Why R-O-R is polar?

Element R takes a slightly positive charge, while O takes and negative charge.

The charge on O is twice R therefore you need two R's to balance R2O.

Because the R's are on either side of the O and the attraction is from to positive, the charges cancel each other out due to symmetry, one goes left 1 unit, the other goes right 1 unit, as they have an equal charge.