Linear polyamides containing heterocyclic rings in their structure are prepared by subjecting to amide-forming conditions in the liquid phase one or more reagents, all the molecules of which, apart from any small proportion of viscosity stabilizing constituents, contain either two or more than two amide-forming groups, the amide-forming groups being present in the molecule in such positions that from each reagent two can be selected to give a series of complementary pairs which are not in a position to form intramolecularly a five-or six-membered amide ring, and having selected them in every possible way and formulated the linear polymer formed by their interaction, the remaining groups are then present in positions to form five- or six-membered rings. Suitable reagents falling within the scope of the invention are a - a 1 - diprimary or a - a 1 - disecondary - amino - dicarboxylic acids, the chain length of which is such that a carboxylic group cannot form a five- or six-membered ring with an amino group in the same molecule. These reagents condense with themselves to form polymers containing diketo-piperazine rings in the chain. Examples are 2.6-diamino pimelic acid; 2.7-diamino-suberic acid; 2.8-diamino azelaic acid, and 2.9-diaminosebacic acid; a -a 1-diamino-dihydracrylic acid or homologues thereof, or acids prepared by reacting diamines (preferably polymethylene diamines), an aldehyde or ketone, and an alkali metal cyanide followed by hydrolysis. Another suitable class of starting materials consists of a reagent containing four reactive radicals, two of which are complementary and in positions to form five- or six-membered rings intramolecularly and the remaining two being available for intermolecular reaction to form the linear polymer. Such a reagent is of the formula <FORM:0610304/IV(a)/1> which reacted with a diamine forms a linear polyamide with simultaneous ring formation. The reagent may be formed by adding acrylonitrile or acrylic ester to nitromethane to form a nitro-tricarboxylic ester or a nitro-trinitrile which may be converted to a nitro-tricarboxylic ester. Reduction of the nitro body yields a lactam di-ester which may be condensed with a diamine to form the polymer or hydrolysed to the amino-tricarboxylic acid which is so condensed. Suitable diamines are ethylene, tetramethylene, pentamethylene, hexamethylene, heptamethylene or decamethylene diamine or a diamine containing a hetero-atom such as oxygen or sulphur, e.g. di-b -aminopropyl ether. Alternatively, a triamino-monocarboxylic acid may be formed by reacting excess nitro-methane with acrylonitrile or acrylic ester to produce g -nitro-butyric acid or ester which is condensed with two molecules of acrylonitrile. The two nitrile groups and the nitro group are then reduced to form a tri-amino monocarboxylic acid of formula <FORM:0610304/IV(a)/2> or the corresponding diamino lactam which may be condensed with a dicarboxylic acid, e.g. oxalic, glutaric, adipic, pimelic, suberic, azelaic, or sebacic acid or diglycollic acid, thiodiglycollic acid or N-methyl-imino-diacetic acid. Methacrylonitrile or methacrylic acid ester may be used in the above syntheses if desired. Alternatively, acetone may be used in place of nitromethane for condensation with 3 mols. of acrylonitrile or acrylic ester and the monoketotricarboxylic acid then converted to the corresponding monoamino tricarboxylic acid by reducing the keto to a carbinol group replacing the hydroxyl by halogen and the halogen by amino which by condensation with the diamine forms the desired polymer. If desired, two complementary tetra-functional reagents of the above kind can be condensed together to give a polymer according to the invention, e.g. CCC-tri-(b -carboxyethyl)-methylamine may be condensed with 1.4.7-triamino-4-b -carboxyethyl heptane. Ethylene-bis-amino-diacetic acid is also a suitable reagent for self-condensation to produce a polymer according to the invention, but the polymer contains no hydrogen attached to nitrogen and is not a preferred one. In a third class of reactions according to the invention, a reagent containing four or a higher even number of reactive groups is reacted with a bifunctional reagent which reacts with pairs of reactive groups in the first reagent to form the rings and the linear chain. Suitable reagents are a tetramine of the general formula H2NR1NHRNHR2NH2. The second reagent may be a dicarboxylic acid or its equivalent, e.g. oxalic, malonic or succinic acid, oxalic ester, urea, urethane, diethyl, dibutyl or diphenyl carbonate. Suitable tetramines for this reaction may be prepared as described in Specification 610,311. The first reagent may be a tetracarboxylic acid or ester, e.g. d d -dicarboxylic azelaic acid which may be reacted, e.g. with hydrazine. More than one mol. of the bifunctional reagent is required per mol. of the reagent with four or more functional groups. Mixed polymers may be formed by reacting reagents of the kind described above with other polyamide-forming reactants. In all the reactions carboxylic groups may be replaced by equivalent radicals, e.g. alkyl or aryl esters, carboxylic amide or nitrile groups (together with water). Similarly, amino groups may be replaced by acidylamino groups or N-carboaryloxy groups, or by isocyano or isothiocyano groups, the reaction with a carboxylic acid then involving elimination of carbon dioxide or carbon oxysulphide. Condensations are carried out in liquid phase, i.e. with molten reagents or in presence of a solvent, e.g. phenol or a cresol or xylenol. When a volatile reagent is employed, initial stages of the reaction may be under pressure or reflux conditions. Vacuum or a current of inert gas may be applied to facilitate removal of volatile materials split off during condensation. Condensation catalysts, e.g. .01-.1 per cent phosphoric acid may be added to the reaction mixture. Air should be excluded. The average molecular weight of the product may be controlled and the viscosity stabilised by using a slight excess of one reagent or by including minor proportions of monofunctional reactants which are preferably of low volatility, e.g. dodecylamine, octadecylamine, lauric acid or stearic acid. Filaments, films and other articles may be produced from the polymers by usual methods. Filaments may be drawn, and products may be aftertreated to modify solubility or melting point, e.g. by treatment with aldehydes, polycarboxylic acids or acid chlorides or sulphuryl chloride. The polymers may be used alone p or with cellulose derivatives, plasticisers, dyestuffs and pigments as softening agents, or coating or film - forming materials. In examples: (I) Purified and esterified a -a 1-diamino-sebacic acid (prepared by liquid ammonia on a -a 1-dibromsebacic acid or by hydrolysis of hexamethylene-bis-phthalimido malonic ester) was heated in m-cresol and an atmosphere of nitrogen for 6 hours at 205 DEG C. The polymer was precipitated with ether, extracted with hot ether and water and dried in vacuo at 50 DEG C. (II) Hexamethylene-bis-imino acetic acid (prepared by treating hexamethylenediamine with formaldehyde and potassium cyanide and hydrolysing the di-nitrile) was heated at 180 DEG C. for 30 hours in a stream of nitrogen in presence of phenol or m-cresol and phosphuric acid. Heating was continued at 218 DEG C., finally at reduced pressure. A film-forming polymer resulted. (III) a -a 1-Diaminosebacic ester and hexamethylene-bis-imino-acetic acid were condensed together in m-cresol. (IV) Polymers were prepared from a -a 1-diaminosebacic ester alone and together with hexamethylene-bis-imino acetic ester. (V) CCC-tri-b -carboxyethyl-methylamine (prepared by adding acrylonitrile to nitromethane, hydrolysing the tri-nitrile produced, and reducing the nitro group with a nickel catalyst) was heated with diformyl-hexamethylene-diamine in hydrogen. A film-forming polymer was obtained. (VI) Tri-(b -carbethoxyethyl)-nitromethane was reduced and the lactam-5.5-di-(b -carbethoxyethyl)-2-pyrrolidone obtained. Heated with deca-methylene diamine in ethanol and hydrogen this yielded a fibre-forming polymer soluble in ethanol, formic acid or m-cresol. (VII) 5.5-Di-(b -carbethoxyethyl)-2-pyrrolidone was heated in hydrogen with diformyl hexamethylene diamine to form a fibre-forming polymer. Samples have been furnished under Sect. 2 (5) of polymers produced from one mol. of the hydrate of 1.2-bis-o -aminohexylaminoethane and 2 mols. of ethyl oxalate in 95 per cent ethyl alcohol. Specifications 570,858 and 577,205 and U.S.A. Specifications 2,071,250 and 2,149,273 also are referred to.ALSO:Examples of raw materials for the synthesis of linear polyamides containing heterocyclic rings (see Group IV (a)) are acids prepared by reacting diamines (preferably polymethylene diamines) an aldehyde or ketone, and an alkali metal cyanide followed by hydrolysis. Another suitable class of starting materials consists of a reagent containing four reactive radials, two of which are complementary and in positions to form five or six membered rings intramolecularly and the remaining two being then available for intermolecular reaction to form the linear polymer. Such a reagent is of the formula <FORM:0610304/IV(b)/1> and may be formed by adding acrylonitrile or acrylic ester to nitromethane to form a nitrotricarboxylic ester or a nitro-trinitrile which may be converted to a nitro-tricarboxylic ester. Reduction of the nitro body yields a lactam diester which may be hydrolysed to the aminotricarboxylic acid. Alternatively a triaminomonocarboxylic acid may be formed by reacting excess nitro-methane with acrylonitrile or acrylic ester to produce g -nitro-butyric acid or ester which is condensed with two molecules of acrylonitrile. The two nitrile groups and the nitro group are then reduced to form a triamino monocarboxylic acid of