Brace Expansion not working bash

I am trying to use brace expansion in a bash script as follows.

#!/bin/bashdocument_root="/var/www/www.example.com"`chmod -R g+w $document_root/{../captcha,../files}`

this gives me the error

chmod: cannot access `/var/www/www.example.com/{../captcha,../files}': No such file or directory

but when I run this in a terminal it works just fine.