I'm trying to perform the following synthesis that I found on a paper : O-palmitoyl
"pullulan (OPP) is prepared by a method described
earlier  by Sunamoto et al. (1992). Briefly, 1g
pullulan is dissolved in 11 ml of dry dimethyl
formamide at 60°C. To the resulting solution, 1 ml
dry pyridine and 0.1 g palmitoyl chloride, dissolved
in 0.24 ml dry dimethyl formamide are added. The
mixture is stirred at 60°C for 2 hours followed by
1hatroom temperature. This mixture is then slowly
poured into 70ml absolute ethanol under stirring.
The precipitate so formed is collected and ished
with 80 ml absolute ethanol and 60 ml dry diethyl
ether. The white solid material obtained is dried in
vacuum at 50°C for 2 h".
I tried the same reaction several times (pure DMF not dry and lauryl chloride instead of palmitoyl chloride), adjusting the concentration of the polymer, or mixing it for more than 3 h (I actually went for 8, 16 and 24 h) but every time I got the same result that is two fractions: one soluble and another which does not dissolve in water and I don't really understand why. I thought that the concentration of the polymer was too high giving a viscous solution which in turns caused a poor diffusion of the acyl chloride but even under more dilute conditions nothing changes. Have you got any idea?