Vinyl ketone‐sec‐allyl alcohol copolymers–open chain‐hemiacetal ring tautomerism

Three poly(vinyl ketones) have been partially reduced to obtain different copolymers of vinyl ketones and secondary allyl alcohols. The first set of copolymers is obtained by partial reduction of poly(phenyl vinyl ketone). UV studies indicated that the Beer-Lambert law cannot be applied to the different carbonyl bands. This is explained by a ring-chain tautomerism between a carbonyl group and a neighboring hydroxyl group. IR and 13C NMR spectroscopy confirmed these results and proved the existence of this tautomerism. The second set of copolymers were obtained by partial reduction of poly(methyl vinyl ketone). UV showed the tautomsrism was strongly shifted toward the ring form; acetal condensation occurs in (tetrahydrofuran) THF-methanol-water mixture solution. The third set of copolymers obtained by partial reduction of chiral poly [(S-2-methyl-butyl)vinyl ketone] were studied by UV, ORD (optical rotary dispersion), and CD (circular dichroism); all three methods giving concordant results. The tautomerism was shifted more toward the chain form than for the reduced poly(methyl vinyl ketones) but less than for reduced poly(phenyl vinyl ketones). The addition of strong acid to the polymer solution shifts the equilibrium toward the chain form; a similar shift is observed when the original polymer is in the solid state.