The effects of the stereospecificity of a polymer chain and of the interaction in polypropylene (PP) solutions on the relation between liquid-liquid phase separation and crystallization were investigated by using an isotactic PP (i-PP) and a syndiotactic PP (s-PP) of high stereoregularity and of similar molar mass. Dialkyl phthalate was used as a solvent. A series of dialkyl phthalates with a different number of carbon atoms in the alkyl chain was employed to control the interaction between polymer and solvent. Phase transition temperatures were measured by optical microscopy with a hot stage. Liquid-liquid phase separation temperature (TL-L) in the system of i-PP and dihexyl phthalate was located below its melting temperature (T-m). However, TL-L for the s-PP system in the same solvent was elevated much above its T-m due to a decreased T-m and increased TL-L. The reduced solubility of s-PP is primarily attributed to enhanced hydrophobicity arising from alternate positioning of the methyl groups along the polymer chain. As the length of the alkyl chain in the phthalate increases, TL-L decreases significantly and T-m decreases slightly, resulting in the value of TL-L shifting below that of T-m for the solution of s-PP and dinonyl phthalate. (C) 1998 John Wiley & Sons, Inc.