A process for halogenating an olefin-based elastomer while the olefinic-based elastomer is in the bulk phase using a system comprising: (i) a first extruder; (ii) a first kneader vessel having an inlet and an outlet downstream of said first extruder and in fluid communication with said first extruder via a conduit and the inlet, where said first kneader vessel is a sealed vessel that is adapted to maintain gaseous reactants that are introduced into said kneader vessel; (iii) a second extruder downstream of said first kneader vessel and in fluid communication with the outlet of said first kneader vessel; (iv) a second kneader vessel downstream of said second extruder and in fluid communication with said second extruder; and (v) a third extruder downstream of said second kneader vessel and in fluid communication with said second kneader vessel; the process comprising: reacting an olefinic-based elastomer substantially in the bulk phase with a halogenating agent within the first kneader reactor to produce halogenated olefinic-based elastomer and by-products of a halogenation reaction, wherein said first extruder is arranged and operated to convert the olefin-based elastomer into a compacted, continuous solid mass that occupies the entire cross-sectional area of at least a portion of the conduit, thereby serving to seal the first kneader vessel at the inlet and wherein said second extruder is adapted and operated to compact and accumulate the halogenated olefinic-based elastomer product discharged from the first kneader vessel to fill a cross-sectional area of the second extruder, thereby providing a seal to the outlet of the first kneader vessel; and separating the halogenated olefinic-based elastomer from at least a portion of the by-products of the halogenation reaction within the second kneader vessel, wherein said second kneader vessel operates at a lower pressure than said first kneader vessel and wherein any water present is present at not more than 10,000 ppm. (i) a first extruder; (ii) a first kneader vessel having an inlet and an outlet downstream of said first extruder and in fluid communication with said first extruder via a conduit and the inlet, where said first kneader vessel is a sealed vessel that is adapted to maintain gaseous reactants that are introduced into said kneader vessel; (iii) a second extruder downstream of said first kneader vessel and in fluid communication with the outlet of said first kneader vessel; (iv) a second kneader vessel downstream of said second extruder and in fluid communication with said second extruder; and (v) a third extruder downstream of said second kneader vessel and in fluid communication with said second kneader vessel; reacting an olefinic-based elastomer substantially in the bulk phase with a halogenating agent within the first kneader reactor to produce halogenated olefinic-based elastomer and by-products of a halogenation reaction, wherein said first extruder is arranged and operated to convert the olefin-based elastomer into a compacted, continuous solid mass that occupies the entire cross-sectional area of at least a portion of the conduit, thereby serving to seal the first kneader vessel at the inlet and wherein said second extruder is adapted and operated to compact and accumulate the halogenated olefinic-based elastomer product discharged from the first kneader vessel to fill a cross-sectional area of the second extruder, thereby providing a seal to the outlet of the first kneader vessel; and separating the halogenated olefinic-based elastomer from at least a portion of the by-products of the halogenation reaction within the second kneader vessel, wherein said second kneader vessel operates at a lower pressure than said first kneader vessel and wherein any water present is present at not more than 10,000 ppm.