In examples (15) N-(N1-isopropyl-amino-propyl)-ethyleneimino acetamide is made by reacting ethyleneimino-acetic acid methyl ester with gamma-isopropylamino propylamine; (27) N - (N1 - isopropylaminopropyl) - alpha - ethy-leneimino butyramide is made by reacting alpha-ethyleneimine-butyric acid methyl ester with gamma - isopropylamino propylamine; (34) omega-ethylene imino heptylamine is made by reacting omega-bromoenanthic nitrile with ethylene imine and hydrogenating the product with lithium alanate.ALSO:Synthetic elastomers are made by cross-linking linear or substantially linear polyesters, polyethers or polythioethers which contain on average more than one ethylene imine group per molecule, have a mean molecular weight of at least 1000 and have an ethylene imine equivalent of at least 500.  Preferably the polymers contain up to 5 ethylene imine groups per molecule and have a molecular weight of 1000-25,000.  The ethylene imine groups may be introduced into the polymers by (a) reacting polymers containing carboxyl groups (or their amide-forming derivatives) with compounds of formula <FORM:1044753/C3/1> where R is a hydrogen atom or an alkyl group and Y is a trivalent radical.  Suitable amide-forming derivatives of the carboxyl groups are the esters (e.g. the methyl esters, aryl esters or "activated esters," e.g. aryl esters with nitro groups or halogen atoms on the benzene ring) or the imidazolides or the acid chlorides; (b) esterifying the polymers containing hydroxyl groups with unsaturated monocarboxylic acids and reacting the products with ethylene imine whereby groups are introduced of formula <FORM:1044753/C3/2> where R is H or alkyl; (c) reacting the polymer containing hydroxyl groups with a diisocyanate and reacting the products with an ethylene imine substituted amine or alcohol whereby groups are introduced of formula <FORM:1044753/C3/3> or <FORM:1044753/C3/4> where Z1 is a bivalent radical (preferably an alkylene group or a bivalent aromatic radical), Y and Y1 are bivalent radicals (preferably alkylene groups which may be interrupted by ether or acid amide groups) and R is H or alkyl; (d) esterifying polymers containing hydroxy groups with halogen substituted carboxylic acids and reacting the products with ethylene imine substituted amines; (e) reacting the polymers containing hydroxyl groups with phosgene to produce the corresponding chloroformates which are then reacted with an ethylene imine substituted amine; (f) reacting polyethers or polyesters containing halogen with ethylene imine; (g) reacting polyethers containing halogen with ammonia or potassium phthalimide to convert the halogen atoms into amino groups and reacting the products with ethylene imine carboxylic esters; (h) reacting polymers containing ethylenic unsaturation with ethylene imine.  Polymers containing carboxyl groups or their amide-forming derivatives may be prepared from polymers containing hydroxyl groups by reacting the latter with (i) isocyanato carboxylic acid esters; (ii) dicarboxylic acids or anhydrides; (iii) phosgene to produce the chloroformates and reacting the latter with amino acid esters. Polymers containing " activated ester " groups may be made from the polymers containing carboxy groups by reacting the latter with suitable diaryl carbonates, diaryl sulphites or triaryl phosphites. Suitable polyesters are those derived by reacting dicarboxylic acids with dihydric alcohols and, possibly, small amounts of tri-or tetra-functional carboxylic acids or alcohols or siloxane based dicarboxylic acids or diols-the polyesters used in the examples being derived by reacting (1) adipic acid, sebacic acid and triethylene glycol; (2) pimelic acid, adipic acid and trialkylene glycol; (3) adipic acid, sebacic acid, triethylene glycol and tetraethylene glycol; (4) adipic acid, sebacic acid, trimethylol propane and triethylene glycol; (5) adipic acid, sebacic acid, triethylene glycol and trimethylol ethane or trimethylol propane; (6) adipic acid, sebacic acid, butane-1:3-diol and hexane-1: 6-diol; (7) adipic acid, sebacic acid, hexene-1: 6-diol and triethylene glycol; (8) adipic acid, maleic anhydride, butane-1: 3-diol, hexane-1: 6-diol and decane-1: 10-diol; (9) adipic acid, azelaic acid and hexane-1: 6-diol; (10) pimelic acid, glutaric acid, propane-1: 3-diol and hexane-1: 6-diol.  Suitable polyethers are polymers and copolymers of ethylene oxide, propylene oxide, trimethylene oxide and, possibly, small amounts of 4,5-epoxy valeric acid methyl ester, 10,11-epoxy-undecanoic acid ethyl ester, glycidyl acrylate, glycidyl crotonate, glycidyl acetate, halogen-substituted epoxides (e.g. epibromohydrin or 4-bromobutene oxide-1,2) or polyoxyalkylation products of tri- or higher polyhydric alcohols.  The polyethers containing ester groups may be saponified to produce polyethers containing hydroxy groups in side-chains, the latter groups being converted into ethylene imine groups as described above.  The polyethers used in the examples are (1) polyethylene glycol; (2) copolymers of tetrahydrofurane and ethylene oxide; (3) polypropylene glycol; (4) a copolymer of tetrahydrofurane, ethylene oxide and 4-bromobutene oxide-1,2; (4) a copolymer of tetrahydrofurane, ethylene oxide and o -epoxy-undecanoic acid methyl ester;. A suitable polythioether is the polymer of thiodiglycol. The polymers containing ethylene imine groups may be cross-linked by heating them (preferably at 50-100 DEG  C.) with dialkyl sulphates, sulphonic acids or their esters, phosphoric acid, oxonium salts or Lewis acids or their complexes.  The cross-linking agent may be thickened with polyvinyl acetate or dispersal silica or may be thinned with a solvent or softening agent.  Cross-linking may be retarded by an amine or alcoholate and may also be effected in the presence of fillers (e.g. silica, calcium silicate, zinc oxide, calcium carbonate, barium sulphate, quartz powder, barytes, fluorspar, calcium phosphate or clay), inorganic pigments, dyes, disinfectants, plasticizers (e.g. phthalates or glycol derivatives) or antioxidants.  Also included may be other ethylene imine derivatives which may be mono-functional or may be based on siloxanes.  The products may be purified by filtration, washing, treatment with ion-exchangers or fractionation. They are useful in the manufacture of shaped articles (e.g. for sealing or coating purposes), dental prosthesis, adhesives, lenses, intermediate layers in safety glass and electric threads.  Foams may be made by blowing the polymers with fluorohydrocarbons.