A seatbelt pretensioning retractor assembly for use in a passenger vehicle, the seatbelt pretensioning retractor assembly comprising: a housing adapted for being mounted to a vehicle structure and having an interior cavity; a tube having an arcuate and curved shape having a first end for fluid communication with a gas generator and a second end in fluid communication with the interior cavity of the housing; a sprocket rotatably mounted to the housing and fixedly coupled to a spindle adapted for taking up seatbelt webbing during pretensioning; a driving element comprising a polymer rod having a proximal end portion that is disposed towards the gas generator and a distal end portion that is opposite the proximal end portion, and is configured to extend in a longitudinal direction from the proximal end portion to the distal end portion, wherein the polymer rod has a recessed portion defining a recess that extends generally in the longitudinal direction, and wherein the polymer rod is disposed within the tube and is translatable through the tube in a first direction toward the sprocket in response to an actuation by the gas generator; a stopper coupled to the proximal end portion of the polymer rod; and a seal member disposed between the gas generator and the stopper, wherein the tube includes a projection extending within the tube adjacent to an exit at the second end to define a constriction portion that has an opening dimension smaller than an inside dimension of adjacent portions of the tube, wherein the recess is aligned with the projection in the first direction and the constriction portion is sized to permit the recessed portion of the polymer rod to pass the constriction portion while restricting the stopper from passing the constriction portion, and wherein the recessed portion of the polymer rod has a proximal end section that is adjacent to the stopper and that includes a chamfered end wall that flares outwardly along a length of the proximal end section in a direction towards the stopper for engagement with the projection during pretensioning. a housing adapted for being mounted to a vehicle structure and having an interior cavity; a tube having an arcuate and curved shape having a first end for fluid communication with a gas generator and a second end in fluid communication with the interior cavity of the housing; a sprocket rotatably mounted to the housing and fixedly coupled to a spindle adapted for taking up seatbelt webbing during pretensioning; a driving element comprising a polymer rod having a proximal end portion that is disposed towards the gas generator and a distal end portion that is opposite the proximal end portion, and is configured to extend in a longitudinal direction from the proximal end portion to the distal end portion, wherein the polymer rod has a recessed portion defining a recess that extends generally in the longitudinal direction, and wherein the polymer rod is disposed within the tube and is translatable through the tube in a first direction toward the sprocket in response to an actuation by the gas generator; a stopper coupled to the proximal end portion of the polymer rod; and a seal member disposed between the gas generator and the stopper, wherein the tube includes a projection extending within the tube adjacent to an exit at the second end to define a constriction portion that has an opening dimension smaller than an inside dimension of adjacent portions of the tube, wherein the recess is aligned with the projection in the first direction and the constriction portion is sized to permit the recessed portion of the polymer rod to pass the constriction portion while restricting the stopper from passing the constriction portion, and wherein the recessed portion of the polymer rod has a proximal end section that is adjacent to the stopper and that includes a chamfered end wall that flares outwardly along a length of the proximal end section in a direction towards the stopper for engagement with the projection during pretensioning.