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4arm PEG, -(（CH2）3NHCO(CH2)2-MAL)4, Mw 20,000
4arm PEG, -(（CH2）3NHCO(CH2)2-MAL)4, Mw 10,000
4arm PEG, -(（CH2）3NHCO(CH2)2-MAL)4, Mw 40,000
