VOL. 15, PP. 481-485 (1977) POLYMER LETTERS EDITION

X-RAY STRUCTURE OF POLY(CY-TRUXILLIC PIPERAZIDE), A PHOTOREACTIVE POLYMER

Pioneering work from Hasegawa's group has recently shown (1 -3) that poly- amides from a-truxillic acid and aliphatic diamines are photoreactive. In fact, they are easily depolymerized by the action of ultraviolet light, showing also a photoreversible behavior as a function of the incident wavelength. Structural information on these polymers are therefore of obvious interest, but these authors (2) reported that their samples were amorphous. We have now suc- ceeded in preparing a highly crystalline polyamide (I) derived from a-truxillic acid and piperazine (4) and we have obtained some details on its fiber struc- ture.

c6H5K?- -0c

I

Only two internal rotation angles per repeat unit need to be considered in order to generate the most likely structural model of polyamide I . In fact, the piperazine rings are rigid units possessing a chair-like shape (5 ) , and the polymer chain can be built without introducing additional internal rotation angles. However, due to the restricted rotation around the amide C-N bond, two amide arrangements (cisoid and transoid) are possible around the piper- azine unit.

cisoid transoid

Both forms were found to be present at the equilibrium in N,N-diacetylpiper- azine in solution (6), so that sequences of both amide arrangements could be present in polyamide I or, more likely, as a consequence of the crystal struc- ture, one would become preferred over the other.

X-Ray examination of untreated polymer powders yielded typical amor- phous pattern, but a solution-cast film yielded diffraction patterns with charac- teristic sharp rings, demonstrating that the film was isotropically crystalline. Hot-stretching of the latter resulted in an oriented crystalline sample, as shown in Figure 1. All observed reflections could be satisfactorily indexed (Table I) by a least-squares procedure on the basis of an orthorhombic unit cell with dimensions a = 8.74 A, b = 5.93 8, and c = 9.22 A (fiber axis). The crystal- lographic density value, obtained assuming only one monomeric unit per cell (1.202 glcm3), agrees well with the experimental value of 1.2 15 glcm3 (floata- tion, Westphal balance).

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0 1977 by John Wiley & Sons, Inc.

482 POLYMER LETTERS EDITION

Fig. 1. X-Ray diffraction pattern of an oriented film of poly(a-truxillic piperazide).

TABLE I

Miller Indices, Lattice Spacings, and Diffracted Intensities for Crystalline Poly(a-

truxillic Piperazide)

h k 1 a(;) Intenaity b (obs) (calcd)

1 0 0 8.69 8.74 9

0 1 0 5.94 5.93 va

2 0 0 4.33 4.37 m

0 1 1 4.97 4.98 m

0 0 2 4.57 4 .60 x

1 0 3 3.99 4 . M w

1 0 3 2.92 2.90 W

aR = 0.0258. bVisually estimated.

These data are consistent with the structural model of polyamide I, shown in Figure 2 . The molecule exists in solid state in the fully extended form with the carbonyls bisecting the cyclobutane ring and with transoid amide sequences along the chain. In fact, if the amide bonds in the piperazine moieties were arranged in cisoid sequences, the repeating unit and the fiber axis would be doubled with respect to the value shown in Figure 2. Furthermore, from ex- amination of Dreiding models, carbonyls eclipsing on the C-C cyclobutane bonds would give a shorter repeating unit (-8.7 A) than the observed.

POLYMER LETTERS EDITION 483

I C: 9.22 A

Fiber 8x1s

I

Fig. 2. Structural model of poly(a-truxillic piperazide).

According to our close-packing model, the chains are arranged in alternate layers: the chains of two adjacent layers are shifted one-half repeat unit along the fiber axis, so that the phenyl rings of polymer chains lying in adjacent layers interpenetrate each other to a certain extent.

The conformation of the two carbonyls with respect to the cyclobutane ring may be inferred also from the experimental dipole moment (DM), 1.90 D, of trans,cis,trans-l,3-di-(N,N-dimethylcarbamyl)-2,4-diphenylcyclobutane (11). A contour map of the calculated DM as a function of d 1 and O 2 angles is re- ported in Figure 3, with the starting conformation and the rotation system. Similar combined DM and “exclusion” maps have been previously used (7) to define the conformational properties of other cyclobutane dicarbonyl deriva-

484 POLYMER LETTERS EDITION

n

t y’

0

n 2

0 - A 4

Fig. 3 . Contour map of calculated DM (Debyes) of diamide 11 as a function of the two internal rotation angles 8 I and O 2 ; overlapped, conformationally allowed area.

tives and details of the procedure can be extracted from that work. Overlapped on the DM map, an energetic “exclusion map” is generated using Dreiding mod- els. Only a narrow range of O and 0 2 , values is compatible with either the experimental DM and the exclusion map. Therefore, it is chosen as the pre- ferred conformational area of 11, showing the two carbonyls ranging from the bisecting position (01r90, O2z9O) to the eclipsed one ( 8 , ~ 1 3 5 , ~ 9 ~ z . 1 3 5 ) . In solution, the molecule may undergo some torsional oscillation around the equi- librium dihedral angle. The observed DM may therefore represent averaged values, and this accounts for the experimental value being higher than that calculated for the bisecting conformation. These a priori data support a simi- lar conformational preference in polyamide I .

Polyamide I was subjected to solution photolysis, in comparison with com- pound I1 and other stereoisomers (8). Further work on the solid-state photo- reactivity of oriented and unoriented films is in progress.

.

Experimental

Polyamide I was prepared and characterized as previously reported (4). The synthesis and characteristics of diamide I1 have already been reported (9).

POLYMER LETTERS EDITION 485

X-Ray diffraction patterns of the film strips were taken with a flat-plate camera using Ni-filtered C u b radiation. From the diagrams, it has seen that the first meridional reflection was absent. The second-order meridional reflec- tion was spread into an arc. To decide whether this is a true meridional reflec- tion or an overlap of two adjacent reflections, the fiber was tilted towards the x-ray beam at the equi-inclination angle for the second-order meridional reflec- tion of spacing 4.57 A (Bragg angle 0 = 9.75’). The resulting photograph led to a strong intensification of the meridional reflection.

The dielectric constant and refractive index of I1 were measured in benzene at 25°C according to techniques already described (7), for w2 = 0.00236 +

0.00786 using the final formula of a Guggenheim procedure (7) to obtain DM = [0.009208 M2(a, - a,)] where a, = 1.4018 and a,, = 0.2743. To ob- tain the overall calculated DM, the amide group moment was taken as 3.84 D, and its direction was assumed to form an angle of 39.6” with the C-N bond (10). The N-C-0 angle was taken as 120” and the cyclobutane ring as planar.

The authors thank the National Research Council of Italy for financial sup- port.

References

(1) H. Takahashi, H. Takahashi, M. Sakuragi, and M. Hasegawa, J. Polym.

(2) H. Takahashi, M. Sakuragi, M. Hasegawa, and H. Takahashi, J. Polym.

(3) M. Hasegawa, Y. Suzuki, H. Nakanishi, and F. Nakanishi, Progr. Polym.

(4) S. Caccamese, P. Maravigna, G. Montaudo, and M. Przybylski, J. Polym.

(5) E. L. Eliel, N. L. Allinger, S. J. Angyal, and G. A. Morrison, “Con-

(6) G. Montaudo and P. Finocchiaro, J. Org. Chem., 37, 3434 (1972). (7) G. Montaudo and S. Caccamese, J. Org. Chem., 38, 710 (1973). (8) S. Caccamese, P. Maravigna, G. Montaudo, A. Recca, and E. Scampor-

(9) G. Montaudo, P. Maravigna, S. Caccamese, and V. Librando, J. Org.

(10) Y. Wada, in “Poly-cw-AminoAcids,” G. D. Fasman, Ed., Dekker, New

Sci. B, 2, 685 (1971).

Sci. A, 10, 1399 (1972).

Sci. Japan, 5, 143 (1973).

Sci. Polym. Chem. Ed., 13, 2061 (1975).

formational Analysis,” Interscience, New York, 1966, p. 250.

rino, J . Polym. Sci. Polym. Letters Ed., l3, 517 (1975).

Chem., 3 , 2 8 0 6 (1974).

York, 1967.

Institutes of General and Industrial Chemistry, University of Catania Catania, Italy

Received January 17, 1977 Revised March 8, 1977

Giovanni Bruno Salvatore C accamese Pietro Maravigna Giorgio Montaudo Antonino Recca