I have tried three methods :

-column chromatography (using basic alumina, did not work very well)

-dialysis using a spectra labs membrane. Effective but leaving the PAM in water for 2-3 days will hydrolyse the bromo chain end via Sn1 water substitution.

-the most common method in literature, precipitation of PAM in acetone. Precipitation occurs: this involves adding the polymer rxn solution in drops, slowly, to a cool, stirring 300 ml volume of acetone => this leads to a white discolouration, a dispersion of white PAM particles, which are then filtered off via vacuum filtration and then dried in an oven. However, the acetone seems to get 'locked' inside the powder during the precipitation process, and the acetone cannot be removed.

Any ideas are really appreciated! :-)