Changes of the temperature at which 2-chloro-1,3-butadiene (chloroprene) is polymerized, with other variables held constant, gives Neoprenes with marked differences in the rate of crystallization and dispersion in solvents of the raw polymer and in the physical properties of the vulcanizates. For example, plastic polychloroprene formed at 10° C crystallizes quickly, forms homogeneous solutions, and exhibits high tensile strength; whereas polymer produced at 100° C fails to crystallize, only swells in solvents, and has low tensile strength. These changes in properties appear to be caused by alterations of the structure of the polymer, notably the side-branching, cross-linking, spacing of the chlorine atoms, and possibly cis-trans-isomerism. The structural differences are small in magnitude and difficult to detect. Ozonolysis reveals that a soluble form of polychloroprene, as well as highly insoluble “popcorn” polymer, is at least 93 to 95 per cent 1,4-addition product and x-ray diffraction shows the polymer chains to have essentially a transconfiguration. Infrared spectroscopy indicates the absence of side vinyl groups. Branching and cross-linking explain the decreased rate of crystallization and other properties of the less soluble polymers, and it is considered that varying minor portions of 4,1-1,4 or 1,4-4,1 addition or of intramolecular cis-isomers are a factor in the crystallization of the soluble polymers. Technical advantages of the variations in Neoprene effected by polymerization temperature are outlined.