Porous ultra-low-k (ULK) dielectrics hold promise for improving microprocessor performance, but these materials suffer from poor mechanical properties  and susceptibility to plasma damage during integration [2, 3). The post-porosity plasma protection (P4) process has emerged as a promising strategy for the protection of ULKs against plasma-induced damage [2, 3), but has not been shown to improve fracture properties. In this work, we transform the P4 process to provide ULKs with dual protection against both plasma damage and mechanical stresses. We infiltrate polymers with a wide range of molecular weights (1 - 103 kg/mol) into porous ULKs, leading to fracture toughness values equal to that of dense silica (10 J/m2). The polymer infiltration process results in a high level of fill (∼100%) and confinement of polymer chains to dimensions far smaller than their bulk radius of gyration. This confinement alters the conformations and inter-molecular interactions of the polymer phase, resulting in novel fracture behavior that has important implications for the reliability and fracture properties of filled ULKs and other materials containing confined polymers.