Ion radical cycloadditions and the synthesis of novel, electron-rich polymer structures
The aminium salt initiated cation radical chain cycloaddition polymerization of various oxygenated difunctional monomers is investigated in order to minimize a limiting factor, a competing carbocation pathway, by applying a dichloromethane/water binary solvent mixture. Polymerizations have also been initiated by electrochemical oxidation and photosensitized electron transfer. The synthesis of cation radical cycloaddition polymers of difunctional carbazole monomers which contain reactive trans-1-propenyl groups for polymerization in the N-, 3- or 6- positions of carbazole, is presented. All polymers containing carbazole units in the polymer main chain show good solubility in halogenated organic solvents, high molecular weights, and high thermal stabilities. The reaction appears to proceed via a highly efficient cation radical chain mechanism which circumvents the usual hole transfer step of the propagation cycle. This polymerization represents the first observation of direct cation radical Diels-Alder cycloaddition polymerization and affords a novel polymer structure. The cation radical Diels-Alder cycloadditions of monofunctional, highly electron-rich substrates such as N-(trans-1-propenyl)carbazoles and N-phenyl-3- (trans-1-propenyl)carbazole to generate monomers of interest in connection with ring-opening metathesis polymerization are also presented. The cycloadditions of various electron-rich monomers to 1,3-cyclopentadiene efficiently generate norbornene monomers, which readily undergo ring-opening metathesis polymerization to yield electron rich polymers. Both ring-opened homopolymers and copolymers containing carbazole groups are synthesized. In addition, investigations of photorefractive effects resulting from a combination of photoconductive and electro-optic effects (change in refactive index in response to an electric field) in composites which utilize new carbazolecontaining polymers as charge transport components are carried out, and their photorefractive performances are compared with the performance of the standard composites based upon poly(N-vinyl carbazole). Finally, some studies of anion radical chain cycloadditions of tethered enones by cathodic reduction, which involve intramolecular anion radical cyclobutanation and unprecedented Diels-Alder cycloaddition, are reported. Evidence for stepwise cycloaddition involving distonic anion radical intermediates has been illustrated.