Abstract: A method of forming white polymers of controlled molecular weight when polymerizing 2-pyrrolidone using an alkaline catalyst and sulfur dioxide as the chain initiator is disclosed. The temperature at which the polymerization is carried out is critical and is carefully controlled. The SO.sub.2 which must also be controlled within limits is diluted with an inert gas to diminish its activity and make it easier to measure the small amounts involved. The viscosity and thermal stability of the polymers formed are as good as or better than those formed when CO.sub.2 is used as the initiator.