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Trapping Tetramethylene Biradical Intermediates in Cycloaddition Reactions of p-Methoxystyrene by Vinyl Copolymerization Reactions
Hall, H.K.,Abdelkader, M.
, p. 2948 - 2953 (2007/10/02)
Tetramethylene biradical intermediates in cycloaddition reactions of p-methoxystyrene (1) with trimethyl ethylenetricarboxylate (2) at 70 - 116 deg C and dimethyl cyanofumarate (10) at 28 deg C were detected through the formation of aternating 1:1 copolymers 5 and 11, identical with those formed by deliberate free-radical-initiated copolymerizations of these monomers.The rates were insensitive to solvent polarity.Free-radical inhibitors supressed copolymerization and led instead to cycloadducts, dihydropyrans 3 and 12, respectively, pointing to common intermediates incopolymerization and cycloaddition.Heating the kinetically favored dihydropyran 3 isomerized it to 1-butene 4.Vacuum distillation of 3 gave 1 and 2, while similar treatment of 12 gave copolymer by cycloreversion and spontaneous copolymerization.An ion-radical mechanism was excluded because deliberate generation of 1+. in presence of 2 gave cyclodimer 8 or homopolymer 7.Reaction of 1 with tetramethyl ethylenetetracarboxylate (14), which cannot copolymerize, led to cyclobutane 15 and traces of homopolymer 7 (some zwitterion character).The reactivity sequence 10 > 2 > 14 is dictated largely by resonance stabilization of and steric hindrance to tetramethylene formation.Consideration of the conformations available to tetramethylenes 16 - 18 provides a reasonable explanation of the results.
