The linker may in fact be trifunctional, that is the linker has at least three functions including two independently selected from hydroxy, amine or carboxy, the amine and hydroxy function(s) being available for esterification/amide bonding with the carboxyl functions of a pair of R2, or the carboxy function(s) on the linker being available for amide bonding with the free ??-amine function of R2.