HELIX JUMP MECHANISMS IN CRYSTALLINE ISOTACTIC POLYPROPYLENE

A proposed 120-degrees helical jump process observed by 2D NMR spectroscopy to occur in the alpha-crystalline form of isotactic polypropylene is analyzed by quasi-static methods of molecular modeling. Two competing mechanisms for describing the chain jump process, denoted herein as the concerted-helix transition and the defect-translation transition, are examined in terms of energetic stability and barriers to activation. Viable transitions of the concerted-helix type comprise a longitudinal translation of the polymer helix along its axis and a screw transition involving a 120-degrees rotation accompanied by a c/3 translation; of these two mechanisms, only the latter would account for the 2D NMR observations. Both mechanisms exhibit energy barriers of roughly 3.6 kcal/mol of propylene units, which increases linearly with the number of propylene units cooperating in the transition process. A viable crystalline defect described by a dispiration of the chain helix distributed over eight propylene units is identified having an energy of formation of 23.4 kcal/mol of defects, which is chain length independent. On this basis, it may be inferred that, in realistic lamellae of thicknesses in excess of 100 angstrom, the defect-translation mechanism represents the most likely mode of 120-degrees helix jumps in the alpha-crystalline modification of isotactic polypropylene.