The effect of stereoregularity and molecular weight(MW)on the dissolution of isotactic poly(acrylonitrile)(PAN)under elevated temperatures was quantitatively studied.Dissolution temperature(Tsol)determined by a laser-optical method was a good measure of this parameter.From systematic experiments,it was concluded that the former was predominantly larger than the latter,and the relative contribution was roughly estimated to be(8:2~7:3)as follows.When the value of Tsol was plotted against the stereoregularity,a simple saturation type of curve was obtained.The results were explained in terms of the configurational entropy(Fig.1).In the inverse plots of Tsol against the stereoregularity,a linear decreasing line was obtained.For example,in homologous dinitrile solvents,CN-(CH2)n-CN(n=1~3),the strongest solvent was MaN(n=1),where these two factors were obtained in two different areas in the plots.Stereoregularity layed on a decreasing line,whereas the MW scattered in a square region crossed over the line(Fig.2).In poor solvent(n=2,3),the straight line shifted toward upward direction with milder slopes and the latter-region depressed and disappeared(Figs.3,4).Under such extreme conditions,the former was only one remaining factor.