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. <EOS>