Multicellular polymers
United States Patent 1 Gruffaz et al.  
 [ Apr. 1, 1975 MULTICELLULAR POLYMERS [75] lnventors: Max Gruflaz, La Mulatiere;  
 Bernard Rollet, Lyon, both of France [73] Assignee: Rhone-Poulence S.A., Paris, France [22] Filed: Apr. 23, 1974 [211 Appl. No.: 463,249  
 [30] Foreign Application Priority Data Apr. 26, 1973 France 73.15175 [52] US. Cl 260/2.5 N, 260/37 N, 260/42, 260/47 CZ, 260/47 UA [51] Int. Cl C08g 53/08 [58] Field of Search 260/2.5 N, 78 UA, 47 CZ, 260/47 UA [56] References Cited OTHER PUBLICATIONS Stillie et al., A Novel Diels-Alder Polymerization, J. P.O.S.. Parta, Vol. 2, pgs. 1487-1491, (1964).  
 Primary Examiner-Morton Foelak Attorney, Agent, or FirmStevens, Davis, Miller &amp; Mosher [57] ABSTRACT Multicellular polymers having excellent chemical inertness and heat resistance are produced by heating at a temperature from 100 to 300C, maleic anhydride and a polyaldimine of the general formula 11 Claims, N0 Drawings MULTICELLULAR POLYMERS The present invention relates to multicellular polymers.  
  The use of multicellular synthetic materials in industry has progressed quite considerably in recent years, largely because of their very low density. This density advantage is greatly valued in applications such as heat insulation and in the manufacture of lightweight constructional elements, particularly in the aeronautical industry.  
  As far as heat insulation is concerned, there is a constantly increasing need for cellular materials which are capable of withstanding severe heat stresses and, in this field, it has already been proposed to make profitable use of the properties of polyimide-type polymers formed from tetracarboxylic acids, or their derivatives, and diamines.  
  The present invention provides a process for preparing rigid multicellular polymers which comprises heating, at a temperature from 100 to 300C, a mixture comprising maleic anhydride and a poly-aldimine of the general formula:  
 A (N =CH G),,  
 in which represents a monovalent organic radical containing less than 35 carbon atoms and at least one aromatic or heterocyclic ring in the a-position to the carbon atom of the imine group in formula (I), A represents an organic radical of valency n containing less than 50 carbon atoms and at least one aromatic or heterocyclic ring situated in the a-position to the nitrogen atom of the imine group in formula (I) and n represents a number at least equal to 2.  
  ln formula (l) each of the symbols G, which can be identical or different, may, in particular, represent an aryl radical such as phenyl or naphthyl, or a heterocyclic radical with at least one ring, containing at least one 0, N or S atom, such as pyridyl, pyrimidinyl, furyl, thienyl, benzo[9 thienyl, indolyl, triazolyl and 3,4-methylenedioxy-phenyl radicals, especially a phenyl, a-hydroxyphenyl or 4-furfuryl radical. The symbol G can also represent a radical containing several identical or different carbocyclic or heterocyclic rings connected to one another either directly or via an inert atom or group such as -O, S, NH, COO-, CONH, SO-,-. N=N.  
  N=N(O) CO or a linear or branched alkylene or a divalent radical containing 12 to 30 carbon atoms, consisting of phenylene radicals bonded to one another by a simple valency bond or by an inert atom or group such as O-, S, -SO- NR,. *N=N, CONH, COO, P(O)R CONHX- NHCO. N=N( O), an alkylene group with l to 3 carbon atoms.  
 NII  
 and which can consist of a naphthalene, pyridine or tri-&#39; azine nucleus, a benzene nucleus optionally substituted by 1 to 3 methyl groups, or several benzene nuclei bonded to one another by an inert atom or group such as those mentioned above, or  
  The polyimines of formula (I) can be considered as being derived from the condensation of one mol of a primary polyamine of the formula with n mols of an aldehyde of the formula H (III) in which formulae the symbols A, G and n are as defined above.  
  Suitable polyamines of formula (ll) which can be used for the preparation of the polyaldimines of formula (I). include 2,6-diaminopyridine, phenylenediamine, para-phenylenediamine. bis-(4- aminophenyl)-methane, 2,2-bis-(4-aminophenyl)- propane, benzidene, bis-(4-aminophenyl) ether, 4,4- diaminophenyl sulphide, bis-(aminophenyll-sulphone, bis-(4-aminophenyl)methylphosphine oxide, bis-( 4- aminophenyl )-phenylphosphine oxide, l,5- diaminonaphthalenm meta-xylylene-diamine. paraxylylcne-diamine, l, l -bis-(para-aminophenyl phthalane, 6,6&#39;-diamino-2.2-dipyridyl, 4.4&#39;-diaminobenzophenone, 4,4-diamino-azobenzene, bis-(4- aminophenol )-phenylmethane, 4-aminophenyl)- phenylmethane. l,l-bis-(4-aminophenyl)-cyclohexane,-  
 l,l-bis-(4-amino-3-methylphenyl)-cyclohexane 2,5- bis-( m-aminophenyl )-l ,3,4-oxadiazole, 2,5-bis-(paminophenyl )-l ,3 ,4-oxadiazole, 2,5-bis-( maminophenyl)-thiazolo (4,5-d)thiazole, 5,5-di-(m- 2,4,4-triamino-3methyl-diphenylmethane, 2,4,4- triaminobenzophenone, N,N,N-tri-(4-aminophenyl)- amine, tri-(4-aminophenyl)-methane, 4,4,4-  
 triaminophenyl orthophosphate, tris-(4-aminophenyl)- phosphine oxide, 3,5,4-triamino-benzanilide, melamine, 3,5,3,5&#39;-tetraaminobenzophenone, I,2,4,5- tetraamino-benzene, 2.3,6,7tetraamino-napthalene, 3,3-diamino-benzidine, 3,3&#39;,4,4&#39;-tetraamine-phenyl ether, 3,3,4,4&#39;-tetraamino-diphenylmethane, 3,34,4&#39;- tetramino-diphenylsulphone, 3,5-bis-(3,4- diaminophenyl)-pyridine, and oligomers of the average formula lTIHz NH: ITIHz l/\ R4 l in which y represents a number ranging approximately from 0.1 to 2 and R represents a divalent hydrocarbon radical of l to 8 carbon atoms which is derived from an aldehyde or a ketone of the general formula:  
 in which the oxygen atom is bonded to a carbon atom metaof the radical R; typical aldehydes and ketones include formaldehyde, benzaldehyde, oenenthal, acetone, methyl ethyl ketone, 2-hexanone, cyclohexanone and acetophenone. These oligomers can be produced in accordance with known processes such as those which are described in French Pat. Nos. 1,430,977, 1,481,935 and 1,533,696.  
  Aldehydes of formula (III) include benzaldehyde, oand p-chlorobenzaldehydes, oand pfluorobenzaldehydes o-, m and p-tolualdehydes, pethylbenzaldehyde, 2,4-dimethylbenzaldehyde, p- (tertiary butyl)-benzaldehyde, cumaldehyde, salicylaldehyde p-hydroxybenzaldehyde, protocatechaldehyde, B-resorcylaldehyde,o-vanillin, vanillin, veratral dehyde. oand p-nitrobenzaldehydes, oand panisaldehydes, 3-ethoxy-4hydroXy-benzaldehyde, 2,4,- 5-trimethoxy-benzaldehyde, 2-furaldehyde, piperonal, pyridine-Z-carbaldehyde and pyridine-4-carbaldehyde, 3-methyl-2-furaldehyde, 3,4-, 3,5- and 4,5-dimethyl-2- furaldehyde, S-benzyl-Z-furaldehyde, 4,6- dimethylpyrimidine-Z-carbaldehyde, indole-3- carbaldehyde, Z-methylindole3-carbaldehyde, a-thiophenaldehyde, benzo[b]thiophene-Z-carbaldehyde, bezo thiophene-3-carbaldehyde, 4-formyll ,2 ,3- triazole, 2-phenyl-l,2,3-triazole-4-carbaldehyde, 1- methyl- 1 ,2,3-triazole-4-carbaldehyde, l-phenyll,2,3triazole-4-carbaldehyde, l-phenyl-S-methyll ,2,3-triazole-4-diphenyl-1,2,3-triazole- 4carbaldehyde, oand p-phenylbenzaldehydes, (p-nitrophenyl)-benzaldehyde, and pphenylbenzaldehydes, p-(p-tolyl)-benzaldehyde, oand p-benzylbenzaldehydes, oand pphenoxybenzaldehydes p-thiophenoxybenzaldehyde, p-cyclohexylbenzaldehyde, p-cyclopentylbenzaldehyde, a-naphthaldehyde, Bnapthaldehyde, l-methyl-4- naphthaldehyde, l,6-dimethyl-4-napthaldehyde, l-hydroxy-4-napthaldehyde, l-hydroxy-Z- napthaldehyde, 2,3-dihydroxyl -naphthaldehyde,  
 4-formyl-azobenzene,  
 4-formyl-diphenyl-sulphone,  
 4-formyl form yl- 4-formyl-azoxybenzene and benzophenone.  
  The polyimines of formula (I) can be prepared from such polyamines and aldehydes by applying the method described by BUU-HOI et al., [Bull. Soc. Chim. Fr. (1956) p. 710].  
  Bis-aldimines, such as those mentioned below, are preferably used: bis-(4-benzylideneaminophenyl)- methane,2,2-bis-(4-benzylideneaminophenyl)- propane, bis-(4-benzylideneaminophenyl)-sulphone, bis(4-benzylideneaminophenyl) ether, bis-4- salicylideneaminophenyl)-methane, bis-(4- salicylideneaminophenyl )-sulphone, bis-(4-0- chlorobenzylideneaminophenyl)-sulph&#39;one, bis-( 3 ,4-  
 methylenedioxy-4-benzylideneaminophenyl)- sulphone, bis-(4p-methoxybenzylideneaminophenyl)- sulphone, bis-(4-thenylideneaminophenyl)-sulphone, bis(4-furfurylideneaminophenyl)-sulphone, bis-(4- furfurylideneaminophenyl)-methane, 2,6-bis- (furfurylideneamino)-pyridine, bis-(4-naphtylmethylideneaminophenyl)-methane, bis-(4-ophenylazobenzylideneaminophenyl)-methane, bis-[4- (2-pyridyl )methylideneaminophenyl- 2 ,6-bis- (benzylideneamino )-pyridine, 1,4-bis- (benzylideneamino )-benzene, 1,3-bis-(benzylideneamino&#39;methyl)-benzene, 1,3-bis- (benzylideneamino)-benzene, 3,5-bis-(4-ben- 2,6-biszylideneaminophenyl )-pyridine,  
  (salicylideneamino)-pyridine (benzylideneamino)-2,2&#39;dipyridyl.  
  For the preparation of the multicellular polymers of the present invention, 0.5 to 5 mols&#39;. especially 0.7 to 3 mols, of maleic anhydride are generally used per imine group in the polyaldimine.  
  It is to be understood that it is possible to use either a single polyimine or a mixture of polyimines. Although it is possible to produce cellular products directly by heating maleic anhydride and the polyaldimine under the specified conditions, it is advantageous. for technological reasons. to prepare these cellular polymers in two steps. In a first step. it is possible to prepare a prepolymer (P) which can be isolated. and, optionally. purified and stored. At ambient temperature, this prepolymer is generally in the form of a solid with a noncellular structure. which can be reduced to powder form, or is in the form of a viscous liquid. During subsequent heating. the prepolymer (P) can be converted and 6.6&#39;-bisinto a multicellular polymer, if desired after if has been shaped.  
  The prepolymers (P) can be prepared in bulk simply by heating a mixture of the reagents. The initiation temperature for reaction can vary within rather wide limits depending on the nature of the&#39;reagents but. as a general rule, it is from 100 to 220C and. most frequently, from 150 to 200C. The reaction is exothermic and is accompanied by the evolution of carbon dioxide. the aldehyde corresponding to the aldimine used being formed. It is advisable to control the way in which the reaction takes place by removing at least a part of the heat produced. It is possible. for example. to keep the reaction temperature in the vicinity of the initation temperature; when it is greater than the latter, it is preferable that the difference should not exceed approximately 50C. Depending on the temperature and on the nature and proportions of the reagents present, and depending on the extent to which it is desired that the reaction should take place. the period of heating is generally from 5 minutes to 3 hours.  
  The prepolymers (P) can also be prepared by heating the reagents in a diluent which is inert and liquid at the temperature used. generally from 100 to 220C and preferably from l50 to 200C. Amongst the diluents which can be used, there may be mentioned polar solvents such as dimethylformamide, N- methylpyrrolidone. dimethylacetamide, N-methylcaprolactam, diethylformamide, N-acetylpyrrolidone and cresols. It is also possible to use an aldehyde which is liquid in at least a part of the range 100 to 220C, such as benzaldehyde, salicylaldehyde, oand panisaldehydes, 2-hydroxy-3-methoxy-benzaldehyde,  
 2-furaldehyde and a-thiophenylaldehyde. According to a particular embodiment, it is possible to use the solutions obtained from a polyamine of formula (II) and an excess of liquid aldehyde of formula (III) directly after removing the water formed.  
 Generally from 0.2 to&#39;20 times the weight of polyaldiperature of from 200 to 300C, more generally from 220 to 280C. generally for ten minutes to 5 hours. When the prepolymer is solid. it is advantageous to reduce it to a powder before it is heated. lt is also possible to extract from it, at least partially, the aldehyde liberated by the reaction, for example by washing with a solvent which does not dissolve, or practically does not dissolve, the prepolymer. Such solvents include alcohols. ethers. ketones and aromatic hydrocarbons,  
  The prepolymer can be combined with liquid or solid adjuvants in the form of, for example a powder. spheres. platelets. granules. fibers or flakes. for the purpose of improving or modifying one or more characteristics of the finished article.  
  It is thus possible to incorporate into the prepolymer adjuvants which make it possible to increase the homogeneity of the cellular structure of the polymers. such as surface-active agents or pulverulent fillers which are inert with respect to the reagents employed. Suitable surface-active agents include organopolysiloxanes comprising organic blocks of the polyoxyalkylene type. Such copolymers are described in. for example. PLASTIC FOAMSQ. C. J. BENNING, Vol. 2, pages 350-325. The proportion of surface-active agent generally represents 0.1 to 2% by weight of the prepolymer (P). Suitable pulverulent fillers, which usually represent 5 to 50% ofthe weight of the prepolymer, include pyrogenic silicas, crude ground silicas, quartz. alumina. titanium oxide, talc, kaolin, mica, calcium carbonate graphite and carbon black.  
  It is also possible to use adjuvants which enable the density of the cellular polymers to be reduced. Examples of such adjuvants include pore-forming agents such as azodicarbonamide and the products mentioned in volume 2, pages 294 to 320 of BENNING loc. cit. The proportion of pore-forming agent can be as much as 10% of the weight of the prepolymer (P).  
  Adjuvants which make it possible to increase the hardness, the mechanical properties or the heat stability of the multicellular polymers can also be mixed with the prepolymer. A particular example of such an adju vant is boric anhydride which, generally used in an amount from 1 to 30% of the weight of the prepolymer. increases the heat-resistance and the flameproof character of the polymers.  
 Before heating the prepolymer to convert it into a cellular polymer, it can be advantageous to effect a preagglomeration of the prepolymer and the adjuvants, optionally in the presence of a liquid such as water, especially when the prepolymer is in powder form and no liquid adjuvant is used.  
  The multicellular polymers of this invention possess,&#34; even for apparent densities as low as 0.1, good mechanical properties, very great inertness to solvents and to chemical agents and excellent resistance to&#39; heat stresses and to fire.  
  Because of these properties, the cellular polymers of this invention are of value in many fields of industry. By way of example, they can be used for the manufacture of insulators in the form of plates or tubes, for example for the building industry and for the aeronautical and space industries.  
  The following Examples further illustrate the present invention. In these Examples, the compressive strength is measured in accordance with ASTM Standard SpecificationD 1621 -59 T.  
 EXAMPLE 1 a. A vesselcontaining a mixture of 150g of bis-(4- benzylideneaminophenyl)-rnethane, 78.4 g of maleic anhydride and 2 g of 2,4-dihydrox-y-azobeniene is immersed in a bath kept at 175C. As soon as the temperature of the mixture reaches 160C, heating isstopped and the mixture is kept at l70175C for 15 minutes.  
  During the operation. 4.7 g of carbon dioxide are evolved. After cooling to 25C, the mass if finely ground and is washed twice with benzene. After drying, 173 g of a prepolymer powder (P,) are thus obtained.  
  .b.-50 g of the powder (p, 1, 2 g of azodicarbonamide, 2 g of water and 0.4 g of a copolymer possessing dimethylpolysiloxane-polyoxyalkylene blocks. the organic blocks of which are terminated by OH groups, prepared in accordance with the technique described in French Pat. No. 1.175.305, as surface-active agent. are mixed intimately;  
 w 16 g ofthis mixture. are compressed for.5 minutes under a pressure of 50 bars to give a disc of height 8.2 mm and diameter. 50 mm. This disc is placed in a heatinsulated ring (internal diameter: 60 mm and height: 45 mm) between two aluminium plates on which there is placed. on the side of the ring, a woven glass fabric with an open mesh so as to permit the escape of air and, where appropriate, volatile products. The whole is placed between the platens of va press preheated to 270C and is left at this temperature for 30 minutes.  
  A multicellular polymer is obtained in the form of a cylinder which possesses a skin and aneapparent density of0.ll. i  
  The compressive strength of the polymer is 9.5 leg/cmv t&#39;ora deformation of 10%.  
  e. 18g of the powder (P 0.4 gof az odicarbonamide, 0.4 g of the surface-active. agent described under (b), 1g of water, and 1 g of maleic anhydride are mixed intimately.  
 &#39; Following the procedure described under (-b), a polymer with a cellular structure, of apparent density 0.14 isobtained in the form of a cylinder of diameter 60 mm and height 45 mm. The compressive strength is 7.2 k&#39;g/c&#39;m for deformation.  
  d. 50 g of the powder (P,), 12 g of boric anhydride, 2.5 g of azodicarbonamide and 2.5 g of water are mixed intimately. A disc is produced by compression, which, when heated under the conditions described under (b), yields a cellular polymer of apparent density 0.16, the compressive strength of which is 1 1.0 kg/cm for a deformation of 10%.  
  e. 17.5 g of the powder (P 2.5 g of boric anhydride and 0.5 g of the surface-active agent described under (b)&#39;are mixed intimately. A disc is formed&#39;by compression, which, when heated for 45 minutes at 275C (in accordance with the technique described under b) yields a cellular polymer of apparent density 0.15. lts compressive strength is 5 kg/cm for a deformation of 10%.  
  Exposure at 250C for a long period does not change the appearance of the polymer. The weight losses are as follows: r i  
 after 216 hours at 250C: 7.5% and after 300 hours at 250C: 10%.  
 EXAMPLE 2 a. 198 g of 4,4&#39;-diamino-diphenylmethan&#39;e are added,  
 over 15 minutes, with stirring, to 1,000 g of benzalde- 8 hyde. After stirring for 30 minutes, 400 &#39;cm of benzene are added. The water-benzene azeotrope is then distilled under apressure of 145 mm Hg., followed by the residual benzen &#39;e. I  
  200 of maleic&#39; anhydride. and 5 g,of 2,4-d ihydroxyazobenzene are incorporated into the solution of bis- (4 benzylideneaminophenyl)-methane obtained, and then the reaction mixture &#39;is heatedat 172-174C for 2 hours 30 minutes. During the heating, 17.9 g of car; bon dioxide are collected. 730 of benzaldehyde are then removed by distillation under 20 mm Hg., and the residual solid mass is then treated as stated in Example (l-a). After drying, 398 g ofa prepolymer powder (P are thus obtained.  
  b. 50 g of the powder (P 10 g of boric anhydride, 3 g of water, 2.5 g of azodicarbonamide and 2 g of the surface-active agent described under lb are mixed intimately. Following the procedure described under lb, a cellular polymer of apparent density 0.20 is produced.  
 EXAMPLE 3 a. The procedure described in Example 1a is followed, but starting from 135.5 g of bis- By compressing a portion of this mixture, a disc is produced which, when heatedat 270-280C, in accordance with the technique described under 1b, yields a cellular polymer of apparent density 0.17.  
 l c. A cylindrical piece ofdiameter 40 mm, manufactured according to (b) is placed 8 cm above the flame of a blow-pipe (900-1,000C) for 2 hours. The temperature on the upper face of the cylinder remained at L 5C for the entire duration of the operation. It is found, at the end of the experiment, that the upper face of the cylinder is unaffected.  
 EXAMPLE 4 a. An operation similar-to that of Example &#34;1 is carried out, starting from 103 g of bis-(4- salicylidenearninophenyl)-methane, 2 g of 2,4-dihydroxy-azobenzene and 102 g of maleic anhydride. During the test, 4.36 &#34;g of carbon dioxide are collected. After grinding and washing in the presence of benzene, followed by drying, 133.3 g (P,,) are isolated.  
  b. 50 g of the powder (P 2.5g of azodicarbonamide, 2.5 g of water and &#39;1 g of the surface-active agent used in Example &#39;lbare mixed intimately. A disc of material is produced from 20 g of this mixture, under the conditions described in Ex&#39;ample&#39;lb. On being heated at 270-280C for 30 minutes, this disc gives rise to a multicel1ular polymer with an apparent density of 0.18. Y  
 EXAMPLE 5 A mixture consisting of 31.7 g of 2,6-bis- (salicylideneamino)-pyridine,&#39; 0.3 g of 2,4-dihydroxyazobenzene and 20 g of maleic anhydride is kept at of a prepolymer powder 180C for minutes. During the operation. 2.55 g of a carbon dioxide are collected. The reaction mixture is treated as indicated in Example lb, and 48.6 g of a powder (P are collected.  
  By working this powder .under the conditions described in Example lb (same adju ants, same proportions), a cellular polymer of apparent density 0.15 is prepared.  
 EXAMPLE 6 8.95 g of an intimate mixture of maleic anhydride (3.45 g) and bis-(4-furfurylideneaminophenyl)- methane (5.5 g). finely ground beforehand, are introduced into an aluminium cylinder (diameter 7 cm and height 6.5 cm). The cylinder is placed in an oven preheated to 200C for 1 hour. The foam obtained has an apparent density of 0.1.  
 We claim:  
  1. Process for preparing a multicellular polymer which comprises heating, at a temperature from 100 to 300C, maleic anhydride and a polyaldimine of the general formula A (N CH G),,  
 in which G represents a monovalent organic radical which contains less than carbon atoms and at least one aromatic or heterocyclic ring in the oz-position to the imine group in formula (l), A represents an organic radical of valency n which contains less than 50 carbon atoms and at least one aromatic or heterocyclic ring in the a-position to the imine group in formula (I), and n represents a number at least equal to 2.  
  2. Process according to claim 1, in which 0.5 to 5 mols of maleic anhydride per imine group in the polyaldimine are employed.  
  3. Process according to claim 2, in which 0.7 to 3 mols of maleic anhydride per imine group in the polyaldimine are employed.  
  4. Process according to claim 1, in which the polyadimine is a bis-aldimine.  
  5. Process according to claim 1, in which at least one of A and G, contains at least one phenyl, pyridyl or furane ring.  
  6. Process according to claim 5, in which G represents a phenyl, a-hydroxyphenyl or 4-furfuryl radical.  
  7. Process according to claim I, in which A represents a 2,6-pyridyl radical or a diphenylenemethane radical.  
 8. Process according to claim 1, which comprises a. preparing a prepolymer by heating the maleic anhydride and polyaldimine at a temperature from 100 to 220C and b. heating this prepolymer at a temperature from 200 to 300C until the desired cellular structure is produced.  
  9. Process according to claim 8, in which, step (a) is carried out for 5 minutes to 3 hours.  
  10. Process according to claim 8, in which at least one of a surface-active agent, pulverulent filler, poreforming agent and a heat-stabilising agent is incorporated.  
 ll. A multicellular polymer obtained by:  
 a. preparing a prepolymer by heating maleic anhydride and a polyaldimine of the general formula:  
 A (N =CH *0) in which G represents a phenyl, napthyl, pyridyl, py-  
 rimidinyl, furyl, thienyl. benzo[b]thienyl. thienyl. indolyl. triazolyl or 3,4-methylenedioxy-phenyl radical or a several said radicals connected to one another directly or via a O, S, NH, COO, CONH, SO N=N, N =N(O), CO- ora linear or branched alkylene or alkenylene group with up to 8 carbon atoms. A represents a phenylene radical. a radical of the formula atoms, consisting of phenylene radicals bonded to N=N( O), an alkylene group with l to 3 carbon atoms,  
 ti ea or and;  
 wherein R represents a hydrogen atom, an alkyl radical with l to 4 carbon atoms a phenyl radical or a cyclohexyl radical, and X represents an alkylene radical with less than 13 carbon atoms or a 3 to 5 valent naphthalene, pyridine, triazine or benzene radical or a 3 to 5 valent radical consisting of several benzene nuclei bonded to one another by a O, S, SO NR N=N,  
 HXNHCO-, N=N O). group with l to 3 carbon atoms,  
 an alkylene V V as a ljlijl -ij NII an n represents a number at least equal to 2 at a temperature from to 200C, and  
 h. heating this prepolymer at a temperature from 200t0 300C so as to produce the desired cellular structure.