  We present 870 $\mu$m ALMA observations of polarized dust emission toward the
Class II protoplanetary disk IM Lup. We find that the orientation of the
polarized emission is along the minor axis of the disk, and that the value of
the polarization fraction increases steadily toward the center of the disk,
reaching a peak value of ~1.1%. All of these characteristics are consistent
with models of self-scattering of submillimeter-wave emission from an optically
thin inclined disk. The distribution of the polarization position angles across
the disk reveals that while the average orientation is along the minor axis,
the polarization orientations show a significant spread in angles; this can
also be explained by models of pure scattering. We compare the polarization
with that of the Class I/II source HL Tau. A comparison of cuts of the
polarization fraction across the major and minor axes of both sources reveals
that IM Lup has a substantially higher polarization fraction than HL Tau toward
the center of the disk. This enhanced polarization fraction could be due a
number of factors, including higher optical depth in HL Tau, or scattering by
larger dust grains in the more evolved IM Lup disk. However, models yield
similar maximum grain sizes for both HL Tau (72 $\mu$m) and IM Lup (61 $\mu$m,
this work). This reveals continued tension between grain-size estimates from
scattering models and from models of the dust emission spectrum, which find
that the bulk of the (unpolarized) emission in disks is most likely due to
millimeter (or even centimeter) sized grains.
