Conformations and configurations of macromolecules. Macromolecule configuration and stereoisomers
1.3. Macromolecular configuration
The concept of configuration includes a certain spatial arrangement of atoms of macromolecules, which does not change during thermal motion. The transition from one configuration to another is impossible without breaking chemical bonds.
Distinguish: 1) link configuration, 2) short-range order - the configuration of connecting links, 3) long-range order - the configuration of large sections (for example, blocks and their alternation, or the length and distribution of branches), 5) the configuration of an elongated chain as a whole.
Link configuration. Examples are the cis and trans configurations of diene polymers
1,4-cis-polyisoprene 1,4-trans-polyisoprene (natural rubber) (gutta-percha) Another example would be l,d-isomerism. For example,
for polymers with ~CH2 –CHR~ units, where R is any radical, the formation of two isomers is possible: l is left-handed, and d is right-handed
Link attachment configuration(short order). The links in the chain can be connected in a head-to-tail and head-to-head fashion:
is head-to-tail attachment, and head-to-head attachment requires overcoming large activation barriers.
For copolymers, the types of structural isomers increase compared to homopolymers. For example, for copolymers of butadiene and styrene, it is possible:
1. sequential alternation of links -A-B-A-B-A-B-,
2. combination of links in the form of dyads and triads–AA–BBB–AA–BBB– ,
3. statistical combination of links–AA–B–AA–BBB–A–B– . Far configuration order spreads on
tens and hundreds of atoms in the main chain. For example, large sequences of blocks in block copolymers or large sequences of units with the same stereoregularity (for example, polymers with isotactic, atactic and syndiotactic structures).
Isotactic Atactic Syndiotactic
Overall circuit configuration is determined by the mutual arrangement of large sequences of links (with a long-range order). For example, for branched macromolecules, various types of configurations are shown in Fig. 4.
Rice. 4. Configurations of macromolecules
1.4. Conformation of macromolecules
A conformation is a variable distribution in space of atoms or groups of atoms that form a macromolecule. The transition from one conformation to another can occur due to rotation, rotation, or oscillation of links around single bonds under the action of thermal motion or external forces and is not accompanied by the breaking of chemical bonds.
Polymers can take various conformations:
Statistical tangle is a folded conformation. It is formed when the intensity of the internal thermal motion prevails over the external influence. Characteristic of linear polymers [PE, PP, PB, PIB and ladder polymers (polyphenylenesiloxane).
Helix - is formed in polymers due to H-bonds (for example, in protein molecules and nucleic acids).
A globule is a very compact particle close to spherical in shape. It is characteristic of polymers with strong intramolecular interaction (for example, in PTFE).
Rod or string found in alkyl polyisocyanates.
Fold conformation. It is characteristic of polymers in a crystalline state (for example, in PE).
Crankshaft Conformation realized in poly-n-benzamide.
Fig.5. Conformations of macromolecules
1.5. Flexibility of macromolecules
Flexibility is one of the most important characteristics of polymers, which determines the highly elastic, relaxation, and thermomechanical properties of polymers, as well as the properties of their solutions. Flexibility characterizes the ability of macromolecules to change their shape under the influence of thermal motion of links or external mechanical influences. Flexibility is due to the internal rotation of links or parts of macromolecules relative to each other. Consider the phenomenon of internal rotation in molecules on the example of the simplest organic compound - the ethane molecule.
In the ethane molecule (CH3 -CH3) carbon atoms are bonded to hydrogen atoms and to each other by covalent (σ-bonds), and the angle between the directions of σ-bonds (valence angle) is 1090 28/. This causes a tetrahedral arrangement of substituents (hydrogen atoms) in space in the ethane molecule. Due to thermal motion in the ethane molecule, one CH3 group rotates relative to the other around the C-C axis. In this case, the spatial arrangement of atoms and the potential energy of the molecule are continuously changing. Graphically, various extreme arrangements of atoms in a molecule can be represented as projections of the molecule onto a horizontal plane (Fig. 6). Let us assume that in position a the potential energy of the molecule is U1, and in position b it is U2, while U1 ≠ U2, i.e. the positions of the molecule are energetically unequal. Position b, in which the H atoms are located one below the other, is energetically unfavorable, since repulsive forces appear between the H atoms, which tend to transfer the atoms to the energetically favorable position a. If accept
U1 =0, then U2 =max.
Rice. 6. Projection formulas for extreme arrangements of H atoms in space in an ethane molecule.
Rice. 7. Dependence of the potential energy of the molecule on the angle of rotation of the methyl group.
When one CH3 group is rotated relative to another by 600, the molecule goes from position a to b, and then after 600 again to position a, and so on. The change in the values of the potential energy of the ethane molecule from the angle of rotation φ is shown in Fig.7. Molecules with lesser symmetry (for example, the dichloroethane molecule) have a more complex dependence U=f(φ).
Potential (U 0 ) or activation barrier rotation
ion is the energy required for the transition of the molecule from the position of the minimum to the position of the maximum potential energy. For ethane, U0 is small (U0 = 11.7 kJ/mol) and at
At normal temperature, CH3 groups rotate around the C-C bond at high speed (1010 rpm).
If the molecule has an energy reserve less than U0, then there is no rotation and only oscillation of the atoms occurs relative to the position of the minimum energy - this is limited or
slow rotation.
In polymers, due to intra- and intermolecular interactions, the dependence U=f(φ) has a complex shape.
If one position of the chain link is characterized by potential energy U1, and the other - by U2, then the energy of transition from one position to another is equal to the difference ∆U= U1 - U2. The difference between the transition energies ∆U from one equilibrium position of a macromolecule unit to another characterizes thermodynamic flexibility. It determines the ability of the chain to bend under the influence of thermal motion.
Another characteristic of flexibility is the speed at which links move from one position to another. The rate of conformational transformations depends on the ratio of U0 and the energy of external influences. The more U0 , the slower the turns of the links and the less flexibility. The flexibility of macromolecules, determined by the value of U0, is called kinetic flexible
Factors that determine the flexibility of macromolecules
These factors include: the U0 value, polymer MM, density of the spatial network, size of substituents, and temperature.
Potential rotation barrier (U 0 ). The value of U0 depends on intra- and intermolecular interactions. Let us consider the factors affecting U0 and chain flexibility in carbon-chain polymers.
Carbochain polymers
In carbon chain polymers, saturated hydrocarbons are the least polar. Their intra- and intermolecular interactions are small, and the values of U0 and ∆U are also small, therefore, polymers have high kinetic and thermodynamic flexibility. Examples: PE, PP, PIB.
The values of U0 are especially low for polymers, in the chain of which there is a double bond next to the single bond.
–CH2 –CH=CH–CH2 – Polybutadiene
lar groups leads to intra- and intermolecular interactions. In this case, the degree of polarity significantly affects
With the introduction of polar groups, three cases are possible in terms of their effect on flexibility:
1. Polar groups are closely spaced and strong interactions are possible between them. The transition of such polymers from one spatial position to another requires overcoming large U0, so the chains of such polymers are the least flexible.
2. Polar groups are rarely located in the chain and there is no interaction between them. The values of U0 and ∆U are small and the polymers have high kinetic and thermodynamic flexibility.
-CF 2 -CF 2 -
Example: Polychloroprene
3.Polar groups are arranged so that the electric fields are mutually compensated. In this case, the total dipole moment of the macromolecule is equal to zero. Therefore, the values of U0 and ∆U are low, and polymers have high kinetic and thermodynamic flexibility.
Example: PTFE
Heterochain polymers
In heterochain polymers, rotation is possible around C–O, C–N, Si–O, and C–C bonds. The values of U0 for these bonds are small and the chains have sufficient kinetic flexibility. Examples: polyesters, polyamides, polyurethanes, silicone rubbers.
However, the flexibility of heterochain polymers can be limited by intermolecular interactions due to the formation of H-bonds (for example, in cellulose, polyamides). Cellulose is one of the rigid chain polymers. It contains a large number of polar groups (–OH) and therefore intra- and intermolecular interactions and high values of U0 and low flexibility are characteristic of cellulose.
Molecular weight of the polymer. An increase in the molecular weight of the polymer increases chain folding and, therefore, long macromolecules
have greater kinetic flexibility compared to short macromolecules. As the MM increases, the number of conformations that a macromolecule can adopt increases and the flexibility of the chains increases.
Spatial mesh density. The more chemical bonds between macromolecules, the less chain flexibility, i.e. as the density of the spatial grid increases, the flexibility decreases. An example is the decrease in chain flexibility with an increase in the number of crosslinks in the resol series.< резитол<резит.
Effect of size and number of substituents. An increase in the number of polar and large substituents reduces the mobility of the macromolecule units and reduces the kinetic flexibility. An example is the decrease in the flexibility of butadiene-styrene copolymer macromolecules with an increase in the content of bulky phenyl substituents in the chain.
If there are two substituents at one carbon atom in the main chain of the polymer (for example, OCH3 and CH3 in PMMA units), then the macromolecule becomes kinetically rigid.
Temperature. As the temperature rises, the kinetic energy of the macromolecule increases. As long as the value of the kinetic energy is less than U0, the chains perform torsional vibrations. When the kinetic energy of the macromolecule becomes equal to or exceeds U0, the links begin to rotate. With an increase in temperature, the value of U0 changes little, while the speed of rotation of the links increases and the kinetic flexibility increases.
test questions
1 General information about polymers, concepts, definitions.
2 Define and give examples of organic, non-
organic and organoelement polymers.
2 Classification of homochain polymers, examples.
3 Classification of heterochain polymers, examples.
4 Thermodynamic and kinetic flexibility of macromolecules. What factors affect the flexibility of macromolecules?
5 What is the configuration of macromolecules and what types of configurations of macromolecules are possible? Examples.
6 What is the conformation of macromolecules and what types of conformations of macromolecules are possible? Examples.
7 What parameters characterize the molecular weight, molecular weight distribution and polydispersity of polymers?
8 Molecular characteristics of oligomers.
9 Fractionation of polymers and construction of molecular curves cular mass distribution.
Introduction
Polymer molecules are a broad class of compounds, main the distinctive characteristics of which are a large molecular weight and high conformational flexibility of the chain. It can be said with confidence that all the characteristic properties of such molecules, as well as the possibilities of their application associated with these properties, are due to the above features.
Therefore, it is of great interest to study the possibility of a priori prediction of the chemical and physical behavior of a polymer based on an analysis of its structure. Such an opportunity is provided by the methods of molecular mechanics and molecular dynamics, implemented in the form of computer calculation programs.
Using these methods, the theoretical calculation of the most probable conformation of some oligomers with the number of monomeric units from 50 to 100 was carried out. Data were obtained that made it possible to determine the most probable conformation of molecules, the size of the Kuhn segment, and the number of monomeric residues in the segment.
Literature review
I. Polymers. Features of the structure and properties.
Polymers are high-molecular substances, the molecules of which consist of repeating structural elements - links connected in chains by chemical bonds, in an amount sufficient for the occurrence of specific properties. The following abilities should be attributed to specific properties:
1. the ability to significant mechanical reversible highly elastic deformations;
2. to the formation of anisotropic structures;
3. to the formation of highly viscous solutions when interacting with a solvent;
4. to a sharp change in properties when adding negligible additives of low molecular weight substances.
The given physicochemical features can be explained on the basis of the understanding of the structure of polymers. Speaking about the structure, one should imply the elemental composition of the substance, the order of the bonds of atoms, the nature of the bonds, the presence of intermolecular interactions. Characteristic of polymers is the presence of long chain molecules with a sharp difference in the nature of bonds along the chain and between chains. Of particular note is that there are no isolated chain molecules. The polymer molecule is always in interaction with the environment, which can have both a polymeric character (the case of a pure polymer) and the character of an ordinary liquid (diluted polymer solutions). Therefore, to characterize a polymer, it is not enough to indicate the type of bonds along the chain - it is also necessary to have information about the nature of intermolecular interaction. It should be borne in mind that the characteristic properties of polymers can only be realized when the bonds along the chain are much stronger than the cross-links formed due to intermolecular interactions of any origin. This is precisely the main feature of the structure of polymer bodies. Therefore, it can be argued that the entire complex of anomalous properties of polymers is determined by the presence of linear chain molecules with a relatively weak intermolecular interaction. The branching of these molecules or their connection into a network introduces some changes in the complex of properties, but does not change the state of affairs in essence as long as sufficiently long chain linear segments remain. On the contrary, the loss of the chain structure of molecules during the formation of globules or dense networks from them leads to the complete loss of the entire complex of properties characteristic of polymers.
The consequence of the above is the appearance of flexibility of the chain molecule. It lies in its ability to change shape under the influence of the thermal motion of the links or the external field in which the polymer is placed. This property is associated with the internal rotation of individual parts of the molecule relative to each other. In real polymer molecules, the bond angles have a well-defined value, and the links are not randomly located, and the position of each subsequent link turns out to be dependent on the position of the previous one.
Polymers that exhibit sufficiently intense torsional vibrations are called flex chain, and polymers in which the rotations of one part of the chain relative to the other are difficult - rigid chain.
This means that molecules can rotate and change their structure without breaking chemical bonds, forming various conformations, which are understood as various spatial forms of a molecule that arise when the relative orientation of its individual parts changes as a result of internal rotation of atoms or groups of atoms around simple bonds, bond bending, etc. .
II. Conformational analysis of polymers.
Conformational analysis is a section of stereochemistry that studies the conformations of molecules, their interconversions, and the dependence of physical and chemical properties on conformational characteristics. Each specific conformation corresponds to a specific energy. Under normal conditions, the molecule tends to move from the energetically least advantageous position to the most advantageous one. The energy required to move a molecule from a position with a minimum value of potential energy to a position corresponding to its maximum value is called potential barrier to rotation. If the level of this energy is high, then it is quite possible to isolate molecules with a certain spatial structure. The set of conformations that are in the vicinity of the energy minimum with an energy below the corresponding potential barrier is a conformer. The change in the conformation of the macromolecule occurs due to the limitation of the rotation of units around the bonds, as a result of which it usually takes the most probable form of a random coil. Various intra- and intermolecular interactions can lead to ordered conformations, as well as to an extremely folded globular conformation. Of exceptional importance is the conformational analysis in biochemistry. The chemical and biological properties of biopolymers depend to a large extent on their conformational properties. Conformational changes are an essential part of almost all biochemical processes. For example, in enzymatic reactions, the recognition of a substrate by an enzyme is determined by the spatial structure and the possibilities of mutual conformational adjustment of the participating molecules.
The following conformations are known:
The conformation of the macromolecular coil, i.e. more or less folded conformation, which the coil can take under the influence of thermal motion;
The conformation of an elongated rigid stick (or rod);
The helix conformation characteristic of proteins and nucleic acids also occurs in vinyl polymers and polyolefins, but they are not stabilized by hydrogen bonds and therefore less stable. The spiral can be either left-handed or right-handed, because it does not affect strength.
Globule conformation, i.e. very compact spherical particle;
Folded conformation, characteristic of many crystalline polymers;
“Crankshaft” or “Crank” Conformation
Each conformation of a macromolecule has a certain size. The theoretical calculation of the size of macromolecules was first made for a freely articulated chain, which, under the influence of thermal motion, can coil into a ball. The distance between the ends of such a macromolecular coil is denoted by h or r. Obviously, it can vary from 0 to L (the length of a fully unfolded chain). To calculate intermediate values of h, the apparatus of statistical physics (methods of molecular mechanics) is used, since there are a very large number of links in one chain.
A similar calculation can be made for a chain with fixed bond angles, replacing it with a freely articulated chain (a chain in which the links do not interact). In a freely articulated chain, the position of each link does not depend on the position of the previous one. In a real chain, the positions of the links are interconnected. However, for a very large chain length between sufficiently distant links, the interaction is negligibly small. If such links are connected by lines, then the directions of these lines are independent. This means that a real chain consisting of n monomer units of length l can be divided into N independent statistical elements (segments, segments) of length A.
It is believed that a statistical element, or a chain segment, of length A, the position of which does not depend on the position of neighboring segments, is called thermodynamic segment or Kuhn segment.
The length of the maximum elongated chain without violation of bond angles is called contour chain length L. It is related to the segment length by the relation
III. Empirical chemical methods of calculation.
For theoretical prediction of the most probable conformation of a molecule, the method of molecular mechanics is used. Molecular mechanics is a computational empirical method for determining the geometric characteristics and energy of molecules. It is based on the assumption that the energy of a molecule can be represented by the sum of contributions that can be attributed to bond lengths, bond angles, and torsion angles. In addition, in the general expression for energy there is always a term that reflects the van der Waals interaction of valence-free atoms, and a term that takes into account the electrostatic interaction of atoms and determines the presence of effective atomic charges.
E \u003d E sv + E shaft + E torus + E vdv + E cool
To calculate the first two terms, Hooke's law known from mechanics is most often used:
E st \u003d S k r (r - r 0) 2
It is assumed that the most thermodynamically stable conformation corresponds to the minimum energy. The method of molecular mechanics makes it possible to obtain information for a complete description of the geometry of various conformers in the ground state.
Classification of polymers according to the chemical structure of the main chain and the macromolecule as a whole. Intermolecular interaction in polymers. Concepts of energy density of cohesion and solubility parameter.
Structure of macromolecules includes their chemical structure and length, length and molecular weight distribution, shape and spatial arrangement of units. According to the chemical structure of the main chain, they are distinguished homochain (with a chain of carbon atoms - carbon chain ) and heterochain polymers, and according to the chemical structure of macromolecules in general - polymers:
· organic - the chain consists of carbon, oxygen, nitrogen and sulfur atoms;
· organoelement - the chain consists of silicon, phosphorus and other atoms to which carbon atoms or groups are attached, or vice versa;
· inorganic - there are no carbon atoms or carbon chains with multiple (double or triple) bonds without side groups.
Most common organic carbon chains polymers, including their various derivatives (halogen-containing, esters, alcohols, acids, etc.), the name of which is formed by the name of the monomer with the prefix "poly". Polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polytrifluorochloroethylene, polyvinyl alcohol, polyvinyl acetate, polyacrylamide, polyacrylonitrile, polymethyl methacrylate, and others belong to the limiting aliphatic carbon chain polymers. Polybutadiene, polyisoprene and polychloroprene are unsaturated, polyethylenephenylene is an example of fatty aromatic polymers, and polyphenylene is an example of aromatic polymers. Number inorganic homochain polymers are limited - carbochain carbine (~C≡C-C≡C~) and cumulene (=C=C=C=), as well as polyser (~S-S-S~), polysilane (~SiH 2 -SiH 2 ~), polygermane (~GeH 2 -GeH 2 ~), etc. More common organoelement homochain polymers from organic chains (carbochain) with organoelement side groups or from inorganic chains with organic radicals: polyvinylalkylsilanes, polyorganosilanes, boron-containing polymers. Organic heterochains polymers are divided into classes depending on the nature of the functional groups in the backbone. They can be aliphatic or aromatic, depending on the structure of the hydrocarbon groups between the functional groups (Table 1.1).
Table 1.1.
Heterochain polymers of various classes:
| Functional group | Polymer | |
| class name | Representatives | |
| Oxygen content | ||
| simple ethereal | Polyethers | Polymethylene oxide (~CH 2 -O~) |
| Polyethylene oxide (~CH 2 -CH 2 -O~) | ||
| Ester | Polyesters | Polyethylene terephthalate ([-CH 2 -CH 2 -O-OC-Ar-CO-O-] n) |
| Polyarylates ([-OC-R-COO-R`-O-] n) | ||
| Polycarbonates ([-O-Ar-CH 2 -Ar-O-CO-O-Ar-CH 2 -Ar-] n) | ||
| Nitrogen content | ||
| Acetal | Acetals | Cellulose (C 6 H 1 0 O 5) n |
| Amidnaya | Polyamides (-CO-NH-) | Polyhexamethylene adipamide |
| Imidnaya | Polyimides | Polypyromellitimide |
| Urea | Polyurea | Polynonamethylene urea |
| Urethane | Polyurethanes (–HN-CO-O) | ~(CH 2) 4 -O-CO-NH-(CH 2) 2 ~ |
| S e r c o n t e n s | ||
| Thioether | polysulfides | Polyethylene sulfide (~CH 2 -CH 2 -S~) |
| Sulfonic | Polysulfones | Poly- n,n`-oxydiphenylsulfone |
Inorganic heterochains the polymers are polyborazole, polysilicic acid, polyphosphonitrile chloride. Organoelement heterochain polymers include a large group of the most sought-after compounds from inorganic chains with organic side groups. These include silicon-containing polymers whose chains consist of alternating silicon and oxygen atoms ( polyorganosiloxanes ) or nitrogen ( polyorganosilazane ). Polymers with a third heteroatom in the main chain - a metal are called polymetallorganosiloxanes (polyaluminoorganosiloxanes, polyboroorganosiloxanes and polytitanorganosiloxanes). There are also polymers with organo-inorganic chains of carbon, silicon, oxygen atoms (polycarbosiloxanes, polycarbosilanes, polycarboranes), which may contain aliphatic or aromatic units. All atoms in the units of the considered polymers are connected chemical covalent bonds . There are also coordination (chelate, intracomplex) heterochain polymers, in which the units are connected by donor-acceptor interaction with a metal ion, forming coordination link (side valence) and ionic bond (main valency). Chemical and metallic bonds with a length of 0.1-0.2 nm significantly exceed the energy of physical bonds and even hydrogen bond (length 0.24-0.32 nm), which occupies an intermediate position between physical and chemical bonds. The polarity of the bonds also depends on the chemical structure and composition of the links, which is quantitatively estimated by the value of the dipole moment μ about, equal to the product of the charge and the distance between the charges (Table 1.3), as well as the level of intermolecular interaction in the polymer. Depending on the polarity of the bonds, the polymer can be polar and non-polar . The dipole moment of all organic carbon chain aliphatic (nonpolar) polymers is close to zero. Depending on the structure of macromolecules, dispersion, orientational, and induction bonds can appear between them. Dispersion bonds are due to the appearance of instantaneous dipoles in atoms during the rotation of electrons around nuclei. Polar macromolecules are characterized by orientation (dipole-dipole) bonds. In the field of dipoles of polar macromolecules, nonpolar macromolecules can also be polarized. Between permanent and induced dipoles arise induction connections.
Intermolecular interaction determines the ability of the polymer to dissolve in low molecular weight liquids, behavior at low temperatures, elastic and other properties. Its level is measured solubility parameter – the ratio of the product of the polymer density to the sum of the attraction constants of individual groups of atoms in the compound link to the molecular weight of the link. For this, they also use cohesive energy density (kJ/mol), which is equivalent to the work of removing interacting macromolecules or groups of atoms from each other over infinitely large distances. At glass transition temperature T s the energy of intermolecular interaction becomes higher than the energy of thermal motion, and the polymer passes into solid vitrified state . Polymers with T With above room is called plastics , and below room temperature and the solubility parameter 14-19 ( M . j/m 3 ) 1/2 – elastomers (rubbers).
Molecular weight of polymers and methods for its determination. Molecular weight distribution and shape of macromolecules. Classification of polymers according to the number and arrangement of constituent units.
Molecular mass(MM) - an important characteristic of the structure of polymers, which determines the level of mechanical properties and belonging to a certain group: oligomers (thermoplastics) - 10 3 -10 4, crystalline thermoplastics - 10 4 -5 . 10 4 , amorphous thermoplastics - 5 . 10 4 -2 . 10 5 , rubbers - 10 5 -10 6 . The lower the MM of polymers, the lower the viscosity of their melts and the easier they are molded. Mechanical properties are determined more by the degree of curing (oligomers) and crystallinity (polyamides, polyesters) or the transition to a glassy state. Rubbers, which are difficult to mold, have the highest MM, but products made from them have high elasticity. Since the same degree of polymerization is not obtained at high molecular weight, the macromolecules differ in size. Polydispersity (polymolecularity) - one of the basic concepts in the physical chemistry of polymers, and the type molecular weight distribution (MWD) is an important indicator that affects the physico-mechanical properties of polymers no less than MM.
Since MM is a statistical average, different methods for determining it give different values. FROM average number methods are based on determining the number of macromolecules in dilute polymer solutions, for example, by measuring their osmotic pressure, and medium-sized - on the determination of the mass of macromolecules, for example, by measuring light scattering. Average number MM ( M n ) is obtained by simply dividing the mass of a polymer sample by the number of macromolecules in it, and average mass MM: M w =M 1 w 1 +M 2 w 2 +…+M i w i , where w 1 , w 2 , w i – mass fractions of fractions; M1 , M2 , M i – mass average MM fractions. medium viscosity MM approaching the mass average MM is determined from the viscosity of dilute solutions. The polymer is called monodisperse , if it consists of one fraction with macromolecular sizes very close to each other, and for it the ratio Mw/M n =1.02-1.05. In other cases, the mass average MM is greater than the number average MM, and their ratio ( Mw/M n =2.0-5.0) is a measure of the polydispersity of the polymer. The more Mw/M n , the wider the MMR. On the polymer MWD curve, the value M n falls to the maximum, i.e. per fraction, the proportion of which in the composition of the polymer is the largest, and Mw shifted to the right along the x-axis.
The large sizes of polymer macromolecules determined one more feature of their structure. They can be linear or branched (with side branches from the main chain or star shape). At close MM values, they become isomers . The properties of polymers consisting of linear and branched macromolecules differ greatly. branching - an undesirable indicator of the structure of macromolecules, which reduces their regularity and hinders the crystallization of the polymer. The connection of macromolecules by chemical bonds leads to the formation mesh structures , further changing the properties of polymers. In accordance with such differences in the structure of macromolecules (Fig. 1.1), polymers are also called linear , branched and reticulated (stitched ).
In the latter case, the concept of "macromolecule" loses its meaning, since the entire cross-linked polymer sample becomes one giant molecule. Therefore, in cross-linked polymers, the average value of the MM of the chain segment between the chemical bonds (network nodes) connecting the macromolecules is determined.
copolymers contain links of two or more different monomers in the main chain (for example, styrene-butadiene rubber) and have a more complex structure than homopolymers consisting of units of one monomer. A copolymer with a random combination of units of monomers in a macromolecule is called statistical , with their correct alternation - alternating , and with a large length of sections (blocks) of links of one monomer - block copolymer . If the blocks of one of the monomers are attached to the main chain of the macromolecule, composed of units of another monomer, in the form of large side branches, then the copolymer is called vaccinated . The structure of a copolymer is characterized by the chemical composition and length of the blocks or grafted chains and the number of blocks or grafts in the macromolecule. Units of the same or different monomers can be combined regularly (end of one - start of another) or irregularly (the end of one is the end of the other, the beginning of the other is the beginning of the third link, etc.), and the substituents in the side groups can have a regular or irregular spatial arrangement. The structure of a macromolecule is also determined by its configuration and conformation.
Configuration of macromolecules and stereoisomers. Conformation and flexibility of macromolecules. Flexible and rigid chain polymers and the shape of their macromolecules.
Macromolecule configuration- this is a certain spatial arrangement of its atoms, which does not change during thermal motion, as a result of which its different types are stable isomers. Cis isomers characterized by the location of different substituents on opposite sides of the double bond in each repeating unit, and trans isomers - the presence of different substituents on one side of the double bond. Examples of such isomers are NK and gutta-percha, natural polyisoprenes identical in chemical structure. Gutta-percha is a plastic with a crystalline structure, melting at 50-70 ° C, and NK is an elastomer in the temperature range from +100 about C to -72 about C, since their macromolecules have different identity periods . AT cis-polyisoprene (NA) unidirectional methyl groups meet through one compound unit, which is equal to 0.82 nm, and in his trance-isomer (gutta-percha) - after 0.48 nm:
cis- 1,4-polyisoprene (NK)
trance-1.4-polyisoprene
From macromolecules optical polymers with an asymmetric carbon atom by special methods of synthesis are obtained stereoregular isomers - isotactic (substituents - on one side of the plane of the macromolecule) and syndiotactic (deputies - on opposite sides):
They differ in properties from atactic polymers with an irregular arrangement of substituents. The mutual repulsion of the substituents leads to their displacement relative to each other in space, and therefore the plane of symmetry is bent in the form of a spiral. Spiral structure is also characteristic of biologically active polymers (for example, the DNA double helix). The structure of macromolecules of stereoisomers is a carrier of information about the methods of their synthesis, and in proteins, double helixes of DNA carry enormous information about their biological heredity.
Conformation of a macromolecule- this is the spatial arrangement of atoms or groups of atoms, which can change under the influence of thermal motion without destroying the chemical bonds between them. The large length of the macromolecule, with the possibility of rotation of its parts around fixed chemical bonds, causes rotational isomerism , which is expressed in the appearance of various conformations. The closer the hydrogen atoms are to each other ( cis-position), the greater their repulsion and, accordingly, the potential energy of the macromolecule. The interaction is enhanced by polar substituents, such as chlorine atoms. AT trance-isomers, the potential energy of the macromolecule is less, the arrangement of atoms is more favorable than in cis-isomers. Energy rotation barrier parts of a macromolecule, which makes it inhibited , consisting of a series of fluctuations, help to overcome thermal energy fluctuations . The totality of vibrations and movements around simple bonds leads to to curvature macromolecules in space, which can go in different directions and change in time. In other words, the macromolecule has flexibility - the ability to change its conformation as a result of thermal movement or the action of external forces. With a large number of atoms, the chain can not only be bent, but even curl up in very loose macromolecular coil , the size of which can be characterized root-mean-square distance between its ends and calculate mathematically, knowing the number of component links in it. Due to the chain structure of macromolecules, the movement of one atom or grouping will lead to the movement of others, resulting in a movement similar to the movement of a caterpillar or worm, which is called repational (fig.1.2). A segment of a chain that moves as a whole in an elementary act of motion is called chain segment . Thermodynamic flexibility characterizes the ability of the chain to change its conformation under the action of thermal motion and can be estimated by the stiffness parameter, the length of the thermodynamic segment, or the Flory flexibility parameter. The lower these indicators, the higher the probability of a macromolecule transition from one conformation to another (Table 1.4). Stiffness parameter evaluated by the ratio of root-mean-square distances between the ends of real and free-jointed chains in dilute polymer solutions. Length of thermodynamic segment A (Kuhn's segment) characterizes such a sequence of links, in which each link behaves independently of the others, and is also related to the root-mean-square distance between the ends of the chain. It is equal to the hydrodynamic length of a macromolecule for extremely rigid chains and the length of a repeating link for extremely flexible chains. Polymers of the diene series and with ~Si-O~ or ~C-O~ bonds in the main chain are characterized by greater flexibility compared to polymers of the vinyl series, since they, due to a decrease in exchange interactions between CH 2 -groups 100 times lower energy of rotational isomers. The nature of substituents has little effect on the flexibility of macromolecules. Flory Flexibility Parameter f about shows the content of flexible bonds in a macromolecule and serves as a criterion for flexibility, according to which polymers are divided into flexible chain (f about>0,63; BUT<10nm) and rigid chain (f about<0,63; BUT>35nm). The latter do not occur in the conformation of a macromolecular coil and have an elongated shape of macromolecules - an elastic string (polyalkyl isocyanate, BUT = 100), crankshaft (poly- P-benzamide, BUT =210) or spirals (biopolymers, BUT =240).Kinetic Flexibility macromolecule reflects the rate of its transition in a force field from one conformation to another and is determined by the value kinetic segment , i.e. that part of the macromolecule that responds to external influences as a whole. Unlike the thermodynamic segment, it is determined by the temperature and the speed of the external influence. With an increase in temperature, the kinetic energy and flexibility of the macromolecule increase, and the size of the kinetic segment decreases. Under conditions where the time of action of the force is longer than the time of transition from one conformation to another, the kinetic flexibility is high, and the kinetic segment is close in size to the thermodynamic segment. Under rapid deformation, the kinetic segment is close to the hydrodynamic length of the macromolecule, and even a thermodynamically flexible chain behaves as rigid. The kinetic flexibility of an isolated macromolecule is determined from the viscoelastic properties of highly dilute solutions with their subsequent extrapolation to zero concentration. Macromolecules of a flexible chain amorphous polymer have ball-shaped both in isolated form and in bulk. At the same time, the structure of the polymer is not similar to the structure of "molecular felt", in which the macromolecules are randomly entangled, as previously thought. The idea of ordered regions in amorphous polymers was put forward in 1948 by Alfrey.
macromolecule configuration otherwise primary structure(English) - spatial arrangement of atoms in . It is determined by the values of bond angles and the lengths of the corresponding bonds.
Description
The configuration of a macromolecule is determined by the mutual arrangement of its constituent monomer units, as well as by their structure. Currently, the term "structure" or "primary structure" is usually used to describe the configuration of macromolecules.
A distinction is made between short-range (configuration of attachment of neighboring units) and long-range configurational order, which characterizes the structure of sufficiently extended sections of macromolecules. A quantitative measure of tact (order) is the degree of stereoregularity. In addition, tacticity can be described by the number of different types of pairs of nearest neighbors (di-, tri-, tetrads), the distribution of which is determined experimentally. A quantitative characteristic of the configuration of statistical network macromolecules, for example, is the crosslink density, i.e., the average chain section between network nodes.
The configuration of macromolecules is determined by the methods of X-ray diffraction analysis, birefringence, etc. As a rule, each method is the most "sensitive" to any configuration characteristic; Thus, NMR in many cases makes it possible to quantitatively characterize the short-range configurational order in
· organic polymers(the composition includes organogenic elements - C, N, O, P, S). They are divided into homochain (the main chain contains only carbon atoms) and heterochain (the main chain includes other atoms) Biopolymers belong to this class of polymers.
· organoelement polymers(in the composition of the main chain, along with carbon atoms, there are atoms of Si, Al, Ti, Ge, B).
· inorganic polymers ( the main chain does not contain carbon atoms, such as silicones).
1. List the types of polymer nomenclature.
2. How is the nomenclature based on the name of the monomers formed?
3. Give examples of the names of polymers according to the nomenclature based on the chemical structure of the polymer chain.
4. Name the types of classification of polymers. Give examples.
5. What types of copolymers exist?
6. How is the chemical classification of polymers carried out?
Tasks for independent solution*
2. Classification and structural formulas of basic polymers
2.1 Classification of polymers
Questions 2501 - 2502, 2403 - 2406, 2307
2.2. Structural formulas of basic polymers
Questions 3501, 3402, 3303 - 3309
*Here and in the future, the tasks are given from the “Collection of test tasks for thematic and final control in the discipline “Chemistry and Physics of Polymers”, M., MITHT, 2009.
Section 3. Main characteristics of macromolecules
Macromolecules are characterized by 4 main parameters:
1. Molecular weight (MM), molecular weight distribution (MWD);
2. Configuration of the macromolecule;
3. Conformation of the macromolecule;
4. Topology (linear, branched).
MM allows you to determine the length, size of macromolecules;
The configuration determines the chemical structure of macromolecules;
Conformation determines the shape of macromolecules.
3.1. Molecular weight (MW), molecular weight distribution (MWD)
The main differences between the concept of MM for Navy and NMS:
MM is a measure of molecular length for linear polymers and can be expressed in terms of the MM of low molecular weight compound repeating units:
https://pandia.ru/text/78/135/images/image040_18.gif" width="12" height="2 src=">m0 is the molecular weight of the compound repeating unit;
Pn - degree of polymerization
Most synthetic polymers are not individual compounds, but consist of a mixture of molecules of different sizes but the same composition.
This leads to:
· for polymers, the effective molecular weight is an average value due to polydispersity - the spread of macromolecules in molecular weight;
In most polymers, the end groups differ from the composition of the polymer chain links;
· certain side branches can exist in macromolecules, this also distinguishes macromolecules from each other;
Most biopolymers are individual compounds (each specific polymer is unique in composition, structure and molecular weight).
Reasons for polydispersity:
1. due to the statistical nature of the polymer production process: during the synthesis, macromolecules of various lengths are obtained;
2. due to the processes of partial destruction of macromolecules, for example, during the operation of the material;
3. due to the difference in the end groups of the polymer molecule;
4. due to the presence of branches in some polymers in different places and different chemical structures.
3.1.1. Methods for averaging molecular weights
1) Averaging over the number of molecules
Average number MM:
Мw=∑(NiMi2)/∑(NiMi) (3.1.1.2)
The mass of a fraction of a given molecular weight is taken into account.
Mw is determined using chromatography, ultracentrifugation, light scattering methods.
Kn=Mw/Mn (3.1.1.3)
For monodisperse (biological) polymers Kn=1.
With a narrow distribution Kn=1.01÷1.05.
In industry, polymers with Kn=3÷10 are most often obtained.
3) Medium viscosity MM:
Mŋ=((∑NiMi)1+α/∑(NiMi))1/α, 0<α<1 (3.1.1.4)
3.1.2. Molecular weight distribution (MWD)
The most complete characteristic of the molecular weights of polymers are distribution functions by molecular weights.
Nitrogen, boron, aluminum can be elements of macromolecular chains in other components of the polymer structure, or enter as heteroatoms into the main chain.
4.3. Carbon
It has a high tendency to form strong covalent bonds, both between its own atoms and with other atoms.
https://pandia.ru/text/78/135/images/image064_12.gif" width="102" height="92"> - two-dimensional carbon-carbon structure of graphene, graphite and carbon black
It is also possible to obtain a linear chain of carbon atoms:
https://pandia.ru/text/78/135/images/image066_10.gif" width="238" height="14 src=">
When heated, it turns into graphite.
Much greater opportunities for building linear macromolecules from carbon atoms open up when 1 or 2 carbon valences are saturated with other atoms or groups.
- polyethylene
- polypropylene
- polytetrafluoroethylene
Also, the main chain can contain various groups containing heteroatoms:
https://pandia.ru/text/78/135/images/image071_11.gif" width="93" height="43 src="> - ester grouping
https://pandia.ru/text/78/135/images/image073_9.gif" width="105" height="45 src="> - carbamide (urea) grouping
https://pandia.ru/text/78/135/images/image076_9.gif" width="185 height=84" height="84">
But they are not very stable chemically, and when oxidized, silicon binds to oxygen, forming very strong silicon-oxygen bonds.
In nature, silicon occurs in the form of quartz:
This is a rigid three-dimensional structure that does not exhibit the "polymeric" properties of linear macromolecules. Linear macromolecules are obtained by replacing two valences at each silicon atom with organic radicals (CH3-, C2H5-, etc.). In this case, silicon-organic polymers appear.
It is possible to synthesize silicon-containing polymers:
- polysiloxanes
Atoms Al, B, Ti, Zn, and some others can be embedded in the chain.
4.5. Phosphorus
Phosphorus atoms can form polymers, but the main chain must also include other atoms (most often oxygen):
- polyphosphates
- polyphosphoric acid
Phosphoric acid residues are included in natural polymers (nucleic acids, DNA and RNA):
Thus, two or polyvalent atoms (C, O, P, N, S, Si, Al, B and some others) can be in the form of elements of the main chain of macromolecules or be in side fragments; monovalent atoms (H, F, Cl, J, Br and some others) can only line up as substituents.
The chemistry of polymers is based on these elements.
4.6. Types of polymers
Polymers are obtained either synthetically, or extracted from living organisms (biopolymers), or by processing already isolated natural polymers.
Some synthetically created polymers exist in nature. Polymers are obtained from monomers - low molecular weight substances or as a result of transformations of finished polymers (synthetic or natural) - polymer-analogous transformations.
1,4-cis-polybutadiene does not exist in nature; it is obtained synthetically from butadiene.
1,4-cis-polyisoprene exists in nature (natural rubber), but is synthesized in nature from glucose and other substances (but not from isoprene, as in industry)
This polyester can be obtained by condensation of poly-β-hydroxybutyrate, at the same time, it is also synthesized by a number of bacteria.
Syntheses of biopolymers will not be considered in this course.
Many natural polymers are very difficult to obtain synthetically. They are obtained in living organisms as a result of complex biochemical reactions.
The most important natural polymers:
Examples are the reactions polyesterification:
HO-R-COOH + HO-R-COOH > HO-R-COO-R-COOH + H2O etc.
polyamidation:
H2N-R-NH2 + ClOC-R"-COCl > H2N-R-NHCO-R"-COCl + HCl, etc.
In this case, unlike polymerization, the elemental composition of the polycondensation products in this case does not coincide with the composition of monomeric compounds, since each chemical act of polycondensation is accompanied by the release of a molecule of a low molecular weight product.
The above general scheme of polycondensation also corresponds to some types of processes that are not accompanied by the release of low molecular weight products. These include, for example, the synthesis of polyurethanes from glycols and diisocyanates:
HO-R-OH + O=C=N-R "-N=C=O > HO-R-O-CO-NH-R"-N=C=O etc.
Such polycondensation processes are often referred to as polyaddition. According to the kinetic regularities, polyaddition reactions are very similar to polycondensation reactions. In both types of polycondensation processes, the growth of macromolecules is carried out by the interaction of functional groups of monomer molecules or the same groups located at the ends of already formed chains of different molecular weights. The intermediate polymeric products obtained as a result of these reactions are quite stable and can be isolated in free form. However, they contain reactive groups at their ends and are therefore capable of further condensation reactions, both with each other and with the corresponding monomeric molecules. Hence it follows that, theoretically, the polycondensation can be considered complete only when all the terminal functional groups have reacted, as a result of which one giant cyclic macromolecule should be formed. In practice, however, this is never achieved.
Questions for self-study:
1. What elements of the Periodic Table are capable of forming polymer chains?
2. Give examples of polymers obtained synthetically.
3. Give examples of natural polymers.
