ABSTRACTThe phenomenon of mesophase-enhanced polymerization of the rodlike polymer poly(pphenylenebenzobisthiazole) in polyphosphoric acid was investigated. When the polymer concentration was greater than 5 wt%, the reacting mixture became anisotropic at an early stage of the polymerization. The reaction rate increased significantly at the isotropic-nematic phase transition as the rods were aligned in positions more favorable for the condensation reaction to occur. At 4.5 wt% of polymer, the reacting mixture remained isotropic for up to 1500 min of reaction time. The chemo-rheological properties of the 15 wt% reaction mixture indicate the occurrence of the phase change in either the shear viscosity or the first normal stress difference measurements.