The bulk polymerization of vinyl chloride initiated by di-tert-butyl- or tert-butyl-tert-pentylmagnesium etherate gives poly(vinyl chloride) (PVC) specimens containing considerable amounts of cyclopropyl, trans-2-tert-butylcyclopropyl, and -CHClCH=CH2 chain ends. This result has no structural precedents among PVC samples that are made by conventional free-radical methods. Formation of the unusual end groups has been rationalized in terms of reactions of various polymeric intermediates having Mg-C bonds, and ethylene formed in situ has been shown to be involved in the process that produces the cyclopropyl chain end. Also observed is a trans-t-BuCH=CHCH2- terminus, whose presence has not been explained. When the polymerization temperature is raised in stages from -20 to 40 degrees C, the unusual chain ends are gradually replaced by structures that are known to result from reactions of free macroradicals. The various end groups were characterized by high-field NMR techniques. These were applied to both original and Bu(3)SnH-reduced PVC specimens, as well as to synthesized end-group models that were used for reference purposes.