fields is due to dust self-scattering. This process becomes important if the dust grain size is comparable to the wavelength, i.e. if a$_{max}$ $\sim$ $\lambda$/$2\pi$ with a$_{max}$ the maximum grain size [@Kataoka2015] and has been mostly studied at millimeter wavelengths. Hence, a low P% is achieved when a$_{max}$ $>$ $\lambda$/$2\pi$ and vice versa. Fig.\[Percentpol\] indicates the variation of the degree of polarization in the nebula at millimeter and sub-millimeter wavelengths. Although there is a clear gradient, which can be associated with a depolarization effect, P% is generally always $\geq$ 2% in most of the nebula except for the region closer to the central star. The relatively high degree of polarization that we find in OH231.8+4.2, compared to the maximum value typically expected from models of self-scattering, about 2.5% at 345 GHz (a$_{max}$=100 $\mu$m) [@Kataoka2015], suggest that self-scattering is not the dominant polarization factor in OH 231.8+4.2. The polarization process occurring in the inner region is less clear though.\
In addition, the good agreement between the 230 GHz and 345 GHz maps in terms of polarization vectors distribution indicates that there is no wavelength dependence. We further convolved the ALMA Stokes [*Q*]{} and [*U*]{} images to the CARMA resolution and confirmed 