Syndiotactic poly (N-n-propylacrylamide)s (PNNPAM) with various racemo (r) diad contents were synthesized, and their phase transition behaviors were studied by high-sensitivity differential scanning calorimetry and FT-IR spectroscopy. The cooperativity of the phase transition increased with the increase in the r diad content. Compared with the atactic PNNPAM, the number of cooperative units, which shows the monomeric units length undergoing cooperative phase transition, increased 2.5- to 4-fold in the syndiotactic polymers. From the results of F-F-IR spectroscopy and quantum chemical calculations, it was determined that the high cooperativity of the syndiotactic polymers resulted from local formation of an ordered structure in a dehydrated state.