I just joined this list so I could answer your question.
I live in North Waterboro Maine which is about 20 miles
inland from Portland. I also live close to the Boston ARTCC
transmitter for the PARSO Sector. I can hear the ground
side of KZBW for the following freqs:
118.550 (Not used much, paired with 285.400)
128.200 (Main freq which is used 99% of the time)
263.050 (Replaced 322.400 paired with 128.200)
273.450 (Heard once for Kennebunk CAP use only on 6/15/03)
285.400 (Not used much, paired with 118.550)
322.400 (Retired - used for discrete backup)
To answer your question. Portland departure on 119.750 hands
off planes headed South/Southwest to Boston ARTCC freq of 128.200
about 99% of the time. On rare occasions, I have heard some flights
heading South at low altitude be handed off to Boston approach (formerly
Manchster app at Pease KPSM) on 125.050
I am not an authority on Portland Airport because my main interest
is listening to Milair, but once in awhile I'll tune Portland in when the
Milair action is slow.
Hope this helps.