Polyethylene glycol substituted acyl borates, methods for their preparation, and the use of the polyethylene glycol substituted acyl borates for the preparation of polyethylene glycol substituted macromolecules. The polyethylene glycol substituted acyl borate has the general formula (I)wherein PEG denotes a polyethylene glycol-based substituent； a is an integer from 1 to 12； X1, X2, and X3 are independently of one another selected from the group made of F, OR, N+R3, N+R2OR, N+R2SR, and N+R2NR2, and are optionally forming a cyclic or a bicyclic structure； and R is an organic substituent or H.
