ACE mechanism

By comparing the problem given by Eqs. (3.186b)-(3.193b) for aCE mechanism with Eqs. (3.186c)-(3.193c) for an EC mechanism, it can be easily inferred that the current corresponding to the EC process can be deduced with a similar procedure to that followed for a CE one (see Appendix D). So, the solution is ... 

The ACE mechanism, developed in the absence of hydroxyl groups, leads to the formation of cyclic oligomers, at a high yield, by the back biting mechanism (reaction 7.20). 

The growing species on which propagation occurs are typically tertiary oxonium ions (although the mechanism involving secondary oxonium ions, the activated monomer (AM) mechanism may also operate as it will be discussed in the following sections). Those tertiary oxonium ions are located at the growing chain end thus, the typical mechanism of cyclic ethers polymerization is called the active chain end (ACE) mechanism. 

As already discussed, propagation in cationic polymerization of cyclic ethers by the ACE mechanism proceeds on tertiary 0x0-nium ion active species. Ionic species in general may exist in the form of ion-pairs (contact or solvent separated) and free ions. The fraction of each form is governed by a corresponding equilibrium constant that depends on the polarity of the medium. The knowledge of the fraction of different ionic forms, which is essential for the proper analysis of kinetics of anionic vinyl polymerization in which different forms show different reactivity, is less crucial in analyzing the kinetics of cationic polymerization of cyclic ethers because available data point out to equal reactivity of ion-pairs and free ions in propagation. 

Propagation proceeds by nucleophilic attack of an oxygen atom in a monomer molecule on a carbon atom in a-position to an oxygen atom bearing formally the positive charge in tertiary oxonium ion located at the chain end. Such a mechanism is known thus as ACE mechanism. 

Cyclization by backbiting cannot be avoided if active species of propagation are tertiary oxonium ions located at the chain end. Several years ago, we proposed that if acid-catalyzed polymerization of oxiranes is conducted in the presence of hydroxyl-group-containing compounds, the other mechanism competes with the ACE mechanism. This mechanism is based on the established mechanism of acid-catalyzed hydrolysis of oxiranes.In analogy to the hydrolysis mechanism, if an excess of oxirane (e.g., EO) is present, successive reactions of proto-nated monomer molecules with hydroxyl groups lead to extension of the chain as shown in Scheme 17... 

Although in photocurable formulations, difunctional oxirane derivatives are employed for mechanistic studies, monofunctional oxiranes are used including cyclohexene oxide, styrene oxide, or phenyl glycidyl ether. These studies indicate that the cationic polymerizations proceeding as a result of photoinitiation by onium salts have typical characteristics of polymerizations initiated by strong protonic adds. Thus, initiation involves protonation of oxirane ring while propagation proceeds on tertiary oxonium ions as active species, that is, by the ACE mechanism. 

THF may be incorporated into oligodiols by the process involving both the AM and ACE mechanisms if a highly strained cyclic ether (e.g., EO or ECH) is continuously fed into the reaction mixture containing THF and the diol as... 

A few propagation steps by ACE mechanism may occur before the whole segment is incorporated into the growing macromolecule by reaction with HO- group. 

Even if the AM mechanism operates in a cationic polymerization of oxiranes in the presence of hydroxyl groups, it does not eliminate the possible contribution of a conventional active chain end (ACE) mechanism (active center oxonium ion located at the macromolecular chain end). In order for an AM-type propagation to prevail, the instantaneous concentration of monomer should be kept as low as possible (e.g. via continuous slow monomer addition). 

Although the thermal efficiencies of various mechanical vacuum pumps and even steam jet ejectors vary with each manufacturer s design and even size, the curves of Figure 6-34 present a reasonable relative relationship between the types of equipment. Steam jets shown are used for surf ace intercondensers with 70°F cooling water. For non-condensing ejectors, the efficiency would be lower. 

Years earlier, Nicholas and Ladoulis had found another example of reactions catalyzed by Fe2(CO)9 127. They had shown that Fe2(CO)9 127 can be used as a catalyst for allylic alkylation of allylic acetates 129 by various malonate nucleophiles [109]. Although the regioselectivites were only moderately temperature-, solvent-, and substrate-dependent, further investigations concerned with the reaction mechanism and the catalytic species were undertaken [110]. Comparing stoichiometric reactions of cationic (ri -allyl)Fe(CO)4 and neutral (rj -crotyl ace-tate)Fe(CO)4 with different types of sodium malonates and the results of the Fe2(CO)9 127-catalyzed allylation they could show that these complexes are likely no reaction intermediates, because regioselectivites between stoichiometric and catalytic reactions differed. Examining the interaction of sodium dimethylmalonate 75 and Fe2(CO)9 127 they found some evidence for the involvement of a coordinated malonate species in the catalytic reactions. With an excess of malonate they... 

Methylamine hydrochloride method. Place 100 g. of 24 per cent, methyl-amine solution (6) in a tared 500 ml. flask and add concentrated hydrochloric acid (about 78 ml.) until the solution is acid to methyl red. Add water to bring the total weight to 250 g., then introduce l50 g. of m%a, and boil the solution gently under reflux for two and three-quarter hours, and then vigorously for 15 minutes. Cool the solution to room tempara-ture, dissolve 55 g. of 95 per cent, sodium nitrite in it, and cool to 0°. Prepare a mixture of 300 g. of crushed ice and 50 g. of concentrated sulphiuic acid in a 1500 ml. beaker surrounded by a bath of ice and salt, and add the cold methylurea - nitrite solution slowly and with mechanical stirring and at such a rate (about 1 hour) that the temperature does not rise above 0°. It is recommended that the stem of the funnel containing the methylurea - nitrite solution dip below the siu ace of the acid solution. The nitrosomethylurea rises to the siuface as a crystalline foamy precipitate. Filter at once at the pump, and drain well. Stir the crystals into a paste with about 50 ml. of cold water, suck as dry as possible, and dry in a vacuum desiccator to constant weight. The yield is 55 g. (5). 

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