In preferred embodiments of the first two aspects of the invention, the bridge comprises the unit A, ##STR3## where: R3 R6 are independently selected from H, a halogen, alkoxy and alkyl groups of {HEAD ~ C5} or less, and tertiary amines; and {TAIL ~ B} is a linker to the maleimido function 
In preferred embodiments of the first two aspects of the invention , the bridge comprises the unit A , # # STR3 # # where : R3 R6 are independently selected from H , a halogen , alkoxy and alkyl groups of { HEAD ~ C5 } or less , and tertiary amines ; and { TAIL ~ B } is a linker to the maleimido function
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