Monomer cation-anionic

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

The surface active agents (surfactants) may be cationic, anionic or non-ionic. Surfactants commonly used are cetyltrimethyl ammonium bromide (CTABr), sodium lauryl sulphate (NaLS) and triton-X, etc. The surfactants help to lower the surface tension at the monomer-water interface and also facilitate emulsification of the monomer in water. Because of their low solubility surfactants get fully dissolved or molecularly dispersed only at low concentrations and at higher concentrations micelles are formed. The highest concentration where in all the molecules are in dispersed state is known as critical micelle concentration (CMC). The CMC values of some surfactants are listed in table below. 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

The polymerization of lactams (cyclic amides) can be initiated by bases, acids, and water [Reimschuessel, 1977 Sebenda, 1976, 1978 Sekiguchi, 1984]. Initiation by water, referred to as hydrolytic polymerization, is the most often used method for industrial polymerization of lactams. Anionic initiation is also practiced, especially polymerization in molds to directly produce objects from monomer. Cationic initiation is not useful because the conversions and polymer molecular weights are considerably lower. 

Polymerization reactions can proceed by various mechanisms, as mentioned earlier, and can be catalyzed by initiators of different kinds. For chain growth (addition) polymerization of single compounds, initiation of chains may occur via radical, cationic, anionic, or so-called coordinative-acting initiators, but some monomers will not polymerize by more than one mechanism. Both thermodynamic and kinetic factors can be important, depending on the structure of the monomer and its electronic and steric situation. The initial step generates... 

As seen from above, the mode of electron trapping in sc CO2 cannot be deduced from the results obtained in the gas phase or matrix isolation studies. It is not obvious whether the solvent radical anion should be similar to multimer cluster anions found in the gas phase, dimer cation(s) in solid matrices, or monomer CO2 anions in inert liquids. Such a situation is typical for other molecular liquids. 

As in the case of hexafluorobenzene solvent anion, EPR and ODMR spectroscopies suggests that no dimerization of monomer radical anions of benzene and toluene occur in liquid benzene and/or in alkane solutions of benzene (whereas the radical cation of benzene is known to dimerize rapidly). The conductivity studies also indicate that there is no volume change associated with the dimerization [45]. 

Polyethers are prepared by the ring opening polymerization of three, four, five, seven, and higher member cyclic ethers. Polyalkylene oxides from ethylene or propylene oxide and from epichlorohydrin are the most common commercial materials. They seem to be the most reactive alkylene oxides and can be polymerized by cationic, anionic, and coordinated nucleophilic mechanisms. For example, ethylene oxide is polymerized by an alkaline catalyst to generate a living polymer in Figure 1.1. Upon addition of a second alkylene oxide monomer, it is possible to produce a block copolymer (Fig. 1.2). 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

Not all monomers are anionically polymerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use such end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymers with polyvinyl and polylactone blocks19 and of copolymers with polyvinyl and polypeptide blocks20-2S). One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene2. ... 

Fig. 7- Intermediate and product species formed in the dimerization and the polymerization of vinylcarbazole in nitrobenzene (N02) and be-nzonitrile (C/V) in the presence of oxygen (VCZ ) monomer cation radical (> cyclodimer (P) polymer [43]...

Ring-opening polymerization is an important field of research in the chemistry of polymer synthesis. Usually, it proceeds by ionic mechanisms, i.e. cationic, anionic and coordinate anionic mechanisms. Research on ring-opening polymerization proceeding via free-radical propagating species in which the so-called molecular design of monomer plays an important role has recently been reported. 

The cation-anion annihilation provides the main mode for termination in extraordinarily dry systems. Traces of water terminate propagation in a most efficient way, and then the termination is first order with respect to the growing polymers. It is interesting to digress here and discuss this writers ideas about the role of water in such processes. Under normally achieved states of dryness the concentration of water in a hydrocarbon monomer is probably sufficiently high to maintain the dissolved water in its dimeric form, (H20)2 or, at least, to allow a second molecule of water to collide with carbonium ion associated with one H20 prior to the decomposition of such an associate. Thus, the termination involves processes such as... 

Conjugated polymers may be made by a variety of techniques, including cationic, anionic, radical chain growth, coordination polymerisation, step growth polymerisation or electrochemical polymerisation. Electrochemical polymerisation occurs by suitable monomers which are electrochemically oxidised to create an active monomeric and dimeric species which react to form a conjugated polymer backbone. The main problem with electrically conductive... 

The vast majority of miniemulsion polymerizations reported in the literature have been stabilized with anionic surfactants, probably because of the widespread application of anionic surfactants in macroemulsion polymerization, and due to their compatibility with neutral or anionic (acid) monomers and anionic initiators. However, Landfester and coworkers [70, 71] have used the cationic surfactants cetyltrimethyl ammonium bromide (CTAB) and cetyltri-methyl ammonium tartrate for the production of styrene miniemulsions. They report that these surfactants produce similar particle sizes to anionic surfactants used at the same levels. Bradley and Grieser [72] report the use of dodecyltrimethyl ammonium chloride for the miniemulsion polymerization of MMA and BA. 

On the other hand, the initiators created by high-energy radiation of monomers initiate radiation polymerization in condensed phases. The initiator that controls the overall radiation polymerization could be cation, anion, or free radical depending on the purity of the monomer. Nevertheless, the concentration of the initiator must be very small and the majority of monomer must remain unaffected otherwise the formation of sufficiently high molecular weight polymers cannot occur. 

The electron ejected from the monomer molecule attaches to the double bond of another monomer molecule, which leads to an anion-free radical. In essence, the irradiation produces cation, anion, and free radical, any of which can initiate the monomer unaffected by the irradiation. Which end of the initiator, i.e., cation, anion, or free radical, initiates the polymerization is dependent on the nature of the double bond (electrophilic or electrophobic) and the purity of the monomer [2]. In some monomers in extremely high purity, polymerization proceeds by all three polymerizations, i.e., cationic, anionic, and free radical, as depicted schematically in Figure 5.1. 

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