Cross-linked hydrophilic polymers

A cross-linked hydrophilic polymer capable of absorbing water to give a clear hydrated polymer containing about 63 to 90% by weight of water and having a tensile strength of at least 10 g./mm.sup.2, the tensile strength being at least (154-1.6x) g./mm.sup.2 where x is the water content as a percentage by weight and the permeability to oxygen and tensile strength of the said hydrated polymer being sufficient for forming contact lenses therefrom, the said polymer comprising hydrophilic units of N-vinyl-2-pyrrolidone and 0. to 30% by weight of the N-vinyl-2-pyrrolidone of 2-hydroxyethyl or 2-hydroxypropyl acrylate or methacrylate and hydrophobic units of a cycloalkyl acrylate, methacrylate or itaconate, the said cycloalkyl containing 5 to 7 ring carbon atoms and being unsubstituted or substituted by alkyl of 1 to 4 carbon atoms, in relative proportions of the said hydrophobic units to the said hydrophilic units by weight from about 86:14 to 97:3;

• and 0 to 50% by weight of the N-vinyl-2-pyrrolidone, of units of one or more alkyl or alkoxyalkyl acrylates, methacrylates, maleates, fumarates, or itaconates, the said alkyl or alkoxyalkyl having up to 7 carbon atoms in a straight or branched chain;
• and units of a copolymerisable cross-linking agent in a proportion of 0.2 to 0.8 parts per 100 parts by weight of the other monomers, the said proportion being sufficient to confer adequate tensile strength and clarity on the hydrated polymer.