Methyl-PEG8-t-butyl ester (methyl-octyloxyethylene-t-butyl ester) is a polyethylene glycol (PEG) derivative containing a t-butyl ester protecting group. Its molecular formula is C₂₂H₄₄O₁₀ with a molecular weight of 468.6. The structure consists of an octaethylene glycol (PEG8) chain, terminated by a methyl group and containing eight ethylene glycol units, linked to a t-butyl ester group. It combines hydrophilicity (from the PEG chain) with chemical protection functionality (from the t-butyl ester). Production typically involves esterification: using methacrylic acid and octaethylene glycol as raw materials, refluxing at 120-130°C for 6-8 hours in the presence of an acidic catalyst (such as p-toluenesulfonic acid) and an inhibitor (such as hydroquinone), followed by purification to obtain a high-purity product. This reagent is widely used in the biomedicine field, such as serving as a carboxyl protecting group for proteins/polypeptides, allowing controlled deprotection under acidic conditions; it is also employed in polymer material modification to enhance biocompatibility and water solubility.