A reactive branched thermoplastic polyether-ester elastomer (TPEE) precursor was synthesized by the esterification reaction of dimethyl terephthalate (DMT) with poly(tetramethylene etherglycol) (PTMEG), 1,4-butadiene, and glycerol as a soft segment, hard segment, and a branching agent, respectively. The high molecular weight TPEE was further synthesized with the prepared branched TPEE precursor, poly(butylene terephthalate) (PBT) and modified methylene diisocyanate (m-MDI, 0.5-2.0 wt%) by the reactive extrusion method. Their chemical structures were determined by Fourier Transform Infra Red (FTIR) and Proton-Nuclear Magnetic Resonance (H-1 NMR). Thermal characteristics and rheological properties of TPEE were measured by Differential Scanning Calorimetry (DSC) and rheometer as a function of m-MDI content. The intrinsic viscosity (M and melt index ratio (MIR) of TPEE increased as the content of m-MDI increased up to 1.5 wt% and remained constant thereafter. The variation of the MIR was consistent with that of the IV. The storage modulus and viscosity did not vary with the measurement time up to 1.0 wt% of m-MDI at the first extrusion, which indicates that the m-MDI reacted fully. However, the viscosity and storage modulus increased with increasing measurement time at m-MDI contents over 1.5 wt%. POLYM. ENG. SCI., 49:1456-1460, 2009. (C) 2009 Society of Plastics Engineers