We have investigated relaxation times and heat capacities of polymer-dimer mixtures of α-methylstyrene by using dielectric relaxation measurements and differential scanning calorimetry. Since both polymer and dimer consists of identical monomers, the local interactions in the system are expected to be uniform. We examined those detailed data to elucidate the polymer concentration dependences of fragilities and cooperativities of molecules at glass transition temperatures. As a result, we found that there is a clear boundary at 20 wt% polymer concentration and that both fragility and cooperativity are large at low polymer concentrations, whereas both fragility and cooperativity decrease rapidly at high polymer concentrations. This suggests that the transition to the ideal glass that exists at the low concentration is lost or weakened at the high concentration side.