The segmental dynamics of PMPS chains are determined in the bulk liquid and crosslinked networks of varying cross-link density by broad-band dielectric relaxation spectroscopy. A large fragility index [tau(T-g*/T) dependence], F-1/2 = 0.77, is found, independent of cross-link density, molecular weight, and T-g. Also independent of the above variables is the relaxation shape (characterized by a KWW beta parameter of 0.45). Since these parameters quantifying the alpha process in PMPS networks are insensitive to cross-linking, it is concluded that the length scale of cooperatively rearranging domains in PMPS networks is smaller than the distance between cross-links, i.e., less than 5 nm, in agreement with the current consensus. An argument based on the temperature dependence of the dielectric relaxation strength, dipole moment, and ensemble average chain configuration was advanced, suggesting that the high fragility of PMPS has an intramolecular origin.