Propagation reactions ionic polymerizations

At the present time the concept of catalytic (or ionic-coordination ) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 

Thus, confirmation of whether the product obtained in an attempted reaction in a true random copolymer is important to clarify the mechanism of the propagation reaction and to correlate structure and reactivity in ring-opening polymerizations. Considering that apparent copolymers may be formed by reactions other than copdymerization, for example, by ionic grafting or by combination of polymer chains, characterization of cross-sequences appears to be one of the best ways to check the formation of random copolymers. 

Several important assumptions are involved in the derivation of the Mayo-Lewis equation and care must be taken when it is applied to ionic copolymerization systems. In ring-opening polymerizations, depolymerization and equilibration of the heterochain copolymers may become important in some cases. In such cases, the copolymer composition is no longer determined by die four propagation reactions. 

Ionic polymerizations are characterized by a wide variety of modes of initiation and termination. Unlike radical polymerization, termination in ionic polymerization never involves the bimolecular reaction between two propagating polymer chains of like charge. Termination of a propagating chain occurs by its reaction with the counterion, solvent, or other species present in the reaction system. 

Water, alcohols, ethers, or amines can cause inhibition of ionic polymerization. However, these substances can act in different ways according to their concentration. For example, in polymerizations initiated by Lewis acids (BF3 with isobutylene) or organometallic compounds (aluminum alkyls), water in small concentrations behaves as a cocatalyst, but in larger concentrations as an inhibitor (reaction with the initiator or with the ionic propagating species). 

In contrast to radical polymerizations, ionic polymerizations proceed at high rates even at low temperatures, since the initiation and propagation reactions have only small activation energies. For example, isobutylene is polymerized commercially with boron trifluoride in liquid propane at -100 °C (see Example 3-16). The polymerization temperature often has a considerable influence on the structure of the resulting polymer. 

In a plasma polymerization process, the growth of low molecular weight monomer species to a high molecular weight plasma polymer network takes place. In a chemical sense, plasma polymerization is different from conventional polymerizations, such as radical or ionic. The term radical polymerization means that propagating reactions of monomers are initiated by radical species. Ionic polymerization means that chemical reactions are propagated by ionic species in the polymerization step. Plasma polymerization involves an energy source to... 

So far as vinyl monomers are concerned, ionic propagation proceeds with carbonium ions (cationic polymerization) or carbanions (anionic polymerization) at the chain ends. The study of the initiation process of radiation-induced ionic polymerization seeks to elucidate how these ions are formed from the primary ionic intermediates. Possible reactions... 

Coordinative initiation differs from ionic polymerization in that the propagating species consists of a covalent bond species. This generally reduces the reactivity and the polymerization rate. Decreased reactivity also leads to fewer amounts of side reactions and the often-living ROP of lactones may take place under these conditions. Chedron, in the early 1960s, showed that some Lewis acids, such as triethylaluminum and water or ethanolate of diethylaluminum, were effective initiators for lactone polymerizations. Tin(IV) alkoxides and phenox-ides, [92,93] aluminum alkoxides, mainly aluminum / so-propoxide, and soluble... 

Assuming a single-ion (carbonium ion) propagation step and based on scavenger studies with ammonia and amines and on electrical conductance measurements, rate constants for the propagation reaction (ion-molecule reaction) can be estimated. These estimates are free of the correction that one has to apply (or ignore) for chemically generated ion pair ionic polymerization, but are subject to other limitations imposed by different assumptions in the treatment of the data. 

In the previous sections, methods of qualitatively controlling the course of propagation were described. Indirect control as well as the quantitative effects caused by intentional control of the other partial processes in polymerization have still to be mentioned. The separation of initiation from propagation alters the kinetic character of the whole reaction. With ionic polymerizations, initiation can be separated from propagation by the selection of conditions suitable for rapid initiation. With radical polymerizations, this is not possible. Therefore both partial processes must be separated in space. Fortunately, radical active centres operate both in polar and in non polar media. Thus it is not difficult to confine initiation and propagation to mutually immiscible components of the medium. Emulsion polymerization remains the most important representative of quantitative control of propagation. 

Approximations based on the concept of relative abundance of catalyst-containing species in trace-level catalysis and of propagation centers in ionic polymerization will be discussed in Chapters 8 and 10, respectively the long-chain approximation in chain reactions will be an important topic in Chapter 9. 

The approximations of a rate-controlling step, quasi-equilibrium steps, and long chains in chain reactions and the concept of relative abundance of catalyst-containing species in catalysis or propagating centers in ionic polymerization can often be used for additional simplification (see Sections 4.1, 4.2, 8.5, 9.3, and 10.4.1). A procedure suited in many cases consists essentially of the following steps [10] ... 

Rates of ionic polymerizations are by and large much faster than in free-radical processes. This is mainly because termination by mutual destruction of active centers occurs only in free-radical systems (Section 6.3.3). Macroions with the same charge will repel each other and concentrations of active centers can be much higher in ionic than in free-radical systems. Rate constants for ionic propagation reactions vary but some are higher than those in free-radical systems. This is particularly true in media where the ionic active center is free of its counterion. 

The high reactivity of ionic active centers which yields fast propagation rates also results in a greater propensity toward side reactions and interference from trace impurities. Low polymerization temperatures favor propagation over competing reactions, and ionic polymerizations are often performed at colder temperatures than those used in free-radical processes, which would be impossibly slow under the same conditions. 

The choice of initiator has no effect on the propagation reactions in free-radical polymerizations but it can influence ionic propagations because the reactivity of the active center is partly determined by the nature of the counterion that is derived from the initiator. 

The effects of solvents on ionic polymerizations have long been misinterpreted, lai ely because of two factors. Firstly the failure to isolate the process of propagation from the other elementary reactions which comprise polymerization, and secondly the failure to compare isolated free ion and ion pair data. It is worthwhile, therefore, to examine these simple considerations from a general point of view. 

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