AbstractThree different samples of polyvinylchloride have been obtained in emulsion, in bulk and in solution in butyraldehyde. The chain structure and conformation of the polymers have been characterized by infrared spectrometry. As expected the solution polymer has a much higher content of s units than the other smaples which are very similar to each other.By dechlorination with zinc in boiling dioxane cyclopropane units are formed; their existence is proved by new infrared absorption bands (3055, 2988, and 1017 cm−1) and by NMR‐analysis (signals 0.2 and 0.3 ppm). Simultaneously unsaturated groups are formed of which the final content is higher for the butyraldehyde polymer.Dehydrochlorination with lithium chloride and dimethyl formamide‐dioxane solution at 80°C was also carried out for the emulsion and the solution polymer; the reaction rate is markedly higher in the first case. The reaction mechanism of the dehydrohalogenation was examined; a concerted reaction mechanism with trans chlorine‐elimination is proposed.