Active centres of polymerizations

The active centres of polymerization are produced by the addition of the primary radical to the monomer, i. e. to a n electron system. Only rarely is this simple process, and almost all branches of theoretical chemistry and chemical physics have contributed to its elucidation. The addition is a bimolecular reaction interpreted kinetically as a second-order reaction [125]. Unfortunately, most studies have been concerned with reaction in the gaseous phase. In the condensed phase, the probability that the excess energy of the reaction product will be removed by collision with a third molecule is very much higher thus the results obtained in the gaseous phase need not be valid generally. 

To date these have not been very important as active centres of polymerizations, mainly because of the limited number of conventional monomer types suitable for building such a centre. Nitrogen cations are an exception. Cyclic imines polymerize on centres [148, 149]... 

Radical ions are, in the main, not very important as active centres of polymerizations. In media suitable for the existence both of radicals and of ions, the latter are usually more reactive. Moreover, the radicals decay by combination their contribution to chain propagation is usually negligible. Radical ions are more important as precursors of active centres, as intermediates generated from initiators and monomers through their radical ends they can combine (disproportionate) yielding active centres, frequently diions. Studies of radical ion behaviour contribute to our knowledge of the processes connected with electron transfer from molecule to molecule. These oxidation-reduction processes are very important in macromolecular chemistry. 

One problem encountered in the field is the apparent irre-producibility of the results of different workers, even those in the same laboratory. This is particularly the case with molar mass distribution and steric triad composition. The explanation of these apparent inconsistencies comes with the realization that the mechanisms are eneidic and the polymer properties are primarily determined by independent active centres of different reactivities and stereospecificities whose relative proportions are set at the initiation step, which is completed in the first seconds of the polymerization. The irreproducibilities arise from irreproducibilities in the initiation step which had not been thought relevant. Ando, Chfljd and Nishioka (12) noted that these rapid exothermic reactions tend to rise very significantly above bath temperature (we have confirmed this effect) and allowed for this in considering the stereochemistry of the propagation reaction. However our results show that the influence on the initiation reactions can have a more far-reaching effect. 

As previously proposed (1,2) stable, persistent active centres of different reactivity and stereospecificity may coexist independently during the course of the polymerization. 

In a very simple case, matrices play the role of microreactors creating a reaction medium near the active ends of the growing macromolecules which differs from that in the rest of the solution. For example, for matrix polymerization of MA on PEO macromolecules, the stereostructure of the PM A daughter chains is similar both in water and in benzene in fact, it is the same as for polymerization of MA without matrix in alcohol media24 . Probably, this is determined by the fact that the dielectric permetivity in the microreactor, where the active centre of the... 

Non-stationary polymerization are complicated from the kinetic point of view. The changing concentrations of active centres, of monomer and possibly even of further components produce conditions unsuitable for an analysis of the process. Even technical and technological difficulties occur. Nevertheless, these have to be solved as most known coordination and cationic, and a considerable number of anionic, polymerizations are non-stationary. Information on the polymerization mechanisms of the more conventional monomers are summarized in Table 3. 

The active centre of anionic polymerization is formed by the addition of an anion to a... 

The activation described is due to an increase in the concentration of active centres. The polymerization rate can, of course, also be enhanced by increased reactivity of the centres. To solvent-separated ion pairs and to free ions, monomers in general are added more rapidly than to contact ion pairs. When we succeed in separating the active centre ions by the addition of a strongly solvating compound, the initiation and propagation rates will be enhanced, and changes affecting the mode of addition may appear [212, 213 ]. 

In my opinion, an active centre of alkene polymerization in the liquid phase is not a single chemical entity to be visualized by a single (and simple) chemical formula. Probably a set of compounds, of complexes with variable composition, a dynamic system where the effects of individual components are mutually complementary or overlapping is really in play. The same macroscopic effect (centres of equal activity and iso-specific regulating ability) can be obtained with various starting organometals and donors. In such a system, subsystems may exist each of which is externally manifested as an individual active centre (rapid or slow, isotactic, with a tendency to transfer or termination, or living, etc.) [225],... 

Active centres of coordination polymerizations can be formed by unusual combinations of elements. We have observed slow polymerizations of VC, MMA, S and AN yielding products of high melting point on complexes such... 

Even carboxylate ions can serve as active centres of / -propiolactone polymerization [316]. Cationic polymerization is characterized by the formation of an oxonium transition salt generated by the reaction of an active centre with an exo- or endocyclic oxygen atom. The reaction mode depends on the kind of initiator and monomer [317]... 

Active centres of ionic polymerizations do not usually decay by mutual collisions as the radical centres. The stationary state, when it exists at all, results from quite different causes, mostly specific to the given system. Therefore the kinetics of ionic polymerizations is more complicated and its analysis more difficult. The concentration of centres cannot usually be calculated. On the other hand, ionic systems with rapid initiation give rise to the kinetically very simple living polymerizations (see Chap. 5, Sect. 8.1). 

Unfortunately, satisfactory analytical techniques are not yet available to precisely define number, chemical and structural composition of the active centres during polymerization. Thus, real capacity of MWD control is still a matter of more or less empirical approach. 

X 10 ]. The occurrence of this absorption band is connected with the presence of a charge-delocalized anion, which is the actual active centre for polymerization, viz. 

Carbocation Intermediates.—Though the nature of the active centres in polymerizations propagated by carboanions has been well known for some time, and reasonably adequately characterized, the corresponding species in cationic propagations has proved difficult to identify with anything like the same certainty. However, the use of super acid media, e.g. FSOaH/SbFj, has now enabled Olah to obtain a n.m.r. spectrum of the elusive styryl carbocation, following experiments such as that in which a racemic mixture of products was obtained from the reaction of (—)-l-phenylethyl chloride with EtaAl, which provided powerful circumstantial evidence for the existence of the ion. More recently a particular definitive paper on and n.m.r. spectra of styryl species has appeared. 

The intrinsic viscosity ( ], at 30 °C in methanol) of the resulting poly(NMMAm) was almost independent of the monomer conversion (Fig. 6 b), although it decreased with increasing temperature. This indicates that a stationary state for the active centres of the polymerization was reached under the present conditions. ... 

The nature of the counterion in an oxonium ion salt initiation affects the stability of the active centres during polymerization. Even relatively stable counterions such as SbCle undergo decomposition in polymerizations of cyclic ethers. ... 

The role of R3AI is reduced to the alkylation of lanthanoids. Some support for this supposition is provided by the formation of carboxylates when the reaction mixture after butadiene polymerization on the NdCl3(THF)2 t3Al system is treated with carbon dioxide [9]. The protolysis of the product gives polybutadiene with COOH terminal groups. Quantum-chemical studies to model the active centres of butadiene polymerization on the Nd-Al catalytic system are also in agreement with this supposition on the mechanism of polymerization. 

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