We report for the first time on a polyurethane system with polyhedral oligomeric silsesquioxanne (POSS) particles chemically bonded along its main chain as nanobuilding blocks. Morphology and molecular dynamics of this novel system are studied with imaging (SEM), thermal (DSC, MDSC) dielectric (TSDC, DRS), and mechanical (DMA) techniques. Two distinct phases occur: one chain extended with POSS and one with chemistry and morphology similar to the matrix. Although T g remains practically unaffected upon incorporation of particles, a relaxation α′ slower than the main dynamic glass transition α and present also in the matrix is enhanced in the presence of POSS. We attribute it to decelerated dynamics in the presence of heavy moieties like particles and rigid hard microdomains. No effect on the time scale of these relaxations is imposed by the presence of POSS, but rubbery modulus is significantly reduced due to the soft POSS-extended phase.