The present invention relates to a polyurethane polymer prepared from a mixture comprising: (a) at least one polyol of formula (I), wherein at least three of X1, X2, X3, X4 and X5 are each independently an OH-terminated polyoxyalkylene group, and the remainder of X1, X2, X3. X4 and X5 are each independently H or absent, and Z is a central linking unit; (b) at least one di- or poly-isocyanate; and (c) optionally at least one OH-terminated chain extender. Further aspects of the invention relate to a process for preparing a polyurethane, and to a process for preparing a contact lens.