We present polarimetric 1.3 cm RadioAstron, 7 mm VLBA, and 3 mm GMVA observations of a sample of blazars probing the innermost jet regions at tens of microarcseconds angular resolution. Comparison of the total and polarized emission across these wavebands allows to determine the magnetic field structure and strength in the vicinity of the central black hole through Faraday rotation analysis. It also allows to probe the brightness temperature, angular sizes, and spectra across and along the innermost jet to determine the physical parameters of the fluid (velocity field, energy density) and that of the non-thermal electron population. This information can be used to understand how AGN jets are formed, accelerated and collimated, what is the role played by the magnetic field in these processes.