A process to form a first composition comprising a functionalized olefin-based polymer comprises: a) polymerizing a composition comprising an olefin to form a reaction product comprising an olefin-based polymer; b) subjecting at least a portion of the reaction product to at least one devolatilization to form a polymer-rich melt, wherein step b) occurs downstream from and in-line with step a); and c) reacting at least a portion of the polymer-rich melt with at least one functionalization agent and, optionally, at least one free-radical initiator to form the first composition, wherein step c) occurs downstream from and in-line with step b), wherein “the viscosity of the functionalized olefin-based polymer (177° C./350° F.)” to “the viscosity of the olefin-based polymer (177° C./350° F.)” is from 0.1 to 5.0.