Trifluoromethanesulfonic esters initiators

Esters of very strong protonic acids (trifluoromethanesulfonic, fluoro-sulfonic, perchloric), however, are sufficiently strong alkylating agents to initiate the polymerization of even weakly nucleophilic monomers (cyclic acetals, ethers) [2-6], Also their anhydrides (e.g., triflic anhydride) are efficient initiators. This last compound is especially interesting, because in the polymerization of cyclic ethers it leads to macromolecules with two identical growing chain ends (difunctional initiator) [30] ... 

Most commonly initiation proceeds as direct addition of initiator to monomer molecule (route 1). Cationic polymerization of cyclic ethers may be initiated by both Bronsted and Lewis acids. Most commonly used initiators include strong protic acids such as trifluoromethanesulfonic (triflic) acid (also its anhydride or esters), fiuorosulfonic acid, perchloric acid, or heteropolyacids, oxonium salts such as triethyloxonium (e.g., EtsC A ), carbenium (e.g., Ph3C A ), or carboxonium (e.g., CeHsCO A ) salts where A should be stable, weakly nucleophilic counterion (e.g., BF4, PFg, and SbFg) or Lewis acids (most commonly used is BF3 -Et20). Several other initiation systems have been used (e.g., rare earth triflates) but the advantages over typically used simple and easily available initiators have not always been shown. 

---