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Base-induced chemistry of alpha,omega-bis(dialkylsulfonium)-p-xylenes: Ylides, quinodimethanes, diradicals, adducts, and high polymers in the precursor route to poly(p-phenylene vinylene)
Abstract
Aqueous reaction of $\alpha$,$\alpha\prime$-bis(dialkylsulfonium)-p-xylenes with base to obtain polyelectrolytes has been known since 1968, when two groups showed thermal annealing of the polymer would yield poly(p-phenylene vinylene) (PPV). Many recent papers have discussed conductive and optical properties of PPV made by this route, but until now, no comprehensive study of the polymerization events has been reported. In the previously accepted mechanism, reaction proceeds by (1) formation of ylides, (2) 1,6-elimination to the quinodimethanes (QDs), (3) generation of diradical initiatiors by tail-to-tail dimerization of QDs, and (4) radical chain propagation. No sulfonium-substituted olefins besides these QDs are known to homopolymerize. This dissertation reviews several areas: organic solid-state properties; history of PPV syntheses; science and technology of the precursor route to PPV; and chemistry of sulfonium compounds, QDs, and related species. UV-vis and NMR spectroscopy experiments gave the strongest evidence to date for the intermediacy of reversibly formed transient ylides and QDs. No equilibrium between QDs and products was detected in the presence of radical traps. Substituent effects on QD stability were pronounced. Exogenous pi-compounds exhibited concentration-dependent effects on QD consumption rates; formation of diradical adducts with the QDs is postulated. Faint UV irradiation caused two effects: accelerated formation of QDs, and acceleration of their reaction rates. Novel methods for converting the polyelectrolyte into analytes suitable for gel permeation chromatography were developed. These involved nucleophilic substitution by mono- or dibasic salts for they utilized counterion exchange; several of the new materials had not been previously described. Characterization of thiophenylated derivatives found large, highly polydisperse molecular weights. The decrease in chain size obtained when radical probes were present during polymerization was consistent with radical propagation. Electron spin resonance studies suggest that diminutive, spontaneously-formed radical populations develop during polymerization. Single electron transfer and nonradical reactions were disproved. Gas chromatograph/mass spectrometry analysis of products from dilute reactions suggest head-to-tail dimerization of QDs occurs; (2.2) paracyclophanes (cyclic dimers) could not be found.
Subject Area
Organic chemistry|Polymer chemistry
Recommended Citation
Denton, Frank Russell, "Base-induced chemistry of alpha,omega-bis(dialkylsulfonium)-p-xylenes: Ylides, quinodimethanes, diradicals, adducts, and high polymers in the precursor route to poly(p-phenylene vinylene)" (1991). Doctoral Dissertations Available from Proquest. AAI9120870.
https://scholarworks.umass.edu/dissertations/AAI9120870