Dormant onium species

Nucleophiles (or electron donors) may react with cationic species in three different ways (see Section VI.B.2 see also Table 2, Section V.A.2 for examples). They can reversibly form onium ions, complexes and covalent species, and if they are basic enough, they may eliminate /3-protons [cf., Chapter 3, Eq. (131)]. In the first reaction, nucleophiles are used to control the polymerization rate because onium ions are inactive dormant species. They can be added at concentrations higher than salts, and can considerably reduce the lifetimes of both unpaired cations and ion pairs by converting them into dormant onium species. In the second reaction, nucleophiles can also affect the polymerization rates by coordination to Lewis acids and reducing their strength. Both reactions are beneficial for controlled polymerization. The third reaction, favored by strongly basic species, should be avoided. 

Dormant Species and Pseudocationic Propagation The majority of propagating chain ends in most cationic polymerizations initiated by protonic acids and/or cocatalyzed by Lewis acids do not exist as carbenium ions, but are instead dormant species. The two major types of dormant species are onium ions and covalent esters or halides. The covalent species are formed by reversible reaction of carbenium ions with nucleophilic anions onium ions are generated by reaction of carbenium ions with noncharged nucleophiles such as ethers, sulfides, and amines. Because the majority of propagating chain ends exist as dormant species, they are often the only species that can be detected spectroscopically ... 

The actual role of the added nucleophile is still under discussion [41,88,92] (cf., Sections VI.B.2 and VILE.4). They may interact with the growing carbocations and stabilize them through the weak solvating interactions [36]. Another possibility is that the added nucleophiles and the growing carbocations form reversibly onium ions that serve as dormant species, as discussed by Penczek [92] and Matyjaszewski [88] (schemati-... 

Trapping agents, such as malonate anions, naphthoxides, and phosphines have been used to determine the concentration of chain carriers in controlled/living and other carbocationic systems [85,249,250]. These strong nucleophiles react with all sufficiently electrophilic species, including not only carbocations but also onium ions and covalent esters. Thus, the discussed measurements can provide only the total concentration of active and dormant end groups. In principle, the kinetics of formation of the product in the trapping experiments could resolve more and less active species but only if they are present at comparable concentrations. As discussed before, carbocations are present in ppm quantities in comparison with dormant species. 

Controlled/living systems can be usually obtained when the polymerization is sufficiently slow and when either nucleophilic anions or additives are present (Sections IV and V). This means that the proportion of carbenium ions should be low and conversion to dormant species, fast. Nevertheless, under such conditions cationic species can be detected by dynamic NMR, by ligand exchange, salt, and solvent effects, and by other methods discussed in Chapters 2, 3, and in this section. Under typical controlled/living conditions, dormant species such as onium ions and covalent esters predominate. It is possible that the active species are strongly solvated by monomer and by some additives. These interactions may lead to a stabilization of the carbocations. However, in the most general case, this stabilization has a dynamic sense and can be described by the reversible exchange between carbocations and dormant species. 

Before going into details, let us make a brief statement that propagation in new controlled/living carbocationic systems has nearly the same mechanism as in the conventional systems discussed in Chapter 3, which consists of the electrophilic addition of carbenium ions to alkenes. The main difference is that carbenium ions are in dynamic equilibria with dormant species (covalent esters and onium ions). The correct choice of structures and concentrations of activators and nucleophilic additives as well as those of initiator allows for the preparation of polymers with predetermined molecular weights, low polydispersities, and controlled end functionality, including block copolymers (see Chapter 5). 

Covalent species are dormant species that do not react directly with alkenes. The ionization process may be spontaneous or facilitated by activators, most often by Lewis acids. In the case of onium ions, the leaving group X is not charged but otherwise Eq. (43) remains the same. 

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. 

The equilibrium position between carbocations and dormant species (covalent or onium) should be adjusted to provide convenient rates (i.e., an appropriately low concentration of the growing carbocation at a given time). For a particular monomer, the equilibrium position (and overall rate) depends on the nature and concentrations of the leaving group X in the initiator RX, of the activator (Lewis acid), and of the deactivator... 

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