Structure–property relationships in metallocene polyethylenes

The development of new polyolefins based on metallocene technologies represent a considerable advance in the performance of polyethylenes available for a wide range of applications. The ability to obtain a homogeneous short chain branching distribution and control molecular weight is remarkable. The added ability to introduce long chain branching (LCB) into what would otherwise be linear polyethylenes has opened up a broad range of processing possibilities. Through a review of this technology, it becomes apparent that significant elements of the fundamentals underlying the materials science of these new polyolefins are in need of clarification. The morphology of metallocene polyethylenes, particularly in the low-crystallinity range, has yet to be conclusively resolved. The occurrence of multiple melting endotherms for ethylene/α-olefin copolymers with, ostensibly, a homogeneous branching distribution is in need of explanation. LCB incorporated into the linear copolymers enhances shear thinning and melt elasticity. Yet to be fully determined, however, are the types of long chain branch architectures that most effectively promote these desirable rheological attributes, and to what degree these types are present in metallocene polyolefins.