A chain saw won't do your job.
If a hedge trimmer on a ladder will work now, a hedge trimmer
on a pole will work.
I suggest you buy a hedge trimmer, a roll of duct tape, and a
20 ft fence rail. Judicious use of a hack saw , a couple bolts,
and the duct tape should give you what you need, and when you
are thru, you can restore it to what it was..... Buying a hedge
trimmer on a pole to use once or twice a year doesn't seem
very cost effective to me...
Personally, I have trouble cutting arbor vitae even with a hedge
trimmer and usually used a ladder and a pair of snippers.
Perhaps you have a better trimmer.
... but, good luck to you on it. If you come up with a good
solution, please post it back here.... Inquiring minds want
to know...... :>)))
Andy in Eureka, Texas
PS Arbor vitae are very susceptable to bag worms. They can
destroy a large tree in two or three days beyond recovery.
Be advised....... Been there, done that......