Ionic polymerization reactions

In ionic polymerizations, reaction rates are faster in solvents with high dielectric constants, which promote the separation of the ion pair. 

Phosphorus pentafluoride is a catalyst in ionic polymerization reactions. 

The formation of polymer, which is by far the most important product, has been attributed to both free-radical and ionic-polymerization reactions. 

Although Friedel-Crafts or Lewis acid catalysts are often used to initiate carbocationic polymerizations and are very important from an industrial viewpoint, very little is known about the active intermediates involved. Such information is important because, in general for ionic polymerization reactions, small changes in the structure of the active center can result in large changes in molecular weight, molecular weight distribution (MWD),... 

Thus NMR studies of monomers are an important tool for understanding the basic problems in radical and ionic polymerization reactions. 

Compressed liquid or supercritical carbon dioxide has been recognized as a useful alternative reaction medium for radical and ionic polymerization reactions (see Chapter 4.5). Many of the benefits associated with the use of SCCO2 in these processes apply equally well to polymerizations relying on a metal complex as the chain-carrying species. However, the solubility of the metal catalyst and hence the controlled initiation of chain growth add to the complexity of the systems under study. Furthermore, many of the environmental benefits would be diminished if subsequent conventional purification steps were needed to remove the metal from the polymer. Nevertheless, the interest in metal-catalyzed polymerizations is increasing, and some promising systems have been described. 

Kutal et reviewed the chemistry of several iron (II) metallocenes that are effective photoinitiators for ionic polymerization reactions. Photoexcitation of ferrocene and 1,1-dibenzoyl -ferrocenes in solutions of ethyl-a-cyanoacrylate produces anionic species that initiate the polymerization of electrophilic monomers. Irradiation of CsHs-Fe (t] - arene) in epoxide containing media generates several cationic species capable of initiating ringopening polymerizations. It was concluded that iron(II) metallocenes exhibit a diversity of photoinitiation mechanisms. 

We have cited several examples which illustrate characteristic ionic polymerization reactions of unsaturated compounds, which may be contrasted with the behavior of alkanes, for which the initial ion-molecule reactions usually lead to stable ion products which do not react further. It was therefore of interest to investigate ionic reactions in cyclobutane, the saturated hydrocarbon isomeric with the unsaturated butenes, to establish whether cyclanes could properly be classified in either of these categories. Additional impetus for such a study was provided by radiolysis data on cyclobutane which suggested that the cyclobutane parent ion rearranges prior to reaction. ... 

The benzyl ester of malolactone was used as the monomer instead of malolactonic acid to eliminate potential problems in the ionic polymerization reactions. The carboxylic acid group, if present, could react with the initiators or cause either chain transfer or termination reactions during the polymerization. The polyester of this pendant ester was readily converted to poly-3-malic acid by hydrogenolysis without change in its molecular weight, according to the following reaction ... 

The direct relationships between activation free energies and tacticity observed for free radical polymerization reactions are seldom encountered in ionic polymerization reactions In these systems, temperature not only affects the propagation rate constant, but it also controls the ion pair equilibrium of the active endgroups. That is, in most ionic polymerization reactions, the active endgroup is in the form of some type of ion pair, and each type of ion pair will most likely have its own ratio of rate constants, kj to k which controls tacticity. As a result, two or more different kinds of active endgroups can exist in dynamic equilibrium with each other and grow concurrently, as indicated below (9)... 

These considerations concerning the mechanism of stereochemical control in ionic polymerization reactions, therefore, can account for the tacticities obtained in most homogeneous anionic and cationic polymerizations as shown in Tables 3 and 6, respectively (3). 

To prove the existence of a Bernoullian sequence, only triad information is needed, but tetrad information is required to verify a first-order Markov sequence. More complicated stereochemical mechanisms are possible of course (such as reactions controlled by penultimate configurations), and these would have to be fitted by more complex statistical analyses requiring knowledge of more than two probabilities. However, most detailed analyses to date on the tactleities of polymers obtained in homogeneous free radical or ionic polymerization reactions have been found to conform to either Bernoullian or first-order Markovian statistics. 

In contrast to radical and ionic polymerization reactions in which the incoming monomer always adds to the end of the growing chain... 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

The ionic liquid process has a number of advantages over traditional cationic polymerization processes such as the Cosden process, which employs a liquid-phase aluminium(III) chloride catalyst to polymerize butene feedstocks [30]. The separation and removal of the product from the ionic liquid phase as the reaction proceeds allows the polymer to be obtained simply and in a highly pure state. Indeed, the polymer contains so little of the ionic liquid that an aqueous wash step can be dispensed with. This separation also means that further reaction (e.g., isomerization) of the polymer s unsaturated ot-terminus is minimized. In addition to the ease of isolation of the desired product, the ionic liquid is not destroyed by any aqueous washing procedure and so can be reused in subsequent polymerization reactions, resulting in a reduction of operating costs. The ionic liquid technology does not require massive capital investment and is reported to be easily retrofitted to existing Cosden process plants. 

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

Giirtler and Jautelat of Bayer AG have protected methods that use chloroalumi-nate(III) ionic liquids as solvents for both cyclization and polymerization reactions of acyclic dienes [52]. They employed the neutral ionic liquid [EMIM][G1-A1G13]... 

The controlled synthesis of polymers, as opposed to their undesired formation, is an area that has not received much academic interest. Most interest to date has been commercial, and focused on a narrow area the use ofchloroaluminate(III) ionic liquids for cationic polymerization reactions. The lack of publications in the area, together with the lack of detailed and useful synthetic information in the patent literature, places hurdles in front of those with limited loiowledge of ionic liquid technology who wish to employ it for polymerization studies. The expanding interest in ionic liquids as solvents for synthesis, most notably for the synthesis of discrete organic molecules, should stimulate interest in their use for polymer science. 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

Phenomenological evidence for the participation of ionic precursors in radiolytic product formation and the applicability of mass spectral information on fragmentation patterns and ion-molecule reactions to radiolysis conditions are reviewed. Specific application of the methods in the ethylene system indicates the formation of the primary ions, C2H4+, C2i/3+, and C2H2+, with yields of ca. 1.5, 1.0, and 0.8 ions/100 e.v., respectively. The primary ions form intermediate collision complexes with ethylene. Intermediates [C4iZ8 + ] and [CJH7 + ] are stable (<dissociation rate constants <107 sec.-1) and form C6 intermediates which dissociate rate constants <109 sec. l). The transmission coefficient for the third-order ion-molecule reactions appears to be less than 0.02, and such inefficient steps are held responsible for the absence of ionic polymerization. 

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