Linear polyimides prepared from bis-imides, tertiary phosphines and dialdehydes
United States Patent [191 Balme [4 1 Apr.1, 1975 1 LINEAR POLYIMIDES PREPARED FROM BIS-IMIDES, TERTIARY PHOSPHINES AND DIALDEHYDES [75] Inventor: Maurice Balme,  
 Sainte-Fov-les-Lyon, France [73] Assignee: Rhone-Poulence S.A., Paris, France [22] Filed: Nov. 6, 1973 [2]] Appl. No.: 413,245  
 [30] Foreign Application Priority Data Nov. 9, 1972 France 72.39730 [52] U.S. Cl 260/47 UA, ll7/l32 B, 117/161 N, 117/161 P, 260/33.4 P, 260/47 CZ, 260/49,  
 [51] Int. Cl C08g 20/32 [58] Field of Search 260/47 CZ, 67.5, 78 UA, 260/47 UA, 326 R, 49, 63 N, 64, 51, 54  
 [56] References Cited UNITED STATES PATENTS 3,737,412 6/1973 Wildi 26()/67.5  
 Primary Examiner-Lester L. Lee Attorney, Agent, or FirmStevens, Davis, Miller &amp; Mosher [57] ABSTRACT The invention provides a linear polyimide which corresponds essentially to the general formula:  
 A represents a divalent organic radical with 2 to 30 carbon atoms, R represents a simple valency bond or a divalent organic radical with l to 24 carbon atoms,  
 a represents a number at least equal to l and T and T, which may be identical or different, each represents either a radical of the formula:  
  l 1 CH2 co in which Y represents a hydrocarbon radical with 4 to 12 carbon atoms, and each of the symbols Y, which may be identical or different, represents a hydrocarbon radical with l to 12 carbon atoms or Y and Y together form a divalent hydrocarbon radical with 2 to 12 carbon atoms; or a maleimido radical of the formula:  
 CH C  
 or an isomaleimido radical of the formula:  
 or a radical of the formula:  
 in which R is defined above. These polyimides are useful as, for example, films, filaments, laminates and moulded articles.  
 12 Claims, No Drawings LINEAR POLYIMIDES PREPARED FROM BlS-lMlDES, TERTIARY PHOSPHINES AND DIALDEHYDES The present invention relates to linear polyimides.  
  Linear polymers which are blocked by maleimido groups and which have the recurring unit of the formula:  
 in which 0 represents a phenylene or hexamethylene radical have already been described [See Stille &amp; Anyos J. Polym. Sci. A, vol. 2, page i487 (1964)]. These polymers were obtained by heating the bismaleimide corresponding to the radical Q with benzalazine. The authors state that the viscosity of the poly- CO- CH CH-CO in which:  
 A represents a divalent organic radical carbon atoms,  
 R represents a simple valency bond or a divalent organic radical with I to 24 carbon atoms,  
 a represents a number at least equal to l and T and T, which may be identical or different, each represents a radical of the formula with 2 to 30 c co  \N- 11,-.- co  
 in which the symbol Y represents a hydrocarbon radical with 4 to 12 carbon atoms, and each of the symbols Y&#39;, which may be identical or different, represents a hydrocarbon radical with l to l2 carbon atoms, or, taken together, form a divalent hydrocarbon radical with 2 to 12 carbon atoms, or a maleimido radical of formula:  
 cnco CH-CO or an isomaleimido radical of formula:  
 LB C  
 CH-CO or a radical of the formula:  
 CO- C=CH-RCHO in which R has the meaning given above.  
  In formula (I), the symbol A can, for example, denote an alkylene radical having less than 13 carbon 5 atoms, a phenylene radical, a cyclohexylene radical,  
 one of the radicals of the formulae:  
 wherein n represents an integer from I to 3, a divalent radical with 12 to 30 carbon atoms, consisting of phenylene or cyclohexylene radicals bonded to one another by a simple valency bond or by an inert atom or group Continued N i n 7 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. The various phenylene or cyclohexylene radicals can be substituted by, for example, methyl groups.  
  The symbols Y and Y can, for example, represent linear or branched alkyl groups with l to 12 carbon atoms, a phenylalkyl group with 7 to l 1 carbon atoms, a cycloalkyl or cycloalkenyl group with to 8 carbon atoms, a phenyl group, an alkylphenyl group with 7 to l l carbon atoms or a naphthyl group. Moreover, the phenyl and naphthyl radicals can optionally be substituted by atoms such as fluorine, chlorine or bromine or by groups such as alkoxy groups with l to 4 carbon atoms.  
  The radicals which the symbol R can represent can be aliphatic, alicyclic, aromatic, carbocyclic or heterocyclic. These radicals can be substituted by atoms such as F, Cl and Br or by groups such as N0 CH OCH or phenolic hydroxyl. More particularly these radicals can be alkylene radicals with l to 12 carbon atoms,  
 alkenylene radicals with 2 to 12 carbon atoms, alkinyin which the symbol A has the meaning given above and the symbol Z represents a maleimido or isomaleimido radical, into contact with a tertiary phosphine of the general formula:  
 in which the symbols Y and Y have the meaning given above and a dialdchyde of the general formula:  
 OHC R CHO in which the symbol R is defined as above.  
  Specific examples of bis-maleimides of formula (ll) include: N,N&#39;-ethylene-bis-maleimide, N,N- hexamethylene-bis-maleimide, N,N-meta-phenylenebismaleimide, N,N&#39;para-phenylene-bis-maleimide, N,-  
 isomaleimide, N,N &#39;-4,4-diphenylmethane-bisisomaleimide, N,N &#39;-4,4-(diphenyl ether)-bisisomaleimide, N,N-4,4&#39;-diphenylsulphone-bisisomaleimide, N,N &#39;-4,4dicyclohexylmethanebisisomaleimide, N,N-oz,a&#39;-4,4-dimethylene-cyclohexanebis-isomaleimide, N,N-meta-xylylene-bisisomaleimide, N,N -para-xylylene-bis-isomaleimide,  
 N,N&#39;-4,4&#39;-( l l diphenyl-cyclohexane )-bisisomaleimide, N,N&#39;-4,4-( l, l -diphenyl-propane)-bis&#39;- isomaleimide, N,N&#39;-4,4&#39;-( 1,1,1-triphenyl-ethane)-bisisomaleimide, N,N&#39;-4,4&#39;-triphenylmethane-bisisomaleimide, N,N-3,5-( l,2,4triazole)- bisisomaleimide and N,N-4,4&#39;-benzophenone-bisisomaleimide.  
  These bis-isoimides can be prepared by applying the method described in, for example U.S. Pat. No. 3,035,065.  
 Amongst the tertiary phosphines which can be used there may be mentioned: tributyl-phosphine, tri-(tbutyl)-phosphine, propyl-dibutylphosphine, trioctyl-, phosphine, tridodecyl-phosphine, triphenyl-phosphine, tri-(p-tolyl)-phosphine, tri-(p-butyl-pheny1)- phosphine, tri-(B-naphthyl)-phosphine, tricyclohexylphosphine, tris-(p-ethoxyphenyl)-phosphine, tris-(ochlorophenyl )-phosphine, tris-( p-bromophenyl phosphine, tris-( m-fluorophenyl)-phosphine, tribenzylphosphine, dimethylphenyl-phosphine, diethylphenylphosphine, dihexylphenyl-phosphine, methyldiphenylphosphine, vinyldiphenyl-phosphine, diallylphenylphosphine and cyclotetramethylenephenyl-phosphine. The preparation of these phosphines is, of course, well known.  
  Amongst the dialdehydes which correspond to the general formula (IV), there may be mentioned: saturated or unsaturated aliphatic dialdehydes such as glyoxal, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, pimelaldehyde, suberaldehyde, azelaldehydc, scbacaldchyde, dodecanedial, l,4-cyclohexaneproximately 0.5 mol of dialdehyde per mol of tertiary phosphine. If, on the other hand, 1 mol of tertiary phosphine and 0.5 mol of dialdehyde per mol of bisimide dihCXllflal. f m r l hy e. ca c yde. Ciare used, a polyimide of the formula:  
 CH-CQ\ /CO- =CHR-CH=C-CO\ /CO-CH f-A-N N-A-N l CH-CO \co n cH co cotraconaldehyde. mesaconaldehydc, 2-pentene-dial, 2- hexcne-dial, 3-hexene-dial, butyne-dial and butal,3- dien-S-yl-2-heptene-dial andaromatic dialdehydes such as: phthalaldehyde, isophthalaldehyde, terephthalaldehyde, naphthalene-l,Z-dicarbaldehyde, naphthalenel ,3-dicarbaldehyde, napthalene-l ,4- dicarbaldchyde, naphthalene-l ,6-dicarbaldehyde, naphthalenel ,8-dicarbaldehyde, naphthalene-2,6- dicarbaldehyde, naphthalene-l ,7-dicarbaldehyde, naphthalene-2,5-dicarbaldehyde, diphenyl-4,4&#39;- dicarbaldehyde, diphenyl-3,3 -dicarbaldehyde, diphenyl-2,3-dicarbaldehyde, diphenyl-2,4- dicarbaldehyde, diphenyl-2,6-dicarbaldehyde, (pterphenyl )-2,2 &#39;-dicarbaldehyde, (o-te rphenyl )-2 ,3- dicarbaldehyde, (m-terphenyl)-2,6&#39;-dicarbaldehyde, (p-terphenyl l ,4&#39;-dicarbaldehyde, (p-terphenyl 4,4&#34;-dicarbaldehyde, (p-terphenyl)-3,3&#34;- dicarbaldehyde, (o-terphenyl)-2,6-dicarbaldehyde, 5,- 5 &#39;-methylene-bis-salicylaldehyde, 4,4-diformyl- (diphenylether),4,4-diformyl-diphenylamine, 4,4- diformyl-( diphenyl thioether 4,4&#39;-diformyldiphenylmethane, 4,4 diformyldiphenylisopropane, 4,4&#39;-diformyl-diphenylsulphonc, 4,4&#39;-diformylbenzophenone, 4,4&#39;-diformyl-azobenzene, phenyl 4,4&#39;-diformyl-benzoate, 4,4-diformyl-azoxybenzene and 2,6-diformyl-pyridine.  
  Nitromalonaldehyde and dibromosuccinaldehyde may also be mentioned. When the dialdehydes exist in polymeric form, these can be used advantageously in place of the corresponding monomer.  
  It is to be understood that it is possible to use a mixture of bis-imides and/or bis-isoimides of the formula (ll) and that, likewise, mixtures of the dialdehydes of formula (IV) and of the phosphines of formula (Ill) can also be used.  
  The polyimides according to this invention can be prepared by intimately mixing the bis-maleimide and- /or bis-isomaleimide, the phosphine and the dialdehyde. 1  
  The temperature can vary to a certain extent depending on the nature of the reagents employed but, as a general rule, the reaction can be initiated between and 80C; this reaction is exothermic and-can be continued advantageously by keeping the temperature within this range.  
  1 to 5 mols of tertiary phosphine per mol of bis-imide are suitably used, and generally from I to 2.5 mols. The dialdehyde is suitablye&#39;mployed in an amount of at least 0.5 mol per mol of bis-imido; .the upper limit is not critical and can range up to of the order of 10 mols per mol of bis-imide. I g  
  The exact proportions of reagents are obviously a function of the molecular weight of the polyimide which it is desired to produce and of the nature of the terminal groups fixed beforehand.  
  Thus polymers of high molecular weight can be pro duced whenthe quantities of reagents used are such that there are approximately 2 mols of tertiary phosphine per mol of bis-imide and, at the same time apwherein the symbols A and R are as defined above is obtained when the bis-imide is a bis-maleimide. If now, the number of mols of tertiary phosphine is greater than I and less than 2, per mol of bis-imide, polyimides terminated by imido groups are obtained when the quantity of aldehyde is at least equal to 0.5 mol of dialdehyde per mol of phosphine. Finally, in order to produce polyimides with terminal aldehyde groups, atleast 2 mols of tertiary phosphine per mol of bis-imide are used, and simultaneously, more than 0.5 mol of dialdehyde per mol of tertiary phosphine.  
  The reaction is generally carried out in a polar organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulphoxide, phenol, cresol, N-methylcaprolactam, diethylformamide and N-acetylcaprolactam. Cresol is preferably used.  
  In practice, the polymers of this invention are prepared by dissolving two of the reagents in the chosen solvent and then introducing the third into the solution obtained.  
  According to a particular embodiment, it is possible, in a first stage, to prepare an addition product possessing groups of formula:  
 from the tertiary phosphine and the bis-imide (ll) in a diluent such as acetic acid, benzene or one of the solvents mentioned above. ln a second stage, after the addition product has been isolated, it is reacted with the dialdehyde under the conditions given above for the preparation of the polymers.  
  Whatever method is adopted, the oxide of the tertiary phosphine employed is formed in addition to the polyimide. The polyimide can be isolated from the final mixture by treatment with a solvent for the phosphine oxide in which the polyimide is insoluble or practically insoluble. Amongst the solvents which can be used, there may be mentioned in particular aromatic hydrocarbons such as benzene, toluene and xylene, organic acids such as acetic and butyric acid, ether, chloroform, acetone, methyl ethyl ketone and lower aliphatic alcohols such as methanol, ethanol and pentanol.  
  The polyimides according to the invention are soluble in cresol, and less soluble or insoluble in the other polar solvents. In the form of solutions, they can be used for coating metal surfaces or woven or non-woven fibrous materials. The products of high molecular weight, the inherent viscosity of which can be greater than 200 cm lg, are very suitable for the manufacture of yarns, films and moulded objects.  
  The polyimides with terminal maleimidol or isomaleimidol groups are very suitable for the production of moulded objects, where appropriate by compression. They can be cured by heating at temperatures of the order of 150 to 350C, optionally whilst they are being shaped.  
  The polyimides according to the present invention are of especial value in the fields of industry which require materials possessing good mechanical and electrical properties, an excellent impact strength and great chemical inertness at temperatures of 200 to 300C.  
  The following Examples, further illustrate the present invention. In these Examples, the properties of the products were determined according to the following standard specifications:  
 flexural tests: ASTM Standard Specification D 79063, tensile strength tests: ASTM Standard Specification D 882-61 T and impact strength tests: D-YNSTAT Pendulum; 7 mm distance between the point where the test piece is fixed and the point of impact.  
  The viscosity of the polymers was determined at 25C using 0.5% strength solutions in o-cresol.  
 EXAMPLE 1 a. 54.58 g of N,N&#39;-4,4&#39;-diphenylmethane-bismaleimide and 20.89 g of terephthalaldehyde are dissolved in 425 g of cresol and 80.1 g of triphenylphosphine are added to the solution thus obtained. The temperature of the reaction mixture rises from 21 to 60C. The mixture is cooled to 25C and is then kept under these conditions for 1 /2 hours. The viscous solution (reaction mixture) is then run into 2 litres of methanol. A precipitate is obtained which is filtered off and then washed continuously with boiling methanol for 15 hours.  
  The precipitate is then dried at 40C/30 mm Hg to constant weight.  
  68.6 g ofa light yellow polymer consisting of a plurality of recurring units:  
 --CH= C CO CH CO CO C13 are obtained.  
  It is a film-forming polymer which softens at about 250270C and which is soluble in concentrated sulphuric acid, cresol and o-chlorophenol and partially soluble in N-methylpyrrolidone. lts inherent viscosity, measured at 25C, is 176 cm lg.  
  Thermogravimetric analysis of the polymer, in pow-. der form, under nitrogen (rate at which the temperature is raised: C/minute) shows that there is no loss This film has a tensile breaking strength of 1,300 kg/cm&#34; and an elongation of 7% at 25C.--At 250C its tensile breaking strength is still 350 kg/em for an elongation of 25% c. Preparation of a moulded object.  
  10.1 g of the polymer are placed in a cylindrical mould of diameter 7.6 cm. The mould is heated to 300C between the platens of a preheated press and a pressure of 25 bars is applied.  
  The whole is maintained (under these conditions) for 1 hour and then the object is released from the mould and subjected to a supplementary heat treatment for 48 hours at 250C.  
  Several samples are subjected to a heat exposure at 300C, and the characteristics which they possess as a function of the duration of the exposure are measured.  
 The results are given in Table l d. Preparation of a laminate. A solution of 3.45 g of polymer in g of cresol is used to coat a sample of glass fabric (1 l X 18 cm) of the satin type, weighing 320 g/m in a first stage, each face of the fabric is coated with the said solution and coc= is then dried at C for 15 minutes under a stream of air, and the coatings are repeated until the entire solution is exhausted. The last coating is completed by 30 minutes of drying at 120C.  
 18 strips of 5.5 X 4.5 cm are cutout of this presol is wet spun (coagulating bath: acetone) with a spinneret containing 13 holes of diameter 15 X l0 mm. After the filament obtained has been dried, it is stretched in a ratio of 1.55 at 320C,  
  A multifilament yarn of gauge 316 d texand tensile breaking strength 0.57 g/dtex for ginfelongation of 5% is obtained. The secant breaking modulus is 12 g/dtex and Young modulus is 16 g/dtex.  
 EXAMPLE 2 3.58 g of N,N&#39;-4,4&#39;-diphenylmethane-bisisomaleimide and 1.36 g of terephthalaldehyde are dissolved in 50 cm of cresol. 5.44 g of triphenylphosphine are added to the solution obtained and the whole is left to stand overnight.  
  The polymer solution obtained is run into methanol. A precipitate is obtained which is filtered off, washed with methanol and dried at 50C/30 mm Hg to constant weight. 4.4 g of polymer, consisting of a plurality of recurring units of the formula:  
  A light brown, transparent and flexible film, of thickness 4.3 X l&#34;&#39;-mm, is produced with a solution of this polymer. The film has a tensile breaking strength of 910 ltg/cm for an elongation of at C.  
 EXAMPLE 4 3.58 g of N,N&#39;-4,4-diphenylmethane-bis-malcimide and 0.56 g of 5,5-methylene-his-salicylaldehyde are 10 dissolved in cm of dimethylformamide and then 5.4  
 g of triphenyl-phosphine are added. The solution is stirred for 16 hours. The viscous solution obtained is poured into methanol. After washing the precipitate with methanol and drying it at C/30 mm Hg, 5.6 g  
 l o tot 1 CH CO CH are thus isolated in the form of a light yellow powder.  
 lts inherent viscosity is cm/g.  
 of polymer, consisting of a plurality of recurring units 25 of the formula:  
 EXAMPLE 3 -CH=C CO CH CO CO CH are thus obtained.  
  The inherent viscosity of this polymer is 152 cm /g. lts softening point is about 270C. Thermogravimetric analysis of this powder, under nitrogen (rate at which the temperature is raised: 5C/minute) reveals a loss in weight of 571 at a temperature of 430C.  
 are obtained.  
  This polymer is in the form of a pink powder which is soluble in cresol and has an inherent viscosity of 92 em/g. Thermogravimetric analysis of this powder, under nitrogen (rate at which the temperature is raised: 5C/minute) shows a loss in weight of 5% at 400C.  
  A transparent red-orange film of thickness 3.2 X 10 mm, is produced from a solution of this polymer in eresol. The tensile breaking strength of the film is 740 45 kg/cm for an elongation of 2.3%.  
 EXAMPLE 5 The experiment described in Example 2 is repeated, but using 3.82 g of N,N&#39;-4,4&#39;-(diphenyl ether)-bismale imide, 1.35 g of terephthalaldehyde and 5.4 g of triphenyl-phosphine. 5g of polymer with the recurring unit:  
 Co-C=cH- are isolated.  
 Its inherent viscosity is 104 cm /g. Following the procedure of Example lb, a film of thickness 2 X l mm is produced. lts tensile breaking and a radical of the formula: strength is 915 kg/cm for an elongation of 5.5%.  
  EXAMPLE 6 CO C CH R CHO The experiment described in Example 2 is repeated, -N I (T but using 3.58 g of N,N-4,4-diphenylmethane-bisco c11 maleimido, 3.32 g of diethylphenyl-phosphine and 1.35 g of terephthalaldehyde. 4.5 g of polymer are isolated, the recurring unit of which is the same as that of the in which R is as defined above. polymer described in Example 1, and the inherent vis- 2. A polyimide according to claim 1 in which A repcosity of which is 106 em/g. resents a phenylene radical or several phenylene radicals bonded to one another by a simple valency bond EXAMPLE 7 or by an inert atom or group. Following the procedure described in Example 6, but 3. A polyimide according to claim 1 in which R repreplacing the diethylphenyl-phosphine by 4.04 g of triresents a radical of the formula:  
  O OH V or -(CH butyl phosphine, 4.84 g of a polymer are obtained, the 4. A process for the preparation of a polyimide as derecurring unit of which is the same as that of the polyfined in claim 1 which comprises reacting a bisimide of mer described in Example 1. its inherent viscosity is the general formula: I06 cm lg. A film of thickness 3 X 10 mm was pro- Z A Z duced in accordance with the procedure of Example lb. Its tensile breaking stren th is 520 k f an in which Z represents a maleimido or isomaleimido elongation of 2.8%. radical, with a tertiary phosphine of the general for- I claim: mula:  
  l. A linear polyimide which consists essentially of a PY(YI)2 product of the general formula:  
 CO C CH-R-CH=C CO A-N A-T&#39; a CO CH CH CO in which: in which Y and Y are as defined in claim 1 and with A represents a divalent organic radical with 2 to 40 a dialdehyde of the general formula:  
  carbon atoms, R represents a simple valency bond or a divalent or- OHC R CH0 ganic radical with l to 24 carbon atoms, in which R is as defined in claim 1.  
 a represents a number at least equal to l and 5. Process according to claim 4 in which the reaction T and T, which may be identical or different, each is carried out in a polar organic solvent.  
 represents a radical selected from a radical of the 6. Process according to claim 4 in which about 1 to formula: 5 mols of tertiary phosphine and at least about 0.5 mol of dialdehyde are used per mol of bis-imide.  
  P C C0 7. Process accordin to claim 6 in which about 2 mols of tertiary phosphine and about 0.5 mol of dialdehyde 2 co are used per mol of bis-imide.  
  8. Process according to claim 6 in which about 1 mol in which Y represents a hydrogen radical with 4 to 12 of tertiary phosphine and about 0.5 mol of dialdehyde carbon atoms, and&#39;each of the symbols Y, which may are used per mol of bis-imide. be identical or different, represents a hydrocarbon rad- 9. Process according to claim 4 which comprises first ical with l to 12 carbon atoms or Y and Y together reacting the tertiary phosphine and the bis-imide to form a divalent hydrocarbon radical with 2 to 12 cargive an addition product possessing groups of the forbon atoms; a maleimido radical of the formula: mula:  
  iH CO\N Y(Y) P c co CH co 2 ncH co an isomaleimido radical of the formula. 2  
 ca co I ll 0 and then reacting the addition product with the dialde- CH eo hyde&#39; 10. A moulded article obtained by curing a linear polyimide as defined in claim 1.  
  11. A polyimide as defined in claim 1 in the form of a film or filament.  
  12. A polyimide according to claim 1 in which A represents an alkylene radical having less than 13 carbon atoms, a phenylene radical, a cyclohexylene radical, 5 one of the radicals of the formulae:  
 Q r @i &#39;t i wherein n represents an integer from I to 3, a divalent radical with 12 to 30 carbon atoms, consisting of phenylene or cyclohexylene radicals bonded to one another by a simple valency bond or by an inert atom or group selected from O, -S, an alkylene group with l 20 to 3 carbon atoms, CO, SO NR,, N=N, CONH, COO, P(O)R,-,  
 kyl group with 7 to l 1 carbon atoms, a cycloalkyl or cycloalkenyl group with 5 to 8 carbon atoms, a phenyl group, an alkylphenyl group with 7 to ll carbon atoms or a naphthyl group, said phenyl and naphthyl radicals optionally being substituted by fluorine, chlorine or bromine atoms or by alkoxy R N-I ,N-N  
 oi o i 1? will 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 l3 carbon atoms, said phenylene and cyclohexylene radicals optionally being substituted by methyl groups,  
 each of Y and Y represents a linear or branched 50 alkyl group with l to 12 carbon atoms, a phenylalbenzene rings bonded to one another by an inert atom or group selected from -O, --S, NH N(CH3) 2 s &#39;C(CH3)2 SO C(O)O-, CONH-, N=N, and -N=N( O)-, said radicals optionally being substituted by a fluorine, chlorine or bromine atom or a nitro, methyl, methoxy or phenolic hydroxyl group.