Cyclotrimerization reactivities of mono- and difunctional cyanates

Abstract

The influence of substituents of several mono- and difunctional cyanates on their cyclotrimerization behaviour has been studied by several methods. Shifts of gel point and gel fractions at full conversion in dependence on the substituent were detected for coreactions of a dicyanate with monofunctional cyanates. Critical fractions of monocyanate groups in the range of 50 mol-% for the absence of gelation were measured and could be described quantitatively by a model that is based on reaction kinetics and network statistics. Reactivity differences in dependence on substituents are also reflected in the distribution of homo- and mixed trimers at full conversion of pairs of different monofunctional cyanates as measured by high-performance liquid chromatography (HPLC). This distribution follows a simple binomial scheme for cyanates with equal reactivity, whereas remarkable deviations occur, if one of the monomers reacts faster than the other. Besides, a qualitative reactivity gradation of the used cyanates was found on the basis of differential scanning calorimetry (DSC) peak temperatures, where the reaction of each cyanate was catalyzed by the same amount of a certain phenol. Finally, the shift of the OH band of several phenols caused by association with cyanate groups, measured by FTIR, was related to the data obtained from gelation, trimer distribution and DSC experiments to establish an easy and reproducible method for comparing the reactivity of cyanate monomers.