The kinetics of radical polymerization is generally very complex; the thing is that she heterogeneous; the kinetic characteristics of the system change quite significantly with increasing process depth. The reason, first of all, is that with an increase in the degree of monomer conversion, the viscosity of the system usually increases significantly and the diffusion rate of large molecules decreases (gel effect, see below). In addition, as the polymer accumulates, the likelihood of chain transfer to the polymer increases, complicating the picture.

However, when low degrees of monomer conversion(not higher than 10%) the kinetics of the process is quite simple; On its basis, quite definite conclusions can be drawn. Next, this option will be considered - kinetics at shallow process depths(it can be called the elementary kinetics of radical polymerization).

Let us first consider the simplest case, when chain transfer reactions can be neglected; This case is real if there are no impurities in the reaction mixture to which transfer can occur and if the monomer is not allylic (then chain transfer reactions to the monomer can be neglected). In this case, we can assume that only initiation, chain growth, and chain termination reactions occur.

where v and is the initiation rate, [I] is the concentration of the initiator, k and is the initiation rate constant, f is the efficiency of the initiator (p. 15); the factor 2 reflects the formation of two radicals from the initiator molecule (the most common option)

Chain growth rate can be expressed by the equation:

where vр is the chain growth rate, kр is the chain growth rate constant, [M] is the monomer concentration, and is the concentration of radicals (“living” chains).

This equation reflects that any chain growth reaction is the interaction of a radical with a monomer (p. 15). It is valid under the assumption that the growth constant kp does not depend on the value of the radical R (this assumption is correct).

Open circuit speed expressed by the equation:

where v o is the chain break rate, k o is the chain break rate constant

This equation reflects that termination occurs during interaction two radicals (“living” chains) (p. 16).

Overall polymerization rate is the rate of monomer consumption (– d[M]/dt) and, therefore, it is equal to the rate of chain growth

The chain growth rate equation involves the concentration of radicals, which is difficult to measure. However, the concentration of radicals can be excluded from the growth rate equation if we assume that during the process the concentration of radicals is constant. This assumption is called condition of quasi-stationarity; at the initial stages of the process (at shallow depths) it works well. With this assumption the rate of formation of radicals is equal to the rate of their disappearance. Since radicals are formed at the initiation stage and disappear at the termination stage, the rates of these reactions are equal, i.e. v and = v o, i.e.:

Thus , the polymerization rate is proportional to the monomer concentration and the square root of the initiator concentration.

(which determines the molecular weight of the polymer) is, to a first approximation, equal to the length of the kinetic chain (p. 17), i.e. the ratio of the rates of chain growth and chain termination reactions:

Thus, the molecular weight of the polymer is proportional to the monomer concentration and inversely proportional to the square root of the initiator concentration.

So, an increase in the monomer concentration leads to an increase in both the polymerization rate and the molecular weight of the polymer, while an increase in the initiator concentration, increasing the rate of the process, reduces the molecular weight. The latter is not difficult to understand and purely qualitatively, because As the concentration of the initiator increases, the concentration of growing chains also increases, which increases the probability of their meeting and chain breakage.

Now let’s complicate the system somewhat and take into account chain transfer reactions (except for transfer to a “dead” polymer, so we continue to consider the kinetics at small depths of polymerization). Typically, chain transfer reactions to foreign molecules, primarily regulators, are of greatest importance; Let's limit ourselves to this type of transmission.

As already indicated, transferring the circuit to the regulator does not affect speed process. Medium degree of polymerization(P r) in this case is equal (to a first approximation) to the ratio of the chain growth rate to sum of speeds break and transmission of the chain (since during transmission they break molecular chains):

The above analysis of elementary kinetics made it possible to determine dependence of the polymerization rate and molecular weight of the polymer on the concentration of the monomer and initiator, and for molecular weight - also on the concentration of the regulator(if present). In addition, the progress and results of polymerization are influenced by a number of other factors, which are discussed below.

Effect of temperature. A.In the most common polymerization option with the participation of initiators an increase in temperature leads to increase polymerization rates decrease molecular weight of the polymer. The increase in speed needs no comment; the decrease in molecular weight is due to the fact that with increasing temperature the rate of initiation increases to a greater extent than the rate of chain growth(since initiation has a higher activation energy). Consequently, according to the condition of quasi-stationarity, the rate of chain termination increases faster than the growth rate, i.e. the ratio v p / v o decreases, and, consequently, the molecular weight decreases.

B. When photochemical initiation with increasing temperature both the speed of the process and the molecular weight of the polymer increase. This is due to the fact that with increasing temperature the rate of photochemical initiation remains virtually unchanged, while the rate of chain growth increases.

Other consequences of increasing temperature (for all polymerization options): 1) increasing temperature reduces the regularity of the structure of polymer macromolecules, because at the same time, the probability of articulation of elementary links according to the “tail to tail” and “head to head” schemes increases (p. 16); 2) Polymerization of vinyl monomers (and dienes) - reaction exothermic(see below); therefore, as the temperature increases, the equilibrium monomer Û polymer moves left; in other words, the role of reactions is growing depolymerization. All this does not allow radical polymerization to be carried out with any efficiency at temperatures above 120 o C.

Effect of pressure. Effect of pressure (P) on speed any chemical reaction is expressed by the Evans–Polyani equation:

where k is the reaction rate constant, ΔV ≠ is the change in volume during the formation of an activated complex (transition state) from reacting particles.

During radical polymerization at the stage chain growthΔV ≠< 0, т.к. реакции роста цепи – bimolecular, and in such reactions the volume decreases during the formation of the transition state; therefore, with increasing pressure the speed chain growth(and, therefore, polymerization in general) increases. On the contrary, for the reaction initiationΔV ≠ > 0, because here the limiting stage is the decay of the initiator - monomolecular reaction, and in such reactions, when a transition state is formed, the volume increases. Consequently, with increasing pressure, the initiation rate, and hence the speed open circuit(according to the condition of quasi-stationarity) decreases. Thus, growing ratio v p /v o , i.e. . polymer molecular weight.

Polymerization at high pressures (about 1000 atm) is used for ethylene (high-density polyethylene is formed).

Influence of process depth(degree of monomer conversion).

The influence of this factor is the most complex and strongly depends on other conditions of the process.

A. In most cases, when small process depths (up to approximately 10%) process speed and molecular weight of the polymer practically do not change. However, as the depth of the process increases, it is observed an increase in both the speed of the process and the molecular weight of the polymer. This may seem unexpected at first glance, because... with increasing degree of monomer conversion, its concentration decreases, which, according to the above kinetic equations (p. 24), should lead to a decrease in both speed and molecular weight. However, here the kinetics are completely different; in particular, the quasi-stationary condition does not apply. The fact is that as polymer macromolecules accumulate, they quickly the viscosity of the system increases(polymer solutions, as is known, have extremely high viscosity, and the higher their concentration and the molecular weight of the polymer, the higher their viscosity). An increase in viscosity leads to a sharp decrease mobility large particles, in particular, "living chains", and, therefore, the probabilities their meetings, i.e. open circuit(chain termination becomes a diffusion-controlled process). At the same time, the mobility of small particles (monomer molecules) is maintained over a fairly wide range of system viscosity, so that the rate of chain growth does not change. A sharp increase in the v p /v o ratio leads to a significant increase in the molecular weight of the polymer. The rate of decomposition of the initiator, as a monomolecular reaction, does not depend on viscosity, i.e. the rate of formation of radicals is higher than the rate of their disappearance, the concentration of radicals increases, and the quasi-stationarity condition is not met.

The changes discussed above associated with an increase in viscosity are called gel effect(sometimes also called the Tromsdorff effect). With a further increase in the depth of the process, the viscosity can increase so much that small particles also lose mobility; this leads to a slowdown in the chain growth reaction, and then to its complete stop, i.e. to stop polymerization. The gel effect is especially pronounced during block polymerization (polymerization of pure monomer); It also manifests itself to a sufficient extent during polymerization in fairly concentrated solutions.

B. If polymerization is carried out in highly dilute solutions and polymers with a relatively low molecular weight are formed, or if the resulting polymer falls out of solution, then the viscosity changes little during the process; in this case, the gel effect is not observed, the speed of the process and the molecular weight of the polymer change little.

In relatively recent times, polymerization processes in the presence of specific initiators have been studied; wherein the molecular weight of the polymer increases relatively uniformly with increasing process depth.

These specific initiators are di- or polyperoxides and iniferters.

The first of them contain two or more peroxide groups in the molecule. When using these initiators, the process proceeds as follows (using the example of an initiator with two peroxide groups):

After the decomposition of such a bis-peroxide, radicals are formed, one of which (16) contains a peroxide group. Radical (16) initiates the growth of the polymer chain; then the chain terminates upon interaction with another “living” chain (indicated in the diagram as R~) and a “dead” polymer is formed (17). This polymer contains a labile peroxide group; under the conditions of the process, this group disintegrates, forming a polymer radical (18), which begins to “complete construction” by reacting with monomer molecules; the situation may repeat itself later. Thus, as the process progresses, the size of macromolecules constantly increases.

Iniferters – peculiar connections that are not only initiators, but also actively participate in the processes transfers chains and cliff chains; hence their name, combined from some letters of the English names of these reactions ( Ini tiation – initiation, Trans fer– transmission, Ter mination - open circuit). The main feature of these initiators: upon decomposition they form two radicals, from which only one active, and second - inactive– it cannot initiate the growth of the polymer chain.

One such inferter is S-benzyl-N,N-diethyldithiourea (19). In its presence the following reactions occur:

Iniferter (19) decomposes to form active radical (20) and inactive radical (21). Radical (20) initiates the growth of the polymer chain. A growing “living” chain can: A) transfer the chain to the initiator; B) terminate by recombination with an inactive radical (21); such recombination is quite probable because inactive radicals can accumulate in quite significant concentrations. Both during transfer and during termination, the “living” chain turns into the same “dead” polymer (22), which contains labile terminal units ~CH 2 -CH(X)-S(C=S)-NEt 2 ; these units easily dissociate into radicals through a reverse recombination reaction, and the “dead” polymer “comes to life” again and is capable of further growth. Therefore, here too the molecular weight increases with increasing conversion depth.

Polymerization processes in the presence of polyperoxides and iniferters make it possible to obtain polymers with lower degree of polydispersity than processes in the presence of ordinary initiators; this has a positive effect on their technical properties.

Effect of preliminary orientation of monomer molecules. It is known that the collision of reacting particles will be effective if they are oriented in a certain way. If the monomer molecules before the start of polymerization linear oriented relative to each other:

then the chain growth rate should increase significantly, because in each growth reaction, the radical is oriented exactly to the “head” of the monomer, and almost every radical-monomer collision will be effective (the value of factor A in the Arrhenius equation increases). The rate of chain termination does not increase, so not only the rate of polymerization increases, but also the molecular weight of the polymer.

Preliminary orientation of monomer molecules can be achieved, for example, during polymerization in inclusion compounds (clathrates), when the monomer molecules are linearly oriented in the crystal channels of the “host” compound. Other options are solid-phase polymerization of single crystals of some monomers or polymerization in monomolecular layers at the interface; these options will be discussed later, in the section “Practical methods for carrying out polymerization”

Radical copolymerization

All the patterns described above were examined using examples of polymerization one monomer (homopolymerization). But, as is known, it is widely used copolymerization– joint polymerization of two or three monomers. It is carried out to obtain polymers with a wider range of properties, to obtain materials with predetermined properties, as well as in fundamental research to determine the reactivity of monomers. The copolymerization products are copolymers.

Basically the mechanism of radical copolymerization is quite similar to the mechanism of radical homopolymerization. However, there are several problems here.

1) Opportunity copolymerization - will units of both (or three) polymers be included in the polymer chain, or will each monomer be polymerized separately and a mixture of homopolymers will be formed?

2) The relationship between the composition copolymer and composition taken for the process mixtures of monomers. What is meant here is differential copolymer composition, i.e. its composition At the moment(if we take the integral composition, i.e. the composition of the entire mass of the copolymer, then it is clear that at a large depth of the process it will approximately coincide with the composition of the mixture of monomers, however, at different depths of the process macromolecules with different ratios of monomer units can be formed).

If the differential composition of the copolymer matches with the composition of the monomer mixture taken for polymerization, then copolymerization is called azeotropic. Unfortunately, cases of azeotropic copolymerization are quite rare; in most cases the differential composition of the copolymer is different on the composition of the monomer mixture. This means that during the polymerization process, monomers are not consumed in the same proportion as they were taken; one of them is consumed faster than the other, and must be added as the reaction progresses to maintain a constant composition of the monomer mixture. From here it is clear how important it is not only quality, but also quantitative solution to this problem.

3) The nature of the structure of the resulting copolymer, i.e. whether a random, alternating or block copolymer is formed (see pages 7-8).

The solution to all these problems follows from the analysis kinetics formation of a copolymer macromolecule, i.e. stages chain growth during copolymerization (since the copolymer macromolecule is formed precisely at this stage).

Let us consider the simplest case of copolymerization two monomers, conventionally designated by the symbols A and B. The chain growth stage in this case, in contrast to homopolymerization, includes elementary reactions of not one, but four types: indeed, during growth, “living” chains of two types are formed - with the terminal radical unit of monomer A [~A, for example, ~CH 2 –CH(X) ] and with the terminal radical unit of monomer B [~B, for example, ~CH 2 –CH(Y) ] and each of them can attach to “its own” and “foreign” monomer:

The differential composition of the copolymer depends on the ratio of the rates of these four reactions, the rate constants of which are designated as k 11 ...k 21.

Monomer A is included in the copolymer according to reactions 1) and 4); therefore, the rate of consumption of this monomer is equal to the sum of the rates of these reactions:

This equation includes difficult-to-determine concentrations of radicals. They can be eliminated from the equation by introducing quasi-stationary condition: concentrations both types radicals (~A and ~B) permanent; as in homopolymerization, the quasi-stationary condition is satisfied only at shallow process depths. From this condition it follows that the rates of mutual transformation of both types of radicals are the same. Since such transformations occur via reactions 2 and 4, then:
This equation is called Mayo-Lewis equations(sometimes called Mayo's equation). This equation reflects the dependence of the differential composition of the copolymer on the composition of the monomer mixture and on the values ​​of r 1 and r 2. The parameters r 1 and r 2 are called copolymerization constants. The physical meaning of these constants follows from their definition: each of them expresses comparative activity of each radical in relation to “its own” and “foreign” monomer(constant r 1 – for radical ~A, constant r 2 – for radical ~B). If a radical attaches more easily to “its” monomer than to a “foreign” one, r i > 1, if it is easier to attach to a “foreign” one, r i< 1. Иначе говоря, константы сополимеризации характеризуют comparative reactivity of monomers.

The left side of the Mayo-Lewis equation is the differential composition of the copolymer. On the right side, two factors can be distinguished: 1) composition of the monomer mixture [A]/[B]; 2) a factor including the copolymerization constants r 1 [A] + [B]/r 2 [B] + [A] = D (we denote it by D). It is easy to see that for D=1 d[A]/d[B] = [A]/[B], i.e. copolymerization is azeotropic. As mentioned above, cases of azeotropic copolymerization are quite rare, i.e. in most cases, D ≠ 1. Thus, the factor D is the factor that determines the difference between the differential composition of the copolymer and the composition of the mixture of monomers. If D > 1, then the copolymer is enriched in monomer A compared to the original mixture (i.e., monomer A is consumed in a greater proportion than monomer B). At D< 1, напротив, быстрее расходуется мономер В.

The value of the factor D is completely determined by the values ​​of the copolymerization constants; therefore it is copolymerization constants determine the ratio of the differential composition of the copolymer and the composition of the mixture of monomers taken for the reaction.

Knowing the values ​​of copolymerization constants also allows one to judge the structure of the resulting copolymer, as well as the possibility or impossibility of copolymerization itself.

Let us consider the main options for copolymerization, determined by the values ​​of copolymerization constants. It is convenient to present them graphically in the form of curves of the dependence of the differential composition of the copolymer on the composition of the mixture of monomers taken for the reaction (Fig. 3).

Rice. 3. Dependence of the differential composition of the copolymer on the composition of the monomer mixture.

1. r 1 = r 2 = 1. In this case d[A]/d[B] = [A]/[B], i.e. at any composition of a mixture of monomers occurs azeotropic copolymerization. This is a rare option. Graphically it is expressed by the dotted line 1 – azeotrope line. An example of such a system is the copolymerization of tetrafluoroethylene with chlorotrifluoroethylene at 60 0 C.

2. r 1< 1, r 2 < 1 . Both constants are less than one. This means that each radical preferentially reacts with strangers monomer, i.e. we can talk about an increased tendency of monomers to copolymerize.

A) Copolymer composition. Differential copolymer composition enriched with the monomer that is low in the mixture of monomers(curve 2 in Fig. 3). This is easy to deduce from the analysis of the factor D in the Mayo-Lewis equation: for [A]<< [B] D < 1, следовательно, d[A]/d[B] < , а при [B] << [A] D >1 and d[A]/d[B] > . Curve 2 intersects the azeotrope line, i.e. at some one In the ratio of monomers, polymerization is azeotropic. This ratio is easy to calculate, because in this case D = 1; from here:

B) Copolymer structure. Since each radical preferentially attaches to to someone else's monomer, in the copolymer there is a tendency towards alternation. If the copolymerization constants are not much less than unity, this tendency is not very pronounced, and the copolymer is closer to random than to alternating [the microheterogeneity coefficient K M (p. 7) is closer to 1 than to 2]. But the smaller the constants, the more the polymer structure approaches alternating. The limiting case is an infinitesimal value of both constants (r 1 → 0, r 2 → 0); this means that each radical reacts only with a “foreign” monomer, in other words, each of the monomers separately does not polymerize, but together they form a copolymer. Naturally, such a copolymer has a strictly alternating structure. An example of such a system is the pair: 1,2-diphenylethylene - maleic anhydride. There are also cases when one of the constants is infinitesimal, and the other has a finite value; in such cases, only one of the monomers does not itself polymerize, but can form a copolymer with a second partner. An example of such a system is styrene-maleic anhydride.

3. r 1 > 1, r 2< 1 или r 1 < 1, r 2 > 1 . One of the constants is greater than one, the other is less than one, i.e. one of the monomers reacts more easily with its “own” monomer, and the second with a “foreign” one. It means that one monomer is more active than the other during copolymerization, because reacts more easily than others both radicals. Therefore, when any composition of the monomer mixture, the differential composition of the copolymer is enriched with units of the more active monomer (in Fig. 3 – curves 3’ for r 1 > 1, r 2< 1 и 3’’ для r 1 < 1, r 2 >1). Azeotropic polymerization is not possible here.

The structure of copolymer macromolecules in this variant is closest to statistical. A special (and not so rare) case: r 1 ×r 2 = 1, i.e. r 1 = 1/r 2 , while the values ​​of the constants are not much more or less than one. This means that the comparative activity of monomers towards both radicals is the same(for example, at r 1 = 2, r 2 = 0.5, monomer A is 2 times more active than monomer B in reactions with both the radical ~A▪ and the radical ~B▪). In this case, the ability of each monomer to enter the polymer chain does not depend on the nature of the radical, which he encounters and is determined simply probability clashes with each of the radicals. Therefore, the structure of the copolymer will be purely statistical (K M ~ 1). This case is called perfect copolymerization- not at all because in this case a copolymer with ideal properties is formed (rather the opposite), but by analogy with the concept of an ideal gas, where, as is known, the distribution of particles is completely statistical. The most famous examples of such copolymerization include the copolymerization of butadiene with styrene at 60 o C (r 1 = 1.39, r 2 = 0.78). In the general case, the option “one constant is greater than one, the other is less” is perhaps the most common.

4. r 1 > 1, r 2 > 1. Both constants are greater than one; each of the radicals preferentially reacts with its “own” monomer; the system has a reduced tendency to copolymerize. Concerning composition copolymer, then it must be impoverished the monomer that few in a monomer mixture. This picture is exactly the opposite of that observed for option r 1< 1, r 2 < 1, а на рис. 3 была бы представлена кривой, зеркально подобной кривой 2. Но этот вариант copolymerization rare; we can only mention the copolymerization of butadiene with isoprene at 50 o C (r 1 = 1.38, r 2 = 2.05), where the constants are only slightly greater than unity. But, unfortunately, there are cases when both constants are infinitely large (r 1 →¥, r 2 ®¥); in this case, copolymerization simply does not occur, each of the monomers is polymerized separately and a mixture of two homopolymers is formed (example - a pair: butadiene - acrylic acid). A very useful option would be where the constants would have a large, but final size; in this case would be formed block copolymers; Unfortunately, no such cases have yet been found.

The term “copolymerization constants” should not be taken too literally: their values ​​for a given monomer can change noticeably with changes in reaction conditions, in particular, with changes in temperature. For example, when copolymerizing acrylonitrile with methyl acrylate at 50 o C, r 1 = 1.50, r 2 = 0.84, and at 80 o C, r 1 = 0.50, r 2 = 0.71. Therefore, when giving the values ​​of constants, it is necessary to indicate the conditions.

Radical polymerization of vinyl monomers CH 2 =CHX underlies the technology for the production of various polymer materials. The mechanism and kinetic patterns of polymerization were intensively studied in the 50s and 60s; A number of monographs have been published on this issue. The following two features distinguish polymerization from other chain reactions. Firstly, as a result of the chain process of sequential addition of monomer molecules to the growing macroradicle, the materialization of repeatedly repeated acts of continuation of the chain occurs in the form of the final product - a macromolecule. Secondly, only one type of active center leads to a chain reaction, namely, a macroradical with a free valence on carbon. The addition of the monomer CH 2 =CHX to the radical R occurs, as a rule, at the CH 2 group, so that the radical RCH 2 C HX is formed, the subsequent addition is of the head-to-tail type, which is energetically the most favorable:

RCH 3 C HX + CH 2 =CHX ® RCH 2 CHXCH 2 C HX

Other types of attachments (head to head, etc.) occur only to a minor extent. For example, when polymerizing vinyl acetate (300-400K), head-to-head addition occurs in no more than 2% of cases.

Initiated polymerization of an unsaturated compound includes the following stages:

r + CH 2 =CHX rCH 2 C HX(R 1)

R 1 + M R 2

Rn + M Rn+1

R n + R m R n -R m

R n + R m R n H + R m-1 CH=CHX

When deriving kinetic relationships, the following 4 assumptions are usually made:

1. The case is considered when polymerization occurs with long chains, i.e. the polymerization rate v>> v i ;

2. It is allowed that k p and k t do not depend on the length of the reacting macroradical, i.e. k p1 = k p2 =... k pn , and the same for k tc and k td. This assumption seems reasonable, especially

for high molecular weight radicals, since the reactivity of a radical is determined by its molecular structure near the free valency, and during homopolymerization the structure of all macroradicals is the same and they differ only in their length.

3. The reaction is assumed to occur in a quasi-stationary mode. This is true for experiments with v i = const and duration t>> t R · , where t R · = (2 k t/ v i) -1/2 . At v i = 10 -8 - 10 -6 mol/l and 2 k t = 10 6 - 10 8 l/mol s The lifetime of macroradicals R · varies in the range of 0.1 -10 s, which is significantly shorter than the reactor heating period (50-200 s).

4. Termination involving primary radicals formed from the initiator is usually neglected (this reaction r · + R · is not in the scheme), since in most cases almost all r · react with the monomer, and the fraction of r · reacting with macroradicals is small, because<< . При таких преположениях для скорости полимеризации v and length of the kinetic chain v the following expressions are obtained:

v= k p[M]( v i/2 k t) 1/2 , (1)

n= v/v i = k p[M](2 k t v i) -1/2 (2)

A variety of peroxide compounds, azo compounds, polyarylethanes, and disulfides are used as polymerization initiators. The mechanism of initiator decay is discussed in Lecture 2.

When the initiator decomposes in the condensed phase, two radicals are formed, surrounded by solvent or monomer molecules (during bulk polymerization). Some of these pairs die in the cell (enter into recombination or disproportionation reactions), and some escape into the volume. If all released radicals react with the monomer, then the rate of initiation is equal to the rate of generation of radicals: v i=2 ek d[I]. If some of the initiator radicals released into the volume react with macroradicals, then v i grows with [M] until it reaches the value 2 ek d[I]. Examples of this kind are described in the literature. The monomer concentration has virtually no effect on the release of radicals into the volume, since the recombination of radical pairs in the cell proceeds immeasurably faster than the reaction of the radical with the monomer.

Usually the initiator decays slowly, so that during the experiment v i = const. However, there are cases when a significant part of it disintegrates during the experiment. In this case, in a quasi-stationary reaction mode, the kinetics of monomer consumption is described by the equation:

The chain continuation reaction determines both the rate of polymerization and the structure of the resulting polymer. Vinyl monomers polymerize in a head-to-tail fashion (see above). Chain continuation rate constant k p is determined by the activity of the monomer and the macroradical leading the chain reaction. Below are the rate constants k p for a number of monomers:

Styrene: k p = 2.4 ´ 10 8 exp(- 37.6/RT), l/mol s;

Methyl methacrylate: k p = 2.5 ´ 10 6 exp(- 22.6/ RT), l/mol s;

Vinyl acetate: k p = 2.0 ´ 10 6 exp(- 19.6/ RT), l/mol s;

Methyl acrylate: k p = 1.1 ´ 10 6 exp(- 17.6/ RT), l/mol s;

Vinyl chloride: k p = 3.3 ´ 10 6 exp(- 36.4/ RT), l/mol s;

Acrylonitrile: k p = 2.3 ´ 10 5 exp(- 16.2/ RT), l/mol s

The addition, naturally, occurs with a decrease in entropy; the pre-exponential factor of 10 6 l/mol corresponds to the activation entropy D ¹ S = - 52 J/(mol l). CH 2 =CHX monomers containing a polar group (ester, nitrile, etc.) form complexes with metal ions. For example, methyl methacrylate forms 1:1 complexes with metal salts Li +, Mn 2+, Fe 3+, Co 2+, Zn 2+, acrylonitrile with metal salts Li +i, Mg +, Fe 3+, Mn 2+ , Co 2+ , Ni 2+ . Such complexes often react with macroradicals more quickly. For example, methyl methacrylate reacts with k p = 2.5 ´ 10 2 l/mol s, and its complex c
ZnCl 2 - c k p = 6.1 ´ 10 2 l/mol s. Zinc chloride accelerates the polymerization of methyl methacrylate.

With increasing temperature, the depolymerization reaction begins to play a noticeable role, i.e. decomposition of a macroradical into a monomer and a radical

R n R n-1 + M

Since the macroradical growth reaction is exothermic, the depolymerization reaction is endothermic and the difference E U- E p = D H 0 . With increasing temperature, a state is reached where the rates of chain growth and depolymerization become equal: k p [M] = k U, and the polymerization rate is zero. This state corresponds to the maximum polymerization temperature equal to:

T max = (4)

For pure monomer (for bulk polymerization) T max = 583K (styrene), T max = 493K (methyl methacrylate), T max = 334K (a-methylstyrene).

Chain termination, as can be seen from the diagram, occurs as a result of a reaction between macroradicals. These radicals enter into two types of reactions with each other, namely recombination:

2 ~ CH 2 - C XY ~CH 2 - CXY- CXY- CH 2 ~~

and disproportionation:

2~ ~ CH 2 -C XY ~~ CH 2 - CHXY + ~~ CH=CXY

The average degree of polymerization depends on the relationship between the rate constants of these two reactions:

P = k p [M] or (5)

This ratio also affects the molecular weight distribution: M w /M n = 1.5 for recombination R · and M w /M n = 2 for their disproportionation.

Rate constants k t = t tc+ k td, depending on the structure of the monomer, varies in the range of 10 8 - 10 6 l/mol s. There is an antibatal relationship between the rate constant of chain termination and the viscosity of the solvent. This indicates that the reaction between two macroradicals is limited by diffusion processes. A number of facts indicate that the progressive diffusion of macroradicals in solution is not the limiting stage of chain termination during polymerization. For macroradicals with a polar group X at the end (~~ CH 2 CHX), there is an obvious symmetry (if not coincidence) between k t and the reorientation frequency of the dipole group (T = 300K).

Apparently, in most cases it is segmental mobility that limits the rate and determines the rate constant for the death of macroradicals.

Polymerization

Polymerization is a process for producing high-molecular compounds in which the growth of a molecular chain occurs as a result of the sequential addition of molecules of a low-molecular substance (monomer) to the active center localized at its end:

M i M* + M M i+1 M*, etc.

where M i is a chain of i links long; M* - active center; M - monomer molecule

Based on the number of monomers involved in polymerization, they are distinguished homopolymerization(one monomer) and copolymerization(two or more monomers).

Depending on the chemical nature of the active centers involved in the formation of molecular chains (radical or ion), there are radical And ionic polymerization.

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: ethylene, vinyl chloride, vinyl acetate, vinylidene chloride, tetrafluoroethylene, acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, styrene, butadiene, chloroprene and other monomers. Radical polymerization usually involves several elementary chemical steps: initiation, chain propagation, chain termination, and chain transfer. Mandatory stages are initiation and chain growth.

Initiation. Initiation consists of the creation of free radicals in the reaction system that are capable of starting reaction chains. The most common method of initiating polymerization is based on thermal homolytic decomposition of unstable substances in the monomer medium - initiators. Various types of peroxides are widely used as initiators: dialkyl peroxides (di- rubs-butyl), hydroperoxides (cumyl hydroperoxide), peresters ( rubs-butyl perbenzoate), acyl peroxide (benzoyl peroxide), etc. Peroxides, for example, decompose when heated according to the polymerization scheme monomer styrene copolymer

In addition to peroxides, azo compounds are widely used as initiators, of which 2,2"-azobisisobutyronitrile (AIBN) is the most widely used:

Initiators of radical polymerization are usually not selective in relation to various 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 rubs-butyl at 120--140°C. The initiation activation energy is usually close to the bond energy that breaks during the decay of the initiators. and ranges from 105 to 175 kJ/mol. The radical formed during the decomposition of the initiator molecule joins the double bond of the monomer and begins the reaction chain:

R* + CH 2 =CHX R--CH2 -CHX*

Redox systems can be used to initiate radical polymerization at room or reduced temperature. The oxidation-reduction reaction is carried out in a medium containing a monomer. Polymerization is caused by free radicals formed as reaction intermediates. You can select oxidizing-reducing pairs that are soluble in water (hydrogen peroxide - ferrous sulfate; sodium persulfate - sodium thiosulfate, etc.) or in organic solvents (organic peroxides - amines; organic peroxides - organic salts of ferrous iron, etc. .). Accordingly, radical polymerization can be initiated in both aqueous and organic media.

A typical example of a redox reaction in an aqueous environment is the interaction of hydrogen peroxide with ferrous iron ions:

Fe +2 + H 2 O 2 Fe +3 + OH - + HO*

The HO radical, joining the monomer molecule, initiates radical polymerization.

An example of a redox reaction that initiates radical polymerization in organic media is the interaction of benzoyl peroxide with methylaniline:

Photochemical initiation Radical polymerization is based on the formation of free radicals as a result of the homolytic cleavage of chemical bonds upon absorption of a quantum of initiating radiation by the monomer or specially introduced photoinitiators or photosensitizers.

At radiation-chemical initiation radical polymerization uses high-energy radiation (-rays, fast electrons, -particles, neutrons, etc.). The activation energy of photochemical and radiation-chemical initiation is close to zero. A feature of the last two initiation methods is the ability to instantly turn on and off the irradiating radiation, which is important for some research work.

Chain growth. The chain grows by sequential addition of monomer molecules to radicals resulting from initiation, for example:

C 6 H 5 -C(O)-O-CH 2 -CHX* + CH 2 =CHX

C 6 H 5 -C(O)-O-CH 2 -CHX-CH 2 -CHX*

C 6 H 5 -C(O)-O-CH 2 -CHX-CH 2 -CHX + CH 2 =CHX*

C 6 H 5 -C(O)-O-CH 2 -CHX-CH 2 -CHX-CH 2 -CH*

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .

C 6 H 5 -C(O)-O-(CH 2 -CHX) n -CH 2 -CHX* + CH 2 =CHX

C 6 H 5 -C(O)-O-(CH 2 -CHX) n+1 -CH 2 -CHX*, etc.

where k p is the chain growth rate constant.

The development of a kinetic chain is accompanied by the formation of a material chain. The activation energies of chain growth reactions lie in the range of 12-40 kJ/mol.

The rate constants and activation energy for chain growth 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 growth reaction of radical polymerization.

Accurate quantum chemical calculation of activation energies for the addition of radicals to double bonds of monomers is difficult in most cases. However, the use of the semi-empirical Evans - Polyany - Semenov Rule, according to which the activation energy E a is related to the thermal effect of the elementary reaction Q by the relation E a = A - Q (where A and are constant values ​​for similar series), allows. In many cases, estimate E a and predict its change in a series of monomers of the same type.

The activation energy for the addition of a monomer to a radical is lower, i.e., the more active the monomer is, the higher the energy of conjugation in the radical, which is obtained as a result of the addition of this monomer to the original radical. On the contrary, the activation energy for the addition of a radical to a double bond is lower, i.e., the lower its conjugation energy, the higher the reactivity of the radical. Thus, the reactivity in the series of monomers and their corresponding radicals changes antibatally. For example, the reactivity in the series of vinyl monomers with substituents

C 6 H 5, -CH=CH 2, -COCH 3, -CN, -COOR, CR, -OCOCH 3, -OR

decreases from left to right. The reactivity of the corresponding radicals decreases from right to left. Therefore, the higher the reactivity of the monomer, the higher the activation energy of the chain growth reaction, i.e., the lower the rate of its radical polymerization.

The above brief qualitative consideration does not take into account polar and spatial effects, which in some cases have a significant impact on the activation energies of radical processes. A theory that considers the reactivity of monomers and radicals only taking into account conjugation energies and not taking into account polar and spatial effects is called theory of ideal radical reactivity.

Open circuit. Reactions that limit kinetic and activation chains are called termination reactions. Termination leads to the disappearance of active radicals in the system or to their replacement by low-active radicals that are unable to attach monomer molecules. Chain termination during radical polymerization mainly occurs when two growing radicals interact as a result of their recombination:

~CH 2 -CHX* + ~CH 2 -CHX* ~CH 2 -CHX-CHX-CH 2 ~

or disproportionation:

~CH 2 -CHX* + ~CH 2 -CHX* ~CH 2 -CH 2 X + ~CH=CHX

The chain termination reaction includes the progressive diffusion of macroradicals with the formation of a united coil, the mutual approach of active terminal units due to segmental diffusion within the united coil, and the direct chemical interaction of reaction centers with the formation of “dead” macromolecules.

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

Chain termination can occur at any length of the growing macroradical. Therefore, during polymerization, macromolecules of different lengths (different degree of polymerization). This explains the polymolecular nature of synthetic polymers, described by the corresponding molecular weight distributions.

Chains can also break when radicals interact with inhibitors. Low-active stable free radicals can be used as inhibitors, for example diphenylpicrylhydrazyl, N-oxide radicals, which themselves do not initiate polymerization, but recombine or disproportionate with growing radicals. Inhibitors can also be substances whose molecules, interacting with active radicals, saturate their free valencies, and themselves turn into low-active radicals. The latter include quinones (for example, benzoquinone, duroquinone), aromatic di- and trinitro compounds (dinitrobenzene, trinitrobenzene), molecular oxygen, sulfur, etc. Inhibitors can also be compounds of metals of variable valence (salts of ferric iron, divalent copper, etc.) , which terminate growing chains due to redox reactions. Often inhibitors are introduced into the monomer to prevent premature polymerization. Therefore, before polymerization, each monomer must be thoroughly purified from impurities and added inhibitor.

Chain transmission. Limitation of material chains during polymerization can occur not only through termination reactions, but also as a result of chain transfer reactions, which are very characteristic of radical polymerization. During chain transfer, the growing radical detaches an atom or group of atoms from any molecule ( transmitter circuit). As a result, the radical is converted into a valence-saturated molecule and a new radical is formed, capable of continuing the kinetic chain. Thus, during transfer reactions the material chain breaks, but the kinetic chain does not.

Chain transfer can occur through monomer molecules. For example, in the case of vinyl acetate

~R* + CH2=CH-OCOCH 3 ~RH + CH 2 =CH-OCOCH 2 *

where k M is the chain transfer rate constant to the monomer.

In this case, the growing radical, instead of joining at the double bond of the vinyl acetate molecule, can tear off one of the hydrogen atoms of the acetyl group, saturating its free valence and converting the monomer molecule into an active radical. The latter can react with another monomer molecule, starting the growth of a new macromolecule:

CH2=CH-OSOSN 2 *+ CH 2 =CH-OSOSN 3 CH 2 =CH-OSOSN 2 -CH 2 -CH*-OSOSN 3

The ability of monomer molecules to participate in the chain transfer reaction is usually characterized self-transfer constant With M, equal to the ratio of the chain transfer rate constant to the monomer. (k M) to the chain growth rate constant (k P), i.e. C M = k M /k P. For most vinyl monomers that do not contain mobile groups or atoms, k M<

In the presence of a solvent, solvent molecules can play the role of chain transmitter, for example in the case of toluene

~CH 2 -CHX* + C 6 H 5 CH 3 ~CH 2 -CH 2 X + C 6 H 5 CH 2 *

where k S is the chain transmission speed constant.

The interaction of a growing radical with a chain transmitter molecule leads to the cessation of the growth of this material chain, i.e., it reduces the molecular weight of the resulting polymer. The ability of solvents to participate in chain transfer during radical polymerization of a given monomer is characterized by the transfer constant C S = k S / k P (Table 1). 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 C S values ​​> 10 -3 are usually used, which are called regulators, For example

~CH 2 --CHX + CC1 4 ~CH 2 --CHXCI + CC1 3 *

Table 1. Chain transfer constants for radical polymerization of styrene at 60 °C.

Kinetics of radical polymerization. The rate of initiation in the presence of initiators that decompose upon heating under conditions under which decomposition occurs by a non-chain mechanism can be expressed by the equation

V in = k in [I] (1.1)

where [I] is the concentration of the initiator; k in -- initiation rate constant.

The rate of chain growth is expressed by the equation

where k ip is the rate constant for the addition of the monomer to the radical with the degree of polymerization n = i; -- concentration of radicals with degree of polymerization i; [M] -- monomer concentration.

In the formation of polymers of large molecular weight, it can be assumed with a good approximation that k p does not depend on the degree of polymerization of the radical (practically, starting from the degree of polymerization n = 3-4). Then the expression for v p is simplified:

where is the concentration of all growing radicals.

The rate of disappearance of radicals as a result of recombination and disproportionation is described by the equation

D[R]/dt = k 0 [R] 2

where k 0 is the termination rate constant (assuming that the reactivity of radicals in termination reactions does not depend on their degree of polymerization).

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

D[M]/dt v p = k p [R][M] (1.2)

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

D[R]/dt = v in - k 0 [R] 2

The concentration of radicals [R], which is difficult to measure by direct experiments, can be eliminated from equation (1.2) by assuming that the rate of formation of radicals is equal to the rate of their disappearance ( quasi-stationary condition), i.e. d[R]/dt = 0. During radical polymerization, this condition is usually practically satisfied within a few seconds after the start of the reaction. That's why

v in = k 0 [R] 2

[R] = (v in / k 0) 1/2

And -d[M]/dt = k p (v in / k 0) 1/2 [M] (1.3)

Thus, the rate of radical polymerization is of the first order in terms of monomer concentration and order of 0.5 in terms of initiator concentration, which is, as a rule, observed experimentally.

Degree of polymerization. From the kinetic data, the degree of polymerization P n of the resulting polymer can be calculated. It is equal to the ratio of the number of monomer molecules included in the polymer chains during polymerization to the number of material chains formed. 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

P n = v p / v 0 + v lane (1.4)

where v 0 is the rate of bimolecular chain termination; v per = (k M [M] + k S [S] x [R] - the sum of the chain transfer rates to the monomer and solvent.

When two radicals recombine, one material chain is formed, i.e., an average statistical doubling of P n occurs, therefore, in the denominator of equation (1.4), before the term corresponding to termination by recombination, it is necessary to place the factor S. In addition, under the assumption that the fraction of polymer radicals terminated by the disproportionation mechanism is equal, and the fraction of radicals dying during recombination is equal to 1-, the equation for P n takes the form

Then for the reciprocal of Р n we get:

Expressing the concentration of the radical in terms of the polymerization rate v p = k P [R] [M] and using the constants C M and C S, we finally obtain:

The resulting equation relates the number-average degree of polymerization to the reaction rate, chain transfer constants, and monomer and transfer agent concentrations. From equation (1.5) it follows that the maximum number-average degree of polymerization of the resulting polymer, achievable at a given temperature, in the absence of other transfer agents, is determined by the chain transfer reaction to the monomer, i.e. P n max C M -1.

The equations derived above are valid for radical polymerization at low degrees of conversion of monomer to polymer (not exceeding 10%). At large conversion depths, deviations are observed associated with an increase in the viscosity of the reaction medium with increasing concentration of the polymer dissolved in it, which leads to a slowdown in the diffusion of macroradicals and sharply reduces the likelihood of their recombination or disproportionation. In this regard, the effective breakage rate constant is significantly reduced. The concentration of radicals in the system increases, and the rate of polymerization increases. This phenomenon is called gel effect. If radical polymerization produces a polymer that is insoluble or has limited swelling in the reaction medium, then the effects associated with the diffusion inhibition of the bimolecular termination reaction appear already starting from very small depths of transformation.

The overall polymerization rate V is determined by the equation:

V= V in + V p + V 0

In a stationary mode, when the numbers of newly formed and disappearing free radicals are equal, i.e. the polymerization rate is constant, we have:

V in =V 0 (4) k in = k 0 2 (5)

= (k in / k 0) 1/2

V = V p (6) V = k p [M] (7)

Determining the value from equation (5) and substituting it into equation (7), we obtain:

V = k p (k in / k 0) 1/2 [M] 1/2

V = k [M] 1/2 (8)

The overall rate of radical polymerization is proportional to the square root of the initiator concentration and the monomer concentration to the first power. In the steady state, the concentration of monomers [M] can be considered constant, then the rate of polymerization depends only on the concentration of the initiator to the power of one half. This dependence is preserved only in the case of bimolecular termination of kinetic chains. In the case of a chain transfer reaction, a deviation from the indicated classical laws of radical polymerization is observed. The equations derived above are valid for radical polymerization at low degrees of conversion of monomer to polymer (not exceeding 10%). At large conversion depths, deviations are observed due to an increase in the viscosity of the reaction medium with increasing concentration of the polymer dissolved in it, which leads to a slowdown in the diffusion of macroradicals and sharply reduces the likelihood of their recombination or disproportionation. In this regard, the effective breakage rate constant is significantly reduced. The concentration of radicals in the system increases, and the rate of polymerization increases. This phenomenon is called the gel effect. If radical polymerization produces a polymer that is insoluble or has limited swelling in the reaction medium, then the effects associated with the diffusion inhibition of the bimolecular termination reaction appear already starting from very small depths of transformation.

Main literature: 1 ; 2 ; 4 ; 6 ; 7

Additional literature:

Control questions

1. What are the characteristics of polymer synthesis reactions?

2. What is the difference between chain and step mechanisms of polymer synthesis?

3. Features of the polyaddition reaction (migration polymerization).

4. Describe the laws of chain polymerization.

5. The ability of monomers to undergo radical polymerization.

6. Main stages of radical polymerization.

7. Initiators of radical polymerization.

8. Kinetics of radical polymerization.

9. What is the peculiarity of the kinetics of radical polymerization at high degrees of conversion?

4 Lecture topic. Radical copolymerization. Copolymerization constants. Methods for determining copolymerization constants. Composition diagram. Scheme "Q-e". Methods of polymerization.

Radical copolymerization. Copolymerization is the process of joint polymerization of two or more different monomers. The high-molecular compounds obtained by copolymerization are called copolymers. The copolymerization process can be described by the equation

n A + m B … – A – A – B – A – B – B – A – A – A – B – A

By using different starting components and changing their ratio, it is possible to change the composition and properties of copolymers. Copolymerization finds great practical application, as it allows the properties of polymers to be varied within a wide range. For example, a copolymer of acrylonitrile and vinyl chloride

– CH 2 – CH – CH 2 – CH – CH 2 – CH – …

It is highly soluble in acetone, while polyacrylonitrile and polyvinyl chloride are soluble only in high-boiling or inaccessible solvents.

Polyacrylonitrile fibers have some properties similar to wool, but do not dye very well. Copolymerization of acrylonitrile with amines, for example, with vinylpyridine, produces a copolymer with a significantly higher affinity for dyes.

… – CH 2 – CH – CH 2 – CH – CH 2 – CH – CH 2 – CH –
ç ç ç ç

Copolymerization is widely used in the synthetic rubber industry. By copolymerizing butadiene with acrylonitrile, butadiene is obtained - SKN nitrile rubber.

n CH 2 = CH – CH = CH 2 + m CH 2 = CHа

à… – CH 2 – CH = CH – CH 2 – CH 2 – CH – CH 2 – CH = CH – CH 2 – …

which is oil and petrol resistant.

Copolymerization of isobutene with a small amount of isoprene produces butyl rubber.

Lecture 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 added, i.e. kinetic chain turns into material .

Main stages radical polymerization:

– initiation

– chain growth

– open circuit

– chain transmission

1 . Initiation consists in the formation of free radicals under the influence of:

– heat (thermal initiation);

– light (photoinitiation);

– ionizing radiation (radiation initiation);

– chemical initiators (chemical initiation)

The first three methods are ineffective, because accompanied by various side reactions (branching, destruction, etc.). Most often, chemical initiation is used, in which the formation of free radicals occurs due to the thermal and photochemical decomposition of various compounds containing unstable (labile) bonds, as well as as a result of redox reaction. The most common initiators are: peroxides, hydroperoxides, iso- and diazo compounds, peresters, acyl peroxides.

Example.

a) benzoyl peroxide

t decay = 70 - 80˚С

Initiation efficiency f = 0.7 - 0.9

b) azobisisobutyronitrile

t decay = 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 free radical production can be achieved.

The radical formed during initiation attaches to the double (=) bond of the monomer and begins the reaction chain.

Since the stability of radicals formed during the decomposition of peroxides, azo compounds and other initiators is different, the rate of their reaction with monomer molecules, and therefore the rate of polymerization, is different. To facilitate the decomposition of initiators and reduce the activation energy of the initiation stage, reducing agents (amines, metal salts of variable oxidation states) are introduced into the reaction.

In order to reduce (from 146 to 42 - 84 kJ/mol) and facilitate the decomposition of initiators, use redox systems .

For example:

https://pandia.ru/text/80/211/images/image008_31.gif" width="231" height="104 src=">

Redox systems are used in aquatic environments or when polymerization in emulsion . Their widespread use in the polymer production industry is associated with a significant reduction in the activation energy of the decomposition of initiators into free radicals and thus a reduction in energy costs under production conditions.

2. Chain growth– consists in the sequential addition of monomer molecules to the resulting active center with its transfer to the end of the chain.

The development of a kinetic chain is accompanied by the formation of a material chain.

(small)

Reaction rate constant kp = 102 – 104 (large)

The activation energy and 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.

A chain break leads to a break in the material and kinetic chain.

The activation energy for chain termination is determined by the activation energy for radical diffusion.

Termination can occur at any length of the growing macroradical. This produces macromolecules of different lengths.

The break occurs most often two ways: by recombination And disproportionation.

Eact ≤ 4.2 kJ/mol

Eact = 12.6-16.8 kJ/mol

A break is also possible during the interaction of growing radicals with low molecular weight substances, present in the system.

Lowering the temperature ↓

You can reduce the rate of circuit breakage

increasing viscosity

4. Chain transmission– occurs by the detachment of an atom or group of atoms from some molecule (chain transmitter) by a growing radical. Wherein:

– the growing radical turns into a valence-unsaturated molecule;

– a new radical develops a kinetic chain

Thus, the chain transfer reaction consists in the fact that the substance introduced into the system - the regulator - breaks the growing chain, but at the same time it itself becomes a free radical and begins a new kinetic chain of polymerization.

Increasing the temperature and increasing the amount of chain transfer agent (for example, halogenated hydrocarbons) lead to a sharp increase in the rate of the chain transfer reaction. This reaction suppresses other stages of polymerization, so that individual low molecular weight substances are formed that can be separated (telomerization reaction). They contain end groups from the cleavage products of the chain transfer agent and are active in various chemical reactions, in particular for the production of new polymers.

Telomeres: oligomers having reactive groups at the ends of the molecules.

etc.

Thus, telomerization of ethylene in a carbon tetrachloride environment occurs with the formation of individual products (tetrachloropentane, tetrachloroheptane, etc.)

Example. Chain transmission via:

a) monomer molecule

b) solvent molecule

start of a new chain

c) specially introduced substances (regulators), for example, mercaptans.

km , ks – chain transmission speed constants.

When a growing radical interacts with a chain transmitter molecule the growth of the material chain stops, i.e. the molecular weight of the resulting polymer decreases; the kinetic chain is maintained.

The ability to participate in chain transfer during radical polymerization is characterized by the chain transfer constant to the monomer Cm, to the solvent Cs, to the initiator Cu.

Cm = (0.1 - 5)*10-4 – small value

For example, during the polymerization of vinyl acetate Cm = 2∙10-3

Of the solvents, the Cs value is high. So during the polymerization of styrene Cs = 9∙10-3

Kinetics of radical polymerization

The speed of the process is described by the equation: https://pandia.ru/text/80/211/images/image026_11.gif" width="44" height="41"> - rate of monomer disappearance

I is the rate of initiation and growth of the chain

When a high-molecular-weight polymer is formed, the number of monomer molecules involved in the initiation stage is much smaller than in the growth stage, and therefore can be neglected.

It's difficult to measure. For a stationary process, the rate of radical formation 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 before the start of the reaction)

It follows from the equation that the rate of polymerization depends on the rate of initiation to the power of 0.5, i.e..gif" width="49" height="25"> times. This is explained by the bimolecular mechanism of chain scission.

During thermal initiation, the polymerization rate V depends on the ratio of three reaction rate constants

A typical kinetic curve describing monomer conversion (i.e., the conversion of monomer to polymer as a result of polymerization) as a function of time is S-shaped.

Fig. 1 Typical kinetic curve of chain radical polymerization:

1 – inhibition; 2 – acceleration of polymerization (speed increases with time); 3 – stationary period (polymerization rate is constant); 4 – slowdown of polymerization (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 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 a monomer into a polymer begins, and the rate increases; 3 – stationary state area, where polymerization of the main amount of monomer occurs at a constant speed (straight-line dependence of conversion on time); 4 - reaction slowdown area , where the reaction rate decreases due to a decrease in the content of free monomer; 5 - cessation of the main reaction after the entire amount of monomer has been exhausted.

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 (initiation stage) is equal to the number of disappearing macroradicals (termination stage) of the reaction and material chains.

Degree of polymerization n(i.e., the number of units of monomer units in one average macromolecule) is, by definition, proportional to the rate of the chain growth 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 = υp /υrev = kp[M] / krev2 = kp[M] / krev = kn / = knI / [I]0.5

In other words, the degree of polymerization and, therefore, the average molecular weight of the polymer in free radical polymerization is inversely proportional to the square root of the initiator concentration.

The influence of various factors on the process of radical polymerization.

1. Effect of temperature

With increasing temperature, the reaction rate of the formation of active centers and the chain growth reaction increases. Thus, the total rate of polymer formation increases. Typically, the polymerization rate increases 2-3 times with an increase in temperature by 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 weight 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 increasing 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 environment, the total rate of polymerization and the molecular weight of the resulting polymer increases with increasing monomer concentration.

When polymerizing in an inert solvent that does not participate in the reaction, the polymerization rate is equal (often x = 1.5).

Most solvents are involved in polymerization (chain transfer reaction). Therefore, much more complex dependencies are obtained.

4. Effect 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 https://pandia.ru/text/80/211/images/image041_1.jpg" align="left" width="217" height="161">Fig. 2 Thermal polymerization of styrene at 100 ˚С in the presence of inhibitors and moderators:

1 – without additives; 2-0.1% benzoquinone (inhibitor); 3 – 0.2% nitrobenzene (inhibitor); 4 – 0.5% nitrobenzene (moderator)

To regulate the polymerization process, polymerization inhibitors and retarders are used. Inhibitors – low-molecular 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 make it possible to completely stop 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 onset of chain initiation, increasing the induction period on the polymerization kinetic curve. The length of the induction period is usually proportional to the amount of inhibitor administered. The same substance can act as an inhibitor, as a retarder, 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-density polyethylene. Oxygen forms peroxides or hydroperoxides when reacting 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 break 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, including dodecyl mercaptan. Due to the large length of the hydrocarbon chain, its molecules are not active enough and are consumed slowly.

Impurities in monomer and solvent : the degree of their influence on the polymerization process is determined by their chemical nature and reactivity towards active particles.

To exclude the influence of these factors, monomers and solvents of “kinetic purity” are taken for synthesis, sometimes instead of https://pandia.ru/text/80/211/images/image050_4.gif" width="23" height="17">, .

Methods of polymerization

Radical polymerization is carried out in a block (mass), solution, emulsion, suspension and gas phase. In this case, the process can occur under homogeneous or heterogeneous conditions. In addition, the phase state of the initial reaction mixture can also change during polymerization.

1. Polymerization in block (in bulk)

Polymerization is carried out without a solvent. Due to the high exothermicity, the polymerization process is difficult to regulate. During the reaction, the viscosity increases and heat removal becomes difficult, resulting in local overheating, leading to the destruction of the polymer and its heterogeneity in molecular weight. The advantage of bulk polymerization is the possibility of obtaining a polymer in the form of a vessel in which the process is carried out without any additional processing.

2. Solution polymerization

Unlike polymerization in the block, in this case there is no local overheating, since the heat of the reaction is removed by the solvent, which also acts as a diluent. The viscosity of the reaction system decreases, which makes it easier to mix.

However, the role (proportion) 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 are dissolved. The resulting polymer is used directly in solution or isolated by precipitation or evaporation of the solvent.

b) In the solvent used for polymerization, the monomer dissolves, but the polymer does not dissolve. As the polymer forms, it precipitates out in solid form and can be separated by filtration.

3. Polymerization in suspension (beads or granules)

Widely used for polymer synthesis. In this case, the monomer is dispersed into the Dispersion" href="/text/category/dispersiya/" rel="bookmark">dispersion is achieved by mechanical stirring 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 microreactors block polymerization.Initiators that are soluble in the monomer are used.

The advantage of this process is good heat removal, the disadvantage is the possibility of contamination of the polymer with stabilizer residues

4. Polymerization in emulsion (emulsion polymerization)

In emulsion polymerization, the dispersion medium is water. Various soaps are used as emulsifiers. For initiation, water-soluble initiators and redox systems are most often used.

Polymerization can occur in a molecular solution of the monomer in, at the interface of a drop of monomer - on the surface or inside soap micelles, on the surface or inside the resulting polymer particles swollen in the polymer.

The advantages of the process are: high speed, formation of a polymer of high molecular weight, ease of heat removal. However, as a result of emulsion polymerization, a large amount of wastewater is generated that requires special treatment. It is also necessary to remove any remaining emulsifier from the polymer.

5. Gas phase polymerization

In gas-phase polymerization, the monomer (for example, ethylene) is in a gaseous state. Peroxides can also be used as initiators. The process takes place at high p.

Conclusions:

Free radical polymerization is one of the types of chain processes in the synthesis of polymers. Polarization of the parent 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 radical polymerization process can be controlled by various techniques, both in terms of the rate of monomer conversion and the molecular weight of the polymer. For this purpose, additives of low molecular weight substances are used that act as inhibitors or moderators of the reaction, as well as transferring the reaction chain or reducing the activation energy for the decomposition of initiators into radicals. Knowledge of the laws 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 producing polymers has found wide application in industry.