  We present an analysis of the linear polarization of six active galactic
nuclei - 0415+379 (3C~111), 0507+179, 0528+134 (OG+134), 0954+658, 1418+546
(OQ+530), and 1637+574 (OS+562). Our targets were monitored from 2007 to 2011
in the observatory-frame frequency range 80-253 GHz, corresponding to a
rest-frame frequency range 88-705 GHz. We find average degrees of polarization
m_L ~ 2-7%; this indicates that the polarization signals are effectively
averaged out by the emitter geometries. We see indication for fairly strong
shocks and/or complex, variable emission region geometries in our sources, with
compression factors <0.9 and/or changes in viewing angles by >10 deg. An
analysis of correlations between source fluxes and polarization parameter
points out special cases: the presence of (at least) two distinct emission
regions with different levels of polarization (for 0415+379) as well as
emission from a single, predominant component (for 0507+179 and 1418+546).
Regarding the evolution of flux and polarization, we find good agreement
between observations and the signal predicted by "oblique shock in jet"
scenarios in one source (1418+546). We attempt to derive rotation measures for
all sources, leading to actual measurements for two AGN and upper limits for
three sources. We derive values of RM = -39,000 +/- 1,000 (stat) +/- 13,000
(sys) rad/m^2 and RM = 420,000 +/- 10,000 (stat) +/- 110,000 (sys) rad/m^2 for
1418+546 and 1637+574, respectively; these are the highest values reported to
date for AGN. These values indicate magnetic field strengths of the order
~0.0001 G. For 0415+379, 0507+179, and 0954+658 we derive upper limits |RM| <
17,000 rad/m^2. From the relation |RM| ~ nu^a we find a = 1.9 +/- 0.3 for
1418+546, in good agreement with a = 2 as expected for a spherical or conical
outflow.
