Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same

Abstract

The present invention provides a new dispersion stabilizer that satisfies, at a high level, the performances required for a dispersion stabilizer used for suspension polymerization. A method of producing a dispersion stabilizer that is useable for suspension polymerization of a vinyl compound and contains a vinyl alcohol polymer (A) includes producing the vinyl alcohol polymer (A) by heat-treating a vinyl alcohol polymer (B) at a temperature of 90 to 180° C. for 0.5 to 20 hours in an atmosphere in which an oxygen concentration is 8000 ppm or lower. The vinyl alcohol polymer (B) has a saponification degree of at least 60 mol % and contains a residual acetic acid group whose block character is in the range of 0.3 to 0.6.

1 . A dispersion stabilizer for use in suspension polymerization of a vinyl compound, the dispersion stabilizer comprising a vinyl alcohol polymer (A′), 
 wherein in a 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A′), a ratio (b)/(a) between absorbance (a) at a wavelength of 280 nm and absorbance (b) at a wavelength of 320 nm is at least 0.7,    the 0.1-wt. % aqueous solution has a transmittance of at least 80% at a wavelength of 500 nm, at 30° C., and    a 1-wt. % aqueous solution of the vinyl alcohol polymer (A′) has a yellow index (YI) of 40 or lower.    
   
   
       2 . The dispersion stabilizer according to  claim 1 , wherein a residual acetic acid group of the vinyl alcohol polymer (A′) has a block character of at least 0.35.  
   
   
       3 . The dispersion stabilizer according to  claim 1 , wherein a residual acetic acid group of the vinyl alcohol polymer (A′) has a block character of at least 0.37.  
   
   
       4 . The dispersion stabilizer according to  claim 1 , wherein a residual acetic acid group of the vinyl alcohol polymer (A′) has a block character of at least 0.4.  
   
   
       5 . The dispersion stabilizer according to  claim 1 , wherein a ratio Mw/Mn between weight-average molecular weight Mw and number-average molecular weight Mn of the vinyl alcohol polymer (A′) is in a range 2.1 to 4.9.  
   
   
       6 . The dispersion stabilizer according to  claim 1 , wherein the ratio Mw/Mn, of weight-average molecular weight Mw to number-average molecular weight Mn, of the vinyl alcohol polymer (A′) is in a range 2.2 to 4.7.  
   
   
       7 . The dispersion stabilizer according to  claim 1 , wherein the ratio Mw/Mn, of weight-average molecular weight Mw to number-average molecular weight Mn, of the vinyl alcohol polymer (A′) is in a range 2.2 to 4.4.  
   
   
       8 . The dispersion stabilizer according to  claim 1 , wherein the ratio (b)/(a) between the absorbance (b) and the absorbance (a) is in the range of 0.7 to 1.5.  
   
   
       9 . The dispersion stabilizer according to  claim 1 , wherein the absorbance (a) at a wavelength of 280 nm of the 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A′) is at least 0.1.  
   
   
       10 . The dispersion stabilizer according to  claim 1 , wherein the absorbance (a) at a wavelength of 280 nm of the 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A′) is at least 0.2.  
   
   
       11 . The dispersion stabilizer according to  claim 1 , wherein the absorbance (a) at a wavelength of 280 nm of the 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A′) is at least 0.25.  
   
   
       12 . The dispersion stabilizer according to  claim 1 , wherein the YI of the 1-wt. % aqueous solution of the vinyl alcohol polymer (A′) is 35 or lower.  
   
   
       13 . The dispersion stabilizer according to  claim 1 , wherein the YI of the 1-wt. % aqueous solution of the vinyl alcohol polymer (A′) is 30 or lower.  
   
   
       14 . The dispersion stabilizer according to  claim 1 , wherein the 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A′) has a transmittance of at least 82% at a wavelength of 500 nm, at 30° C.  
   
   
       15 . The dispersion stabilizer according to  claim 1 , wherein the 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A′) has a transmittance of at least 85% at a wavelength of 500 nm, at 30° C.