AbstractThe structure and optical compensation of polymers having asymmetric carbon atoms in the main chain are discussed. When rectus (R) and sinister (S) isotactic (it) polymers mix in a 1:1 ratio giving a racemic crystalline sample, three kinds of optical compensation can be con sidered; (a) optical compensation in a unit cell, (b) compensation in a crystallite, and (c) intercrystallite compensation. it‐Poly(tert‐butylethylene oxide) form I, it‐poly(tert‐butylethylene sulfide), and it‐poly(isopropylethylene sulfide), are of case (a). it‐Poly(isopropylethylene oxide), it‐poly(propylene sulfide), it‐Poly‐β‐hydroxybutyrate, it‐Poly(β‐ethyl‐β‐propiolactone), and it‐poly(tert‐butylene oxide) form II correspond to case (c). The results of the x‐ray analyses of these polymers clearly show a stereoselective mechanism of polymerization in