Branched polyethylene glycol linked with diacyl glycerol, process for producing the same, and polyethylene glycol modified liposome

A method for producing branched polyethylene glycol which is represented by the following formula (3) and which is bound to a diacylglycerol:

• where R1 is a hydrocarbon group containing 1 to 7 carbon atoms, R2 is an alkyl or alkenyl group containing 9 to 21 carbon atoms, n is the average number of moles of added polyethylene glycol chains, and 10≦n≦100, the method comprising:
• (A) a step of adding ethylene oxide to a compound represented by the following formula (4) to obtain a polyethylene glycol derivative represented by the following formula (5):
• where n is the average number of moles of added polyethylene glycol chains and 10≦n≦100,
• (B) a step of substituting hydrogen atoms of terminal hydroxyl groups of ethylene oxide groups in the polyethylene glycol derivative represented by Formula (5) by hydrocarbon groups to obtain a polyethylene glycol derivative represented by the following formula (6):
• where R1 is a hydrocarbon group containing 1 to 7 carbon atoms, n is the average number of moles of added polyethylene glycol chains, and 10≦n≦100,
• (C) a step of subjecting the polyethylene glycol derivative represented by Formula (6) to acid hydrolysis at a pH of 1.0 to 3.0 to obtain a polyethylene glycol derivative represented by the following formula (7):
• where R1 is a hydrocarbon group containing 1 to 7 carbon atoms, n is the average number of moles of added polyethylene glycol chains, and 10≦n≦100, and
• (D) a step of subjecting terminal hydroxyl groups in the polyethylene glycol derivative represented by Formula (7) and an aliphatic acid to dehydrocondensation to obtain the branched polyethylene glycol represented by Formula (3).