Substances that stimulate polymerization.
In the past, polymerization catalysts referred to any reagent that promotes polymerization. With the study of specific processes, it became clear that some reagents are irreversibly consumed at the stage of excitation of polymerization and enter (in the form of end groups) into the composition of the resulting polymer, for example, during radical (often anionic) polymerization. Such reagents are called initiators.
The term “polymerization catalysts” is usually referred to as causative agents of cationic, coordination-ionic, and less commonly anionic polymerization, although in these processes the mechanism does not always correspond to the classical definition of catalysis.
The main role of polymerization catalysts is the creation of active centers on which the macromolecule grows. Along with the nature of the monomer and medium, the nature of the catalyst determines the mechanism of the process, the kinetic characteristics of elementary acts, the molecular weight, MWD, and spatial structure of the resulting polymer.
Depending on the nature of the active sites, ionic (cationic and anionic), metal complex, organometallic, and metal oxide polymerization catalysts are distinguished.
TO ionic
cationic polymerization catalysts include protic and aprotic acids (HF, H 2 SO 4, AlCl 3, BF 3, FeCl 3 and others), carbonium salts, for example Ph3C + SbCl 4 -, oxonium (R 3 O + SbF 6 -) and others . All of them are acceptors of electrons and electron pairs. Microimpurities of water, alcohols, and other proton donors play an important role in the formation of active centers.
Efficiency cationic
The polymerization catalysts depend on the acidity of the complex formed during the interaction of the catalyst components with the monomer. In industry, using these catalysts, a number of polymers are synthesized. So, polyisobutylene with a molecular weight of 150-225 thousand is obtained by polymerization of isobutylene in the presence of BF 3 at temperatures from -80 ° C to -100 ° C, butyl rubber - by copolymerization of isobutylene and isoprene at temperatures from -80 to -95 ° C in the presence of AlCl 3 or complexes of ethylaluminum sesquichloride (C 2 H 5) 3 Al 2 Cl 3, polyformaldehyde - by polymerization of trioxane in the presence of BF 3 complexes or carbonium salts. To obtain coumarone-indene resins, H 2 SO 4 is usually used as catalysts (the reaction is exothermic, proceeds instantly), anhydrous AlCl 3 (reaction time 20-40 minutes, temperature 100-120 ° C) or BF 3 etherates.
Catalysts anionic
Polymerizations - alkali metals, their alcoholates, Naphthylide Na, NaNH 2 , Ph 3 CNa, Grignard reagents, organolithium compounds and other agents of a basic nature. In their presence, monomers with a reduced electron density at the double bond CH 2 =CHX, where X=NO 2, CN, COOR, C 6 H 5 , CH=CH 2, as well as some lactones, siloxanes and others, are polymerized.
Processes involving anionic polymerization catalysts are in some cases characterized by low chain transfer and termination rates, which leads to the formation of so-called living polymers. In industry, such catalysts are used for the synthesis of rubbers, polyamides, polysiloxanes, and others. So, the synthesis of rubber from butadiene can be carried out under the action of metallic Na (according to Lebedev) or Li, the industrial synthesis of polyisoprene - under the action of metallic Li, the synthesis of poly-e-caproamide - in the presence of hydroxides, carbonates or hydrides of alkali metals at 140-260 ° WITH.
Metal complex
Polymerization catalysts are obtained by reacting compounds of transition metals of groups IV-VIII (for example, TiCl 3 , TiCl 4 , VC1 4 , VOCl 3 , ZrCl 4 , NiCl 2 and others) with organic derivatives of metals of groups I-III (for example, AlR 3 , AlR 2 Cl, ZnR 2 , RMgCl and others). Such polymerization catalysts are called Ziegler-Natta catalysts.
Are widely used metal complex catalytic systems
Fixed on inorganic and organic carriers. When using solid and supported complex polymerization catalysts, their dispersed composition, surface area, pore volume, and strength are of great importance. On solid microspherical catalysts, polymer particles of a given size can be obtained during synthesis.
The most promising for the polymerization of olefins are Ziegler-Natta catalysts obtained by fixing Ti and V halides on the surface of supports containing Mg (for example, MgO, MgCl 2 , polyethylene with grafted fragments of MgR and MgCl). For example, using such titanium-magnesium catalysts, it is possible to obtain several tons of polyethylene and about 100 kg of polypropylene per 1 g of catalyst.
Using metal complex polymerization catalysts, stereoregular polymers are obtained. For example, polymerization catalysts based on soluble Zr compounds and methylalumoxanes 6-20 exhibit high activity in the polymerization of ethylene (25.10 6 g of polyethylene per 1 g of Zr); in their presence it is possible to obtain polyolefins with special properties. So, in the polymerization of propylene in the presence of methylalumoxane and bis-cyclopentadienylzirconium dichloride, atactic polypropylene is formed, in the presence of alumoxane and chiral ethylene-bis-tetrahydroindenylzirconium dichloride, isotactic polypropylene is formed, and under the action of an optically active isomer of zirconocene and alumoxane, an optically active polymer.
The stereospecificity of the action of metal complex polymerization catalysts is determined by the nature of the transition metal, the ligand environment of the central atom, the type of the catalyst and support lattice, and the like.
organometallic
Polymerization catalysts - organic derivatives of metals of IV-VIII groups. Used for the polymerization of dienes, acetylenes, cycloolefins. The active sites of diene polymerization are p-allyl complexes of metals, the structure of which determines the microstructure of the resulting polymer. The polymerization of cycloolefins proceeds with the participation of active sites, including carbene complexes of the ~CH2:MX type.
metal oxide
polymerization catalysts usually contain oxides of Cr, Ca and Mo. They are used, like organometallic polymerization catalysts, for the polymerization of olefins and dienes. For example, for the polymerization of ethylene (130-160°C; pressure 4 MPa), a chromium oxide catalyst is used with a Cr content on the carrier (usually aluminosilicate) of about 25% by weight. The stereospecificity of these polymerization catalysts is much lower than that of metal complex catalysts.
Gladkova Natalya
Announcements for the purchase and sale of equipment can be viewed at
You can discuss the advantages of polymer grades and their properties at
Register your company in the Business Directory
A systematic study of the anionic polymerization of unsaturated compounds began in the 1920s by Lebedev, Ziegler et al.
active center during anionic polymerization, it carries a partial or full negative charge.
Monomers prone to anionic polymerization, have a reduced electron density of the C=C bond with an electron-withdrawing substituent (acrylates, acrylonitrile, ethylene oxide, aldehydes, lactones, lactams, siloxanes) or have an increased conjugation energy (styrene, dienes). In addition, many of the carbonyl-containing compounds and heterocycles with C=C, C=O, C=N, etc. bonds are also prone to anionic polymerization.
Catalysts are strong bases, Lewis bases, i.e. electron donors - alkali metals, derivatives of metals of groups I and II (alkyls, aryls, alcoholates, amides). Processes that develop with the participation of transition metals are usually referred to as coordination-ionic polymerization. In addition, anionic polymerization can be induced by electric current and high energy radiation.
Initiation reaction can be done in two ways:
According to the type of acid-base interaction, as a result of the addition of an anion or a compound containing an anion to the monomer, for example, initiation of anionic polymerization of vinyl monomers with sodium amide in liquid ammonia at low temperature:
- according to the type of oxidation-reduction, as a result of electron transfer between monomer and catalyst molecules; for example, in the reaction of metals of group I, as well as organometallic compounds of elements of groups I and II. The act of initiation involving the metal is preceded by the stage of formation of a charge transfer complex (CTC) between the catalyst and the monomer:
or between metal and arena:
At the second stage, the monomer displaces naphthalene from sodium naphthylinide, resulting, as in the first system, the radical anion of the monomer. Further, the recombination of radical anions occurs with the formation of dianions, to which the monomer is attached.
Processes of the type of oxidation-reduction are also characteristic of electrochemical and radiation-chemical initiation. In principle, with such a mechanism, the parallel development of anionic and radical reactions is possible; however, no cases with a significant participation of radical processes have been found in the really studied systems.
A feature of the anionic polymerization of nonpolar monomers is the association of the catalyst and growing chains in nonpolar solvents and the dissociation of ion pairs into free ions in polar media.
The activity of the anionic catalyst - metal alkyl MeR is directly dependent on the polarity of the Me-C bond, as well as on the solvent, and the active centers can exist in the form of covalent polarized molecules (II), their associates (I), ion pairs that differ in reactivity and stereospecificity different degrees of solvation (III, IV), free ions (V):
The polymerization of nonpolar monomers (styrene, butadiene, isoprene) in hydrocarbon solvents is often accompanied by inductive effects due to insufficient initiation rate. These effects can be eliminated by introducing into the system promoters electron-donor type, which form stoichiometric complexes with the initiator (ethers, alkylamines, etc.):
R-Me + nD R-Me×nD.
The presence of an electron donor in the coordination sphere of the metal leads to depletion of electrons and weakening of the Me-C bond. In reactions proceeding with the cleavage of the Me-C bond, this is equivalent to an increase in the activity of the catalyst. For example, this phenomenon favors the 1,2(3,4)-addition of dienes and the formation of syndiotactic polymethyl methacrylate.
chain growth for anionic polymerization is characterized by the relative stability of the active centers. For example, for non-polar polymers in hydrocarbon solvents, the total process practically includes only the stages of chain initiation and growth (the so-called “living polymers”, first described by M. Schwartz (USA)). This makes it possible to create conditions for studying the mechanism of anionic polymerization, as well as for solving various synthetic problems: obtaining polymers with a given MWD, including practically monodisperse ones; synthesis of polymers and oligomers with terminal functional groups capable of further transformations of the polycondensation or polymerization type, as well as block copolymers, graft copolymers and various polymers with a controlled type of branching, etc.
The participation of the counterion in the events of chain growth makes it possible to influence the microstructure of the polymer, up to the formation, in some cases, of stereoregular and optically active polymers. The orienting effect of the counterion is most pronounced in the hydrocarbon medium, where Li is present, the most stereospecific of the alkali metals, 1,4-polyenes are formed (with a predominance of the cis structure in the case of isoprene or with an equal content of cis and trans structures in the case of butadiene) and isotactic polymethyl methacrylate. Among alkaline earth metals, the formation of cis-1,4-polydienes and isotactic PMMA is most favored by Ba.
Chain termination and transfer reactions characteristic of the anionic polymerization of monomers with polar functional groups. This is always a more complex process, accompanied by the deactivation of active centers upon interaction with the functional groups of the monomer and polymer. The activation energy of side reactions (as well as chain transfer to the solvent in the case of substances with a mobile hydrogen atom, for example, toluene), as a rule, is higher than the chain propagation energy; therefore, lowering the temperature usually contributes to the suppression of side reactions.
The most common chain termination reaction is the transfer of a hydride ion to a counterion or monomer:
Kinetics. Anionic polymerization is characterized by a wide variety of reaction mechanisms and kinetic schemes. In each specific case, the choice of initiators and process conditions is due to the need to synthesize a polymer of a certain structure and MWD. The rate of anionic polymerization, especially at moderate temperatures, is much higher than the rate of radical polymerization. This is due to the higher active concentration of active particles (in the limit, it can be equal to the initial concentration of the initiator). For example, for styrene at 30°C, the order of the absolute chain growth rate constant (in l/mol×s) in the transition from lithium associates II to free anions (V) changes from 10 -1 to 10 5 .
The general kinetic picture of anionic polymerization is significantly complicated by the multiplicity of forms of existence of active centers mentioned above. In addition to these, more complex formations also play a role, for example, ionic tees of the type P - , Me + , P - . Therefore, even in the case of living polymers at a fast stage of initiation, when the total concentration of growing chains is equal to the initial concentration of the initiator [С 0 ], the total rate of the chain growth reaction (u р) is by no means always described by a simple equation:
u p = k p [M]
,
where and [M] are the initial and current concentrations of the monomer, x=1-[M]/ is the degree of monomer conversion, n is the number of growing ends in the macromolecule.
More complex general dependencies are often observed:
,
where the contribution of various forms of active centers is taken into account.
Usually the order of the reaction according to the initiator varies from 1 to 0, and the order according to the monomer is in most cases equal to 1.
In anionic polymerization, the appearance of an active center is associated with the formation of a carbanion. Anionic polymerization is often divided into anionic and anionic-coordination ones. The latter includes polymerization in the presence of organometallic compounds, proceeding through the stage of formation of an intermediate complex of catalysts - a monomer, in which the catalyst is linked to the monomer by coordination bonds. Depending on the polarity of the medium and other reaction conditions, the polymerization mechanism can change from purely ionic to ionic-coordination and vice versa.
During the polymerization of styrene in the presence of potassium amide in liquid ammonia, each resulting polystyrene macromolecule contains an NH 2 group. In this case, the molecular weight of the polymer does not depend on the catalyst concentration and is directly proportional to the monomer concentration. As the polymerization temperature increases, the molecular weight of the polymer decreases. Chain termination occurs when a carbanion interacts with ammonia as a result of the addition of an ammonia proton with the regeneration of an amide ion.
Acrylic acid derivatives - methyl methacrylate, acrylonitrile, methacrylonitrile - also polymerize with alkali metal amides. These monomers contain electronegative substituents, i.e. are electron acceptors and therefore very active in anionic polymerization.
A feature of this polymerization is the bifunctional addition of the monomer. BM joins one function at a time. The chain growth reaction during polymerization involves 2 catalyst centers - metal and alkyl (two-center polymerization mechanism).
The mechanism is not fully understood and is very complex. It is assumed that the formation of a complex with a catalyst precedes the connection of a monomer molecule.
In such complexes, the metal is bound to the monomer by a coordination bond; therefore, polymerization proceeding with the formation of such complexes is called anionic-coordination polymerization.
In anionic polymerization, chain growth is carried out with the participation of a carbonion or an ion pair; while the terminal group of the growing macromolecule, having high activity at the same time, is quite stable. Therefore, anionic polymerization in the absence of impurities can lead to chain termination, in many cases it can proceed without chain termination until the monomer is completely exhausted. As a result of such polymerization, polymers are formed, the macromolecules of which contain active centers and are capable of initiating polymerization. These polymers are called "living" polymers. When a new portion of the monomer is added to such a polymer, its molecular weight increases.
Feature of "living" polymers:
- - when another monomer is added to “living” polymers or oligomers, block copolymers can be obtained (method for determining “living” macromolecules);
- - A "living" polymer for chain termination can be introduced with various compounds and polymers with various terminal functional groups can be obtained, which opens up great opportunities in the synthesis of block copolymers with heterochain oligomers.
In recent years, anion-coordination polymerization in the presence of complex Ziegler-Natta catalysts has become widespread. (This method is used in the industrial synthesis of stereoregular polymers.) Ziegler-Natta catalysts include organometallic compounds of groups I-III and chlorides of groups IV-VII with transitional valence. The most commonly used are organometallic compounds of aluminum and titanium chlorides, which easily form coordination bonds. Such complex catalysts are insoluble and their structure has not been established, but it is assumed that they represent a bimetallic complex with coordination bonds.
The dependence of the polymerization rate on the conformation of the molecular chain of synthetic polymers was first shown by the example of the polymerization of N-carboxyl anhydrides of amino acids with the formation of polypeptides. In this case, the reaction proceeds in 2 stages, which differ in speed. Stage 1 proceeds relatively slowly until an oligomer capable of coiling into a coil is formed, then the reaction proceeds at a high rate with the formation of a high molecular weight polypeptide. The presence of isomeric amino acids in the reaction mixture sharply reduces the rate of polymerization.
Then, ideas about the guiding role of the conformation of the resulting molecular chain during polymerization were transferred to vinyl monomers. From this point of view, the effect of the nature of the solvent and temperature on the stereospecificity of the polymerization of vinyl compounds is considered. Thus, it was shown that the polymerization of styrene in the presence of triphenylmethylpotassium in benzene leads to the formation of atactic polystyrene, and with the same catalyst in hexane a stereoregular polymer is obtained. From the standpoint of the so-called helical polymerization, this is explained by the high stability of the helical conformation of the growing polystyrene macromolecules during polymerization in a poor solvent compared to benzene - hexane. The formation of stereoregular polystyrene during polymerization in the presence of butyllithium at -30°C in a hydrocarbon environment and the lack of stereospecificity in the polymerization of styrene with this catalyst at a higher temperature are explained in a similar way. Such a new direction in the study of the mechanism of stereospecific polymerization is extremely interesting, although there is still little experimental data to create a harmonious concept.
The process of anionic polymerization proceeds with the participation of basic substances: alkali metals; alkali metal derivatives (alcoholates, amides, Me-organic compounds); most often sodium naphthalene complex.
Mechanism of chain growth in the Li-organic compound during the formation of the microstructure during the anionic polymerization of diene hydrocarbons:
It can be seen from the reaction scheme that the preliminary orientation of the monomer molecules and its introduction at the site of the polarized bond are carried out.
Chain termination in anionic polymerization reactions can proceed according to the following mechanisms of deactivation of active centers:
- transfer of dehydrogenated H from the end of the growing chain
~CH 2 -C - H-R + Me + → ~CH \u003d C - H-R + MeH
- the capture of a proton of a growing chain and the limitation of chain growth are observed during polymerization in liquid ammonia or a solvent capable of splitting a proton.
- the termination of the growing macroion due to its transformation into an ion with a reduced reactivity is possible due to the isomerization of the terminal group
~CH 2 -C - CH 3 -COOCH 3 Na + → ~CH 2 -CCH 3 \u003d C-O - OCH 3 Na +
During anionic polymerization, the process can proceed selectively and a microstructure can be formed. For example, isoprene during polymerization on an alkali metal in a solvent of pentane.
Mechanism of polymerization in the presence of alkali metal amides.
Initiation
KNH 2 → NH 3 K + + N - H 2
N - H 2 + CH 2 \u003d CH-R → NH 2 -CH 2 -C - H-RK +
chain growth
NH 2 -CH 2 C - HRK + → CH 2 CHR NH 2 -CH 2 CHR-CH 2 -C - HR
chain break
~CH 2 -C - HRK + + NH 3 → ~CH 2 -CH 2 R +N - H 2 K +
Mechanism for organometallic catalysts.
- Initiation
MeR '+ CH 2 = CH-R → R '-CH 2 -C - HRMe +
chain growth
R '-CH 2 -C - HRMe + → CH 2 =CHR R 'CH 2 -CHR-CH 2 -C - HRMe +
chain break
~CH 2 -C - HRMe + → ~CH=CHR + MeH
Lecture #6
Anionic-coordination polymerization: polymerization of dienes, polymerization on complex Ziegler-Natta catalysts on P-allyl complexes; obtaining stereoregular polymers.
Ion coordination polymerization differs from ionic polymerization in that the act of addition of the monomer is preceded by its coordination on the active center or catalyst. Monomer coordination can take place in both anionic and cationic polymerization, but it is more characteristic of anionic polymerization.
The numbers in the named isomeric units indicate the number of the carbon atom,
included in the main chain of the isoprene molecule. For the first time polymerization of isoprene on metallic Na in 1932 was carried out by Lebedev. Subsequently, isoprene is polymerized on Li-organic compounds in a hydrocarbon medium. Monomer coordination occurs on a polar but non-dissociated active center - C - Li - as a result of which the monomer unit takes on a configuration corresponding to
1.4 cis
The addition of only a few percent of electron-donor compounds (ether, tetrahydrofuran, alkylamine) dramatically changes the microstructure of the resulting polyisoprene - 1,4-trans (80-90%) and 3,4-structure (10-20%) become predominant. The electron-donating compound polarizes the C-Li bond prior to separation into ions
In this case, the microstructure of the polymer chain is determined by the coordination of the Li ion with the terminal unit of the macroion, which has an allyl structure. In the allyl structure, π-electrons are delocalized and, therefore, the two extreme carbon atoms are equivalent in electron density. For the carbon anion, this is expressed as follows:
With this in mind, the coordination of the Li ion with the final unit of the isoprene chain, which carries the charge, can be represented by a cyclic structure:
The monomer can add both to the 1st and to the 3rd C atom, which leads to a 1,4-trans or 3,4-structure.
In 1955, the German chemist Ziegler proposed a catalytic system consisting of 3-ethylaluminum and titanium chloride ((C 2 H 5) Al + TiCl 4) for the synthesis of polyethylene under mild conditions (50-80 C and p = 1 MPa). The Italian chemist Natta applied this system to the synthesis of polyethylene and polystyrene, and explained the mechanism of action of these catalysts. Currently, the group of Ziegler-Natta catalysts includes catalytic systems formed by the interaction of organic compounds of non-transition elements (1-3 gr.) and salts of transition elements (4-8 gr.). Heterogeneous and homogeneous Ziegler-Natta catalysts are known. At the first, mainly isotactic polymers are obtained, and at the second, iso- and syndiotactic ones. The detailed mechanism of olefin polymerization on Ziegler-Natta catalysts is still being discussed, however, it has been established that TiCl 4 alkylation with 3-ethylaluminum occurs at the first stage, and then the monomer is added via the labile TiC bond.
There are 2 points of view:
According to the first, an active center Ti 3+ is formed on the surface of crystalline TiCl 4 , on which the monomer is coordinated and then introduced into the Ti-C bond.
Coordination contributes to the weakening of the Ti–C bond, and also ensures the attachment of the monomer in a certain spatial position.
According to the second point of view, the interaction mechanism provides for the participation of R Al in the active center, which is a coordination complex in which the Ti atom forms a 3-center 2-electron bond with the allyl group, and the Al atom forms a 2-center coordination bond with the Cl atom titanium chloride (bridge connections).
In the initiation reaction, the monomer is coordinated on the positively polarized Ti atom, forming a π-complex, which then transforms into a σ-complex; as a result of these transformations, the monomer is introduced into the Ti-C bond and the structure of the active center is successively reproduced.
Subsequent acts of growth proceed similarly. It can be seen from the diagram that there is a negative charge at the active end of the chain, so polymerization on catalysts
Ziegler-Natta is referred to as an anion-coordination.
Chain termination during polymerization on these catalysts occurs as a result of the same reactions as in anionic polymerization, in particular, as a result of the transfer of a hydride ion to a monomer or a counterion. At present, stereoregular rubbers and polyolefins are obtained by anion-coordination polymerization.
Lecture #7
Copolymerization, its importance as a method of modifying polymers. Types of copolymerization: ideal, block copolymerization, grafting. composition of the copolymer. Regularities of the copolymerization process.
Copolymerization - joint polymerization of two or more monomers. It is widely used in practice, as it is a simple and very effective method for modifying the properties of bulk polymers. The most studied two-component or binary copolymerization. During copolymerization, the best properties of each of the homopolymers are achieved.
For example, polyethylene has high elasticity, frost resistance, but poor adhesive properties. The introduction of up to 30% vinyl acetate units into the polyethylene macromolecule gives the polymer the property of a hot melt adhesive.
To increase the frost resistance of polypropylene, butyl rubber units are introduced into the macromolecule (the brittleness temperature drops to -40 0).
Copolymerization consists in obtaining HMC from a mixture of two or more monomers, which are called comonomers. Copolymer macromolecules consist of units of all monomers present in the initial reaction mixture. Each comonomer imparts its own properties to the polymer, and the properties of the polymer are not the sum of the individual homopolymers. The patterns of copolymerization are more complicated than those of homopolymerization. If during homopolymerization there is one type of growing radical and one monomer, then during binary copolymerization there are 4 types of growing radicals. For example, if 2 monomers A and B interact with free radicals R · , arising from the decay of the initiator, primary R · , one of which has an end link A, and the second - B.
R · +A R A ·
R · +B R B ·
R A · and R B · can react with A and B:
A + RA A · (K AA)
B+ RA B · (K AB)
A+ RВ A · (KVA)
B+ RB B · (K VV)
The ratio of the reaction rate constant of each R · with “own” monomer to the rate constant of the reaction with “alien” monomers are called copolymerization constants, or relative activities r of monomers.
r A = K AA / K AB
r B = K BB / K VA
The values of r A and r B determine the composition of the copolymer macromolecules to a greater extent than the ratio of monomers in the initial reaction mixture. If the relative activities of the comonomers are approximately equal to 1, then each R · interacts with both its own and foreign monomers with equal probability. The attachment of the monomer to the chain is random and a random copolymer is formed. This is an ideal copolymerization. Copolymerization reactions can proceed by radical and ionic mechanisms. In ionic copolymerization, the constants are affected by the nature of the catalyst and solvent, so polymers obtained from the same monomers, but in the presence of different catalysts, have different chemical compositions. For example, a copolymer of styrene and acrylonitrile, synthesized from an equimolecular mixture of monomers in the presence of benzoyl peroxide, contains 58% styrene units, and during anionic copolymerization on a C 6 H 5 MgBr catalyst - 1%, and during cationic polymerization in the presence of SnCl 4 - 99%.
In practical terms, block and graft copolymers are of interest. In their macromolecules, there are areas of great length and links of each copolymer. Block copolymers are obtained by various methods. First, during the anionic polymerization of one monomer, the emerging “living” chains can initiate the polymerization of another monomer:
AAA - + n B \u003d - AAA (B) n-1 B -
Secondly, under intense mechanical action on a mixture of different polymers, the chains and resulting macroradicals are destroyed. Macroradicals, interacting with each other, form block copolymers. Block copolymers can also be formed from oligomers due to the interaction of terminal functional groups. Graft copolymers are produced by reacting a monomer with a polymer, and less commonly by reacting two different polymers. Since these processes use a chain transfer reaction with the transformation of polymer molecules into macroradicals, atoms or groups with increased mobility (Br, which accelerates the chain transfer reaction) are introduced into the composition of macromolecules. If the reaction medium contains a polymer based on the monomer CH 2 =CH-X, CH 2 =CH-Y, then the process of formation of the graft copolymer proceeds in a complex manner. First, the middle macroradical arises:
This macroradical then initiates the polymerization of the monomer with the formation of side branches:
The preparation of block and graft copolymers is always accompanied by the formation of a homopolymer from the monomer present in the reaction zone.
composition of the copolymer.
The composition of the copolymer is not equal to the composition of the original monomer mixture. The relationship between them can be established by kinetic and statistical methods.
1. Kinetic method. In most cases, the reactivity of centers at the ends of the chains is determined only by the nature of the end unit, therefore, when deriving the composition equation, four chain propagation reactions between monomers A and B and growing active chains, as well as the copolymerization constant, are taken into account. The differential equation for the composition of the copolymer looks like this:
d[A]/d[B]=[A](r A [A]+[B])/[B](r B +[A])
The equation relates the current or instantaneous concentrations of monomers in copolymers and monomer mixtures through the values of the relative activities of the monomers. The graphical form of this equation is the copolymer composition curves, the form of which is uniquely determined by r A and r B .
1- the composition of the copolymer is equal to the composition of the monomer mixture r A =r B =1 (type of ideal copolymerization), the distribution of links is statistical.
2- r A >1, r B<1
3-r A<1, r B <1
4-r A<1, r B >1. The copolymer is enriched with a more active monomer in the entire composition area.
5- r A → 0, r B → 0. In a copolymer, there is a strict alternation of monomer units for any composition of the monomer mixture. A 1:1 copolymer is formed.
6- r A → 0, r B<1. Для сополимеров также характерно чередование мономерных звеньев, но оно не является регулярным.
ANION POLYMERIZATION
General information
Anionic polymerization- This is ionic polymerization, in which the carriers of the kinetic chain are anions. active center during anionic polymerization carries partial or full negative charge. The general scheme for the initiation of polymerization through the formation of a carbanion can be represented as follows:
In the general case, in anionic systems, the existence of various forms of active centers in a state of equilibrium (polarized molecule–ionic pair–free ions) is possible:
This is the reason for the significant dependence of the kinetics of the process and the microstructure of the resulting polymer on the properties of the reaction medium and the counterion.
Monomers, most easily polymerized by the anionic mechanism, are unsaturated compounds containing electron-withdrawing substituents (-COOR, -NO 2, -CN, -CH \u003d CH 2, -C 6 H 5, etc.). Carbonyl-containing and heterocyclic compounds having C=C, C=O, C=N bonds (oxides, lactones, lactams, siloxanes, etc.) are also capable of anionic polymerization. The most common monomers by relative activity in anionic polymerization reactions can be lined up:
Initiators anionic polymerization can be substances basic O of an obvious nature are alkali and alkaline earth metals and their derivatives (alkyls, alcoholates, amides, aryls). It can also be caused by the action of an electric current or ionizing radiation. Typical examples anionic polymerization initiators - sodium amide (NaNH 2), alkali metals (Li, Na, K, Rb, Cs) and their alkyls, complexes of alkali metals with aromatic hydrocarbons (naphthyl Na, naphthyl K).
Anionic polymerization has certain benefits compared to radical polymerization: it is applicable to a wider range of monomers; provides incomparably greater opportunities for the synthesis of stereoregular, including optically active, polymers; due to the formation of "living" polymers in a number of anionic systems, it makes it possible to obtain polymers with a given molecular weight, a narrow molecular weight distribution and to synthesize block and graft copolymers of a given structure.
Process mechanism
General scheme Anionic polymerization includes initiation, growth, transfer, and chain termination. The last two reactions, which are often referred to as chain-limiting reactions, do not accompany anionic polymerization in all cases. They are more characteristic of polar media (chain transfer reaction) and polar monomers (chain termination reaction), and may also be due to the presence of random impurities in the reaction system, in particular, substances that deactivate organometallic compounds.
Initiation
The formation of an active center during anionic polymerization can occur by various mechanisms.
1. Initiation by free anion addition mechanism is realized during the polymerization of monomers in solvents with high polarity, for example, in liquid ammonia during catalysis alkali metals and their amides:
2. By electron transfer mechanism polymerization occurs from the initiator to the monomer when using alkali metals in a weakly polar environment, such as the polymerization of butadiene. The initial act of initiation is the formation of the radical anion of the monomer:
As a result of the recombination of two anion radicals, a biion is formed, which is the active center of polymerization (at low temperature and in a non-polar medium):
Better control of polymer molecular weight can be achieved using a catalytic complex alkali metal - naphthalene in a polar solvent (tetrahydrofuran). This type of initiation of anionic polymerization is called electron transfer polymerization. Process diagram:
The complex transfers its electron to the monomer, and naphthalene is regenerated into the original molecule:
Such catalytic systems are able, in the absence of impurities, to ensure chain growth without termination until the monomer is completely depleted, when the so-called. "living" polymers whose macromolecules are negatively charged ions.
3. By the mechanism of attachment to the monomer double bond initiator molecules act alkali metal alkyls(R-Me). In some cases, initiating systems of this type differ significantly from those considered above, since they have the ability to coordinate monomer molecule to give it a certain spatial position, which is preserved in the structure of the emerging macromolecule. This is especially evident in the polymerization of isoprene with n-butyl lithium:
As can be seen from the reaction scheme, in the process of chain growth, an isoprene molecule intercalates between a negatively charged alkyl residue and a positively charged lithium ion. In the resulting six-membered complex of isoprene with butyllithium, the isoprene molecule assumes a cis-conformation (according to the arrangement of the methylene groups relative to the double bond plane), which is retained during subsequent events of chain growth. The resulting polyisoprene with a high content of cis-1,4 units is a synthetic analogue of natural rubber and has the property of high elasticity.
During the polymerization of ethylene derivatives with these catalysts, the following cycle of reactions proceeds:
Since polymerization with organometallic catalysts proceeds in the field of two catalyst sites (alkali metal and alkyl), it is called two-center polymerization. The coordination of the monomer in the field of two centers makes it possible to distinguish this type of polymerization from others proceeding according to a purely anionic mechanism, and therefore it was called anionic-coordination polymerization.
chain growth
For most systems, the chain growth stage can be represented by a scheme in which the introduction of each new monomer molecule occurs between an alkyl residue with a negative charge and a counterion, most often a metal ion:
Implementation of such a mechanism provides strictly regular attachment of molecules according to the "head to tail" type, since the polarized monomer molecule is oriented before attachment under the influence of an ion pair. However stereoregularity while, as a rule, not achieved.
Stereoregular polymers obtained using organolithium catalysts, which is due to the formation cyclic complexes with an ion pair, in which the monomer has cis-conformation. This is due to the fact that among alkali metals, lithium has the smallest ionic radius and a high ionization potential, which causes the lowest polarity of the Li–C bond. This relationship is also preserved in the transitional complex. The higher the stability of the complex, the higher the stereoregularity.
Chain growth during anionic polymerization is characterized by the relative stability of active centers, which makes it possible to create conditions for studying the mechanism of anionic polymerization, as well as for solving various synthetic problems: obtaining polymers with a given MWD, incl. practically monodisperse; synthesis of polymers and oligomers with terminal functional groups capable of further transformations of the polycondensation or polymerization type, as well as block copolymers, graft copolymers and various polymers with a controlled type of branching, etc.
chain break
Deactivation of active sites can occur by several mechanisms.
1. Hydride ion transfer H - or another anion from the end of a growing chain to a monomer or counterion:
2. proton detachment growing chain from a monomer or solvent capable of donating protons (toluene, liquid ammonia, organic alcohols, acids, water):
3. Reducing the reactivity of the active center, for example, as a result of isomerization reactions:
As with cationic polymerization, in many cases polymerization proceeds without breaking the kinetic chain by the mechanism of "living" chains. The activity of such polymers can be maintained for a long time, and if, after the complete consumption of the monomer, it is added to the system again, the polymerization reaction will resume. Most often, these are polymers of hydrocarbons (styrene and its derivatives, dienes) during the polymerization of which the role of side reactions is minimal. According to the mechanism of "living" chains, the polymerization of heterocyclic compounds (ethylene oxide, cyclosiloxanes) can proceed.
The presence of long-lived active centers makes such systems very convenient objects for studying the mechanism of the polymerization process, as well as for various syntheses (block copolymers, star and graft polymers, polymers with terminal functional groups, etc.). The industrial application of these methods is limited the purity of the starting compounds and the need to maintain stringent synthesis conditions.
Anionic polymerization is characterized by a wide variety of reaction mechanisms and kinetic schemes. In each specific case, the choice of initiators and process conditions is due to the need to synthesize a polymer of a certain structure and MWD. The rate of anionic polymerization, especially at moderate temperatures, is much higher than the rate of radical polymerization. This is due to the higher active concentration of active particles (in the limit, it can be equal to the initial concentration of the initiator).
The general kinetic picture of anionic polymerization is significantly complicated by the multiplicity of forms of existence of active centers. Therefore, even in the case of living polymers at a fast stage of initiation, when the total concentration of growing chains is equal to the initial concentration of the initiator, the overall rate of the chain propagation reaction is by no means always described by a simple equation
living polymerization. Block copolymers
Living anionic polymerization was first observed by Abkin and Medvedev in the 1930s, but this process was systematically studied by Schwartz in the 1950s. XX century, and to date, anionic polymerization has become widespread. Carbanions are relatively more stable and therefore less reactive species compared to carbocations, so in anionic polymerization non-polar monomers in non-polar and low-polar solvents, for example, styrene or 1,3-butadiene in benzene, tetrahydrofuran, or 1,2-dimethyloxyethylene, all major material and kinetic chain termination reactions are absent. Polymerization continues until the complete exhaustion of the monomer, and upon its completion, the active centers (anions) of macromolecules remain for 1-2 weeks. During this period, polymerization can be resumed by adding a new portion of the monomer. It is believed that the slow deactivation of active sites is associated with reactions that begin with the transfer of a hydride ion to a counterion:
The allyl anion formed as a result of the last reaction is unable to initiate anionic polymerization.
Living polymerization of polar monomers is carried out at very low temperature in order to avoid transfer and termination reactions. All the main features of living chain polymerization - a linear increase in molecular weight with conversion, a narrow molecular weight distribution, the possibility of obtaining block copolymers - are most pronounced for living anionic polymerization. In particular, monodisperse polymers (usually polystyrene) used as standards in gel chromatography are obtained by this method in practice. At k O = 0 and at an initiation rate much higher than the growth rate k and >>k p, the rate and degree of polymerization are expressed by simple dependencies:
where [Mo] and [M] are the initial and current concentrations of the monomer; [I] is the initial concentration of the initiator; q = 1 – [M]/ is the degree of monomer conversion; P is the number of growing ends in a macromolecule. When initiating polymerization n-C 4 H 9 Li P= 1; in the case when electron transfer and the formation of radical ions take place at the initiation stage, P = 2.
Living ion polymerization is used in industry to obtain block copolymers. The general method is that at the end of the polymerization of one monomer, another monomer is added to its living chains. In some cases, the order is important, i.e. order of polymerization of different monomers. Thus, living chains of polystyrene can initiate the polymerization of methyl methacrylate, but not vice versa. Hence it follows that there are only two- and three-block (depending on the initiator) block copolymers of these monomers. In general, by sequential living anionic polymerization of different monomers, multiblock copolymers containing many different blocks can be obtained. The best known of the block copolymers are the so-called thermoplastic elastomers, in which one block refers to elastomers, the other to plastics. Thermoplastic elastomers have a complex of unusual properties intermediate between those of rubbers and plastics. Among thermoplastic elastomers, block copolymers of styrene with butadiene and isoprene are most common.
Influence of process conditions on the rate and degree of polymerization
It is known that anionic polymerization is usually carried out in a solvent environment. Depending on the nature of the solvent (mainly its polarity), the initiator or initiator complex is solvated differently by the solvent. As a result, ion pairs have different degrees of separation; accordingly, they coordinate the monomer differently in their field and initiate the polymerization process. At high polarity solvent, and, consequently, its high solvating ability, occurs ion pair separation and education free anions, whose activity is hundreds of times higher than that of ion pairs in chain propagation reactions. However, in this case, the coordinating ability of the ion pair is lost and regularity is broken structures of macromolecules.
The type of solvent affects the chain termination step and, as a result, the molecular weight of the polymers. So, for example, when polymerizing isoprene and butadiene in toluene (which readily undergoes chain transfer reactions), the molecular weight is much lower than in benzene:
The activation energy of side reactions (as well as chain transfer to the solvent in the case of substances with a mobile hydrogen atom, for example, toluene), as a rule, is higher than the chain propagation energy; That's why lowering the temperature usually contributes to the suppression of side reactions.
plays an important role in the rate of polymerization. the nature of the alkali metal ion: Generally, the chain growth rate increases with increasing ionic radius of the cation. The solvating ability of the solvent is maximum during solvation organolithium compounds due to the small ionic radius of lithium and decreases in the series of alkali metals: Li + > Na + > K + > Rb + > Cs + . Metal-carbon bond polarity c is inversely related, i.e., the polarity of the Li–C bond is also minimal due to the smallest ionic radius of Li + . This promotes better coordination of the monomer at the Li–С bond, in contrast to the Na–C and K–C bonds, where the polymerization mechanism is close to purely anionic. In accordance with the above, the polymerization rate is minimal for lithium derivatives of catalysts, but the monomer conversion is maximum and approaches 100%.
