Processes of polymerizing olefin monomers and catalyst systems. The catalyst systems include a non-hydrogen-generating post-metallocene procatalyst; a co-catalyst; and a hydrogenation procatalyst having the formula Cp2TiXnTiCp2 or Cp2TiXn, in which each Cp is cyclopentadienyl substituted with at least one (C1-C10)alkyl; each X is independently monoanionic or neutral, wherein each X is independently (C1-C40)hydrocarbon, (C1-C40)heterohydrocarbon, (C1-C40)hydrocarbyl, (C1-C40)heterohydrocarbyl, or a halogen atom; and n is 1 or 2.