Abstract: To elucidate the mechanism of the polyme rization reactions of polyether polyols with N 100,NMR spectroscopy was used to study the associations and complex formations between reactants and catalysts.Five catalysts were employed in the study:dibutyltin dilaurate(DBTDL),triphenyl bismuth(TPB) and three isomers of tris (phenyl ethoxyl) bismuth.It was found that a relatively stable complex was formed between hydroxyl and isocyanate.DBTDL was able to form a complex with hydroxyl througha bonding of tin to hydr oxyloxygen. There was hydrogen bonding type association between TPB and hydroxyl hydrogen;In the case of three isomers of tris (phenyl ethoxyl) bismuth,the association was enhanced in the sequence:meta