Polymerization(redirected from cross-linked polymerization)
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the process of producing macromolecular substances in which polymer molecules (macromolecules) are formed by the successive addition of low-molecular-weight substances (monomers) to the reactive site at the end of the growing chain. Upon becoming part of the chain, a monomer molecule forms the monomer unit of the chain. The number of such units in the macromolecule is called the degree of polymerization.
A distinction between homopolymerization (one monomer) and copolymerization (two or more monomers) is made on the basis of the number of monomer types participating in the polymerization process. A distinction is also made between free-radical and ionic polymerization on the basis of the nature of the reactive sites forming the chain. In free-radical polymerization, the reactive site is a free radical, and the act of chain growth is a homolytic reaction. In ionic polymerization, the reactive sites are ions or polarized molecules, and the opening of double bonds or rings takes place heterolytically. Ionic polymerization, in turn, is subdivided into anionic polymerization, if the terminal atom of the growing chain carries a full or partial negative charge, and cationic polymerization, if the terminal atom is positively charged. The reactive sites in ionic polymerization are rarely free ions. An oppositely charged component (counterion) is usually present at the active center in addition to the growing end of the chain. In many cases, the addition of a monomer to the growing end of the chain is preceded by the formation of a coordination complex with the counterion. This type of polymerization is called ionic coordination polymerization. Because of the moderating effect of the counterion in ionic coordination polymerization, the formation of polymers with a high degree of ordered structure is possible. In this case, the polymerization is called stereospecific. The capacity of a particular monomer for polymerization is determined by both thermodynamic and kinetic factors, that is, the presence of a suitable initiator and the selection of conditions. The polymerization of most monomers occurs through the scission of multiple C=C, C≡C, C=0, C≡N bonds
n A=B→[— A — B —]n
or by means of cyclic groupings
where A, B, and X are various atoms or groups. Thus, the composition and structure of the monomer unit in the macromolecule correspond to those of the initial monomer (with the exception, of course, of the bond that is opened during polymerization). However, several cases are known in which the monomer units formed during polymerization differ from the initial monomer in structure and sometimes in composition—for example, because of the formation of new bonds within the monomer unit, the shift of an atom or group during the addition of the monomer to the growing chain, and the liberation of low-molecular-weight substances.
Polymerization is a special type of chain reaction in which the development of the kinetic chain is accompanied by the growth of a material macromolecular chain. Several basic stages, called elementary steps, may be distinguished in polymerization: initiation, chain growth, termination, and transfer.
Initiation is the conversion of a small fraction of the monomer molecules into reactive sites, which are capable of adding further monomer molecules. Special compounds called initiators or polymerization catalysts are added for this purpose (initiators are incorporated into the structure of the polymer; polymerization catalysts are not). Polymerization also may be induced by the action of ionizing radiation, light, or electric current.
Chain growth consists of a series of repeating similar addition reactions of the monomer molecules (M) to the reactive site (M*):
As a result of this process, the initial low-molecular-weight reactive site grows into a macromolecule.
Termination of chain is the deactivation of the reactive site by its reaction with another active center or some other compound or as a result of rearrangement to an unreactive product. In transfer, the reactive site is transferred from a growing macromolecule to some other species X (a monomer, solvent, or polymer), which begins the growth of a new macromolecule:
In some cases, transfer results in the formation of a stable compound, which does not annex monomers. Such a reaction, which is kinetically equivalent to termination, is called inhibition, and a compound producing this effect is called an inhibitor. When sufficiently large amounts of efficient transfer agents are added to the system, only low-molecular-weight compounds are formed; in this case, the process is called telomerization.
In the absence of transfer, the length of the kinetic chain of the process (the number of monomer molecules that react with the active center from initiation to termination) is equal to the length of the molecular chain (the number of units in the resultant macromolecule). In the presence of transfer, the length of the kinetic chain exceeds the length of the molecular chain. Thus, each initiation step leads to the formation of one macromolecule if there is no transfer or of several macromolecules if transfer reactions take place.
Since a growing reactive site of any length may enter into a chain-growth, termination, or transfer reaction with a certain probability, the degree of polymerization and molecular weight of the polymer are statistical quantities. The nature of the size distribution of the macromolecules is determined by the mechanism of the polymerization process and may, in principle, be calculated if the kinetic scheme of the process is known.
The equations relating the rate of polymerization to the concentrations of the major components may take on a very wide variety of forms, depending on the mechanism of the actual processes. However, the general principle for deriving these equations is the same in all cases and is based on a few simplifying assumptions. The most important such assumption is that the reactivity of the growing chains does not depend on chain length when the latter exceeds a certain limit (three to four units). The steady-state principle—that is, the assumption that the concentration of the growing chains does not change over time or that the rates of chain initiation and termination are equal—is used to calculate processes in which the lifetime of the growing chains is short compared to the total time of the polymerization process.
Polymerization may be carried out by various processes, which differ in the state of aggregation of the system being polymerized. The most common processes are (1) polymerization of a liquid monomer in the absence of solvent (bulk polymerization) or in a solvent under the action of a radical or ionic initiator or of a disperse or granulated solid catalyst, (2) polymerization in aqueous emulsions and suspensions, (3) polymerization in the solid phase under the action of ionizing radiation, and (4) polymerization of a gaseous monomer under the action of ionizing radiation or on the surface of a solid catalyst.
Polymerization was discovered as early as the mid-19th century, virtually simultaneously with the isolation of the first monomers capable of polymerization (styrene, isoprene, and methacrylic acid). However, the concept of polymerization as a unique chain process for the formation of true chemical bonds between the monomer molecules was understood only in the 1920’s and 1930’s through the work of S. V. Lebedev, H. Staudinger, K. Ziegler, and the American scientist F. Whitmore.
Polymerization accounts for about three-quarters of world production of polymers. The industry based on the production of polymers by polymerization is one of the largest and probably fastest-growing branches of the organic synthesis industry. The introduction of ionic coordination polymerization, which is characterized by high efficiency, high stereoregulatory capacity, and the possibility of flexible control of the properties of the products, is a current feature of this industry.
REFERENCEEntsiklopediia polimerov, vols. 1–3. Moscow, 1972–77.
A. A. AREST-IAKUBOVICH
an increase in the number of homologous organs or organelles in the course of evolution. The concept of polymerization of organs as an important morphophysiological priniciple in the evolution of Protozoa was introduced by V. A. Dogel’ in 1929. In contrast to multicellular organisms, in which involution of organs plays the major role, an increase in the number of organelles is observed in unicellular organisms in all phylogenetic progressions (Infusoria, Foraminifera, and Ra-diolaria). An example of polymerization of organs is polykaryon —that is, the multiplicity of nuclei.
REFERENCESPolianskii, Iu. I. “Evoliutsiia prosteishikh i morfo-fiziologicheskie zakonomernosti evoliutsionnogo protsessa.” In Zakonomernosti progressivnoi evoliutsii Leningrad, 1972.
Dogiel, V. “Polymerisation als ein Prinzip der progessiven Entwicklung bei Protozoen.” Biologisches Zentralblatt, 1929, vol. 49, pp. 451–69.