A good metric to use would be the glass transition temperature, which marks the point where a polymer's amorphous phase transitions from a glassy state (at lower temperature) to a rubbery state (at higher temperature). Polymers tend to be more brittle below the Tg
. The Tg
is usually between 0 and -20 C for polypropylene (PP), and it can depend a lot on the type of PP and any additives/plastizers the plastic contains. Thus it may range from above to is slightly below the temperature of most household freezers.
Most freezer bags are made out of polyethylene (PE), which usually has a much lower Tg
than PP. So while some PP products may get brittle in a household freezer, most PE products will be fine. Because of this, for applications at temperatures at or around the freezing point of water, PP is not usually a preferred material.
For more information see this article
about the link between glass transition temperature and mechanical properties of polymers.
This of course, as Enthalpy mentions, does not account for changes due to chemical or photo aging, to which PP is particularly susceptible.