Syndiotactic poly(methyl methacrylate) (st-PMMA) is known to form an inclusion complex with fullerenes in toluene. We investigated in detail the influence of the primary structure of st-PMMA, such as the molecular weight and stereoregularity, on the inclusion complex formation with C-60. The formation of an inclusion complex with C-60 was accompanied by gelation that occurred in toluene by using st-PMMAs with an rr content of 89% and an M-n of over 7.8 X 10(3) in the polymer, with C-60 concentrations of 10 and 2 mg/mL, while no gelation was observed when using st-PMMA with an 4, of 4.6 x 10(3). The st-PMMA gelation ratio and the C-60 encapsulation ratio increased with the increasing molecular weight and became constant at an M-n of over 2.9 X 10(4). The guest C-60 molecule also had a large influence on the formation of the helical structure of st-PMMA. To obtain a gel quantitatively without fullerene, it was necessary to use st-PMMA with a high M-n of 5.8 X 10(5), whereas in the presence of C-60, stPMMA with an M-n of 2.9 X 10(4 )was sufficient. The influence of stereoregularity on the inclusion complex formation was also investigated using st-PMMA with a sufficiently high molecular weight (M-n > 3.0 X 10(5)) and different rr contents. Even when the rr content of st-PMMA decreased from 97 to 89%, the C-60 encapsulation ratio did not decrease. Since the main chain of st-PMMA with an rr content of 89% incorporates approximately one defect (m-diad) per helical turn, it was concluded that the helical structure of the polymer chains is maintained when the defects are at this level.