AbstractPoly(vinylidene chloride) (PVDC) separates from the reaction mixture during the free‐radical polymerization of vinylidene chloride. The morphology of the “as‐polymerized” powder is determined by the degree of undercooling existing in the reaction medium at the time of crystallization. Crystallization occurs simultaneously in the process of polymerization and subsequently by the precipitation of polymer which formed in solution. As the medium becomes more favorable, an increasing portion of the polymer forms in solution and then crystallizes. The morphology developed by polymerization in borderline solvents such as dioxane can be nearly duplicated by recrystallization under similar conditions. The simultaneous occurrence of solution and solid phase polymerization leads to a double DTA endotherm. The upper peak at ca. 200°C represents the melting point of PVDC. The size and temperature of the lower peak vary with polymerization conditions. The results can be correlated with the degree of under‐cooling as estimated by the difference between the polymerization temperature and the temperature at which the crystalline polymer dissolves in the reacti