4.2. (HO)2–PEG–(OH)2
Using a similar synthetic strategy, a four-functional PEG was also synthesized. α,ω-hydroxy PEG (HO–PEG–OH, Mn = 1050 g/mol, Mw/Mn = 1.08) was reacted with vinyl acrylate (3.0 equivalent per –OH of PEG) in the presence of CALB for 24 h at 50 °C. The resulting PEG diacrylate was then reacted with diethanolamine (1.0 equivalent per acrylate group of PEG) in DMSO at 50 °C for 24 h via CALB-catalyzed Michael addition to obtain (HO)2–PEG–(OH)2 (99.5% yield by 1H NMR). Fig. 19 shows the 1H and 13C NMR spectra of the (HO)2–PEG–(OH)2 product. The integral ratio of the chain end hydroxyl protons (8) to the methylene protons next to the ester linkage (3) was 1:1 confirming the expected structure. The 13C NMR spectrum of (HO)2–PEG–(OH)2 (Fig. 19(B)) was identical to that of (HO)2–TEG–(OH)2 (Fig. 18(B)).