Thermodynamic parameters for isotactic and atactic poly‐l‐pentene

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Abstract
Precipitation temperature measurements have been the exclusive source of thermodynamic parameters used for the comparison of polymers differing in tacticity. The Flory theta temperatures differ for the various tactic forms, although not in a manner which exhibits any obvious consistency. The precipitation temperature entropy parameters, on the other hand, have been larger for the isotactic polymer in every case but one. The theoretical treatment underlying the evaluation of ψ1 from precipitation temperature data is, unfortunately, not strictly valid. Thus, the interpretation of this behavior is in some doubt. We report the results of an osmotic study of isotactic and atactic poly‐l‐pentene in phenetole at three temperatures. For the isotactic and atactic modification the respective values found were Θ = 329.0 and 321.5°K., while for ψ1 the values were 0.45 and 0.72. The theta temperatures fall in the same order as those previously obtained for a closely related system, poly‐l‐butene–anisole, by precipitation temperature measurements. The osmotic ψ1 values are larger than those reported here‐tofore for other systems, but of more importance, they fall in the reverse of the order consistently obtained from precipitation temperatures. This suggests that the regular variation of the precipitation temperature ψ1 values can not have its origin in local pair interaction effects; instead, it must reflect some difference in the concentration dependence of the chemical potential for solutions of the various tactic forms.