Growing centers

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

When the initiation is slow, the number of growing centers as a function of time must be deterrnined in a separate step before the kinetic analysis can be carried out. Several different methods are available (6,31,66,69—71). 

The propagation step of polymerization involves an addition of monomeric units to the growing centers followed by regeneration of these centers. A series of consecutive propagation steps yields eventually a long polymeric molecule. 

Ions and ion pairs interact strongly with the solvent, and hence an ionic polymerization is greatly influenced by the environment. Solvation tends to separate the ions and thus the system approaches a state which would be expected in a hypothetical solution deprived of gegen ions. At the same time formation of a solvation shell around the growing center probably slows down the addition. This effect is particularly notable in the termination step and will be discussed further in the next section of this paper. 

All of these examples explain why such a variety of phenomena are observed in ionic or coordination polymerization. What we need to understand is the cause which gives to a particular center this or other properties, e.g., why dissociation into isolated ions leads to one and not another change in the reactivity and the specificity, how changes in solvation shell change the behavior of the growing center. This whole field is still uncharted, and calls for a thorough academic research. 

This relation was verified experimentally7 49 and it was shown that the degree of polymerization in a system containing "living polymers is independent of concentrations of initiator or monomer and of temperature. Furthermore, if all the growing centers were formed in a time much shorter than the time of polymerization, a Poisson molecular weight distribution would be obtained. Indeed, by using this technique samples of polystyrene were obtained for which MjMn = 1.04. 

Number of monomers combined per growing center in a system yielding the Poisson distribution (Chap. VIII). 

Spontaneous polymerization of 4-vinyl pyridine in the presence of polyacids was one of the earliest cases of template polymerization studied. Vinyl pyridine polymerizes without an additional initiator in the presence of both low molecular weight acids and polyacids such as poly(acrylic acid), poly(methacrylic acid), polyCvinyl phosphonic acid), or poly(styrene sulfonic acid). The polyacids, in comparison with low molecular weight acids, support much higher initial rates of polymerization and lead to different kinetic equations. The authors suggested that the reaction was initiated by zwitterions. The chain reaction mechanism includes anion addition to activated double bonds of quaternary salt molecules of 4-vinylpyridine, then propagation in the activated center, and termination of the growing center by protonization. The proposed structure of the product, obtained in the case of poly(acrylic acid), used as a template is ... 

The various kinds of growing species differ not only in their propagation but also in their stereochemical preferences. Professor Hogen-Esch will review this subject in his talk on anionic oligomerization of some vinyl monomer, and mechanisms of anionic, stereospecific polymerization of 2-vinyl pyridine will be discussed by Dr. Fontanille. In this context, the interesting paper of Schuerch et al.(12) deserves attention. Their work clearly reveals the effect of cation solvation upon the mode of monomer s approach to the growing centers. 

Commonly in radical polymerizations, initiation occurs continuously at a steady rate and is balanced by termination so lhal a steady concentration of growing centers (usually in the region of 10-8 mole/1) is established. The number of propagation reactions greatly exceeds the number of reactions of other types so that macromolecules are built up. The life-time of an active center is very much less than the duration of the whole process of polymerization and so the macromolecules are produced even in the earliest stages there is not a continuous rise in the molecular weight of the polymeric product as found in polymerizations of certain other types. It is instructive to consider in some detail the component reactions in the overall process of radical polymerization. 

Successive additions of monomeric molecules to the reactive center of a growing polymer characterize the propagation steps of an addition polymerization. The propagation process continues until the reactivity of the growing center is lost, an event known as the termination of a polymerization — or until the supply of the monomer is exhausted. In a termination process the reactivity of a growing center may be destroyed without creating a new reactive center and such a reaction may be referred to as a proper termination. On the other hand, if a new reactive center is formed in the course of destruction of the previous one, the termination process is known as a chain transfer. Both types of termination will be discussed in this article. 

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. 

Up to now our attention has been focused on termination reactions resulting either from an intramolecular reaction of the growing end or from an interaction involving two growing centers. We will consider now the termination reactions caused by a solvent or by another suitable reagent present in the solution. 

The lack of termination has many important consequences. First of all, one has to realize that living" polymers are not infinitely long. In any polymerization system the amount of the available monomer is finite, hence if n growing centers are formed in the initiation process — the number average degree of polymerization of the living" polymer is... 

The end view of a log (Figure 1) exposes the wood and bark portion of a tree trunk. Each year a growing center located between the wood and bark inserts a new layer of wood adjacent to the existing wood. In addition, new bark is deposited next to the pre-existing bark. Wood occupies the largest volume of a tree stem because more wood cells are produced than bark cells and also because the wood cells are retained and thus accumulate while the outermost bark cells are continually discarded. 

If transfer reactions result in formation of species with a much lower reactivity with a monomer than that of the growing center, a substantial part of active centers may not be operative Z6,Z7). 

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