A polymer comprising units α, β, γ and δ wherein: unit α is present at 30 mole % to 60 mole % and is an optionally substituted arylene; unit β is present at 1 mole % to 30 mole % and is a unit comprising an optionally substituted fluorene; unit γ is present at 1 mole % to 40 mole % and comprises aryl substituted nitrogen, or an optionally substituted triazine; unit δ is present at 0.5 mole % to 15 mole % and comprises an iridium complex; and optionally up to 20 mole % of other units if the total of α, β, γ and δ is less than 100 mole %.