Strict RAW, No.
Sentinel negates Disengage when it is used before leaving your reach:
Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.
However this enemy has taken the Disengage action before entering your reach - as such this benefit of Sentinel will not apply.
In my personal opinion, RAI, Yes.
In the general case, opportunity attacks only happen when you leave an enemy's reach. The very naming of Disengage underlies the lack of consideration for opportunity attacks in other situations. In my opinion, one of the intended benefits of Sentinel is to negate Disengage actions, and the "leaving your reach" rider is only there as a clarification (since that's when opportunity attacks generally occur), and was not intended as a restriction.
Polearm Master's opportunity attacks are a special case not considered by any of this wording, and in my opinion should still be able to benefit from this effect of Sentinel.