Multi-frequency Polarization Properties of Blazars

We have obtained total and polarized intensity images of 15 AGNs with the VLBA at 7 mm at 17 epochs, accompanied at many epochs by nearly simultaneous measurements of polarization at 1.35/0.85 mm and at optical wavelengths. Analysis of the data shows an obvious connection between the polarized emission at high frequencies and the most strongly polarized features in parsecscale jets. The largest deviations between polarization parameters at different frequencies are detected when a new compact knot emerges from the core, and are likely caused by very short time-scale (<1 week) polarization variability during the events.