Anionic kinetics

A general description of anionic polymerization kinetics is complicated by the associations that may occur, particularly in nonpolar (hydrocarbon) solvents. The rate of propagation is proportional to the product of the monomer concentration and the concentration of active living chains [ ] 

With negligible association (as in tetrabydrofuran solvent, for example, or hydrocarbons at BuLi concentrations less than 10 molar), each initiator molecule starts a growing chain, and in the absence of terminating impurities, the number of active living chains equals the number of initiator molecules added, 

Unlike the case for free-radical addition, this can be readily integrated for a batch reactor ([/]o is constant) to give 

Because anionic polymerizations are generally carried out in rather dilute solutions in inert solvents, volume changes with conversion tend to be much smaller than when undiluted monomer is polymerized. This often justifies the neglect of volume change (s = 0), for which (11.9) becomes 

Example 3, For an isothermal reaction (constant fcp) subject to the assumptions in Example 2, obtain an expression that relates x to time. Neglect volume change. 

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Triphenylmethyl, tropylium, xanthylium, acylium and diazonium salts have also been used successfully in the study of the polymerization of THF [50]. In fact the true equilibrium conversions were first demonstrated using p-chlorophenyl diazonium hexafluorophosphate [41, 115], and at the same time the polymerizations were shown to be living . The lower equilibrium yields [50] observed when other counter-ions are employed, e.g. SbClg, appear to arise because of a termination mechanism associated with the anions. Kinetic studies [50] of the polymerization of bulk THF with Phg C SbCl have established the apparent rate law... 

Each anionic kinetic experiment is disturbed to some extent by two side reactions brought about by traces of impurities. Traces of water in the monomer solution cause a rapid killing of some living ends simultaneously with the... 

In a reaction under kinetic control, the composition of the product mixture is determined by the relative rates of formation of each product. No equilibrium among possible alternative structures is set up. In the case of formation of enolate anions, kinetic control refers to the relative rates of removal of the alternative a-hydrogens. The less hindered a-hydrogen is removed more rapidly thus, the major product is the less substituted enolate anion. Because a slight excess of base is used, there is no ketone to serve as a proton donor and the less stable enolate anion cannot equilibrate with a more stable one. 

Multidentate Leaving Groups.—The hydrolysis of [Co(ox)a] - and of [Co(ox)2(OH2)2], which ultimately produces cobalt(n) and carbon dioxide, involves the formation of an intermediate containing a unidentate oxalate ligand previous to the rate-determining step. Free radical intermediates are thought unlikely in the decomposition of these oxalato-complexes, but malonate ion-radicals are thought to be intermediates both in the thermal and photochemical hydrolysis of the [Co(mal)3] anion. Kinetics are reported for a third example of these aquation-redox processes, [Co(acac)2] in acidic solution. ... 

As an alkaline environment is encountered during interesteriflcation, the catalyst (which is nucleophilic) attacks the slightly positive carbonyl carbon at one of three fatty acid-glycerol ester bonds to form a tetrahedral intermediate. A fatty acid methyl ester is released, leaving behind a glycerylate anion. Kinetics of base-catalyzed hydrolysis of esters are second-order reactions dependent on both ester... 

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