*2.1. Materials*

Poly-tetrahydrofuran (PTMG, Mn = 1000), Triethylamine (TEA), 2,2-Dimthylolpropionic acid (DMPA), Dibutyltin dilaurate (DBTDL) and bis (2-alkylethyl) disulfide (HEDS) were obtained from Shanghai Aladdin Company, Shanghai, China. Hexamethylene diisocyanate (HDI) was purchased from Bayer Technology, Germany. PTMG and DMPA should be dehumidified under vacuum at 100 ◦C for 2 h before the experiment.
