Branched polyalkylene glycols

A branched polyalkylene glycol represented by formula (I):

• (R1—Mn—X1)mL(X2—X3—R2)q (I)
• {wherein L is
• M represents OCH2CH2, OCH2CH2CH2, OCH(CH3)CH2, (OCH2CH2)r—(OCH2CH2CH2)s (in which r and s, which may be the same or different, each represent an arbitrary positive integer) or (OCH2CH2)ra—[OCH(CH3)CH2]sa (in which ra and sa have the same meanings as the above r and s, respectively);
• n represents an arbitrary positive integer;
• m represents an integer of 3 or more;
• q represents an integer of 1 to 3;
• R1 represents a hydrogen atom, lower alkyl or lower alkanoyl;
• R2 represents a group having reactivity with an amino acid side chain, the N-terminal amino group or the C-terminal carboxyl group in a polypeptide or a group convertible into the group having reactivity;
• X1 represents R4—NH—C(═O)—R5 [in which R4 represents a bond, alkylene or O(CH2)tc (in which tc represents an integer of 1 to 8) and R5 represents a bond;
• X2 represents a bond, O or (CH2)teO (in which te has the same meaning as the above tc);
• X3 represents a bond or alkylene;
• and
• three or more R1—Mn—X1's may be the same or different, and when two or three X2—X3—R2's are present (when q is 2 or 3), they may be the same or different}.