I present an analysis of the linear polarization of six active galactic nuclei (AGN). We monitored our targets from 2007 to 2011 in the observatory-frame frequency range 80-253 GHz with the Plateau de Bure Interferometer (PdBI). We find average degrees of polarization in the range 2-7%; this indicates that the polarization signals are effectively averaged out by the emitter geometries. We see indication for the presence of strong shocks and/or variability of the emitter geometries. We attempt to derive rotation measures for all sources, leading to actual measurements for two targets which find the highest rotation measures reported to date for AGN.