polyethylene, polyisobutylene, and polystyrene, are good dielectrics. As the molecular weight of the polymer increases, its dielectric properties improve. During the transition from glassy to highly elastic viscous-flow states, the specific electrical conductivity of polymers increases.
The electrical conductivity of dielectrics is determined by the movement of ions formed during the destruction of polymers, as well as the dissociation of impurities, including low molecular weight condensation products, solvents, emulsifiers, initiators and polymerization catalysts. Therefore, to improve dielectric properties, it is necessary to remove impurities from polymers.
Some functional groups, such as hydroxide groups, make polymers hydrophilic. Such polymers absorb water. The presence of water leads to an increase in the electrical conductivity of polymers, so hydroxide groups tend to bond with each other or with other groups (condensation reaction).
Polymer dielectrics are widely used in electrical and radio engineering as materials for various electrical products, protective coatings for cables, wires, insulating enamels and varnishes.
Organic semiconductors and electrolytes. Semiconductors include substances whose electrical conductivity lies in the range of 10 –10 ÷10 –3 Ohm –1. cm–1. The electrical conductivity of semiconductors increases with increasing temperature and when exposed to light. Some polymers have semiconducting properties. Typically these are polymers with a system of conjugated double bonds. The semiconducting properties of such polymers are due to the presence of nonlocalized π-electrons of conjugated double bonds.
In an electric field of a certain voltage, these electrons can move along the circuit, providing charge transfer. Examples of organic semiconductors include polyacetylene [-CH=CH-] n, polyvinylenes [- HC = C -] n, polynitriles [- N = C -] n,
polyacrylonitrile heat treatment products
In recent years, the phenomenon of a sharp increase in the electrical conductivity of polyacetylene, polyaniline (-C 6 H3NH 2 -) n, polypyrrole (-C 4 H3N-) n and other polymers during their chemical or electrochemical oxidation or reduction has been discovered. During electrochemical oxidation, anions are introduced into the polymer composition, for example C1O4 –, during reduction - cations, for example Li +. At a certain concentration of additives, the electrical conductivity increases abruptly, for example, for polyacetylene from 10 -6 to 10 1 Ohm -1. cm -1 .
Organic semiconductors doped with ions can be used as electrode materials for batteries, capacitor plates, sensor materials, and in the future, to replace metals (organic metals).
A mixture of some polymers in an amorphous state, for example, polyethylene oxide (-CH2-CH2-O-) n with metal salts, for example, LiClO 4, has ionic conductivity, so such solid electrolytes can be used in batteries. Gel-like mixtures of solvent polymer and salt have acceptable ionic conductivity.
Thus, the physical and chemical properties of polymers depend on their composition and structure.
Questions for self-control
6. Which polymer is more resistant to aging: fluoroplastic or polyethylene?
7. Which of the two polymers has higher electrical conductivity: polypropylene or polyvinyl chloride?
