New copolymers of a tautomerizable beta-ketonitrile monomer: synthesis, characterization and solution tautomerism

Abstract

The monomer 2-methyl-3-oxo-5-phenyl-4-pentenonitrile (MOP) was prepared by reaction of ethyl cinnamate and propionitrile in alkaline mixture. This monomer exhibits three possible tautomeric forms. The tautomeric equilibria of MOP and its copolymers with styrene in different solvents were analyzed by 1H NMR spectroscopy. The bulk and solution radical copolymerization behavior initiated with azobisisobutyronitrile was carried out at 60 ºC. The products were characterized by 1H NMR, 13C NMR, HSQC NMR, HMBC NMR and FTIR spectroscopies. The average molecular weight and polydispersity index were analyzed with size exclusion chromatography (SEC). The monomer reactivity ratios were obtained with the Fineman-Ross method, obtaining a value of r1r2 = 0.301. MOP copolymer composition as well as the nature of the solvent significantly affected the tautomeric equilibrium. Regression analysis of the copolymer composition with solvatochromic parameters showed a good linear correlation, as quantitatively expressed by means of the linear salvation energy relationship using the empirical set of Kamlet-Taft solvent parameters. This behavior could be attributed to polymer-polymer or polymer-solvent interactions prevalent in solvents of different polarity, which are responsible for changes in macromolecular chain conformations, as confirmed by FTIR and viscometric studies.