Segmented block copolymers from different grades of hydroxyl terminated liquid natural rubber (HTNR) ((M) over bar (n) 3000, 8800, 10,000, and 17,000) and polypropylene oxide (PPO) ((M) over bar (n) 1000, 2000, 3000, and 4000) have been synthesized and characterized by spectral analysis, thermal analysis, scanning electron microscopy (SEM), and mechanical testing. The glass transition temperature of NR block was found to be at about -64 degrees C, which is independent of the PPO whose transition is around 15 degrees C. The thermogravimetric analysis (TGA) shows that the thermal degradation of the samples proceeded in two steps characteristic of the immiscible components. The inability of PPO segments to provide physical crosslinking and the subsequent formation of hard domains is reflected in the low tensile properties and tear properties. The amorphous nature of the PPO phase and its immiscibility with NR phase are evidenced by the SEM studies. The effect of molecular weight of PPO as well as HTNR on the properties of the block copolymers has also been discussed. (c) 2006 Wiley Periodicals, Inc.