Tritylium ions as initiators and co-initiators in cationic polymerizations

On the other hand, the less stabilized tris(4chlorophenyl)methyl cation (1 b) was reported to initiate the polymerization of isobutene3). In accordance, p-methoxy and p-methyl substituted tritylium ions do not initiate the polymerization of a-methylstyrene, in contrast to the less stable 1 a and 1 b4). Only for few reactive monomers, however, the initiation was proven to proceed via addition of trityl cations 1 to the monomer5), and in most of these cases the site of attack at the trityl cation has not been determined”. As some nucleophiles were shown to attack the para-position of trityl cations’), the reaction might occur either at the central carbon or at one of the aromatic rings. Due to their sluggish addition towards alkenes, tritylium ions have been introduced as Lewis acidic catalysts for the addition of acetals and aldehydes to (silyl) enol ethers and allyltrimethylsilanes by Mukaiyama et al.*). In this work, we have studied electrophilic alkylations of isobutene (2) and allyltrimethylsilane, in which tritylium ions alternatively act as electrophilic alkylating agents or as Lewis acidic catalysts. As in previous investigations, conditions were used under which the reactions terminate after the inclusion of one alkene unit”.