For example, if one or both of the end groups X11 and X22 in a polymer of formula I2 is a reactive group or a protected reactive group, like for example an optionally protected hydroxy or amine group, they can be reacted (after removing the protective group) with the end group of another polymer of formula I2 (e.g. with different groups R1 and/or X11 and/or X22), or with a polymer of different str