CHIRALITY 3:285-291 (1991)

Cooperation in a Deep Helical Energy Well MARK M. GREEN, SHNEIOR LIFSON, AND AKIO TERAMOTO

Department of Chemistry and Polymer Research Institute, Polytechnic University, 333 Jay Street, Brooklyn, New York 11201 (M.M.G.), Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76150, Israel (S.L.), and Department of Macromolecular Science, Faculty of

Science, Osaka University, Toyonaka, Osaka 560, Japan (A.T.)

ABSTRACT In contrast to random coil polymers, polyisocyanates maintain a highly extended helical conformation in solution. This structural characteristic causes unusually large chiral optical properties to arise from copolymerization of tiny proportions of optically active monomer isocyanates with achiral isocyanates or even from stereospecific placement of deuterium in the side chain of poly(n- hexyl isocyanate). These effects can be understood as phenomenologically related to the optical activity amplification properties of vinyl polymers studied by Pino and his co-workers and ascribed to breaking the energetic degeneracy of the oth- erwise equally populated left- and right-handed helical states of the backbone. Statistical thermodynamic calculations, based on this model, and analogous to those carried out earlier on the vinyl polymers, allow matching the temperature and molecular weight dependence of the optical activity in poly((R)-1-deuterio-1- hexyl isocyanate) to the approximate responsible energy terms.

KEY WORDS: macromolecular, optical activity, polyisocyanate, deuterium

INTRODUCTION

Polyisocyanates created first at DuPont about 30 years ago,' have been the subject of wide interest fol- lowing the discovery of their stiff properties. Various studies, most notably the light scattering dimension, intrinsic viscosity and the dipole moment, all as a func- tion of molecular weight, demonstrate that these poly- mers scale as rigid rods a t low degree of polymeriza- tion.' At higher molecular weights the polyisocyanates may be described as wormlike, which may arise from various potential sources of flexibility accumulating with higher probability among the many monomer units in the hai in.^,^

Stiff polymer properties must arise from one confor- mational state of great preference over all others and in the case of the polyisocyanates, as is typical for bi- ologically interesting macromolecules, this is a helix.5 The interesting difference in the polyisocyanates is the absence of a preference for one helical sense. There is no stereogenicity in the isocyanate monomer unit. This fact of symmetry brought the suggestion that helix re- versals could contribute to the flexibility of the chain: which could then be described as a broken rod rather than a worm.

Questions of polymer chain flexibility arising from accumulated torsional and angular motions and/or from specific interruptions, as in the helix reversals, have been of interest for many years for various stiff polymers including polypeptides and DNA, with the conclusion that the experimental tools of polymer phys- ics were inadequate to distinguishing these source^.^ Nevertheless, in the polyisocyanates small local tor-

0 1991 Wiley-Liss, Inc.

sional motions can be shown theoretically to be consis- tent with the chain dimensional properties.8

The discussion above can be related to the work of the Pino School on the optical activity properties of certain vinyl polymers. One of Pino's interests was the relation of the conformation of vinyl polymers in solu- tion to that in the solid state. In the solid, extensive x-ray work had shown helical conformations for ste- reoregular vinyl polymers and Pino was convinced, by many experimental results, that such helical confor- mations interrupted by numerous helix reversals oc- curred in solution and were responsible for the en- hanced chiral optical properties of these polymer^.^^^^

It was even possible to demonstrate an increased di- mension in solution in an optically active vinyl poly- mer, a result required if the helical segment lengths were increased by favoring one helical sense. l1

As reviewed below, this identical qualitative view," differing in the fact of far fewer helix reversal states in the stiff polyisocyanates, allows understanding of the extraordinary chiral optical properties of these macro- molecules and an answer to the question posed about the role of helix reversals and torsional motion to the chain f le~ib i l i ty .~

DISCUSSION

Goodman and Chen, in Brooklyn, were the first to incorporate stereogenicity in the side chain of a poly- isocyanate and discovered an unusually large optical

Received for publication February 26, 1991; accepted April 8, 1991. Address reprint requests to Mark M. Green at the address given above.

286 GREEN ET AL.

activity, associated by circular dichroism spectroscopy (CD) with the main chain conformation.12 Much later, with a view of preparing an optically active water sol- uble stiff polymer, poly[(S)-2,2-dimethyl-l,3-dioxolane- 4-methylene isocyanate] (1) was prepared and its light scattering radius of gyration was discovered to be ex- ceptionally large for the degree of p01ymerization.l~ This increased dimension could be assigned to the ab- sence of helix reversals, since the optical activity prop- erties suggested one helical sense. There was though the question of the size of the cyclic pendant in 1 which could cause large changes in local torsional motion compared to a simple n-alkyl pendant polyisocyanate. Ultraviolet (UV) and carbon-13 NMR showed these tor- sional restrictions. l4

The desire to prepare an optically active polyisocy- anate of identical steric properties to poly(n-hexyl iso- cyanate) to overcome the uncertain source of the in- creased dimension in l ,13 combined with an interest in cooperative effects in polymers,

0

I PCC

FERMENTING YEAST

1 . p-bromobenzenesulfonyl chloride/

2. NaN3/DMF/H2O/phase transfer cat. pyridine/CHC13

1 77x

[ -0.039 (neat!

dry HCI

COcl2 I 90%

stimulated the idea to prepare a polyisocyanate stereo- specifically substituted by deuterium in the side chain. The monomer target (2) was prepared as outlined in Scheme 1 and was obtained with the expected, ex- tremely low, D-line optical r0tati0n.l~

Polymerization of 2 yielded several molecular weights. All had large and highly temperature- dependent optically activities (Fig. l), and ultraviolet spectra identical to poly(n-hexyl isocyanate).15

The shape of the circular dichroism and the shape and intensity of the ultraviolet spectra of poly[(R)-1- deuterio-1-hexyl isocyanate] (3) (Fig. 2) are molecular weight independent, and show that the chromophore associated with the backbone structure is the source of the optical activity. This demonstrates that an ex- cess of one helical sense has resulted from the deute- rium substitution. This result demands that the deu- terated stereocenters are not acting individually, but rather cooperatively, in influencing the helical sense since only a few calories per mole per deuterium stere- ocenter favoring one helical sense could be reasonably expected.

The cooperative interactions necessary for the helix sense excess observed in (3) could be understood based on the conformational properties developed by the many physical chemical of poly(n-hexyl isocyanate).

If the poly(n-hexyl isocyanate) could be described by two conformational states, helical and helix reversal, with the latter state necessarily rarely encountered so as to be consistent with the stiff chain character, one could understand a cooperative mechanism. Since the

1 18S'C. 2 hrs + - - - - y c o

~ *.* ----xNa3c' ,.-. 3S-68% D n D H

lal~'+0.6S (neat)

Scheme 1. Synthesis for (R)-1-deuterio-1-hexyl isocyanate. (2)

helical units between reversal states must take on the same helix sense, any chiral perturbation favoring one helical sense would be summed over these units there- fore magnifying the per unit influence.

This idea predicts an interesting molecular weight dependence for the optical activity in (3). If the degree of polymerization is much larger than the number of units between reversal states the optical activity must be independent of degree of polymerization, i.e., the cooperation is limited by the units between reversals. Alternatively, if the degree of polymerization is low so that many chains have no reversal states, the cooper- ation and therefore the helix sense excess induced op- tical activity would be directly a function of chain length.

The data in Figure 1 support this idea. The three highest molecular weight samples (3) with average de- grees of polymerization, 4600, 6800, and 7800, exhibit identical optical activity temperature dependencies while the two lowest molecular weights, differing only slightly, yield optical activities different from each other and from the higher molecular weights. Further, the lower molecular weight has the lower optical activ- ity. These results (Fig. 1) suggest, in line with the con-

COOPERATION IN A DEEP ENERGY WELL 287

,/-' CD(-

poly ((R)-1-deutero-n-hexyl isocyanate) - -1 and UW-) spectra of

\

\ in hexanea 1.88 xlO% '-. 8 - t I I \ . \

\ Re-# 0 ' I

I f 0 0 \ '\ -4 I I1 I fN-C),

0 -

Temperature ('C)

Fig. 1. Optical rotation as a function of degree of polymerization (DP) for poly((R)-1-deuterio-1-hexyl isocyanate) (3).

- 4

, O

- - -2 '-0

2 0

formational cooperative picture drawn above, that the lowest molecular weights samples (Fig. 1) are in the range where the degree of polymerization is not large compared to the number of units between helix rever- sal states. The opposite conclusion can be drawn for the three higher molecular weight samples.

This cooperative idea is expressed pictorially in Fig- ure 3 where the excess energy of a helix reversal state is designated by E, and the diastereomeric energy dif- ference per monomer unit between the helical senses is

designated by 2 Eh. The symbol E designates a free energy. Cooperativity occurs if E, is significantly larger than the thermal energy RT, and if 2 Eh is sig- nificantly smaller than RT.

Figure 3 allows a quantitative interpretation of the long and short chain regimes as related to the compar- ison of the degree of polymerization to the exponential of E, divided by RT. Moreover the effort was taken to develop the partition function for this model using the methods of statistical thermodynamics.16 This theoret- ical work yields the relationships among the defining energies, 2 Eh, the chiral bias, and E,, the excess cost of a helix reversal state, and the temperature, degree of polymerization, and the optical activity. The two ex- tremes are presented in Figure 3, defined as Case 1, the short chain model, and Case 2, the infinite chain model. Equations (1) and (2) relate these parameters in quantitative form and, as can be seen, E, does not ap- pear in Eq. (1) while the degree of polymerization N does not appear in Eq. (2).16 The theoretical approach taken here16 is qualitatively analogous to that for vinyl optically active polymer^,'^ but differs substantially because the energetically deeper helical energy well of the polyisocyanates causes far fewer helix reversal states.

[a] = x/(x* + 1)'/2 where x = (Eh/Rqe"'R' (2)

Experimental support for this cooperative picture (Fig. 3) is found in the behavior of the copolymers of n-hexyl isocyanate and the optically active isocy- anates: ( - )-(S)-2,2-dimethyl-l,3-dioxolane-4-

[a],

288 GREEN ET AL.

helix reversal (I )

E

e '' << degree of polymerization (N)

helix (H)

E %T

la] = f (N,E,)

e )> degree of polymerization (N) h f f (Er)

Fig. 3. Cooperative model for polyisocyanates.

methylene isocyanate and ( + )-(R)-2,6-dimethyl- herptyl isocyanate. These copolymers, in analogy to work of Carlini, Ciardelli, and Pino on copolymers of optically active and inactive olefins," show optical ac- tivities which are both disproportionately high com- pared to the percentage of chiral comonomer and also highly temperature dependent. l9 Some of these datalg are shown in Table 1.

The results in Table 1 can be seen to fit into the expectations of the model in Figure 3. Since all the monomer units between helix reversal states, i.e., the cooperative length, must take on the same helix sense, incorporation of even a few among them favoring one helical sense will cause a bias for the entire cooperative length toward one helical sense. In this way the chiral comonomers (Table 1) can be seen as sergeants control- ling the otherwise indifferent soldiers, the hexyl isocy- anate derived units.

Further support for the cooperative model (Fig. 3) comes from analysis of the lowest molecular weight samples of (3) (Fig. 1). Gel permeation chromatography (GPC) shows that the lowest molecular weight poly- mers (Fig. 1) are highly polydisperse. This allowed US to use GPC to obtain highly monodisperse samples of a

wide molecular weight range by fractionation. The tiny amounts of these samples, obtained by collection from the GPC eluent, necessitated measurement of their op- tical activities at a highly sensitive lower wavelength, 300 nm. The degrees of polymerization, obtained by

TABLE 1. Specific D-line rotations of polyisocyanates"

% comonomer [aID *

X Y z -20°C +20"c M"

A 0 100 0 -514 -500 500,000 B 1 0 99 -251 -138 700,000

0.5 99.5 -140 -66 1.1 X lo6 c o D 0.12 0 99.88 -64 -26 1.3 X lo6

a See ref. 19 for further details.

COOPERATION IN A DEEP ENERGY WELL

-200 -

-300 -

4-

-500-

289

-600- -70 -60 -54 4 -30 -20 -10 0 10 20 30 40 50 60

3500

3000

3 2500 T

A

I

6 2000

k P

5

1500

% 1000

500

0

0

0

0 50 100 150 200 2 5 0 300 350 400 4 5 0

DEGREE OF POLYMERIZATKN

Fig. 4. Specific rotation at 300 nm at 19.9"C in chloroform versus degree of polymerization for almost monodisperse samples of poly[(R)-1-deuterio-1-hexyl isocyanate] (3) obtained by GPC fractionation.

calibration against known samples of poly(n-hexyl iso- cyanate)," are plotted against the taken at 19.9" in chloroform (Fig. 4).

Although work is in progress to expand the fraction- ation to higher molecular weights, and to measure the optical activity as a function of temperature, all of this to allow a firm quantitative test of Eqs. (1) and (2),16 nevertheless the data in Figure 4 are consistent with the cooperative model (Fig. 3).16 As predicted,16 the lowest degrees of polymerization of 3 show a strong dependence of optical activity on chain length while this dependence falls off as chain length increases to degrees of polymerization beyond about a few hundred. This interpretation of the data is supported by the data for various degrees of polymerization of polymer A in Table 1 where over much of the same range the optical activity is invariant.21 When this is added to the small temperature dependence of sample A (Table 1) we can understand the uniform helical sense of this material compared to the changing ratio of helical senses in 3, i.e., changing as a function of both temperature and degree of polymerization at low molecular weights, and only temperature at high molecular weights.

This so-far qualitative support seen in Figure 4 can be corroborated by the experimental data in Figure 1. Here we see (Fig. 1) that the three highest molecular weight samples show identical optical activities requir- ing that they fit into the infinite chain model of the cooperative theory,16 and therefore be described by Eq. (2). The necessary numerical analytical work showed only a narrow range of values of E, and Eh which could produce the shallow sigmoidal experimental data (Fig. 1) for the middle sample, i.e., degree of polymerization of 6800. The overlays of the produced theoretical curve for Eh = 0.8 cal/mol (chiral bias equals 2 X Eh) and E, = 3766 cal/mol are shown in Figure 5 .

Although we have also fit a general equation for all

r 4

0

Fig. 5. Experimental points (0 and theoretical line (--f obtained by setting E!, = 0.8 cal/mol, E, = 3766 cal/mol, and [a], = -568". Plots of specific rotation versus temperature rC).

molecular weights to the same data and obtained sig- nificantly different values ofEh and ER,16 this equation shows far larger covariances, i.e., it is a much more difficult equation for numerical analytical analysis. Moreover the value of E, obtained from the fit to the alternative general equation16 is inconsistent with the preliminary data shown for the fractionated materials in Figure 4 and also with the temperature dependence

290 GREEN ET AL.

of the polydisperse low-molecular-weight samples shown in Figure 1.

We see therefore that although the assignment of exact values to the chiral energy bias 2 Eh and to the excess helix reversal energy E, must wait until work is completed on the fractionated narrow dispersity sam- ples and their optical activity and temperature depen- dencies, nevertheless the cooperative model is well sup- ported. In addition from the results so far, discussed above, we know the approximate values for these crit- ical energies.

CONCLUSIONS The approximate value of the free energy difference

between the helix reversal and helical state of 3700 cal/mol is higher than necessary to exclude helix rever- sals as major contributions to chain flexibility in poly(n-hexyl isocyanate). For the stated value, for ex- ample, at 300 K, a helix reversal would appear on av- erage every 476 units or therefore about once every 925 A.5 Since the persistence length in chloroform, the sol- vent used in Figure 1, is only about 250 clearly other sources of chain flexibility must contribute sig- nificantly if this value of E, is near correct. In this way the study of the optical activity properties of the poly- isocyanates is able to help answer a theoretical ques- tion posed for this ~ y s t e m . ~

The small number of calories/mole for the value of the chiral bias energy, 2 Eh, can be related to recent studies of conformational equilibrium isotope ef- f e c t ~ . ~ ~ , ~ ~ In these studies it was shown that the isotope effect arises from different carbon hydrogen stretching and angular motions associated with the substituted diastereotopic hydrogens. As is theoretically predicted, the lower zero point energy of the carbon deuterium bond drives the equilibrium in the deuterated mole- cules to place this isotope in the stronger diastereotopic position, i.e., substituted for the diastereotopic hydro- gen with the higher stretching frequency. In cyclohex- ane, the stretching mode of the equatorial carbon hy- drogen bond is higher than the axial by 32 wavenum- bers and the free energy for the equilibrium in 1-deuteriocyclohexane favors the equatorial deuterium by close to 6 cal/mol.”

A force field calculation on the nonamer of poly((R)- 2,6-dimethylheptyl isocyanate) which models a poly- mer of single helical sense (A, Table 1) (see discussion above) shows an overwhelming energy bias toward the left handed helix. l6 Since the circular dichroism spec- tra of poly((R)-1-deuterio-1-hexyl isocyanate) (3) and poly((R)-2,6-dimethylheptyl isocyanate) are of identi- cal shape and sign over the ultraviolet region15 we must accept that the deuterated polymer (3) is predom- inately left handed.

From the knowledge of the source of the conforma- tional equilibrium isotope e f f e ~ t ~ ~ , ~ ~ and from the con- clusion that 3 has an excess of the left-handed helix we can therefore know that the pro-(R) hydrogen on (2-1 of poly(n-hexyl isocyanate) in the left-handed helical re- gions has a slightly higher vibrational frequency than

the diastereotopic pro-(S) hydrogen. Based on the free energy difference this can only be several wavenum- bers and it is impossible with current theory to connect such a tiny difference to a structural interpretation.

The cooperativity, therefore, associated with the polyisocyanates allows an energy, which is far too small to be understood by structural theory on a per unit basis, to nevertheless be capable of controlling, in a powerful way, the structural properties of the poly- mer. This is not a phenomenon restricted to the poly- isocyanates but can be expected in all organized sys- tems where tiny local energy differences are leveraged through cooperation. As chemical science moves fur- ther into supramolecular structuresz4~z5 where cooper- ative effects, long known to be of importance in poly- mer properties,26 predominate, so will the necessity in- crease to understand the structural basis of small local energy differences. Examples of this can be found in polymer complexes where minimum chain lengths are necessary to amplify the small per unit differences in attractive energies,” in polymer blends where unusual methods must be used to detect the tiny necessary co- hesive forces,z8 in membranes where very small per- turbations in water solvation may account for re- stricted appr0ach,2~ in liquid crystals where low con- centrations of chiral molecules cause large changes in proper tie^,^' and in cooperative transitions of side chains confirmation of polysa~charides.~~

ACKNOWLEDGMENTS

Much of this work is taken from the Ph.D. theses of R.A. Gross (19861, M.P. Reidy (19901, and C. Andreola (19911, Polytechnic University, Brooklyn, New York. We wish to thank the Petroleum Research Fund, Ad- ministered by the American Chemical Society and the National Science Foundation (USA) for their generous financial support. We are also grateful to the Interna- tional Program of the National Science Foundation for support for the scientific exchange between Brooklyn and Osaka.

This paper is dedicated to the memory of Piero Pino who was so much respected and who is so much missed. We were happy to discover that the Milano School, which trained Pino in his interest in polymer science, also had an early interest in the macromolecules which are the subject of this article.3z

LITERATURE CITED 1. Shashoua, V.E., Sweeney, W., Tietz, R.F. The homopolymeriza-

tion of monoisocyanates. J . Am. Chem. SOC. 82:866-873, 1960. 2. Bur, A.J., Fetters, L.J. The chain structure, polymerization and

conformation of polyisocyanates. Chem. Rev. 76:727-746, 1976. 3. Cook, R., Johnson, R.D., Wade, C.G., O’Leary, D.J., Munoz, B.,

Green, M.M. Solvent dependence of the chain dimension of poly(n- hexyl isocyanate). Macromolecules 23:3454-3458, 1990.

4. Takada, S., Itou, T., Chikiri, H., Einaga, Y., Teramoto, A. Dielec- tric dispersion of narrow distribution poly(hexy1 isocyanate) in dilute solution. Macromolecules 22:973-979, 1989; Kuwata, M. Murakami, H., Norisuye, T., Fujita, H. Macromolecules 17:2731- 2734, 1984.

5. Shmueli, V., Traub, W., Rosenheck, K. Structure of poMn-buty1 isocyanate). J. Polym. Sci.: Part A-2 7:515-524, 1969.

COOPERATION IN A DEEP ENERGY WELL 291

6. Tonelli, A.E. Conformational characteristics of the poly(n-alkyl isocyanates). Macromolecules 7:628-631, 1974.

7. Mansfield, M.L. Broken wormlike chain model of semiflexible polymers. Macromolecules 19:854-859, 1986.

8. Cook R. Flexibility in rigid rod poly(n-alkyl isocyanates). Macro- molecules 20:1961- 1964, 1987.

9. Farina M. The stereochemistry of linear macromolecules. Topics Stereochem. 17:l-111, 1987.

10. Pino, P. The control of main chain conformation in vinyl and related polymers. Polym. Preprints (ACS) 30:433-434, 1989.

11. Neuenschwander, P., Pino, P. The unperturbed dimensions of poly-(S)-4-methyl-l-hexene and poly-4-methyl-1-pentene. Eur. Polym. J. 19:1075-1079, 1983.

12. Goodman, M., Chen, S. Optically active polyisocyanates. Macro- molecules 3:398-402, 1970.

13. Green, M.M., Gross, R.A., Crosby, C. III, Schilling, F.C. Macro- molecular Stereochemistry: The effect of pendant group structure on the axial dimension of polyisocyanates. Macromolecules 20:992-999, 1987.

14. Green, M.M., Gross, R.A., Cook, R., Schilling, F.C. Broken worm and wormlike models for polyisocyanates. Macromolecules 202636-2638, 1987.

15. Green, M.M., Andreola, A., Munoz, B., Reidy, M.P. Macromolec- ular stereochemistry: a cooperative deuterium isotope effect lead- ing to a large optical rotation. J . Amer. Chem. SOC. 110, 4063- 4065, 1988.

16. Lifson, S., Andreola, C., Peterson, N.C., Green, M.M. Macromo- lecular stereochemistry: Helical Sense Preference in Optically Active Polyisocyanates. Amplification of a Conformational Equi- librium Deuterium Isotope Effect. J. Am. Chem. Soc. 111:8850- 8858, 1989.

17. Luisi, P.L., Pino, P. Conformational Properties of optically active poly-a-olefines in solution. J . Phys. Chem. 72:2400-2405, 1968. See also Allegra, G., Corradini, P., Ganis, P. A model of the chain conformation of an isotactic vinyl polymer, having optically ac- tive side groups. Makromol. Chem. 90:60-65, 1966.

18. Carlini, C., Ciardelli, F., Pino, P. Optical activity and confor- mation in solution of stereoregular copolymers of (S)-4-methyl-l- hexane with 4-methyl-1-pentene. Makromol. Chem. 119:244- 248, 1968.

19. Green, M.M., Reidy, M.D., Johnson, R.D., Darling, G., O’Leary,

D.J., Willson, G. Macromolecular stereochemistry: The out-of- proportion influence of optically active comonomers on the con- formational characteristics of polyisocyanates. The sergeants and soldiers experiment. J . Am. Chem. Soc. 111:6452-6454, 1989.

20. Itou, T., Chikiri, H., Teramoto, A., Aharoni, S.M. Wormlike chain parameters of poly(hexy1 isocyanate) in dilute solution. Polym. J . 20:143-151, 1988.

21. Unpublished work of Y. Nagamura, K. Hatada, Y. Okamoto, and M.M. Green, Osaka University, Faculty of Engineering Science.

22. Anet, F.A.L., Kopelevich, M. Deuterium isotope effects on the ring inversion equilibrium in cyclohexane: A value of deuterium and its origin. J. Am. Chem. SOC. 108:1355-1356, 1986.

23. Forsyth, D., Hanley, J.A. Conformationally dependent intrinsic and equilibrium isotope effects in N-methylpiperidine. J. Am. Chem. Soc. 109:7930-7932, 1987.

24. Lehn, J.M. Some future aspects of stereochemistry. In: Euchem Conference on Stereochemistry Burgenstock (1965- 1989). Schef- fold, M., Scheffold, R. eds. Frankfurt. Otto Salle Verlag, 1989:28- 30.

25. Whitesides, G. What will chemistry do in the next twenty years. Agnew. Chem. Int. Ed. 29:1209-1218, 1990.

26. Green, M. Cooperation. Curr. Polym. Res. Polym. SOC. Jpn. 36:l- 2, 1990.

27. Wang, Y., Morawetz, H. Fluorescence study of the complexation of poly(acry1ic acid) with poly (N,N-dimethylacrylamide-co- acrylamide). Macromolecules 22:164-167, 1989.

28. Larbi, F.B.C., Teloup, S., Halary, J.L., Monnerie, L. Phase dia- grams of poly(vinylmethy1 etherbdeuterated polystyrene: An original route to the precise nature of polymer-polymer interac- tions. Polym. Commun. 27:23-25, 1986.

29. Rand, R.P., Parsegian, V.A. Hydration forces between phospho- lipid bilayers. Biochim. Biophys. Acta 988:351-376, 1989.

30. Solladi6, G., Zimmermann, R.G. Liquid crystals: A tool for studies on chirality. Agnew. Chem. Int. Ed. 23348-362, 1984.

31. Hirao, T., Sato, T., Teramoto, A,, Matsuo, T., Suga, H. Solvent effects on the cooperative order-disorder transition of aqueous so- lutions of schizophyllan, a triple-helical polysaccharide. Biopoly- mers, 29:1867-1876, 1990 and references therein.

32. Natta, G., DiPietro, J . , Cambini, M. Crystalline polymers of phe- nyl and n-butylisocyanate. Makromol. Chem. 56200-207, 1962.