  We present a polarization variability analysis of a sample of 26 $\gamma$-ray
blazars monitored by the Steward Observatory between 2008 and 2018 in the
optical band. We investigate the properties and long-term variability of their
optical polarization, searching for differences between blazar types. We
observe that BL Lac objects are typically less polarized and less variable than
flat spectrum radio quasars (FSRQs). Moreover, BL Lacs display a distribution
of their polarization angle typically oriented in a preferential direction,
contrary to the rather random distribution of FSRQs. For the latter blazar
type, as well as those sources showing a bright stellar emission, we take into
account the depolarizing effect introduced by the broad line region and the
host galaxy on the measured polarization degree. In this sample we also observe
that BL Lacs present an uncorrelated evolution of the flux and the
polarization. Contrary, FSRQs show a correlation before the depolarization
correction, that is lost however after considering this effect. In addition, we
study the behaviour of the polarization angle, searching for angle rotations in
its long-term evolution. We derive that the FSRQs studied here show rotations
more frequently than BL Lac objects by a factor $\sim$1.5. During these periods
we also observe a systematic decrease of the polarization fraction, as well as
a marginal flux increase, not significant however to connect rotations with
optical flares. We interpret these results within the extended shock-in-jet
scenario, able to explain the overall features observed here for the
polarization of the blazar sample.
