Gel Formation during the Gilch Synthesis of Poly(p‐phenylene vinylenes): Evidence of an Unexpected Explanation

Abstract

Gelation of the reaction mixture and insolubility of the poly(p‐phenylene vinylenes) (PPVs) when isolated at this stage, but complete redissolution of the gel and excellent solubility of the resulting PPVs after further stirring for hours or days, is a phenomenon in Gilch polymerizations that has not been explained so far. It is verified that, in agreement with the literature, specific additives prevent gelation. However, it is also shown that chemical crosslinking is certainly not the reason for gel formation. Instead, it seems to be the consequence of a very high entanglement density in the pristine PPVs, which requires time for relaxation. The mentioned additives seem to support this dis‐entanglement process.magnified image

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

“…In the course of our studies, we have observed some evidence in favor of attractive (p-p, donor-acceptor, or dipolar) interactions between additive molecules and chain segments, which may lead to preferential solvation of the polymer chains by the additive molecules. [62] In combination with the lower overall concentration of the polymer chains in the reaction mixture, because of the additive-induced increase of [2.2]paracyclophane formation, these additive-segment interactions may help to prevent gel formation, or support re-dissolution of formed gels, by the mechanism proposed in the following: irrespective of the amount and efficiency of the additive, the formed PPV chains have very high molar masses, and they also have coil dimensions and entanglement densities far away from the equilibrium state immediately after their formation, which are the consequences of i) the extremely rapid chain propagation, which includes regular 1,6-type radical monomer addition steps and polyrecombination events, and the fact that ii) the PPX intermediates 4 start their macromolecular dehydrohalogenation cascade instantaneously after their formation. The latter conversion is very rapid as well and transforms the still rather flexible and coiled PPX 4 into the semiflexible and comparably rod-like PPV 2.…”

“…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.…”

“…This again needs identification of the reasons for the apparently transient crosslinking process. [62] In most other polymer syntheses that lead to gel formation, the latter is caused by chemical crosslinking. For PPVs 2, which contain many unsaturated vinylene moieties, and for their precursors, the halogenated poly( pxylylene) derivatives 4, various reactions may be quoted which could induce chemical crosslinking.…”

“…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.…”

Polymerization of bifunctional dithioester and the conversion into poly(p-phenylene vinylene)s

“…The physical gel was a consequence of extremely high molecular weight and uncontrolled elimination reaction which proceeded rapidly in the presence of base, forming rod-like conjugated structure that induced gelation [35]. These disadvantages were avoidable using xanthates and dithiocarbamates as the starting materials.…”