This paper concerns the rheological behavior of an unpasteurized sample of xanthan as a function of the polymer concentration and of the conformation of the polysaccharide. The evidence of critical concentrations (C* and C**) is discussed from steady shear and dynamic experiments. The behavior of the xanthan solutions is characteristic of polymeric solutions without any evidence of gellike character in contrast with the results obtained previously on commercial samples.

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Chain transfer constants (C,,) for three phosphonated thiols in methyl methacrylate (MMA) polymerization at 6OoC were determined from measurements of the degree of polymerization.These thiols were: diethyl 2-mercaptoethyl phosphate (l), diethyl 2-mercaptoethylamidophosphate (2), and tetraethyl mercaptomethylethylene bisbhosphate) (3). Transfer efficiencies of the thiols were found to be dependent upon their structures. The C, value of thiol 2 with MMA was the lowest in the present series, which can be attributed to a possible charged tautomeric form which eventually may reduce the electron density on the thiol, thus rendering the chain transfer reaction with the PMMA radical less feasible. C, values of thiol 1 were evaluated at 55,60,65,70, and 75 "C. This thiol behaves as an "azeotropic" transfer agent for MMA at %OC.

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