Episulfides are reacted with mono, di or trithiols in the presence of tertiary aliphatic amines or tertiary aliphatic phosphines, to form corresponding thioether thiols by a highly selective anionic mechanism. Increasing episulfide-thiol ratios result in polythioether thiols of increasing molecular weights. Using two or more episulfides, block copolymers can be prepared. The reactive terminal thiol functionality and the segmented crystalline and amorphous block structure of the new polymers makes them particularly useful for castable rubber applications.