The Effect of Persistent TEMPO Radicals on the Gilch Polymerization

Abstract

The mechanism of the Gilch polymerization leading to poly(p‐phenylene vinylenes) is still a matter of controversial discussion. Similar to some other research groups, we strongly favor a basically radical process. Moreover, we believe it is initiated by spontaneously formed diradicals. Here, we describe further experimental evidence which clearly supports the assumed initiation step: it is shown how the polymerization process is affected quantitatively when different amounts of 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) are added as a scavenger. In full agreement with our expectations, the chain growth is either retarded or completely prevented, depending on the respective molar ratio of monomer and scavenger.magnified image

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

“…This is certainly a sound argument, independent of the fact that we are not in agreement with the offered explanation of why the PPV chains might be shorter in the presence of the mentioned additives: an anionic propagation mechanism was assumed for the Gilch polymerization, and the additives were believed to act as additional carbanionic initiators after their in-situ deprotonation (option A in Scheme 1). In contrast to this, we [20][21][22][23] and others [14,24] are convinced that this polymerization is based on radical chain growth. Nevertheless, for this latter option there is also a way available to explain the decreasing PPV chain lengths with increasing quantities of monofunctional benzylhalogenides: the mentioned additives are known to act as chain-transfer agents in radical polymerizations through their mobile benzylic hydrogen atoms (option B in Scheme 1).…”

“…General procedures and monomer syntheses were as described recently. [20][21][22][23] Deuterated tetrahydrofuran (THF-d 8 ) was purchased from Deutero GmbH, Kastellaun, Germany, and used as received.…”

“…Spectroscopic data of the obtained polymers and oligomeric fractions were as described in recent publications. [20][21][22][23] Results and Discussion 1,4-Bis(halomethylene)benzene derivatives 1a and 1b were applied as the starting materials, the standard monomer concentration was 10 mmol Á L À1 , and the number of moles of 1 used in an entry was defined as '1 equivalent'. The conversions were carried out at room temperature, at 0 8C, or at 60 8C, and the solvent was very dry THF.…”

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

“…of TEMPO. [66] The most distinctive observation was an increase in the induction period of PPV formation with increasing amounts of TEMPO. Moreover, while all entries that contained less than 1.0 equiv.…”

“…7 min, and even after that time PPV was formed only very slowly, and less than 30% of the theoretically expected quantity of PPV was obtained after completed conversion. [66] In a subsequent step, representative samples of the obtained product mixtures were analyzed after complete conversion using SEC. Figure 14 shows some characteristic molar-mass distribution curves from Gilch reactions of 1a carried out at 0 8C and in the presence of various amounts of TEMPO.…”