The extension of efficient cation radical chain cycloaddition chemistry to the difunctional context has recently provided a novel and efficient mechanism for the polymerization of monomers possessing readily ionizable, electron-rich alkene moieties. The new mechanism, cation radical chain cycloaddition polymerization, appears to provide the most efficient method presently available for cycloaddition polymerization and is unique in affording cyclobutapolmers, i.e., polymers in which the monomer units are interconnected via cyclobutane linkages. In addition to cyclobutapolymerization, the new mechanism is also capable of yielding Diels-Alder polymers when one functionality is an ionizable alkene moiety and the other is a conjugated diene moiety. Further Diels-Alder cation radical chain cycloaddition polymerization can proceed either directly or via cyclobutapolymerization followed by facile cation radical vinylcyclobutane rearrangement. The new method is also exceptional in that propenyl functionalities are usually preferred over vinyl moieties. Reactions are typically initiated via catalytic amounts of a stable cation radical salt, tris(4-bromophenyl)aminium hexachloroantimonate in dichloromethane solvent, and are extremely fast, reaching high (often > 100 000)molecular weights in 2-5 minutes. In several cases, polymerizations have also been initiated by electrochemical oxidation and photosensitized electron transfer. Although homopolymerization is usually by far the most efficient approach to cation radical cycloaddition polymerization, a number of copolymerizations have also been investigated and will be discussed in the present review. The recently established use of cation radical Diels-Alder cycloadditions of monofunctional, highly electron-rich substrates such as propenylcarbazoles to generate monomers of interest in conection with ring-opening metathesis polymerization will also be presented. The cycloadditions of various electron-rich monomers to 1,3- cyclopentadiene efficiently generate norbornene monomers , which readily undergo ring-opening metathesis polymerization (ROMP) to yield electron rich polymers.
cation radical cycloadditions, cyclobutapolymer, diels-alder, romp, ring-opening metathesis polymerization
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin,TX, USA.