Reactivity of mono‐epoxides toward silk fibroin

The addition reaction of various mono‐epoxides such as epichlorohydrin, glycidol, allylglycidyl ether, butyl glycidyl ether, phenyl glycidyl ether, propylene oxide, styrene oxide, hexadecene oxide, and vinyl cyclohexene monoepoxide with silk fibroin was investigated in the presence of some salts in different solvents at 30–80°C. Histidine, lysine, arginine, aspartic acid and glutamic acid, serine and tyrosine reacted. The weight gains decreased with increasing solubility parameter of the solvents, reached a minimum value at about 10 or at the solubility parameter of the epoxide, and then increased with the parameter. The salt effects on the epoxide‐silk reaction were described partly by the hydration ability or by the acidity of the cations, and also by the basicity or the nucleophilicity of the anions. The substituent effects of the epoxide on the add‐ons were elucidated by the modified TAFT equation, (log k − log k 0 )/σ * = ρ p + ρ s E s /σ * , where k 0 and k are the rate constants for the reaction of a given compound and for the same reaction of one of its derivatives; σ * is the polar substituent constant characteristic of the substituent, E s is the steric substituent constant, and ρ p and ρ s are the reaction constants as measure of the susceptibility of the reaction to the polar and steric effects of substituents.