Copolycondensed vinylphosphonates and their use as flame retardants

A flame retardant polymeric resin composition containing at least one copolycondensed vinylphosphonate said copolycondensed vinylphosphonate comprising the product resulting from the reaction in stoichiometric ratio of from about 1:10 to 10:1 between a bis-(2-haloalkyl) vinylphosphonate and at least one pentavalent phosphorus ester of the structure ROP(.dbd.O)XY where R is selected from the class consisting of C.sub.1 -C.sub.20 alkyl, C.sub.2 - C.sub.20 alkenyl, and C.sub.1 - C.sub.20 haloalkyl groups and X and Y are groups selected from the class consisting of RO -, C.sub.1 -C.sub.20 alkyl, C.sub.2 -C.sub.20 alkenyl, phenyl, phenoxy, amino, C.sub.1 -C.sub.20 alkyl substituted amino, phenyl substituted amino, C.sub.2 -C.sub.20 alkylene bonded to the same or to another ROP(.dbd.O) moiety and C.sub.2 -C.sub.20 alkylenoxy and C.sub.2 -C.sub.20 alkylenedioxy bonded to the same or to another ROP(.dbd.O) moiety, wherein said reaction is carried out at an elevated temperature for a period of time which is sufficient to evolve R-halide as a by-product and to form a P(O) -- O-- alkylene-O-P(O) linkage with the proviso that said product is not the homopolycondensed product of said bis-(2-haloalkyl) vinyl phosphonate.