Initiation with Strong Acids

Spectroscopic (NMR, IR) studies show, that protonation (or coordination with Lewis acids) occurs predominantly at the oxygen atom as depicted in Eq. (11-3)5,6). In the absence of other strong bases the positively charged C-atom in the protonated monomer is attacked by the N-atom of another lactam and primary amino- and N-acyl lactam end-groups are formed (11-3)  

In the next step proton is transferred to the incoming monomer, which becomes ready to participate in chain growth. Propagation involves the most basic primary amino group and protonated (activated) monomer. After protonation of the monomer, the reactive intermediate is formed in which the proton may be located either on the exo-N-atom (A), endocyclic N-atoms (Q or on hydroxyl O-atom (B)  

The intermediates most likely react intramolecularly by breaking the C—N bonds. If Lewis acid is used instead of the protonic acid, it coordinates with the O-atom and the process proceeds similarly as described for proton 7). 

According to the Delongchamps theory of stereoelectronic control8), the cleavage of a given bond is facilitated when at the neighbouring heteroatoms the electron pairs are located in the antiperiplanar positions7,9 . This is illustrated below, the arrow shows the bond in the antiperiplanar position  

Therefore, depropagation (from A) is unlikely, because the electron pair on the endocyclic N-atom cannot be located at the antiperiplanar position (the required position for the electron pair is occupied by a bond that is a part of the ring). The breaking of the C—O bond (B) and propagation (C) are both allowed stereoelec-tronically for the same reason. This is visualized by the scheme below, which shows the corresponding conformations and subsequent reaction steps  

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In the polymerization initiated with strong acids under anhydrous conditions the major part of monomer is incorporated into the polymer via highly reactive intermediates [243]. In the presence of strong acids, protonation of -substituted lactams can occur both at the oxygen and nitrogen (see Section 3). Similarly as with the unsubstituted lactams (Section 5.1), the iV-protonated form (XXXI) resulting from the equilibrium... 

Other early match-like devices were based on the property of various combustible substances mixed with potassium chlorate to ignite when moistened with strong acid. More important was the property of chlorates to form mixtures with combustibles of low ignition point which were ignited by friction (John Walker, 1827). However, such matches containing essentially potassium chlorate, antimony sulfide, and later sulfur (lucifers), mbbed within a fold of glass powder-coated paper, were hard to initiate and unreHable. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

A road tanker was loaded with l-chloro-2,3-epoxypropane and then driven 250 miles overnight to the delivery point. On arrival, the contents were found to have self heated (undoubtedly from polymerisation initiated by some unknown contaminant) to the boiling point (115°C at ambient pressure) and soon afterwards the relief valve lifted and discharged large volumes of vapour. Cooling with water sprays eventually restored thermal control over the remaining tanker contents [1]. The material is incompatible with strong acids, caustic alkalies, zinc, aluminium, aluminium chloride or iron(III) chloride, all of which catalyse exothermic polymerisation [2],... 

As was shown previously, reduction of contact time and specific surface may be helpful in lowering sorption losses, and acidification with strong acid will generally prevent the problems of losses by sorption. However, it must be emphasised that the use of acids may drastically change the initial composition of aqueous samples, making unambiguous interpretation of the analytical results cumbersome or even impossible [55]. 

Several strong protonic acids are commercially available. Trifluoro-methanesulfonic (triflic) acid, fluorosulfonic acid, and perchloric acid may be obtained and stored in a pure state. The first two can be conveniently purified by distillation (b.p. 162° C and 165° C, respectively) [12], perchloric acid is less frequently used due to its oxidative properties and difficulties in handling (explosive). Complex acids HPF6 (HF + PF5) and HSbF6 (HF + SbF5) are available as complexes with ethers. Acids of H + BF30H- type are often the real initiators of polymerization initiated with Lewis acids (e.g., BF3) if water is not rigorously excluded from the system. 

Cationic polymerization, initiated with strong protonic acids (e.g., triflic acid), occurs purely by opening of the Si—O bond giving high molecular weight regular polysilaethers. For the former monomer, yields up to 80% and M up to 40,000 were reported, whereas the latter polymerized with yields up to 90% of polymer and with M 34,000. 

Polymerizations initiated under anhydrous conditions with carboxylic acids [194—196] should follow a similar mechanism as polymerizations with strong acids. Lactam cations (XXIX) formed in the equilibrium... 

Isoquinolines are often synthesized by the Bischler-Napieralski cyclization of an N-acyl-2-phenylethyl amine with strong acid and P2O5, followed by oxidation of the initially formed dihydroisoquinoline. Suggest a mechanism for the cyclization. 

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