The present invention relates to a novel and improved halogen-free flame retardant compound having the structure of formula (I) wherein R 1 and R 2 are independently hydrogen, C 1 -C 6 alkyl, -P (O) (OR 3 ) 2 , -P (O) OR 3 R 4 , or -P (O) R 32 wherein R 3 and R 4 are independently C 1 -C 4 alkyl, C 6 -C 12 aryl, C 7 -C 15 aralkyl or C 7 -C 15 alkaryl, or; Or R 1 and R 2 together form an unsaturated cyclic ring, said ring being optionally substituted by an alkyl group; k is independently an integer of 1 to 2; X is each independently oxygen (O) or sulfur (S); v is 0 or 1; Y is each independently selected from the group consisting of C 1 -C 4 alkylene, C 6 arylene, C 7 -C 15 aralkylene, C 7 -C 15 alkarylene, oxygen (O) or nitrogen (NR) It is H or C 1 -C 4 alkyl; n is 0, 1 or 2, provided that when Y is oxygen (O) or nitrogen (NR), n is 1; Each Z is independently C 1 -C 4 alkylene, C 6 arylene, C 7 -C 15 aralkylene or C 7 -C 15 alkarylene; m is independently 0, 1 or 2; With the proviso that when Y is oxygen (O) or nitrogen (N), m can not be 0; Q are each independently C 1 -C 4 alkylene; t is an integer from 1 to 2; W is oxygen (O) or sulfur (S). These compounds are particularly suitable as flame retardant additives for thermoplastic polyesters.
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