Traditionally, in each CT there are tasks for polymers which are obtained by polymerization and polycondensation reactions. But not all applicants cope with these tasks. So I decided to fill this gap in your knowledge.

I made a selection of all the polymers that were found in CT of all years taking into account the reactions of their production. And also made a video solution of one of the most difficult tasks on polymers from CT in chemistry 2008.

Polymerization(ancient Greek πολυμερής - consisting of many parts) - the process of formation of a high molecular weight substance (polymer) by repeated addition of molecules of a low molecular weight substance (monomer, oligomer) to active centers in a growing polymer molecule. The monomer molecule that is part of the polymer forms the so-called monomeric (structural) unit. Elemental composition ( molecular formulas) monomer and polymer are approximately the same.

Polycondensation- the process of synthesizing polymers from polyfunctional (most often bifunctional) compounds, usually accompanied by the release of low molecular weight by-products (water, alcohols, etc.) upon interaction functional groups.

That is, during polycondensation, in addition to the polymer, some other low-molecular substance is formed, but during polymerization, only polymer!

Polymerization produces:

1) plexiglass: made of methyl methacrylate

2) polystyrene

4) chloroprene rubber (polychloroprene): from 2-chlorobutadiene-1,3

5) butadiene rubber

6) Teflon (polytetrafluoroethylene)

7) polypropylene, polyethylene, etc.

By polycondensation we get:

1) lavsan (polyethylene terephthalate): terephthalic acid + ethylene glycol

2) Kevlar: phenylene-1,4-diamine (para-phenylenediamine) + terephthaloyl chloride (terephthalic acid dichloride)

3) nylon: adipic acid + hexamethylenediamine

4) polypeptides: from amino acids

5) phenol-formaldehyde resins: phenol + formaldehyde

During the polymerization reaction, only polymers are obtained as a result. During polycondensation, the reaction product becomes polymers and low molecular weight substances.

Definition

In progress polymerization both identical and different monomer molecules are sequentially connected, building one complex polymer molecule (high molecular weight substance) without isolating and forming by-products - low molecular weight compounds. Therefore, the output is a polymer with exactly the same elemental composition as the monomer.

In progress polycondensation molecules of one or more monomers, connecting with each other, form a polymer macromolecule and by-product release one or another low-molecular product (water, alcohol, hydrogen chloride or ammonia). Polycondensation underlies the biosynthesis of cellulose, nucleic acids and, of course, proteins.

Comparison

These two processes are similar in that at the beginning of the reaction the original monomer enters into the reaction. And then during polymerization in the reaction system at all stages of the current process there are increasing active chains, the original monomer and macromolecules that have completed growth. And in the process of polycondensation, the monomer, as a rule, is exhausted at the initial stages of the ongoing reaction, and subsequently only polymers (oligomers) remain in the system, interacting with one another.

For polymerization and polycondensation, the reactivity of the desired monomers and, of course, their structure are equally important. During polymerization, reactions that occur between increasing molecules usually end in chain termination.

And during polycondensation, the reactions occurring between increasing molecules are the main reactions of growth of polymer chains. Long chains are formed due to the interaction of oligomers. Polymerization occurs in three stages: initiation, chain growth and chain termination. In this case, the centers of growth of the polymer chain are cations, free radicals or anions. Functionality (the number of reaction centers in a molecule) influences the formation of three-dimensional, branched or linear macromolecules.

Conclusions website

  1. Polycondensation is characterized by the release of by-products - low molecular weight substances such as water or alcohol.
  2. During polymerization, only polymers become reaction products.
  3. The biosynthesis of cellulose, proteins and nucleic acids is possible due to the polycondensation reaction.

Condensation is the basis for the creation of polymer synthetic materials: polyvinyl chloride, olefins. When using basic versions of monomers, it is possible to obtain millions of tons of new polymer substances by copolycondensation. Currently, there are various methods that make it possible not only to create substances, but also to influence the molecular weight distribution of polymers.

Process Features

The polycondensation reaction is the process of producing a polymer by the stepwise addition of molecules of polyfunctional monomers to each other. In this case, low molecular weight products are released.

The basis of this process can be considered Due to the release of by-products, there are differences in the elemental composition of the polymer and the original monomer.

The amino acid polycondensation reaction is associated with the formation of water molecules during the interaction of the amino and carboxyl groups of neighboring molecules. In this case, the first stage of the reaction is associated with the formation of dimers, then they are converted into high-molecular substances.

The polycondensation reaction, an example of which we are considering, is distinguished by the ability to form stable substances at each stage. The dimers, trimers and polymers obtained by the interaction of amino acids can be isolated at all intermediate stages from the reaction mixture.

So, polycondensation is a stepwise process. For its occurrence, monomer molecules are needed, which contain two functional groups capable of interacting with each other.

The presence of functional groups allows oligomers to react not only with each other, but also with monomers. This interaction characterizes the growth of the polymer chain. If the original monomers have two functional groups, the chain grows in one direction, which leads to the formation of linear molecules.

Polycondensation is a reaction that results in products capable of subsequent reaction.

Classification

Polycondensation reaction, an example of which can be written for many organic matter, gives an idea of ​​the complexity of the ongoing interaction.

Currently, such processes are usually classified according to certain criteria:

  • type of connection between links;
  • the number of monomers taking part in the reaction;
  • process mechanism.

How does the polycondensation reaction differ for different classes of organic substances? For example, in polyamidation, amines and carboxylic acids are used as starting components. During the stepwise interaction between monomers, the formation of polymer and water molecules is observed.

In esterification, the starting materials are alcohol and carboxylic acid, and the condition for receiving ester is the use of concentrated sulfuric acid as a catalyst.

How does polycondensation occur? Examples of interactions indicate that, depending on the number of monomers, homo- and heteropolycondensation can be distinguished. For example, during homopolycondensation, substances having similar functional groups will act as monomers. In this case, condensation is the combination of starting substances with the release of water. An example is a reaction between several amino acids, which will result in the formation of a polypeptide (protein molecule).

Process mechanism

Depending on the characteristics of the process, reversible (equilibrium) and irreversible (nonequilibrium) polycondensation is distinguished. This division can be explained by the presence or absence of destructive reactions, which involve the use of low-molecular processes, different activities of monomers, and also allow differences in kinetic and thermodynamic factors. Such interactions are characterized by low equilibrium constants, low process speed, reaction duration, and high temperatures.

In many cases, irreversible processes are characterized by the use of monomers that are highly reactive.

The high process speeds using this type of monomer explain the choice of low-temperature and interfacial polycondensation in solution. The irreversibility of the process is determined by the low temperature of the reaction mixture, the production of low-active chemical substance. IN organic chemistry There are also variants of nonequilibrium polycondensation that occur in melts at high temperatures. An example of such a process is the process of obtaining polyesters from diols and dihalogen derivatives.

Carothers equation

The depth of polycondensation is associated with the thorough removal from the reaction medium of low-molecular-weight products that prevent the process from shifting towards the formation of a polymer compound.

There is a relationship between the depth of the process and the degree of polymerization, which has been combined into a mathematical formula. During the polycondensation reaction, two functional groups and one monomer molecule disappear. Since a certain number of molecules are consumed during the process, the depth of the reaction is related to the proportion of reacted functional groups.

The greater the interaction, the higher the degree of polymerization. The depth of the process is characterized by the duration of the reaction and the size of macromolecules. What is the difference between polymerization and polycondensation? First of all, the nature of the course, as well as the speed of the process.

Reasons for stopping the process

The growth of the polymer chain stops due to various chemical and physical reasons. As the main factors contributing to stopping the process of synthesis of a polymer compound, we highlight:

  • increasing the viscosity of the medium;
  • reducing the speed of the diffusion process;
  • reducing the concentration of interacting substances;
  • temperature drop.

With an increase in the viscosity of the reaction medium, as well as a decrease in the concentration of functional groups, the probability of molecular collisions decreases, followed by a stop in the growth process.

Among the chemical reasons for the inhibition of polycondensation, the leading ones are:

  • change chemical composition functional groups;
  • disproportionate amount of monomers;
  • the presence of a low molecular weight reaction product in the system;
  • balance between forward and reverse reactions.

Specifics of kinetics

Polymerization and polycondensation reactions are associated with a change in the rate of interaction. Let us analyze the main kinetic processes using the example of the polyesterification process.

Acid catalysis occurs in two stages. First, protonation of the acid, the initial reagent, is observed with an acid acting as a catalyst.

During the attack of the alcohol group by the reagent, the intermediate decomposes to the reaction product. For a direct reaction to occur, it is important to promptly remove water molecules from the reaction mixture. A decrease in the rate of the process is gradually observed, caused by an increase in the relative molecular weight of the polycondensation product.

If equivalent amounts of functional groups are used at the ends of the molecules, the interaction can last for a long period of time until a giant macromolecule is created.

Process options

Polymerization and polycondensation are important processes used in modern chemical production. There are several laboratory and industrial methods carrying out the polycondensation process:

  • in solution;
  • in the melt;
  • in the form of an interfacial process;
  • in emulsion;
  • on matrices.

Melt reactions are necessary to produce polyamides and polyesters. Basically, equilibrium polycondensation in a melt occurs in two stages. First, the interaction is carried out in a vacuum, which avoids thermal-oxidative destruction of monomers, as well as polycondensation products, guarantees gradual heating of the reaction mixture, complete removal of low-molecular-weight products.

Important facts

Most reactions are carried out without the use of a catalyst. Evacuation of the melt at the second stage of the reaction is accompanied by complete purification of the polymer, so there is no need to additionally carry out the labor-intensive process of reprecipitation. A sharp increase in temperature at the first stage of interaction is not allowed, since this can lead to partial evaporation of monomers and a violation of the quantitative ratio of interacting reagents.

Polymerization: features and examples

This process is characterized by the use of one starting monomer. For example, by such a reaction it is possible to obtain polyethylene from the starting alkene.

A feature of polymerization is the formation of large polymer molecules with a given number of repeating structural units.

Conclusion

By polycondensation, it is possible to obtain many polymers that are in demand in various modern industries. For example, phenol-formaldehyde resins can be isolated during this process. The interaction of formaldehyde and phenol is accompanied by the formation of an intermediate compound (phenol alcohol) in the first stage. Then condensation is observed, leading to the production of a high-molecular compound - phenol-formaldehyde resin.

The product obtained by polycondensation has found its application in the creation of many modern materials. Phenoplastics based on this connection, have excellent thermal insulation characteristics, therefore they are in demand in construction.

Polyesters and polyamides obtained by polycondensation are used in medicine, technology, and chemical production.

Polyamides. Let's consider the process of formation of polyamides, representatives of which are numerous varieties of nylon. Some of them are formed by the condensation of diamines with chloro derivatives of dicarboxylic acids. For example, nylon-6,6 is formed by heating hexane-1,6-dioyl dichloride (adipic acid dichloride) with hexane-1,6-diamine:

Each monomer contains two functional groups. The process is accompanied by the release of a low molecular weight compound – HC1. The composition of the elementary unit of a polymer molecule does not correspond to the composition of the molecule of the original monomers. Nylon-6,6 is used either as a fiber or as a plastic (brushes, making gears and parts in mechanisms, etc.).

Polyesters are also polycondensation products. They are used as synthetic fibers. For example, “terylene” (“lavsan”, “dacron”) is formed by heating 1,2-ethanediol (ethylene glycol) with terephthalic acid. Both of these monomers are bifunctional. The first of them is a dihydric alcohol, and the second is dicarboxylic acid:

Phenol-formaldehyde resins is obtained by the polycondensation reaction of phenol C6H5OH and formaldehyde CH2O. Depending on the ratio of components and the conditions of the polycondensation process, novolac or resol resins are formed.

Novolac resins are formed with a slight excess of phenol with a catalyst - hydrochloric acid when heated. First, predominantly o-hydroxybenzyl alcohol is obtained, and then, as a result of its polycondensation, novolac resin is obtained:

Resol resins are obtained with a slight excess of formaldehyde with an alkaline catalyst:

When resol resins are heated to 150–170°C, chain molecules are cross-linked through CH2 bridges and a resite structure appears:

Curing of novolac resins can be carried out by adding a hardener - methenamine (CH2)6N4 and heating.

An example of stepwise polymerization that takes place without isolating low molecular weight compounds is the production of polyurethanes.

Scheme of the reaction for the production of linear polyurethanes:

Urea - urea-formaldehyde and melamine-formaldehyde resins.

Urea is also capable of condensation with formaldehyde, resulting in urea-formaldehyde resins. The reaction proceeds similarly to the formation of phenol-formaldehyde resins. In this case, mono- and dimethylol derivatives are obtained, which then, reacting with urea, form the final resin structure:

The final scheme is as follows:

The hydrogen atoms of the imide group of a linear polymer can be further replaced by methylol groups in the presence of excess formaldehyde:

Structure final product, as with the condensation of phenol-formaldehyde resins, depends on the ratio of urea and formaldehyde in the initial mixture. Thus, when a linear polymer is heated in the presence of excess formaldehyde, a three-dimensional polymer is formed:

Melanin and formaldehyde can also react to form methylol derivatives of melamine:

Condensation of methylol derivatives of melamine with a large amount of melamine results in a linear polymer. This polymer, upon further condensation with excess formaldehyde, forms a three-dimensional network polymer, insoluble in many solvents:

Non-crosslinked urea-formaldehyde and melamine-formaldehyde resins are water-soluble and are used as binders, for example in the production of plywood.

Melamine resins are used in the production of particle boards and fiberboards.

Melamine-formaldehyde resins have higher heat and moisture resistance compared to urea-formaldehyde resins

Epoxy polymers

Epoxy polymers. - These are simple polyesters. One of the epoxy polymers (or epoxy resins) is obtained from ethylchlorohydrin and bisphenol A. The reaction is carried out in an excess of epichlorohydrin

Instead of bisphenol A, glycols, glycerin, resorcinol and their derivatives can also be used.

The resulting epoxy resins are highly viscous liquids or solids with high melting points. Epoxy resins can be further cured by adding amines, polysulfides, and polyamides. Epoxy resins find a very wide and varied application due to their chemical resistance and good adhesion. Epoxy resins are structural adhesives. Once fully cured, epoxy resins are durable materials, which allows them to be used for flooring in industrial buildings as sealing compositions.

Synthetic polymers are obtained as a result of polymerization reactions, poly-condensation and transformations in chains of macromolecules.

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

Polycondensation is the process of connecting molecules of one or more monomers containing two or more functional groups (OH, COOH, COCl, NH 2, etc.) with each other, capable of chemical interaction, in which the elimination of low molecular weight products (H 2 O, HCl, etc.) occurs. Polymers obtained by the polycondensation method do not correspond in elemental composition to the original monomers, therefore the structure of their macromolecules is considered from the point of view of a repeating unit rather than a monomer unit.

Polymerization of monomers with multiple bonds occurs according to the laws of chain reactions as a result of the rupture of unsaturated bonds. During chain polymerization, a macromolecule is formed very quickly and immediately acquires its final size.


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

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



The transformation of functional groups in polymers occurs at a lower rate than in low molecular weight substances. This is due to the impact on reactivity functional groups of polymers, structure of their chains, steric factors, shape of macromolecules (loose or dense coil), phase state of polymers (crystalline or amorphous), diffusion processes. The listed factors determine the accessibility of the functional groups of macromolecules to the chemical reagent.

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

Polymer-analogous transformations are reactions of polymers with low-molecular substances, as a result of which some functional groups are replaced in polymers by others without changing the length of the main chain of macromolecules. For example, polyvinyl alcohol cannot be obtained by polymerization of the monomer - vinyl alcohol, since the latter is unstable and upon production immediately isomerizes into acetaldehyde:


By reaction of polymer-similar transformations, various polyvinyl acetals, cellulose ethers, etc. are produced in industry.

Reactions in polymer chains, accompanied by a change in their molecular weight, occur in three cases: when a monomer interacts with a polymer that serves as its initiator; during the interaction of different polymers or oligomers (interpolymer interaction) due to the reactive functional groups they contain; during the recombination (combination) of two macroradicals that arise during irradiation or mechanical action on a mixture of polymers. In industry, impact-resistant polystyrene and ABS plastics are produced by similar reactions.

The destruction of polymers is accompanied by a decrease in molecular weight due to the rupture of the main chain of the macromolecule. Factors causing destruction are heat, light, oxygen, penetrating radiation, mechanical stress, etc. During destruction, the molecular weight of the polymer decreases and its physical and mechanical properties deteriorate. The resistance of polymers to degradation depends on their chemical structure, shapes of macromolecules, degrees of crystallinity, spatial grid frequencies.

Destruction reactions occur predominantly by a radical (less often ionic) mechanism. There are thermal, thermo-oxidative, photochemical, radiation, mechanical and chemical destruction. Destruction reactions underlie the aging of polymers, during which they deteriorate or lose their beneficial properties.