Non-parallel chains in crystalline γ-isotactic polypropylene

Abstract

CRYSTALLINE synthetic polymers generally exhibit highly anisotropic arrangements of the polymer chains, which tend to lie parallel to each other. Indeed, chain-axis parallelism has become virtually an unchallenged principle of polymer crystallization, essentially for two reasons. First, it seems consistent with the requirements of the close packing of rods, which may be assumed to approximate crystallized polymer chains, and second, most polymer crystal structures have been determined from samples that are fibrous. As part of a study¹–¹ of crystal structures of polymer phases that do not readily form fibrous specimens, we have determined the crystal structure of the γ-form of isotactic polypropylene (γ-iPP)^5-8. We show that the structure comprises layers which are two chains wide, reminiscent of the α-phase⁹, but with the chain-axis directions in adjacent bilayers at an angle of 80° to one another. This feature recalls the arrangement, at the same inter-helical angle, that is proposed to occur at the branching points in α-iPP crystals^10-12. Similar packing at large interaxial angles between isochiral chain fragments is also often found in globular proteins¹³. The γ-iPP crystal structure is unusual, however, in that non-parallel chains appear in a fully ordered array. The driving force for such packing seems to be the favourable registration between helices that occurs for this tilt angle.