Polymer

A light-emitting polymer comprising a repeat unit of formula (I):

• wherein:
• R1-R4 are each independently H or a substituent;
• and
• Ar1 and Ar2 are each independently an aromatic or heteroaromatic group selected from formulae (IIa) and (IIb):
• wherein:
• X1 is O, S, NR5 or PR5;
• X2-X4 independently in each occurrence is selected from CR6 and N;
• R5 is H or a substituent;
• R6 independently in each occurrence is H, a substituent or a direct bond to C1 or C2;
• Ar3 is an aromatic or heteroaromatic group;
• and
• either:
• one of X2-X4 is CR6 wherein R6 is a direct bond to C1 or C2;
• or
• an aromatic carbon atom of Ar3 is bound directly to C1 or C2.