Polymerization heterolytic mechanisms

The differences between the homolytic and heterolytic mechanisms of polymerization are reflected in the influence of the following factors on the course of the reaction ... 

The 13C NMR study of the polymerization of pent-l-ene with 95% H2SO4 points to the carbenium-ion mechanism, which involves the formation of sulfuric acid esters and their further heterolytic dissociation to generate aliphatic carbenium ions, whose steady-state concentration is low. By contrast, polymerization in 60-70% H2SO4 proved to occur via oxonium (rather than carbenium) ions.38... 

Aqueous alcohol iron protoporphyrin solutions with pH > 10 (14 in the limit), required for preventing their interaction with one another, are most frequently used in the synthesis and reactions with biomimics. On the contrary, H202 dissociation was implemented in the presence of 5,10,15,20-tetrakis-(2,6-dimethyl-3-sulfonatophenyl)porphynato-Fe3+ H20 [(P)Fe3+(H20)] in a pH range of between 1 and 12 [65], This porphyrin is easily soluble in water and does not form dimeric and polymeric associates. Of interest are transition states shown in Figure 7.2. It is the author s opinion that the mechanism with heterolytical O—O bond break is the most preferable. 

Marks and co-workers employed organolanthanide complexes of the type Cp2Ln-R, where metal-centered redox processes cannot be involved. They studied PhSiH3 polymerization using kinetics and thermochemical measurements and proposed a four-center, heterolytic bond-scission/bond-forming sequence analogous to the Tilley mechanism.62b... 

Coordination chemistry has become a powerful tool for the control and the living nature of radical polymerization [79,80]. Various examples show that the role of initiator and counter radical can be played by organometallic species with an even number of electrons. Besides aluminum complexes used by Matyjaszewski, several other transition metals, metallocenes, and organolan-thanides with various ligands have been studied in controlled radical polymerization [79-97]. In some cases, a controlled polymerization was achieved [81,83-85,87,90-94,97]. However, the mechanism of the polymerization is not always known and it may happen that heterolytic cleavage of the active bond... 

Phenacylammonium Salts Phenacyl anilinium salts have light absorption 300-350 nm that make them preferable for photoinduced cationic polymerization. Upon irradiation, these salts undergo either a heterolytic cleavage or a homolytic cleavage followed by an electron transfer to yield a cation intermediate that in turn can initiate polymerization of appropriate monomers [16]. Scheme 11.14 shows the mechanism in detail. As can clearly be seen, the photolysis of phenacylammonium salts are irreversible and different than their sulfonium analogs [43]. 

Recently, Yagci and coworkers investigated the initiation mechanism of a new type of cationic photoinitiator, namely, /V-phenacyl-/V,/V-dimethylanilinium hexafluoroan-timonate (PDA+ Sbl fi ) which initiates the polymerization of appropriate monomers [116]. The proposed mechanism includes irreversible fragmentation of the absorbent salt to yield the initiating species either via a heterolytic cleavage or via a homolytic cleavage followed by subsequent electron transfer between the preformed species still, forming the same cation that initiates cationic polymerization (Scheme 11.30). 

The term "ionic polymerization" basically involves the chemistry of heterolytic cleavage of chemical bonds, as opposed to the homolytic reactions that characterize the well-known free-radical polymerization mechanism. Hence, essential and profound differences exist between these two mechanisms of polymerization. Although these differences are also found between radical and ionic mechanisms in ordinary reactions, they exert a much more drastic influence on the result, that is, the growth of a long chain molecule to macro dimensions. Thus, one would expect that the two mechanisms could lead to quite different results in most simple reactions, in terms of rate, yield, or mode of the reaction. In the case of polymerization, however, such differences, can, in fact, decide whether any high polymer is obtained at all. 

The development of solid-state polymerization was largely due to the use of high-energy radiation such as y-rays 115,123), x-rays 124>, electron beams 125,126) or a-partic-les ll6). y-Rays are used most frequently. It is generally accepted that y-ray-induced polymerization of TXN proceeds by a cationic mechanism. Ions or radical-ions are formed by electron transfer from TXN, the loss of hydrogen atoms or the heterolytic cleavage of the ring 127>. 

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free-radical and ionic species. When this application of shear is carried out in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the viscoelastic state. Both homolytic and heterolytic scissions are possible. The first yields free-radical and the second ionic species. Heterolytic scissions require more energy but should not be written off as completely unlikely." Early work was done with natural rubber. It swells when exposed to many monomers and forms a viscoelastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers. Later, the technique was applied to many different polymers with many different monomers. ... 

Pudelski JM, Manners I (1995) A heterolytic cyclopentadienyl carbon-silicon bond cleavage mechanism for the thermal ring-opening polymerization of silicon-bridged [l]ferroceno-phanes. J Am Chem Soc 117 7265-7266... 

Solid-state polymerization of TOX can be initiated by different kinds of radiation, including y-rays, X-rays, electron beams, or a-particles.The mechanism of initiation is not well understood. It is, however, generally accepted that radiation induces cationic polymerization. Ions or radical ions are generated by electron transfer, the loss of hydrogen ions, or the heterolytic cleavage of TOX rings. 

Leaving Group Effects on the Condensation Polymerization Process. In studies of partially polymerized reaction mixtures, we find that only relatively high molecular weight polymers 50,000) and unreacted monomer are observed in the early stages of the reaction 4), Thus, the condensation polymerization of N-silylphosphoranimines appears to involve a chain growth mechanism probably initiated by heterolytic cleavage of the polar P-X bond. 

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