Alkoxycarbenium ions, initiation

The simultaneous presence of secondary and tertiary oxonium ions in the polymerization of 1,3-dioxolane initiated with trifluoromethanesulfonic acid was alas shown by trapping a proton (from the secondary oxonium ion) and a carbenium ion (from either the tertiary oxonium ion or the alkoxycarbenium ion) by the ion-trap-ping technique. 

Microsystem-controlled cationic polymerization technology requires extremely reactive initiators and cation pools serve as effective initiators for this technology. Usually carbocations are generated by a reversible process from their precursor. Yoshida et al. developed the cation pool method [35], in which carbocations are generated irreversibly by low-temperature electrolysis and are accumulated in relatively high concentration in the absence of nucleophiles. N-Acyliminium ions, alkoxycarbenium ions [36-40] and diarylcarbenium ions [41] have been generated by this method. Such cation pools are expected to serve as extremely reactive initiators for cationic polymerization. 

By reacting 1,4-butane diol with paraformaldehyde in the presence of sulfuric acid at 150-180" , the seven-membered 1,3-dioxepane is prepared. With other aldehydes, homologous 2-alkyl-substituted 1,3-dioxepanes have also been prepared using a cationic ion exchange resin instead of sulfuric acid. This latter technique was introduced by Astle [42]. The dioxepanes have been converted to polymers in methylene dichloride or in 1,2-dichloro-ethylene. The initiator used was boron trifluoride etherate the reaction temperatures ranged from -10° to +10°. The reactions were carried out under anhydrous conditions by techniques suitable for reaction kinetics studies. The work indicated that, at least for this class of compounds, the polymerization propagation step involves linear alkoxycarbenium ions [47]. 

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