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

A consistent picture is presented of the mechanistic details and intermediates of the Gilch polymerization leading to poly(p-phenylene vinylenes) (PPVs). In-situ generated p-quinodimethanes are shown to be the real monomers, and spontaneous formation of the initiating radicals is effected by dimerization of some of these monomers to dimer diradicals, the latter also being the reason why significant amounts of [2.2]paracyclophanes are formed as side-products. Chain propagation predominantly proceeds by radical chain growth, occasionally interrupted by polyrecombination events between the growing α,ω-macro-diradicals. Based on this knowledge, oxygen is identified as a very efficient molar-mass regulating agent, and the temporary gelation of the reaction mixtures is interpreted to be the consequence of a very high entanglement of the polymers immediately after their formation. Last but not least, it is rationalized why the usually considered constitutional defects in Gilch PPVs might not be the only and most relevant ones with respect to the efficiency and durability of the organic light emitting devices produced thereof, and why cis-configurated halide-bearing vinylene moieties should be perceived as being among the most critical candidates. These considerations result in the recommendation of straightforward measures that should lead to clearly improved PPVs.