Materials under space confinement can alter their physical and chemical properties, paving the way for new science and novel applications. Nanoparticles can enforce spatial constraints on the polymers densely grafted on their surfaces, which can effectively change polymers' thermal/mechanical properties and finally lead to significant mechanical reinforcement for the obtained nanocomposites. Here, we describe the confinement effect that metal-organic polyhedron (MOPs), nanoparticles with monodispersed and ultrasmall sizes (similar to 2 nm), impose to the 24 poly(N-isopropylacrylamide) (PNiPAM) chains grafted on their surface. The curved surface of the MOP brings unique confinement to the grafted PNiPAMs: the inner fraction of PNiPAM chains is under ultraconfinement, while the outer fraction is almost free. Suggested from small angle neutron scattering studies, the outer layer polymers stay hydrated and help solvate the nanocomposites in water, while the inner polymer layer prohibits the water penetration into MOP area and contributes to the high stability of the MOPs. Such a confinement effect further influences the thermoresponsiveness of PNiPAM-MOPs in solutions and enhances their efficiency for controlled separation and release of small molecules.