Kinetics of oxime-blocked isocyanates

Studies on the synthesis and reactivity of oxime-blocked isocyanates with alcohols and amines are presented.; Oxime blocked aromatic isocyanates were more reactive with n-butyl amine than blocked aliphatic isocyanates, which is in accordance with the generally observed reactivity for blocked isocyanates. At elevated temperatures ({dollar}>{dollar}60{dollar}spcirc{dollar}C), relative reactivities for acetone oxime blocked adducts are o-oxime {dollar}>{dollar} p-oxime {dollar}>{dollar} TMI/AOX {dollar}>{dollar} CHI/AOX (60:24:8:1). However, when a sterically crowded isocyanate (TMI) is blocked with a sterically crowded oxime (diisopropyl ketoxime) the reactivity of the resulting adduct is intermediate with respect to the blocked aromatic adducts; o-oxime {dollar}>{dollar} TMI/i-POX {dollar}>{dollar} p-oxime (60:38:24).; Unlike the reaction at elevated temperatures, the reaction rates of oxime-blocked aromatic isocyanates with amine at 35{dollar}spcirc{dollar}C are dependent on amine concentration. Competing mechanisms, one being independent of amine (such as elimination-addition), and the other being first order in amine (such as addition-elimination or base catalyzed elimination) are consistent with the observed rates of reaction.; When the above blocked adducts are reacted with n-butyl alcohol the observed relative reactivities are p-oxime {dollar}>{dollar} TMI/i-POX {dollar}simeq{dollar} o-oxime {dollar}>{dollar} TMI/AOX {dollar}>{dollar} CHI/AOX (13:10:10:2.5:1). Once again the sterically crowded TMI/i-POX adduct is intermediate in reactivity relative to the blocked aromatic isocyanates. TMI/i-POX offers the high reactivity of the blocked aromatic adducts allowing low cure temperature as well as good long-term weatherability, characteristic of aliphatic isocyanates.; Data presented herein for the high temperature reaction of oxime-blocked isocyanates with alcohols and amines support an elimination-addition mechanism. The reaction rates of alcohols with oxime blocked adducts are less than those with amine. The observed reactivity is attributed to the decreasing order of reactivity of amine {dollar}>{dollar} oxime {dollar}>{dollar} alcohols with the intermediate isocyanate generated by the elimination reaction.; The catalysis of the reaction of oxime-blocked isocyanates with nucleophiles was investigated. With amine, the addition of DABCO and DBTDA as catalyst did not significantly increase the observed rates of reaction. On the other hand, addition of these catalysts when alcohol is used, significantly increased the observed rates of reaction. This increase has been attributed to catalysis of the rate limiting addition reaction.