The phosphorus comonomer [(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)methyl]-methyl butane-dioate (DOP-MBDA) was synthesized through the addition reaction of dimethyl itaconate (DMI) with 9,10-dihydro9-oxa-10-phosphaphenan threne-10-oxide (DOP). A series of novel flame retardant poly(butylene terephthalate)s (PBTs) containing different amounts of phosphorus were prepared using DOP-MBDA as a comonomer. These novel polymers were characterized by $^{1}H-NMR$, IR, and differential scanning calorimetry (DSC). The novel phosphoruscontaining poly(butylene terephthalate)s, referred to as FR-PBTs, exhibited interesting thermal and mechanical behavior, as well as superior flame retardancy. These properties are attributed to the effect of incorporating the rigid structure of DOP-MBDA and the pendant phosphorus group into the poly(butylene terephthalate) (PBT) homopolymer. The UL 94-V2 rating could be achieved with this novel flame retardant PBT, which has a phosphorus content as low as $0.5 wt\%$, and the FR-PBTs emitted less fumes and toxic gases than the PBT homopolymer.
