Radical polymerization of styrene. Synthetic polymers

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 bond C=heteroatom enter into radical polymerization. Cyclic monomers do not undergo radical polymerization.

Common monomers that undergo radical polymerization include ethylene, vinyl chloride, vinyl acetate, vinylidene chloride, tetrafluoroethylene, acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, styrene, butadiene, chloroprene, etc. The listed monomers form high molecular weight products, while vinyl ethers and allyl monomers form oligomers. Vinylene monomers (CHX=SPC, with the exception of X=F) do not enter into radical polymerization due to steric hindrance.

Initiation of polymerization 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 initiation methods are thermal homolytic decomposition of initiators, initiation by redox systems, photochemical initiation, and 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 the most widespread is 2,2"-azo-bms-isobutyronitrile (DAK or AIBN)

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

Polymerization at high temperatures can be caused without introducing 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 carry out polymerization in an aqueous or organic environment at room temperature.

Here are typical redox initiating systems:

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

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

Photochemical initiation occurs under the influence of UV light. In this case, the radical can arise both in a system containing a pure monomer and during 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 is that the polymerization process can be carried out at room temperature.

Radiation-chemical initiation(under the influence 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 initiation methods is the ability to instantly turn on and off the irradiating radiation.

Initiation includes two elementary acts:

a) generation of radicals R" from ininiatope I:

b) interaction of radical R* with monomer M:

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

Of these two stages, in most cases the limiting stage is the hemolytic degradation of the initiator, i.e. reaction(s).

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 participating in reaction (b) to the number of radicals formed in reaction (a).

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

Where k p - chain growth rate constant.

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

Rate constants and activation energy E a chain growth 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 reaction of radical polymerization chain growth.

The activation energy for the addition of the monomer to the heme radical is lower, i.e. the more active the monomer is, 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 changes antibatally.

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 an unsaturated group of the substituent, i.e. with high conjugation energy. In inactive monomers there is no conjugation or its (conjugation) energy is low. The higher the reactivity of the monomer, the higher the activation energy of the chain growth reaction, those. the lower the rate of its radical polymerization.

Chain breakage leads to limitation of the kinetic and material chain, i.e. to the death of the active center (the disappearance of the active radical or its replacement by a low-active radical that is 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- kinetic termination constants according to the mechanism of recombination and disproportionation, respectively.

The chain termination reaction occurs in three stages:

1) progressive diffusion of macroradicals with the formation of a united coil;
2) mutual approximation of active terminal links due to segmental diffusion within 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 wide molecular weight distribution.

Another option for chain termination is termination on inhibitor molecules. Inhibitors can be low-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 transform into low-active 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, divalent copper, etc.), which break off 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 added inhibitor.

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

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

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

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

Separately, it should be noted 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 position to the double bond leads to the formation of a resonance-stabilized, inactive allylic radical, which is not capable of initiating further polymerization:

Allyl radicals recombine to form dimers. In this case, unlike conventional transmission, not only material, but also kinetic values are interrupted. This type of transmission is called degradation chain transfer. Degradative transmission, 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 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 growth reaction kp:

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

Table 5.6

Transfer constants for radical polymerization

The ability of solvents to participate in chain transfer during 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 values of Cs> 10 3, which are called regulators.

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

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

Chain growth rate V n expressed by the equation

Where k-- rate constant for the addition of the monomer to the radical, degree of polymerization r; | R* | - concentration of radicals, degree of polymerization g; [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), it can be assumed that k if) does not depend on the degree of polymerization of the radical. Then the expression for V p simplifies:

where | R* | - concentration of all growing radicals.

Taking into account the assumption that the reactivity of growth 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 total rate of polymerization equal to the rate of monomer disappearance 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, then 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 eliminated from equation (5.10) by accepting that the rate of formation of radicals is equal to the rate of their disappearance (quasi-stationary condition), those. dR"]/dt= 0. In radical polymerization, this condition is usually met 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) the constants of the elementary stages do not depend on the degree of polymerization of growth radicals (Flory's principle);
3) if the lifetime of active particles is short compared to the polymerization time, then use principle of quasi-stationary parity, according to which the change in the concentration of macroradicals over time is zero, i.e. the initiation rate is equal to the circuit break rate;
4) the process is considered at the initial conversions of the monomer.

Thus, the order of the reaction rate by monomer concentration is

is unity, in terms of initiator concentration - 0.5. In order to evaluate the effect of temperature on the rate of polymerization, we consider the total activation energy of this process. Effective polymerization rate constant

Then the effective (total) activation energy of the process

Activation energy for growth reaction E= HR40 kJ/mol, activation energy for termination reaction? 0 = (R6 kJ/mol, activation energy of the initiation reaction E IN= 105-H75 kJ/mol for thermal decomposition of the initiator and E nn= 0 for photo- or radiation initiation. So anyway 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.

Kinetic chain - the number of monomer molecules per one R* radical formed before its death due to chain termination.

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 one act of death of the radical R’ during chain termination and transfer.

When terminated 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.

When terminated by recombination (&), one macromolecule is formed from two kinetic chains, and R p = 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 quasi-stationary conditions in the absence of an inhibitor, then at a sufficiently small depth of conversion, when there is still little polymer in the system and, therefore, the rate of chain transfer to the polymer and monomer consumption can be neglected:

Where V a- rate of bimolecular chain termination; - the sum of the price transfer rates for monomer M and 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) before 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 that terminate via the disproportionation mechanism, X, then the fraction of radicals that die 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 Cs, finally we get

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

k"/2 k t and [I] 0 - current and initial concentrations of the initiator; t- time; Kd- rate constant for the reaction of the initiator decomposition into free radicals.

Task. Determine the rate constant for the decomposition of benzoyl peroxide in dioxane at 80°C if its initial concentration was 1.1%, and after 10 minutes 1.07% benzoyl peroxide was iodometrically detected 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 select 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 synthesis below 85°C, it is advisable to use AIBN. At higher temperatures, better results are obtained by using benzoyl peroxide, etc.

Table 5.1. Kinetic characteristics of some polymerization initiators

It is advisable to carry out the polymerization reaction at temperatures below 70°C using inorganic peroxides.

The duration of the initiation stage decreases as the amount of free radicals increases.

To increase the rate of decomposition of initiators, for example peroxides, “promoters” - reducing agents - are introduced into the reaction mixture. Redox initiating systems are widely used for the synthesis of various carbon chain polymers. Initiation of the polymerization process through the use of redox systems is characterized by a small temperature coefficient (relatively low apparent activation energy).

Thus, under the influence of physical or chemical factors, free radicals appear in the system, having, for example, unpaired p-electrons and, as a result, have high chemical activity. Collisions of free radicals lead to the formation 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 a diagram

R* + R* → R: R; R* + M → R: M*.

The tendency to undergo addition reactions limits the lifetime of free radicals. For example, the half-life of the H 3 C * radical is 10 -4 s. However, pairing the unpaired p-electron [for example, in triphenylmethyl (C 6 H 5) 3 C * ] or screening with its substituents included in the free radical, for example in diphenylpicrylhydrazyl

dramatically increases the stability of free radicals.

As a result of chemical initiation, the free radical becomes the terminal group of the growing polymer chain.

The time required for the initiation of a chain is called the induction period. Substances that increase the induction period are called inhibitors. Not all free radicals interact with monomers and initiate a reaction. Some of them are deactivated after a mutual collision. The ratio of the number of radicals attached to the monomer and initiating the reaction to the total number of all radicals formed is called the efficiency of the initiator f e. The effectiveness of the initiator can be assessed by one of three methods:

by comparing the rate of decomposition of the initiator and the rate of formation of polymer molecules (this technique requires accurate measurement of the average molecular weight of the polymer);

comparing the amount of initiator combined with the polymer with the amount of decomposed initiator;

using an inhibitor that breaks kinetic chains.

For example, the use of diphenylpicrylhydrazyl makes it possible to terminate the chain 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 in 20 minutes of reaction 80 cm 3 of nitrogen was released per 100 g of monomer (based on normal conditions). The degree of monomer conversion reached 5%. The molecular weight of the resulting polymer is 2500 (determined by the 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 initiator at the beginning of the reaction per 100 g of monomer:

[I] 0 = 1.1/164 = 0.007 = 7 10 -3.

The amount of nitrogen released will be

80/(22.4 1000) = 3.5 10 -3.

Thus, in 20 minutes of reaction, 3.5 × 10 -3 mol of initiator decomposed and, consequently, 7 × 10 -3 mol of radicals were formed. At a conversion rate of 5% and an average molecular weight of 2500, the number of moles of polymer formed is

5/2500 = 2 · 10 -3.

Let us assume that all kinetic chains ended in the recombination of radicals and, therefore, 1 mole of initiator was consumed per 1 mole of polymer. From here we find the efficiency of the initiator f e:

f e = 2.0 · 10 -3 /(3.5 · 10 -3) = 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 initiator, monomer, solvent, etc.

For example, when initiating the polymerization of acrylonitrile with dinitrile azodiisobutyric acid in dimethylformamide and a 51.5% aqueous solution of NaCNS, the value K d f e in the second case turns out to be significantly less due to the large manifestation of the “cage 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] - initiator concentration;

Where Kd is the decay rate constant of the initiator; TO p is the growth rate constant of the polymer chain; TO 0 - circuit break 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 the possible reason for this phenomenon.

Answer. Benzoyl peroxide is very slightly soluble in water. Therefore, the rate of initiation reaches a noticeable value only after the concentration of initiator particles in the dispersion is sufficiently large [see. equation (5.3)]. Azodiisobutyric acid dinitrile is better soluble in water, and 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 a kinetic chain are called elementary

stages of a chain reaction that occur with the preservation of free valence and lead to the consumption of starting substances and the formation of reaction products. During polymerization, this sequence of reactions causes the growth of the polymer chain:

Chain growth is a rapidly occurring stage of the polymerization process, described by equation (5.3). The rate of polymerization also increases with increasing monomer concentration in the reaction medium.

Circuit break. The termination of the kinetic chain is the stage of the chain process that leads to the disappearance of free valence. The kinetic chain can be broken:

as a result of recombination, i.e. interaction of two identical or different free radicals,

or disproportionation, i.e. transfer of a proton from one radical to another, with loss of 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 activation energy of disproportionation reaches values of 16-18 kJ/mol.

The cessation 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) meets 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 kinetic chain transfer (“radical otropy”). This process can result in 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 accordingly TO n i , K n s , K p m, K p p.

Question. In the process of free radical polymerization, branched macromolecules are formed along with linear macromolecules. 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:

Breaking the kinetic chain leads to a decrease in the degree of polymerization of the resulting high-molecular compound. Sometimes, to regulate the speed of the process and the molecular weight of the polymers, special substances (hydroquinone, nitrobenzene, etc.), called polymerization inhibitors, are added to 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 r and Vt- the rate of growth and chain breakage, respectively.

Using 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 maximum molecular weight are formed. As the degree of monomer conversion (polymer yield) increases, its molecular weight usually decreases. Explain the probable cause of this phenomenon.

Answer. As the degree of conversion increases, the number of growing kinetic chains in the reaction medium increases, which causes 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 certain by-products. However, if the kinetic chain length is sufficiently long, polymerization can be described with sufficient approximation by a single stoichiometric equation. Chain reaction speed v equal to the product of the chain initiation rate v i and length of the kinetic chain v:

At the same time v= (1 - β)/β, where β is the probability of chain breakage at each stage of growth. Kinetic chain length v can be calculated based on the relation

Task. Define value TO r/ TO
can be determined from the equation for the steady-state rate of polymerization, which well describes the process at its initial stage [equation (5.3)]. After transforming equations (5.3) and (5.4), we obtain
ln([M] 0 /[M] t) = (K p/ K
)V i t. In the presence of free radical scavengers, the process slows down (inhibits). If WITH ing is the inhibitor concentration, then the rate of initiation reaction can be calculated from the dependence
V i= C ing t i.

According to this empirical dependence, for any arbitrarily chosen inhibitor concentration (for example, 0.2 mol/dm 3), the corresponding value can be calculated t, and therefore the initiation rate:

t= 2 · 10 -5 + 2857 · 0.2 = 571 min;

V i= 1 · 10 -1 /571 = 5.83 · 10 -6 mol/(dm 3 s).

For two points in time ≥ t i you can calculate the value TO r/ TO
= = 0,25.
In accordance with equations (5.3) and (5.4) we have

Where f e - efficiency of the initiator; Kd- rate constant of decomposition of the initiator; [M] - monomer concentration; [I] is the concentration of the initiator.

It was noted earlier that the values f e and Kd can be measured separately. Also determined experimentally V p , [I], [M]. Having thus found 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 Δ E i, Δ E p and Δ E o are the apparent activation energies of the initiation, growth and chain termination stages, 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 length of the kinetic chain v under isothermal synthesis conditions is determined only by the nature of the monomer.

The polymerization reaction involves compounds that contain at least one multiple bond or rings. The reactivity of a monomer depends on its structure, the conjugation of the double bond in the monomer molecule, the number and relative arrangement of substituents, and their polarization phenomenon on the double bond.

Radical polymerization occurs via a chain mechanism and is described by the kinetics of an unbranched chain reaction.

The main stages of the chain reaction:

Initiation- formation of active centers;

Chain growth- sequential addition of monomers to the active center;

Open circuit- death of the active center;

Chain transmission- transfer of the active center to another molecule.

I. Chain initiation (nucleation)

This stage is the most energy-intensive. Distinguish physical And chemical initiation.

Physical initiation:

Chemical initiation

This initiation method is used most often. The principle is to use initiating substances(peroxides, azo compounds, red-ox systems), in which the energy of breaking a chemical bond is significantly less than that of monomers. In this case, the process occurs in two stages: first, initiator radicals are generated, which then join the monomer molecule, forming a primary monomer radical.

The initiator is very similar in properties to the catalyst, but its difference is that the initiator is expended during a chemical reaction, but a catalyst does not.

Examples of initiators:

II. Growth of the Chain

Monomers alternately attach to the active center of the primary monomer radical.

III. Open circuit

Chain termination occurs as a result of the death of active centers (kinetic chain termination).

Break in the kinetic chain- active centers disappear;

Break in the material chain- when a given chain stops growing, but the active center is transferred to another macromolecule or monomer (chain transfer reaction).

Reactions leading to the death of the kinetic and material chain - reactions recombination And disproportionation.

The type of chain termination reaction (recombination or disproportionation) depends on a number of factors, in particular on the structure of the monomer molecule. If the monomer contains a substituent that is bulky in size or electronegative in chemical nature, then such growing radicals do not collide with each other and chain termination occurs through disproportionation. For example, in the case of methyl methacrylate:

As the radicals grow, the viscosity of the system increases, and due to the mobility of macroradicals, the rate of chain termination by recombination decreases. An increase in the lifetime of macroradicals with an increase in the viscosity of the system leads to an interesting phenomenon - acceleration of polymerization at later stages ( gel effect) due to an increase in the concentration of macroradicals.

IV. Chain transmission

Chain transfer occurs by the detachment of an atom or group of atoms from a molecule by a growing radical. The chain transfer reaction leads to the break of the material chain, and the growth of the kinetic chain continues.

Chain transmissions are distinguished:

Features of radical polymerization:

High polymerization rate;

Branching;

Connections g-g, g-xv, xv-xv are possible;

Polymolecular polymers.

Kinetics of radical polymerization

Chemical kinetics is a branch of chemistry that studies the mechanism and patterns of a chemical reaction over time, and the dependence of these patterns on external conditions.

To study the kinetics of radical polymerization, it is necessary to consider the dependence of the reaction rate and degree of polymerization on the concentration of starting substances, pressure and temperature.

Designations:

I. The influence of the concentration of starting substances on the reaction rate.

The overall reaction rate depends on the rate of formation of radicals V in (rate of initiation), on the rate of chain growth V r and its termination V o.

We will consider the reaction of free radical polymerization, when initiation is carried out using chemical initiators.

Let's look at each stage:

Consideration of kinetics is greatly facilitated if the reaction occurs under conditions close to stationary mode, at which the rates of appearance and disappearance of free radicals can be considered equal. In this case, the concentration of active centers will be constant.

As can be seen from the curve graph, five sections can be distinguished according to the rates of the main reaction of converting a monomer into a polymer as a result of polymerization:

1 - inhibition site, 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 converting monomer into polymer begins, and the speed increases;

3 - stationary area, where polymerization of the main amount of monomer occurs at a constant speed (straight-line dependence of conversion on time);

4 - reaction slowdown section, where the reaction rate decreases due to a decrease in the free monomer content;

5 - cessation of the main reaction after exhaustion of the entire amount of monomer. The stationary mode is usually observed at the initial stage of the reaction, when the viscosity of the reaction mass is low and cases of chain nucleation and chain termination are equally likely.

Thus, the rate of the chain growth reaction is:

II. The influence of the concentration of starting substances on the degree of polymerization.

The degree of polymerization depends on the ratio of the growth and chain termination rates:

Let us take into account the corresponding expressions for speeds

The degree of polymerization is:

III. Effect of temperature on the rate of chain propagation reaction.

Let us substitute the Arrhenius equation into the chain growth rate equation:

Let us take the logarithm of the resulting expression:

The numerator (6+15-4 = 17) is greater than zero, which means that the higher the temperature, the higher the rate of radical polymerization reaction. However, as the temperature increases, the probability of radicals colliding with each other (chain termination by disproportionation or recombination) or with low molecular weight impurities also increases. As a result, the molecular weight of the polymer as a whole decreases, 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. 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.

Growth activation energy ~ 6 kcal/mol;

Initiation activation energy ~30 kcal/mol;

The termination activation energy is ~8 kcal/mol.

The numerator (6-15-4 = -13) is less than zero, which means that with increasing temperature the degree of polymerization decreases. As a result, the molecular weight of the polymer as a whole decreases, and the proportion of low molecular weight fractions in the polymer increases.

V. Effect of pressure on the polymerization rate

Le Chatelier's principle: If a system is exposed to an external influence, then processes are activated in the system that weaken this influence.

The higher the pressure, the higher the rate of radical polymerization. However, to influence the properties of condensed systems, pressure of several thousand atmospheres must be applied.

A feature of polymerization under pressure is that the increase in speed is not accompanied by a decrease in the molecular weight of the resulting polymer.

Polymerization inhibitors and retarders.

The phenomena of open circuit and transmission are widely used in practice for:

preventing premature polymerization during storage of monomers;

to regulate the polymerization process

In the first case, they add to the monomers inhibitors or stabilizers, which cause chain termination and themselves turn into compounds that are unable to initiate polymerization. They also destroy peroxides formed when the monomer reacts with atmospheric oxygen.

Inhibitors: quinones, aromatic amines, nitro compounds, phenols.

Regulators polymerization causes premature termination of the material chain, reducing the molecular weight of the polymer in proportion to the amount of regulator introduced. An example of these are mercaptans.

Thermodynamics of radical polymerization

The chain growth reaction is reversible; along with the addition of the monomer to the active center, its elimination-depolymerization can also occur.

The thermodynamic possibility of polymerization, like any other equilibrium chemical process, can be described using the Gibbs and Helmholtz functions:

However, the Gibbs function is closest to real conditions, so we will use it:

Also, the change in the Gibbs function is related to the equilibrium constant of the reaction by the equation:

The constant of polymerization-depolymerization equilibrium at a sufficiently large molecular weight of the resulting polymer (p>>1) depends only on the equilibrium concentration of the monomer:

Whence it follows that

From equation (a) you can find the temperature at which the polymerization reaction will not occur, and from equation (b) you can find the equilibrium concentration of the monomer, above which polymerization will occur.

Effect of temperature

To determine the effect of temperature on the equilibrium concentration, we present equation (b) as follows:

In the case where ΔH°<0 и ΔS°<0 с ростом температуры увеличивается равновесная концентрация мономера. Верхний предел ограничен концентрацией мономера в массе. Это значит, что есть некоторая верхняя предельная температура - Т в.пр. , выше которой полимеризация невозможна.

In the case when ΔH°>0 and ΔS°>0 an inverse relationship is observed: with decreasing temperature, the equilibrium concentration of the monomer increases. Consequently, for monomers with a negative thermal effect there is a lower limiting temperature T n.a.

There are also known cases when these dependencies do not intersect, but they are not of practical interest.

Thermodynamic probability

Now consider the thermodynamic possibility of a reaction occurring, the condition for which is the equality ΔG<0. Оно определяется как изменением энтальпии так и энтропии, причем вклад энтропийного члена будет изменяться с температурой реакции.

During polymerization along multiple bonds, the entropy of the system always decreases, i.e. the process is unprofitable for entropic reasons. The weak dependence of ∆S° on the nature of the monomer is due to the fact that the main contribution to ∆S° comes from the loss of translational degrees of freedom of the monomer molecules.

But monomers are also known for which an increase in entropy occurs during polymerization. This change in ∆S° is typical for some unstressed cycles. Moreover, since polymerization turns out to be beneficial from an entropic point of view, it can occur even with negative thermal effects (polymerization of the S 8 and Se 8 cycles with the formation of linear polymers)

Calculations and entropy measurements for the polymerization of most vinyl monomers show that ∆S° is about 120 J/K mol.

On the contrary, ∆Н° varies depending on the chemical structure of the monomer over a fairly wide range (∆Q° = −∆Н° varies from several kJ/mol to 100 kJ/mol), which is due to the difference in the nature of the multiple bond and its substituents. Negative values of ∆Н° indicate that polymerization is beneficial from the point of view of the enthalpy factor. At ordinary temperatures of the order of 25°C, polymerization is thermodynamically resolvable for monomers whose thermal effect exceeds 40 kJ/mol. This condition is met for most vinyl monomers. However, during polymerization at the C=O bond, the thermal effects are below 40 kJ/mol. Therefore, the condition ∆G<0 соблюдается только при достаточно низких температурах, когда |TΔS°|<|ΔH°|.

Let us consider the phenomenon of discrepancy between the theoretical and practical enthalpy of polymerization

Less energy is released, where does it go?

The coupling effect is destroyed;

Steric repulsion (during the synthesis of polystyrene, a helical molecule is formed due to steric repulsion).

The reason for the increase in Q during the polymerization of rings is the thermodynamically unfavorable bond angle between hybridized orbitals and the repulsion of lone electron pairs of the substituent.

Cycle opening (ΔS 1° > 0)

Chain growth (ΔS 2°< 0)

ΔS° = ΔS 1° + ΔS 2°, ΔS° can be greater or less than zero.

METHODS OF SYNTHESIS OF BMC

Synthetic IUDs are obtained as a result of 2 types of reactions - polymerization And polycondensation.

POLYMERIZATION – the reaction of joining monomer molecules, which occurs due to the breaking of multiple bonds and is not accompanied by the release of by-products of low molecular weight substances (H 2 O, HCl, NH 3, etc.). Polymerization of monomers occurs via a chain mechanism. Unsaturated monomers with a double bond enter the polymerization reaction. Polymerization in which molecules of one substance enter is called homopolymerization:

n H 2 C = CH 2 ¾® [¾CH 2 ¾CH 2 ¾] n nH 2 C=O ¾®[¾CH 2 ¾O¾] n

formaldehyde polyformaldehyde

If different monomers enter into a polymerization reaction, it is called copolymerization, for example copolymerization of styrene with methyl methacrylate:

The polymerization reaction does not lead to a change in the elemental composition of the monomer. Like any chemical reaction, polymerization begins with the breaking of some chemical bonds and the formation of others. Such rupture can occur by a hetero- or homolytic mechanism. In the first case, ions are formed, in the second - free radicals. Thus, radical and ionic polymerization differ in the nature of the active center, which begins and leads the macromolecular chain. Polymerization that occurs through the formation of ions is called ionic polymerization (cationic or anionic), and that occurs with the participation of free radicals is called radical.

RADICAL POLYMERIZATION –

the process of polymer formation by a free radical mechanism with the sequential addition of monomer molecules to the growing macroradical. In this case, the active center is a carb radical, i.e. a carbon atom with 1 unpaired electron. Such a radical easily takes away one of the electrons of the p-bond and forms a pair of electrons, i.e. new s-connection:

The radical located at the end of the growing chain is called a growing radical. The diagram shows that the addition of the monomer to the growth radical is accompanied by regeneration of the active center at the end of the chain. The sequence of chemical events excited by one active particle is called a kinetic chain. Like any chain process, the radical polymerization reaction consists of 3 stages: initiation, chain growth and chain termination.

Initiation of radical polymerization

The initiation reaction includes 2 sequential acts: the formation of primary free radicals and the addition of radicals to monomers: I ¾® 2R ·

R + CH 2 =CHX ¾® RCH 2 ¾C HX

The rate of the first reaction is much less than the rate of the second, so it determines the rate of the initiation stage. Depending on the method of formation of free radicals, several types of initiation are distinguished: material, photochemical, radiochemical and thermal.

Real initiation. It uses substances that break down to form free radicals. These compounds contain unstable chemical bonds in their molecules (O¾O, N¾N, S¾S, O¾N, etc.). Peroxides and azo compounds are used as such substances. Among peroxides, acyl-, alkyl-, hydroperoxides and peresters are widely used. The most famous among azo compounds is isobutyronitrile, which decomposes to release nitrogen:

Due to the latter circumstance, it is used in industry not only as an initiator, but also for foaming plastics in the production of foam plastics.

Photochemical initiation. When the monomer is irradiated with UV light, the molecules that have absorbed the light quantum are excited and decompose into radicals that initiate polymerization:

M+ hv¾® M* ¾® R 1 + R 2

However, direct irradiation of the monomer is ineffective, because most monomers do not absorb UV light. In this case, a photosensitizer (Z) is used - a compound that transfers excitation energy to other molecules: Z + hv¾® Z*,

Z* + M ¾® Z + M* ¾® R 1 + R 2 + Z

The most effective photoinitiators are aromatic ketones and their derivatives, due to the wide absorption range of the UV spectrum.

Photopolymerization is used for applying polymer coatings to metal, wood, ceramics, in dentistry for curing dental filling compositions, in photolithography, which is used to produce integrated circuits in microelectronics, as well as printed circuit boards (matrices) in modern phototypesetting technology, which eliminates the use of lead.

The disadvantage of photoinitiation is the rapid decline in its efficiency with increasing thickness of the irradiated layer. Therefore, photochemical initiation is effective in inducing polymerization in thin layers, on the order of several millimeters.

Radiochemical initiation. Unlike photoradiation, radioactive radiation is ionizing and has a much greater penetrating ability, which is explained by the greater energy of its particles (a-particles, neutrons, electrons, hard electromagnetic radiation and radiation from radioactive Co 60 sources). Ionization of a monomer is a consequence of knocking out electrons from its molecules by high-energy particles: M + radiation ¾® M + · + ē

Thermal initiation. There are very few examples of this process (polymerization of styrene and vinyl pyridines).

Chain growth

The chain growth reaction consists of repeated addition of monomer molecules to a radical while retaining a free electron at the terminal link of the growing macromolecule:

RCH 2 ¾C HX+ CH 2 =CHX ¾® RCH 2 ¾CHX¾CH 2 ¾C HX

The growth radical attacks the methylene group of the double bond, i.e. "tail" of the monomer. This order of addition is called “head” (radical) to “tail” (monomer).

Open circuit

is associated with the disappearance of a free electron at the final link of a macromolecule:

Free radicals interact not only with monomers and the resulting macromolecules, but also with the solvent and impurities. Such reactions are called chain transfer reactions. In this case, the active center can transfer to any molecule, for example, a solvent molecule, which, turning into a radical, gives rise to a new macromolecule:

In this case, chain transfer occurs through a solvent - carbon tetrachloride. In this case, the rate of the polymerization reaction does not decrease, but the degree of polymerization of the resulting polymer decreases. Therefore, by changing the ratio of the amount of monomer and solvent, it is possible to obtain polymers with different molecular weights.

IONIC POLYMERIZATION

proceeds with the formation of either a carbonium ion or a carbanion, followed by the transfer of “+” or “-” charge along the growing chain. Depending on this, a distinction is made between cationic (carbonium) and anionic (carbanionic) polymerization. Ionic polymerization takes place in the presence of catalysts that promote the formation of ions. Therefore it is also called catalytic polymerization.

Lecture outline:

1. Radical polymerization.

2. Ionic polymerization

The vast majority of high-molecular compounds are obtained as a result of polymerization and polycondensation reactions.

Polymerization

Polymerization is a process for producing polymers in which the construction of macromolecules occurs by sequential addition of molecules of a low molecular weight substance (monomer) to the active center located at the end of the growing chain. For polymerization, the stages of initiation and chain growth are mandatory.

Initiation - This is the transformation of a small fraction of monomer molecules M into active centers AM*, capable of attaching new monomer molecules. For this purpose, pathogens are introduced into the system ( initiators I or catalysts) polymerization. The initiation of polymerization can be represented as follows:

If one monomer participates in polymerization, then we get homopolymers, if two or more then copolymers. Depending on the nature of the active center, there are radical And ionic polymerization And copolymerization.

Radical polymerization

Radical polymerization always occurs via 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, chloroprenide).

Radical polymerization is characterized by all the signs of chain reactions known in the chemistry of low-molecular compounds (for example, the interaction of chlorine and hydrogen in light). Such signs are: the sharp influence of a small amount of impurities on the speed of the process, the presence of an induction period and the course 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 until the formation of a polymer macromolecule.

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

Reaction (1b) proceeds many times faster than reaction (1a). Therefore, the rate of initiation of polymerization 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 physical impact per monomer upon 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 material initiation.

Thermal initiation is self-initiation at high temperatures of polymerization of pure monomers without introducing 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, so-called block polystyrene is obtained in this way. However, the method of thermal initiation of polymerization has not found widespread use, since it requires large amounts of thermal energy, and the rate of polymerization in most cases is low. It can be increased by increasing the temperature, but this reduces the molecular weight of the resulting polymer.

Photoinitiation 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 goes into an excited energy state. Colliding with another monomer molecule, it is deactivated, transferring part of its energy to the latter, and both molecules turn into free radicals. The rate of photopolymerization increases with increasing irradiation intensity and, unlike thermal polymerization, does not depend on temperature.

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

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

Chemical initiation is carried out by introducing into the monomer medium low-molecular unstable substances containing low-energy bonds - initiators that easily decompose into free radicals under the influence of heat or light. The most common initiators of radical polymerization 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.

Peroxide tert-butyl(alkyl peroxide):

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

Effective initiators that allow the radical polymerization process to be carried out at room and low temperatures are redox systems. Peroxides, hydroperoxides, persulfates, etc. are usually used as oxidizing agents. Reducing agents are metal salts of variable valency (Fe, Co, Cu) in the lowest oxidation state, sulfites, amines, etc.

Self-test questions:

1. What substances are the initiators of radical polymerization?

2. What does the initiation of radical polymerization come down to?

3. Types of initiation.

4. What is polymerization?

Lecture 6. Copolymerization.

Lecture outline:

1.Copolymerization

2. Technical methods for carrying out homo- and copolymerization.

Copolymerization

Copolymerization is the production of high molecular weight substances from a mixture of two or more monomers, which are called comonomers, and the substance itself - copolymer. Macromolecules of copolymers consist of elementary units of all monomers present in the initial reaction mixture. Each comonomer imparts its own properties to the copolymer it is part of, and the properties of the copolymer are not a simple sum of the properties of the individual homopolymers. Thus, the content of a small amount of styrene in polyvinyl acetate chains increases the glass transition temperature of the latter, eliminates the property of cold flow and increases its surface hardness.

The laws of copolymerization are much more complex than the laws of homopolymerization. If in homopolymerization there is one type of growing radical and one monomer, then in binary copolymerization, which involves only two monomers, there are at least four types of growing radicals. Indeed, if two monomers A and B interact with free radicals R" generated during the decomposition of the initiator, primary radicals are formed, one of which has a terminal unit A, and the second - B:

Each primary radical can react with both monomer A and monomer B:

The ratio of the rate constant of the reaction of each radical with its “own” monomer to the rate constant of the reaction with the “foreign” monomer is called copolymerization constants or relative activities monomers:

The values of r A and r B determine the composition of the macromolecules of the copolymer to a greater extent than the ratio of monomers in the initial reaction mixture. For example, in a vinyl acetate (A)-styrene (B) pair, the copolymerization constants are r A = 0.01, r B = 55. This means that when a copolymer is produced by polymerization in bulk and solvent, the macromolecules contain significantly more styrene units than vinyl acetate. If the relative activities of comonomers are close to unity, then each radical interacts with both “its own” and “foreign” monomer with equal probability. The inclusion of monomers in the chain is random in nature, and statistical copolymer. This copolymerization is called perfect. An example of a system close to ideal is the butadiene-styrene pair.

Copolymerization reactions can occur by both radical and ionic mechanisms. In ionic copolymerization, the copolymerization constants are influenced by the nature of the catalyst and solvent. Therefore, copolymers obtained from the same comonomers at the same initial ratio in the presence of different catalysts have different chemical compositions. Thus, a copolymer of styrene and acrylonitrile, synthesized from an equimolar mixture of monomers in the presence of benzoyl peroxide, contains 58% styrene units. At the same time, during anionic copolymerization on a C 6 H 5 MgBr catalyst, the content of styrene units in macromolecules is 1%, and during cationic polymerization in the presence of SnCl 4 - 99%.

In practical terms, interesting block- And vaccinated copolymers. In the macromolecules of these copolymers there are long sections of units of each comonomer.

Block copolymers are prepared by different methods. Firstly, during anionic polymerization of one monomer, the resulting “living” chains, that is, macroanions, can initiate the polymerization of another monomer:

Secondly, with intense mechanical action on a mixture of different polymers, chain destruction occurs and macroradicals are formed. Macroradicals interact with each other to form a block copolymer.

Block copolymers can also be formed from oligomers due to the interaction of end groups.

Graft copolymers are usually obtained by the interaction of a monomer with a polymer and, less commonly, by the interaction of two different polymers with each other. Since these processes use a chain transfer reaction with the conversion of polymer molecules into macroradicals, atoms or groups with increased mobility (for example, bromine) are often introduced into the macromolecules, which accelerates the value transfer reaction. Thus, if the reaction medium contains a polymer based on the CH 2 =CHX monomer, the CH 2 =CHY monomer and an initiator, the process of formation of the graft copolymer proceeds as follows. First, the middle macroradical appears:

This macroradical then initiates polymerization of the monomer to form side branches:

The production of block and graft copolymers is almost always accompanied by the formation of a tomopolymer from the monomer present in the reaction zone.