Model polyethers IV—Vapour pressures of oligomers of polytetramethylene oxide

Model Polyethers lV VapourPressures of Oligomers of Polytetramethylene Oxide T. P. HOBIN Vapour pressures of polytetramethylene oxide oligomers H[(CHz)40]n(CH~)4H, where n = 2 to 4, have been measured over the range 80 ° to 200°C. They are compared with those o] n-paraffins and of polymethylene oxide oligomers and discussed in terms o] cohesive energies and chain configurations.

BOYD1 obtained vapour pressure data for oligomers of polymethylene oxide (PMO) and used them to estimate the incremental molar cohesive energy per repeat group of the amorphous high polymer. The present work was undertaken in order to derive similarly the incremental molar cohesive energy per repeat group of amorphous polytetramethylene oxide (PTMO). EXPERIMENTAL (1) Materials Model polytetramethylene oxides were prepared and purified as described in Parts I-IIP -~.

(2) Apparatus A manometric apparatus was used, immersed in a silicone oil bath controlled to + 0 1 deg. C. Mercury cut-off valves were preferred to greased taps to avoid interaction between the vapour and the grease and to ensure freedom from leakage. The system could be pumped down to high vacuum at any time; volatile decomposition products, if formed at the higher temperatures, could have been removed prior to the measurement of the vapour pressure. No such problem was encountered over the periods of the experiments. The degassing facility was used to withdraw vapour during cooling cycles in order to prevent undesired condensation inside the apparatus-particularly on the meniscus of the mercury manometer, where it could have interfered with the manometric reading. The system required at least five minutes to attain equilibrium following a degassing operation. Fuller details of the apparatus will be given elsewhere. (3) Results The results are set out in Table 1 and illustrated in the form of a log P versus 103/T plot in Figure 1. Also shown in Figure 1 are data, taken from the literature, for selected n-paraffins5, for Boyd's PMO oligomers1 and for di-n-butyl etheaz. The latter, the monoether of the PTMO series, is reported to obey the relationship log P [mm Hg] = 8-002 - 2106-059/T [°K] over the range 50 ° to 150°C. 65

T. P. HOBIN

Table 1. Vapour pressures of polytetramethylene oxide oligomers.

H [(CH2)40]n (CH~)~ H Pressure P in mm Hg. Temperature T in *C. n=2

n=3

T

P

T

80 87 92 102 107 116 125 130 135 145 150

2"0 3'0 3"6 6"8 8"9 13"5 20"0 24"5 31 '6 46"8 51 "3

119 127 137 143 152 160 167 174 182 188

n=4 P

"

1"6 2"1 3'6 4"9 7"6 9"1 11 "2

15"1 19'5 29"5

T

P

140 145 152 158 164 170 178 182 186 192 197 203

1'9 2"5 3'0 3"6 4"4 5"9 6"9 8"1 9"6 12"0 13"2 17"4

C8 C7 Pent a - ~ ~

3'0

Hexa~ ~, Tetr,a '\..~,, ~~ko~ °~~°~

2"0 -

° ~ o Tri-

\\

\\

\

o

\ ox

oo

•

o

~..x, I

I

I

I

,,

\

\',~ooo-

Tetra-~c21 ~ %~ - C ~~, , C1~/cD~3 ~C11 ~C: 2-0 2-5 103// [°K]3-0 3.5 I

I

I

I

I

I

I

I

I

I

I

I

I

L

I

I

I

Figure /--Pressure versus reciprocal temperature data.

• This work; <3 Boyd's data for PMO oligomers HCHz [OCH2]~ H. Full lines - n-Paraffins; Dashed line - - di-n-butyl ether

Th e following features are evident f r o m Figure 1. (a)

T h e n-paraffins f o r m a regular family of curves.

(b)

B o y d ' s P M O oligomers a p p e a r to fit in with the paraffin family; each lies very close to the curve for the n-paraffin with the s a m e n u m b e r of chain atoms. 66

M O D E L P O L Y E T H E R S IV

(c)

The mono-, di- and tri-ethers of the PTMO series also appear to fit in with the paraffin family. Their nearest neighbours in the paraffin series, however, have slightly fewer chain atoms. (d) The tetra-ether of the PTMO series deviates from the family formed by the lower oligomers of the series. It exhibits a slight decrease in slope in comparison to the triether and is considerably displaced from the n-paraffin with the same number of chain atoms. DISCUSSION

(1) Comparison of PMO oligomers with the n-paraffins The close proximity in Figure 1 of the PMO oligomers and their corresponding n-paraffins raises the question as to whether both series possess comparable average molar cohesive energies per chain atom. Boyd1 by the use of a computer on his data, arrived at a value of 2"36 kcal for the incremental molar cohesive energy per ---CH~O-- group of the liquid polymer at 25°C. He recognized that this value was very close to Billmeyer's6 value of 2"34 kcal/mole of ---CHiCHi-- for the n-paraffins at 25°C. On this basis, wherever plots of log P versus 1/T for any two series of oligomers form a single regular family of curves, there would be justification for the estimation of the heats of vaporization of members of one series by comparison of their positions with those of members of the other series, provided the heats of vaporization of the members of the other series are known.

Comparison of PTMO oligomers with the n-paraO~ns and estimation of their cohesive energies Computation of vaporization energies from the present results by the method used by Boydx would not be justifiable because of the shorter range of the present studies and the greater molecular weights of the PTMO oligomers, which reduce the differences in slopes, e.g. between diether and triether. Howex,er, comparison of the positions "of the PTMO oligomers up to triether with those of the nearest paraffins suggests that they would have the same vaporization energies as the following hypothetical paraffins: monoether--Cs.5, diether--C~3.2, and triether--C,7.2. The vaporization energies of these hypothetical paraffins can be obtained from a plot of vaporization energy at 25°C versus number of C atoms which, using the data of Bristow and Watson~, is a smooth curve, almost linear. The results in kcal/mole at 25°C are: C8.~=10.5, C~3.~=16"0 and C~7.~=205. The corresponding cohesive energies obtained by subtraction of the expansion energy RT are 9'9, 15.4 and 19"9 kcal/mole respectively. Assignment of these values to the PTMO oligomers gives an average value of 5"0 kcal/ mole of --(CH~)40-- as the estimated cohesive energy associated with the repeat group of liquid PTMO at 25°C. (2)

(3) Empirical calculation o] the cohesive energy per mole of (CH2)~O By use of the empirical molar attraction constants (F) claimed by Small8 to be constitutively additive, ~F values can be calculated for PTMO oligomers at 25°C. The molar cohesive energies would then be given by the 67

T. P. HOBIN

quotient (EF)~/V~ where V~5 refers to the molar volumes at 25°C. Molar volumes at 20°C can be calculated from densities~ and these will differ from values at 25°C by only a fraction of one per cent. The estimated molar cohesive energies calculated from (EF)~/V~ are as follows: Monoether: (1296)8/169 = 9.94 kcal Diether :

(1898)z/240= 15.1 kcal

Triether:

(2500)2/312=20"0 kcal

Tetraether : (3102)~/387 = 24-9 kcal These are in good agreement with the experimental values estimated in the previous section. They give an average incremental cohesive energy per mole of (CH~),O of 4.93 kcal which compares favourably with the experimental estimate of 5.0 kcal (4) Cohesive energy density of polytetramethylene oxide The molar volumes of the PTMO oligomers are a linear function of the number of repeat groups and yield an average volume of 73"0 cm~ per mole of --(CH~)~O-- in the liquid state at 20°C (equivalent to a polymer density of 0"986 g/cm 3 in the same state). The cohesive energy density of the liquid high polymer in the region of 25°C is therefore given by 5-0 x 11Y/73-0= 69 cal/cm 3. This result is of the same order as that of polyethylene~,9 but differs considerably from the value of 104 cal/cm~ for polymethylene oxide1. (5)

General comparison of the cohesive energies of n-paraffins, PMO oligomers and PTMO oligomers

Replacement of every fifth methylene group of a paraffin by an oxygen link gives rise to a small reduction in molar cohesive energy (Figure 1) but, because the - - O - - link is shorter than the --CH~-- link, there is not a very significant difference in the cohesive energy densities of the corresponding high polymers. By analogy it might be expected that replacement of every second ----CH~-- by - - O - - to give a PMO oligomer would result in a substantial reduction in molar vaporization energy. In practice, however, there is scarcely any change. The probable explanation is that suhstitution of every nth --CH~-- by - - O - - occurs in positions that are all on one side o f the planar zig-zag of the paraffin chain when n is even but on alternate sides when n is odd. Hence there will be considerable differences in dipole moments. This is reflected in the high c.e.d, of PMO in comparison to that of PTMO. The dipole moments of these polymers and oligomers have been discussed1°. (6) The deviation shown by the PTMO tetraether The PTMO tetraether appears to have a slightly lower heat of vaporization than the triether. It is likely that its four - - O - - links render it more flexible than the corresponding paraffin with the result that the molecule, because it is longer than the triether, can adopt a configuration in the vapour state that involves significant intramolecular cohesion. The energy released by .such cohesion would offset some of the energy required for vaporization. 68

MODEL POLYETHERS IV The possibility of a reduction in vaporization energy- in cases where intramolecular cohesion could occur in the perfect gas state has been discussed by Scatchard 11, who has quoted experimental evidence to show that increases in cohesive energies on ascending a homologous series have been found to be lower when calculated from vaporization energies than when calculated from polarizability data. Rose TM has shown that relatively large yields of a strain-free cyclic ~etraether (16 chain atoms) can be obtained by polymerization of trimethylene oxide. This indicates that there is considerable flexibility in the tetraether of the polytrimethylene oxide series and also that in this series a chain length of 16 atoms is necessary before the ends can meet. Presumably in members of the same series with more than 16 atoms the ends could do more than m e e t - - t h e y could overlap and lie side by side with considerable intramolecular cohesion. Crown Copyright, reproduced with the permission of the Controller, Her Majesty's Stationery Office. Explosives Research and Development Establishment, Waltham Abbey, Essex (Received April 1967) REFERENCES 1BOYD, R. H. 1. Polym. ScL 1961, 50, 133-141 HOBIH,T. P. Polymer, Lond. 1965, 6, 403--409 3 HOB~, T. P. and LOWSON,R. T. Polymer, Lond. 1966, 7, 217 4 HOmN,T. P. Polymer, Lond. 1966, 7, 223 JOgDAN, T. E. Vapor Pressures of Organic Compounds. Interscienc¢: New York, 1954 6 BILLMEY~,F. W. I. appl. Phys. 1957, 28, 1114 7 B~aSTOW,G. M. and WATSON,W. F. Trans. Faraday Soc. 1958, 54, 1733 8 SMALL,P. A. 1. appl. Chem. 1953, 3, 71 9ALLEN,G., GEE, G. and WILSON,G. J. Polymer, Lond. 1960, 1, 465 10FtX)RY,P. J. and M.~.K, J. E. 1. Amer. chem. Soc. 1966, 118,3702-3707 M~'~:, J. E. I. Amer. chem. Soc. 1966, 88, 3708-3711 11SCATCrISd~D,G. Chem. Rev. 1949, 44, 24 12RosE, J. B. I. chem. Soc. 1956, 542-555

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