Telechelic oligo- and poly(dimethylsiloxanes) 1 and 2, with two ureidopyrimidone (UPy) functional groups, have been prepared via a hydrosilylation reaction. The compounds have been characterized in solution by H-1 NMR and viscometry and in the solid state by H-1 NMR and C-13 NMR, FTIR, and rheology measurements. The measurements show that the UPy groups of 1 and 2 are associated via quadruple hydrogen bonds in a donor-donor-acceptor-acceptor (DDAA) array. In many aspects, the materials behave like entangled, high molecular weight polymers. Compound 2 has a T-g at -119 degrees C and shows melting of microcrystalline domains of associated UPy units at -25 degrees C. Compound 1 has a crystalline form (T-m = 112 degrees C) and an amorphous modification with a T-g of 25 degrees C. Solid-state NMR was used to investigate the mobility of these phases. WISE spectra show a higher mobility of the UPy groups in the amorphous phase than in the crystals of 1. Amorphous 1 and 2 behave like entangled polymers. Their mechanical behavior is characterized by a rubbery plateau and a relatively high activation enthalpy for stress relaxation (Delta H 127 kJ/mol for 1; Delta H 54 kJ/mol for 2), which was derived from the temperature dependence of the zero-shear viscosity. Estimates for the degree of polymerization (DP) of 1 and 2, based on the mechanical properties, give DP > 100 for 1 and approximately 20 for 2. Like in condensation polymerization, the DP＇s of reversible supramolecular polymers are presumably limited by the presence of small amounts of monofunctional impurities.