Polysulfide polymer was found in the 19th century. At present, this polymer consists of the repetition of disulfide linkage and diethyl formal and has SH terminals. This liquid polymer (LP) is cured by lead dioxide. We studied polysulfide polymerization and elastomeric mechanical properties of cured LP. The condition of polymerization greatly influenced its elastomeric mechanical properties. These phenomena could not be explained from the point of residual unreacted chlorine or produced OH terminals. We found that additional crosslinking sites are produced during polymerization besides crosslinking agents trichloropropane (TCP) by using the Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) method. We proposed their chemical structures and reaction scheme of their formation. TCP and also these additional crosslinking sites influence the modulus of cured LP. The sum of the intensity of their crosslinking sites by C-13-NMR had good correlation with 300% modulus of cured LP.