We report preliminary results of a 3-year multifrequency polarization campaign of 15 AGNs of blazar type, including bimonthly total and polarized intensity imaging at 7 mm with the VLBA. We find that the sources have two different dependences of degree of polarization on wavelength: sub-mm peaked or straight "polarization spectra". The fractional polarization of the latter group increases with frequency. All sources with sub-mm peak polarization (6 objects) have similar degree of polarization in the optical region and in the VLBA core. In high-optically polarized sources (5 objects) the EVPAs at all frequencies align with the pc-scale jet direction within the 1σ uncertainties. This implies that the high frequency emission is co-spatial with the VLBI core and/or transverse shocks in the jet.