Living Active Centers (LACs)

A. M. KAPLAN and N. I. CHEKUNAEV

N.N. Semenov’s Institute of Chemical Physics, Russian Academy of Sciences, Kosygin Street 4, Moscow 119991, Russian Federation, E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

DEFINITION OF LACS CONCEPT

The classic active center (A) (an ion, a radical, or an ion-radical) which is localized in “friable” zone of solids was named by authors as the living active center (LAC). It is known that the friable zones with the scale of nanosize exist near the structural defects (D) with the excess of a free volume (vacancies, dislocations, and microcracks in crystals and nanocavities in glasses). When approaching by a small distance (3-3.5 nm) the classical active center (A) and defect (D) the peculiar complex (C)-the complex (A...D) arises. Activation of the chain solid-state reaction (ChSR) occurs just in friable zones (FZs) of nanoparticles of the complex (C) (NPCs). Almost orientationally immobile and therefore chemically inert in dense zones of solids classic active centers (A) turn into LACs, when the centers (A) fall into the FZ of the NPC. The increasing of orientational mobility of reactants in NPC zones provides removal of the steric obstacles to the chemical interaction of LACs with neighboring particles of reagents. Activated in this manner, LACs provide realization of ChSRs.

HISTORICAL ORIGIN AND SCIENTIFIC DEVELOPMENT OF LACS CONCEPT

The introduction into the chemical science of a new LAC concept has been dictated by the need of the adequate description of the nontrivial

(according to traditional notions) kinetic peculiarities of chain reactions in solids.

The LAC concept was proposed by Anatoly Kaplan in 1969. It should be noted the difference of sense of two close by the title of chemical concepts. The first one is the known concept in polymer chemistry “living polymerization” refers to the description of some process. The second concept (author’s LAC concept) refers to a certain nanoparticle. The credibility to the author’s concept was first confirmed by the results of experimental study of solid state polymerization (SP) of acrylonitrile in a regime of the samples heating with stops (Kaplan et al., 1969). Subsequently, the author’s concept has been repeatedly confirmed in experimental studies of polymerization in crystals (e.g., Chachaty & Forchioni, 1972), in the glasses (e.g., Gerasimov Henry et al., 1973) and in studies of other ChSRs (e.g., Barkalov Igor et al., 1980).

In the development of new applications of the LAC concept for the analysis of peculiar nontrivial kinetic features of ChSRs, Anatoly Kaplan in 1989 has noted one additional important active agent in such processes. This is a mobile structural defect (MD), which is capable of transmuting orien- tationally motionless chemically inert active center A in the LAC (Kaplan, 1996).

Authors’ further development of the LAC concept application for analyzing of ChSRs was compiling of the complete system of differential equations describing of the change of all active agents in such processes. To describe the ChSR kinetics, it is necessary to add to the traditional system of equations describing the chain reactions in the gas or in liquid phase two new differential equations. These are the equations describing variation with time of the concentration of the two most important active agents in the ChSR (LACs and MDs). Numerical solution to the complete system of equations with the help of modern computer technology has allowed analyzing the majority of nontrivial kinetic features of the SP already at the quantitative level (Kaplan & Chekunaev, 2012).

NANOCHEMICAL APPLICATION OF LACS CONCEPT

Because the LAC is the important nanoobject, determining the main features of the ChSR, the LAC concept was the basis for the authors’ original kinetic model of the chain reactions development in solids

23.3.1 MAIN PROVISIONS OF THE KINETIC MODEL

The main provisions of the kinetic model of the chain reactions development in solids. Along with the LAC important object of the discussed model is a “friable” zone of NPC. In the discussed model as a structural defect involved in the formation of NPC was taken vacancy (Vac), because the Vac concentration is substantially higher than the concentrations of other structural defects in solids.

In accordance with the data (Rabinovich, 1968) diameter (d) of the “friable” “zone of NPC is equal d = 2/<=( 5-6)7. (where 7. = 0.5-0.6 nm is intennolecular distance). The rate constant for the interaction of LAC with neighboring reagent particles in the “friable” zone of solids is taken equal to the rate constant of the bimolecular chemical process: K = K0exp(-EJRT). Here, K0 and Ex are, respectively, pre-exponential factor and the activation energy of a chemical act of interaction of indicated reagents. The development of ChSRs is performed in a large number of “friable” zones of NPCs in the systems studied. Classic active center, which left the “friable” zone of the NPC becomes orientationally immobile and chemically inert. If this takes place the chain process is stopped. However, the rapprochement with the active center of the mobile structural defect (MD) can revitalize such center. This will provide the continuation of the investigated process (Kaplan & Chekunaev, 2012).

The simplest non-trivial kinetic features of ChSRs will be considered in this article below on the example of the SP.

23.3.2 THE ROLE OF LAC IN GROWTH OF POLYMER CHAINS IN THESP

Figure 23.1 shows a section of a sphere of radius r = (2.5-3) A. The area inside the sphere occupies a “friable” zone of NPC. The growth of the individual polymer chain in the “friable” zone of the NPC carried by the mechanism of “chemical diffusion.” According to this mechanism, the image of the polymer chain part formed in the NPC zone is a broken line in three-dimensional space. For clarity, Figure 23.1 shows only the monomer molecules, incoiporated into the growing polymer chain, which propagate in two-dimensional space (in a plane).

The mean number N of new links of the polymer chain, which appeared in the NPC zone were determined using the Einstein-Smoluchowsky relation (E-SR) for diffusing particles L2 = X2N. HereL = 2° V = 2.57.-205 is the average and most probable length of a straight line between points A*n_j and A*n+k

(see Figure 23.1) on the surface of the sphere (with radius r) surrounding the NPC zone. By use of E-SR fonnula was calculated the number of links of the polymer chain which are formed in one NPC zone:

Here are shown p and A - monomer density and the distance between

r m m J

the monomer molecules in the solid state; p и A - the same characteristics

vp p

for the polymer.

The mechanism of “chemical diffusion” of active center A* . (m varies

n+m v

from 1 to к + 1) disposed at the end of the growing polymer chain in a zone of the NPC (a sphere at Figure 23.1), leads after some time to moving away of the active center An k l from the center of vacancy to a distance greater than r. The active center An+k+1 becomes chemically inert (“congeals”), and chain process stops. However, the rapprochement of a mobile vacancy with this active center can reanimate it. The presence in the system of sufficient numbers of mobile vacancies causes the possibility of multiple returning of congealed polymer chain to its active growth.

An image of LAC action

FIGURE 23.1 An image of LAC action.

Thus, use of the LAC concept allowed predicts the previously unknown phenomenon of cyclic (with stops) growth of individual polymer chains in the solid state polymerization.

In conclusion, we note that the successful application of the LAC conception for the explanation of nontrivial kinetic features of the solid state polymerization is noted in an extensive review of the radiation polymerization (Abram et al., 1973), in a monograph (Mark & Sergei, 1990) and in a textbook for universities (Evgeniy, 1978).

KEYWORDS

  • classic active centers (ions, radicals, ion-radicals)
  • congealing and reanimation of chains
  • living active centers
  • mobile and immobile nanodefects (vacancies, dislocations, microcracks)
  • solid-phase chain reactions

REFERENCES

Abkin. A. D., Scheinker, A. P., & Gerasimov, G. N.. (1973). Chapter 1, Radiation polymerization. In: Kargin, V. A., (ed.). Radiation Chemistry’ of Polymers (pp. 7-107). Publishing house “Science.” Moscow.

Barkalov, I. M., Goldanskii, V. I.. Kiryukhin, D. P, & Zanin, A. M., (1980). Kinetics and mechanism of the low- temperature chain hydrobromination of ethylene. Chem. Phys. Lett., 75(2), p. 273.

Brooke, M. A., & Pavlov, S. A., (1990). Polymerization on the Surface of Solids (p. 183). Publisher “Chemistry.” Moscow. Soviet Union.

Chachaty, C., & Forchioni, A., (1972). NMR and ESR studies of the solid state polymerization of vinyl monomers. J. Polymer Sci., Part A—1, 10, p. 1905.

Denisov, E. T., (1978). Kinetics of Homogeneous Chemical Reactions (p. 367). Publisher "High School.” Moscow. Soviet Union.

Gerasimov, G. N.. Bespyatkina. T. A., et al., (1973). Peculiarities of Growth and Termination of Chains in Polymerization in Glassy Matrices (Vol. 209, p. 628). Dokl. Academy of Sciences of the USSR.

Kaplan, A. M., & Chekunaev, N. I., (2012). About the role played by mobile nanovoids in the stimulation of solid-state processes. Russian Journal of Physical Chemistiy B, 6(3), pp. 407-415. © Pleiades Publishing, Ltd.

Kaplan, A. M., (1996). Thesis for the degree of doctor of chemical sciences. "Peculiarities of radiation and mechanical- stimulated processes in nonequilibrium condensed systems.” Inst. Chem. Phys. R.1S, Moscow; 14.03.

Kaplan, A. M., Kiryuliin, D. P., Barkalov, I. M, & Goldanskii. V. I., (1969). About the phenomenon of "congealing'’ and “reanimation” of polymer chains during post-polymerization of low-temperature polymorph modification of solid acrylonitrile. V sokomoiek. Compound, 11, Brief Communications, 9. p. 639.

Rabinovich, M. K., (1968). Strength, Temperature, Time (p. 160). Publisher “Nauka.” Moscow. Soviet Union.

 
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