**2.2 Monomer reactivity conditions**

As explained in the above section, the PAA solution is formed by nucleophilic substitution of carbon atom in carbonyl group of the dianhydride with a diamine. Therefore, electrophilicity of carbonyl group carbon atom and nucleophilicity of amino group nitrogen atom can be controlled using electron affinity measurement of both dianhydride and diamine. PMDA has the highest electron affinity and shows strong reactivity with diamines compared with other dianhydrides such as DSDA, BTDA, BPDA, and ODPA. The change in the structure of the diamines and dianhydride can affect the reaction significantly. These highest electron affinity dianhydrides can easily absorb moisture; therefore, they must keep moisture-free environment.
