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[求助] PEG羟基改性遇到了麻烦，求大神

8臂PEG-OH溶解于无水二氯甲烷，在氮气保护下，加一定量的NaH（有40%矿物油，百灵威）搅拌，但搅拌一个小时左右就交联了，磁子也无法转动，这是什么情况？各位朋友有什么好的建议吗？ 3Q~~~~

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PEG改性

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PEG除水了吗?
建议严格参照原文献步骤，
··Synthesis of PEG Vinyl Sulfone (PEG-VS). Multiarm PEG-VSs were synthesized by coupling PEG-OHs with an excess of divinyl sulfone (Aldrich, Buchs, Switzerland), in contrast to a previously published synthesis of PEG-VS via PEG-chloroethyl sulfone.11 PEG-OH (ca. 5 g) was either used as received and dissolved directly in 300 mL of dry dichloromethane (previously dried over molecular sieves) or, in some cases, PEG was dried by azeotropic distillation in toluene using a Dean Stark trap before starting the reaction. To the PEG dissolved in dichloromethane, NaH was added under argon, at 5-fold molar excess over OH groups. After hydrogen evolution, divinyl sulfone was added very quickly at 50- to 100-fold molar excess over OH groups. The reaction was carried out at room temperature for 3 days under argon atmosphere with constant stirring. Afterward the reaction solution was neutralized with concentrated acetic acid, filtered through paper until clear, and reduced to a small volume (ca. 10 mL) by rotary evaporation. PEG was precipitated by adding the remaining solution dropwise into ice-cold diethyl ether. The polymer was recovered by filtration, washed with diethyl ether, and dried under vacuum. The dry polymer was then dissolved in 200 mL of deionized water containing ca. 5 g of sodium chloride and extracted three times with 200 mL of dichloromethane. This solution was dried with sodium carbonate, and the volume was again reduced by rotary evaporation. Finally, the product was reprecipitated and thoroughly washed with diethyl ether to remove all remaining divinyl sulfone. The final product was dried under vacuum and stored under argon at −20 °C. Derivatization was confirmed with 1H NMR (CDCl3): 3.6 ppm (PEG backbone), 6.1 ppm (d, 1H, CH2), 6.4 ppm (d, 1H, CH2), and 6.8 ppm (dd, 1H, −SO2CH). The degree of end group conversion, as shown by NMR, was found to range from 95 to 98%. Gel permeation chromatography was used to confirm that the starting material (PEG-OH) and the end-functionalized PEG-VS have identical molecular weight distributions.··
Ref. Synthesis and Physicochemical Characterization of End-Linked Poly(ethylene glycol)-co-peptide Hydrogels Formed by Michael-Type Addition M. P. Lutolf and J. A. Hubbell *
http://pubs.acs.org/doi/abs/10.1021/bm025744e