Yes, POLARITIES is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, L 1, A 1, R 1, I 1, T 1, I 1, E 1, S 1.

Is it a valid word?

POLARITIES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a relation between two opposite attributes or tendencies
having an indicated pole (as the distinction between positive and negative electric charges)

Also: mutual opposition.