Solid-state synthesis of a ladder polymer

Solid-state Synthesis of a Ladder Polymer

K. C. YEE,* Materials Research Center, Allied Chemical Corporation, Morristown, New Jersey 07960

Synopsis

Solid-state synthesis of a cyclically bound ladder polymer from a cyclic tetradiyne (cyclodotriaconta-l,3,9,11,17,19,25,27-octayne) is described. Irradiation of the colorless, needlelike monomer crystals with interstitially incorporated chloroform with 50 Mrad of 6oCo y-ray radiation results in red-brown polymer fibers in nearly quantitative yield. Infrared, Raman, and x-ray diffraction analyses of the polymer are consistent with polymerization by a 1,4-addition reaction a t each diacetylene linkage to produce four fully conjugated chains joined together in pairs by a total of four -(CH2)*- interchain linkages per 4.8-A polymer repeat unit, Conformationally, it appears that the cyclic tetradiyne monomer polymerizes via a chair form. The results of the mechanical and thermal analyses indicate the presence of unreacted diacetylene functionality in the ladder polymer crystals.

INTRODUCTION

The solid-state synthesis of polydiacetylene single crystals was brought to light by Wegnerl in 1969. The reaction proceeds by a 1,4-addition polymerization at each diacetylene (RC=C-C=CR) to provide a trans fully conjugated polymer (A or B), as developed by Raman2,3 and x-ray ana ly~es .~-~ X-ray single-crystal analyses show that most diacetylene polymers have an acetylenicP6 (A) structure with a 4.9-A repeat unit along the chain axis:

RC=C-C=CR - +RC-C=C-CR+n - +RC=C=C=CRF,, A B

This solid-state polymerization of diacetylenes is a powerful tool for the syntheses of unique polymers which cannot be obtained by conventional methods; for ex- ample, single crystals of highly stereospecific backbone reinforced polydiacetylene have been synthesized8 from a cyclic diacetylene monomer [R = -C6H40CO(CH2)30COC6H4-] in the solid state. In addition, we have found that a new diacetylene polymer with stereoregular polarizable groups in the side chains (DCHD polymer, R is carbazolylmethyl)s can also be obtained by solid- state polymerization.

In contrast to conventional polymerizations, the solid-state polymerization of diacetylenes is controlled by the lattice packing of the monomers. Only those monomers that pack in proximity are solid-state polymerizable.1° It has been shown that the solid-state reactivities of certain linear diacetylenes can be modified by crystallizations under various conditions. Wegnerl has shown that needlelike crystals (Mod-I) of 2,4-hexadiyn-1,6-diol bisphenylurethane (HDU, R = -cH&CONHC6H5) obtained from a solution of dioxane-water are highly reactive. Alternately, the platelike crystals (Mod-11) of HDU obtained from a solution of anisole are relatively inactive in the solid state. The high solid-state

* Address correspondence to the author a t Sandoz Inc., Route 10, Bldg. 502, East Hanover, New Jersey 07936.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 17,3637-3646 (1979) Q 1979 John Wiley & Sons, Inc. 0360-6376/79/0017-3637$01.00

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reactivity for Mod-I has been attributed to the incorporation of dioxane into the monomer phase, which modifies the lattice packing and results in a reactive phase.

Demand for fibers of high modulus and high thermal stability has led to the development of ladder polymers.ll It is known that the stereoregularity of a polymer has a pronounced effect on the physical properties in the solid state. If the degree of crystallinity of a ladder polymer can be increased, then the resulting polymer will have improved mechanical strength as well as improved thermal stability. With respect to this aspect, solid-state polymerization appears to be advantageous in synthesizing ladder polymers. In this article, we report on the synthesis of a cyclically bound ladder polymer with carbon atoms in the backbone via solid-state polymerization of a cyclic tetramer. The structure of the cyclic monomer is shown schematically:

~ + C ~ ( C H Z ) ~ C = C + ~ ~

EXPERIMENTAL

Preparation of the Cyclotetradiyne Monomer Cyclodotriaconta- 1,3,9,11,17,19,25,27-Octayne

To a 2000-ml, three-necked, round-bottomed flask fitted with a mechanical stirrer, thermometer, additional funnel, and reflux condenser were added 225 g (1.14 mole) of cupric acetate monohydrate and 1500 ml of pyridine. The resulting mixture was stirred vigorously, and 15.0 g (0.141 mole) of 1,7-octadiyne was added in one portion at 43°C (no increase in reaction temperature occurred). After this addition the reaction mixture was stirred and heated at 4343°C for 4 hr, cooled, and filtered to separate solids from liquids. The solids retained on the filter were washed with three 300-ml portions of benzene. The benzene washings were combined with the pyridine filtrate, and the solvents were removed under reduced pressure to near dryness. The resulting dark brown residue was taken up with 1500 ml of 40% (v/v) water-benzene solution. The benzene layer was separated from the aqueous layer. The benzene solution was washed in sequence with 100 ml of water, two 200-ml portions of 3N HCl solution, and 200 ml of water and dried over MgS04. Concentration of the solution and drying under reduced pressure (0.1 mm) gave 7 g (50% yield) of a solid product of cyclic polyacetylenes.

The desired cyclotetradiyne monomer was isdated from the cyclic polyacet- ylene by liquid column chromatographic separation. The results are summarized in Table I. The structure of the cyclodimer and cyclotrimer were confirmed by mass spectrographic analyses. Because the cyclotetramer decomposed under the conditions of mass spectrographic analysis, a sample of the cyclotetramer was hydrogenated in dioxane with 10% Pt/C catalyst to yield the corresponding cycloalkane. The results of the mass spectrographic analysis showed that the resulting cycloalkane had a molecular weight of 448.5, in good agreement with the calculated value of 448.8 (C32H64). The cyclotetramer obtained from the chromatographic separation melted at 152"C, in agreement with the reported value (155°C).12 The infrared (IR) spectrum (KBr) showed absorptions at 2160 and 2250 cm-l for the C = C stretchings. NMR (CDC13) showed resonance peaks at 6 2.30 (triplets, C=C-CH~-CH~-CH~-CHF-C<) and 1.65 (multiplets, C=C-CH~-CH~-CH~-CH~-C~C) with a nearly equal integrated area. Raman spectroscopy of the solid showed an intense band at 2256 cm-I and weak

SOLID-STATE SYNTHESIS OF A LADDER POLYMER 3639

TABLE I Chromatographic Separation of Cyclic Polyacetyleness

Volume Weight mP Compounds (ml) (g) ("C)

... ... Voidb 3150 Dimer 3420 0.29 162 Void 3020 z * . Trimer 5600 0.62 167 Void 5500 Tetramer 11,550 0.48 152 Void 5250 Mixture' 6300 0.07 130-135 Hexamer' or 9000 0.07 162

... ... ... ... ...

heptamerC

a Crude product (7 g) was dissolved in 160 ml of benzene. N-hexane was added until a slight white precipitate appeared, which was then taken up by adding toluene in sufficient amount to redissolve the precipitate. The resulting solution was filtered through a column (48-cm length X 10-cm di- ameter) slurry packed with 2200 g of silica gel (Mallinckrodt CC-7) and 75% hexane-toluene. The column was eluted with 75% hexane-toluene at a flow rate of 2.5 ml/min.

Void volume is volume between components. As indicated by thin layer chromatographic results.

bands at 1429, 1349, 1236, and 994 cm-l. Elemental analysis was satisfac- tory.

ANAL. Calcd for C32H32: C, 92.3%; H, 7.7%. Found: C, 92.26%; H, 7.97%.

Solid-state Polymerization of the Cyclotetradiyne Monomer

Needlelike crystals of the monomer were obtained by crystallization from 25% (v/v) chloroform-petroleum ether (60-110). Crystallization may be effected conventionally by room temperature evaporation of solutions that contain 0.001-0.01 parts of monomer per part of solvent blend. These needlelike crystals contain interstitial chloroform and are highly reactive in the solid state. The monomer was polymerized by using 50 Mrad of 6oCo y radiation at a dose rate of 1 Mradhr at ambient temperature. Extraction of the as-polymerized sample with excess chloroform showed that the weight percent conversion of monomer to polymer was nearly quantitative. C and H analyses show that the polymer contains one chloroform molecule per monomer unit.

ANAL. Calcd for (C32H32)x: C, 92.3%; H, 7.69%. Calcd for (C32H3&HC1& C, 73.97% H, 6.16%; C1, 19.86%. Found: C, 74.08%; H, 6.56%.

To determine the presence of CHC13 in the polymer 2.7 mg of polymer crystals was used for C1 analysis. The results showed 14% C1 in the polymer, in reasonable agreement with the theoretical value of 19.9% considering the small size of the sample used for the analysis. Characterization of the polymer with IR, Raman, and x-ray diffraction is described in the following section.

RESULTS AND DISCUSSION

Synthesis of Monomer The monomer cyclodotriaconta-1,3,9,11,17,19,25,27-octayne was prepared

according to the method of Sondheimer et a1.12 with modifications. Briefly, this

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monomer can be prepared by oxidative coupling of 1,7-octadiyne with cupric acetate in pyridine. The result is a mixture of cyclic dimer, trimer, tetramer, pentamer, hexamer, and so on. These cyclic polyacetylenes were isolated by liquid column chromatography.

Monomer Phase

As reported by Sondheimer,12 we have also noted that the cyclodimer, cyclotrimer, and cyclotetramer obtained from the column are crystalline colorless materials which gradually change to yellow and then brown on exposure to light a t room temperature. Apparently these phases of cyclic polyacetylenes are not suitable for solid-state synthesis of crystalline polymer because they do not show the bright (red or blue) color changes observed by Wegner for the linear dia- ~ety1enes.l.l~ We found, however, that crystallization of the cyclotetramer from chloroform or from mixtures of inert solvents with chloroform (such as petroleum ether-chloroform) at room temperature produces colorless needlelike crystals, which change readily to pink on light exposure at room temperature. This ob- servation indicates that these crystals are solid-state polymerizable. As shown in the following sections, the change in reactivity can be attributed to the mod- ification of the monomer packings by adding chloroform molecules to the monomer phase. As shown in elemental analysis, the amount of interstitially incorporated solvent in the monomer lattice is about one chloroform per cyclotetramer unit. In contrast to the HDU system in which half the dioxane molecule is hydrogen bonded to each monomer molecule4 the interstitial chloroform molecules are weakly bonded by van der Waals forces in monomer or polymer structures. If these colorless crystals are kept under reduced pressure for hours or are allowed to sit a t room temperature in the dark for days, they change to a yellow powder that is not solid-state polymerizable but has nearly the same melting point as the freshly prepared needlelike monomer. We have also found that it is not true that all cyclic polyacetylenes with solvent molecules incorporated in the lattice are solid-state polymerizable. Specifically, we have noted that crystallization of the cyclotetramer from benzene or benzene-petroleum ether by slow evaporation at room temperature yields colorless platelike crystals that are photochemically inert. IR and lH-NMR studies show that these crystals contain one benzene molecule per cyclotetramer unit. They stay colorless (unreactive) as long as the benzene molecule remains. These findings clearly demonstrate that the lattice packing of the monomer phase is an important factor in governing the solid-state polymerizability of cyclic polyacetylenes.

Solid-state Polymerization

It is essential that during polymerization the interstitially incorporated chloroform be present in the crystal structure. Hence solid-state polymerization is preferably carried out under vapor pressure of chloroform to prevent evaporation of the interstitially incorporated chloroform. This may be accomplished conveniently by storing the crystallized monomer in closed vessels in the presence of chloroform.

Solid-state polymerization of monomer crystals (needlelike) that contain interstitially incorporated chloroform may be effected by subjecting them to high-energy radiation, heat, or mechanical stress. We have found that irradiation


with ~ C O y rays is a convenient method of effecting solid-state polymerization. Irradiation of colorless needlelike crystals (0.05 g in 13 ml of 23% C H C l a E ) with 50 Mrad of y rays at a dose rate of 1 Mradhr a t room temperature resulted in red-brown fibers. The 50-Mrad sample was extracted with excess chloroform to remove unreacted monomer. Concentration of the chloroform extractant produced a small number of pink monomer crystals, which indicated that the weight percent conversion of monomer to polymer is nearly quantitative. We have noted that freshly prepared polymer contains one chloroform molecule (22.3% wt) per monomer unit. When the polymer was dried under vacuum, however, the number of chloroform molecules incorporated in the polymer de- creased. Therefore the chloroform is interstitially incorporated as in a clarthrate. Because the chloroform molecules are weakly bonded to the polymer by van der Waals force, it is difficult to analyze them quantitatively. Accordingly, we did not measure the conversion4osage curve by the extraction method. In contrast to the y-ray polymerization of diacetylene DCHD,S which contains aromatic substituents and shows an S-shaped conversion-dosage curve, cyclotetramer, which contains only aliphatic substituents, is expected to show a C-shaped conversion-dosage curve in y-ray irradiation. Alternatively, thermal polymerization at 25-30°C for a two year period yielded about 40% polymer.

Characterization of Polymers

The resulting polymers are infusible and insoluble in common organic solvents andhave almost the same shape as the precursor monomer crystals. They are strongly dichroic, with the axis of dichroism parallel to the polymer needle axis. The IR spectra of the polymer are essentially the same as those of the monomer, which indicates that the polymer possesses the same functional groups as the monomer. Raman spectral evidence is consistent with a 1,4-addition reaction at each of the four diacetylene groups in the monomer molecule. Raman intense vibration at 2255 cm-l( vc-c) of the monomer disappears during polymerization and is replaced by Raman intense vibration at 2105 cm-l(vc-c) and 1490 cm-l(vc=C) in the polymer, which corresponds to the multiple-bond vibrations in the backbone. X-ray diffraction analyses show that the polymer is crystal- lographic only in the chain axis projection.* The probable symmetry of the chain axis projection is Pgg. The unit-cell parameters are a = 12.4 A, b = 24.8 A, and c = 4.8 A for an orthorhombic unit cell, where c is the polymer chain direction. Assuming that there are two monomer molecules and two molecules of chloroform per unit cell, the calculated density is 1.206 g/cm3, compared with the measured bulk density of 1.14 g/cm3. These results are consistent with the solid-state polymerization of the cyclotetramers (needlelike crystals) by a 1,4- addition reaction which gives rise to a cyclically bound ladder polymer.

There are a t least three possible isomeric structures for the resulting ladder polymer, depending on whether the monomer reacts from a crown-, boat-, or chairlike conformation. As shown in Figure 1, if the cyclic tetramer reacted by a 1,4-addition reaction from a crownlike conformation, the resulting polymer would have a helical backbone structure.? The repeat unit along the chain axis

* Characterization of the ladder polymerization of a crystalline cyclotetradiyne monomer, A. Banarjee, J. B. Lando, K. C. Yee, and R. H. Baughman, J. Polym. Sci. Polym. Phys. Ed., in press.

t For helical conformational structure see L. E. Alexander, in X-Ray Diffraction Methods in Polymer Science, Wiley, New York, 1969, Chap. 6.

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R = -(CH2)4-

Fig. 1. Schematic representation of solid-state polymerization of the cyclotetradiyne monomer by a crownlike conformation that gives rise to a ladder polymer by a 1,4-addition reaction at each diacetylene functionality; b is the repeat unit along the reaction chain axis of the monomer.

would be much larger than 4.9 A in contrast to the x-ray diffraction analysis. In addition, a molecular model constructed from the Corey-Pauling-Koltun (CPK) models indicates that the polymer structure is highly strained. Consequently, we have rejected this form of polymerization. As shown in Figure 2, if the monomer reacted from a boatlike conformation, the resulting polymer' would be highly chain aligned with four fully conjugated chains joined together in pairs by a total of four tetramethylefie linkages per 4.9-A polymer unit in agreement with the x-ray diffraction analyses. Because the polymer structure constructed from the CPK models is quite strainless, this polymerization method is highly probable. Alternately, the cyclic tetramer could also react from a chairlike conformation, as shown in Figure 3. The resulting polymer is also highly chain

hY A

R = -(CH2)4-

Fig. 2. Schematic representation of solid-state polymerization of the cyclotetradiyne monomer by a boatlike conformation that gives rise to a ladder polymer by a 1,4-addition reaction a t each diacetylene functionality; b is the repeat unit along the reaction chain axis of the monomer and along the polymer chain axis of the polymer.


T b

R ' - (CH2 14-

Fig. 3. Schematic representation of solid-state polymerization of the cyclotetradiyne monomer by a chairlike conformation that gives rise to a ladder polymer by a 1,4-addition reaction a t each diacetylene functionality.

aligned with four fully conjugated chains joined together in pairs by a total of four -(CH2)4- linkages per 4.9-81 polymer unit. Because the resulting polymer structure is also strainless, as indicated by the CPK models, and in general a chair is conformationally more stable than a boat,* it appears that polymerization of the cylocotetramer by a chair conformation is the most favorable process.

Mechanical and Thermal Properties of Cyclotetradiyne Polymer

In theory, this type of highly chain-aligned cyclically bound ladder polymer would have good mechanical properties and thermal stability. The mechanical strength of polydiacetylene fibers, such as poly(HDU),14 has been shown to be inversely proportional to its cross-sectional area per chain (Ac) . In the chloroform-containing ladder polymer crystals the molecular weight per chain length (4.8 81) is 536.0 g/mole and the density is 1.14 g/cm3. Therefore A, is 162.7 A2. In theory a ladder polymer with the same realized per chain strength and modulus as poly(HDU) would have an ultimate strength of 1.0 X lo3 N/mm2 and a modulus of 2.7 X lo4 N/mm2. Experimentally, we have found that the maximum ultimate tensile strength and dynamic modulus are 2.2 X lo2 and 5.5 X lo3 N/ mm2, respectively, for polymer fibers with a 10-5-10-4-~m2 cross-sectional area. These poor mechanical properties could be attributed to the chain misalignment and gross structural imperfections in the polymer. X-ray diffractions (Debye-Schemer, with Ni-filtered Cu radiation) showed that the polymer fiber was only 36% crystalline in the equatorial direction (i.e., perpendicular to the polymer chain direction). These findings suggest that the ladder polymer might have unreacted diacetylene groups that would result in structural imperfections or disorder.

Typical results of the thermogravimetric analysis (TGA, DuPont 951,

* Cyclotetradeca-1,3,8,lO-tetrayne has been reported to exist in chairform. See F. Sondheimer, Y. Amiel, and R. Wolvsky, J . Am. Chem. So,., 79,6263 (1957).

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1O0C/min heating rate, in N2) of cyclotetradiyne polymer are shown in Figure 4. The polymer crystals were prepared by y-ray polymerization (50 Mrad) and stored at room temperature for several days before analysis. The TGA thermogram indicated an early weight loss of 9% at 100-200°C, followed by a stable plateau up to 390OC. Between 400 and 500°C an exothermic degradation occurred with a 48% weight loss a t 500°C. The early-stage weight loss (with a maximum weight loss a t 15OOC) can be attributed to the volatilization of the incorporated chloroform in the polymer. Elemental analyses show that the polymer consisted of 85.92% C, 8.09% H, and 7.7% of C1 (total 101.71%). These findings also show that this ladder polymer had one-third of a chloroform molecule per monomer unit; that is, [( C8H8)4+(CHCl3)ln. The incorporated chloroform can be completely removed by heating at 15OOC for 2 min. The resulting ladder polymer was stable up to 4OOOC with a 41% weight loss a t 50OOC (see Fig. ~5). To our knowledge this ladder polymer had the highest thermal

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Fig. 5. Typical TGA thermogram of a chloroform-free cyclotetradiyne polymer at a heating rate of lO"C/min in Nz.


5 - s x 4 - - -1

E

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DSC 197

Fig. 6. Typical DSC scan of a cyclotetradiyne polymer containing 9 wt % of incorporated chloroform at a heating rate of 10"C/min in N2.

stability among the diacetylene polymers investigated. The most stable polydiacetylene previously reported is DCHD polymer: which is stable up to 300°C. Apparently the unusual thermal stability of the ladder polymer is due to unusual cyclically bound ladder polymer structure.

Differential scanning calorimetry (DSC, DuPont 990, in Nz, 10°C/min heating rate) shows that the ladder polymer with 9% incorporated chloroform displayed a significant exotherm, starting from 105OC with a peak temperature a t 197OC (see Fig. 6 ) . Instead of a usual endotherm due to the volatilization of solvent, the presence of an exotherm suggested the occurrence of a reaction. This reaction was due to the 1,4-addition polymerization of the (unreacted) diacetylene groups.* Apparently after the volatilization of chloroform the unreacted dia-

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TEMPERATURE, OC

Fig. 7. Thermomechanical analyses of a cyclotetradiyne polymer showing the specific change in length parallel to the fiber axis as a function of temperature. See text for details; (0) initial heating, (n) first cooling, ( X ) first reheating, (0) second cooling.

* Solid-state polymerization of diacetylene via l,.l-addition reaction is an exothermic reaction. See R. R. Chance, G. N. Patel, E. A. Turi, and Y. P. Khanna, J. Am. Chem. Soc., 100,1307 (1978); E. M. Barrall, 11, T. C. Clark, and A. R. Gregges, J. Polym. Sci. Polym. Phys. Ed., 16, 1355 (1978).

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cetylene groups can pack to a closer proximity, hence polymerize accordingly. In addition, the chloroform-free ladder polymer crystals show new Raman vibration bands at 2113 cm-l ( V C ~ C ) and 1508 cm-l ( v c , ~ ) due to a change in the crystal-packing environment. Thus these findings provided evidence that the polymer crystals obtained by y-ray polymerization (50 Mrad) have unreacted diacetylene groups.

Thermal mechanical analyses (TMA, Perkin-Elmer TMS-1, heating rate lO"C/min, in He) shows that the cyclotetradiyne polymer crystals exhibit positive thermal expansion coefficients along the chain direction in contrast to the unusual negative thermal expansion coefficient observed in the polydia~etylenes.~J~ The results are shown in Figure 7. During the initial heating the ladder polymer with 9% incorporated chloroform shows a near linear expansion coefficient (4.46 X 10-60C-1) in the temperature range of -50 to +4OoC. After heating at 100°C for 30 min (presumably to volatilize the incorporated chloroform) the first cooling and reheating portions of a cycle are not completely reversible. After this treatment, however, the repeated cooling-heating cycles are reproducible. This positive thermal expansion behavior is consistent with the presence of unreacted diacetylene groups, which are responsible for the generation of unaligned amorphous fractions, stable voids, or chain misalignments in the ladder polymer crystals.

Further characterization of the polymer with respect to the degree of unreacted diacetylene functionality is in progress. The results will be reported else- where.

The author thanks R. H. Baughman for useful suggestions and criticisms. Thanks are also due to J. D. Witt for Raman spectra, L. Palmer for chromatographic separation, A. W. Hanson and P. A. Apgar for single-crystal x-ray diffraction measurements, J. Sidun for elemental analyses, B. H. Vrooman for mechanical strength and density measurements, and E. Turi, D. Richardson, and W. Wenner for TGA, DSC, and TMA measurements.

References

1. G. Wegner, 2. Naturforsch., 24b, 824 (1969). 2. A. J. Melveger and R. H. Baughman, J. Polym. Sci. A-2, 11,603 (1973). 3. R. H. Baughman, J. D. Witt, and K. C. Yee, J. Chem. Phys., 60,4755 (1974). 4. E. Hadicke, H. C. Mez, C. H, Krauch, G. Wegner, and J. Kaiser, Angew. Chem., 83, 253

5. D. Kobelt and E. F. Paulus, Acta Crystallogr., B30,232 (1974). 6. P. A. Apgar and K. C. Yee, Acta Crystallogr., B34.957 (1978). 7. D. Day and J. B. Lando, J. Polym. Sci. Polym. Phys. Ed., in press. 8. R. H. Baughman and K. C. Yee, J. Polym. Sci. Polym. Chem. Ed. , 12,2467 (1974). 9. K. C. Yee and R. R. Chance, J. Polym. Sci. Polym. Phys. Ed. , 16,431 (1978).

(1971).

10. R. H. Baughman, J. Polym. Sci. Polym. Phys. Ed. , 12,1511 (1974). 11. J. W. Powell and R. P. Chartoff, J. Appl. Polym. Sci., 18, 83 (1974); V. N. Salaurov et al.,

12. F. Sondheimer, Y. Amiel, and R. Wolovsky, J. Am. Chem. SOC., 81,4600 (1959). 13. G. Wegner, Makromol. Chem., 154,35 (1972). 14. R. H. Baughman, H. Gleiter, and N. Sendfeld, J. Polym. Sci. Polym. Phys. Ed. , 13, 1871

15. R. H. Baughman and E. A. Turi, J. Polym. Sci. Polym. Phys. Ed., 11,2453 (1973).

Makromol. Chem., 175,757 (1974).

(1975).

Received May 3,1978 Revised November 6,1978

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Solid-state synthesis of a ladder polymer