General considerations regarding the isocyanate structure

The isocyanate group represents a strained structure able to perform addition reactions when interacting with various compounds that can be generally considered as phyloisocyanaic reaction partners. As a rule, the addition performs at the N=C double bond and only exceptionally at the C=0 group [88,89]. 

The isocyanate group reactivity is based on the polarisation of these groups represented by the following resonance limit structures [1] (equation 1.1)  

The problem of the electron density distribution in the case of the isocyanate group has been investigated. There still is large disagreement between researchers. This element is very important as it determines the reaction mechanism by specifying which of the double bonds, C=N or C=0 is the first affected in the initial stage of the reaction [89]. 

With regard to the urethane group formation, Kozak et al [90] concluded that the electronic deficit at the NCO carbon is not a sufficient condition to promote the 

The NCO group deviation from the linearity although apparently of a smaller importance, induces the possibility of the existence of two planar isomeric forms 

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