A seatbelt pretensioning retractor assembly for use in a vehicle is provided. The pretensioning assembly includes a tube in fluid communication with a gas generator and a driving element disposed therein that travels in a first direction in response to actuation of the gas generator. The driving element includes a polymer rod. A stopper is coupled to an end portion of the polymer rod. In one example, a seal member disposed between the gas generator and the stopper includes a head section and a skirt section. In another example, the tube has an opening that is adjacent to an exit to facilitate venting gas from the gas generator.