Chlorinated polyethylene (CPE)-modified polyester prepolymers with NCO end groups were synthesized. The chain-extension reactions of the prepolymers with 3,3’-dichloro-4,4’-diaminodiphenylmethane were studied by Fourier transform infrared spectrometry and found to change from second-order reaction at lower temperature to first-order reaction at higher temperature for these reaction systems. The properties of CPE-modified polyurethane (PU) elastomers was also studied by dynamic mechanical property analysis. The experimental results showed that the CPE content increase would lead to a decrease of the mechanical properties of PU elastomers and result in a prolongation of the cream time of curing reaction systems, but would make the manufacturing process easier than before. It is suggested that the CPE may be selected as a delayed-action activator of curing reaction.