Termination reactions ionic polymerizations

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. 

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 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. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Since the required activation energy for ionic polymerization is small, these reactions may occur at very low temperatures. The carbocations, including the macrocarbocations, repel one another hence, chain termination does not occur by combination but is usually the result of reaction with impurities. 

The polymerization of olefinic materials by organometallic catalysts involves different types of initiation, transfer and termination reactions. In the past discussion we have referred to chain transfer which involved the exchange of one organometallic bond for another in the active catalyst species. This involved the equilibrium exchange of alkyl metals. However, another group of important reactions include the formation and destruction of alkyl bonds. These reactions follow the same ionic factors which apply to polymerization reactions. 

The most important mode of termination of a radical polymerization is the bimolecular interaction of its reactive centers. Radicals rapidly recombine or disproportionate, and these reactions anihilate the growing ends. On the other hand, neither recombination nor disproportionation takes place in an ionic polymerization, and hence a collision between two ionically growing centers usually does not lead to termination. 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

These early examples of macromonomer synthesis illustrate some of the chief principles that have subsequently led to a great variety of methods. Ionic polymerization is often preferred because of the long lifetime of the active sites. Transfer reactions in free-radical processes are also used quite often, yielding both acceptable molecular weights and an adequate proportion of terminal functions. 

Perhaps the best qualitative and quantitative scavenger studies in these ionic polymerizations have been made with ammonia and several amines. Busier, Martin, and Williams (8) have shown that, at —78°C. the initial polymer yield from irradiated cyclopentadiene increases linearly with the ratio of [C5H6]/[NH3] over the range 1-10. This would be consistent with a mechanism in which all growing chains are terminated by reaction with ammonia, according to ... 

The difficulties involved in the direct determination of the momentary concentration of active centres are the most serious shortcoming in studies of termination itself. With radical polymerizations we at least know the most probable method of centre decay, and thus the molecular scheme of the termination reaction. In ionic and coordination polymerizations, the termination mechanism is mostly unknown. Quite generally we can write... 

Equation (175) can be amplified by a term involving the termination reactions. In this way they acquire a much more general character, permitting analysis even of complicated cases of non-stationary ionic polymerizations. Their solution is not simple, of course. 

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