Polymeric cyanuric and/or isocyanuric acid esters are prepared by reacting at least one organic halogen-containing compound having at least two halogenomethylene or halogenomethylgroups, as hereinafter defined, in the molecule and no acidic or basic group, with an alkali cyanate, as hereinafter defined, in the presence of solvent quantities of a dialkyl sulphoxide, alkylene sulphoxide, dialkyl sulphone, alkylene sulphone or compound of the formula R1 NYZ wherein R1 is alkyl, Y is alkyl, R1CO, aryl or together with R1 forms a heterocyclic ring which may contain a further hereto atom, Z is R1CO or R11 SO2 where R1 is hydrogen, an aliphatic or an aromatic radical and R11 is an aliphatic or aromatic radical. The products may contain rings having a mixed cyanurate/isocyanurate structure. The halogenomethyl or halogenomethylene groups must contain chlorine, bromine or iodine; if fluorine is also present it does not take part in the reaction. The alkali cyanates are defined as lithium, sodium, potassium and ammonium cyanates or their mixtures. The organic halogencontaining compound(s) may be saturated or unsaturated. Inorganic iodides act as promoters. Temperatures of 30 to 300 DEG C. are mentioned. The products may be stabilized by treatment with substances having active hydrogen to react with end groups. e.g. water, alcohols, glycols, phenols, aldehydes, organic or inorganic acids, metal hydroxides, organo silanols, ammonia, amines, inorganic bases, mercaptans or polymers; specified are diethylamine, water, aqueous NaOH, and a two stage use of water and acetic anhydride. Polymers are prepared in examples from 1, 4-dichlorobutane, 1, 2-dibromoethane, trimethylene chlorobromide, propylene dichloride, bis-(b -chlorethyl) ether, bis-(4-chlorobutyl) ether, bis-(b -chlorethyl) sulphone, chlorinated kerosene, chlorinated pentane, polychlorinated xylene (mainly m-and p-chloromethylbenzyl chlorides), tris-(4-chlorobutyl)-iso-cyanurate, tris-(5-chloropentyl)-isocyanurate, tris-(b chloroethyl)- cyanurate and a mixture of tris(4-chlorobutyl)-isocyanurate and tris-(5-chloropentyl)-isocyanurate: solvents used are dimethyl formamide and sulphoxide. In examples (20) a porous resin is obtained, mainly poly (butylenediisocyanurate) in the pores of which styrene is polymerized in the presence of 2% peroxide, and cured at 60 DEG C. for several days. In example (29) polystyrene, polyvinyl acetate or polyacrylonitrile powder is added to a poly-kerosene cyanurate for moulding. In Example (32) tricresyl phosphate is added to a polycyanurate. An optional additive mentioned is glass fibres. Uses. Lubricants: plasticizers: pastes: paints: fillers: packing; sponges: films: synthetic rubber: laminates; porous insulation.ALSO:Cyanuric and isocyanuric acid esters are prepared by reacting at least one organic halogen-containing compound having at least one halogenomethylene or halogenomethyl group, as hereinafter defined, in the molecule and no acidic or basic group, with an alkali cyanate, as hereinafter defined, in the presence of solvent quantities of a dialkyl sulphoxide, alkylene sulphoxide, dialkyl sulphone, alkylene sulphone or compound of the formula R1 NYZ wherein R1 is alkyl, Y is alkyl, R1CO, aryl or together with R1 forms a heterocyclic ring which may contain a further hetero atom, Z is R1CO or R11SO2 where R1 is hydrogen, an aliphatic or an aromatic radical and R11 is an aliphatic or aromatic radical, The products may contain rings having a mixed cyanurate/isocyanurate structure. The halogenomethyl or halogenomethylene group must contain chlorine, bromine or iodine ; if fluorine is also present it does not take part in the reaction. The alkali cyanates are defined as lithium, sodium, potassium and ammonium cyanates or their mixtures. The organic halogen-containing compound(s) may be saturated or unsaturated. Inorganic iodides act as promoters. Temperatures of 30 to 300 DEG C. are mentioned. Esters containing 3 alkyl groups of 3 or more carbon atoms each are useful as plasticisers. Esters prepared in examples include trimethyl-, triethyl-, tri-isobutyl-, triallyl-, tribenzyl-, tris-(4-chlorobutyl)-, tris-(5-chloropentyl)- and tris-(4-fluorobutyl) isocyanurates, mixed allyl/butyl cyanurates and cyanuric esters from the alkyl chlorides derived from palm oil alcohol.ALSO:A spongy resin, mainly poly (butylene-diisocyanurate), is obtained by reacting potassium cyanate and 1, 4-dichlorobutane in dimethylformamide solution, filtering, distilling the filtrate, and blending the residue, after solvent removal, in air, and leaving it in a mould at room temperature for several days, during which it expands to a spongy cake-suitable for building materials.