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Model polyethers II—Synthesis by polycondensation of glycols in the presence of alcohols
Model Polyethers II Synthesis by Polycondensation of Glycols & the Presence of Alcohols T. P. HOBINand R. T. LOWSON The acid-catalysed condensation o] glycols in the presence of alcohols has been investigated as a potential synthesis for model polyethers H[(CH,2)xO],~(CH~)zH where n=2, 3, 4, etc. The method proved to be satisfactory where x = 6 and 10 but failed where x = 4 and 5.
THE use of the Williamson reaction for the stepwise synthesis of model polyethers was described in Part 11. A batch synthesis yielding a homologous series of model polyethers was required and a possible method of achieving this was by a polycondensation of glycols in the presence of alcohols; the latter to provide terminal alkoxy groups on the condensed glycols. Earlier work by Hibbert and Perry 2 had shown that ethylene glycol would undergo polycondensation in the presence of iodine to give a homologous series of model polyethylene glycols; the same catalyst was also effective for the depolymerization of polyethyleneglycol. Later, Rhoad and Flory~ reported that sulphuric acid catalysed the polycondensation of 1,10decanediol but decomposed lower glycols. Subsequently, Lal and Trick" found that sulphuric acid used in conjunction with borontrifluoride etherate was a successful catalyst for the polycondensation of both 1,6-hexanediol and 1,10-decanediol. The various catalysts used by the previous workers were tried in the course of the present work. Optimum conditions for condensation were ascertained from a preliminary study of the condensation of n-hexanol to di-n-hexylether. These conditions were employed in attempts to achieve polycondensation of glycols, both alone and in the presence of alcohols. EXPERIMENTAL
(1) Optimum conditions for the condensation of n-hexanoI n-Hexanol was heated with the catalyst in a two-necked fask. A thermometer with a ground-glass conical joint was inserted into one of the necks of the flask to record the temperature of the reactants. A Dean and Stark apparatus, inserted into the other neck of the flask, was used to isolate the water produced in the reaction. A series of experiments was carried out using various concentrations of several catalysts (sulphuric acid alone and sulphuric acid, sulphamic acid or p-toluene sulphonic acid in conjunction with borontrifluoride etherate). Some results are summarized in Table 1. The yields quoted were only approximately reproducible because the rise in temperature as the nhexanol was converted to di-n-hexylether was not necessarily the same in 217
T. P. HOBIN and R. T. LOWSON all the experiments and because the composition of the condensate in the Dean and Stark separator was not controlled. In all cases some unreacted hexanol remained in the separator at the end of the experiment. The following conclusions were drawn from the results.
Efficiency of the catalysts--Comparison of half-reaction times for runs 5, 11 and 12 shows that in the presence of borontrifluoride etherate the order of catalytic activity was sulphuric acid > p-toluene sulphonic acid sulphamic acid. Comparison of runs 2, 4, 6 and 7 shows that the presence of borontrifluoride etherate had very little effect on the rate of reaction or on the yield of hexylether. Effect of catalyst concentration~Comparison of the half-reaction times for runs 8, 9, 10 and 4 shows that increase of catalyst concentration increased the rate of reaction. Effect of temperature--The temperatures at half-conversion ranged from 162 ° to 181°C. This range of temperature is therefore adequate for the condensation. In the later stages of the reactions, however, the temperatures rose as high as 208°C but at the same time there was some loss of n-hexylether, presumably by acid-catalysed degradation (compare runs 2 to 5). In keeping with this it was observed that there was considerable darkening of the reaction mixtures when the temperatures exceeded 180°C. By-products--In all cases a small amount of hexene was obtained (average 1 '7 per cent, maximum 3"0 per cent) and an involatile residue remained after distillation of the products (average 6.7 per cent, maximum 10 per cent). In addition, reactions in the presence of borontrifluoride etherate produced a white deposit in the condenser. The latter contained both silicon and fluorine but it had no catalytic activity. (2) Attempted polycondensation of glycols (i) Catalysis by iodine--Some preliminary experiments showed that iodine is a general catalyst for the condensation of glycols. 1,4-butanediol and 1,5-pentanediol, when heated with half per cent by weight or iodine, were readily converted into cyclic ethers. Under the same conditions, 1,6hexanediol and 1,10-decanediol were converted into low polymers. The latter developed a yellow colouration on storage. (ii) Catalysis by sulphuric acid--Sulphuric acid was found to be a more active catalyst than iodine for the condensation of glycols but it caused some charring of the products (less darkening was observed when the reactions were carried out under vacuum). Catalyst concentrations of half per cent by weight and a reaction temperature of 180°C were found to be suitable for the polycondensation of both 1,6-hexanediol and 1,10-decanediol, although, with the former, about ten per cent of the glycol was convetted to cyclic ether. All the polymers, after decolourization with charcoal, exhibited no discolouration on storage. 218
MODEL POLYETHERSII
Table 1. Runs Run Run Run
4
Condensation of n-hexanol to di-n-hexylether 1 to 10 Sulphuric acid catalyst 11 p-Toluenesulphonic acid catalyst 12 Sulphamic acid catalyst
Catalyst (% by wt)
Hall-reaction poMtion Time, h
Acid catalyst
BF3, Et20
0'6
0-9
0"6
0"9
0'6
0.9
0"6
0"9
12
0"6
0-9
13
12
Temp. °C
Final position
Yields %
Time, h
Temp. °C
Hexylether
Water 93
15
172
84
20
180
86
88
25
194
74
95
181
3O
208
64
77
168
63
210
68
94
2O
180
86
95 81
169
0'6 7
0'6
178
32
204
65
8
0"1
0"3
104
175
129
181
62
87
9
0"2
0"3
39
162
56
190
83
64
10
0"4
0"6
2O
165
52
206
7O
96
lq
1"2
0"9
253
174
73
213
42
59
12
0"6
0'9
45
178
72
208
57
66
Two solvents were tried namely, n-decane (b.pt 171 ° to 175°C) and, for the reactions involving hexanediol, di-n-hexylether (b.pt 223°C). There was no evidence that these interfered with the reactions, except for the expected reduction in rate. Two comparative experiments were carried out in n-decane for the purpose of ascertaining the effect of borontrifluoride etherate on the polycondensation of 1,6-hexanediol. In these 1,6-hexanediol (75g) and n-decane (50 ml) were reacted with sulphuric acid (0-45g) in one case and sulphuric acid (0.45g)+borontrifluoride etherate (0.9g) in the other. The rate at which water collected was found to be the same in both experiments and both reactions approached completion after five hours. In each case about 7 g of hexamethylene oxide and 52 g of low polymer were obtained. (3) Attempted poIycondensation of glycols in the presence o[ alcohols (i) Buta~ediol--l,4-Butanediol (1 mole), n-butanol (2 mole) and sulphuric acid (1.5 g)reached 118°C on heating and steadily evolved a vapour which passed over at 95 ° to 100°C. Most of the reactants had been converted to product after eight hours. The distillate contained n-butanol, tetrahydrofuran and water. (ii) Pentanediol--l,5-Pentanediol (1 mole), n-pentanol (2 mole) and sulphuric acid (1 "5 g) attained 140°C at reflux and evolved water at the rate of I g/h. A considerable quantity of tetrahydropyran was formed (b. pt 88°C) and because this tended to lower the temperature of the reactants it was distilled over and collected from time to time. The reaction was stopped after 40 h when 19 g of water had been collected. The product was neutralized with aqueous sodium carbonate solution and on fractional distillation yielded the following: n-pentanol (l18g), tetrahydropyran 219
T. P. HOBIN and R. T. LOWSON
(48 g), di-n-amylether (7 g), pentyloxypentanol (7 g) and a higher-boiling residue (7 g). (iii) Hexanediol--The ease of condensation of n-hexanol at its boiling point (158°C) and the reluctance of 1,6-hexanediol to undergo intramolecular condensation contributed to the success of attempts to prepare model hexamethylene oxide polymers from condensations involving 1,6-hexanediol and n-hexanol. The results of two experiments are given in Table 2. Table 2. Condensation of hexanediol (1 mole) with hexanol. Sulphuric acid catalyst (0"6% by wt) Final Oosltion
H exano l (moles)
Time, h
Te~o, *C
Water Hexene, lormed 70*-90* (760 g ram)
5
180
50
3
190
34
4"6 15
Crude yields (g) i Cyclic Hexanol, Hexane- Monoether, i 140"dtol, ether, 105 °160" 250*200"125" (760 265* 225* (760 ram) (760 (760 ram) mm) mm) 6'5 7
DIether, 140 * 160 * (2 ram)
Trtether, 190"220 ° (2 ram)
Tetraether, 250*275 ° (2 ram)
-
148
--
234
60
38
21
25
20
160
6s-
34
21
It can be seen that increase in the ratio of hexanol to hexanediol from 3 : 1 to 5 : 1 did not appreciably affect the yields of diether ~[(CH0~O]~(CH~)6H, triether or tetraether but produced a substantial increase in the yield of di-n-hexylether. The crude diether and triether referred to in Table 2 were at least 80 per cent pure by vapour phase chromatography (VPC). The ethers were further purified by repeated fractional distillation from sodium and characterized by boiling point and refractive index (diether 135°C/1 mm n~2°=1"4381, triether 190°C/0"7 mm nD2°=1"4457 and tetraether 240°C/0.4 mm n~°=1"4511). The diether and triether were also mixed with the same compounds prepared by the Williamson method 1 and subjected to VPC; the mixtures were unresolvable. (iv) Decanediol--As might have been expected, model decamethylene oxide polymers were satisfactorily obtained by the polycondensation of 1,10-decanediol in the presence of n-decanol. A mixture of decanol (1 mole), de:~_nediol (½ mole) and sulphuric acid (0-6% w/w) at 180°C gave 12 g water in eight hours. The product was neutralized with aqueous sodium carbonate solution and all material of boiling point less than 180°/5 mm was distilled off and discarded. Fractional distillation of the residue gave the following: 180 ° 172 ° 235 ° 240 ° 300 °
to 190°C/5 to 220°C/1 to 240°C/1 to 300°C/1 to 305°C/1 Residue
mm mm mm mm mm
28 g 10 g 29 g 5.5 g 15"5 g 32 g
(decylether) (diether) (triether) (tetraether, etc.)
The diether H[(CH~)100]~(CH0x0H and triether were fractionally distilled several times from sodium and characterized by melting points, boiling point 220
M O D E L P O L Y E T H E R S II
and refractive index (diether m.pt 40°C, b.pt 235 °/0-4 mm, n ~ ° = 1'4366, triether m.pt 50°C, b.pt 305°/1-0 mm, nD6°= 1.4416). The values of these physical constants agree with those of the analogous compounds prepared by the Williamson reaction. DISCUSSION
The preliminary experiments on the condensation of n-hexanol had indicated that in order to minimize degradation of the products the least quantity of sulphuric acid catalyst consistent with a convenient time scale should be employed (e.g. 0"5 per cent H~_SO~w/w) and that the temperature should not be allowed to rise much above 180°C. This was satisfactorily achieved by the use of n-decane (b.pt 171 ° to 175 °) as a carrier for the removal of water from the reactants. Rhoad and Flory~ used more drastic conditions (2 per cent H~SO~, 200°C) and so it is perhaps understandable that 1, 6-hexanediol decomposed in their experiments. Lal and Trick ~ apparently did not try sulphuric acid alone; they used a mixture of sulphuric acid with borontrifluoride etherate but did not explain why they used this combination. The polycondensation of 1, 6-hexanediol is accompanied by some intramolecular condensation to hexamethylene oxide and it is known that borontrifluoride is a catalyst for the polymerization of some cyclic ethers:'. There have been no reports, however, of hexamethylene oxide having been so polymerized. The nearest cyclic ether to have been polymerized is tetrahydrofuran and the polymerization of this has a ceiling temperature of 83°C 6. The ceiling temperature for the polymerization of hexamethylene oxide would have to be well above 200oc (the temperature used by Lal and Trick) for there to be~ any appreciable polymerization at this temperature. In the present work it was found that the same quantity of hexamethylene oxide was formed whether borontrifluoride etherate was used or not. In fact borontrifluoride etherate was found to have very little effect on the sulphuric acid catalysed condensation of either n-hexanol or of I, 6-hexanediol. There is a theoretical objection to the use of borontrifluoride in that it might conceivably be converted to boric acid which is known to be an excellent reagent for the conversion of alcohols to olefins 7, the yield of 1-hexene from n-hexanol in reaction with boric acid is of the order of 90 per cent. The possibility of obtaining high-molecular-weight polyethers by the acid catalysed condensation of diols is doubtful. The experiments on the condensation of hexanol showed that some hexene was formed in the reaction; a similar reaction in a diol during polycondensation would produce inactive end-groups which would result in limitation of the molecular weight. Another snag is that towards the end of the reaction the ratio of catalyst to hydroxyl groups increases rapidly and at this stage degradation of the ethereal products becomes significant. This might possibly be overcome by first preparing and purifying the low polymer and then applying very low catalyst concentrations and extended reaction times in an attempt to convert the low polymer to material of higher molecular weight. The main objective of the present work, the synthesis of model polyethers from glycols and alcohols, was successfully achieved with 1,6-hexanediol and 1,10-decanediol. The method failed for 1,4-butanediol and 221
T. P. HOBIN and R. T. LAWSON 1, 5-pentanediol, n o t b e c a u s e these dials resisted c o n d e n s a t i o n b u t because of their preference for i n t r a m o l e c u l a r condensation. O n this basis it is expected t h a t t h e synthesis w o u l d be satisfactory f o r all dials higher t h a n h e x a n e dial.
The author is grateful to M r I. Tunstall of the E.R.D.E. Anctlyticat Section for carrying out V P C analysis. Explosives Research and Development Establishment, Waltham A b b e y (Received September 1965) REFERENCES i HOBIN, T. P. Polymer, Land. 1965, 6, 403 2 HIBBERT, H. and PERRY, S. 7_, Canad. J. Res. 1936, 1411, 77 a RHOAD,H. J. and FLORY,P. J. J. Amer. chem. Sac. 1950, 72, 2216 4 LAL, J. and TRICK, G. S. J. Polym. Sci. 1961, 50, 13 5 EASTHAM,A. M. Advanc. Polym. ScL 1960, 2, 18 6 SIMS, D. J. chem. Sac. 1964, 864 7 BRANDENBERG,W. B. and GALAT,A. J. Amer. chem. Sac. 1950, 72, 3275
222
THE use of the Williamson reaction for the stepwise synthesis of model polyethers was described in Part 11. A batch synthesis yielding a homologous series of model polyethers was required and a possible method of achieving this was by a polycondensation of glycols in the presence of alcohols; the latter to provide terminal alkoxy groups on the condensed glycols. Earlier work by Hibbert and Perry 2 had shown that ethylene glycol would undergo polycondensation in the presence of iodine to give a homologous series of model polyethylene glycols; the same catalyst was also effective for the depolymerization of polyethyleneglycol. Later, Rhoad and Flory~ reported that sulphuric acid catalysed the polycondensation of 1,10decanediol but decomposed lower glycols. Subsequently, Lal and Trick" found that sulphuric acid used in conjunction with borontrifluoride etherate was a successful catalyst for the polycondensation of both 1,6-hexanediol and 1,10-decanediol. The various catalysts used by the previous workers were tried in the course of the present work. Optimum conditions for condensation were ascertained from a preliminary study of the condensation of n-hexanol to di-n-hexylether. These conditions were employed in attempts to achieve polycondensation of glycols, both alone and in the presence of alcohols. EXPERIMENTAL
(1) Optimum conditions for the condensation of n-hexanoI n-Hexanol was heated with the catalyst in a two-necked fask. A thermometer with a ground-glass conical joint was inserted into one of the necks of the flask to record the temperature of the reactants. A Dean and Stark apparatus, inserted into the other neck of the flask, was used to isolate the water produced in the reaction. A series of experiments was carried out using various concentrations of several catalysts (sulphuric acid alone and sulphuric acid, sulphamic acid or p-toluene sulphonic acid in conjunction with borontrifluoride etherate). Some results are summarized in Table 1. The yields quoted were only approximately reproducible because the rise in temperature as the nhexanol was converted to di-n-hexylether was not necessarily the same in 217
T. P. HOBIN and R. T. LOWSON all the experiments and because the composition of the condensate in the Dean and Stark separator was not controlled. In all cases some unreacted hexanol remained in the separator at the end of the experiment. The following conclusions were drawn from the results.
Efficiency of the catalysts--Comparison of half-reaction times for runs 5, 11 and 12 shows that in the presence of borontrifluoride etherate the order of catalytic activity was sulphuric acid > p-toluene sulphonic acid sulphamic acid. Comparison of runs 2, 4, 6 and 7 shows that the presence of borontrifluoride etherate had very little effect on the rate of reaction or on the yield of hexylether. Effect of catalyst concentration~Comparison of the half-reaction times for runs 8, 9, 10 and 4 shows that increase of catalyst concentration increased the rate of reaction. Effect of temperature--The temperatures at half-conversion ranged from 162 ° to 181°C. This range of temperature is therefore adequate for the condensation. In the later stages of the reactions, however, the temperatures rose as high as 208°C but at the same time there was some loss of n-hexylether, presumably by acid-catalysed degradation (compare runs 2 to 5). In keeping with this it was observed that there was considerable darkening of the reaction mixtures when the temperatures exceeded 180°C. By-products--In all cases a small amount of hexene was obtained (average 1 '7 per cent, maximum 3"0 per cent) and an involatile residue remained after distillation of the products (average 6.7 per cent, maximum 10 per cent). In addition, reactions in the presence of borontrifluoride etherate produced a white deposit in the condenser. The latter contained both silicon and fluorine but it had no catalytic activity. (2) Attempted polycondensation of glycols (i) Catalysis by iodine--Some preliminary experiments showed that iodine is a general catalyst for the condensation of glycols. 1,4-butanediol and 1,5-pentanediol, when heated with half per cent by weight or iodine, were readily converted into cyclic ethers. Under the same conditions, 1,6hexanediol and 1,10-decanediol were converted into low polymers. The latter developed a yellow colouration on storage. (ii) Catalysis by sulphuric acid--Sulphuric acid was found to be a more active catalyst than iodine for the condensation of glycols but it caused some charring of the products (less darkening was observed when the reactions were carried out under vacuum). Catalyst concentrations of half per cent by weight and a reaction temperature of 180°C were found to be suitable for the polycondensation of both 1,6-hexanediol and 1,10-decanediol, although, with the former, about ten per cent of the glycol was convetted to cyclic ether. All the polymers, after decolourization with charcoal, exhibited no discolouration on storage. 218
MODEL POLYETHERSII
Table 1. Runs Run Run Run
4
Condensation of n-hexanol to di-n-hexylether 1 to 10 Sulphuric acid catalyst 11 p-Toluenesulphonic acid catalyst 12 Sulphamic acid catalyst
Catalyst (% by wt)
Hall-reaction poMtion Time, h
Acid catalyst
BF3, Et20
0'6
0-9
0"6
0"9
0'6
0.9
0"6
0"9
12
0"6
0-9
13
12
Temp. °C
Final position
Yields %
Time, h
Temp. °C
Hexylether
Water 93
15
172
84
20
180
86
88
25
194
74
95
181
3O
208
64
77
168
63
210
68
94
2O
180
86
95 81
169
0'6 7
0'6
178
32
204
65
8
0"1
0"3
104
175
129
181
62
87
9
0"2
0"3
39
162
56
190
83
64
10
0"4
0"6
2O
165
52
206
7O
96
lq
1"2
0"9
253
174
73
213
42
59
12
0"6
0'9
45
178
72
208
57
66
Two solvents were tried namely, n-decane (b.pt 171 ° to 175°C) and, for the reactions involving hexanediol, di-n-hexylether (b.pt 223°C). There was no evidence that these interfered with the reactions, except for the expected reduction in rate. Two comparative experiments were carried out in n-decane for the purpose of ascertaining the effect of borontrifluoride etherate on the polycondensation of 1,6-hexanediol. In these 1,6-hexanediol (75g) and n-decane (50 ml) were reacted with sulphuric acid (0-45g) in one case and sulphuric acid (0.45g)+borontrifluoride etherate (0.9g) in the other. The rate at which water collected was found to be the same in both experiments and both reactions approached completion after five hours. In each case about 7 g of hexamethylene oxide and 52 g of low polymer were obtained. (3) Attempted poIycondensation of glycols in the presence o[ alcohols (i) Buta~ediol--l,4-Butanediol (1 mole), n-butanol (2 mole) and sulphuric acid (1.5 g)reached 118°C on heating and steadily evolved a vapour which passed over at 95 ° to 100°C. Most of the reactants had been converted to product after eight hours. The distillate contained n-butanol, tetrahydrofuran and water. (ii) Pentanediol--l,5-Pentanediol (1 mole), n-pentanol (2 mole) and sulphuric acid (1 "5 g) attained 140°C at reflux and evolved water at the rate of I g/h. A considerable quantity of tetrahydropyran was formed (b. pt 88°C) and because this tended to lower the temperature of the reactants it was distilled over and collected from time to time. The reaction was stopped after 40 h when 19 g of water had been collected. The product was neutralized with aqueous sodium carbonate solution and on fractional distillation yielded the following: n-pentanol (l18g), tetrahydropyran 219
T. P. HOBIN and R. T. LOWSON
(48 g), di-n-amylether (7 g), pentyloxypentanol (7 g) and a higher-boiling residue (7 g). (iii) Hexanediol--The ease of condensation of n-hexanol at its boiling point (158°C) and the reluctance of 1,6-hexanediol to undergo intramolecular condensation contributed to the success of attempts to prepare model hexamethylene oxide polymers from condensations involving 1,6-hexanediol and n-hexanol. The results of two experiments are given in Table 2. Table 2. Condensation of hexanediol (1 mole) with hexanol. Sulphuric acid catalyst (0"6% by wt) Final Oosltion
H exano l (moles)
Time, h
Te~o, *C
Water Hexene, lormed 70*-90* (760 g ram)
5
180
50
3
190
34
4"6 15
Crude yields (g) i Cyclic Hexanol, Hexane- Monoether, i 140"dtol, ether, 105 °160" 250*200"125" (760 265* 225* (760 ram) (760 (760 ram) mm) mm) 6'5 7
DIether, 140 * 160 * (2 ram)
Trtether, 190"220 ° (2 ram)
Tetraether, 250*275 ° (2 ram)
-
148
--
234
60
38
21
25
20
160
6s-
34
21
It can be seen that increase in the ratio of hexanol to hexanediol from 3 : 1 to 5 : 1 did not appreciably affect the yields of diether ~[(CH0~O]~(CH~)6H, triether or tetraether but produced a substantial increase in the yield of di-n-hexylether. The crude diether and triether referred to in Table 2 were at least 80 per cent pure by vapour phase chromatography (VPC). The ethers were further purified by repeated fractional distillation from sodium and characterized by boiling point and refractive index (diether 135°C/1 mm n~2°=1"4381, triether 190°C/0"7 mm nD2°=1"4457 and tetraether 240°C/0.4 mm n~°=1"4511). The diether and triether were also mixed with the same compounds prepared by the Williamson method 1 and subjected to VPC; the mixtures were unresolvable. (iv) Decanediol--As might have been expected, model decamethylene oxide polymers were satisfactorily obtained by the polycondensation of 1,10-decanediol in the presence of n-decanol. A mixture of decanol (1 mole), de:~_nediol (½ mole) and sulphuric acid (0-6% w/w) at 180°C gave 12 g water in eight hours. The product was neutralized with aqueous sodium carbonate solution and all material of boiling point less than 180°/5 mm was distilled off and discarded. Fractional distillation of the residue gave the following: 180 ° 172 ° 235 ° 240 ° 300 °
to 190°C/5 to 220°C/1 to 240°C/1 to 300°C/1 to 305°C/1 Residue
mm mm mm mm mm
28 g 10 g 29 g 5.5 g 15"5 g 32 g
(decylether) (diether) (triether) (tetraether, etc.)
The diether H[(CH~)100]~(CH0x0H and triether were fractionally distilled several times from sodium and characterized by melting points, boiling point 220
M O D E L P O L Y E T H E R S II
and refractive index (diether m.pt 40°C, b.pt 235 °/0-4 mm, n ~ ° = 1'4366, triether m.pt 50°C, b.pt 305°/1-0 mm, nD6°= 1.4416). The values of these physical constants agree with those of the analogous compounds prepared by the Williamson reaction. DISCUSSION
The preliminary experiments on the condensation of n-hexanol had indicated that in order to minimize degradation of the products the least quantity of sulphuric acid catalyst consistent with a convenient time scale should be employed (e.g. 0"5 per cent H~_SO~w/w) and that the temperature should not be allowed to rise much above 180°C. This was satisfactorily achieved by the use of n-decane (b.pt 171 ° to 175 °) as a carrier for the removal of water from the reactants. Rhoad and Flory~ used more drastic conditions (2 per cent H~SO~, 200°C) and so it is perhaps understandable that 1, 6-hexanediol decomposed in their experiments. Lal and Trick ~ apparently did not try sulphuric acid alone; they used a mixture of sulphuric acid with borontrifluoride etherate but did not explain why they used this combination. The polycondensation of 1, 6-hexanediol is accompanied by some intramolecular condensation to hexamethylene oxide and it is known that borontrifluoride is a catalyst for the polymerization of some cyclic ethers:'. There have been no reports, however, of hexamethylene oxide having been so polymerized. The nearest cyclic ether to have been polymerized is tetrahydrofuran and the polymerization of this has a ceiling temperature of 83°C 6. The ceiling temperature for the polymerization of hexamethylene oxide would have to be well above 200oc (the temperature used by Lal and Trick) for there to be~ any appreciable polymerization at this temperature. In the present work it was found that the same quantity of hexamethylene oxide was formed whether borontrifluoride etherate was used or not. In fact borontrifluoride etherate was found to have very little effect on the sulphuric acid catalysed condensation of either n-hexanol or of I, 6-hexanediol. There is a theoretical objection to the use of borontrifluoride in that it might conceivably be converted to boric acid which is known to be an excellent reagent for the conversion of alcohols to olefins 7, the yield of 1-hexene from n-hexanol in reaction with boric acid is of the order of 90 per cent. The possibility of obtaining high-molecular-weight polyethers by the acid catalysed condensation of diols is doubtful. The experiments on the condensation of hexanol showed that some hexene was formed in the reaction; a similar reaction in a diol during polycondensation would produce inactive end-groups which would result in limitation of the molecular weight. Another snag is that towards the end of the reaction the ratio of catalyst to hydroxyl groups increases rapidly and at this stage degradation of the ethereal products becomes significant. This might possibly be overcome by first preparing and purifying the low polymer and then applying very low catalyst concentrations and extended reaction times in an attempt to convert the low polymer to material of higher molecular weight. The main objective of the present work, the synthesis of model polyethers from glycols and alcohols, was successfully achieved with 1,6-hexanediol and 1,10-decanediol. The method failed for 1,4-butanediol and 221
T. P. HOBIN and R. T. LAWSON 1, 5-pentanediol, n o t b e c a u s e these dials resisted c o n d e n s a t i o n b u t because of their preference for i n t r a m o l e c u l a r condensation. O n this basis it is expected t h a t t h e synthesis w o u l d be satisfactory f o r all dials higher t h a n h e x a n e dial.
The author is grateful to M r I. Tunstall of the E.R.D.E. Anctlyticat Section for carrying out V P C analysis. Explosives Research and Development Establishment, Waltham A b b e y (Received September 1965) REFERENCES i HOBIN, T. P. Polymer, Land. 1965, 6, 403 2 HIBBERT, H. and PERRY, S. 7_, Canad. J. Res. 1936, 1411, 77 a RHOAD,H. J. and FLORY,P. J. J. Amer. chem. Sac. 1950, 72, 2216 4 LAL, J. and TRICK, G. S. J. Polym. Sci. 1961, 50, 13 5 EASTHAM,A. M. Advanc. Polym. ScL 1960, 2, 18 6 SIMS, D. J. chem. Sac. 1964, 864 7 BRANDENBERG,W. B. and GALAT,A. J. Amer. chem. Sac. 1950, 72, 3275
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