A seatbelt pretensioning retractor assembly for use in a passenger 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 a proximal end portion of the polymer rod. A piston-safety valve member is disposed between the gas generator and the stopper and is configured to allow gas from the gas generator to pass therethrough when the gas is at or above a predetermined pressure.