Toward controlled Gilch synthesis of poly(p-phenylenevinylenes):: Synthesis and thermally induced polymerization of α-bromo-p-quinodimethanes

It is general consensus that in Gilch polymerizations the 1,4-bis(halomethylene)benzene starting material first changes into an alpha-halo-p-quinodimethane intermediate which then acts as the real active monomer in the subsequent chain growth process. Recently, we could verify the formation of alpha-chloro-p-quinodimethane directly via in-situ NMR spectroscopy at low temperatures. However, quantitative formation of this p-quinodimethane was not possible there. Now, we show that even such quantitative conversion into the active monomer is possible if bromomethylene-functionalized starting materials are used instead of their chloromethylene counterparts. Moreover, it is even possible to induce chain growth leading to PPV in a very controlled way by carefully warming the obtained solution of p-quinodimethane. In this manner, the temperature can be determined where chain growth startsand hence thermal energy is sufficient for the initiating process. Finally, we could reconfirm that the chain growth is a radical polymerization here as well, initiated by diradicals formed via spontaneous dimerization of a low number of alpha-bromo-p-quinodimethane monomers. This proof could be provided by quantitatively analyzing the effect of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO): depending on the molar ratio of monomer and scavenger, the chain growth is either retarded or completely prevented.