Efficient Oxygen‐Induced Molar‐Mass Control of Poly(p‐phenylene vinylenes) Synthesized via the Gilch Route

Abstract

Oxygen is shown to act as an efficient molar‐mass regulating agent in Gilch syntheses of PPV. As a scavenger, it undergoes instantaneous recombination with the initiating diradicals as soon as they appear in the system. Regular polymer formation can only start when all oxygen has been used, proceeding predominantly as chain‐growth polymerization of the p‐quinodimethane monomers. Since all radical species involved in this Gilch process are diradicals, some polyrecombination events occur in parallel. Therefore the initially formed peroxy diradicals are also incorporated into the resulting chains. Later, they break under very mild conditions, thereby causing a systematic decrease of the final molar mass of PPV.magnified image

The Gilch Synthesis of Poly(p‐phenylene vinylenes): Mechanistic Knowledge in the Service of Advanced Materials

“…In order to achieve a maximum of significance, the set of experimental parameters selected for our investigations [62][63][64][65][66][67][68] was quite similar to those used in the majority of mechanistic studies on the Gilch reaction published in the literature.…”

“…Chain propagation proceeds predominantly by 'regular' 1,6-type addition of monomers 3 to the radical chain ends, but recombination events of the a,v-macro-diradicals 4 Ã and/or 2 Ã also contribute to the chain growth. [63] also designed in a way that they break easily, either thermally, chemically, or by another stimulus, the desired chain-length reduction should be accomplished (Scheme 13).…”

“…Based on these considerations, oxygen was tested as a regulating agent: [63] it is well-known that oxygen, when present during a radical polymerization, is incorporated into the chains by a twofold recombination with active chain termini, and thus as a labile peroxy moiety. [141,142] For validation of this strategy, different volumes of ambient air (which corresponded to 0.005 up to 0.5 equiv.…”

“…Therefore, we started a research program dedicated explicitly to the elucidation of all open mechanistic facets of the Gilch synthesis and their implications on i) the control of molar masses, ii) the prevention of gel formation during polymer synthesis, and iii) the minimization of critical defects and chain termini in the resulting PPVs. [62][63][64][65][66][67][68] The first step in this program was the careful review and evaluation of all available mechanistic information not only for the Gilch reaction but also for all 'Gilch-related' PPV syntheses. a Guided by this groundwork, a step-by-step analysis was carried out of the intermediates, products, and side-products, and how the involved reactions proceed when syntheses are carried out either under the standard conditions or affected by specific modifications and additives.…”

“…Due to their diradical nature, recombination of two growing a,v-macrodiradicals 4 Ã and/or 2 Ã always leads back to another active diradical 4 Ã and/or 2 Ã which is able to continue chain propagation further (Scheme 12). [63] Moreover, as no efficient pathway is obvious for chain termination by disproportionation events (appropriate hydrogen atoms are missing at the chain ends of 4 Ã and 2 Ã ) this polymerization should essentially lead to infinite chain lengths. In reality, termination seems effected by slow rearrangement processes, impurities that are always present in the reaction mixture to some extent, or another very slow deactivation reaction.…”