ANALYSIS OF THE ADJACENT REENTRY FOLDS IN THE FORM-I OF SYNDIOTACTIC POLYPROPYLENE

Models of possible adjacent re-entry chain folds for the most stable f orm (form I) of syndiotactic polypropylene in the body-centered orthor hombic unit cell have been studied by molecular mechanics. Folds along (100), (010), and (110) planes connecting chains having both the same or the opposite chirality have been investigated. Various models of f olds with different number of bonds have been built up and optimized b y conformational energy calculations. Tight folds are the lowest in en ergy. Low-energy folds are possible along the three fold planes with s ome restrictions on the chirality of the chains. The energy values obt ained for the best folds are in good agreement with the work of the fo ld deducible from experimental determinations. The energy calculations also indicate that configurational defects can be well tolerated in t he fold.