Kinetics and mechanism of cyclic acetal polymerizations initiated by stable carbonium salts. I. 1.3‐Dioxolan interaction with Ph3C⊕ SbCl6⊖ and SbCl5

Abstract 1.3‐Dioxolan (DiOX) reacts with SbCl5 giving 1.3‐dioxolenium hexachloroantimonate (Dn), HCl and SbCl3. Dn decomposes in solution, forming β‐chloroethylformate and reproducing SbCl5. magnified image magnified image When DiOX is reacted with Ph3C⊕ SbCl6⊖ Dn and PhCH are formed according to MEERWEIN's reaction in spite of SbCl5 being in equilibrium with the trityl salt. Although neither HCl alone ner HCl + SbCl3 (HSbCl4) initiate the polymerization of DiOX, HCl readily reacts with DiOX, giving ethylene glycol monochloromethylether according to expectations. The polymerization can thus be initiated when this ether reacts further with SbCl5 or when H⊕ SbCl6⊖ (HCl + SbCl5) reacts directly with DiOX. As an alternative PLESCH's ring expansion mechanism, involving a protonated monomer, should be considered. When a model compound, ethoxycarboxonium hexachloroantimonate (CH3CH2OCH2⊕ SbCl6⊖), is reacted with DiOX some of the SbCl6⊖ disappears and instead SbCl3 is formed, indicating the interaction between SbCl5 and DiOX also in this case. If however, prior to this reaction, the ethoxycarboxonium hexachloroantimonate, is “stabilized” by addition of a linear acetal (e.g. CH3OCH2OCH3) and formation of the corresponding tertiary oxonium ion: magnified image then its reaction with DiOX is not accompanied by any hydride transfer reaction and [SbCl6⊖] is fully preserved. A detailed UV study of the reaction mixture and all the components involved enabled us to determine λmax and εmax for the majority of intermediate compounds formed in the course of initiation, i.e., when the initiator precursor (Ph3C⊕ SbCl6⊖ or SbCl5) is transformed into the true initiator.