Condensation is the basis for the creation of polymer synthetic materials: polyvinyl chloride, olefins. Using basic monomers, it is possible by copolycondensation to produce millions of tons of new polymeric substances. Currently, there are various methods that allow not only creating substances, but also affect the molecular weight distribution of polymers.
Process features
The polycondensation reaction is the process of producing a polymer during the stepwise addition of polyfunctional monomers to each other. In this case, the release of low molecular weight products.
The substitution reaction can be considered as the basis of this process . Due to the isolation of by-products, there are differences in the elemental composition of the polymer and the starting 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 turn into high molecular weight substances.
The polycondensation reaction, an example of which we are considering, is characterized by the ability to form stable substances at each stage. Obtained by the interaction of amino acids, dimers, trimers and polymers can be isolated at all intermediate stages from the reaction mixture.
So, polycondensation is a stepwise process. For its course, monomer molecules are needed, which include two functional groups capable of interacting with each other.
The presence of functional groups allows oligomers to react not only among themselves, but also with monomers. A similar interaction characterizes the polymer chain growth. If the initial monomers have two functional groups, the chain grows in one direction, which leads to the formation of linear molecules.
Polycondensation is a reaction that will result in products capable of subsequent interaction.
Classification
The polycondensation reaction, an example of which can be written for many organic substances, 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 involved in the reaction;
- process mechanism.
What is the difference between the polycondensation reaction for different classes of organic substances? For example, in polyamidation, amines and carboxylic acids are used as starting components. During the stepwise interaction between the monomers, the formation of a polymer and water molecules is observed.
In the esterification, the starting materials are alcohol and carboxylic acid, and the condition for obtaining the ester is the use of concentrated sulfuric acid in the form of 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 a combination of starting materials with the release of water. An example is the reaction between several amino acids, as a result of which a polypeptide (protein molecule) will be formed.
Process mechanism
Depending on the flow characteristics, reversible (equilibrium) and irreversible (nonequilibrium) polycondensation are distinguished. Such a division can be explained by the presence or absence of destructive reactions, which involve the use of low molecular weight processes, different activity of monomers, and also allow differences in kinetic and thermodynamic factors. Such interactions are characterized by low equilibrium constants, an insignificant rate of the process, the duration of the reaction, and high temperatures.
In many cases, irreversible processes are characterized by the use of monomers that are highly reactive.
High process rates using this type of monomer explain the choice of low-temperature and interfacial polycondensation in solution. The irreversibility of the process is caused by the low temperature of the reaction mixture, obtaining a low-activity chemical. In organic chemistry, there are also such non-equilibrium polycondensation variants that occur in melts at high temperatures. An example of such a process is the process of preparing polyesters from diols and dihalogen derivatives of dicarboxylic acids .
Carozers equation
The depth of polycondensation is associated with the thorough removal of low molecular weight products from the reaction medium, which interfere with the shift of the process towards the formation of a polymer compound.
There is a relationship between the depth of the process and the degree of polymerization, which was combined into a mathematical formula. In 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, the size of the 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 terminating the process
The arrest of polymer chain growth is caused by various chemical and physical causes. As the main factors contributing to the halt of the synthesis of the polymer compound, we highlight:
- increasing the viscosity of the medium;
- a decrease in the speed of the diffusion process;
- decrease in the concentration of interacting substances;
- lowering the temperature.
With an increase in the viscosity of the reaction medium, as well as a decrease in the concentration of functional groups, there is a decrease in the probability of collision of molecules with the subsequent stop of the growth process.
Among the chemical causes of inhibition of polycondensation are in the lead:
- change in the chemical composition of functional groups;
- disproportionate amount of monomers;
- the presence in the system of a low molecular weight reaction product;
- balance between direct and reverse reactions.
Kinetics specificity
Polymerization and polycondensation reactions are associated with a change in the rate of interaction. Let us analyze the basic kinetic processes using the example of the polyesterification process.
Acid catalysis proceeds in two stages. First, protonation of the acid, the initial reactant, with an acid acting as a catalyst, is observed.
During the attack with the reagent of the alcohol group, the intermediate decomposes to the reaction product. For a direct reaction to occur, it is important to remove water molecules from the reaction mixture in a timely manner. Gradually, a decrease in the process rate caused by an increase in the relative molecular weight of the polycondensation product is observed.
In the case of the use of equivalent amounts of functional groups at the ends of the molecules, the interaction can take a long 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 for carrying out the polycondensation process:
- in solution;
- in the melt;
- in the form of an interphase process;
- in emulsion;
- on matrices.
Reactions in melts are necessary to obtain polyamides and polyesters. Basically, in the melt, equilibrium polycondensation proceeds in two stages. At first, the interaction is carried out in a vacuum, which avoids the thermo-oxidative degradation of monomers, as well as the products of polycondensation, guarantees gradual heating of the reaction mixture, and the complete removal of low molecular weight products.
Important facts
Most reactions are carried out without the use of a catalyst. Evacuation of the melt in the second stage of the reaction is accompanied by complete purification of the polymer; therefore, there is no need to additionally carry out a laborious reprecipitation process. A sharp increase in temperature at the first stage of interaction is not allowed, since this can lead to partial evaporation of the monomers, a violation of the quantitative ratio of the interacting reagents.
Polymerization: features and examples
This process is characterized by the use of a single starting monomer. For example, by such a reaction, polyethylene can be obtained 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, you can get many polymers 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 at the first stage of an intermediate compound (phenol alcohol). Then condensation is observed, leading to the production of a high molecular weight compound - phenol-formaldehyde resin.
The product obtained by polycondensation has found its application in the creation of many modern materials. Phenoplasts, which are based on this compound, have excellent thermal insulation characteristics, and therefore are in demand in construction.
Polyesters, polyamides obtained by polycondensation are used in medicine, technology, and chemical production.