Living cationic ring-only

This mechanism represents a novel type of living polymerization in that the polymer "dies after each addition of a monomer tmit. The chain "lives and grows only as long as monomer is present to be activated by the catalyst. However, block copolymers derived from the addition of other monomers (e.g., isoprene) via this mechanism have not yet been rqwrted. Breaks in the fused ring structure may occur by cationic 1 2 or 1,4 initiation. 

CROP is only reported for a limited number of cyclic ethers that exhibit enough ring strain to be readily opened. In addition, the rather similar nucleophilicity of the ether moieties in the monomers and the ring-opened polymers together with the reactive cationic oxonium species, often leads to the occurrence of transetherification reactions, complicating the development of living CROP methods. This section will focus on the living CROP of ethylene oxide, oxetane, and tetrahydrofuran. 

Non-empirical MO calculations for 1,2- and 1,3-dithiolium cations suggest that the d-orbitals are involved in bonding only to a trivial extent. A series of papers has appeared describing CNDO calculations of the electronic spectra of 1,2-dithiolium cations, and presenting a theoretical treatment of the species produced by cathodic reduction of dithiolium salts, particularly the radical form (32). Flash photolysis of 2,5-diphenyl-1,2-dithiolium salts in ethanol yields two products, of which the less stable has the absorption spectrum of the radical (32 R = Ph), and the longer-lived product appears to be the ring-opened anion (33) thus, the reaction is a photoreduction involving solvent. 

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