Free-Radical Polymerization of Dibenzofulvene Leading to a π-Stacked Polymer:  Structure and Properties of the Polymer and Proposed Reaction Mechanism

Abstract

Free-radical polymerization of dibenzofulvene (DBF) was carried out under various conditions. The radical polymerization proceeded exclusively in vinyl fashion without isomerization of the growing radical or aromatic substitution by the initiator fragment. The polymerization was highly conformation-specific (stereospecific), giving a polymer with a π-stacked conformation in which the main-chain C−C bonds are nearly all trans and the side-chain fluorene moieties are stacked on top of each other. The conformation specificity (stereospecificity) of the DBF radical polymerization was almost similar to that in the anionic polymerization although the radical polymerization products appeared to have a small amount of irregular, defective conformation incorporated into the mostly π-stacked chain. The conformation specificity was affected by the initial monomer concentration and reaction temperature. This observation was explained by a proposed mechanism where two types of the growing radicals having different conformations mediate the polymerization. The poly(DBF) obtained by the radical polymerization showed lower fluorescent intensity and higher solubility possibly due to a small amount of conformational defects.

Synthesis, structure and function of π-stacked polymers

“…The radical polymerization of DBF has also been studied in detail. 17 Radical polymerization products have been found to have virtually the same chemical structure as that of anionic polymerization products. The conformation specificity (stereospecificity) of DBF radical polymerization was also similar to that of anionic polymerization.…”

“…17 This polymerization is assumed to be mediated by radical growing species. On standing in a flask under air or pure O 2 atmosphere, DBF crystals produce a polymer.…”

“…[41][42][43][44][45][46][47] However, there has been no clear examples of solid-state polymerization of vinyl monomers, except for the reactions using solid matrices (inclusion polymerization) [48][49][50][51][52] and the DBF polymerization described here. 17 Although the details are still under investigation, cationic initiators can also effectively polymerize DBF. 14 …”

“…Stable and regulated p-stacked structures in polymers have been realized for DNAs, 3 oligo(m-phenyleneethynylene)s, 4 oligomers consisting of alternating donor and acceptor monomeric units, 5,6 oligomers consisting of acceptor monomeric units in the main chain that interact with a low-molecular-weight donor molecule, 7 oligomers having perylene moieties in the main chain, 8,9 polymers consisting of cyclophane units, [10][11][12] a polymer of bezofulvene derivative, 13,14 and polymers of dibenzofulvene (DBF) and its derivatives (Chart 1). [15][16][17][18][19][20][21][22] In a DNA duplex, paired bases are regularly stacked inside the helical strand. In oligo (m-phenyleneethynylene)s, phenylene groups in the main chain closely overlap because of helical folding of the polymer chain.…”

A comprehensive study of substituent effects on poly(dibenzofulvene)s

“…1), which gives rise to a number of characteristic observations in the photophysics and electrochemistry profiles of the polymer. [1][2][3][4] In terms of practical application, poly(DBF) has a great potential to serve as an excellent organic transistor which has stacked fluorenes as a delocalization tunnel for charge transport, contrary to typical conducting polymers such as polythiophene and polyaniline that delocalize charges through the conjugated main chain. 5,6 For instance, poly(DBF) exhibits a high hole mobility of B3 Â 10 À4 cm 2 V À1 s À1 which is of the same order of magnitude as the inorganic selenium semiconductor.…”