α,ω-diisocyanatocarbodiimides, -polycarbodiimides, and their derivatives

Volume 20 - Number 10 October 1981

Pages 81 9-898

International Edition in English

a,w-Diisocyanatocarbodiimides, -Polycarbodiimides, and Their Derivatives

By Kuno Wagner, Kurt Findeisen, Walter Schafer, and Werner Dietrich[*]

Dedicated to Professor Herbert Griinewald on the occasion of his 60th birthday

Research in the field of low-molecular weight, oligomeric and polymeric a,@-diisocyanatocarbodiimides and -polycarbodiimides has been fruitful, not only in connection with these compounds themselves, but also-as so often happens in chemistry-with quite different problems. Novel synthetic methods, discoveries concerning the properties of low-molecular weight carbodiimides and phosphane imide derivatives, as well as results on the fragmentation reactions of four-membered heterocyclic compounds containing oxygen, phosphorus, and nitrogen, and a better understanding of the diisocyanate polyaddition process are among the many by-products of this research. The “high- and low-temperature formation” of polycarbodiimides and the homogeneous and heterogeneous catalysis of this process are described, and the fundamental importance of four-membered ring fragmentation mechanisms resulting in the formation of phosphane imide derivatives is outlined. Inter- esting building blocks for the diisocyanate polyaddition and polycondensation processes can be synthesized by many derivatization reactions of oligomeric and high-molecular weight polycarbodiimides and polyuretonimines. The in situ production of polycarbodiimides via matrix reactions in flexible polyurethane foams leads to a cellular arrangement of the material due to the pronounced symmetrical growth processes. Combination-foams with increased carbonation tendencies are formed in this way. Attention is drawn to several industrial applications of apdiisocyanatopolycarbodiimides, of high-molecular weight cross-linked polyuretonimines, and of polycarbodiimide foams.

1. Old and New Methods for the Synthesis of Low-Molecular Weight Carbodiimides

foundrz1. The cumulative double bonds make these compounds highly reactive, especially in addition and cycloaddition reactions, with the result that they are versatile building blocks for the synthesis of heterocyclic corn- pounds.

The carbodiimide family was first described more than 100 years ago by Weith”]. This was the beginning of an intensive study of these reactive compounds, for the production of which a great variety of methods were subsequently R-NH-C-NH-R + HgO -+ R-N=C=N-R + HgS + HZO

I1 S

(11 (21

[*j Dr. K. Wagner, Dr. K. Findeisen, Dr. W. Schsfer Zentralbereich Forschung und Entwicklung, Bayer AG, D-5090 Leverkusen, Bayenverk (Germany) Dr. W. Dietrich Anwendungstechnische Abteilung der Sparte Polyurethane, Bayer AG, D-5090 Leverkusen, Bayenverk (Germany)

Carbodiimides, especially dicyclohexylcarbodiimide, are important dehydrating reagents-in many cases on polymeric carriers (in Merrifield synthe~is~~’)-in the synthesis of pep tide^'^].

Angew. Chem. Int. Ed. Engl. 20, 819-830 (1981) 0 Verlag Chemie GmbH. 6940 Weinheim, 1981 0570-0833/81/1010-0819 S 02.50/0 819

Many methods for the synthesis of low-molecular weight carbodiimides were developed by E. Schmidt et al.[5-'01. N,N-dialkylthiourea derivatives (3) are oxidized to carbodiimides very efficiently by alkaline hypochlorite solution at temperatures below O'C, a reaction which is characterized by a wide range of applications. However,

R-NH-C-NH-R' + 4 NaOCl + 2 NaOH - II S

(31

the preparation of oligomeric or high-molecular weight polycarbodiimides with more than two carbodiimide units in the molecule is beset with considerable difficulties, however, even when highly effective phase transfer catalysts are used"'].

Grigat and Putter"" succeeded in obtaining high yields of carbodiimides by reacting aromatic cyanates (5) with N,N-substituted thiourea derivatives (1).

ArylOCN + R-NH-C-NH-R-

ArylOC-NH, + R-N=C=N-R (5 ) (1)

II S (61 (2)

As discovered by Appel et u I . [ ' ~ ~ , the urea derivatives (3) and (7) can react with Ph3P/Et3N/CCl4 to give carbodiimides in yields exceeding 90%.

R-NH-C-NH-R' + Ph,P + Et3N + CClh - II X

(3), x = s (7)> x = 0

R-N=C=N-ti + Ph3PX + HCClS + Et ,N*HCl

(4 )

One method for the preparation of unsymmetrically substituted carbodiimides, which was subsequently developed by us, is the reaction of isocyanide dichlorides (9) with amine hydrochlorides (8) in an inert organic

c1 C 1,C=N Cl

c1

c1'

We found that hydrocyanic acid is readily cleaved by aliphatic carbodiimides (2) from a-aminonitriles (ll)[I5'. In this way it is possible to obtain the hitherto unknown 2- propanimine (12a), which can be removed by distillation from the simultaneously formed cyanoformamidine derivative (13).

820

For a variety of reasons, all these methods for the preparation of low-molecular weight carbodiimides were unsuc- cessful when applied to the synthesis of oligomeric or polymeric polycarbodiimides.

A special study of problems relating to the diisocyanate polyaddition process and an intensive investigation of the side-reactions involved in the preparation of polycarbodiimides from diisocyanates resulted in the development of thermal condensation processes by means of which only low-molecular weight polycarbodiimides with unidentified terminal groups, together with insoluble by-products, were initially obtained.

2. Improvements in the Synthesis of Oligomeric a,o-Diisocyanatocarbodiimides and -Polycarbodiimides

In connection with the diisocyanate polyaddition process, which was devised by Otto Buyer in 1937 and subsequently developed by his colleaguesr'6.'7J, questions were soon raised about the storage stability of diisocyanatocarbodiimides-questions were even raised concerning their ability to exist at all-as well as about the nature of the side reactions involved in the production of polyisocyanates.

Polyurethane chemistry, the foundations of which were laid by 0. Buyer et al. more than forty years ago, has since acquired great technical importance throughout the world, polyurethanes now being among the most versatile of plastics. The low-molecular weight and oligomeric building blocks which participate in the diisocyanate polyaddition process form a large group of interesting functional or heterofunctional compounds. 0. Buyer himselc'81 was already interested to ascertain whether isocyanatocarbodiimides could be synthesized and in what manner diisocyanatocarbodiimides, i. e. compounds having three cumulative double bond systems in the molecule, can be stabilized by means of cycloaddition processes to form interesting new monomers with heterofunctional groups having different degrees of reactivity. Initially it appeared quite impossible to predict whether a diisocyanate polycondensation process could produce high-molecular weight a,o-diisocyana- topol ycarbodiimides.

It was discovered that monomeric a,w-diisocyanatocarbodiimides and polymer homologous series of a,w-diisocyanatopolycarbodiimides could not be formed by thermal condensation of diisocyanates in the presence of highly basic catalysts. The OHQ-catalyzed reaction leads, via the uretedione derivatives (15), to the isocyanurate derivatives (I 7) and to derivatives of (16) (Scheme 1).

Angew. Chem. Int. Ed. Engl. 20. 819-830 (1981)

0

t Dimerization and Trimerization of

180-2OO0C / -N=C=N- OCN-X-NC 0 OCN-XfN=C=N-Xf, NCO

Linear polymerization to polyguaridines

\ (16 )

I P

114) /j_ Scheme 1. Side reactions in the production of polyisocyanates; X=aryl, alkyl, or cycloalkyl.

Polyisocyanates are metastable systems having high energy contents; they are only stable and capable of being handled on an industrial scale because the formation of systems (15)-(I 7) requires a relatively large amount of ac- tivation energy (1 5-25 k~al/mol[’~~).

In the absence of diluting agents and in the presence of highly basic catalysts, cross-linked polyisocyanurates (cf. (17)) are formed at temperatures between 40 and 210 “ C as a result of exothermic reactions which are often violent and in which the NCO groups are trimerized.

One of the best models for the mechanism of purely thermal “high temperature carbodiimidization” was proposed by StaudingerrZo1 in the course of his fundamental work on ketened2’l. A similar mechanism was postulated independently in 1955 by Gaylord and Snydedzzl. The symmetrical constitution (15) proposed by Staudingerlz” for crystalline 1,3-diazetidine-2,4-dione derivatives (“uretediones”) was confirmed by X-ray crystallographic analyses performed by Brown‘231 in 1955. According to Staudinger, molten uretediones and liquid monoisocyanates are present, to a small extent, as asymmetrical four-membered ring systems (19) at high temperatures; their fragmentation results in the formation of the carbodiimides (2).

In the course of kinetic investigations we found that the exclusively thermal high-temperature carbodiimidization (at 180- 195 “C) of industrially available aliphatic diisocyanates is an extremely slow and entirely unpromising method for the production of low-molecular weight isocyanatocarbodiimides. In the case of hexamethylene diisocyanate, for example, even temperatures of 180- 195 “ C for 20 h only result in the formation of ca. 4-6% of oligomeric isocyanatocarbodiimides. Isocyanatopolyisocyanu- rates are formed in such reactions, the yield being 18- 20%[2~1.

4-Cyclohexylimino-l,3-diazetidin-2-one-derivatives (“uretonimines”) (20). which represent an interesting family of four-membered ring heterocyclic compounds, were first synthesized in the years 1955-1957 by Hofmann, Reichle, and Moosmiiller in Dormagen, and by E. Schmidt‘251 in Miinchen, from carbodiimides such as (2a) and isocyanates such as (18a). (20) is formed in an equilibrium reaction which is highly dependent on temperature. This makes it pos~ ib Ie~’~*’~~-when use is made of a mono- isocyanate R‘NCO whose boiling point is higher than that of RNCO-for the uretonimine (20) to be fragmented in a reversed manner to its formation, provided that RNCO is removed from the reaction mixture by distillation.

R I x

R-N=C=N-R + RLNCO R-N N-R’-+R“N=C=N-R + R-NCO

(2a) . R = C&,1 0 8 a ) 0 (20) (41 I IRh) K

The application of this reaction by Fisched26J to diisocyanates (14) led to “refragmentation processes”, which, de- pending on the molar ratio of the reactants, give different products (Scheme 2).

( I401 - OCNfcH2fsN=C=NfCHz~6NC0 --+ - HiiCsNCO (2,‘)

Scheme 2.

Angew. Chem. Inf. Ed. Engl. 20. 819-830 (1981) 821

The products obtained in this way included polymer-homologous series of uretonimine polyisocyanates (23) of hexamethylene diisocyanate (26); they were produced in several addition-elimination steps from (2a) and (14a) via the uretonimines (20a) and (206). cyclohexyl isocyanate being removed continuously by distillation.

Recent investigations by modern analytical methods, such as gel chromatography, show that, as expected from the laws of Elory and Schultz, polymolecular mixtures are always obtained. If a large excess of (14a) is used the monomeric uretonimine (23a), x = 1, is obtained in the polymer homologous series of the polyuretonimine polyisocyanates at concentrations of up to 60 mol %.

High temperature carbodiimide formation from 4,4‘-diisocyanatodiphenylmethane (1 46) was performed success- fully for the first time in 1959, special catalysts being used and solutions of the uretonimine triisocyanate (24) being ~ b t a i n e d [ ~ ~ . ~ ’ ~ .

N Y R

0

!24)* R = C,H4CH,C,H4NC0 i /4h)

(24) is formed in a thermally reversible equilibrium reaction during the cooling phase, through addition of the isocyanate group to the already formed a,o-diisocyanatocarbodiimide.

Polymer-homologous series of polyisocyanates with the biuret structure (25) (Desmodur N, Bayer AC), whose synthesis mechanisms are given in Scheme 3[29-311, are par- ticularly useful non-basic catalysts in the high temperature formation of carbodiimides[281 for the liquefaction of 4,4‘- diisocyanatodiphenylmethane.

+ H20, 100°C

- C02 J ? H 4 OCNfCH, N-C-NfCH, NCO

i N H +=O I (25) . x = 1-6

Scheme 3.

The in situ production of the catalysts from aliphatic, cy- cloaliphatic, or aromatic polyisocyanates can be achieved by adding small amounts of water, tert-butyl alcohol, or mono- or diamines.

The isocyanatocarbodiimides and polyisocyanatoure- tonimine synthesized by high temperature carbodiimide formation are, however, contaminated by by-products (iso- cyanatopolyisocyanourates, isocyanatouretediones), the amounts of which depend largely on the reaction temperature.

Later Neurnann and Fi~cher‘~~] , using the technique of OHe-catalyzed thermal condensation, were also able to synthesize monomeric and oligomeric a,o-diisocyanatocarbodiimides from mono- and diisocyanates with high degrees of steric hindrance, in a way which excluded side reactions.

R R R

k k 126) (27)

R = CH,, C,H,, CH(CH,),

R I s

The bulky isopropyl moieties prevent quantitative formation of uretedione, uretonimine, and polyisocyanu- rate, as well as the uretonimine cross-linking of the oligomeric diisocyanatopolycarbodiimides formed during the process.

The degrees of polymerization obtainable by this method are, however, low. The oligomeric a,o-diisocyanatopolycarbodiimides obtained are typical aligomers with mean molecular weights of less than 2000.

A “low temperature polycarbodiimidization” at ca. 0- 60°C, in which linear polymers with ten to one hundred -N=C=N-units are obtained, remained impossible until the 1960’s.

3. Catalytic “LOW Temperature Carbodiimidization” of Mono- and Polyisocyanates

More than 60 years ago Staudinger et a1.[33-371 working in Zurich found two important classes of compounds; the substituted phosphorus ylides and the phosphane imides, e.g. (30) and (35) respectively, both of which are characterized by high reactivity towards, e. g . , phenyl isocyanate (31).

(30) and (35) react with reagents containing cumulative double bonds to form four-membered ring heterocyclic compounds (Scheme 4). When heated, these compounds decompose in a manner opposite to that of their formation (a process referred to as “refragmentation”), the decomposition products being the diphenylketenimine derivative (34), the phenyl isocyanate (31), and the diphenylcarbodi- imide (2a). Phosphane imides and phosphane oxides play a fundamental part as catalysts in carbodiimidization reactions.

Addition of carbon dioxide to (35) to form the heterocyclic compound (38) and the fragmentation of the latter to triphenylphosphane oxide and phenyl isocyanate reflect the high energy content of phosphane imide derivatives and the affinity of phosphorus for the oxygen. This is also the key to understanding the four-membered ring mechanism of carbodiimide formation.

“Low temperature polycarbodiimidization” and the formation of polyuretonirnines, starting from diisocyanates, were developed in the years following 1959 after CampbeN et aZ.[38,391 of DuPont, and Schliebs and Block of Bayer had synthesized phospholene oxides, which proved highly active as catalysts for the formation of polycarbodiimides.

822 Angew. Chem. Int. Ed. Engl. 20. 819-830 (1981)

Schliebs and Block optimized the process by which the isomer mixture was obtained from the 1-methyl substituted phospholene oxides (40) and (41), a mixture which was

n 0 H3C-N, ,N-CH3 P P

H3C' "0

140) (41) (42)

found to be the most effective catalyst of the carbodiimidization in our studies. But the diazophospholidine oxide (421, synthesized by Dabritz and Her€inger[401, is also an ef- ficient catalyst at temperatures of 60°C and above.

The mechanism of the polycarbodiimidization, as catalyzed by the mixture of (40) and (41), is in accord with the assumptions made by Staudinger (Scheme 5).

Repetition of this reaction sequence finally leads to high- molecular weight a,o-isocyanatopolycarbodiimides.

With aromatic polyisocyanates the polycarbodiimidization with the isomer mixture of the phospholene oxides(40)/ (41) is extremely fast and occurs with continuous elimination of C 0 2 . In the absence of solvents and at a catalyst concentration of about 3% by weight it takes only a few minutes at 20-40 "C. Sterically unhindered aromatic polyisocyanates and oligomeric isocyanate prepolymers react exceptionally rapidly with active phospholene oxides to form polyisocyanatopolycarbodiimides or polyisocyan- ato(po1yurethane)polycarbodiimides. Even when only a few ppm of these were used we were able to ef- fect the smooth partial carbodiimidization of polyisocyanates. As expected, these carbodiimide isocyanate mixtures

Scheme 5. Mechanism of formation of polycarbodiimides from diisocyanates. In (43/-(45) the double bond isomerism is represented by broken lines, X =awl, alkyl, cycloalkyl.

The isocyanatophosphane imide (44), which is capable of undergoing cycloaddition, is formed just as rapidly as the a,w-diisocyanatocarbodiimide (16), n = 1, even at temperatures in the vicinity of 0 C, due to refragmentation.

undergo changes upon storage. The carbodiimide bands disappear from the IR spectrum, an indication that isocya- natouretonimines such as (46) have been formed from the carbodiimides and isocyanates in a thermally reversible equilibrium reaction. The stability of the isocyanatoureton- imines in solution is high, and it is possible to deactivate the catalysts adequately with a few ppm of, e.g. , anhy- OCN-X-N=C=N-X-NCO + OCN-X-N=C=O

( I 6 ) . n = 1 (14) OCN-X-N-f-X-NCO drous HCl: 146)

(40)/(41) + 2 HCI [(40)/(41) . 2 HCll. 0 ~ N - X - N C O

Angew. Chem. Int. Ed. Engl. 20, 819-830 (1981) 823

4. Recent Developments

With the advent of highly active phospholene oxide catalysts it become possible for the first time to produce linear, high-molecular weight a,--diisocyanatopolycarbodiimides under mild conditions. In this connection we were interested both in the molecular weights obtainable and in the uniformity of the end-groups. Numerous polymer-analogous reactions of high-molecular weight polycarbodiimides appeared to us to be promising, in view of the changes which could be effected in the material properties of the high polymer products. It was also necessary to investigate problems connected with the homogeneous and heterogeneous catalyization. In view of the exceptionally rapid polycarbodiimidization of solventless aromatic polyisocyanates or of highly concentrated solutions of these compounds, we decided to initiate the polycarbodiimide formation in flexible polyurethane foams impregnated with, and swelled by, aromatic polyisocyanates, i. e. to perform it as a series of matrix reactions on interconnecting cell walls solvated with polyisocyanates.

5. Rates of Formation of a,o-Diisocyanatopolycarbodiimides

The a,w-diisocyanatocarbodiimidization and -polycarbodiimidization from 4,4'-diisocyanatodiphenylmethane (146) and 2,4-diisocyanato- 1-methylbenzene (47) in toluene occur rapidly at temperatures of 5-40°C (Fig. 1).

out evaporating, with polar resonance forms of dimeric isocyanatocarbodiimides and of the dimeric hexamethylene diisocyanate even at room temperature (cf. Section 9).

6. Linearity of High-Molecular Weight and Oligomeric a,--Diisocyanatopolycarbodiimides

In the early part of our investigations we were concerned with discovering whether the powdery polycarbodiimides from 4,4'-diisocyanatodiphenylmethane (146), which apparently have very high-molecular weights, con- tained isocyanate end-groups or small amounts of branching and cross-linking sites (uretonimine segments as in (49), and cross-linking isocyanourate or triazine structures, which could have been formed from trimerized carbodiimide units). Since these species are insoluble in all solvents it was at first impossible to determine the average molecular weight. Free NCO groups (1.1- 1.4% NCO) were iden- tified using special titration methods. IR spectroscopic data was inconclusive. The oxygen content of 0.4-0.6% found in the elemental analysis was consistent with polymers of the structure shown in (48) and (49).

L

(48), x = 25-27

@ = 2500 B = 2500 OCN- NCO

PCD = Polycarbodiimide segment

1, a 12 16 20 2~ 28 t h -

Fig. I. Carbodiimide formation from (14b) (I), (47) (II), and (14a) (111); (14b) and (47) were used as 18% solutions in toluene, and (14a) as pure substance. The carbodiimide equivalents (calculated from the amount of COI released) are plotted as a function of time.

Although we are concerned here with a sterically hindered diisocyanate, the high rate of condensation of (47), both at 40°C and at 5"C, and the small differences between the half-lives are remarkable.

The curve plotted for hexamethylene diisocyanate (14a) (111) does not correspond to the actual progress of carbodiimide formation; liberated COz reacts immediately, with-

824

& ? = 2500 @ = 2500 OCN- N N P NCO

PCD PC D

From the NCO content found in the end-group analysis, and from the relations between this content and the amount of C 0 2 liberated in the polycarbodiimidization, and the oxygen content (high-molecular weight polycarbodiimides with a=25000 contain only ca. 0.13% of oxygen), it was concluded that thermally stable isocyanurate structures could hardly be present in the powdery polycarbodiimides. But the presence of small amounts of cross- linking uretonimine segments could not be excluded.

Thus the highest attainable molecular weights of the white, insoluble polycarbodiimide powders from (146) could only be determined by the classical methods which Staudinger had used to investigate the constitution of the polyoxymethylenes. In this connection we were helped by discoveries concerning insoluble polymethylenethioureas which had been made in the 1950's142,43!

At 80°C, E-caprolactam (50) is a liquid of low viscosity which readily dissolves polymethylenethioureas of the type (51) and (52), which had long been considered to be cross- linked and insoluble.

Angew. Chem. Inr. Ed. Engl. 20, 819-830 (1981)

NH (SO), m. p. = 7OoC

(SI), R = H, CH,OH, x = 4-12

S

H HZOH (52)

Surprisingly, we found that not only is E-caprolactam an excellent solvent for the otherwise insoluble high-molecular weight polycarbodiimides of (14b), but also that when it is heated to 120 "C (without polymerization of E-caprolactam), the lactam in the molten material rapidly adds quantita- tively to all the carbodiimide groups and to the isocyanato end-groups of the polymer moleculefaJ. In the resulting polymers (53) the covalently bound lactam content leads to

7. Molecular Weight Jumps in the Melting of High- Molecular Weight a,o-Diisocyanatopolycarbo- diimides

At a temperature of about 180-270 O C the polycarbodiimides described in Section 6 undergo changes which are attributable to the presence of phosphane imide end- groups as impurities in the polymers'441; structures like (54) (where only one phospholene isomer is represented) being present. In the processing of a,o-diisocyanatopolycarbodiimides as thermoplastics at 270 "C, these polymeric impurities act as catalysts for the molecular weight multiplica- tion. The polycarbodiimide/phopshane imide derivatives with relatively high molecular weights generally have an NCO-content of 1.3%, a P-content of cu. 0.05%, and an 0- content of cu. 0.6%.

The phosphane imide end-groups cause pronounced molecular weight jumps when the powdery polycarbodiimides melt at 240--270°C. in accord with the mecha-

a dramatic increase (from about 0.6 to 5.7%) in the oxygen content of the powdery final product. The polyaddition product (53) melts at 210-214"C, i. e. at a temperature cu. 70°C lower than the melting point of the polycarbodiimide. The lactam ring (acylated guanidine segment), which functions as a spacer in the polymer, now renders the polyaddition product soluble in dimethylformamide or N- methylpyrrolidone. In this polymer-analogous conversion, chain cross-linking uretonimine segments are quantita- tively eliminated by linearization reactions with E-caprolactam. The linearized polymers can be isolated by precipi- tation in acetone. Very little uretonimine cross-linking takes place in carbodiimidization at room temperature, i. e. about 6000 segment units, at the most, are formed per uretonimine segment. According to Hoffmann and K r O ~ n e r ' ~ ~ ~ , the polymers treated with E-caprolactam have a fairly broad molecular weight distribution and a mean molecular weight of cu. 11000. The highest obtainable molecular weights of powdery a,o-diisocyanatopolycarbodiimides of (14b), with the idealized constitution respresented in (48), a=30, are never exceed 6000-7000. The increase in the average molecular weight to 10000- 11 000 in the reaction with E-caprolactam i s in good accord with the oxygen content.

Our results on powdery polycarbodiimides can be sum- marized as follows. The end-groups of these polycarbodiimides are mainly a,w-NCO groups; but the highest obtainable average molecular weights of the polycarbodiimides, which plastisize at 270 "C, are less than 6000 to 7000. The limiting factors to chain growth ("molecular weight barrier") in many of these compounds are the solvents in which they are synthesized because the solubility limits of the polymers are reached. Even after long reaction times (e.g. 40 h at 100--140°C) the polymers undergo no more chain growth due to topochemical reactions of their end- groups.

Angew. Chem. Int. Ed. Engl. 20, 819-830 (1981)

nism of carbodiimide formation shown in Scheme 5. The processing of these materials to transparent thermally stable linear polymers, e.g. as plastics and fibers is diffi-

cult because the flowability of these systems decreases continuously, while the softening points rise dramatically; not only do the chain lengths increase, but the materials also undergo cross-linking.

At high temperatures, the pronounced polymerization tendencies of the accumulated carbodiimide segments in these powdery polycarbodiimides lead to branching and cross-linking. Transparent molded sheets produced at 270 "C and 250 bar have considerably greater high-temperature bending strength than polycarbonates, but because they have high cross-link densities they have less notched impact strength.

The assumption that cross-linking reactions take place is supported by the fact that polycarbodiimide powders which are molded at 300°C are no longer soluble in, but are merely swelled by E-caprolactam. Addition of caprolactam to unpolymerized carbodiimide units is then very difficult.

Thus, the processing of linear a,w-diisocyanatopolycarbodiimides with average molecular weights of 4000- 8000 to produce thermoplastics with the typical character- istics of linear macromolecular materials, e. g. with average molecular weights of 20000-40 000, high degrees of hard- ness, the ability to cobweb in the molten state, and low brit- tleness, is prevented by cross-linking polymerization of the numerous carbodiimide groups of the polymer at high temperatures.

825

The experience gained in connection with polymer-analogous reactions involving E-caprolactam led us to prepare other polycarbodiimides and mixed condensates with par- ticularly high degrees of thermal stability, e. g. mixed condensates from p-phenylene diisocyanate and 1,5-naphthy- lene diisocyanate with softening points of approximately 300°C. The average molecular weights of these a,m-diisocyanato mixed polycarbodiimides are very greatly reduced because polymers with three to six carbodiimide units reach their solubility limits in inert solvents, with the result that they do not undergo further condensation. In contrast, ternary mixed carbodiimides from 2,4-diisocyanato-l-methylbenzene (47) and the aforementioned aromatic diisocyanates in the ratio 1 : 1 : 1 can acquire average molecular weights of up to ca. 3500.

a,@-Diisocyanato mixed polycarbodiimides can be made elastic with a,o-diisocyanato polyether-or polyester-urethanes (NCO-prepolymers) and their softening points can be greatly reduced. In aromatic solvent mixtures and isopropanol the chains of polymers containing apdiisocyanatopolyurethane or polyurea groups and having three to four carbodiimide units can be extended with sterically hindered diamines without premature addition of the diamines to the carbodiimide groups, with the result that useful film-forming agents of high-molecular weight are obtainedLa1.

8. Condensation and Polycondensation of 2,4-Diisocyanato-l-methylbenzene in Toluene or Hexane/Petroleum Ether

Polymer-homologous compounds of oligomeric a,o-diisocyanatopolycarbodiimides from 2,4-diisocyanato-1- methylbenzene (47), which are formed at room temperature using phospholene oxides as catalysts, are readily soluble in many chlorinated and aromatic hydrocarbons because of the considerable reduction in mutual association of the chain molecules within certain molecular weight limits. We were also able to show that in such cases the molecular weight is controlled by the solvent. The highest obtainable molecular weight compatible with linearity, which is less than 8000, was determined as described in Section 6; the polymers may be considered to have the constitution given in (55). Using such precipitating agents as cyclohexane and petroleum ether/hexane, we were able to prepare a homologous series of carbodiimides from (47), beginning with the bis(3-isocyanato-4-methylphenyl)carbodiimide (56), using the techniques of fractionating condensation and polycondensation.

These compounds and their addition products to polyethers or polyesters with terminal hydroxy groups, i. e. their NCO-containing prepolymers, are useful building blocks in the diisocyanate polyaddition process.

Despite their molecular non-uniformity these polymers are strictly bifunctional; they can be added to all the com- ponents used in the diisocyanate polyaddition process by means of selective NCO reactions. Since these powdery products contain 200-400 ppm of phosphane imide end- groups, i. e. highly active catalysts, it is necessary to stabil-

826

ize them with a few ppm of HCl, BF,, or benzoyl chlo- ride.

OCN N c o

(57). M = 434, Yo NCO = 19.3

NCO

OCId N c o

(58). M = 564, yo NCO = 14.9

Because the apdiisocyanatocarbodiimides (56)-(58) are adequately to freely soluble in many solvents, they have been used for a large number of polymer-analogous reacti~ns~~’-’~J.

These reactions give many interesting polyaddition products with polyurea-, polyisourea-, polyisothiourea-, polyformamidine-, polyacylurea-, polyguanidine-, polyphosphonoformamidine-, and polycyanoformamidine-segments (Table 1).

In this way-through the use of ppm amounts of bases-even such carboxylic acids as acrylic acid and methacrylic acid can readily be added (e in Table l), without substantial anhydride formation; the polyaddition products obtained being easily polymerizable. Route i in Table 1 leads to N-alkoxysilylmethyl-N-alkylamino-substituted polyguanidines, which are among the most reactive of those polyaddition products which cross-link via siloxane bridges in the presence of atmospheric moisture.

9. Possible Errors in the Interpretation of the Reaction Curve for Low-Temperature Polycarbodiimidization of Hexamethylene Diisocyanate

Whereas, we originally thought that only aromatic diisocyanates react at room temperature to form carbodiimides, we later discovered using hexamethylene diisocyanate (14a) that this assumption was based on the incorrect interpretation of the respectively determined C 0 2 Since CO, was not formed in the phospholene oxide catalyzed reaction of (14a) at room temperature, and the characteristic carbodiimide and uretonimine bands were not present in the IR spectra, condensation had not occurred. From the results of NCO-titrations performed at definite time intervals, gel chromatography, and IR spectroscopic investigations of the reaction products, it is clear that ure-

Angew. Chem. Int. Ed. Engl. 20, 819-830 (1981)

Table 1. Polymer-analogues reactions on oligomeric and high-molecular polycarbodiimides.

Educts Polyaddition products

a) -X+N-C-N-Xj-, + H 2 0

b) -X+N--C-N-Xj, + R 4 H

C) -X+N==C-N-Xj, + R-SH

L.

--+ -X N-C-N-X

i " A H f" - -X NH-C-N-X t b R t

/ d) -X+N--C-N-Xj, + CH2

'Y Z=COOCzHs, COCH,; Y=COOCZHs

f) -XfN-C-N-Xjx + R-NH2 R-prim. and sec. alkyl

0 OR

h) -X+N--C-N-Xj, + HCN - -x N H - ~ ~ N - X ~ € i) -X+N-C-N-X j, + H, ,Cb-N-CH2--Si(OEt), -r - X + N H - C - N - X d

H or H I IC6-N-CH2--Si(OEt)2

H

/ \

polyurea segments

polyisourea segments

polyformamidine segments

polyisothiourea segments

polyacylurea segments

polyguanidine segmentes

polyphosphonoformamidine segments

polycyanoformamidine segments

pol y-N-alkoxysilylmethyl- N-alkyl-amino-substituted guanidine segments

tonimine or carbodiimide units are not found even at room is therefore misleading with respect to the rate of carbodi- temperature, because the C 0 2 produced cycloadds imme- imide formation of hexamethylene diisocyanate. diately after its formation to dimeric, polar intermediates At 150- 160 " C, however, even aliphatic polyisocyan- of biscarbodiimides and diisocyanates, leading to com- ates, such as (14a) and 3-isocyanatomethyl-3,5,5-trimethyl- pounds with the idealized constitutions given in (59) and cyclohexyl isocyanate (14c) (isophorone diisocyanate) (60). react in the presence of phospholene oxide derivatives to

At temperatures up to 80 "C phospholene oxide derivatives are therefore not selective catalysts of the carbodiimidization; instead they catalyze the oxadiazinetrione ring formation and also the trirnerization, thus behaving like phosphane derivatives. The curve reproduced in Figure 1

Angew. Chem. Inf . Ed. Engl. 20, 819-830 (1981)

form mainly polycarbodiimides (61) with the polycarbodiimide-polyuretonimine structure. Provided that a large excess of monomeric diisocyanate is avoided, these polymers participate in the formation of three-dimensional cross-links, accompanied by uretoneimine formation as

827

160T

- 3 x co* ( 1 4 ~ ) , X = fCH,f,

the temperature falls, but, however, readily form polyisocyanates again if the temperature is raised. These systems represent masked polyisocyanates which can be used together with polymerization, polyaddition, and polycondensation products containing Zerewitinoff-active hy- drogen for many cross-linking reactions at elevated tem-

pera ture~ '~~] . The advantage of these compounds, especially of polycarbodiimide-polyuretonimines from (14c) is that volatile decomposition products, which do not participate in the cross-linking, are not formed.

10. Heterogeneously Catalyzed NCO Condensations Leading to Oligomeric Isocyanatopolycarbodiimides in Swellable Matrices

With the aid of special phospholene oxide derivatives ionogenically or covalently bound to insoluble but swellable matrices, we investigated whether swellable catalysts are effective in the heteregeneous catalysis of polycarbodiimide formation and how the reaction depends on temper- a t~ re [ '~ - '~~ . The h. tgh-molecular weight matrix shown in Scheme 6 can be produced by salification of commerical basic ion-exchange resins with phosphonic acid derivatives of the phospholene oxides. Using this matrix those polyisocyanates, which among oligomeric technical polyisocyanates have the lowest molecular weights, can be converted without participation of trifunctional and tetrafunctional polyisocyanates of high molecular weight into isocyanatocarbodiimides provided that they reach the catalytically active anchor group in the matrix. Temperatures of 130 to 165 "C are needed for this purpose.

In the reaction step C, the diisocyanatocarbodiimide formed is released from the matrix and hence the high-molecular catalyst is again fully effective.

If the process is properly controlled, the oligomeric polyisocyanatocarbodiimides and polyisocyanatoureton- imines obtained are completely free of phosphane imide end-groups and can, therefore, be stored.

The use of pore diameters of ca. 100-200 A for the matrices and adjustable porosity, permit the selective formation of the lowest molecular weight polyisocyanatocar-

bodiimides. Thus, heterogeneous catalysis enables derivatives of polyisocyanate mixtures which are free of phosphane imide end-groups to be produced without loss of catalyst. Another considerable advantage to the process is that the solid catalysts can be removed from the reaction mixture at any time, and can be used again if proper care is taken.

c OC N-X-N=C=N-X-NC 0

+

Scheme 6.

Phospholene oxide moieties bound covalently to matrices with pores of 100-200 A are also catalytically active. They can be produced in the ways indicated in Scheme 7, and also permit the selective formation of polyisocyanatocarbodiimides of low-molecular weight.

Matr ix A CH?

"

I - 1 ) CH2=CCH=CH2

2) H a 0 1;;9 +

Q CnH,

M a t r i x B

(40) / ( 4 / ) + Oligomer ic maleic acid p o l y e s t e r s

Radical milators 1 Mixed polymers Matr ix C

Scheme I.

Isomer

828 Angew. Chem. Int. Ed. Engl. 20. 819-830 (1981)

11. High-Molecular Weight Polycarbodiimides and Polyuretonimines in Swellable, Impregnated Flexible Polyurethane Foams

A review article published almost ten years ago[601 re- ported on growth processes caused by reactions in swellable, cellular matrices and described a universally valid cell arrangement principle which can be applied to any de- sired organic, cellular substrate and can be effected by any type of organic or inorganic chemical reaction, provided that the reactants exert a sufficient swelling action on the matrices employed. The physical swelling pressure and its irreversible fixation by cellular solid formation were recog- nized as the driving forces responsible for the observed growth phenomena[61-661.

Results gained so far, indicate that matrix reactions are potentially useful when reactive swelling agents with high energy contents are employed. The essential condition for rapid and irreversible fixation of the state of swelling is that the swelling agents must be able to react very rapidly to form solids in the absence of solvents. For example, since they have high energy contents, flexible polyurethane foam matrices swollen with liquid polyisocyanates, but when previously charged with catalytical amounts of phospholene oxides, need no further energy input to be converted from the cellular, swollen state into a cellular, three- dimensional, and entirely symmetrical solid because of rapid polycarbodiimide formation.

This process gives interesting open-celled, carbonized combination-rigid foams, which show enhanced flame resistance. The interconnecting cell walls of the matrix are considerably stretched by the cellular solid arrangement of the high-molecular weight polycarbodiimides and these forms are fixed irreversibly. It is not until the elongations have acquired the maximum, irreversible degree of fixation that the surfaces of the interconnecting cell walls become-to an increasing extent-the site of the reaction.

A cellular solid arrangement of polyisocyanatopolycarbodiimides arises when the polyurethane matrix is charged with phospholene oxides and then impregnated with any, although preferably aromatic, polyisocyanates and oligomeric and elastifying isocyanate prepolymers of hydroxy-terminated polyethers or polyesters; in this way it is possible to produce rigid or semi-rigid combination- foams whose cells are entirely open and which have carbonization properties and enhanced flame resistance.

12. Industrial Applications of Polycarbodiimides, a,w-Diisocyanatomono-, -Polycarbodiimides, and their Derivatives, and of Polyuretonimines

Through the use of special associates prepared from phospholene oxides and monoalcohols or p o l y a l c ~ h o l s ~ ~ ~ ~ it has been possible to overcome all the technical difficulties involved in the production of highly cross-linked polycarbodiimide foams with extremely low densities, good high-temperature bending strength, and high degrees of flame resistance. For this purpose it was necessary to

Angew. Chem. Int. Ed. Engl. 20. 819-830 (1981)

specifically increase the stirring, waiting, and rising times of the systems, together with sufficiently short setting times, by systematic variation of the catalyst associates.

The industrial applications of isocyanatocarbodiimides, polycarbodiimides, and uretonimines cover a wide range. This includes anti-hydrolysis agents for polyester-polyurethane plastics and chain extending and cross-linking agents for the diisocyanate polyaddition process. Many of the isocyanato polymers and derivatives described above are interesting diisocyanate liquefiers, reactive fillers, light- fast raw materials for electrostatic powder spraying proc- e ~ s e s [ ~ ~ ] , pigment binders, thixotropic agents, starting materials for the manufacture of film'461, coatings'471, or micro- encapsulation d e v i c e ~ [ ~ ~ . ~ ~ ] , as well as for catalysts used in isocyanate chemistry.

13. Conclusion

The chemistry of isocyanates is characterized by the enormous variety of the building blocks that can be used for polyaddition; it can be combined with ring cleavage polymerization and copolymerization processes, and with grafting reactions involving a very wide range of vinyl monomers. In addition, the formation of polycarbodiimides and mixed polycarbodiimides provides a variable polycondensation structural principle; a characteristic of this type of polycondensation is its high rate relative to that of familiar polycondensation processes.

The possibilities of simple in situ production of a,o-isocyanatopolycarbodiimides in monomeric vinyl compounds, together with the ease with which the polymerizable acrylic or methacrylic acid can be added to carbodiimide g r o ~ p s [ ~ ~ , ~ ' ] to form acylated urea segments, makes interesting polymerizable and graftable polymer combina- tions feasible.

Owing to the great variability of the diisocyanate polyaddition and diisocyanate polycondensation processes used to produce polycarbodiimides, together with the op- portunities for combining these processes with methods involving polymer-analogous reactions on polycarbodiimides, ring cleavage polymerization, copolymerization processes, and with grafting reactions involving many vinyl monomers, solutions to problems encountered in numerous industrially interesting fields of macromolecular chemistry, especially those of polymer blends and multiphase plastics, may be expected.

We wish to thank Professor K . Buchel, Professor W . Swodenk, and Dr. H. Rudolph for the help and encourage- ment they have given us. We also wish to thank Drs. G. Baatz, H.-D. Block, W. von Bonin, E. de Cleur, E. Dabritz, M. Dahm, R . Dhein, U . von Gizycki, E. Grigat, P. Fischer, H. Heitzer. H.-J. Hennig. R. Holm, H.-J. Kreuder, La Spi- na, D . Liebsch. F. Meisert, H.-J. Muller, P. Muller, U. Nehen, W . Oberkirch, G. Oertel, L. Preis, and R . Schliebs, the members of the Anwendungstechnische and Produktions- abteilungen der Polyurethan- and AC-Sparte, the Analy- tische and Polymerphysikalische Abteilungen des Zentralbe- reichs Forschung and Entwicklung and the Zentralbereich

829

Patente, Marken and Lizenzen of Bayer AG for close inter- disciplinary cooperation.

Received: February 2, 1981 German version: Angew. Chem. 93, 585 (1981)

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830 Angew. Chem. Int. Ed. Engl. 20, 819-830 (1981)

α,ω-diisocyanatocarbodiimides, -polycarbodiimides, and their derivatives

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