trifluoroacetic acid (TFA), ethylene glycol (EG), and N-(tert-butoxycarbonyl) glycine (Boc-Gly-OH), are of analytical grades, and were purchased from Aladdin (China, Shanghai). Potassium methylate solution in methanol (CH 3 OK, 20 wt %), glycidol (96%), trimethylolpropane (TMP), poly(vinyl alcohol) (PVA, M n = 89 KDa), polyethylene glycol (PEG, M n = 20 KDa), and poly(acrylic acid) (PAA, M w = 1.8 KDa), 4-dimethylamino-pyridine (DMAP), and