Synthetic resins of hydrophilic character are produced by polymerizing a mixture of one or more liquid monomers and one or more polymers and/or copolymers soluble and/or swellable in the liquid monomer or monomers, the polymerization being effected in the presence of water and one or more suitable fillers under the influence of an accelerator, the suitable fillers being compounds which swell in water, dissolve colloidally in water, or take up physically substantial quantities of water, such as gelatine, alginic acid, cellulose ethers, pectin, colloidal silicic acid, polyvinyl alcohol, viscose, or a copolymer made up principally of acrylic and/or methacrylic acid.  The liquid monomer may be an acrylate or methacrylate, particularly methyl methacrylate, and the polymer and/or copolymer is preferably formed principally from an acrylic or methacrylic acid ester or mixture of such esters, preferred copolymers being those derived from a mixture of methyl methacrylate and methacrylic acid.  The accelerator is preferably a combination of a peroxide such as benzoyl peroxide and a tertiary amine such as dimethylaniline and dimethyl-p-toluidine, and the reaction mixture is preferably emulsified before polymerization by incorporation of an emulsifier such as triethanolamine, an alkylaryl sulphonate or an alkali-metal salt of a higher fatty acid. The mechanical strength of the resins may be improved by the incorporation therein of fibres, woven materials or knitted fabrics, and the resins may be made porous (see Group V).  The resins are suitable for the production of surgical casts, splints and supports.ALSO:Porous synthetic resins of a hydrophilic character are produced by introducing gas into the mixture during the polymerization of a mixture of one or more liquid monomers and one or more polymers and/or copolymers soluble and/or swellable in the liquid monomer or monomers, the polymerization being effected in the presence of water and one or more suitable fillers under the influence of an accelerator, the suitable fillers being compounds which swell in water, dissolve colloidally in water, or take up physically substantial quanties of water, such as gelatine, alginic acid, cellulose ethers, pectin, colloidal silicic acid, polyvinyl alcohol, viscose or a copolymer made up principally of acrylic and/or methacrylic acid.  The monomer may be methyl methacrylate, and the polymer may be polymethyl methacrylate or a copolymer of methyl methacrylate and methacrylic acid.  Gas may be introduced into the mixture by polymerizing in the presence of sodium bicarbonate and an acid-reacting substance such as a copolymer containing carboxylic groups or aluminium sulphate or by including in the mixture a substance such as azodiisobutyric acid dinitrile, which decomposes on heating.ALSO:Surgical casts, splints and supports are made from synthetic resins of hydrophilic character produced by polymerizing a mixture of one or more liquid monomers and one or more polymers and/or copolymers soluble and/or swellable in the liquid monomer or monomers, the polymerization being effected in the presence of water and one or more suitable fillers under the influence of an accelerator, the suitable fillers being compounds which swell in water, dissolve colloidally in water or take up physically substantial quantities of water, such as gelatine, alginic acid, cellulose, ethers, pectin, colloidal silicic acid, polyvinyl alcohol, viscose or a copolymer made up principally of acrylic and/or methacrylic acid (see Group IV (a)).  In an example a powdered mixture of a copolymer of methyl methacrylate and ethyl acrylate, benzoyl peroxide, sodium bicarbonate and a copolymer of methacrylic acid and methyl methacrylate is formed into a paste with a mixture of methyl methacrylate, dimethyl-p-toluidine, ethyl alcohol, water and triethanolamine, the mass so obtained being then poured into a slotted funnel, through which are led two muslin bandages corresponding to the width of the slot; when the muslin bandages are evenly withdrawn there is obtained a strip covered on both sides with muslin and consisting of a foamed soft mass of synthetic resin which is applied to a member to be splinted, covered with a foil of synthetic resin and hardens after three to four minutes.