Disclosed are oligomeric phosphonates including oligophosphonates, random or block co-oligo(phosphonate ester)s and co-oligo(phosphonate carbonate)s produced using a condensation process terminated with hydroxyl, epoxy, vinyl, vinyl ester, isopropenyl, isocyanate groups, and the like. These materials can be used as a reactive additive to other polymers, oligomers or monomer mixtures to impart flame resistance without diminishing melt processability which is important in the fabrication of polymers for many applications.