. . . "A coupling agent of the formulas ##STR18## wherein E is a group capable of reacting with a site on a protein; L is an organic linking radical which may contain a cleavable site; R and R' are independently hydrogen or lower alkyl; m, n, p and q are integers which are independently selected from the group consisting of 2 and 3; v and w are integers which are independently selected the group consisti" .