Propagating reactions, kinetics

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

Termination. The conversion of peroxy and alkyl radicals to nonradical species terminates the propagation reactions, thus decreasing the kinetic chain length. Termination reactions (eqs. 7 and 8) are significant when the oxygen concentration is very low, as in polymers with thick cross-sections where the oxidation rate is controlled by the diffusion of oxygen, or in a closed extmder. The combination of alkyl radicals (eq. 7) leads to cross-linking, which causes an undesirable increase in melt viscosity. 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

The kinetics of copolymerisation are rather complex since four propagation reactions can take place if two monomers are present... 

The influence of penultimate units on the kinetics of copolymerization and the composition of copolymers was first considered in a formal way by Merz et al and Ham.8 They consider eight propagation reactions (Scheme 7.2). 

Terpolymerizations or ternary copolymerizations, as the names suggest, are polymerizations involving three monomers. Most industrial copolymerizations involve three or more monomers. The statistics of terpolymerization were worked out by Alfrey and Goldfinger in 1944.111 If we assume terminal model kinetics, ternary copolymerization involves nine distinct propagation reactions (Scheme 7.9). 

In early work, it was assumed that the rate constant for termination was determined by the monomer unit at the reacting chain ends. The kinetics of copolymerization were then dictated by the rate of initiation, the rates of the four propagation reactions (Scheme 7.1) and rales of three termination reactions... 

At the initial stage of bulk copolymerization the reaction system represents the diluted solution of macromolecules in monomers. Every radical here is an individual microreactor with boundaries permeable to monomer molecules, whose concentrations in this microreactor are governed by the thermodynamic equilibrium whereas the polymer chain propagation is kinetically controlled. The evolution of the composition of a macroradical X under the increase of its length Z is described by the set of equations ... 

Thus propagation must be much faster than isomerization, and the product will be determined by thermodynamics, rather than by reaction kinetics. The net results of the two processes may be quite similar, however, in that polymers of unexpected structures may be obtained, and copolymers may be prepared by polymerization of a single monomer. 

The reaction kinetics In the case of HMDS suggests a two stage mechanism for the major dichloride consuming reaction. First a slow Na surface dependent build up of long lived active centers then a propagation step In which the Na surface is not rate determining. The reaction which occurs on the surface must be fast. 

To elucidate the chain propagation mechanism of alternating copolymerization of TBSM with MA and to quantitatively estimate the contribution of complex-bound monomers to the propagation reaction, a kinetic approach described in Ref.91) was employed. 

Propagation problems. These problems are concerned with predicting the subsequent behavior of a system from a knowledge of the initial state. For this reason they are often called the transient (time-varying) or unsteady-state phenomena. Chemical engineering examples include the transient state of chemical reactions (kinetics), the propagation of pressure waves in a fluid, transient behavior of an adsorption column, and the rate of approach to equilibrium of a packed distillation column. 

Radical polymerizations have three important reaction steps in common chain initiation, chain propagation, and chain termination. For the termination of chain radicals several mechanisms are possible. Since the lifetime of a radical is usually less than 1 s, radicals are continuously generated and terminated. Each propagating radical can add a finite number of monomers between its initiation and termination. If a divinyl monomer is in the monomer mixture, the reaction kinetics changes drastically. In this case, a dead polymer chain may grow again as a macroradical, when its pendant vinyl groups react with radicals, and the size of the macromolecule increases until it extends over the whole available volume. 

P. H. Plesch, The Propagation Rate-Constants of the Cationic Polymerisation of Alkenes, Progress in Reaction Kinetics, 1993, 18, 1. 

Kennedy and Thomas attempted to construct a formal kinetic theory to account for these phenomena. They started from the assumption, for which there is no evidence, that the propagation reaction takes place with free ions only, but that the chain breaking reactions involve ion-pairs. 

The role of water is two-fold. It is a co-catalyst, and its consumption during the reaction is, or is associated with, a kinetic termination. Moreover, there is evidence that the reaction in which water is consumed is of a lower order with respect to monomer than the propagation reaction, which therefore is most probably of first order with respect to monomer. 

At this point we leave the conventional kinetic theory and turn to the substance of our Section 2.5.2. The principal point made there is that for polymerisations in bulk the propagation reaction is not bimolecular but unimolecular, so that the rate is given by Equation (39) ... 

In the previous chapters, the fundamental areas of thermodynamics and chemical kinetics were reviewed. These areas provide the background for the study of very fast reacting systems, termed explosions. In order for flames (deflagrations) or detonations to propagate, the reaction kinetics must be fast—that is, the mixture must be explosive. 

Szwarc, M. and J. Smid, The Kinetics of Propagation of Anionic Polymerization and Copolymerization, Chap. 5 in Progress in Reaction Kinetics, Vol. 2, G. Porter, ed., Pergamon Press, Oxford, 1964. 

The derivation of the terminal (or hrst-order Markov) copolymer composition equation (Eq. 6-12 or 6-15) rests on two important assumptions—one of a kinetic nature and the other of a thermodynamic nature. The Erst is that the reactivity of the propagating species is independent of the identity of the monomer unit, which precedes the terminal unit. The second is the irreversibility of the various propagation reactions. Deviations from the quantitative behavior predicted by the copolymer composition equation under certain reaction conditions have been ascribed to the failure of one or the other of these two assumptions or the presence of a comonomer complex which undergoes propagation. 

The kinetic chain length is given by the number of monomer molecules consumed per initiation step. Since the efficiency of most initiators is not known quantitatively it is necessary to compare the rate of the propagation reaction with either the rate of initiation or the rate of termination. If there is no chain transfer, the kinetic chain length v for termination by disproportionation is equal to the number-average degree of polymerization ... 

It is instructive to sketch this reaction as in Figure 10-1. The chain is fed by initiation reactions that create CHs and terminated by reactions that destroy CHy- to form C2H6. However, the major processes by which the reaction proceeds are the two propagation steps that alternately create and destroy the two chain-propagating radicals CH3 and CHsCOv Thus we have the notion of a chain of reactions, which is a kinetic chain of propagation reaction steps that feed reactants into the chain and spit out stable products. This is a kinetic chain reaction, which is different from the polymerization chain of reactions, which we wiU introduce in the next chapter (although those reactions form both kinetic and polymer chains). 

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