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Vinyl group capped urethane acrylate (UA) was prepared with diisocyanate (IPDI), hydroxyethyl methacrylate (HEMA) and dimethylolpropionic acid (DMPA). The UA was grafted onto sodium alginate via free-radical polymerization with KPS/Na 2 SO 3 as initiator and then crosslinked by Ca 2+ . Grafted polymers (UA- g- CaA) were characterized by FT-IR, SEM, TGA, XRD and tension gauge. The swelling behavior was also investigated. It is found that urethane acrylate grafted calcium alginate possesses lower swelling degree and is improved in thermal stability as well as mechanical performance. The reason is that hydrophobic urethane acrylate provides alginate with physical crosslinking and forms partial crystal regions. The optimum condition for synthesis of UA- g- CaA with lower swelling degree is that weight percentage of initiator to monomer is 0.8%, mass ratio of reductant to oxidant is 0.45:1, monomer concentration is 0.0385 mol/L and grafting reaction temperature is 55 °C. [ABSTRACT FROM AUTHOR]