Thermal radiation initiation can proceed by several methods. Radical polymerization

Monomers for radical polymerization

As monomers for radical polymerization, substituted alkenes CH 2 \u003d CH-X and CH 2 \u003d CX 2 (X \u003d H, Hal, COOH, COOR, OCOR, CN, CONH 2, C 6 H 5, C 6 H 4 Y), CH 2 =CXY (X=Alk, Y=COOH, COOR, CN); CH 2 -CX=CH-CH 2 and CH 2 -CH=CH-CHX (X=H, Alk, Hal, polar group).

Kinetic scheme

The radical polymerization process includes 4 stages:

Initiation

At this stage, primary radicals of the monomer are formed. For chain initiation, both physical (photolysis, radiolysis, thermolysis) or chemical (decomposition of radical initiators - peroxides, hydroperoxides, azo compounds) methods are used. At the first stage, initiator radicals are generated, which are attached to the monomer molecule, forming the primary monomeric radical:

The rate of the initiation stage is given by the equation

Where f is the coefficient of efficiency of initiation, the ratio of the number of radicals formed in reaction 1a to the number of radicals that entered into reaction 1b. f = 0.6-0.8. [I] - molar concentration of initiator

Thermal initiation

Rarely used. It is observed during the polymerization of butadiene at elevated temperatures, as well as styrene and methyl methacrylate, which form relatively stabilized radicals.

The rate of thermal polymerization is directly proportional to the square of the monomer concentration. It also depends on temperature. The formation of radicals occurs due to bimolecular initiation:

Photoinitiation

The essence of the process of photoinitiation of polymerization without the use of initiators or photosensitizers is to irradiate the reaction mass with ultraviolet radiation with a certain wavelength, depending on which double bond is to be broken. Thus, by irradiating the allyl ester of methacrylic acid, it is possible to selectively break the methacrylate double bond without affecting the allyl double bond.

In this case, two processes are possible:

• Excitation of a monomer molecule upon absorption of a light quantum, collision with another monomer molecule and, with a certain probability, the formation of a biradical followed by disproportionation into radicals:

• The decay of an excited monomer molecule into free radicals:

Not all absorbed light quanta cause photoinitiation. The degree of initiation is determined by the quantum yield of photoinitiation β, which is derived from the relation

where v i is the rate of initiation, I a is the intensity of the absorbed light. The photoinitiation yield β depends on the radiation wavelength and on the type of monomer used.

In addition to direct photoinitiation, photosensitizers are used, the molecules of which absorb radiation, go into an excited state and transfer excitation to a photoinitiator or monomer molecule:

Radiation initiation

Polymerization is initiated by irradiation with ionizing radiation (α-, β-, γ-rays, accelerated electrons, protons, etc.)

chain growth

The stage of chain growth consists in the sequential addition of monomer molecules to the growing macroradical:

The chain propagation reaction rate is expressed by the formula

It is assumed that the constant k 2 does not depend on the length of the macroradical (this is true for n>3-5). The value of k 2 depends on the reactivity of the monomer and macroradical.

chain break

Chain termination in radical polymerization consists in the bimolecular interaction of two macroradicals. In this case, two reactions can occur - disproportionation or recombination. In the first case, one macroradical splits off a hydrogen atom from another, in the second case, both radicals form one molecule:

The chain termination reaction rate is given by the equation

Due to the quasi-stationarity of the polymerization process, the reaction proceeds to a depth of 10% or more at a practically constant rate , while the concentration of macroradicals is determined by the formula

chain transfer

The chain transfer step consists in the transfer of the active center of the macroradical to another molecule present in solution (monomer, polymer, initiator, solvent). In this case, the macromolecule loses the possibility of further growth:

If the formed new radical is able to continue the kinetic chain, then the polymerization reaction continues further at the same rate. If the new radical is inactive, then either the rate of polymerization slows down or the process stops. This is used to inhibit radical polymerization.

In general, the chain transfer reaction results in the formation of a polymer with a low degree of polymerization. Chain transfer to macromolecules leads to the formation of branched, cross-linked and grafted polymers.

Chain growth and chain transfer reactions compete with each other. The quantitative characteristic of their ratio is determined by the equation

Initiators

The following radical polymerization initiators are most common:

• Benzoyl peroxide
• Dicyclohexylperoxydicarbonate
• Di-tert-butyl peroxide
• Potassium persulfate
• Cumyl hydroperoxide

Inhibitors

Application

Literature

• V. A. Kabanov (editor-in-chief), Encyclopedia of polymers, vol. 3, Soviet Encyclopedia, 1977, article " Radical polymerization", S. 260-271
• Zefirov N.S. and etc. v.4 Half-Three // Chemical Encyclopedia. - M .: Great Russian Encyclopedia, 1995. - 639 p. - 20,000 copies. - ISBN 5-85270-092-4

Links

• Article " Radical polymerization» on Macrogallery, an educational popular science site developed by the University of Southern Mississippi (English) Russian

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IN radical polymerization the functions of active intermediates (active centers) are performed by free radicals. Monomers with a multiple C=C bond and monomers with a polarized multiple C=heteroatom bond enter into radical polymerization. Cyclic monomers do not undergo radical polymerization.

Among the common monomers entering into radical polymerization are ethylene, vinyl chloride, vinyl acetate, vinylidene chloride, tetrafluoroethylene, acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, styrene, butadiene, chloroprene, etc. The listed monomers form high-molecular products, while vinyl esters and allyl monomers form oligomers. Vinylene monomers (CHX=SPX, with the exception of X=F) do not enter into radical polymerization due to steric hindrance.

Polymerization initiation is the transformation of a small fraction of monomer molecules into active centers (radicals) under the action of specially introduced substances (initiators) or high-energy radiation (radiation polymerization), or light (photopolymerization), etc.

The most common methods of initiation are thermal homolytic decomposition of initiators, initiation by redox systems, photochemical initiation, radiation initiation.

Thermal homolytic decomposition of initiators carried out through initiators, which include various types of peroxides: alkyl peroxides (peroxide tert- butyl), hydroperoxides (cumene hydroperoxide), peresters (from/d?ti-6utilperbenzoate), acyl peroxides (benzoyl peroxide)

and azo compounds, among which 2,2 "-azo-bms-isobutyronitrile (DAK or AIBN) is most widely used

These initiators usually do not differ in their selective action with respect to different monomers, so the choice of the initiator is most often determined by the temperature at which the desired rate of free radical generation can be achieved in each particular case. So, DAK is used at 50-70°C, benzoyl peroxide - at 80-95°C, and peroxide tert- butyl - at 120-140°C. The activation energy of initiation is usually close to the bond energy that breaks during the decay of initiators and ranges from 105-175 kJ/mol.

Polymerization at high temperatures can also be induced without the introduction of special initiators into the system. In this case, the formation of radicals occurs, as a rule, due to the decomposition of small amounts of peroxide impurities, which are often formed during the interaction of the monomer with atmospheric oxygen, or other random impurities. The possibility of thermal self-initiation has been proven only for a limited range of monomers (styrene and some of its derivatives, methyl methacrylate, and a number of others).

Initiation by redox systems has the advantage of being able to polymerize in an aqueous or organic medium at room temperature.

Here are typical redox initiating systems:

(also, instead of iron salts, salts of Cr 2+, V 2+, Ti 3+, Co 2 "are used)

The disadvantage of redox initiation is the low efficiency of initiation.

Photochemical initiation proceeds under the influence of UV light. In this case, the radical can arise both in a system containing a pure monomer, and during the photolytic dissociation of the initiator, or in a system containing a photosensitizer, for example, benzophenone. The rate of photoinitiation is proportional to the amount of light absorbed. The convenience of this initiation method lies in the fact that the polymerization process can be carried out at room temperature.

Radiation-chemical initiation(under the action of high-energy radiation) causes radical polymerization at temperatures above 0 ° C, and at lower temperatures ionic polymerization occurs more often. The advantages of this process include the ease of adjusting the dose rate and polymerization time and the high purity of the resulting polymer.

The activation energy of photochemical and radiation-chemical initiation is close to that of a bullet. A feature of the last two methods of initiation is the possibility of instant switching on and off of the irradiating radiation.

Initiation includes two elementary acts:

a) generation of radicals R" from ininiatope I:

b) interaction of the radical R* with the monomer M:

Here & and and k"H- kinetic constants of initiation reactions.

Of these two stages, in most cases the stage of hemolytic decomposition of the initiator is the limiting one; reaction (a).

Part of the radicals R" can be spent on side reactions; to take this into account, the parameter "initiation efficiency" / is introduced, equal to the ratio of the number of radicals involved in the reaction (b) to the number of radicals formed in the reaction (a).

The increase in value is carried out by the sequential addition of monomer molecules to the radicals resulting from initiation, for example:

where kp - chain growth rate constant.

The development of the kinetic chain is accompanied by the formation of the material chain of the macroradical. The value of the activation energy of chain growth reactions lies in the range of 10-40 kJ/mol.

Rate constants and activation energy E a chain propagation reactions primarily depend on the nature of the monomer. Solvents that are not prone to specific interactions with monomer molecules and growing radicals do not affect the chain propagation reaction of radical polymerization.

The activation energy of monomer addition to the heme radical is lower; the more active the monomer, the higher the conjugation energy in the radical, which is obtained as a result of the addition of this monomer to the original radical. Thus, the reactivity in the series of monomers and their corresponding radicals change antibatically.

Reactivity vinyl monomers with substituents decreases in the series:

where R is alkyl.

The reactivity of the corresponding radicals decreases from right to left.

Active monomers include monomers in which the double bond is conjugated with the unsaturated group of the substituent, i.e. with high coupling energy. Inactive monomers have no conjugation or its (conjugation) energy is low. The higher the reactivity of the monomer, the higher the activation energy of the chain propagation reaction, those. the lower the rate of its radical polymerization.

Chain termination leads to the limitation of the kinetic and material chain, i.e. to the death of the active center (disappearance of the active radical or its replacement by a low-active radical, unable to attach monomer molecules). Chain termination during radical polymerization mainly occurs when two growing radicals interact as a result of their recombination-.

where A and to OL are the kinetic termination constants for the mechanism of recombination and disproportionation, respectively.

The chain termination reaction proceeds in three stages:

• 1) translational diffusion of macroradicals with the formation of a united coil;
• 2) mutual approach of the active terminal links due to segmental diffusion inside the united coil;
• 3) direct chemical interaction of reaction centers with the formation of "dead" macromolecules.

The activation energy of the termination reaction does not exceed 6 kJ/mol and is mainly determined by the activation energy of mutual diffusion of radicals.

The chain termination reaction involves macroradicals of different lengths, therefore, during polymerization, macromolecules of different lengths (degrees of polymerization) are formed. The final polymerization product is a polymer with a broad molecular weight distribution.

Another variant of chain termination is termination at inhibitor molecules. Inhibitors can be weakly active stable free radicals (for example, diphenylicrylhydrazyl, N-oxide radicals), which themselves do not initiate polymerization, but are able to recombine or disproportionate with growing radicals. Inhibitors can also be substances whose molecules, when interacting with active radicals, themselves turn into inactive radicals: these are quinones (benzoquinone, duroquinone), aromatic di- and trinitro compounds (for example, dinitrobenzene, trinitrobenzene), molecular oxygen, sulfur, etc. Inhibitors compounds of metals of variable valency also serve

(salts of ferric iron, bivalent copper, etc.), which break the growing chains due to redox reactions. Often an inhibitor is introduced into the monomer to prevent its premature polymerization. Therefore, before polymerization, each monomer must be thoroughly purified from impurities and the added inhibitor.

In extremely rare cases, chain termination can occur unimolecularly on the walls of the vessel.

The value transfer also leads to the limitation of material chains during polymerization, but in this case the active center does not die, but passes to another molecule. Chain transfer reactions are very characteristic of radical polymerization. The essence of these reactions is the detachment by a growing radical of an atom or a group of atoms from some molecule (chain transfer agent).

As a value transfer agent, a compound with a mobile atom or group of atoms, specially added to the reaction system, as well as a monomer, polymer or solvent can act:

Here I m is the kinetic constant of the chain transfer reaction on the monomer; k u-kinetic constant of the chain transfer reaction on the polymer; k s - kinetic constant of the chain transfer reaction to the solvent.

Special attention should be paid to the features of polymerization amyl monomers. In this case, the chain transfer reaction to the monomer with the abstraction of the mobile atom II in the double bond position leads to the formation of a resonance-stabilized, inactive allyl radical, which is unable to initiate further polymerization:

Allyl radicals recombine to form dimers. In this case, unlike the usual transmission, not only material, but also kinetic values ​​are cut off. This type of transmission is called degradation chain transfer. The degradation transfer, competing with the growth reaction, leads to extremely low rates of polymerization of allyl monomers and the formation of products with low molecular weights - oligomers.

The tendency of monomer molecules to participate in the chain transfer reaction is usually characterized by self-transfer constant C m equal to the ratio of the rate constant of the chain transfer reaction to the monomer k M to the rate constant of the chain propagation reaction kp:

For most vinyl monomers that do not contain mobile groups or atoms, k M k . Meaning S s usually lies within 10 "4 -10 for allyl monomers C m\u003e 100 (Table 5.6).

Table 5.6

Transfer constants for radical polymerization

The ability of solvents to participate in chain transfer during the radical polymerization of a particular monomer is characterized by transfer constant-.

Chain transfer reactions are widely used in the synthesis of polymers to control their molecular weights. To reduce the molecular weight of the synthesized polymer, transmitters with the values Cs> 10 3 , which are called regulators.

Kinetics of radical polymerization. Initiation rate when using thermally decomposing initiators, can be expressed by the equation

where / is the efficiency of the initiator, which is usually from 0.5 to 1.0; ^p decay - the rate constant of the decay of the initiator; |1| - concentration of the initiator.

Chain growth rate V n expressed by the equation

where k-- the rate constant of monomer addition to the radical of the degree of polymerization r; | R* | - concentration of radicals of degree of polymerization r; [M] is the concentration of monomer molecules.

However, when macromolecules of large molecular weight are formed (the degree of polymerization is greater than 5–10), we can assume that kif) does not depend on the degree of polymerization of the radical. Then the expression for Vp simplified:

where | R* | is the concentration of all growing radicals.

Taking into account the assumption that the reactivity of propagating radicals does not depend on the degree of their polymerization, the rate of disappearance of radicals as a result of the termination reaction is described by the equation

where kn- break rate constant.

The overall rate of polymerization, equal to the rate of disappearance of the monomer in the system, provided that the degree of polymerization of the resulting macromolecules is quite high and the monomer is consumed only for polymerization, is identical to the chain growth rate, i.e.

If there is no inhibitor in the system, active radicals disappear as a result of their recombination or disproportionation. In this case, the change in the concentration of radicals is described by the equation

The concentration of radicals, which is difficult to measure by direct experiments, can be excluded from equation (5.10), assuming that the rate of formation of radicals is equal to the rate of their disappearance (quasi-stationarity condition), those. dR"]/dt= 0. In radical polymerization, this condition is usually satisfied within a few seconds after the start of the reaction. That's why

As a result, we get the equation

Thus, the assumptions necessary and sufficient to derive equation (5.11) for the rate of radical polymerization can be formulated as follows:

• 1) the degree of polymerization must be much greater than unity;
• 2) constants of elementary steps do not depend on the degree of polymerization of propagating radicals (Flory principle);
• 3) if the lifetime of active particles is short compared to the polymerization time, then use the principle of quasi-stationarity, according to which the change in the concentration of macroradicals over time is zero, i.e. the initiation rate is equal to the chain termination rate;
• 4) the process is considered on initial monomer conversions.

Thus, the order of the reaction rate with respect to monomer concentration

is one, according to the concentration of the initiator - 0.5. In order to estimate the effect of temperature on the rate of polymerization, let us consider the total activation energy of this process. Effective polymerization rate constant

Then the effective (total) activation energy of the process

Activation energy of the growth reaction E= HR40 kJ/mol, activation energy of termination reaction? 0 = (R6 kJ/mol, activation energy of the initiation reaction E Ying= 105-Н75 kJ/mol for the thermal decomposition of the initiator and E nn= 0 for photo or radiation initiation. Thus, in any case the total activation energy of the radical polymerization reaction is positive, and the rate of the process increases with increasing temperature.

degree of polymerization. From the kinetic data, the kinetic chain length (v) and the average degree of polymerization can be calculated (R p) the resulting polymer. Let's define these concepts.

The kinetic chain is the number of monomer molecules per one formed R* radical before it dies when the chain is terminated.

Thus, the expression for the kinetic chain has the form

Under the condition of quasi-stationarity, using equation (5.11), one can obtain the expression

Material chain (number average degree of polymerization) - the number of elementary acts of addition of monomers per act of death of the radical R 'during chain termination and transfer.

When breaking by disproportionation (& od), one macromolecule is formed from one kinetic chain, and the length of the material chain is equal to the length of the kinetic chain: R p= v.

Upon termination by recombination (&), one macromolecule is formed from two kinetic chains, and P n = 2v. With a mixed break (& op + to ol) the length of the material chain also does not coincide with the length of the kinetic chain:

Let us derive an equation for the degree of polymerization from the kinetic data. If polymerization proceeds under conditions of quasi-stationarity in the absence of an inhibitor, then at a sufficiently small depth of transformation, when there is still little polymer in the system and, therefore, the rate of chain transfer to the polymer and the consumption of the monomer can be neglected:

where Va is the rate of bimolecular chain termination; is the sum of the value transfer rates for the monomer M and the solvent S;

When two radicals recombine, one material chain is formed, i.e. there is an average doubling R p, therefore, in the denominator of equation (5.13) in front of the term corresponding to termination by recombination, it is necessary to take into account the factor 0.5. If we denote the fraction of polymer radicals terminating by the disproportionation mechanism, x, then the fraction of radicals dying during recombination is equal to (1 - x) and the equation for R p will take the form

Then for the reciprocal R p, we get

Expressing the radical concentration in terms of the polymerization rate and using the quantities Cm And C s , finally we get

The resulting equation relates the number average degree of polymerization to the reaction rate, transfer constants, and monomer and transfer agent concentrations. Equation (5.15) implies that the degree of polymerization is directly proportional to the concentration of the monomer, inversely proportional to the concentration of the initiator in the degree 1/2, a maximum degree of polymerization of the resulting polymer in the absence of other transfer agents determined by the chain transfer reaction on the monomer(Cm).

k "/2 k 1/2. As for the degree of polymerization, i.e., the molecular weight, it is inversely proportional to the square root of the initiator concentration n = K`[M]/1/2. The physical meaning of this provision is that that with an increase in the concentration of the initiator, the number of radicals formed in the system also increases. These radicals react with a large number of monomer molecules and thereby increase the rate of their transformation into growing macroradicals. However, with a general increase in the concentration of radicals, the probability of their collision with each other also increases, i.e. polymerization chain termination This leads to a decrease in the average molecular weight of the polymer.

Similarly, one can consider the effect of temperature on the kinetics of radical polymerization. Typically, the rate of polymerization increases by a factor of 2-3 with a 10° increase in temperature. An increase in temperature increases the rate of polymerization initiation, since it facilitates the decomposition of initiators into radicals and their reaction with monomer molecules. Due to the greater mobility of small radicals, with increasing temperature, the probability of their collision with each other (chain termination by disproportionation or recombination) or with low-molecular impurities (inhibitors) increases. In all cases, the molecular weight of the polymer decreases, i.e., the average degree of polymerization decreases with increasing temperature. Thus, the number of low molecular weight polymer fractions increases in the overall balance of distribution of macromolecules by their molecular weights, the proportion of side reactions leading to the formation of branched molecules increases, chemical irregularity in the construction of the polymer chain appears due to an increase in the proportion of monomer connection types "head to head" and "tail to tail". ".

Radical polymerization always proceeds by a chain mechanism. The functions of active intermediates in radical polymerization are performed by free radicals. Common monomers that undergo radical polymerization include vinyl monomers: ethylene, vinyl chloride, vinyl acetate, vinylidene chloride, tetrafluoroethylene, acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, styrene, and diene monomers (butadiene, isoprene, chloroprenidr.).

Radical polymerization is characterized by all the signs of chain reactions known in the chemistry of low molecular weight compounds (for example, the interaction of chlorine and hydrogen in the light). Such signs are: a sharp effect of a small amount of impurities on the rate of the process, the presence of an induction period and the flow of the process through a sequence of three stages dependent on each other - the formation of an active center (free radical), chain growth and chain termination. The fundamental difference between polymerization and simple chain reactions is that at the growth stage, the kinetic chain is embodied in the material chain of a growing macroradical, and this chain grows to form a polymer macromolecule.

The initiation of radical polymerization is reduced to the creation of free radicals in the reaction medium capable of starting reaction chains. The initiation stage includes two reactions: the formation of primary free radicals of the initiator R* (1a) and the interaction of the free radical with the monomer molecule (16) to form the radical M*:

Reaction (1b) proceeds many times faster than reaction (1a). Therefore, the rate of polymerization initiation is determined by reaction (1a), as a result of which free radicals R* are generated. Free radicals, which are particles with an unpaired electron, can be formed from molecules under the influence of physical influence - heat, light, penetrating radiation, when they accumulate energy sufficient to break the π-bond. Depending on the type of physical effect on the monomer during initiation (formation of the primary radical M*), radical polymerization is divided into thermal, radiation, and photopolymerization. In addition, initiation can be carried out due to the decomposition into radicals of substances specially introduced into the system - initiators. This method is called real initiation.

Thermal initiation consists in self-initiation at high polymerization temperatures of pure monomers without the introduction of special initiators into the reaction medium. In this case, the formation of a radical occurs, as a rule, due to the decomposition of small amounts of peroxide impurities, which can arise during the interaction of the monomer with atmospheric oxygen. In practice, the so-called block polystyrene is obtained in this way. However, the method of thermal initiation of polymerization has not found wide distribution, since it requires large expenditures of thermal energy, and the polymerization rate in most cases is low. It can be increased by increasing the temperature, but this reduces the molecular weight of the resulting polymer.

Photoinitiation of polymerization occurs when the monomer is illuminated by the light of a mercury lamp, in which the monomer molecule absorbs a quantum of light and passes into an excited energy state. Colliding with another monomer molecule, it is deactivated, transferring the last part of its energy, while both molecules turn into free radicals. The rate of photopolymerization increases with increasing irradiation intensity and, in contrast to thermal polymerization, does not depend on temperature.

Radiation initiation of polymerization is similar in principle to photochemical initiation. Radiation initiation consists in exposing monomers to high-energy radiation (γ-rays, fast electrons, α - particles, neutrons, etc.). The advantage of photo- and radiation-chemical methods of initiation is the possibility of instant "turning on and off" radiation, as well as polymerization at low temperatures.

However, all these methods are technologically complex and may be accompanied by side undesirable reactions in the obtained polymers, such as degradation. Therefore, in practice, chemical (material) initiation of polymerization is most often used.

Chemical initiation is carried out by introducing low-molecular unstable substances into the monomer medium, which have low-energy bonds in their composition - initiators that easily decompose into free radicals under the influence of heat or light. The most common radical polymerization initiators are peroxides and hydroperoxides (hydrogen peroxide, benzoyl peroxide, hydroperoxides mpem-butyl and isopropylbenzene, etc.), azo- and diazo compounds (azobisisobutyric acid dinitrile, diazoaminobenzene, etc.), potassium and ammonium persulfates. Below are the decomposition reactions of some initiators.

Tert-butyl peroxide (alkyl peroxide):

The activity and applicability of radical polymerization initiators is determined by the rate of their decomposition, which depends on temperature. The choice of a particular initiator is determined by the temperature required for polymer synthesis. Thus, azobisisobutyric acid dinitrile is used at 50-70°C, benzoyl peroxide - at 80-95°C, and tert-butyl peroxide - at 120-140°C.

Redox systems are effective initiators that make it possible to carry out the process of radical polymerization at room and low temperatures. Peroxides, hydroperoxides, persulfates, etc. are usually used as oxidizing agents. Reducing agents are salts of metals of variable valence (Fe, Co, Cu) in the lowest oxidation state, sulfites, amines, etc.

The oxidation-reduction reaction takes place in a medium containing the monomer, with the formation of polymerization-initiating free radicals. You can choose a pair of oxidizing agent, soluble in water (for example, hydrogen peroxide-iron (II) sulfate) or in organic solvents (for example, benzoyl peroxide - dimethylaniline). Accordingly, radical polymerization can be initiated in both aqueous and organic media. For example, the decomposition of hydrogen peroxide in the presence of iron (II) salts can be represented by the following equations:

The radicals HO* and HOO*, joining the monomer molecule, initiate radical polymerization.

Chain growth is carried out by successive addition of monomer molecules to radicals (2) that have arisen in reaction (1b), for example:

In the chain process of radical polymerization, the growth of the kinetic chain occurs almost instantaneously with the formation of the material chain of the macroradical and ends with its termination.

Chain termination is the process of stopping the growth of kinetic and material chains. It leads to the disappearance of active radicals in the system or to their replacement by low-active radicals that are not capable of attaching monomer molecules. At the termination stage, a polymer macromolecule is formed. Circuit breakage can occur by two mechanisms:

1) two growing macroradicals, colliding, combine with each other into a single chain, that is, they recombine (Za);

2) colliding macroradicals turn into two macromolecules, one of which, donating a proton, turns into a macromolecule with a double C=C bond at the end, and the other, accepting a proton, forms a macromolecule with a simple terminal C-C bond; such a mechanism is called disproportionation (3b):

When chains are terminated by recombination, initiator residues are located at both ends of the macromolecule; when the chains are broken by disproportionation - at one end.

As the chains of macroradicals grow, the viscosity of the system increases and their mobility decreases, as a result of which chain termination becomes more difficult and the overall rate of polymerization increases. This phenomenon is known as the gel effect. The gel effect causes an increased polydispersity of polymers, which usually leads to a deterioration in their mechanical properties. Limitation of material chains during radical polymerization can also occur by attaching a macroradical to the primary radical (termination at the initiator) and as a result of chain transfer reactions.

Chain transfer consists in the detachment by a growing macroradical of a mobile atom from a molecule of any substance - a solvent, monomer, polymer, impurities. These substances are called chain transmitters. As a result, the macroradical is converted into a valence-saturated macromolecule and a new radical is formed that is capable of continuing the kinetic chain. Thus, during transfer reactions, the material chain breaks, but the kinetic chain does not.

The chain transfer reaction to a solvent (e.g. carbon tetrachloride) can be represented as follows:

The free radicals formed in this case from the solvent molecules can attach monomer molecules, that is, continue the kinetic chain:

If their activity differs from the activity of primary radicals, then the rate of polymerization also changes.

When the chain is transferred to the polymer, branched macromolecules are formed:

The probability of chain transfer to the polymer increases at high monomer conversion, when the concentration of macromolecules in the system is high.

The role of the chain transfer agent in some cases can be played by the monomer itself, if its molecules contain a mobile hydrogen atom. In this case, the growing radical does not attach a new monomer molecule to itself via a double bond, but removes a mobile hydrogen atom from it, saturating its free valence and simultaneously converting the monomer molecule into a monomeric radical. This takes place during the polymerization of vinyl acetate:

The reactions of chain transfer to a solvent underlie the production of telomers. If the polymerization of any monomer is carried out at high concentrations of a solvent whose molecules contain mobile hydrogen or halogen atoms, then the reaction product will be substances with a low molecular weight, consisting of several monomer units containing fragments of solvent molecules at the ends. These substances are called telomeres, and the reaction of their production is called telomerization.

Chain transfer reactions can be used to control the molecular weight of polymers and even prevent their formation. This is widely used in practice, often using chain regulators during polymerization, and inhibitors during storage of monomers.

Chain regulators are substances that, while terminating the growing polymer chains, practically do not affect the overall speed of the process. Typical chain regulators are mercaptans containing a mobile hydrogen atom in the mercapto group. The chain transfer to them can be represented as follows:

The polymers synthesized in the presence of chain regulators are distinguished by the average molecular weight and MWD that are optimal for processing.

Inhibitors are substances that terminate the growing chains of the polymer, thus turning into compounds that are not able to initiate polymerization. As inhibitors, substances are usually used, the transfer of the chain to which leads to the formation of inactive (stable) radicals. In practice, hydroquinone, benzoquinone, aromatic amines, and nitrobenzene are often used to inhibit radical polymerization and store monomers.

The formation of free radicals is possible under the action of chemical and physical factors, therefore, the initiation of radical polymerization is divided into physical and chemical (see diagram)

In polymer production technology, chemical methods of initiation are predominantly common, when initiators (J) - substances that, under certain conditions, easily decompose into radicals.

The group of initiators includes the following substances (table 10)

Table 10 - Types of radical polymerization initiators

Type of initiator	Formula (general)	The mechanism of decomposition into radicals
Dihydroperoxides
Alkyl hydroperoxides
Dialkyl peroxides
Diacyl peroxides
disulfides
Persulfates
Azo compounds

Specific representatives of initiators, conditions and mechanism of their decay are given in Table 11.

Easy and fast decomposition of initiators into radicals occurs via oxygen-oxygen or carbon-nitrogen bonds, since these bonds have the lowest strength (bond energy).

The lowest decomposition temperatures (50÷85 0 C) and, accordingly, the lowest activation energy (E a ​​u) for such initiators as ammonium persulfate, azobisisobutyric acid dinitrile, benzoyl peroxide. These initiators are most commonly used in radical polymerization.

Redox systems (OR systems) are complex systems that include an oxidizing agent and a reducing agent (promoter). The role of the oxidizing agent is most often played by the above initiators J (peroxides, hydroperoxides, etc.). A promoter is a substance that accelerates the breakdown of initiators into radicals. . The promoters in OB systems are salts of metals of variable valency in the lowest oxidation state, such as ferric chloride of ferrous FeCl 2 , copper chloride of monovalent CuCl, cobalt naphthenates, nickel naphthenates ( ) and others. In addition to them, the role of promoters is played by amines, sulfites, etc. In the paint and varnish industry, promoters - polymerization accelerators are called driers .

The mechanism of action of redox systems is different. The simplest OB system is Fenton's reagent - a mixture of hydrogen peroxide H-O-O-H and ferric chloride FeCl 2 . In this system, the unstable Fe 2+ cation easily loses an electron ` e and goes to the highest oxidation state. Fe3+. The released electron accelerates the decomposition of hydrogen peroxide H-O-----O-H into the HO ion - and radical HO · .

The decomposition of initiators into radicals in the presence of OB systems is very fast, and the activation energy of the initiation stage when using OB systems is lower than when using only chemical initiators (E a ​​and ( OV) < Е а и (J ) ) . Usually, the activation energy of redox initiation E a and ( OV) is 42÷84 kJ/mol, and the activation energy of initiation using only initiators E a and ( J ) is equal to 112÷170 kJ/mol.

RP initiation, i.e. free radical formation goes in two stages, which can be represented schematically as follows:

but) radical formationJ · as a result of the decay of initiator moleculesJ:

b) formation of active growth radicalsJ-M · from monomer moleculesM:

Using the vinyl chloride monomer as an example, this reaction looks like this:

The initial stage (a) always requires the expenditure of activation energy E a u, proceeds at a low rate and limits the entire process. Not all radicals initiator J · may call the second stage b) and generate growth radicalsJ-M · .

Part of the initiator radicals disappears as a result of the reverse recombination reaction J · + J · =J2.

Initiation rate constant k and with RP small and is k and =0.8÷ 5.0× 10 -5 with -1 .

Photochemical initiation less commonly used than chemicals. During photochemical initiation, monomer molecules M absorb photon energy ( hn ) radiation with a wavelength of 100 nm

The activation energy of photochemical initiation E a and ( f/h) is much lower than the activation energy of purely chemical initiation E a and (E a ​​and ( f/h) <<Е а и (J)) and is close to 0 (E a ​​and ( f/h) » 0 kJ/mol). As a result, photochemical radical polymerization can proceed at low and even negative temperatures.

However, the rate of decomposition of monomers into radicals under the action of UV rays or visible radiation (VI) is low. To accelerate photochemical polymerization, 2 methods are used:

1. Substances are introduced into the monomer - photoinitiators

2. Substances are introduced into the monomer - photosensitizers.

Photoinitiators are substances that, under the influence of the energy of UV or VI quanta, are more easily decomposed into radicals than the monomers themselves. Photoinitiators are haloalkyls (carbon tetrachloride СCl 4, 1,2-trichloroethaneC 2 Cl 6), organometallic compounds. The mechanism of action of CCl 4 is as follows:

Photosensitizers are substances that absorb and accumulate the energy of UV or VI quanta in a wider wavelength range than the monomer itself, then transfer the accumulated energy to the monomer in larger portions. Monomer molecules quickly pass into an excited state and decompose into radicals. The role of photosensitizers is played by substances containing a conjugated double bond in their structure ( chromophore groups) or aromatic rings, such as dibenzphenone or fluorescein

dibenzphenone fluorescein

The mechanism of action of photosensitizers (F) is as follows:

F + hn F* F* + M F + M* M* M + M

Table 11 - The main groups of radical polymerization initiators, the mechanism and conditions for their decomposition

Organic	Decay conditions	water soluble	Decay conditions
1. Benzoyl peroxide (PB) 2. Ditretbutyl peroxide 3.Isopropylbenzene (cumene) hydroperoxide - HYPERIS 4. Dinitrilazobisisobutyric acid - DINIZ 5. Diazoaminobenzene	Tr=85 0 C Ea =113 kJ/mol Tr=130 0 C Ea=150 kJ/mol Tr=160 0 C Ea=130 kJ/mol Tr= 60 0 C Ea=112 kJ/mol Tr= 60 0 C	Ammonium persulfate (K, Na) Hydrogen peroxide Redox systems	Тр= 50-70 0 С Тр= 50 0 С Тр can be less than 0 0 С

Thermal initiation - this is a variant of the formation of growth radicals from monomer molecules, which manifests itself when heated to a temperature of T = 100 0 C and above. However, this type of initiation has been studied only for the polymerization of methyl methacrylate and styrene. The activation energy of thermal initiation E a and ( T) = 146 kJ/mol. Thermal initiation proceeds specifically, through the stage of formation of the monomer biradical.