PURPOSE: To obtain a graft polymer having resistance to dripping and puddling on exposure to fire from a thermoplastic polymer, s-triazine and a specific heterocyclic compd. under a controlled initial depolymerization conditions.

CONSTITUTION: A graft polymer is produced by a process mixing a thermoplastic material (A), an s-triazine compd. (B) and a heterocyclic compd. having N or 0-ring atoms (C), a process allowing 50 wt.% or more of the component A to be present based on the sum total of the mixture A, B, C and forming a graft polymer from the components B, C under initial depolymerization conditions and allowing about 5-45 wt.%, to the sum total mixture, of the component B to be present and allowing the component C to be present in an amt. of about 0.1-5 wt.% of the mixture and a process exposing the mixture to the controlled initial depolymerization to form a graft polymer of the components B, C with the component A and a process cooling the graft polymer to prevent further depolymerization.