Stable Conformations of Polymer Chains and ModelCompound MoleculesBY TAKEHIKO SHIMANOUCHIDepartment of Chemistry, Faculty of Science, University of Tokyo, Hongo, Tokyo,JapanReceived 5th January, 1970In order to have information about the conformational energy of polymer chains, the structuresof small molecules which have chemical configurations similar to those of polymer chains are studied.The results for polyethylene, poly(vinylchloride), polypropylene, 1,4-polyisoprene, 1 &polybutadiene,nucleic acid and polypeptides show that the conformations of small model molecules are closelycorreIated with those of polymer chains.For several years we have studied the structures of small molecules which havechemical configurations similar to those of various polymer chains.In the presentpaper I describe the basic idea underlying this group of studies, summarize the resultsso far obtained, and discuss the facts derived from these results.BASIC IDEAOne of the main purposes of the structural chemistry of high polymers is to knowthe conformation of polymer chains in various circumstances. The first step of thisproblem is to have knowledge about the energy of internal rotation for componentsingle bonds. This knowledge can be obtained by studying the energy of simplemolecules and by estimating the difference in the conformational energy between thesimple molecules and the polymer chains.In this process it is generally recognized that the energy of internal rotation about abond axis of a polymer chain is seriously affected by the conformations of neighbour-ing bonds.Accordingly, the energy is usually expressed by the two-dimensionalconformation map or diagram or by the statistical weight matrices for inter-dependentb0nds.l However, it is not easy to have accurate information about the energy ofsuch interactions because many kinds of factors are involved. This difficulty oftencauses the gaps between the structure study and the explanation of high polymerproperties.The purpose of our present research is to overcome the difficulty by studying theconformations of those small molecules which have two or more successive bond axessimilar to those found in polymer chains. We often encounter the cases in which theenergy is different from what we expect from the energies of independent bonds.This fact shows that the present kind of experimental work is important in the studyof high polymer conformations.NOTATIONSIn this paper the conformation is expressed with respect to the main chain andFlory’s notations,l t, gf and g-, are used.The notations g and g’ are also used, the6TAKEHIKO SHIMANOUCHI 619 --, , I , ,latter denoting the opposite g. Thus, gg represents g+g+ and g-g- and gg’ representsg+g- and g-g+.When we discuss the conformation of polyisoprene chain, we need c (cis) and s(skew).2 When the rotational angle is 0,60, 120, 180,240 and 300°, the conformationis expressed by t, s+, g f , c, g- and s-, respectively. We also use s’ which is oppositeto s. Thus, ss’ represents s+s- and s-S+ and gs‘ represents g+s- and g-s+.f - 0 0I I I t 1POLYETHYLENEThe conformational energy of a polymethylene chain has been studied by manyauthors The simplestmodel compound is n-pentane.The conformation of this molecule has been studiedby infra-red and Raman ~pectra.~‘~ They show the existence of t t , tg, and gg.However, the band due to gg’ has not yet been found.and the conformation diagram shown in fig. la is given.t1 I9- t 9 +09+t t 0The combination of t and g gives various shapes for the polymethylene chain.6is approximately given by For instance, the structure of the (CH& molecule 7*fig. 2 which is expressed byAs discussed by Newman and Kay, the structure ttggtggtt is a possible conformation ofa folded polymethylene chain and may furnish an explanation for the chain-folding inthe polyethylene crystal.’ The relative orientation of the two polymethylene planesin fig.2 is more or less different from that found for the neighbouring chains in poly-ethylene crystals. However, it may not be difficult to adjust the orientation by slightlychanging the angles of internal rotation.(t)l 29stgg(t)12s’s’tg’g’62 CONFORMATIONS OF MODEL COMPOUND MOLECULESFIG. 2.-Approximate conformation of (CH&+VINYL POLYMERSFor poly(viny1 chloride) chain there are two kinds of conformation diagrams.One expresses the interaction energy for the two bonds connected by a CH2 group andthe other for the two bonds connected by a CHCl group. A model compound,2,4-dichIoropentane CH3CHClCH2CHCICH3, has been studied both for meso andDL configuration^.^ Syndiotactic, isotactic and heterotactic 2,4,6-trichloroheptanesYwhich are related to both of the two diagrams, have also been studied spectroscopic-ally.lo* Schneider et al.also studied these compounds and 2-chloropentaneCH3CH2CHClCH2CH3.l2-l4 The results are shown in fig. 1.As for the conformation of a syndiotactic polymer chain, the problem is the stab-ility of tt and gg given in fig. lb. For PVC and its model compounds, tt is far inorestable than gg and the syndiotactic chain takes only (tt), conformation. With poly-propylene the situation is different. For the model compoundtt is identical with gg. This result explains the fact that the syndiotactic polypropylenechain takes the forms (tt), and (ttgg),.The (gg)n conformation is not stericallyallowed. The mixtures of tt and ttgg are allowed. An example is shown in fig. 3.CH3CH(CH3)CH2CH(CH3)CH3,For the isotactic chain PVC and polypropylene have the same situation. Themodel compounds take the forms tg or g t and the polymer chain the (tg)n or (gt)nhelix. The purely isotactic chain is allowed to have only one joint connecting theright-handed and the left-handed helices. The joint can migrate in the chain and thetransition from the right-handed helix to the left-handed one and the reverse transitioncan take place easily by this migration.There may be other conformations, which are similar to the gg' form of n-pentane,both for the syndiotactic and isotactic chains.However, they are far less stable andare not discussed hereTAKEHIKO SHIMANOUCHI 631,4-POLYISOPRENE A N D lY4-POLYBUTADIENEFor polyisoprene the model compounds shown in table 1 have been studied byinfra-red spectra.2 The vibrational frequencies were calculated for variousconformations ; they were compared with the observed infra-red bands in the regionTABLE STABLE CONFORMATIONS OF MODEL COMPOUND MOLECULES OF CIS- ANDTRANS-1,4-POLYISOPRENES amolecule solid liquidS+, s- s+, s-H CH2CHZCHgctct ( W + Y c9-1\ * // \c=c(s+ty s-t, s+g-, s-gf)CH3 Hin parentheses are less stable.a Conformations are expressed with respect to the carbon atoms with an asterisk. Conformations700-200 cin-l, and the stable conformations were determined.The results are shownin table 1 and a few examples of the spectra are shown in fig. 4.These results lead to the conformations for cis and trans 1,4-polyisoprenes shownin table 2. The fact that CH3-CH=C(CH3)CH,CH3 takes the forms cs+ and cs-(corresponding to the cis polymer) and tc, tsf and ts- (corresponding to the transTABLE 2.-ACCESSIBLE CONFORMATIONS OF CIS- AND TRANS- 1,4-POLYISOPRENESbond axis cis polymer trans polymerCH2-CH Sf, s- Sf, s-CH=C(CH,) C tCHZ-CHZ t, 9 + Y 9-2 t, s+, 9-C(CHS)-CH2 s+, s- c7 (S+Y s-laa skew is less stable.b trans and gauche are almost equal in energy for this structure. s+g+, s-g-, g+s+ and g-r arenot sterically allowed64 CONFORMATIONS OF MODEL COMPOUND MOLECULES1------II I I6 0 0 4 00 2 00U S6 00 4 00 2 00(4 cm-I (6)FIG 4.-Infra-red spectra of (a) 3-methyl-trans-2-pentene and (6) 3-methyl-cis-2-pentene.S and Ldenote the solid and liquid states, respectively. For (a) both the cis and skew forms coexist in theliquid state.TABLE 3.-sTABLE CONFORMA'MONS OF MODEL COMPOUND MOLECULES OF CIS- ANDTRANS-1 ,~-POLYBWTADIENES amolecules solid liquidH CHZCH3\ / c=c/ \H H\ / c=c/ \CH3 H\ // \C=CH HH CH2CH3\ / c=c/ \CHSCH2 Hc, s+, s-ts+, ts-cs+, cs-c, s+, s-tc, ts+, ts-cs+, cs-ctsf, cts-sfts-, s-tsc s+ts+, s-ts-s+ts-, s-ts+CH3CH2 CH2CH3C=CH Hs+csf, s-cs- s+cs+, s-cs-s+cs-, s-cs+ s+cs-, s-cs+\ // \a unpublished results by Yasuhide Alkai and the author.2TAKEHIKO SHIMANOUCHI 65polymer) and that tc is more stable than tsf and ts- may explain the difference betweennatural rubber and gutta-percha.At lower temperatures, the number of the acces-sible conformations for the cis polymer is larger than that for the trans polymer. Athigher temperatures, however, the number for the former is smaller than that for thelatter.TABLE 4.-ACCESSIBLE CONFORMATIONS OF CIS- AND TRANS-1,4-poIybutadienes abond axis cis polymer trans isomerCH=CH C tCH-CH2 Sf, s- c, s+-, s-CH2-CH2 t, Qf, 9- t, 9+, 9-a see notes of table 2.For polybutadiene the model compounds studied and the stable conformations areshown in table 3. The conformations for the cis and trans polymers are shown intable 4. From these results we can count the number of accessible conformations anddiscuss the relationship between the number and elasticity.Table 5 shows the averagenumber n, of accessible conformations per axis, which is defined byn, = ( n [ n i ) l ' P ,iwhere n1 is the number of stable conformations for each axis or for each set of axesfound in the unit polymer chain, andp is the number of axes in the unit. It providesthe intramolecular factor for rubber elasticity.TABLE 5.-AVERAGE NUMBER OF ACCESSIBLECONFORMATIONS PER AXIS apolymercis-l,4-polyisoprenetrans-l,4-polyisoprenepolyisobutylenepolyethylenepolytetrafluoroethylenepoly(viny1 chloride)isotactic polypropylenenc1.86H.T. 2.06L.T. 1.571.86H.T. 2.28L.T. 1.86-3H.T. 2.65L.T. 11110 H.T. and L.T.give the number at higher and lower temperatures, respectively. For L.T. wetake only the conformations which have the lowest energy.NUCLEIC ACIDThe only model compound for this substance so far studied is dimethylphosphateanion CH,O(PO ;)OCH3. l7 The infra-red and Raman spectra of barium dimethyl-phosphate were measured in the solid state and in aqueous solution. The normalvibration frequencies were calculated for the tt, tg and gg conformations. The resultshows that the anion takes only the gg form, which is also found for the phosphategroup in the DNA double helix. The conformation of the anion were confirmedafterwards by the X-ray diffraction analysis.l866 CONFORMATIONS OF MODEL COMPOUND MOLECULESPOLYPEPTIDESAs the model compounds of polypeptide chains, we have chosen acetylamino acidN-methylamides CH3CONHCHRCONHCH3, where R is the amino acid residueside chain.This compound was first synthesized by Mizushima et al. for some aminoacids, the near infra-red spectra in dilute carbon tetrachloride solution were measuredand the two conformations, one with an intramolecular hydrogen bond and onewithout it, were found.lg Recently, the model compounds were prepared forR=CH3 and DL), CH2CH3 a and DL), CH2CH2CH3 (L and DL), CH2CH2CH2CH3(DL), CH2CH2SCH3 (L and DL), CH2CH(CH3), (L and DL), CH2COOH (L) andCH2C6H5 (DL) and were studied.20TABLE 6.-CRYSTAL MODIFICATIONS OF ACETYLAMINO ACID N-METHYLAMIDESCH3CONHCHRCONHCH3 aR m.p. ("C) form A form B-CH3 L 182DL 162DL 161-CH;CHzCH3 L 197DL 160-CH2CH2SCHS L 181DL 133-CH2CH3 L 205-6-CH2CHzCH2CH3 DL 173-4-CH2CHCH3 L 165-6Ifrom melt aa -- aabaaaaa-------CH3 DL 152-3 from melt a-CHZCOOH L 190 from melt d-CH2CgH5 DL 183 C e, from melta In form A and form B, a, b, c, d and e mean that the crystal is obtained by the recrystallizationfrom ethylacetate, ethanol+ether, acetone, methanol + ether and water, respectively.The crystalobtained from melt is metastable.In the course of the study of infra-red spectra of these compounds we found thatthere are two kinds of crystalline modifications. For acetyl-L-alanine N-methylamide,for instance, we have " form B " when the crystal is grown from solutions and '' formA " when it is melted at high temperatures and the crystal grown from melt.Whenform A is kept at room temperature for a few days, it becomes form B, showing thatthe former is metastable. The situation is different when we chose a different aminoacid. It also depends on whether the compound is pure L or racemic. The result isshown in table 6. In many cases we have only form A.These two modifications are distinguished by the infrared spectra in the region700-500 cm-l, where the C-0 in-plane and out-of-plane bending vibrations (amide-IV and VI) appear (fig. 5). In this region the form B crystals have two definitely-separated bands near 630 and 600 cm-l, usually the former being weaker. On theother hand, the form A crystals have only one band or two almost overlapping bandsnear 600 cm-l.These two kinds of crystals are also distinguished by their physicalappearance, the latter being soft and fibrous and the former being hard and crystalline.The infra-red spectra of these modifications in the other wave-number regionsare also different from each other. We calculated the normal frequencies of theacetylalanine N-methylamide molecule for the various conformations, e.g., for thevarious values of II/ (the rotational angle for the Ca-CO axis) and + (that for thTAREHIKO SHIMANOUCHI 67NH-C'-axis) and compared the values with the observed frequencies of the infra-redbands. The result suggests that the conformation of the molecule in the form Acrystal is different from that in the form B crystal and that $ is near 300" and 4 is near60" for form B, and both II/ and 4 are near 120" for form A.Ichikawa and Iitaka 21studied the crystal structure of acetyl-DL-leucine N-methyl-amide which takes theform B modification. The result shows that the conformation of this molecule is$ = 319" and 4 = 86", thereby supporting our suggestion. The results are sum-marized in the conformation diagram shown in fig. 6. The model compound takesthe conformations corresponding to the stable forms of polypeptide chains.We have also studied the conformation of the acetylglycine N-methylamidemolecule CH3CONHCH2CONHCH3 .22 The compound has also two modifications,although the transition behaviour is not so definite as for the systems mentionedabove. The infra-red spectra and the calculated frequencies show that the conform-ation in one modification (form A) is t,b = 180" and $ = 120" and that in the order(form B) is $ = 0" and # + 120".In other words, $ is trans or cis, and $ is gauche.We have also studied the molecular conformations of N-methyl-propionamideCH3CH2CONHCH3, N-ethylacetamide CH3CONHCH2CH3, and N-methylchloro-acetamide ClCH2CONHCH3.23 For the last compound the X-ray crystal analysisForm A0004P e0, , I700 5 0 00I , ,700 5 0 0Form Bcm-IFIG. 5.-Infra-red spectra of form A and form B crystals. A, acetyl-L-alanine N-methylamide;B, acetyl-L-leucine N-methylamide ; C , acetyl-DL-leucine N-methylamide ; D, acetyl-I-aspertic acidN-methylamide ; E, acetyl-DL-phenylalanine N-methylamidc. The bands with a circle are the keybands68 CONFORMATIONS OF MODEL COMPOUND MOLECULESshows that the Cl atom is almost cis to the NH We are now doubtful of theaccepted concept that the inherent potential of internal rotation about the Ca-COaxis of polypeptide chains has three minima.The two minima of the trans and cismay explain the experimental results more ~traightforwardly.~~A P'iFIG. 6.-Conformation diagram of polypeptide chain. A and B are the suggested conformation forform A and form By respectively. 1 , a-helix; 2. antiparallel 6 : 3. acetyl-DL-leucine N-methylarnide.2On the other hand, the experimental results support the three potential minima(trans and gauche) for the NH-Ca axis.25 The fact that we often have the gaucheconformation may suggest that the gauche form is far more stable than the trans formfor this axis.23CONCLUSIONAll the above results show that the conformations of model compound moleculesare closely related with those of the polymer chains.This fact strongly suggests thatthe short-range forces are dominant when the polymer chain chooses its conformations.The intermolecular forces or the molecular packings may select one of the stableconformations when the short-range forces give more than one accessible conforma-tions. However, the intermolecular interactions cannot largely change the angles ofinternal rotation from those of the stable conformations, unless there are exceptionallystrong intermolecular forces or exceptionally small barriers to internal rotation.P. J .Flory, Statistical Mechanics of Chain Molecules (Interscience, New York, 1969).T. Shirnanouchi and Y. Abe, J. Polymer Sci. A , 2, 1968,6, 1419.S. Mizushima and H. Okazaki, J. Amer. Chem. Soc., 1949, 71,3411 ; N. Sheppard and G. J.Szasz, J. Chem. Phys., 1949, 17, 86.R. G. Snyder, J . Chem. Phys., 1967, 47, 1316.A. Tomonaga and T. Shimanouchi, Bull. Chem. SOC. Japan, 1968, 41, 1446.S. Mizushima and T. Shimanouchi, J. Amer. Chem. SOC., 1964, 86, 3521. ' B. A. Newman and H. F. Kay, J. Appl. Phys., 1967,38,4105.H. F. Kay and B. A. Newman, Acta Crysf. By 1968, 24, 615.T. Shimanouchi and M. Tasumi, Spectrochim. Acta, 1961, 17,755.lo T. Shimanouchi, M. Tasumi and Y. Abe, Makromol. Chem., 1965,86,43TAKEHIKO SHIMANOUCHI 69'l 2 D. DoskoEilova, J. 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Tatsuno, Biupofymevs, to be published.21 T. Ichikawa and Y. Iitaka, Acta Cryst. B, 1969, 25, 1824.22 Y. Koyama and T. Shimanouchi, Biopolymers, 1968, 6, 1037.23 Y. Koyama, Thesis (University of Tokyo, 1970).24 Y . Koyama, T. Shimanouchi and Y. Iitaka, to be published.25 S. Mizushima and T. Shimanouchi, Adv. Enzymol., 1961, 23, 1.26 T. Shimanouchi, Y . Abe and Y . Alaki, Polymer J., to be published