Polymers, monomers and methods of forming polymers

A crosslinkable polymer having a conjugated polymer backbone and comprising a repeat unit substituted with at least one crosslinkable unit of formula (I):

• wherein the repeat unit has formula (IIIa) or (IIIb):
• wherein:
• RU represents a conjugated core repeat unit comprised in a backbone of the polymer, wherein RU is at least partially conjugated to one or both adjacent repeat units in the polymer backbone;
• Sp2 represents a spacer group;
• m is 0 or 1;
• p is at least 1;
• Ar is aryl or heteroaryl which is unsubstituted or is substituted with one or more substituents independently selected from monovalent substituents and a divalent linking group linking the unit of formula (I) to the core unit;
• and
• R is independently in each occurrence H, a monovalent substituent, or a divalent linking group linking the unit of formula (I) to the core unit, with the proviso that at least one R is a linear or branched C3-20 alkyl,
• and wherein the polymer backbone is substituted with a crosslinkable double bond unit.