Non-dissociated propagating species

Polymer Synthesis via Non-Dissociated Propagating Species Formed by... 

This chapter reviews reactions at high temperature via the non-dissociated propagating species carrying an oxo acid-derived counteranion. 

To visualize the synthetic versatility of the non-dissociated propagating species, two chapters are devoted to selective linear dimerization and living cationic polymerization, both using non-MX initiators and the nature of the non-MX -type counteranions has been discussed. Further comparison of MX and non-MX initiators is made in living cationic polymerization, and it has been proposed that the mechanisms to yield living polymers may differ, depending on the type of initiator. 

Figure 29A-D shows a set of GPC traces of PIBs obtained in AMI experiments by the H20 /TiCl4 initiating system at — 60°C at different [TEA]s (5-10 -6-10 mol/1) as a function of time (0.5-10 min). At low conversions (short times), the GPC traces are multimodal, indicating the presence of different kinds of growing species. The peaks at the lowest MWs correspond to 6-8 monomer units. These products show UV activity (see Fig. 29E), which may be due to the olefinic end groups due to chain transfer to monomer. The intermediate peaks tend to narrow with time in all groups of GPC traces. The higher the [ED], the narrower the MWDs at the same or similar conversions because of the increasing contribution of non-dissociated living species to propagation.

The results of Enikolopyan and co-workers [27, 28] on the polymerisation of styrene by perchloric acid at high pressures shed some new light on the problem. Essentially their kinetic results agree with those of Pepper and Reilly and of ourselves. The important feature of their findings is that the extent of acceleration by pressure is merely that which can be attributed to increase of dielectric constant of the solvent. There was no effect which could be attributed to increasing abundance of free ions by increased dissociation of ion-pairs. This means that, if the propagating species are ions, then they are all free ions even at normal pressure (which is reasonable), or the propagating species is non-ionic. 

The step of cationic initiation can be subdivided into two separate reactions. The first one consists of formation of ionic species and the second one of reactions of these ionic species with the olefins, a cationization process. This reaction, termed priming by Kennedy and Marechal, is a process of ion formation in a non-nucleophilic media through (1) dissociation of protonic acids to form protons and counterions, (2) reactions of Lewis acids with Bronsted acids, (3) dissociation of dimeric Lewis acids, (4) complexation of Lewis acids with water or with alkyl halides or with ethers, and so on. These reactions may take place through a series of complicated steps. The second reaction, the cationization of the olefins, may also include several intermediate steps that will eventually lead to propagating species. 

The bimodal MWD and its change with the polarity of polymerization solvent clearly indicate the existence of two propagating species in different ionic dissociation states One of them is a dissociated species forming the high polymer, and the other is a non-dissociated species giving the low polymer. Equation (5) shows the formation of polymers with a bimodal MWD from these two intermediates ... 

The counteranions derived from MX initiators, in contrast, do not interact as strongly with carbocations as the oxo-acid counteranions, as previously discussed. Therefore, even when two propagating species are formed by these initiators, the interconversion between them is as fast as to allow a propagating chain to take both dissociated and non-dissociated forms during its kinetic lifetime. Under these conditions, product polymers show a rather broad, unimodal MWD whose peak lies at an average of the molecular weights corresponding to the two dissociation states (cf. Fig. 2). 

It is therefore concluded that the iodine-initiated polymerizations of pMOS and IBVE involve long-lived propagating species in a non-dissociated state. However, these species are not truly living as judged from the M of the formed polymers being not directly proportional to conversion. 

This review, mainly based on our recent investigations, has discussed the nature of the propagating species in cationic polymerization of vinyl monomers, classified cationic initiators into metal halides (MXJ and nonmetal halides (non-MX ), and pointed out clear differences between the propagating species derived from these two classes of cationic initiators. The article also emphasizes the unique characteristics of the non-MX initiators that have eluded systematic studies for a long time. In particular, it should be noted that these initiators specifically form two independent propagating species (dissociated and non-dissociated species). 

An electron-conducting material brought into contact with an ionically conducting phase establishes an electrode. In case of semiconductors in addition to electrons, holes may act as means of charge propagation. The ionically conducting phase may be an electrolyte solution composed of a dissociated electrolyte and a solvent, an ionic liquid, a molten salt, or a solid electrolyte. At the established interphase an equilibrium is established. Assuming at the instance of contact a non-equilibrium between both phases and the chemical potentials of the involved species, two ]u, possibilities may be considered one is depicted below (Fig. 1) ... 

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