  The polarization of fast radio bursts (FRBs), bright astronomical transients,
contains crucial information about their environments. We report polarization
measurements of five repeating FRBs, the abundant signals of which enable
wide-band observations with two telescopes. A clear trend of lower polarization
at lower frequencies was found, which can be well characterized by a single
parameter rotation-measure-scatter ({\sigma}RM) and modeled by multi-path
scatter. Sources with higher {\sigma}RM have higher RM magnitude and scattering
timescales. The two sources with the most substantial {\sigma}RM, FRB 20121102A
and FRB 20190520B, are associated with a compact persistent radio source. These
properties indicate a complex environment near the repeating FRBs, such as a
supernova remnant or a pulsar wind nebula, consistent with their arising from
young populations.
