The propagation-rate constant of vinylidene chloride (VDC) was determined at 40 and 50 degreesC, respectively, by applying the so-called Ugelstad plot to the polymerization-rate data of the seeded and unseeded emulsion polymerizations of VDC. The values of the propagation-rate constant k(p) thus determined are k(p) = 64 dm(3)/mol s at 50 degreesC and k(p) = 52 dm(3)/mol . s at 40 degreesC, respectively. From these k(p) values, the activation energy for propagation reaction was determined to be E-p = 4.2 kcal/mol, which is close to that of vinyl chloride (3.7 kcal/mol). (C) 2001 John Wiley & Sons, Inc.