optically active hydrocarbon polymers with aromatic side chains, 5. conformational and chiroptical...

Die Makromolekulare Chemie 114, 27-37 (1974)

Istituto di Chimica Organica Industriale ed Istituto di Chimica Organica della Facoltk di Scienze M.F.N. della Universitk di Pisa, Via Risorgimento 35, 56100 Pisa, Italy

Optically Active Hydrocarbon Polymers with Aromatic Side Chains, 5 *)

Conformational and Chiroptical Properties of Stereoregular Polymers of Phenyl-a-olefins

CARLO CARLINI, FRANCESCO CIARDELLI, LUCIANO LARDICCI, and RITA MENICAGLI

(Date of receipt: May 17, 1973)

S U M M A R Y : The chiroptical properties of stereoregular polymers of optically active 4-phenyl-1 -hexene

(1) and 5-phenyl-1-heptene (2) as well as of the copolymers of 1 with 4-methyl-1-pentene (3) were investigated. The comparison of the rotatory power and of the CD spectra in the spectral region of the x+x* electronic transitions of the aromatic chromophore of these polymers with the corresponding properties of optically active phenylalkanes allows the conclusion that the macromolecules of the homopolymer of 1 as well as of its copolymer with 3 contain chain blocks in helical conformation with a predominant direction of the screw. No evidence of this type was obtained for the polymers from 2. Conformational analysis based on molecular models shows that in the case of 1 having R absolute configuration, the favoured direction of the screw is the right handed one as found for poly[(R)-4-methyl-l-hexene]. The relationships between the conformation of the macromolecules and CD of the phenyl chromophore between 275 and 185 nm are consistent with previous results .concerning copolymers of styrene with optically active a-olefins.

Z U S A M M E N F A S S U N G :

Die chirooptischen Eigenschaften von stereoregularen Polymeren aus optisch aktivem 4-Phenyl-1-hexen (1) und 5-Phenyl-I-hepten (2) sowie von Copolymeren aus 1 und 4-Methyl- I-penten (3) wurden untersucht. Der Vergleich des Drehwerts sowie der CD-Spektren im Bereich der x +x*-Absorptionsbanden des aromatischen Chromophors dieser Polymeren mit den entsprechenden Eigenschaften der optisch aktiven Phenylalkane erlaubt den SchluB, daB die Makromolekiile von Homopolymeren aus 1 und von Copolymeren aus 1 und 3 Seg- mente enthalten, die in Helixform mit vorherrschendem Drehsinn vorliegen. Kein Beweis fur das Vorliegen dieses Typs wurde im Fall der Polymeren aus 2 erhalten. Die Konformations- analyse von Molekulmodellen zeigt, daB Rechtshelices in den Polymeren aus 1 mit absolu- ter R-Konfiguration bevorzugt sind, wie beim Poly[(R)-4-methyl-l-hexen] gefunden wurde. Die Beziehungen zwischen der Konforrnation der Makromolekule und den CD-Kurven im Be- reich von 275-185 nm sind im Einklang mit friihern Ergebnissen, die im Fall von Copoly- meren aus Styrol mit optisch aktiven a-Olefinen erhalten wurden.

*) Part 4: CJ lo), preceding paper, page 15.

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C. CARLINI, F. CIARDELLI, L. LARDICCI, and R. MENICAGLI

Introduction

In previous papers I*’) it has been reported that isotactic polymers, obtained from optically active aliphatic cr-olefins, show properties in agreement with the existence of sections of the macromolecules having helical conformation with a predominant direction of the screw. The absence of chromophoric systems absorbing above 185 nm in these hydrocarbon polymers did not allow to get spectroscopic evidence of this hypothesis. Further studies were devoted to poly(viny1 ethers) 3, and poly(viny1 ketone^)^). In the former case the CD has been recently ’) investigated down to 185 nm. However, the optically active bands in these polymers are centered at the limits of the accessible spectral region and it has been not possible to obtain clear spectroscopic evidence of helical conformations in solution with a predominant direction of the screw results obtained are consistent with this conformational model. In the latter case the COTTON effect at about 290 nm showed a larger amplitude than that of low molecular weight structural models, which increased with stereoregularity, whereas the low dissymmetry factor hindered the investigation in the region of COTTON effects related to the shorter wavelength transitions6). In both cases the difficulty of having a satisfying description of the conformational situation makes it difficult to draw definitive conclusions.

In the present paper we report the chiroptical properties of the linear polymers of (R)-4-phenyl-l-hexene (1) and (S)-5-phenyl-l-heptene (2). These monomers are the analogues of 4-methyl-1-hexene and 5-methyl- 1-heptene, in which

(jH=CHZ YH = CH2 FH=CH,

the methyl group is substituted by a phenyl group. For the isotactic macromolecules of these last monomers a helical conformation with a predominant direction of the screw in solution has been proposed and experimentally found in the crystalline state7,*). The phenyl ring is known9) to have three optically active bands in the accessible spectral region. The presence of this group in the side chains of the macromolecules from 1 and 2 could give spectroscopic evidence of the existence of chiral conformations of the main chain and then confirm the conformational model, proposed for the isotactic macromolecules of optically active vinyl monomers in solution z).

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Optically Active Hydrocarbon Polymers with Aromatic Side Chains, 5

Results

The structure of the polymers from 1 and 2 as well as of the copolymers of 1 with 4-methyl-l-pentene (3) has been thoroughly discussed in the preceding paper ”). Here, some points are summarized in order to help the understanding of the relationships between chiroptical properties and primary and/or secondary structure in this type of macromolecules. IR, NMR, and X-ray data suffice to demonstrate the linear substantially isotactic structure of the homopolymers of 1 and the coisotactic structure of its copolymers with 3. In fact X-ray diffraction showed that both are crystalline and that the diffraction maxima for blocks of units derived from 3 are the same as in the isotactic poly[4-methyl- 1-pentene]. Considering also the isospecificity 1 1 ) of the catalytic system used, it seems reasonable that also units from 1 are enchained in an isotactic way. The same seems to be at least partially true for polymers of 2 as supported by NMR data, even if these polymers are amorphous at X-ray examination.

Analysis of optical rotation data

Information given by rotatory power at sodium-D-line is in general very limited; in fact [@ID is the result of the contributions of several optically active electronic transitions located at shorter wavelength 12).

Rotatory power data are usefully replaced by CD data when the compound under examination contains, as in our case, a chromophoric system absorbing in the accessible spectral region.

In the case of aliphatic poly(wo1efins) a comparison of [@ID for polymers and low molecular weight models allowed l) to get interesting information about the conformational situation. Similar considerations have been extended to polymers of 1 and 2. The polymers of 2 (Tab. l), which has S absolute configuration, show positive optical activity at 589 nm, the value of which does not change appreciably for fractions having different degree of stereoregular- ityl’’. The order of magnitude of [@ID per monomeric unit is larger than that of flexible low molecular weight structural models, such as 3-phenylhep- tane 3), and similar to that of more rigid phenylalkanes such as 2-methyl-3- phenylbutane 14* 15) and 2,2-dimethyl- 3-phenylbutane 16) for which a single conformational isomer seems to prevail largely over the possible others 9).

More information can be obtained from the polymers of 1. The homopolymer fractions show negative optical rotation at 589 nm, which is larger in absolute value than that of more or less rigid model compounds. According to previous data1.2) the absolute value of [@ID seems to increase with increasing degree of stereoregularity, that is from the fraction extracted by acetone to the fraction extracted by chloroform (Tab. 1). The rotatory power of the insoluble

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Tab. 1. Rotatory power at 589 nm of poly[(R)-4-phenyl-1-hexene] (poly-1), pOlY[(s)-5- phenyl-1-heptene] (poly-2) and of some low molecular weight models

Polymer [@I’d”’ Structural [@I’d b, Conformationally [@]gb) models rigid models

~~ ~

poly-1 A -66,2 (S)-3-phenyl- -4,83 (R)-2-methyl-3- -40,37

B -105 hexane phenylbutane

p0ly-2~) A +41,1 (R)-3-phenyl- (R)-2,2-dimethyl-3-

B +46,4 heptane -10,83 phenylbutane --40,42

C +41,5

a) Calculated as [a]$5. M/IOO, where M is the molecular weight of the monomeric unit. b)’ Neat; extrapolated for the optically pure antipode; CJ 1 3 - 1 6 ) .

Fractions extracted successively with A = acetone, B = chloroform; monomer optical purity: 88-91 %.

dl Fractions extracted successively with A = acetone, B = diethyl ether and C = cyclohexane; monomer optical purity: 90-92 %.

residue has not been determined, but taking into account that the increase of the degree of stereoregularity from the chloroform fraction to the residue seems to be not very large lo), also the rotatory properties should not change markedly.

The copolymers of (R)-Cphenyl- 1-hexene with 4-methyl-1-pentene have also a negative [@I,, which is in absolute value very close and even larger than that of the homopolymer from 1 (Tab. 2). This result clearly demonstrates that, as in the case of the coisotactic copolymers of (S)-4-methyl-l-hexene with 4- methyl-1-pentene, the units of the non chiral monomer contribute to the optical rotation of the polymer, and the contribution has the same sign as that due to the chiral monomer. The molar rotatory power for units from 3 (Tab. 2) in- creases with increasing the content of monomeric units deriving from l 1 7 ) .

It is worth mentioning that the rotatory power per unit from 3 ( is appreciably larger in absolute value than that of units from 1. The rotatory power of copolymers containing about 80% of units from 1 is quite close to that found for the homopolymer of 1 soluble in chloroform.

Circular Dichroism between 300 and 185 nm

Optically active phenylalkanes show in the spectral region of 300-185 nm at least three optically active electronic transitions 9*1s). In general, because of the low dissymmetry factor only the first one, at about 265 nm, has been observed by CD in phenylalkanes and only very recently all the three bands have been clearly detected ’).

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Tab. 2. Optical rotation at 589 nm of copolymers of (R)-4-phenyl-l-hexene (1) with 4-methyl- 1-pentene (3)

Run Frac- Mole fraction N1 [a I'd [@1p [@I3 tionsa) of units from 1 (of the units from 3)

HP 1 B 0,190 -91,O -89,7 -86,2 C 0,264 -80,9 -87,8 -92,3 A 0,470 -1 14 -137 -165 D 0,803 -86,l -125 -208

HP 2 D 0,793 -69,3 -101 -84,5 C 0,178 -80,4 -79 -85,2 B 0,203 -104 -104 -103 A 0,366 -1 14 -128 -141

Fractions extracted successively with ethyl acetate (A), diethyl ether (B), cyclohexane (C). and chloroform (D). Calc. as [@I2 = [a]&5. i@/lOO, where h is the molecular weight of the average monomeric unit (cf. given by N1 + N3 M3 ( M = molecular weight).

p ~ ~ 1 2 5 N Calc. as [ @ I 3 = D- taking ([@]31 = -105' (cf. 17)).

N3

In the case of poly [(S)-5-phenyl-l-heptene] also the most stereoregular fraction did not allow to detect in its CD spectrum optically active bands associated with the aromatic chromophore in spite of the fact that the phenyl group is directly bound to the asymmetric carbon atom.

The CD spectrum of poly [(R)-4-phenyl-l-hexene] has been measured only until 240nm because its low solubility did not allow to use a solvent more transparent than chloroform. A CD band can be observed in this region between 272 and 245 nm (Fig. 1). This band has a negative sign and its ellipticity islargerthan that of corresponding low molecular weightflexiblemodels15*1g-21~ and of poly-2 and poly Ip-sec-butylstyrene] 22).

The higher solubility of the 1/3 copolymers makes it possible to dissolve them in heptane, thus permitting to measure the CD spectrum down to 185 nm. The band of the longest wavelength between 272 and 245 nm is also negative and its ellipticity is slightly higher than that of the corresponding homopolymer from 1. In the spectral region of 'La-transition of the benzene chromophore, two bands are present, the first one, between 245 and 218 nm is positive with the maximum centered at 220 nm, the second one at about 215 nm is negative (Fig. 1).

Finally in the region of 'B-transition of the benzene chromophore two bands having similar ellipticity and an opposite sign are present, the former negative

31


and the latter positive centered at about 197 and 190 nm, respectively. The extremely low dissymmetry factor has not allowed an accurate determination of the ellipticity values of the last two bands even if no doubt exists on their existence and sign.

- m - 20 c! sr

15

10

5

0

-5

- 10

ru

4 Y

3

2

1

3

-1

-2 210 230 250 270

Wavelength in nm

Fig. 1 . UV spectra (upper curves) and CD spectra (lower curves) of poly[(R)-Cphenyl-l- hexene] (dotted line) in chloroform and of the (R)-4-phenyl-l-hexene/4-methyl-l-pentene 1/4 copolymer (solid line) in heptane. The values of log E and [@I for the copolymer are based

on one aromatic residue (s. Experimental Part)

Discussion

In the case of poly[(S)-5-phenyl-1-heptene] (poly-2) the CD data show that the phenyl group is very slightly dissymmetrically perturbed, although directly bound to the asymmetric carbon atom. This result, analogous to that found for poly[p-sec-butylstyrene] 22), might be related to the fact that the macromolecules conformational equilibrium is not markedly shifted towards conformations of a single chirality type. In the case of poly[(S)-5-methyl-l- heptene], where the asymmetric carbon atom of the side chains is also in the y-position with respect to the main chain, a small prevalence of one direction of the screw over the opposite seems to exist'). However, in both polymers a

32


large number of allowed conformations, having similar statistical weight, are possible for the side chain; in the case of poly-2 therefore the CD of the phenyl group in the different side chain conformations can be of opposite sign thus giving a very low average ellipticity. The values of [@]is of the (R)-4-phenyl- I-hexene (l)/Cmethyl- 1-pentene (3) copolymers can be explained admitting that the optically active units with aromatic side chains induce the units from 3 to assume a preferential chiral conformation. A substantiation of this model derives from the evaluation of the rotatory power of 3-units ([@I3) in the copolymer. In fact, the values (Tab. 2) are of the same order of magnitude as those previously found for the copolymers of 4-methyl-1-pentene with (S)-4- methyl- 1-hexene, where the former is included in left-handed helical sections which are induced by the latter”). As in that case the sign of [@I3 was positive, the negative sign found in the 1/3 copolymer indicates that the predominant direction of the screw should be now the right-handed one (Tab. 2).

Admitting a 72 helix, as that found 7 * 23) in the solid state for isotactic poly[4- methyl-I-pentene], the chain sections should have predominantly the conformation reported in Fig. 2. The prevalence of the right-handed direction of the screw in the 1/3 copolymer macromolecules is due to the chiral units from (R)-4-phenyl- 1 -hexene ; the same direction, therefore, should prevail in the macromolecules of the homopolymer of this last monomer.

Fig. 2. Projection perpendicular to the major axis of a 72 right-handed helical section of a copolymer macromolecule containing (R)-4-phenyl-l-hexene and

4-methyl-1 -pentene units

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The larger population of a single screw direction with respect to the opposite in polymers of optically active 4-methyl- 1-hexene, was explained considering the cooperative effect among chiral side chains ’* z4). This effect was attributed to the fact that in the preferred screw direction the ethyl group, bound to the asymmetric carbon atom of the side chain, is farther from the main chain than the methyl group and thus allowed to assume two conformations; whereas in the case of the unfavoured opposite screw direction, only one conformation is allowed. Therefore, it was assumed that the entropy difference was respon- sible for the prevalence of a single screw direction whereas the enthalpy difference was assumed to be very low and negligiblez4). In the polymers from 1 the methyl group is substituted by a phenyl group, but again the favoured screw direction is that in which the ethyl group is farther from the main chain (Fig. 3, conformations R, and R,) rather than the phenyl group (Fig. 3, conformation &). Taking into account the free energy values in conformational equilibria involving methyl, ethyl and phenyl substituted cyclohexane 5, a larger stability could be expected when the phenyl group is farther from the main chain. In order to explain our result, we have to admit that the conformational mobility of the phenyl ring in the polymers investigated is very low. Accordingly, the construction of molecular models indicates the phenyl group to be “fixed” in one conformation, which, in agreement to the literature 9,z6*z7),

should be that, in which the phenyl group is eclipsed to the hydrogen atom bound to the asymmetric carbon atom.

The CD spectra, in the region of the ‘Lb electronic transition of benzene, substantiates this hypothesis showing (Fig. 1) a band with ellipticity much larger than that of flexible phenylalkanes and of the same order of magnitude of conformationally rigid 2,2-dimethyl-3-phenylbutane 9, and of coisotactic

Fig. 3. Allowed conformations of the monomeric units of (R)-4-phenyl-l-hexene inserted in an isotactic chain. Sl: left-handed helix; R1 and R2: right-handed helices

34


copolymers of styrene with optically active cc-olefins ' ' 9 "I. The ellipticity of this band per phenyl group is slightly higher in the copolymer than in the homopolymer (Fig. 1). This result might be at least in part ascribed to the higher average stereoregularity degree in the copolymer fractions ' O). However, this indicates also that the rotational strength of the longest wavelength CD band is mainly determined by the conformational rigidity of the isolated phenyl ring, which is strongly perturbed by the dissymmetric environment of the helical chain.

This does not exclude the possibility of interactions between the aromatic side chains in macromolecules having ordered conformation. These intefh actions cannot probably be detected in this spectral region. The CD spectrum of the copolymer below 240nm shows that two bands of opposite sign are present both in the region of the 'La (Fig. 1) and 'B electronic transitions of benzene chromophore. Analogous results were obtained in the case of the coisotactic copolymers of styrene with 3,7-dimethyl-l-octene ' ') for which the splitting of the CD bands below 240 nm was attributed to the interaction between the phenyl groups of styrene units, included in helical chains with a predominant screw direction.

Conclusions

The investigation of the chiroptical properties of stereoregular polymers of optically active cc-olefins bearing a phenyl group bound to the asymmetric carbon atom has given the following picture of the conformational situation of the macromolecules in solution :

(A) When the asymmetric carbon atom of the side chains is not farther than the p position from the main chain, the phenyl chromophore shows chiroptical properties which can be related to the prevalence of one screw direction of the helical sections of the main chains.

(B) In the case of a right-handed helix, that is turning away from the observer when turned clockwise, a negative optical rotation at 589 nm is found as for aliphatic poly-ct-olefins') and the sign of the successive CD-bands is in agreement with that found for styrene/cc-olefins copolymers ' '. 2 8 ) .

(C) Conformational rigidity of the monomeric units in the helical chains suf- fices to explain optical rotation data at 589 nm as well CD data over 240 nm. The splitting of the bands in the region of the 'La and 'B electronic transitions of benzene seems to indicate the existence of interactions of aromatic side chains along the macromolecules having an ordered chiral conformation as proposed for copolymers of optically active cc- olefins with vinyl aromatic monomers "1.

35


Experimental Part

Preparation of the polymeric products and their characterization are reported in the preceding paper lo). Polarimetry was also carried out as reported lo).

UV and CD spectra were obtained at 27OC by a Cary 14 Spectrophotometer and by a Jouan-Roussel Dichrograph 11, respectively. Spectrograde chloroform and heptane were used as solvents between 300 and 240 nm, and between 300 and 185 nm, respectively.

For copolymer samples the values of log E and [@I are calculated as follows:

log E = log (A.lOO/cl), where A = recorded absorbance, I = spectral light path of the cell in cm and c = (g of copolymer. wt- % of unit 1)/

[@I = Ae.3300, where A& = d.10-5/cl, d = recorded deflection in mm, c and I as indi- (mol. weight of unit 1. dm3)

cated above.

The authors wish to express their appreciation to Mr. C. BERTUCCI and M. POGGIANTI for the assistance in obtaining UV and CD spectra. The financial support from the CONSIGLIO NAZIONALE DELLE RICERCHE (CNR), Roma, Italy, is also gratefully acknowledged.

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optically active hydrocarbon polymers with aromatic side chains, 5. conformational and chiroptical...

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