Cross-linked polymer networks are widely used as structural and protective materials, which require strength and toughness. Experiments have shown that cross-linked poly(dicyclopentadiene) (pDCPD) networks provide similar strength but superior fracture toughness relative to commonly-used network chemistries like epoxy. To better understand pDCPD, we use atomistic molecular dynamics to study the properties of pDCPD networks across the glass transition as a function of molecular weight between crosslinks. Moreover, we identify molecular mechanisms that potentially control mechanical and transport properties. The alpha-relaxation (the glass transition) is linked to intra-chain motions and large-scale segmental motions, while sub-T-g relaxations are linked with more localized motions. (C) 2015 Published by Elsevier B.V.