The mechanism of this synthesis method was established back in the 1930s in the works of S.S. Medvedev and G. Staudinger. Polymerization is initiated by free radicals generated by thermal, light or radioactive effects, which are ineffective or are accompanied by side effects. Therefore, chemical initiators are used (benzoyl peroxide, isopropylbenzene hydroperoxide, azoisobutyric acid dinitrile, etc.):
(С6Н5СОО)2>2С6Н5СОО*>2С*6Н5+2СО2,
To accelerate the decomposition of initiators into radicals, reducing agents (amines, sulfites, thiosulfates, hydroxy acids, ferrous salts) are introduced. Redox systems reduce the activation energy of the initiation stage from 146 to 50-84 kJ/mol. During the decomposition of hydroperoxide in the presence of Fe2+ salts, rongalite (HO-CH2-SO2Na) makes it easy to convert Fe3+ ions to Fe2+, and the initiator decomposition cycle is repeated:
ROOH+Fe2+>RO*+HO-+Fe3+;
2Fe3++2HO-+HO-CH2-SO2Na>2Fe2++HO-CH2-SO3Na+H2O.
The inorganic persulfate-thiosulfate system operates according to the scheme:
S2O8-2-+ S2O3-2-> SO4-2-+ S*O4-+ S*2O3-; S*O4-+Н2О > НSO4-+О*Н.
The resulting free radicals initiate the polymerization of the monomers.
At the stage of chain termination, neutral macromolecules are formed during the recombination (collision) of macroradicals or as a result of their disproportionation to two neutral macromolecules:
R-(-CH2-CHX-)n-CH2-XHC*+R-(-CH2-CHX-)m-CH2XHC*>
>R-(-CH2-CHX-)n-CH2-CHX-CHX-CH2-(-CHX-CH2-)m-R (recombination),
R-(-CH2-CHX-)n-CH2-XHC* + XHC*-CH2-(-CHX-CH2-)m-R>
>R-(-CH2-CHX-)n-CH2-CH2X+XHC=CH-(-CHX-CH2-)m-R (disproportionation).
The type of chain termination reaction depends on the structure of the monomer molecules. If the monomer contains an electronegative or bulky substituent (methyl methacrylate), then the chain is terminated by disproportionation:
In the polymerization of styrene, the recombination of macroradicals predominates:
As the chain grows, the viscosity of the system increases, the mobility of macroradicals and the rate of their recombination decrease, their lifetime and concentration increase, which leads to an acceleration of polymerization at later stages (gel effect) and a deterioration in the mechanical properties of the polymer. To regulate the MW of the polymer, a chain transfer reaction is used by introducing a regulator into the system, such as mercaptan (RSH), or a solvent, especially a halogen-containing one, such as carbon tetrachloride:
~CH2-HXC*+RSH>~CH2-CH2X+RS* (material circuit break),
RS*+CH2=СHX>RSCH2-HXC* (beginning of a new material chain); or
~CH2-HXC*+CCl4>~CH2-HXCCl+C*Cl3 (material chain break),
CH2=CHX+C*Cl3>Cl3C-CH2-HXC* (beginning of a new material chain),
or increasing the concentration of the initiator to its induced decay:
~CH2-HXC*+ROOR > ~CH2-CHX-OR+RO*;
RO*+CH2=CHX > RO-CH2-HXC* etc.
Unlike the chain termination reaction, they terminate only the material chain - the number of links in the macromolecule ceases to grow. At the same time, they themselves become a free radical and continue the kinetic chain, which is measured by the number of elementary acts of attachment of monomer molecules to the active center per one free radical formed during the initiation of the polymerization reaction. With an increase in temperature and the amount of the regulator, due to the acceleration of chain transfer reactions and the suppression of chain growth reactions, low molecular weight substances (telomerization reaction) are formed, which can be isolated and used to obtain new polymers.
The kinetics of chain polymerization in terms of conversion (degree of conversion) of the monomer is characterized by an S-shaped curve with five sections (Fig. 7):
· site of inhibition, when the concentration of free radicals is low, and they cannot start the chain process of polymerization (1);
polymerization acceleration section, where the main reaction of monomer-to-polymer transformation begins, while the reaction rate increases (2);
· a section of the stationary state (rectilinear section), where the main amount of the monomer is consumed at a constant rate (3);
· area of slowing down polymerization due to a sharp decrease in the concentration of the monomer (4);
termination of the main reaction due to the exhaustion of the entire monomer (5).
Fig.7.
The rate of the initiation reaction is proportional to the concentration of the introduced initiator [I]: vi=ki[I], where ki is the rate constant of the initiation reaction. The chain propagation reaction rate is proportional to the product of the concentrations of growing macroradicals [M*] and free monomer molecules [M]: vр=kр[М*][М], where kр is the chain propagation reaction rate constant. The rate of the chain termination reaction is proportional to the square of the concentration of colliding macroradicals: vrev=krev[M*]2. The rate of polymerization is the algebraic sum of the rates of its three stages: vtotal=vi+vp-vrev.
For kinetic analysis, the stationary period of the reaction is of interest, when polymerization proceeds at a constant rate, and the number of newly formed free radicals is equal to the number of disappearing macroradicals upon chain termination (vi=vobr): ki[I]=kobr[M*]2. It follows that the rate of monomer conversion is proportional to the square root of the initiator concentration. The degree of polymerization is proportional to the chain growth rate and inversely proportional to the chain termination rate, since a macromolecule is formed by the collision of two macroradicals. In other words, the degree of polymerization and the average molecular weight of the polymer are inversely proportional to the square root of the initiator concentration:
Thus, the process parameters and the size of macromolecules for the stationary period can be expressed in terms of the concentration of the chemical initiator.
With an increase in temperature by 10°C, the rate of polymerization increases by a factor of 2-3, and with a decrease in temperature, the regularity of the alternation of links and the MM value increase, the proportion of low molecular weight fractions, the branching of macromolecules, and side reactions decrease. To increase the rate of polymerization at low temperatures, promoters are introduced that activate the breakdown of initiator molecules. The kinetic laws of the polymerization process can be controlled by changing:
time before polymerization (length of the induction period) by introducing inhibitors that react with the initial radicals;
· inclination of the rectilinear section of the kinetic curve to the abscissa axis by introducing polymerization inhibitors (benzoquinone, nitrobenzene), which reduce the concentration of radicals and reduce their lifetime, which leads to a decrease in the length of the polymer chain. The inhibitor does not affect the rate of polymerization, but lengthens the induction period. Depending on the nature of the monomer, the same substance can be both an inhibitor, a moderator, and a polymerization regulator. Benzoquinone works according to the scheme:
Most commercially available ethylene and diene monomers are capable of participating in radical polymerization. The activity of ethylene series monomers depends on the chemical nature of the substituents at the double bond and is determined by the activity of the free radical formed upon breaking the p-bond. The activity of the radical depends on the electron-withdrawing properties of the substituent group and increases with an increase in its ability to delocalize the electron cloud. The best electron acceptor is the benzene ring of styrene, and more electron donors are the alkoxy groups of vinyl alkyl ethers. The radicals of these monomers give the reverse (antibatic) sequence of activities: the lifetime of the radical is the shorter, the more active it is and the less the effect of conjugation of its unpaired electron with the electronic structure of the substituent in the monomer molecule. Therefore, in descending order of activity, vinyl monomers are arranged in the following row:
The activity of radicals can be quantitatively determined and increases with increasing ratio of the constants kobr/kr. For example, the activities of vinyl acetate, methyl methacrylate, and styrene radicals in the chain propagation reaction are quantitatively related as 20:2:1. The activity of radicals is also affected by polymerization conditions, and the activity of monomers is affected by the number of substituents. The presence of two benzene rings at one carbon atom in the monomer molecule completely suppresses its ability to polymerize due to the strong stabilization of the unpaired electron.
One of the features of free radical polymerization is that along the length of one macromolecule there can be different types of connection of monomer units - “head to tail” (a), “head to head” (b), since the radical can attack the monomer molecule from any of its end:
a) CH2-HC-CH2-HC b) HC-CH2-CH2-HC-HC-CH2.
There is also no order in the spatial arrangement of substituents on monomeric units due to the lack of coordinating action when attaching each next monomer molecule. Vinyl series polymers are characterized by the alternation of units in the head-to-tail position, which provides a high level of polymer properties, despite the lack of spatial regularity of their macromolecules. Therefore, the method of free radical polymerization produces the bulk of industrial polymers of this type - polystyrene, polyacrylonitrile, polymethyl methacrylate, polyvinyl chloride, polyvinyl acetate.
Compared to vinyl series monomers, diene monomers provide the greatest variety of macromolecular structures, since each molecule contains two double bonds. There are five main types of connection of links in a macromolecule - in positions 1,4; 1.1; 4.4; 1.2 and 3.4. In the last two cases, they can be considered as polymers of the vinyl series:
For asymmetric dienes (isoprene, chloroprene), when their units are connected in positions 1,1 and 4,4, the regularity of their alternation may be disturbed:
As noted above, 1,4-polydienes can differ in the spatial arrangement of CH2 groups in the chains relative to the plane of the double bond:
All kinds of polydiene structures can exist along the chain length, which leads to instability and irreproducibility of their properties. Structures-1,4 are formed predominantly in the trans position, especially during the polymerization of active and polarized chloroprene, so polychloroprene is produced on an industrial scale by free radical polymerization. Polybutadiene and polyisoprene are most valuable mainly as cis-1,4-isomers; therefore, they are increasingly produced in industry by ion-coordination polymerization methods.
Home > LectureLecture 4 radical polymerization.
Radical polymerization flows through chain mechanism . As a result of each elementary act, a new radical is formed, to which a new neutral molecule is attached, i.e. kinetic chain turns into material . Main stages radical polymerization:- initiation
chain growth
open circuit
chain transfer
- heat (thermal initiation); light (photoinitiation); ionizing radiation (radiation initiation); chemical initiators (chemical initiation)
Example.
a) benzoyl peroxide
decay t = 70 - 80˚С
Initiation efficiency f = 0.7 - 0.9
b) azobisisobutyronitrile
decay t = 60 - 75˚С
Initiation efficiency f = 0.5 - 0.7
c) potassium persulfate
t decay = 40 - 50˚С
The choice of initiator is determined by its solubility in the monomer or solvent and the temperature at which a certain rate of production of free radicals can be achieved.
The radical formed upon initiation adds to the double (=) bond of the monomer and starts the reaction chain. Since the stability of the radicals formed during the decomposition of peroxides, azo compounds, and other initiators is different, the rate of their reaction with monomer molecules, and, consequently, the rate of polymerization are different. To facilitate the decomposition of initiators and reduce the activation energy of the initiation stage, reducing agents (amines, metal salts of variable oxidation state) are introduced into the reaction. For the purpose of downgrading 
(from 146 to 42 - 84 kJ / mol), to facilitate the decomposition of initiators use redox systems
. For example:
Redox systems are used in aquatic environments or when emulsion polymerization . Their wide distribution in the polymer production industry is associated with a significant decrease in the activation energy of the decomposition of initiators into free radicals and thus a decrease in energy costs under production conditions. 2. chain growth- consists in the sequential attachment of monomer molecules to the resulting active center with its transfer to the end of the chain. The development of a kinetic chain accompanied by the formation of a material chain.
(small)
Reaction rate constant k p = 10 2 – 10 4 (large)
The activation energy and the reaction rate constant depend on the nature of the monomers and the parameters of the reaction medium.
3. Open circuit- occurs as a result of the death of active centers.
The break of the chain leads to the break of the material and kinetic chain.
The activation energy of chain termination is determined by the activation energy of diffusion of radicals. The termination can occur at any length of the growing macroradical. In this case, macromolecules of different lengths are obtained. Breakdown most often occurs two ways: by recombination And disproportionation.
E act ≤ 4.2 kJ/mol
E act \u003d 12.6-16.8 kJ / mol
It is also possible to break in the interaction of growing radicals with low molecular weight substances, present in the system. by lowering the temperature ↓ You can reduce the chain termination rate by increasing the viscosity
- chain transfer- occurs by tearing off an atom or a group of atoms from a molecule (chain transmitter) by a growing radical. Wherein:
- the growing radical turns into a valence-unsaturated molecule; new radical develops a kinetic chain
etc. Thus, ethylene telomerization in a carbon tetrachloride medium proceeds with the formation of individual products (tetrachloropentane, tetrachlorheptane, etc.) Example. Chain transfer through: a) monomer molecule b) solvent molecule start of a new chain
c) specially introduced substances (regulators), for example, mercaptans.
k m , k s are the chain transfer rate constants.
When the growing radical interacts with the chain transfer molecule the growth of the material chain stops, i.e. the molecular weight of the resulting polymer is reduced; the kinetic chain is preserved. The ability to participate in chain transfer during radical polymerization is characterized by the chain transfer constant to the monomer C m , to the solvent C s , to the initiator C u .
C m \u003d (0.1 - 5) * 10 -4 - a small value
For example, during the polymerization of vinyl acetate, C m = 2∙10 - 3 From solvents, a high value of C s 
. So during the polymerization of styrene C s = 9∙10 - 3
Kinetics of radical polymerization
The process speed is described by the equation: 
, Where 
is the rate of disappearance of the monomer and 
- the rate of chain initiation and growth In the formation of a high-molecular polymer, the number of monomer molecules involved in the initiation stage is much less than in the growth stage, so it can be neglected. 
hard to measure. For a stationary process, the rate of appearance of a radical is equal to the rate of their death, and the rate of change in the concentration of radicals ( 
) 
For a stationary process, the polymerization rate equation will take the form: 
initiator concentration (known and set prior to the start of the reaction) It follows from the equation that the polymerization rate depends on the initiation rate to the power of 0.5, i. a doubling leads to an increase 
V 
once. This is explained by the bimolecular mechanism of chain scission. During thermal initiation, the polymerization rate V depends on the ratio of the three reaction rate constants 
A typical kinetic curve describing monomer conversion (i.e., the conversion of a monomer to a polymer as a result of polymerization) as a function of time is S-shaped. R 
fig.1 Typical kinetic curve of chain radical polymerization:
1 - inhibition; 2 - acceleration of polymerization (the rate increases with time); 3 – stationary period (polymerization rate is constant); 4 - polymerization slowdown (speed decreases with time)
As can be seen from fig. 1 on the curve, five sections can be distinguished according to the rates of the main reaction of the transformation of the monomer into a polymer as a result of polymerization: 1 - site of inhibition where the concentration of free radicals is low. And they cannot start the chain polymerization process; 2- polymerization acceleration section , where the main reaction of the transformation of the monomer into a polymer begins, and the rate increases; 3- steady state area where the polymerization of the main amount of the monomer occurs at a constant rate (rectilinear dependence of the conversion on time); 4 - retardation site , where the reaction rate decreases due to the decrease in the content of free monomer; 5 - cessation of the main reaction after the exhaustion of the entire amount of monomer. Of greatest interest is the stationary period of the polymerization reaction, when polymerization of the bulk of the monomer occurs at a constant rate. This is possible when the number of newly formed free radicals (stage of initiation) is equal to the number of disappearing macroradicals (stage of termination) of the reaction and material chains. Degree of polymerization n(i.e., the number of links of monomeric units in one average macromolecule) is, by definition, proportional to the rate of the chain propagation reaction and inversely proportional to the rate of the chain termination reaction, since a neutral macromolecule is formed as a result of the collision of two growing macroradicals. n = υ p / υ arr = k p [M] / k arr 2 = k p [M] / k arr = k n / = k n I / [I] 0.5 In other words, the degree of polymerization and, therefore, the average molecular weight of the polymer in free radical polymerization, it is inversely proportional to the square root of the initiator concentration.
Influence of various factors on the process of radical polymerization.
1. Effect of temperature With an increase in temperature, the rate of the reaction of the formation of active centers and the reaction of chain propagation increases. Thus, the overall rate of polymer formation is increased. Typically, the polymerization rate increases by a factor of 2-3 with a temperature increase of 10 °C. However, with a general increase in the concentration of radicals, the probability of their collision with each other (chain termination by disproportionation or recombination) or with low-molecular impurities also increases. As a result, the molecular weight of the polymer as a whole decreases (the average degree of polymerization decreases with increasing temperature), and the proportion of low molecular weight fractions in the polymer increases. The number of side reactions leading to the formation of branched molecules increases. The irregularity in the construction of the polymer chain increases due to an increase in the proportion of head-to-head and tail-to-tail monomer connection types. 2. Effect of initiator concentration.With an increase in the concentration of the initiator, the number of free radicals increases, the number of active centers increases, and the total rate of polymerization increases.
However, with a general increase in the concentration of radicals, the probability of their collision with each other also increases, i.e. chain termination, which leads to a decrease in the molecular weight of the polymer. 3. Effect of monomer concentration. During polymerization in a solvent medium, the total polymerization rate and the molecular weight of the resulting polymer increase with increasing monomer concentration. During polymerization in an inert solvent that does not participate in the reaction, the polymerization rate is equal to 
(often x = 1.5). Most solvents participate in polymerization (in a chain transfer reaction). Therefore, much more complex dependencies are obtained. 4. Influence of pressure. High and ultra-high pressure of 300-500 MPa (3000-5000 atm) and above significantly accelerates polymerization. Example. Polymerization of methyl methacrylate in the presence of 
air at 100˚C and p = 0.1 MPa lasts 6 hours, under p = 300 MPa - 1 hour, i.e. the total polymerization rate increases by about 6 times. Likewise, the impact p affects the polymerization of styrene, vinyl acetate, isoprene, etc. NB!
polymerization feature under p is that the increase in speed is not accompanied by a decrease in the molecular weight of the resulting polymer.
polymerization inhibitors and regulators
The chain breaking and transfer phenomena are widely used in practice for:
- preventing premature polymerization during storage of monomers;
- to control the polymerization process
fig.2 Thermal polymerization of styrene at 100 ˚С in the presence of inhibitors and retarders: 1 – without additives; 2-0.1% benzoquinone (inhibitor); 3 - 0.2% nitrobenzene (inhibitor); 4 - 0.5% nitrobenzene (moderator) Inhibitors and retarders of polymerization are used to regulate the polymerization process. Inhibitors
- low molecular weight substances that change the duration of the induction period, slowing it down. This is often necessary in polymer technology to prevent premature polymerization under uncontrolled conditions. Inhibitors: quinones, aromatic amines, nitro compounds, phenols, organic salts 
, 
, 
, 
etc. Example: hydroquinone Quinone interacts with free radicals, turning them into inactive products. The death of radicals increases the length of the induction period. Along with inhibitors that allow you to completely stop the polymerization, there are polymerization retarders
, which only reduce its speed. Moderator
performs a dual role: it reduces the concentration of radicals and reduces their lifetime, which leads to a decrease in the length of the polymer chain. The inhibitor does not affect the rate of polymerization, but prevents the initiation of the chain by increasing the induction period on the polymerization kinetic curve. The length of the induction period is usually proportional to the amount of inhibitor administered. One and the same substance can act both as an inhibitor, and as a moderator, and as a polymerization regulator, depending on the nature of the polymerized monomer. For example, oxygen, which slows down the polymerization of vinyl acetate and accelerates the polymerization of styrene. At high pressures and high temperatures, oxygen promotes the polymerization of ethylene. This phenomenon is used in the industrial production of high pressure polyethylene. Oxygen forms peroxides or hydroperoxides upon interaction with monomers or growing chains. hydroperoxide peroxide Depending on the stability of intermediate peroxides or hydroperoxides, they can either increase the concentration of radicals and accelerate polymerization, or deactivate existing radicals and slow down or even inhibit polymerization. Fig.1.3 p.28 Kuleznev Example: aromatic nitro and nitroso compounds. polymerization regulators
cause premature breaking of the material chain, reducing the molecular weight of the polymer in proportion to the introduced amount of the regulator. An example of these are mercaptans, including dodecylmercaptan. Due to the long hydrocarbon chain, its molecules are not active enough and are consumed slowly.
, 
. Methods for carrying out polymerization
Radical polymerization is carried out in block (mass), solution, emulsion, suspension and gas phase. In this case, the process can proceed under homogeneous or heterogeneous conditions. In addition, the phase state of the initial reaction mixture may also change during polymerization.
Polymerization in block (in bulk)
Solution polymerization
Unlike polymerization in the block, in this case there are no local overheatings, since the heat of reaction is removed by the solvent, which also acts as a diluent. The viscosity of the reaction system decreases, which facilitates its mixing.
However, the role (share) of chain transfer reactions increases, which leads to a decrease in the molecular weight of the polymer. In addition, the polymer may be contaminated with solvent residues, which cannot always be removed from the polymer. There are two ways to carry out solution polymerization. a) A solvent is used in which both the monomer and the polymer dissolve. The resulting polymer is used directly in solution or isolated by precipitation or evaporation of the solvent. b) The solvent used for polymerization dissolves the monomer but does not dissolve the polymer. The polymer precipitates out in solid form as it forms and can be separated by filtration.
Suspension polymerization (bead or pellet)
in the form of small drops. The stability of the dispersion is achieved by mechanical mixing and the introduction of special additives - stabilizers into the reaction system. The polymerization process is carried out in monomer drops, which can be considered as block polymerization microreactors. Monomer soluble initiators are used. The advantage of this process is good heat removal, the disadvantage is the possibility of contamination of the polymer with stabilizer residues. Emulsion polymerization (emulsion polymerization)
Gas-phase polymerization
- Free radical polymerization is one of the types of chain processes of polymer synthesis. Polarization of the initial monomer molecules facilitates their reactions with initiator radicals during chemical initiation or physical methods of generating radicals. Electron-withdrawing substituents contribute to greater stability of monomer radicals and growing chains. The process of radical polymerization can be controlled in various ways, both in terms of the rate of monomer conversion and in terms of the molecular weight of the polymer. To do this, additives of low molecular weight substances are used that act as inhibitors or retarders of the reaction, as well as transfer the reaction chain or reduce the activation energy of the decomposition of initiators into radicals. Knowledge of the patterns of free radical polymerization makes it possible to control the structure of the polymer and, consequently, its physical and mechanical properties. Due to its simplicity, this method of obtaining polymers has found wide application in industry.
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 with 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.
5.1.1. Radical polymerization
polymerization mechanism
The simplest scheme of the kinetic chain during chemical initiation can be represented as the following sequence of reactions:
1. Initiation (chemical):
a) the formation of primary free radicals
b) the origin of the material chain
2. Growing material chain:
3. Broken material chain:
a) recombination
b) disproportionation
c) chain transfer (growth of the kinetic chain);
per polymer molecule
per monomer molecule
per solvent molecule
When describing the polymerization process, the following assumptions are made:
- the activity (reactivity) of a free radical depends only on the nature of the atom on which the unpaired electron is localized, and on its nearest environment;
- the average length of the kinetic chain is large;
- a steady state is established in the reaction medium, i.e. the rate of initiation is equal to the rate of destruction of free radicals.
The interaction of free radical particles refers to fast chemical reactions. However, the approach of reacting particles proceeds much more slowly. On fig. Figure 5.1 shows a one-dimensional diagram illustrating the decisive role of diffusion processes at all stages of the polymerization reaction.
Active particles are surrounded by solvent molecules (environment), forming
Rice. 5.1. Scheme for the implementation of the "cell effect":
I- solvent; II- "hole"; III- reacting particle
"cage", within which their movement is possible as a series of "jumps" into vacant voids - "holes". The approach of reacting particles and the release of reaction products from this "cage" is a diffusion-controlled process. This feature of the process is defined as the "capture effect" or "cell effect" (the Frank-Rabinovich effect).
Question. 2.2"-azo- bis-isobutyronitrile (azodiisobutyric acid dinitrile - DAK, porophore ChKhZ) is widely used in experimental and industrial practice as a substance that easily generates active free radicals as a result of thermal decomposition according to the scheme
This process begins in solution already at 40°C. At the same time, it was found that an increase in the viscosity of the reaction medium leads to a decrease in K d. What is the possible reason for this effect?
Answer. A decrease in the rate of decomposition of the initiator with an increase in the viscosity of the reaction medium may be due to the two-stage nature of this reaction: first, radicals are formed
With an increase in the viscosity of the medium, the "cage effect" slows down the separation of the initially formed radical pair, thereby preventing the release of active particles from the "cage". In this regard, the second stage of the process of complete decomposition of this compound into free radicals proceeds at a slower rate.
The origin of the chain (initiation). The process of formation of active centers proceeds relatively slowly and requires the expenditure of a certain amount of energy. This initial stage of radical polymerization is called chain nucleation (initiation) and leads to the formation of free (secondary) radicals from valence-saturated monomer molecules. Free radicals in a polymerizing system can be formed in various ways: under the influence of heat, light, ultrasound, hard radiation (X-rays, α-, β- and γ-rays - physical initiation),
as well as with the introduction of chemical polymerization initiators, i.e. substances that easily break down into free radicals. The excitation of the polymerization reaction with the introduction of initiators has been widely used in the production of fiber-forming polymers, since this facilitates the regulation of the synthesis process. The decay of the initiator requires the supply of external energy and proceeds at a certain rate. For example, the initiators of free radical polymerization are compounds that can decompose: at the ~O~O~ (I) bond or at the bond 
(II).
I. The dissociation energy of this bond is 150-160 kJ/mol. Connections of this type include:
II. The dissociation energy of this bond is 295 kJ/mol. The use of azo compounds as initiators is predetermined
2.2"-azo- bis-isobutyronitrile
NC-C(CH 3) 2 -N=N-C(CH 3) 2 -CN.
The rate of formation of primary free radicals is described by the first order reaction rate equation:
As a result of integration and subsequent transformation, we have
where [I] t and [I] 0 - current and initial concentration of the initiator; t- time; Kd- rate constant of the reaction of decomposition of the initiator into free radicals.
Task. Determine the rate constant of the decomposition of benzoyl peroxide in dioxane at 80°C, if its initial concentration was 1.1%, and after 10 min iodometrically 1.07% benzoyl peroxide was found in the system.
Solution. According to equation (5.2),
ln=exp/ kd) = 151.9 kJ/mol.Estimation of Δ values E d allows you to choose the most appropriate temperature range for the synthesis of fiber-forming polymers. In table. 5.1 shows the values of the apparent activation energy Δ E d and rate constants Kd for some initiators. When carrying out the synthesis below 85°C, it is advisable to use DAA. At higher temperatures, benzoyl peroxide, etc., gives better results.
Table 5.1. Kinetic characteristics of some polymerization initiators
The polymerization reaction at temperatures below 70°C, it is advisable to carry out using inorganic peroxides.
The duration of the initiation stage is reduced with an increase in the number of free radicals.
To increase the rate of decomposition of initiators, such as peroxides, "promoters" - reducing agents - are introduced into the reaction mixture. Redox initiating systems are widely used to carry out the synthesis of various carbon chain polymers. The initiation of the polymerization process by the use of redox systems is characterized by a small temperature coefficient (comparatively low apparent activation energy).
Thus, under the influence of physical or chemical factors, free radicals appear in the system, for example, having unpaired p-electrons and therefore possessing high chemical activity. Collisions of free radicals lead to the emergence of a covalent bond between them with the formation of an inactive molecule. When a free radical interacts with an inactive molecule, a reaction product is formed, which also has one unpaired electron and has almost the same activity as the original free radical. These processes can be illustrated by the diagram
R* + R* → R: R; R* + M → R: M*.
The propensity for addition reactions limits the lifetime of free radicals. For example, the half-life of the H 3 C * radical is 10 -4 s. However, the conjugation of the unpaired p-electron [for example, in triphenylmethyl (C 6 H 5) 3 C *] or screening with its substituents that are part of the free radical, for example, in diphenylpicrylhydrazil
dramatically increases the stability of free radicals.
As a result of chemical initiation, the free radical becomes the end group of the growing polymer chain.
The time it takes for the circuit to start is called the induction period. Substances that increase the induction period are called inhibitors. Not all free radicals, interacting with monomers, initiate a reaction. Some of them are deactivated after mutual collision. The ratio of the number of radicals attached to the monomer and initiating the reaction to the total number of all formed radicals is called the efficiency of the initiator f e. The effectiveness of an initiator can be assessed by one of three methods:
- comparing the rate of decomposition of the initiator and the rate of formation of polymer molecules (this technique requires an accurate measurement of the average molecular weight of the polymer);
- comparing the amount of initiator bound to the polymer with the amount of decomposed initiator;
- using a chain terminating inhibitor.
For example, the use of diphenylpicrylhydrazyl allows chain termination according to the scheme
Task. Calculate the efficiency of 2.2 "-azo- bis-isobutyronitrile, if during the polymerization of styrene the initial concentration of the initiator was 1.1%, and 80 cm 3 of nitrogen were released per 100 g of monomer in 20 minutes of reaction (in terms of normal conditions). The degree of monomer conversion reached 5%. The molecular weight of the resulting polymer is 2500 (determined by osmometric method).
Solution. When the initiator molecule decomposes, two free radicals are formed and a nitrogen molecule is released. We calculate the number of moles of the initiator at the beginning of the reaction per 100 g of monomer:
[I] 0 \u003d 1.1 / 164 \u003d 0.007 \u003d 7 10 -3.
The amount of released nitrogen will be
80/(22.4 1000) = 3.5 10 -3.
Thus, in 20 min of the reaction, 3.5 × 10 -3 mol of the initiator decomposed and, consequently, 7 × 10 -3 mol of radicals were formed. At a conversion of 5% and an average molecular weight of 2500, the number of polymer moles formed is
5/2500 \u003d 2 10 -3.
Let us assume that all kinetic chains ended with the recombination of radicals and, therefore, 1 mol of initiator was consumed per 1 mol of polymer. From here we find the efficiency of the initiator f e:
f e \u003d 2.0 10 -3 / (3.5 10 -3) \u003d 0.6.
In general, the decay rate of the initiator V 0 = Kd[I].
For most used initiators f e is in the range of 0.3-0.8, i.e. almost always f e f e varies depending on the medium: the nature and amount of the initiator, monomer, solvent, etc.
For example, when initiating the polymerization of acrylonitrile with azodiisobutyric acid dinitrile in dimethylformamide and 51.5% NaCNS aqueous solution, the value K d f e in the second case turns out to be significantly smaller due to the strong manifestation of the "cell effect" (the viscosity of the medium increases, and specific solvation effects also appear).
Numerous experimental data have established that at a constant monomer concentration, the polymerization rate is proportional to the square root of the initiator concentration ("square root rule"):
Where TO- total polymerization rate constant; [M] - monomer concentration; [I] - concentration of the initiator;
Where Kd is the decay rate constant of the initiator; TO p is the rate constant of polymer chain growth; TO 0 is the chain termination rate constant.
Question. Heterophase polymerization of vinyl chloride in the presence of benzoyl peroxide proceeds under isothermal conditions 6-8 times slower than in the presence of azodiisobutyric acid dinitrile. Explain a possible reason for this phenomenon.
Answer. Benzoyl peroxide is very slightly soluble in water. Therefore, the initiation rate reaches a noticeable value only after the concentration of initiator particles in the dispersion is sufficiently high [see. equation (5.3)]. Azodiisobutyric acid dinitrile is more soluble in water; therefore, the induction period of the polymerization process, which determines the overall duration of the process, will be shorter in this case.
Continuation (growth) of the chain. Reactions of continuation (growth) of the kinetic chain are called elementary
stages of a chain reaction that proceed with the preservation of free valence and lead to the consumption of starting materials and the formation of reaction products. During polymerization, this sequence of reactions causes the growth of the polymer chain:
Chain growth is a fast flowing stage of the polymerization process, described by equation (5.3). The rate of polymerization also increases with increasing monomer concentration in the reaction medium.
Chain break. Breakage of the kinetic chain is the stage of the chain process, leading to the disappearance of free valency. Breakage of the kinetic chain can occur:
as a result of recombination, i.e. the interaction of two identical or different free radicals,
or disproportionation, i.e. transfer of a proton from one radical to another, with the loss of the activity of the reaction products, i.e.
The activation energy of the first reaction - recombination - is close to zero and, in any case, does not exceed 0.5-1.5 kJ/mol, while the disproportionation activation energy reaches 16-18 kJ/mol.
Termination of macromolecule growth can occur as a result of recombination and disproportionation of macroradicals.
At the same time, the same effect is observed when a polymer radical (macroradical) encounters an inactive molecule. The cessation of growth of a macromolecule as a result of the transfer of an unpaired electron to an inert molecule is called the transfer of a kinetic chain ("radical tropium"). This process can lead to the addition of a hydrogen atom to the growing polymer chain:
Molecules of an initiator, solvent, monomer, inactive polymer or macroradical, etc. can act as RH. The rate constants of these reactions will be respectively TO P i , K P s , K p m, K p p
Question. In the process of free radical polymerization, along with linear macromolecules, branched ones are formed. Write a probable scheme for the formation of such branches during the polymerization of vinyl acetate in the presence of benzoyl peroxide.
Answer. At high degrees of conversion, the resulting macromolecules (and macroradicals) can be exposed to mobile free radicals. The most vulnerable part of the macromolecule is the hydrogen atoms at the tertiary carbon atoms:
Breakage of the kinetic chain leads to a decrease in the degree of polymerization of the resulting high-molecular compound. Sometimes, to control the rate of the process and the molecular weight of polymers, special substances (hydroquinone, nitrobenzene, etc.), called polymerization inhibitors, are introduced into the reaction mixture. Their action is based on binding
active centers of the kinetic chain. The length of the kinetic chain v is
Where V p and V t are the rates of chain growth and chain termination, respectively.
With the help of polymerization inhibitors, the yield and properties of the resulting polymer (average molecular weight, degree of polydispersity) can be varied over a relatively wide range.
Question. In the initial periods of free radical polymerization, polymers with a maximum molecular weight are formed. As the degree of conversion of the monomer (polymer yield) increases, its molecular weight usually decreases. Explain the likely cause of this phenomenon.
Answer. As the degree of conversion increases, the number of growing kinetic chains in the reaction medium increases, which leads to an increase in the probability of recombination processes.
Polymerization is a complex process and often cannot be described by a single stoichiometric equation, since in some cases chain termination leads to the appearance of some by-products. However, for a sufficiently large length of the kinetic chain, polymerization can be described with sufficient approximation by a single stoichiometric equation. chain reaction speed v is equal to the product of the chain initiation rate v i and the length of the kinetic chain v:
Wherein v= (1 - β)/β, where β is the probability of chain termination at each stage of growth. The length of the kinetic chain v can be calculated from the ratio
Task. Define value TO R / TO
can be determined from the equation of the stationary polymerization rate, which describes the process well in its initial stage [equation (5.3)]. After transforming equations (5.3) and (5.4), we obtainlog([M] 0 /[M] t) = (K p/ K
)V i t. In the presence of free radical acceptors, the process slows down (is inhibited). If WITH ing is the concentration of the inhibitor, then the rate of the initiation reaction can be calculated from the dependenceVi= C ing t i.
According to this empirical dependence, for any arbitrarily chosen inhibitor concentration (for example, 0.2 mol / dm 3), it is possible to calculate the corresponding value t, and hence the rate of initiation:
- t\u003d 2 10 -5 + 2857 0.2 \u003d 571 min;
- Vi\u003d 1 10 -1 / 571 \u003d 5.83 10 -6 mol / (dm 3 s).
For two times ≥ t i value can be calculated TO R / TO
= = 0,25.In accordance with equations (5.3) and (5.4) we have
Where f e - efficiency of the initiator; Kd is the decomposition rate constant of the initiator; [M] - monomer concentration; [I] - concentration of the initiator.
It was previously noted that the values f e and Kd can be measured separately. Also determined experimentally V p , [I], [M]. Finding thus K
= 2.34 10 -7 .At low degrees of conversion, the total polymerization rate V is satisfactorily described by equation (5.8). Temperature dependence V, characterized by the apparent activation energy of the synthesis process, is described by the equality
Δ E rev = 1/2Δ E i - Δ E p + 1/2Δ E o ,
where ∆ E i, Δ E p and Δ E o are the apparent activation energies of the stages of chain initiation, growth, and chain termination, respectively.
For most vinyl monomers
- Δ E i= 130 ± 10 kJ/mol; Δ E p = 25 + 5 kJ/mol;
- Δ E o = 6 ± 2 kJ/mol.
This means that with increasing temperature in all cases, the rate of the polymerization reaction increases.
The kinetic chain length v under isothermal synthesis conditions is determined only by the nature of the monomer.
2.1.1. Initiation of radical polymerization
The initiation process is the first step in free radical polymerization:
The primary radicals necessary to initiate radical polymerization can be obtained as a result of chemical reactions and by physical action on the monomer.
Real initiation. In chemical or material initiation, substances are used that decompose with the formation of free radicals, or mixtures of substances that react with each other with the formation of free radicals. As such substances, peroxides and azo compounds are usually used, as well as combinations of substances that form a redox system.
Among peroxides, acyl-, alkyl-, hydroperoxides and perethers have found wide application. The range of azo compounds practically used as initiators is more limited. The most famous among them is 2,2 "-azobis(isobutyronitrile), which decomposes with the release of nitrogen:
Due to the latter circumstance, this and similar azo derivatives are used in industry not only as initiators, but also for foaming plastics in the production of foams.
The most commonly used initiators in modern research and production practice are given in Table. 2.1 along with the characteristics of their decay. The table is closed by high-temperature initiators that decompose with bond breaking S-S.
Redox systems are divided into two groups; organic and water soluble. The first group includes numerous combinations of peroxides with amines, of which the most studied is the benzoyl peroxide–dimethylaniline system. As a result of the redox reaction in this system, the primary act of which is the transfer of an electron from the amine to the peroxide, a benzoate radical is formed, which further initiates the polymerization process:
Table 2.1
The most important initiators of radical polymerization
|
Initiator |
E dist, kJ/mol |
A rasp, s -1 |
Temperature, ºС for τ 1/2 |
||
|
Bis(2-ethylhexyl)peroxydicarbonate
| |||||
|
| |||||
|
(isobutyronitrile)
| |||||
|
benzoyl peroxide | |||||
|
peroxybenzoate | |||||
|
| |||||
|
peroxide
| |||||
|
Cumyl hydroperoxide | |||||
|
Dicyclohexylperoxydicarbonate
| |||||
|
3,4-Dimethyl-3,4-diphenylhexane
| |||||
|
2,3-Dimethyl-2,3-diphenylbutane
| |||||
Lauryl peroxide
2.2"-Azobis
tert-Butyl
cumyl peroxide
Di- tert-butyl
In this example, the formation of a redox system leads to an increase in the rate of polymerization and a decrease in the temperature of its initiation compared to the process initiated by only one peroxide. These advantages are also characteristic of other redox systems.
Water-soluble redox systems originate from the classical system:
often called Fenton's reagent. They can also be formed by other metal ions of variable valence and peroxides. Hydroperoxides are usually used instead of the latter in aqueous solutions.
By now, the redox systems containing persulfates as an oxidizing agent, and metal ions of variable valence or thiosulfates as a reducing agent have received the greatest distribution:
They are widely used in industry to initiate emulsion and solution polymerization.
For the correct choice of the polymerization initiator, it is necessary to have data characterizing the rate of its decomposition at the reaction temperature. The most universal characteristic is the half-life of the initiator τ 1/2 (Table 2.1.). Most often, the initiator is characterized at a temperature at which the half-life is 10 hours. This temperature is usually in the range from 20 to 120°C and depends on the structure of the initiator. Usually, initiators are used to initiate polymerization, the half-life of which is commensurate with the duration of the process. Since for first order reactions τ 1/2 = ln2/ k rasp, then, knowing the value τ 1/2 , you can calculate the concentration of the initiator at any time of polymerization in accordance with the equation:
, (2.1)
Where k rasp is the rate constant of the monomolecular decomposition reaction of the initiator;
[ I 0 ] And [ I] - initial and current concentrations of the initiator.
Photochemical initiation. When a monomer is irradiated with UV light, the molecules that have absorbed a quantum of light are excited and then decompose into radicals capable of initiating polymerization:
M+ hν→ M * → R 1 + R 2
However, direct irradiation of the monomer is ineffective, since quartz glass usually does not transmit UV light in the region corresponding to its absorption by the monomer ( π-π* -transition, 200-220 nm), or passes it to a small extent.
In the case when the monomer does not absorb transmitted light, it is necessary to use photosensitizer(Z) - a compound that transfers excitation energy to other molecules:
Z+ hν→Z*
Z * + M → Z + M * → R 1 + R 2 + Z .
The use of dyes as photosensitizers makes it possible to use the visible region of light for photoinitiation.
For practical purposes, photopolymerization is usually carried out in the presence of photoinitiators- substances that decompose in the required region of the UV spectrum with a sufficiently high quantum yield. An ideal photoinitiator should meet the following criteria:
1. First, decompose when irradiated with a light source with a certain wavelength, which is not absorbed by the monomer;
2. the efficiency of the initiator should be high and close to unity;
3. it is better if one type of radicals is formed in this case.
According to the mechanism of action, photoinitiators can be divided into 2 types: decomposing into radicals upon irradiation, for example, compounds containing a benzene group (acetophenol type) ( I) and interacting with co-initiators to form radicals (azo compounds ( II)).
The mechanisms of action of azo compounds have features that consist in changing their configuration during irradiation with cis- on trance-. This is of particular importance in the case of pulsed exposure to light.
The most effective photoinitiators are aromatic ketones and their derivatives, due to a fairly wide absorption region of the UV spectrum and a high quantum yield of radicals. Aromatic ketones are believed to undergo photochemical transformation along two pathways:
the latter of which is realized only in the presence of hydrogen donors.
In industry, benzoin (1), benzylketal (2) and their numerous derivatives are used as photoinitiators:
Knowing the number of absorbed photons ( n abs) and irradiated volume ( V), one can determine the concentration of primary radicals ( WITH) formed during monochromatic light irradiation:
, (2.2)
where Ф is the primary quantum yield.
The number of absorbed photons can be calculated using the well-known Lambert-Beer expression:
,
(2.3)
Where E p is the absorbed energy, E λ – energy of one mole of photons at wavelength λ , - extinction coefficient, WITH- concentration, d- optical density.
Photopolymerization is used for applying polymer coatings in a continuous way on metal, wood, ceramics, light guides, in dentistry for curing dental filling compositions. Particularly noteworthy is the use of photopolymerization in photolithography, which is used to produce large integrated circuits in microelectronics, as well as printed circuit boards (matrices) in modern phototypesetting technology, which makes it possible to eliminate the use of lead.
The main advantage of photoinitiation in polymerization processes is the ability to accurately determine the start and end of the process through the duration of exposure to light. In addition, the decomposition rate of the initiator is practically independent of temperature, while the irradiation intensity makes a decisive contribution.
A significant disadvantage of photoinitiation is the rapid drop in its efficiency with increasing thickness of the irradiated layer due to radiation absorption. For this reason, photochemical initiation is effective in initiating polymerization in sufficiently thin layers, on the order of several millimeters.
radiochemical initiation. Radiation from radioactive sources of Co 60 , as well as various kinds of accelerators, includes a set of particles, such as α-particles, neutrons, electrons and hard electromagnetic radiation. Unlike photoradiation, radioactive is ionizing and has a much greater penetrating power, which is explained by the greater energy of its particles.
The ionization of the irradiated substance is a consequence of the knocking out of electrons from its molecules, for example, a monomer, by high-energy particles:
Radicals capable of initiating polymerization arise as a result of further transformations in the system with the participation of excited ions, radical ions and electrons, for example:
The presence of free radicals and ions in the irradiated monomer predetermines the possibility of developing both radical and ionic polymerization. In most cases, the result is radical polymerization, however, at low temperature in the absence of water and other impurities that deactivate ions, it was possible to observe both cationic and anionic polymerization of individual monomers.
Thermal initiation. There are very few examples of thermal initiation of polymerization. These include, first of all, the spontaneous polymerization of styrene and vinylpyridines. It is believed that the mechanism of the formation of free radicals during thermal initiation is bimolecular, but it has been reliably revealed only with respect to styrene. The first step of the reaction is the formation of a Diels-Alder adduct from two styrene molecules:
At the second stage, the transfer of a hydrogen atom from the adduct to the styrene molecule takes place, which leads to the formation of radicals capable of initiating polymerization:
The self-initiated polymerization of styrene has a high activation energy. So 50% conversion of the monomer is observed at 29ºС for 400 days, at 127ºС the reaction takes place within 4 hours. The advantage of this method is that the final polymers do not contain initiator impurities.
In most other cases, spontaneous thermal polymerization is due to initiation by peroxides, which are easily formed in the light even during short-term contact of the monomers with atmospheric oxygen.
Plasma initiation. Here, as in the previous case, ions and radicals are formed. The polymerization process is difficult. This method is used to obtain thin polymer films.
Electrical initiation. Occurs during the electrolysis of a mixture containing an organic solvent, a monomer and an inorganic compound that conducts electric current. In this case, ions and radicals are formed.
Initiation efficiency. Initiation efficiency f is equal to the proportion of radicals initiating polymerization of their total number, which corresponds to the spontaneous decomposition of a certain amount of initiator. Usually 0.3< f < 0,8, т.е. заметно меньше единицы. Это объясняется двумя причинами – индуцированным распадом инициатора и побочными реакциями в «клетке».
The induced decomposition of the initiator occurs as a result of its reaction with the propagating radical, i.e. as a result of chain transfer to the initiator. This leads to a decrease in the number of radicals of the decomposed peroxide that initiate polymerization:
The “cage” effect consists in the fact that two radicals formed as a result of the decomposition of the initiator, in the case under consideration, benzoyl peroxide, cannot separate for some time, since their diffusion is hindered by the surrounding monomer and solvent molecules. This moment is very favorable for the occurrence of side reactions leading to their deactivation. One of them is shown below (radicals in the "cage" are indicated by brackets):
Primary benzoate radicals leave the "cage" by diffusion and as a result of reaction with the monomer. Then they can be decarboxylated
as a result, the reaction with the monomer (initiation) is carried out with the participation of both benzoate and phenyl radicals:
Side reactions that reduce the effectiveness of initiation, in addition to the above reaction in the "cage", include the following two reactions:
In general, the efficiency of initiation is determined by the nature of the initiator, monomer, solvent, and conversion. The microviscosity of the medium is of great importance; the viscosity of the monomer or monomer-solvent mixture. It determines the mobility of the "cell": with its increase, the release of radicals from the "cell" becomes more difficult, and the efficiency of initiation decreases. The efficiency of initiation decreases even more with an increase in conversion, i.e. fraction of monomer converted to polymer.
