Conformational properties of poly(alkene sulphone)s in solution: 1. Relation between the dielectric properties in solution and the structure of the repeat unit

Conformational properties of poly(alkene sulphone)s in solution: 1. Relation between the dielectric properties in solution and the structure of the repeat unit A. H. Fawcett and K. J. Ivin Department of Chemistry, The Queen's University of Be~fast. Be~fast BT9 5AG, UK (Received 19 September 1974; revised 13 December 1974)

A survey of the available evidence shows that the central C-C bonds in/]-disulphones and polysulphones have a mainly trans, and sometimes an exclusively trans, conformation. Calculations of the coulombic interactions in the 27 possible conformational sequences in -SO2CH2CH2SO2- show that certain sequences having gauche C-C bonds are unlikely to be significantly populated. Others such as (t, g-, g+) and (g-, g-, g+) involve a certain amount of steric interaction but are likely to be comparable in energy with those having a trans C-C bond. One alkyl substituent or two at the same carbon atom eliminates some but not all of the accessible sequences having a gauche C - C bond. Substituents at both carbon atoms cause such strong steric interactions that only trans C-C bonds are allowed. It is shown how such considerations account satisfactorily for the two types of dielectric behaviour of poly(alkene sulphone)s in solution. Preliminary experiments indicate that poly(norbornene sulphone), unlike polysulphones of other 1,2-disubstituted alkenes, does have a permanent dipole,because of the inability of adjacent C-S bonds to take up anti-parallel positions. A close resemblance is noted between the conformation about the main-chain C - C bond in poly(cyclohexene sulphone) and that about the main-chain C-C bond in poly(but-2-ene sulphone) in its most stable configuration.

INTRODUCTION

CONFORMATIONS IN SULPHONES

The dielectric behaviour of six poly(alkene sulphone)s in solution has been reported l-a. The polymers fall into two distinct groups: group A in which the alkene unit is hex-1ene or 2-methylpent-l-ene; and group B in which the alkene unit is but-2-ene, hex-2.ene, cyclohexene or cyclopentene. Members of group A show a dispersion in the range 103106 Hz and the critical frequency is strongly dependent on molecular weigh0 '2, indicating relaxation by overall motion of the molecule. Members of group B show no dispersion in the range 102-107 Hz, indicating zero dipole moment. The essential difference between the two groups of polymers is thought to reside in the proportion of gauche conformations taken up by the main-chain C - C bonds. The zero dipole moment for group B implies that all the mainchain C - C bonds are in the trans conformation, while the finite dipole moment for group A implies that at least a proportion of the main-chain C - C bonds are in a gauche conformation at any one time. The purpose of the present paper is first, to summarize the available evidence concerning conformations about C - C bonds in ~-disulphones and polysulphones, and also about C - S bonds in sulphones; second, to consider the factors which operate to determine the conformational populations in sulphones and polysulphones, and to examine the importance of coulombic interactions between adjacent sulphone groups; and third, to consider the relationships between the structures of polymers in group B.

Apart from the dielectric evidence on the polysulphones in solution there are five pieces of information concerning the conformation of the central C - C bonds in/~-disulphones and polysulphones. First, the magnitude of one of the vicinal coupling constants (~9.5 Hz) in the 1H n.m.r, spectrum of poly(propene sulphone) 4 indicates a preference for the trans conformation about the main-chain C - C bond, as in poly(propene sulphide) s. Second, for prnSO2CH2CH2SO2Pr n in the polar solvent dimethyl sulphoxide-d 6 the 13C side band of the 1H n.m.r. spectrum of the central protons indicates a preference for the trans C - C conformation6; the enthalpy difference is estimated to be about 6 kJ/mol in favour of the trans conformation 7. Third, prnSO2CH2CH2SO2Pr n in the solid state obeys the rule of mutual exclusion for the Raman and infrared bands, showing that it has a centre of symmetry s. Intermolecular forces in the crystal thus assist to ensure that all the central C - C bonds adopt a trans conformation. Fourth, the dipole moments of prnSO2CH2CH2SO2Pr n and prnSO2CHMeCH2SO2Pr n in dioxane solution are both 3.6 D 7, which is not equal to any of the values which would be expected (0, 5.8 or 8.2 D) were any one of the 27 possible conformations of the S - C - C - S bonds exclusively adopted. In both of these molecules there must be present in the moderately polar solvent dioxane several conforma-

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Dielectric properties and repeat unit structure of polysulphones: A. H. Fawcett and K. J. Ivin

S--C

bond

C--C

bond

CIS

C

S

C

c

s

c

H

H

O

C

S

C

H

H

O

bond

g*

ral angle positions to be affected by steric repulsions, dispersion attractions and dipole-dipole interactions. Sulphones have relatively large dipole moments 2 so that dipole-dipole interactions between two sulphone groups separated by two carbon atoms may be quite large. Thus, taking each dipole as 4.4 D and representing it as a pair of electronic charges separated by 100 pm, a pair of parallel dipoles separated by 400 pm in a medium of relative permittivity 2 is calculated to have an energy 17 kJ/mol in excess of that for a pair of anti-parallel dipoles at the same distance. This is considerably more than RT so that the orientation of each dipole in the chain will be strongly correlated with those of its neighbours. More detailed calculations of the coulombic interactions are described below and we then go on to consider how the situation is modified by the introduction of substituents. CALCULATIONS OF COULOMBIC INTERACTION ENERGIES BETWEEN ADJACENT SULPHONE GROUPS IN THE STRUCTURE -SO2CH2CH2SO 2 -

c

s

c

Figure 1 Rotational states about the three bonds in -SO2-CH2--CH2--SO 2-

tions of comparable enthalpy. Fifth, in the 13C-{IH} n.m.r, spectrum of poly(but-2ene sulphone), the two chemical shifts observed for the methyl carbon are consistent with an all-trans conformation for the main chain C - C bond and two alternative relative configurations about adjacent main-chain carbon atoms9; also see final section. Thus all the evidence indicates a substantial preference for the trans conformation of the central C - C bonds in /3-disulphones and polysulphones. However, gauche conformations are significant in some molecules and their energies must then lie fairly close to those for the trans conformation. We may note in passing that in compounds of the type RSO2CH2-CH2CH 3 the temperature dependence of the n.m.r, spectra again indicates a preferred trans conformation, the gauche conformations about the a-j3 C - C bond having an excess enthalpy of the order of 4 kJ/mol (R = CH2CH2SO2Prn , Me, Prn , Bu t , OH) 7'~°. Knowledge of the potential energy function for rotation about C - S bonds in sulphone is limited to the determination of the barrier height to internal rotation in dimethyl sulphone. This has been estimated from third law arguments to be 14 kJ/mol, assuming a three-well cosine function ~. Substituents at the carbon atom may be expected to remove the symmetry of the energy function but without affecting the angular positions of the minima. FACTORS CONTRIBUTING TO THE POTENTIAL FUNCTIONS FOR ROTATION ABOUT THE MAIN-CHAIN BONDS Since we are dealing with main-chain atoms that are each bonded to four other atoms, we assume that the potential function for rotation about each of the main-chain bonds will show three minima, situated to a first approximation at dihedral angles of 0, 27r/3 and -2n/3 and corresponding to conformational states t, g+, g - . The rotational states of the three bonds in a repeat unit are depicted in Figure 1. We may expect the energies of the minima and their dihed-

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POLYMER, 1975, Vol 16, August

It is assumed that the rotational states are as shown in Figure 1, the bond lengths being taken as: C-S, 180 pm; C - C , 154 pm; and S-O, 144 pm, in accordance with published data 12. The dipole moments of n-propyl and isopropyl sulphones 2 in benzene have a mean value of 4.42 D. In calculating coulombic interactions between adjacent sulphone groups we have assumed that each dipole may be represented by charges of 8.87 x 10-2°C and 17.74 x 10-20C located on the oxygen and sulphur atoms respectively. The total coulombic energy E c arising from the interaction of adjacent sulphone groups in the 27 possible sequences of conformations has been calculated from:

Ec = E i

(1)

E (qiqj/4rrri] ereo) i

and the results are shown in Table 1. qi and q! are the charges associated with the interacting atoms i and j on different sulphone groups separated by a distance ri] calculated from the geometry of the system, er is the relative permittivity of the medium, taken as 2.0 (close to the value of dioxane and benzene) and eo is the permittivity of a vacuum. It is likely that the true value ofer is less than 2.0 when the interacting atoms are very close together. It may also be affected by specific solvent interactions, as will be shown in another paper ~7. It will be seen from Table I that there are in fact only nine distinct values o f E c ; sequences with the same value of E c bear a structural relationship to each other through simple symmetry operations. It may also be seen that when the C - C bond has agauche conformation, there is a larger range of energies than when the C - C bond has the trans conformation. This is because the sulphone groups are brought closer together in the former case so that Table 1 Dipole--dipole interaction energies (k J/tool) for a pair of sulphone groups in the structure -SO2-CH2--CH2-SO 2 - (positive values imply repulsion; negative values attraction)

t

C-C

C-S

t

~'~_+0.8 S-C 5.9 ~.g 5.9

g+

g-

g+

g-

t

g+

5.9 9.8 5.85

5.9 5.8s 9.8

25.8 29.3 -0.4

29.3 - 0 . 4 25.8 - 0 . 4 29.3 25.0 8.8 - 0 . 4 25.0 8.8 8.8 25.0 29.3 8.8 25.0

g-

t

g+

g-

Dielectric properties and repeat unit structure of polysulphones: A. H. Fawcett and K. J. Ivin

Z

a

O

~Y CI

S~t

I

of 2.0 for er. Even neglecting this factor, sequences of high energy (>25 kJ/mol) in Table I are not likely to be appreciably populated, and can be eliminated from further consideration. The sequences of real interest to the present discussion are therefore those with the C - C gauche conformation which have energies such as those of the sequences shown in Figures 2b and 2c, together with those with C - C trans conformations. The uncertainties in the calculations mean that one cannot be sure of the actual relative energies of these conformational sequences; some light will be thrown on this problem in the next paper 17 STERIC FACTORS AND EFFECT OF SUBSTITUENTS

O Z

b

(.~

>y

X

O z

C I

)y

X

Figure 2 (a) (t,g--.

O Conformational sequences in - S O 2 - C H 2 - C H 2 - S O 2 - : t); (b) (t,g-,g+); (c) (g-.g--,g+)

neighbouring dipoles are then more strongly correlated in orientation. In Figure 2 we show three different sequences in which the C - C bonds have a gauche conformation: Figure 2a illustrates one (t, g-, t) which is typical of the high energy sequences, while the others are examples of lower energy (t, g-, g+), (g-, g-, g+). In the first case two oxygen atoms of adjacent sulphone groups are brought very close together, and the repulsive contribution to E c from this pair of atoms is probably underestimated by the use of the value

If we examine the 'low energy' sequences depicted in Figures 2b and 2c in more detail we find that the interatomic distance between C4 and the nearest oxygen atom to it attached to S 1 is only 171 pm, which is considerably less than the sum of the van der Waals radii of an oxygen atom doubly bonded to a sulphur atom (140 pm) and a methylene group (200 pm) la. This would cause a considerable repulsive force, which will, however, be relieved by a relatively small change in the dihedral angles and perhaps by some change in the bond angles 14. From the different laws of force of coulombic interaction on the one hand and steric repulsion on the other, small changes in the angles may relieve steric strain in the segment under consideration, while not greatly changing coulombic energies. Thusgauche states of the C - C bonds in the polymers may be significantly populated. The states of the bond can still be designated g+ and g - , though it is likely that the minima will occur at dihedral angles somewhat different from 2rr/3 and -27r/3; the C - S bond rotational minima may likewise be displaced from the 'ideal' dihedral angles. Such distortion will be less significant when the C - C bond has the trans conformation. Next we consider the effect of replacing the hydrogen atoms by one or two alkyl groups on the same carbon atom. Poly(2-methylpent-l-ene sulphone) and poly(hex-l-ene sulphone) are known to have a head-to-tail structure is so that we may place the alkyl substituent(s) on C1 and C3 in Figures 2b and 2c. It can be seen that this will not greatly affect the conformational energy. The sequences (t, g-, g+) and (g-, g-, g+) are thus accessible in the sense of not having much higher energies than certain other sequences; likewise the symmetry-related sequences (t, g÷, g - ) and (g÷, g÷, g - ) are also accessible. If, however, we place the alkyl substituents on C2 and C4 in Figures 2b and 2c, an additional and severe steric hindrance arises between the alkyl group(s) on C4 and the oxygen of the adjacent sulphone group; the sequences (t, g - , g+), (g-, g - , g+), (t, g+, g - ) and (g+, g+, g - ) are then inaccessible. This is equivalent to demonstrating that with substituents on C1 and C 3 the symmetry-related sequences (g÷, g-, t), (g+, g-, g - ) , (g-, g+, t), and (g-, g+, g÷) are inaccessible. Thus with one or two substituents on the same carbon atom, the number of accessible sequences having agauche C - C bond is more limited. Coming now to the case of one substituent on each mainchain carbon atom we may predict from the above considerations that all the sequences with gauche C - C (main chain) conformations will be of high energy as a result of steric effects, leaving only the trans C - C conformations significantly populated. The zero dipole moment in poly(but2-ene sulphone) is thus accounted for, as well as the 13C chemical shifts of the methyl carbons (see next section).

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Dielectric properties and repeat unit structure of polysulphones: A. H. Fawcett and K. J. Ivin

a

s

b

s

trans-but-2-ene is observed to occur to some extent when

I

the polymer is made at very low temperature and the resulting structure is as shown in Figure 3b. Here the methyl groups are all in a trans position relative to one another so that the 2e-effect will be absent. In accordance with this the chemical shift for the methyl group is 3 ppm downfield from that for the structure in Figure 3a. It is worth noting that the structure of the more stable configuration, shown in Figure 3a, is analogous to that of poly(cyclohexene sulphone) a, shown in Figure 3c. Poly(cyclopentene sulphone) also shows no dielectric dispersion in solution s , which again indicates trans addition to the double bond of cyclopentene, with the C - S bonds distorted into near antiparallel positions. The question arises as to whether poly(norborn-2-ene sulphone) would have a net dipole moment, and therefore a dielectric dispersion. The structure of this polymer is shown in Figure 3d and no matter whether the sulphone groups are attached to the bridged ring system at the positions shown, or in another way, it is impossible for the components of the sulphone dipoles along the C - S bonds to cancel. Preliminary experiments on an unfractionated sample of this polymer in benzene solution showed that there was a dielectric loss in the region 102 to 107 Hz, as expected 19. An attempted fractionation using dioxane/ acetone as the solvent/non-solvent system gave a sample ([77] = 17.2 cm3/g in benzene) which showed two dispersion regions in this frequency range. This appears to have been caused by a bimodal distribution in the sample, as indicated by gel permeation chromatography 2°, rather than by the operation of two relaxation mechanisms. Thus it appears that poly(norborn-2-ene sulphone) exhibits group A behaviour. Further work is planned.

I M

Me /C e~CZf.-H

'

~

M

e ~C/I...H

I

I

S

S S

C

Me

0

S

/s

I

S

Figure3

(a) Structure about the main-chain C - C bond of poly(but2-ene sulphone) in its most stable configuration (and conformation); (b) structure about the main-chain C - C bond of poly(but-2-ene sulphone) for its less stable configuration; cf (a); (c) structure about the main-chain C - C bond of poly(cyclohexene sulphone) in its most stable conformation; (d) unit of poly(norbornene sulphone)

Summarizing the above arguments, the population of main-chain gauche C - C states may be expected to decrease through the series: -SO2CH2-CH2SO2- > -SO2CH2-CRIR2SO2- > (one of R1R 2 can be H) -SO2CHR1-CHR2SO 2 - (zero) (R1R2 both alkyl) The experimental observations on disulphones and polysulphones summarized at the beginning of this paper can be understood in terms of this conformational analysis. It should be noted that those matrix schemes of Flory ~6, which express Markov correlations of bond conformations over pairs of bonds by use of statistical weight matrices of order r x r (r is the number of rotational states per bond), will not be adequate to account for vector properties of the polysulphones in group A (see introduction), where correlation over three bonds is clearly important. However, the all-trans conformation o f the main-chain C - C bond for the group B polymers is a simplification which allows the method to be applied 17without recourse to the use of statistical weight matrices of order r 2 x r 2 and a more elaborate generator matrix formulation. COMPARISON OF CONFORMATIONS IN POLYSULPHONES OF 1,2-DISUBSTITUTED ALKENES The 13C-{IH} n.m.r, spectrum of poly(but-2-ene sulphone) indicates that the most stable configuration is formed as a result o f trans-addition to cis-but-2-ene 1°. This information, coupled with the known trans conformation of the mainchain C - C bond 3, means that the structure is as shown in Figure 3a. The methyl groups are thus all in a gauche position relative to one another. In such circumstances the chemical shift is expected to be upfield from that of the methyl carbon in poly(propene sulphone) by about 3 - 4 ppm (7-effect) Is, as is indeed the case. Trans addition to

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POLYMER, 1975, Vol 16, August

ACKNOWLEDGEMENT A.H.F. thanks the Inter-University Council for the award of a Fellowship. REFERENCES 1 Bates,T. W., lvin, K. J. and Williams, G. Trans. Faraday Soc. 1967, 63, 1964 2 Bates,T. W., lvin, K. J. and Williams, G. Trans. Faraday Soc. 1967, 63, 1976 3 Fawcett, A. H. and Ivin, K. J. Polymer 1972, 13,439 4 Ivin, K. J. and Navra;til, M. J. Polym. Sci. (A-I) 1970, 8, 3373 5 Ivin, K. J. and Navr~til, M. J. Polym. Sci. (A-I) 1971, 9, 1 6 Fawcett, A. H., lvin, K. J., Navr~til, M. and Walker, N. A. IUPA C Int. Syrup. Macromol. Chem. Toronto 1968, Paper A4.4 7 Fawcett, A. H. and lvin, K. J. unpublished results 8 Fawcett, A. H. and Stobart, S. R. unpublished results 9 lvin, K. J., Stewart, C. D. and Watt, P. IUPAC Int. Syrup. Macromolecules, Madrid 1974 10 Fawcett, A. H. unpublished results 11 Cleaver,H. L. and Westrum, E. F. J. Phys. Chem. 1970, 74, 1309 12 Sutton, L. E. 'Tables of Interatomic Distances and Configurations in Molecules and Ions', The Chemical Society, London, 1958 13 Bondi, A.J. Phys. Chem. 1964,68,441 14 Hendrickson, J. B. J. Am. Chem. Soc. 1961, 83, 4537 15 Ivin, K. J. and Navr~til, M. Prepr. XXtlllnt. Congr. Pure AppL Chem., Boston 1971, p 755 16 Flory, P.J. Proc. Nat. Acad. Sci. 1964,51,1060;Flory, P.J. 'Mechanics of Chain Molecules', Interscience, New York, 1969 17 Fawcett, A. H. and lvin, K. J.Polymer 1975, 16, 573 18 Grant,D.M. andCheney, B.V.J. Am. Chem. Soc. 1967, 89, 5319 19 Walker,N. A. PhD Thesis Queen's University Belfast (1969) 20 Meyerhoff, G. personal communication