  The X/$\gamma$ ray polarimetry of the Crab pulsar/nebula is believed to hold
crucial information on their emission models. In the past, several missions
have shown evidence of polarized emission from the Crab. The significance of
these measurements remains however limited. New measurements are therefore
required. POLAR is a wide Field of View Compton-scattering polarimeter
(sensitive in 50-500 keV) onboard the Chinese spacelab Tiangong-2 which took
data from September 2016 to April 2017. Although not designed to perform
polarization measurements of pulsars, we present here a novel method which can
be applied to POLAR as well as that of other wide Field of View polarimeters.
The novel polarimetric joint-fitting method for the Crab pulsar observations
with POLAR, allows us to obtain constraining measurements of the pulsar
component. The best fitted values and corresponding 1$\sigma$ deviations for
the averaged phase interval: (PD=$14\substack{+15 \\ -10}$\%,
PA=$108\substack{+33 \\ -54} ^{\circ}$), for Peak 1: (PD=$17\substack{+18 \\
-12}$\%, PA=$174\substack{+39 \\ -36} ^{\circ}$) and for Peak 2:
(PD=$16\substack{+16 \\ -11}$\%, PA=$78\substack{+39 \\ -30} ^{\circ}$).
Further more, the 3$\sigma$ upper limits on the polarization degree are for the
averaged phase interval (55\%), Peak 1 (66\%) and Peak 2 (57\%). Finally, to
illustrate the capabilities of this method in the future, we simulated two
years observation to the Crab pulsar with POLAR-2. The results show that
POLAR-2 is able to confirm the emission to be polarized with $5\sigma$ and
$4\sigma$ confidence level if the Crab pulsar is polarized at $20\%$ and $10\%$
respectively.
