Cationic reactions depolymerization

The running of parallel reactions of hydrolysis, ammonolysis and depolymerization of apple pectin in aqueous solution of ammonia (IM) at 25 C were investigated. It was examined the effects of monovalent cations (Na, K", NH4 ) and divalent cations (Ca, Mg ) when they were added as chloride salts. It was found that the relative rates of the above mentioned reactions, depend on the nature and concentration of the added salts as well. The chlorides of sodium, potassium and calcium accelerate hydrolysis and depolymerization, while magnesium chloride delays these reactions. Ammonolysis was increased in cases of ammonium chloride addition. 

As for (i-0-4 ethereal bond cleavage, reaction of the primary cation-radical with solvent water under the same conditions of bio-oxidation was shown to form an arylglycerol and the corresponding phenoxy radical (Kirk et al. 1986, Fabbri et al. 2005) (Scheme 8.22). Since the p-0-4 ethereal bond is the most abundant type of interunit linkage in the lignin polymer, this ethereal bond cleavage represents an important depolymerization reaction. 

Tn the cationic polymerization and copolymerization of trioxane in the - melt or in solution, an induction period usually exists, during which no solid polymer is formed and the reaction medium remains clear. Nevertheless, reactions are known to occur during this period. By using BF3 or an ether ate as catalyst, in homopolymerization, Kern and Jaacks (I) reported the formation of formaldehyde via depolymerization of polyoxymethylene cations. 

One of the most prominent features in the heterogeneous copolymerization of trioxane is the occurrence of two different kinds of active centers—dissolved and crystalline copolymer cations. They have different copolymer reactivity ratios and different tendencies to depolymerize, i.e., different formaldehyde equilibrium concentrations. At first the formation of soluble copolymer with high dioxolane content did not raise much hope for obtaining a crystalline copolymer of good thermal stability from trioxane and dioxolane but the gradual depolymerization of the soluble copolymer proved to be a useful side reaction which greatly improved the situation. Eventually, the entire complicated process turned out to be quite favorable for the formation of a stable crystalline copolymer with the desired random distribution. 

In115 116) the cationic polymerization of cyclic ethers was examined theoretically and experimentally with regard to the nature of MWD variation. A theoretical analysis was made of how MWD is affected by the depolymerization reactions, monomole-cular deactivation of active centers, recombination of active centers, chain transfer by hydroxyl-containing compounds, chain transfer to the monomer, and ether oxygen of the polymer chain, as well as via the end hydroxyl group. 

Type of Cation and Cation Ratio. The polymerization-depolymerization equilibrium conceivably can be influenced by the stabilizing effect of certain cations on certain precursors. The cations present also may determine the way in which these building blocks are joined to give a framework structure. Reaction mixtures, identical except for the cation introduced with the phosphate, yield different products at the same pH. 

Depolymerization of the high polymer to cyclic oligomers can occur by back-biting reactions in which active cationic end units attack middle units of the same chain. 

The oxidation of various sulfides 424 with iodosylbenzene in the presence of catalytic amounts of quaternary ammonium bromides affords the respective sulfoxides 425 in high yields (Scheme 3.171) [521]. The best catalytic effect in this reaction is observed when oxidation is carried out in a nonpolar solvent (toluene, hexane, dichloromethane) in the presence of trace amounts of water and 10 mol% of cetyltrimethylammonium bromide (CTAB). Iodosylbenzene can also be activated in the solid state by pulverization with natural clays or silica gels [522, 523], The oxidation of various alkyl aryl sulfides with (PhIO) supported on natural (montmorillonite, KSF and bentonite clays) as well as cation-exchanged KlO-montmorillonite clays affords sulfoxides in excellent yields. A mechanism involving depolymerization of (PhIO) by the acidic SiOH sites on the clay is proposed for this reaction [522], Organic sulfides are also selectively oxidized to sulfoxides by the solid reagent system PhI(OAc)2-alumina [524], or by Phl(OAc)2 in water in the presence ofKBr [525],... 

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