Abstract

A sample of poly(1-octadecene), synthesized with a highly active heterogeneous Zieger-Natta catalyst, has been fractioned with heptane, giving soluble and insoluble fractions. Both fractions and the original polymer have been characterized by size exclusion chromatography, solution and solid-state 13C NMR, DSC and X-ray diffraction. The results show that the fractionation occurs on the basis of both molecular mass and tacticity differences with the atactic content concentrated in the lower molecular mass chains. Thus, the soluble fraction, having a lower average molecular mass than the original sample, consists predominantly of atactic chains, whereas the insoluble fraction is mainly isotactic. The analysis of the solid-state structure reveals that both atactic and isotactic fractions are able to crystallize, although their crystalline structures are different. The NMR and X-ray data together support the "most probable" structure for the isotactic polymer advanced by Turner-Jones. That structure is characterized by an orthorhombic crystal form, where a) the backbone crystallizes in a quarternary helical conformation, b) the sidechains are packed in a way analogous to orthorhombic poly ethylene, and c) successsive sidechains are conformationally inequivalent. Support for points b) and c) are respectively found in the chemical shift of the sidechains and in the splitting observed for backbone carbons and for some sidechain carbons located near the points of attachment. In addition, there is evidence that the mobility of sidechain sites at points near both the bonded and free ends are not uniform from chain to chain. On the other hand, the crystal form for the atactic polymer shows only sidechain order with some support for the notion that this order approximates the disordered hexagonal rotator phase of the alkanes.