Ionic polymerization kinetic scheme

Ionic-polymerization Kinetics. The kinetics of ionic polymerization share some common principles with that of the free-radical reaction. Both are based on the basic steps of initiation, propagation, termination, and chain transfer, and in both the ultimate average molecular weight depends on the ratio of the reaction rates of propagation and termination. There are, however, important differences. In ionic polymerization the termination step appears to be unimolecular, while it is bimolecular in free-radical type polymerization. The dependence of the kinetic scheme of the reaction on the various parameters is therefore different in the two reactions. Likewise, the fact that a cocatalyst has to be brought into the ionic reaction scheme has to be taken into account. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

About thirty years ago, all cases of polymerization kinetics used to be solved as statinary reactions. Hayes and Pepper [27] were the first to call attention to the non-stationary character of ionic polymerizations. They noticed the premature decay of styrene polymerization initiated by H2S04 (see Fig. 8). This was a simple case of non-stationarity caused by the slow decay of rapidly generated active centres [27, 28]. They assumed that the polymerization proceeds according to a rather conventional scheme represented in simplified form (without transfer) by the reactions... 

Condensation reactions follow kinetic schemes similar to those of small molecule reactions. They are simple first-order, second-order, etc. reactions. In contrast, the kinetics of chain reactions, such as free-radical polymerization or ionic polymerization, are much more complicated. 

So the kinetic scheme and parameters not only depend on M, but they may differ considerably according to the initiator used. Furthermore, the active centers are not uniform as in free-radical polymerization, but because of their ionic nature there usually exists a complex chemical equilibrium between different species, even if the initiator has a unique stmcture. This equilibrium [Eq. (3)] between free ions, solvent-separated ion pairs, contact ion pairs, covalent polarized bonds, and between associated and non-associated species, as well as the concentration of these species strongly depend on the polarity or solvating power of the solvent system, the solvent itself, and the presence of other salts. 

However, from a kinetic and modeling point of view, this site-based nature of ionic polymerization also has some disadvantages. Because the reactivity of the active center strongly depends on the nature of the initiator and on all the other factors in the polymerizing system, the kinetic scheme and parameters of every system are different, and so in general they must be determined again for every change in the system. 

Free Radical Polymerization Free radical polymerization is one of the most widely used polymerization chemistries owing to its ability to polymerize a wide variety of monomers (those with carbon-carbon double bonds) and, unlike ionic polymerization, the rather mild conditions under which it can be carried out. The complete kinetic scheme for free radical polymerization can be described as follows ... 

Early studies (1 ) of the kinetics of polymerization of styrene, isoprene and butadiene in hydrocarbon solvents indicated a half-order rate dependency on growing chain concentration, although there were conflicting data at that time (10, 11) which suggested even lower fractional orders for the dienes. Since the apparent half-order dependency could not be rationalized, as in the case of the polar media, by an ionic dissociation mechanism, some other form of association-dissociation phenomenon offered a possible answer. In view of the known tendency of organolithium compounds to undergo molecular association in non-polar media, the following scheme was proposed by us (l) ... 

The first viewpoint contradicts the autocatalytic character of the reaction, conductometric measurements in the polymerization system and some other facts (see below). Scheme (33) can be considered as completely experimentally substantiated. The following important proofs were obtained A direct experimental discovery of a quaternary ammonium alcoholate in the reaction system, 42) a full agreement of the nature of the active propagating site with all the existing kinetic and structural data l4,149 153 157 I58) establishment of the ionic behaviour of the propagating sites by comparison of the kinetic curves of the process with the character of the electric... 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

The solution for specific cases is greatly simplified when one of the reactions (87) or (88) is much slower than the other and thus controls the initiation rate. [In radical polymerizations, this is usually reaction (87).] We know, of course, that reaction (87) can be reversible, that R° can decay by secondary decomposition to R j (the reactivity of which generally differs from that of R°), and both reactions can only be a part of a much more complicated set of interactions, especially in ionic and coordination polymerizations. An exact kinetic analysis must be based on a proved scheme with identified intermediate transition states and products, and a knowledge of the rate constants and of the rates of various initiation stages. Such a complete and complex analysis does not yet exist. 

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