A meta-stable state nitrogen-containing polymer formed by reacting Compound (A) and Compound (B) is described. Compound (A) is a monomer having a reactive terminal functional group. Compound (B) is a heterocyclic amino aromatic derivative as an initiator. The molar ratio of Compound (A) to Compound (B) is from 10:1 to 1:10. The meta-stable state nitrogen-containing polymer has a variance less than 2% in its narrow molecular weight distribution after being retained at 55° C. for one month.