Abstract: A process for making an infusible polyolefin includes the steps of: a) contacting the polyolefin in a sulfonation reactor with a sulfonation mixture comprising sulfur trioxide to produce the infusible polyolefin; b) recovering from the sulfonation reactor a recovery stream having sulfur dioxide; c) oxidizing at least a portion of the recovered sulfur dioxide to produce a recycle stream; and d) combining at least a portion of the recycle stream with the sulfonation mixture of step (a). Another aspect of the invention is for making a carbonized fiber from an infusible polyolefin of the present invention and further includes the step of carbonizing the infusible polyolefin to produce a carbon fiber. Another aspect of the invention is an apparatus for preparing an infusible polyolefin. The apparatus includes a plurality of compartments in fluid communication wherein at least one compartment is adapted for contacting a polyolefin with sulfur trioxide.