Polymerization cation stabilization

Another reaction that has been applied to the generation of highly functionalized polymers is cationic polymerization [12-15]. Catalysts for cationic polymerizations are aprotic acids, protic acids, or stable carbocation salts. In these processes, the catalyst generally reacts with a cocatalyst to form an active initiated species. Initiation takes place by protonation of the monomer (Fig. 2A). Monomers that possess cation stabilizing groups, such as electron rich olefins, are preferred as they more readily undergo the desired polymerization process... 

The polymerization and stabilization of ribosomes are influenced by the presence of potassium and other monovalent cations in the medium, and also by Mg2+. Potassium is thus essential for protein synthesis in that it controls the structural integrity of the ribosomes, but it also appears to facilitate amino acid incorporation by transfer from the RNA to the polypeptide chain in the ribosome. 

A direct organocatalytic Michael reaction of ketones or aldehydes with /3-nitrostyrene has been reported in brine solution, using a bifunctional catalyst system proline-derived diamine (70) and TFA.203 In some cases the conversion, yield, de, and ee all exceeded 95%. Results in water were poor, mainly due to polymerization, which is catalysed by amines. It is proposed that sodium cations stabilize the anionic intermediate formed from (70) and /3-nitrostyrene, thus minimizing polymer formation. While organic co-solvent is not required, an organic-rich phase is proposed to concentrate the Michael reactants and catalysts, thus accelerating the reaction. 

Sufficiently nucleophilic alkenes such as vinyl ethers, styrenes, and isobutylene polymerize cationically to generate somewhat stabilized carbenium... 

Alkenes polymerize cationically by electrophilic addition of the monomer to a growing carbenium ion. Therefore, the monomer must be nucleophilic and capable of stabilizing the resulting positive charge. In addition, the double bond must be the most nucleophilic functionality in the monomer. Some vinyl monomers which can polymerize cationically are listed in Eq. (34) in their order of reactivity, which corresponds to the electron-donating ability of their substituents. 

Electron-donating substituents in ortho and especially in para positions additionally stabilize carbenium ions. Therefore, p-methyl and p-me-thoxystyrene readily polymerize cationically. 

The tertiary propagating carbenium ions in isobutene polymerizations are stabilized by eight /3-H atoms through hyperconjugation. Therefore, 1,1-disubstituted alkenes polymerize cationically much more readily than monosubstituted a-olefins. 

Figure 9-1 illustrates inductive influences in cationic polymerizations. The electron-releasing inductive influence of alkyl groups causes isobutene to polymerize very quickly at low temperatures where propylene reacts inefficiently and ethylene is practically inert. For similar reasons, a-methylstyrene (9-12) is more reactive than styrene, and substitution of an electron-withdrawing halogen for an ortho- or/ ara-hydrogen, decreases the monomer reactivity still further. As a corollary, ortho- and para-electron-releasing substituents (RO—, RS—, aryl) increase cation stability and monomer reactivity. 

Cationic polymerization is one of the most fundamental methods for synthesizing polymers.A general scheme for the cationic polymerization of vinyl monomers having a cation stabilizing group (R) is shown in Scheme 9.4. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions are used as... 

The resins are prepared by chloromethylating the polymer matrix, followed by treatment with the appropriate amine. The quaternary resin in both hydroxyl and salt forms is ionic, hence it may be described as a strongly basic resin which may be regarded as an insoluble polymeric cation associated with an equivalent number of active exchangeable hydroxide or halide ions. Anion exchangers are normally supplied in the chloride form rather than the hydroxide, due to the former s greater stability. 

Controlled/Living Cationic Polymerization of Vinyl Monomers Based on Cation Stabilization... 

Thermal stabilization this step is necessary to change intercalated species into pillars . These intercalated species have to be thermally stable and to be kept fixed in the interlayer space. Polymeric cations are convenient for this purpose if used as intercalating agents, they form irreversibly "oxide pillars" in the interlayer space, by thermal treatment. 

Propylene can only be polymerized cationically because of the electron-donating methyl group, since an attack on the a-carbon atom by an initiating anion is improbable because of steric hindrance. Acrylic esters, with the electron accepting acrylic ester group, on the other hand, can only be polymerized anionically (Table 15-2). Vinyl ethers obviously do not polymerize free radically because of resonance stabilization ... 

Trioxane can be polymerized cationically (catalyst BF3, HCIO4, etc.) or anionically (R3N, etc.). In the cationic polymerization, the hydrogen ion from the HCIO4, for example, protonates the acetal oxygen and forms an oxonium ion. The ring opens because the newly formed open-chain species is resonance-stabilized. The trimer eliminates formaldehyde up to an equilibrium concentration of about 0.07 mol of formaldehyde/liter. The actual chain growth probably involves the addition of formaldehyde, not trioxane. Thus, if the reaction is not too fast, an induction period is observed. The formaldehyde consumed in polymerization is replenished via the depolymerization of the trioxane ... 

Chloral, CCI3CHO, can be polymerized cationically or anionically. Phosphine and lithium t-butoxide are especially suitable initiators for anionic polymerization, whereas tertiary amines produce poly(chlorals) of lower thermal stability. The polymerization is first initiated above the ceiling temperature (58°C) and is subsequently allowed to proceed well below the ceiling temperature however, yields of only 75-80% are obtained because of the unfavorable polymerization equilibria. Since the unconverted monomer cannot be removed completely by heating, heating must be supplemented by extraction to remove the residual monomer. 

Cationic polymerization is one of the most fundamental methods for synthesizing polymers [19, 20]. Although there are several types of cationic polymerization, the most important one is cationic polymerization of vinyl monomers having a cation stabilizing group (Y) (Scheme 14.1). The initiation usually involves the addition of a cationic species (A ) to a vinyl monomer to produce a carbocationic intermediate associated with a counter anion (X ), which is derived from the initiator. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions [21-23] are used as initiators. 

Controlled/Living Cationic Polymerization Based on Cation Stabilization Using Microflow Systems [28]... 

Scheme 14.4 An example of controlled/living cation polymerization based on cation stabilization.

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