Re: why do you need acetic anhydride and acetic acid for cellulose acetate

Assuming that your acetic anhydride was of good quality (it is moisture 
sensitive) and that you allowed sufficient time for the reaction, I see no 
reason why it would not work.  

Raising the temperature may help speed up the reaction, and there are various 
degrees of acetylation, the lower levels of acetyl group incorportion may not 
change the properties of cotton enough to be noticeable.  Catalyst may help too, 
I would suggest a bit of toluenesulfonic acid, or dimethylformamide or even 
4-dimethylaminopyridine.  Acetic acid as a solvating agent may help to open up 
the cellulose structure but otherwise I see no reason why it would be essential. 
 (And of course, acetic acid is a co- product of the reaction anyway.)

J. McCormick