Lower critical solubility temperatures (LCST) have been measured for polyethylene (PE), polypropylene, ethylene-propylene copolymers, polybutene-1, polypentene-1 and poly 4-methylpentene-1 (P4MP1) in twenty-five isomeric alkanes. The LCST for PE are much lower than for branched polyolefins, especially in branched alkanes. The results are not predicted by the Prigogine-Flory-Patterson theory but are understood in terms of correlations of molecular orientations in the PE melt. Fusion thermodynamics of P4MP1 confirm the existence of ordered conformations in the melt. The thermoreversible gelation of P4MP1 solutions depends on solvent and thermal history. It is studied by calorimetry, viscosimetry and wide angle X-ray diffraction. The gel crosslinks are presumably of a fringed-micellar nature and their formation is tentatively associated with the existence of helical conformations of P4MP1 in solution. The latter is also responsible for the obtaining of four different crystalline structures of P4MP1 from solutions and gels. Preliminary measurements of dynamic mechanical properties of P4MP1 gels are reported.

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