Block-type architectures for Poly(p-Phenylene Vinylene) derivatives: a reality or an illusion?

Abstract

Poly(p-Phenylene Vinylene) derivatives are synthesized mostly making use of the polymerization behavior of pquinodimethane systems. Over the last forty years different synthetic routes have been developed, e.g. Wessling, Gilch, Xanthate and Sulphinyl route. For all these routes mechanistic studies are rather scarce and lead to a controversy between two possible mechanisms: anionic and radical polymerization. In this contribution it becomes clear that high molecular weight materials are associated with a self-initiated radical chain polymerization and low molecular weight materials are obtained via an anionic mechanism. This will be demonstrated for the model system in which a sulphinyl pre-monomer is polymerized in N-Methyl-Pyrrolidone. In this model system both these mechanisms are competing with each other. The observed effects on the product distribution of concentration of reagents, temperature and order in which the reagents are added, are consistent with the conclusion above. The question whether living polymerization can occur will be addressed for the radical mechanism. An experiment with a set of sequential polymerizations gives rise to an evolution of molecular weight consistent with the effect of simple dilution of the reaction medium. The conclusion is that a termination reaction is active, which can be identified as related to traces of oxygen. In these conditions the synthesis of block-type copolymers can not be achieved. For the anionic mechanism an argumentation against such possibility will be presented on the basis of relative acidities.

A deeper Insight into the Dithiocarbamate Precursor Route: Synthesis of Soluble Poly(thienylene vinylene) Derivatives for Photovoltaic Applications

“…To verify this observation, a reversed addition, meaning that the monomer is added dropwise to a solution of the base, has been done (Figure ). Such an addition should favor the anionic polymerization. , The experiment confirms this hypothesis, the molecular weight dropped extremely ( M w , 5.5 × 10 3 ; PD, 3.1).…”

“…From previous experience it is indeed to be expected that due to the lower base strength, proton abstraction and formation of the p -quinodimethane system is much slower. In case of a competition between a radical and an anionic mechanism the competition occurs at the level of initiation kinetics . As the rate of initiation for the radical mechanism scale with the square and for the anionic mechanism in first order in the concentration of the p -quinodimethane system, initiation kinetics are for the former more sensitive to concentration of the true monomer than the latter.…”

Kinetic Monte Carlo Modeling of the Sulfinyl Precursor Route for Poly(p-phenylene vinylene) Synthesis

“…In particular, for the Gilch, the sulfinyl, and the dithiocarbamate route, a series of detailed studies performed by the groups of Rehahn ,,− ,, and Vanderzande ,,− has revealed mechanistic similarities in precursor polymer formation. There are convincing indications ,,,,,,, that in polar aprotic solvents, radical and anionic quinoid polymerization can proceed in parallel; higher molar mass polymer is thought to originate form a radical chain mechanism while a parallel anionic polymerization mechanism produces lower molar masses leading to a precursor polymer which displays a bimodal molar mass distribution. ,,,,,,, In protic solvents, the anionic mechanism seems to be suppressed effectively and high molar mass precursor polymer is produced mainly via a free radical polymerization ,,,,,,,,,− that is initiated by spontaneous dimerization of the active p-quinodimethane monomer to dimer diradicals. Experimental and theoretical studies ,,− ,,,,− indicate that the preferred mode of initiation involves formation of the diradical which has both radicals located at terminal CHP groups, see Figure .…”

“…First, C–C-bond formation between the two CH 2 groups is least sterically hindered and, second, both radicals centers are stabilized by the presence of the polarizer P in the terminal CHP-group. Next to chain initiation, the diradical can undergo intramolecular radical recombination leading to the formation of [2.2]paracyclophane sideproducts. − , It has been suggested − ,,,,, that radical chain propagation is not merely a conventional free radical polymerization but that a more complex binary mechanism is operative in which, next to the “conventional” 1,6-type addition to the radical chain ends, recombination of growing α,ω-macro-diradicals can contribute to chain growth too. In contrast to conventional free radical polymerization, recombination of two α,ω-macro-diradicals does not lead to a dead polymer but to another α,ω-macro-diradical which can grow further by addition of monomer.…”

“…According to Schwalm et al, − this type of polymerization would therefore lead to essentially infinite chain lengths. However, in practice, termination reaction can occur due to the presence of oxygen ,,,, or other impurities ,− ,, in the reaction mixture or by cyclization of the α,ω-macro-diradical.…”