Hydrophilic polymer derivative having cyclic benzylidene acetal linker

A method for chemical modification of a biofunctional molecule with a hydrophilic polymer compound for improving in vivo and intracellular kinetics in environments having different pH in a living body, which comprises reacting a functional group in the biofunctional molecule with a chemically reactive functional group in the hydrophilic polymer,

• wherein the hydrophilic polymer compound has a cyclic benzylidene acetal linker represented by formula (1):
• wherein R1 and R6 are each independently a hydrogen atom or a hydrocarbon group;
• R2, R3, R4 and R5 are each independently an electron-withdrawing or electron-donating substituent or a hydrogen atom;
• X1 is the chemically reactive functional group and is selected from the group consisting of an active ester group, an active carbonate group, an aldehyde group, an isocyanate group, an isothiocyanate group, an epoxy group, a maleimide group, a vinyl sulfone group, an acryl group, a sulfonyloxy group, a carboxy group, a thiol group, a dithiopyridyl group, an a-haloacetyl group, an alkynyl group, an allyl group, a vinyl group, an amino group, an oxyamino group, a hydrazide group and an azide group;
• P is a hydrophilic polymer;
• s is 1 or 2, t is 0 or 1, and s+t is 1 or 2;
• m is an integer of 1 to 8;
• and
• Z1 and Z2 are each independently a selected divalent spacer.