Organic Crystal Engineering of Thermosetting Cyanate Ester Monomers: Influence of Structure on Melting Point

Key principles needed
for the rational design of thermosetting
monomer crystals, in order to control the melting point, have been
elucidated using both theoretical and experimental investigations
of cyanate esters. A determination of the thermodynamic properties
associated with melting showed that the substitution of silicon for
the central quaternary carbon in the di­(cyanate ester), 2,2-bis­(4-cyanatophenyl)­propane,
resulted in an increase in the entropy of melting along with a decrease
in the enthalpy of melting, leading to a decrease in the melting temperature
of 21.8 ± 0.2 K. In contrast, the analogous silicon substitution
in the tri­(cyanate ester), 1,1,1-tris­(4-cyanatophenyl)­ethane, resulted
in no significant changes to the enthalpy and entropy of melting,
accompanied by a small increase of 1.5 ± 0.3 K in the melting
point. The crystal structure of 1,1,1-tris­(4-cyanatophenyl)­ethane
was determined via single crystal X-ray diffraction, and the structures
of these four di­(cyanate esters) and tri­(cyanate esters) were examined.
Although both the empirical models of Lian and Yalkowsky, as well
as Chickos and Acree, provided reasonable estimates of the entropy
of melting of 2,2-bis­(4-cyanatophenyl)­propane, they successfully predicted
only certain effects of silicon substitution and did not capture the
difference in behavior between the di­(cyanate esters) and the tri­(cyanate
esters). Semiempirical molecular modeling, however, helped to validate
an explanation of the mechanism for the increase in the entropy of
melting of the silicon-containing di­(cyanate ester), while providing
insight into the reason for the difference in behavior between the
di­(cyanate esters) and tri­(cyanate esters). Taken together, the results
assist in understanding how freedom of molecular motions in the liquid
state may control the entropy of melting and can be utilized to guide
the development of compounds with optimal melting characteristics
for high-performance applications.