check pH etc. What type of polymer is it? Is the end group ok for functionalization, should it be activated etc?

Hi Alexandre. It's a poloxamer, with one -OH group at each end. I wondered if it could be the pH, but the reaction occurs in dichoromethane with DCC (it is in fact a Steglich esterification) that will act first as nucleophile and then as electrophile to form the O-acylisourea reactive rhodamine intermediate, wright? So I think there is no need of an acidic millieu. The presence of DMAP will allow the formation of the ester with poloxamer. Should I add the poloxamer to the reaction some time later, so the reactive intermediate is formed without the presence of it?

Thanks, Christoph. I played a bit with the molarities of the compounds and found that the reaction occured in some extent but the yield is really low. Maybe adding a reactive spacer as you suggested might improve the yield of the ligation.