mats = cmds.ls(sl=1)
#Then you can loop over it and disconnect like this:

for mat in mats:
    conns = cmds.listConnections(mat, p=True) or []
    for conn in conns:
        matA = '%s.%s'%(mat,'diffuse_weight')
        try:
            cmds.disconnectAttr(conn, matA)
        except:
            pass