ISOCYANATE-DERIVED POLYMERS Introduction Organic diisocyanates and polymeric diisocyanates are widely used as monomers in reaction polymerization reactions (1,2). By far their widest use is in the manufacture of polyurethanes produced by an addition reaction from macroglycols and isocyanates. Modiﬁcation of polyurethanes (qv) by partial trimerization of the isocyanate group is used in the manufacture of rigid cellular plastics (polyurethane isocyanurate foams), and in the molding of solid cross-linked elastomers, using the RIM (reaction injection molding) process. Also, poly(urethane ureas) are produced by using a diamine extender or a macrodiamine as one of the comonomers. An example is the use of diethyltoluenediamine as an extender in the RIM of automotive parts. The use of amine monomers, which react very rapidly with isocyanates, increases the reaction rates in the corresponding molding processes. Also, diamine extender or macrodiamines are used in spray applications and in the manufacture of elastic Spandex ﬁbers. Poly(urethane isocyanurates) and poly(urethane ureas) are treated adequately under polyurethanes (qv), and they are not included in this article (see POLYURETHANES). The homopolymerization of diisocyanates is only useful for specialty diisocyanates, such as aliphatic 1,2- or 1,3-diisocyanates (3) and aromatic o-diisocyanates (4), which polymerize via cycloaddition processes. Anionic homopolymerization of monoisocyanates takes place by addition across the C N bond to form nylon-1 polymers. Polyamides are also obtained from diisocyanates and enamines or ketenaminals. This reaction proceeds by a [2 + 2] cycloaddition reaction with subsequent ring opening to form polyamides. [2 + 4] cycloaddition polymerization to form heterocyclic polymers is observed with carbonyl diisocyanate (5). Ring-opening polymerization occurs in the reaction of bis-epoxides Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
with aromatic diisocyanates to give poly(2-oxazolidinones). Vinyl isocyanates undergo addition polymerization across their vinyl group to give polyisocyanates with a very high isocyanate content, which are difﬁcult to handle. Copolymerization of vinyl isocyanate with other vinyl monomers allows the synthesis of useful polyisocyanates. Other difunctional nucleophiles, such as bis-mercaptans, hydrazine, dihydrazides, or dioximes, are also used as comonomers in polyaddition processes. Reaction polymerization reactions of isocyanates with suitable monomers can be performed in an extruder or in a RIM machine. In the latter reaction thermosets (cross-linked polymers) are produced. In an extruder usually linear polymers are manufactured. For example from methylene di-p-phenylene isocyanate (MDI), with some macroglycols and 1,4-butanediol as extenders, segmented polyurethane elastomers are produced in an extruder (6). However, linear condensation polymers are also produced in a vented extruder. For example from MDI, with macrodicarboxylic acids and dicarboxylic acids as extenders thermoplastic block copolyamide elastomers are produced. The by-product of the condensation reaction, carbon dioxide, is removed in the vented extruder. The polycondensation process can also be performed in solution. For example, MDI can be added to a solution of dicarboxylic acids in tetramethylene sulfone, with simultaneous removal of the carbon dioxide. Tetramethylene sulfone is the solvent of choice for solution polymerization of isocyanates (7). In addition to dicarboxylic acids trimellitic acid anhydride and benzophenonetetracarboxylic acid dianhydride (BTDA) are utilized as monomers for condensation polymers. With these monomers poly(amide imides) and poly(imides) are produced. The diisocyanate-derived commercial polycondensation products are listed in Table 1. Often it becomes necessary to lower the melting points of the derived polymers to allow processability. This is best achieved by using mixtures of the diisocyanates. For example, high temperature ﬁbers (PI 2080) are produced using 80% 2,4-TDI and 20% MDI and BTDA. Also, poly(amide imides) are modiﬁed with 20% isophthalic acid, and azelaic acid is often used as comonomer to reduce the crystallinity of the derived polymers. The utilization of comonomers Table 1. Condensation Polymers Derived from Diisocyanates and Suitable Monomers Isocyanate MDI
Polyester dicarboxylic acids, azelaic acid Isophthalic acid, azelaic acid
Trimellitic acid anhydride, isophthalic acid Benzophenonetetracarboxylic acid dianhydride —a
MDI, TDI MDI, TDI aA
Polymers Segmented poly(amide) elastomers Engineering thermoplastic poly(amides) Semicrystalline engineering plastic poly(amide) Copoly(amide imide) ﬁlms Poly(imide) ﬁbers and ﬁlms Poly(carbodiimide)
phospholene oxide catalyst is used in this polycondensation reaction.
allows the manufacture of numerous condensation polymers from diisocyanates. Segmented thermoplastic polyamide elastomers and some engineering thermoplastic polyamides are available from Dow, and Lenzing AG in Austria has produced PI 2080 ﬁbers. Oligomeric carbodiimides obtained from sterically hindered aromatic diisocyanates are used as stabilizers for polyesters, polyester-based polyurethanes, and polyether-based poly(urethane ureas) (8). The mechanism of stabilization of polyesters with oligomeric carbodiimides is based on the rapid reaction of carbodiimides with carboxylic acids generated in the hydrolysis of a polyester. Oligomeric carbodiimides can also function as chain-mending agents, preventing loss of molecular weight. The carbodiimide oligomers derived from sterically hindered aromatic diisocyanates are commercially available under the trade name Stabaxol from Rhein Chemie, a subsidiary of Bayer. Diisocyanates have also been used as monomers in the manufacture of thermally stable addition/condensation heterocyclic polymers. For example, reaction of dihydroxyaryl-dicarboxylic acid esters with diisocyanates produce poly(benzoxazindiones), which are useful as high performance wire coatings (9). The addition/condensation procedure was also used in the manufacture of poly(parabanic acid) from MDI and hydrogen cyanide (10). The use of the toxic hydrogen cyanide as monomer has prevented the commercialization of this useful polymer.
Monomers Diisocyanates are usually synthesized by phosgenation of the corresponding diamines. A tabulation of diisocyanates synthesized up to 1972 with pertinent methods of synthesis is available (11). Instead of the toxic phosgene gas, the liquid trichloromethyl chloroformate (diphosgene) (12) or the solid bistrichloromethyl carbonate (triphosgene) (13) can be used in the laboratory (see PHOSGENE). Also, oligomeric t-butylcarbonates are used to convert diamines into diisocyanates (14). The principal commercial aromatic isocyanates are toluene diisocyanate (TDI), polymeric isocyanate (PMDI), and its coproduct (MDI), but some specialty aromatic isocyanates and some aliphatic diisocyanates are also available. The latter are used in the manufacture of color stable polyurethanes. TDI is mainly produced as a 80:20 mixture of the 2,4- and the 2,6-isomers, but a 65:35 mixture of the same isomers, pure 2,4-TDI and a crude partially trimerized product, are also available. PMDI is a mixture of the MDI isomers and their higher oligomers, and MDI is mainly the 4,4 -diisocyanate. The principal commercially available isocyanates are listed in Table 2. Sulfonyl diisocyanates, such as m-phenylenedisulfonyl diisocyanate, are used as monomers for base-soluble polymers (15). 4-Isocyanatobenzenesulfonyl isocyanate can also be used as a monomer for base-soluble polymers. Reaction of p-toluenesulfonyl isocyanate with polymers having amide units in the backbone structure affords sulfonyl urea derivatives, which on hydrolysis with 1 N NaOH degrade the polymer to carboxylic acids (16). Likewise, polymerization of the monomer derived from acrylamide and p-toluenesulfonyl isocyanate affords a homopolymer, which on hydrolysis with 1 N NaOH affords poly(acrylic acid) (17).
Table 2. Major Aromatic Isocyanates Name
Boiling point, ◦ C/kPaa
Melting point, ◦ C
BASF, Bayer, Dow, Lyondell
BASF, Bayer, Dow
BASF, Bayer, Dow
convert kPa to mm Hg, divide by 7.5. to dark brown liquid.
Some aliphatic diisocyanates are also commercially available and they are listed in Table 3. Because of its relatively low boiling point HDI is currently only supplied in the form of derivatives (trimer, allophanate, biuret, etc) (18). Polymeric aliphatic isocyanates are obtained by copolymerization of 2-isocyanatoethyl methacrylate and styrene (19). Hydroxyalkyl acrylates and methacrylates are used as monomers to form hydroxyl group containing polymers, which are cross-linked with diisocyanates or blocked diisocyanates (20). The elusive parent diisocyanate, O C N N C O, is only stable at −75◦ C, and therefore it is not suitable as a monomer or comonomer in polymerization reactions (21). The market for polyurethanes in 2000 was of the order of 8 million tons per year. Therefore, diisocyanates and polymeric isocyanates are readily available at reasonable cost, and their use as monomers for other addition and condensation polymers is indicated. Blocked isocyanates can be used in the cross-linking of hydroxyl group containing polymers in the formulation of powder coatings. Blocking agents include phenol, caprolactam, ketoximes, and 3,5-dimethylpyrazole. The latter has the lowest dissociation temperature (22). Currently, virtually all automobiles produced use a blocked isocyanate in the formulation of cationic electrodeposition coatings (23). The [2 + 2] cycloadduct derived from MDI and diarylketenes is used as an initiator in the nylon RIM process (24).
Table 3. Major Aliphatic Isocyanates Name HDI TMHDI
Boiling point, ◦ C/kPaa 130/98 149/75
Structure OCN(CH2 )6 NCO OCNCH2 C(Me)2 CH2 CH(Me)CH2 CH2 NCO OCNCH2 CH(Me)CH2 C(Me)2 CH2 CH2 NCO
Producer Bayer Huels
convert kPa to mm Hg, divide by 7.5. of stereoisomers.
Also, isocyanates are used in the formulation of interpenetrating polymer networks (IPNs) (25). A polyester–polyurethane IPN with a ratio typical for sheet molding compound (SMC) is obtained from MDI, poly(caprolactone triol), an unsaturated polyester resin, and styrene monomer (26). IPNs are also reviewed under polyurethanes. Isocyanates in general are toxic chemicals and require great care in handling. Oral ingestion of substantial quantities of isocyanates can be tolerated by the human body, but acute symptoms may develop from inhalation of much smaller amounts. The suppliers Material Safety Data Sheets (MSDS) have to be consulted for the most current information on the safe handling of isocyanates.
Homopolymerization The anionic homopolymerization of monoisocyanates proceeds by addition across their C N double bond to give nylon-1 polymers (27). The reaction is generally conducted in dimethyl formamide (DMF) at −50◦ C, using sodium cyanide as the initiator. The infrared spectra of the nylon-1 homopolymers show a strong absorption band at 1700 cm − 1 (C O) and a band from 1280 to 1390 cm − 1 , indicating a disubstituted amide structure. The nylon-1 polymers obtained in the anionic polymerization reaction are listed in Table 4.
Table 4. Nylon-1 Polymers Obtained by Anionic Polymerization of Monoisocyanates Isocyanate
Vinyl Ethyl Allyl n-Propyl n-Butyl Isobutyl n-Amyl n-Hexyl n-Nonyl n-Undecyl n-Octadecyl 9-n-Decenyl Benzyl Phenyl m-Tolyl p-Tolyl p-Methoxyphenyl m-Dimethylaminophenyl a Temperature
48 10 70 75 18 67 85 100 70 2 75 50 28 60 25 35
Softening temp., a ◦ C
180 180 180 173 145 120 45 40 75
Melt temp., ◦ C 300 250 260–290 250 209 210–220 209 195 155 94 180–240 250 197 200 212–214 315
at which the polymer ﬁrst become plastic without sticking on a metal temperature-
The melt temperature of nylon-1 polymers depends on the legth of the side chain. The longer the side chain, the lower the melting point of the polymer. Steric requirements for the homopolymerization of monoisocyanates are severe. No homopolymers are obtained from isopropyl, cyclohexyl, o-methoxyphenyl, and αnaphthyl isocyanate. All alkyl nylon-1 polymers, with the exception of poly(allyl isocyanate), have a low order of crystallinity as shown by X-ray diffraction. However, the polymerization of n-butyl isocyanate and phenyl isocyanate in various solvents at −78◦ C, initiated by lithium or sodium alkyls, produces crystalline polymers (28). The mechanism of the sodium cyanide catalyzed polymerization in DMF involves intermediates having a spiro structure (29). The solution properties of alkyl poly(isocyanates) in a variety of solvents indicate an unusual chain stiffness similar to the α-helical poly(α-amino acids) (30). Optically active poly(isocyanates) are obtained by anionic polymerization of the optically active monomers (31). The temperature-dependent optical activity and circular dichroism suggest a large preference of one of the helical senses (32). Vinyl isocyanate is polymerized across the isocyanato group to give the nylon1 polymer 1. Treatment of 1 with azoisobutyronitrile and UV light at room temperature produces the ladder polymer 2 (33).
The double bond also survives in the homopolymerization of 2isocyanatoethyl methacrylate (34). The nylon-1 polymers are thermally not stable because unzipping occurs on heating, giving rise to the formation of monomers and cyclic trimers. The decomposition temperature of poly(alkyl isocyanates) is about 140◦ C. The main fragmentation products from alkyl nylon-1 polymers are the cyclic trimers, while aryl nylon-1 polymers afford mainly the monomers. The unzipping of the homopolymers can be prevented by using Cl3 TiOCH2 CF3 to stabilize the living ends of the homopolymer. When the homopolymers contain functional groups, CpTi(OCH2 CF3 )Cl2 can be used for this purpose (35). Poly(isocyanatocarboxylic acid esters) are obtained by conducting the polymerization at −40◦ C in toluene in the presence of n-butyllithium (36). Silylated isocyanato carboxylic acids are likewise polymerized, and the protecting group is removed using HF in ethanol (37). The rapid degradation of the nylon-1 polymers has been used to create raster images upon treatment with electron beams (38). Poly(isocyanates) with chiral azo chromophores can undergo photoisomerization of the chiral side chain. This reaction can be used as a switch, whose effect is ampliﬁed in the polymer backbone (39). A review article on nylon-1 polymers appeared in 1976 (40). The homopolymerization of aliphatic diisocyanates across their C N bond is limited to aliphatic 1,2- and 1,3-diisocyanates and cyclohexane-1,2 and 1,3diisocyanate (3). These diisocyanates are cyclopolymerized by an alternating, intermolecular–intramolecular propagation mechanism, using sodium cyanide as the catalyst. The cyclopolymers obtained in this manner are listed in Table 5. The cyclopolymerization of 1,2-diisocyanatodecane (3) in the presence of CpTiClN(CH3 )2 affords the cyclopolymer 4, which is soluble in many organic solvents (41).
Aromatic o-diisocyanates, such as 2,3-TDI, are also homopolymerized via a cycloaddition mechanism (4). The anionic homopolymerization of 2,4-TDI in DMF, using sodium cyanide as the initiator, occurs only through the nonhindered isocyanato group to give linear polymers containing free isocyanato groups (42). Homopolymers are also obtained by coupling polymerization of MDI, using samarium iodide to induce the electron transfer necessary to achieve the polymerization (43). Condensation homopolymerization of PMDI affords low density poly(carbodiimide) foams (see later under Condensation Polymers). 4-Vinylphenyl isocyanate also undergoes homopolymerization across the C C double bond. This reaction is conducted in toluene at 60◦ C, using 3 mol% of AIBN as the catalyst (44). Ruthenium catalysts are used to convert p-nitropolystyrene in the presence of methanol into the corresponding methyl carbamate, which is a masked polyisocyanate (45).
Table 5. Cyclopolymers Derived from 1,2- and 1,3-Diisocyanatoalkanes Mp, C
Inherent viscosity,a dL/g
a Concentration b In
0.5% w/v in tetrachloroethane/phenol (40/60). conc. sulfuric acid.
When a group reactive with the isocyanate group is attached to an aryl isocyanate A–B homopolymers are obtained. For example, from isocyanatophenols, polyurethanes are obtained. Substituents next to the isocyanate or hydroxyl group increase the stability of isocyanatophenols (46). Polyurethanes are also obtained from α,γ -aminoalcohols and di-t-butyldicarbonate, and the intermediate α,γ -hydroxyalkyl isocyanates are polymerized using Zr(acac)4 as catalyst (47). Polyamides are obtained from sterically hindered aliphatic aminoisocyanates (48) and poly(imides) are obtained from 4-isocyanatophthalic anhydride (49). Polyamides are also obtained by interfacial polymerization of 4-isocyanatobenzoyl chloride. Reaction of isocyanatocarboxylic acid chlorides with glycols gives poly(ester urethanes) (50). Similarly, reaction of 4-isocyanatobenzenesulfonyl chloride with diamines affords poly(sulfonamide ureas) (51). When two reactive groups are attached to the aryl isocyanate, hyperbranched homopolymers are obtained. For example, 3,5-dihydroxyphenyl isocyanate,
generated from 3,5-dihydroxybenzoyl azide, undergoes polymerization with network formation (52). Hyperbranched polyurethanes are also constructed using phenol blocked 1,3-diisocyanatobenzene with a CH2 OH group in the 5-position in combination with 4-methylbenzyl alcohol for end capping. Homopolymerization proceeds by thermally unblocking the isocyanato groups (53).
Copolymers Copolymerization of 4-vinylphenyl isocyanate and styrene at 60◦ C in toluene in the presence of AIBN affords the expected copolymers (44). Also, 1:1 copolymers from vinyl isocyanate and maleic anhydride are known (54). The copolymeriation of n-butyl isocyanate with a variety of oleﬁns is conducted in toluene/THF at −80◦ C, using sodium biphenyl as initiator (55). Anionic copolymerization of styrene and hexyl isocyanate affords rod–coil block copolymers. The styrene polymer forms the coil block, while the polyisocyanate block assumes the rod shape (56). Vinyl, 9-decenyl-, or β-allyloxyethyl isocyanate undergoes copolymerization reactions with styrene or methyl methacrylate (57). The anionic copolymerization of isocyanates and ketenes is conducted in DMF at −60◦ C, using sodium cyanide as initiator. This method gives copolymers of dimethylketene and isocyanates (58) and diphenylketene and phenyl isocyanate (59). The structure of the copolymers 5 was elucidated by spectral and degradative studies (60).
The sterically hindered o-methoxyphenyl isocyanate does not copolymerize with ketenes. Copolymers derived from isocyanates and ketenes are listed in Table 6. Alkoxyallenes 6 and aryl isocyanates undergo copolymerization at 80◦ C to give alternating polyamide copolymers 7 (61).
The same alternating copolymers are obtained from methoxyallene and pchlorophenyl isocyanate. Initiators are not required in this copolymerization reaction (62). Copolymers of diynes 8 with phenyl isocyanate afford poly(2-pyridones) 9. This copolymerization reaction uses nickel(0) complexes as initiators (63).
Table 6. Copolymers from Isocyanates and Ketenes Ketene Diphenyl Phenylmethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl Phenylethyl
Isocyanate Phenyl Phenyl Phenyl Phenyl Phenyl Phenyl p-Chlorophenyl m-Chlorophenyl p-Methoxyphenyl p-Methoxyphenyl p-Methoxyphenyl m-Methoxyphenyl m-Methoxyphenyl m-Methoxyphenyl
Melt temp., ◦ C
0.84:1 1.3:1 2:1 3.2:1 1:1 1:1 3:1 4:1 6.5:1 1:1 1.5:1 2:1
70 30 4 12 40 51 45 18 22 35 63 7 29 46
179–186 200–204 170–179 186–197 189–199 201–220 152–167 148–156 174–185 178–192 203–209 167–175 180–192 167–182
From diethynylbenzene and aromatic or aliphatic isocyanates also copolymers are obtained. Aryl isocyanates produce 1:1 copolymers using equimolar amounts of the monomers. An excess of aliphatic isocyanates is necessary to give the 1:1 copolymers (64). Similarly, from 1,4-bis(phenylethynyl)benzene and alkylor aryl isocyanates rigid poly(2-pyridones) are obtained (65). Soluble ladder poly(2pyridones) are obtained from macrocyclic diynes and aryl- or alkyl isocyanates (66). Anionic copolymerization of isocyanates with chloral (67), cyanopropionaldehyde (68), and acrolein dimer (69) is also known. For example, reaction of phenyl isocyanate with chloral in DMF at −68◦ C, using potassium cyanide as initiator, affords chloroform-soluble copolymers 10, which melt at about 230◦ C (67).
Copolymers produced in diethyl ether with n-butyllithium as initiator are insoluble and melt as high as 329◦ C. The infrared and melting point data indicate a marked difference in crystallinity of the differently initiated copolymers. The reaction of cyanopropionaldehyde with methyl isocyanate, initiated with the benzophenone dilithium complex at −78◦ C affords a 1:1 copolymer (68). Also,
copolymerization of acrolein dimer with methyl-, ethyl-, or phenyl isocyanate at 15◦ C in toluene affords 1:1 copolymers (69). The copolymerization of ethylene oxide with aromatic isocyanates is conducted in toluene at −78◦ C, using triethylaluminum as initiator.The shown acetal structure 11 is proposed for the copolymer (70).
Alkyl isocyanates do not copolymerize with ethylene oxide. Block copolymers are obtained from aromatic diisocyanates and vinyl monomers, such as styrene, isoprene, and methyl methacrylate via anionic polymerization, using n-butyllithium as initiator (71). Since TDI undergoes selective reaction through the nonhindered isocyanato group poly(butadiene-block-imide) block copolymers are obtained (72).
Addition Polymers The reaction of MDI with enamines produces polyamides by a [2+2] cycloaddition reaction across the C C bond. Ring opening in the initial reaction forms an amide bond and regenerates the double bond, which undergoes the same reactions again (73). With formic acid in DMF the enamine polymer hydrolyzes to give the corresponding keto polymer (74). From MDI 12 and ketene aminals 13, polyamides 14 are formed in a similar manner (75). These reactions can be performed in a RIM machine.
Glycidyl ethers derived from bisphenol A (15) or diglycidyl tetrephthalate can also be used as monomers in diisocyanate reactions. The reaction of 2,4-TDI with 15 proceeds by ring-opening [2+3] cycloaddition polymerization across the 1,3-dipol generated from the bis-epoxides to form a linear poly(2-oxazolidinone) (16) (76).
The reaction of bis-epoxides with aromatic diisocyanates can also be conducted in DMF in the presence of lithium chloride (77). Diisocyanates also react with other difunctional nucleophiles to give addition polymers. The use of hydrazine as extender in the manufacture of Spandex ﬁbers
is discussed under polyurethanes. Reaction of MDI with isophthalic dihydrazides 17 in DMSO affords poly(semicarbazides) 18 (78). Dioximes and diisocyanates react in a similar manner to give polyaddition products (79).
The polyaddition reaction of MDI with benzyloxamine in dimethylacetamide (DMAc) in the presence of 1,8-diazabicyclo [5-3-0] undec-7-ene (DBU) affords a poly(biuret) (80).
Condensation Polymers Polycarbodiimides 19 are obtained from diisocyanates by the phospholene oxide catalyzed condensation–elimination sequence (81).
The reaction proceeds at room temperature at a sufﬁciently fast rate to allow fabrication of polymers from liquid monomers. However, only thermosets are produced because of cross-linking of the linear poly(carbodiimides). The generated carbon dioxide can be utilized as a blowing agent and low density cellular products are readily obtained from PMDI (82). Copolymerization of diisocyanates and monoisocyanates, the latter as chain stoppers, affords processable oligomeric carbodiimides. For example, polymerization of MDI with controlled amounts of phenyl isocyanate in xylene at 120◦ C, using 1-phenyl-3-methyl-2-phospholene oxide as the catalyst, affords low molecular weight linear carbodiimides (83). The reaction of HDI with a phospholene oxide catalyst affords low molecular weight trifunctional oligomers, in which the isocyanate and the carbodiimide groups are cotrimerized with the generated carbon dioxide (84). Carbodiimide groups undergo a [2+2] cycloaddition reaction with isocyanates to form four-membered ring cycloadducts. This reaction is used to convert the low melting MDI into a liquid commercial product (Isonate 143-L) (85) The liquid MDI is used in RIM applications. The manufacture of polyamides by reaction polymerization of MDI with macrodicarboxylic acids, dicarboxylic acids, and trimellitic acid anhydride was pioneered in the 1970s at the D.S. Gilmore Research Labs of the Upjohn Co (86). Some of these products are commercially available from Dow. For example, segmented polyamide elastomers are obtained in the reaction of MDI with dicarboxylic acids and a carboxylic acid terminated aliphatic polyester, polycarbonate, or polyether prepolymer with an average molecular weight of M n 500–5000 (87). The dicarboxylic acid serves as the hard segment chain extender and the macrodicarboxylic acid forms the soft-segment matrix for the polyamide elastomers. The reaction is conducted in a polar solvent, using a phospholene oxide catalyst. Preferrably, a
continuous reaction in a vented extruder is used. The polyamide elastomers produced in this manner have light yellow to brown color and they are soluble in polar solvents. Molecular weights in inherent viscosity terms range between 0.8 and 1.1 dL/g measured as 0.5 wt% solution in N-methylpyrrolidinone (NMP). Polyamide elastomers retain useful tensil properties where most other thermoplastic elastomers cannot even be tested. Polyesteramide elastomers are also resistant to dry heat aging at 150◦ C. Alloys of poly(ethylene adipate) (PEAs) with nylon-66 and poly(butylene terephthalate) (PBT) display useful high temperature properties and improved impact properties, respectively. Homopolymers derived from MDI and azelaic acid are semicrystalline engineering plastics with a T g of 135◦ C and a T m of 290◦ C (88). Copolymers of MDI with azelaic acid, containing 20–30 mol% of adipic acid show a eutectic T m of approximately 240◦ C. These amorphous or slightly crystalline copolymers have mechanical properties comparable to transparent nylons or polycarbonates. Although injection molded samples are transparent, they will crystallize and turn opaque. Copolyamides derived from MDI and aromatic dicarboxylic acids are more difﬁcult to prepare. Because of the very high T m (420◦ C) of the isophthalic acid/MDI block it was necessary to prevent the formation of any appreciable crystalline blocks, which was accomplished by prereacting a portion of the isophthalic acid (15–20 mol%) with 2,4-TDI. In this manner crystallization of the isophthalic acid/MDI blocks was surpressed (89). Thus, copolyamides containing IPA/azelaic acid (50:50) are obtained with thermal and mechanical properties similar to polysulfone. The preparation of poly(amide imides) from isocyanates was explored by Rhone Poulenc, Amoco, and Upjohn (now Dow). For example, MDI was reacted with trimellitic acid anhydride in NMP and the solutions were directly converted into ﬁlms (Rhodeftal) or ﬁbers (Kermel). When the polymer is isolated it crosslinks, and it can no longer be redissolved. Upjohn produced poly(amide imides) from MDI and trimellitic acid anhydride or from MDI, trimellitic acid anhydride, and various dicarboxylic acid as comonomers in tetramethylene sulfone (90). In this manner high molecular weight polymers are obtained, which can be redissolved in polar solvents, such as DMF or NMP after isolation. As an example, PAI 8020 was made by reacting a 80:20 mol% mixture trimellitic acid anhydride/isophthalic acid with MDI. PAI 8020 would not injection mold by itself unless modiﬁed by copolymerization or alloying with other thermoplastics. Its high temperature properties can be further enhanced by blending with polyimide 2080 (see below). Films of PAI 8020 are useful as cable and wire wrap and in other high temperature applications. Poly(amide imide) foams have also been prepared from PMDI and trimellitic acid anhydride (91,92). Polyimides are also obtained in one step in the reaction of diisocyanates with tetracarboxylic acid dianhydrides. As an example, BTDA is reacted with a mixture of 2,4-TDI and MDI in DMF (93). This copolyimide, PI 2080, is used as a high temperature ﬁber and it is also sinter molded into solid shapes with exceptional mechanical properties. From PMDI and benzophenone-tetracarboxylic acid dianhydride polyimide foams with outstanding thermal properties and ﬂame resistance are produced (94). Polyimides derived from p-phenylene diisocyanate (PPDI) or naphthalene diisocyanate (NDI) and pyromellitic dianhydride or BTDA are also synthesized using DMAc as solvent (95).
An A–B polyimide is also obtained from 4-isocyanatophthalic anhydride (49). Processable polyimides 21 are obtained from the diisocyanate 20 and pyromellitic dianhydride (96).
Poly(arylene ethers) are obtained from n-propyl isocyanate blocked highly aromatic bisphenols and diﬂuoroarenes. In this manner polymers containing hole (electron) transport moieties are obtained (97).
Addition/Condensation Polymers High temperature resistant heterocyclic polymers are obtained from diisocyanates and suitable monomers containing groups which undergo addition polymerization, followed by condensation polymerization reactions with neighboring groups. For example, dihydroxydicarboxylic esters 22 react with MDI in DMSO, in the presence of a tertiary amine catalyst, to form linear addition polymers, which undergo subsequent cyclization to produce poly(benzoxazindiones) 23 (9).
The addition/condensation polymers derived from dihydroxybiphenyl dicarboxylate and diphenylether-4,4-diisocyanate have outstanding thermal stability (−180 to 300◦ C). The hydrolytic stability of poly(benzoxazinedione) ﬁlms to concentrated acids and dilute bases is excellent. Similarly, 4,4 -diaminobiphenyl-3,3 -dicarboxylic acid (24) reacts with diisocyanates to produce linear polyureas 25, which on dehydration at 160–180◦ C give the condensation polymers 26. Further heating of 26 to 230–250◦ C causes
rearrangement to give poly(quinazolinediones) 27, a polymer exhibiting outstanding thermal stability (98). The polymerization can also be conducted in polyphosphoric acid.
Reaction of MDI with hydrogen cyanide gives linear cyanoformamidines 28, which cyclize to give poly(iminoimidazolidinones) 29. The latter are hydrolyzed to poly(parabanic acids) 30 (10).
The reaction of diisocyanates with difunctional glycinates 31 affords linear polymers 32, which cyclize to form poly(hydantoins) 33. Phenol and cresol are the preferred solvents, and the cresol solutions of 33 can be used directly for wire coating and other applications (99).
Another approach to poly(hydantoins) involves cyclization of α-cyanoalkyl substituted polyureas, obtained from aromatic diisocyanates and the corresponding nitriles. Hydrolysis of the resultant poly(iminoimidazolidinones) affords poly(hydantoins) (100).
The reaction of aromatic diisocyanates with N,N-arylenedioxamide acid esters 34 affords poly(2,4,5-trioxoimidazolidines) 35 (101).
Poly(oxazolininediones) 36 are obtained from α-hydroxy acid esters and aromatic diisocyanates (99).
In the reaction of N,N -bis(2-carbomethoxyethyl)-1,6-hexanediamine (37) with aromatic diisocyanates poly(hydrouracils) 38 are formed. The intermediate linear poly(ureas) are cyclized in polyphosphoric acid (100).
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