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 with a recessed portion. A stopper is coupled to an end portion of the polymer rod. The tube includes a projection extending within the tube adjacent to the exit to define a constriction portion. The constriction portion is sized to permit the recessed portion of the polymer rod to pass therethrough while preventing the stopper from passing therethrough. The polymer rod has a retention feature for releasably engaging the projection.