The state of aggregation of cholesterol at various temperatures in CHCl, and in CCl, solutions has been determined by vapor pressure osmometry (VPO). There is hydrogen bonding by the CHC1, to the cholesterol, so the self-association of cholesterol is weaker in CHC1, than in CCl,, where no solvent hydrogen bonding occurs. With both solvents the self-association increased as the temperature decreased. In CHC1, the self-association is described by two models-an indefinite self-association with all odd species beyond monomer absent, or a monomer-dimer association. In CC14 the self-association was best described by a monomer-trimer-hexamer model. Values of the thermodynamic functions for these self-associations are reported. Proton nuclear magnetic resonance studies on these solutions at 33.0 "C parallel the VPO studies. The chemical shift of the hydroxyl proton showed linear concentration dependence in CHCl,, whereas a parabolic concentration dependence was observed in CC4. These downfield chemical shifts indicated that hydrogen bonding was involved in the self-association.

Tsukiji 2-chome, Chuo-ku, Tokyo, 104 Japan) The branching degree of natural rubber (NR) estimated by GPC/LALLS measurement is higher than that of synthetic isoprene rubber (IR). Branched polymer chains and the gel networks of NR are easily decomposed by various means such as mechanical force, solvent power and enzymic deproteinization reaction. This decomposition leads to the increase of linear polymer chains.13C NMR spectrum of the low molecular weight fraction of NR gives small signals which are characteristics of an ester group. From these facts we estimate that the NR gel consists of weak linkages such as hydrogen bonds between an ester group in NR and an amino group in protein. Since the IR gel is formed by chemical bonds, it can be randomly decomposed only by mechanical force; this results in the increase of branched polymer chains. The gel structures of NR and IR are directly observed by transmission electron microscopy. The networks of NR gel show looser than those of IR gels.

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