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Reactions between perchlorate salts and tetrahydrofuran
J inorg, nucl Chem. Vol. 43, pp. 23-27 Pergamon Pre~s l . t d 1981 Printed in Great Britain
REACTIONS BETWEEN PERCHLORATE SALTS AND TETRAHYDROFURAN Y. ECKSTEIN and P. DREYFUSS Institute of Polymer Science. The University of Akron. Akron, OH 44325, U.S.A. tReceived 11 September 1979; received forpublication 6 March 1980) Abstract--Reactions between tetrahydrofuran (THF) and 22 different perchlorate salts, most often in the presence of organic halides were studied. The use of the organic halides including acetyl, benzoyl and substituted allyl chlorides aided in arranging the perchlorates according to their ability to give good yields of tertiary oxonium ion and polymer from THF. The approximate order, based on the yield of poly(tetrahydrofuran) obtained after 24 hr was Ag +, Hg 2" > Pb 2*. TP + > Fe 3+, AP +, Na +, Fe 2+ > Cr 3+, Mnz+, Ni 2+, Cu 2+, Co 2~, Cd z+ ~>Ce st, Mg2+, Ca" ", Ba-~ , Zn 2*. Li ÷, K +, NH~ = 0. Only Ag+ and Hg:+ perchlorates gave polymer when allylhalides were used. With one other perchlorates the order of reactivity of the halide was acetyl > benzoyl ~ allyl = 0.
received. It did not dissolve or react with dioxane and was only washed and then dried as above. The amount of complexed dioxane was determined by analytical determination of the metal. Typically, 1-3 mol of dioxane was complexed with each mole of perchlorate salt. With the exception of NH4CIO4, all perchlorates used were from Ventron Corp., AIfa Division. Polymerizations were carried out at room temperature in screw cap vials as previously described[5]. Polymerization times varied from 1 to 7 days. Longer times were used when no precipitation or increase in viscosity was apparent. The polymers were characterized as previously described [6].
INTRODUCTION
Tetrahydrofuran (THF) is both an important industrial solvent and an important monomer for the preparation of polymeric materials. Metal salts with complex ions are often dissolved in T H F to serve either as supporting electrolytes or to initiate polymerization. When T H F is used as a solvent, it is desirable that the T H F should be totally inert. If the T H F is to be polymerized, fast, efficient generation of tertiary oxonium with a stable counterion is preferred. In this paper we have screened interactions between T H F and perchlorates with 22 different cations selected from almost every group of the periodic table. New insights into the reactions that occur and into the relative reactivities of the perchlorates have emerged. The reactivity of the salts is related to the position of the cation in the periodic table.
RESULTS AND DISCUSSION
The polymerization of a heterocycle such as tetrahydrofuran (THF) can be initiated by adding both a reactive halide and a suitable salt [7-13]: R-X
+ MY +
EXPERIMENTAL
Typically, the commercial perchlorates contain 3-9mol of water per mole of salt. We expected and confirmed experimentally that removal of this water is necessary for THF polymerization to occur. Drying of these salts by heating is a very tedious procedure, and is not always successful, as some of them either decompose before being totally dehydrated (e.g. Zn(CIO4)2J or as in the case of LiCIO4 the water is retained even after heating the sample for 12 hr at 300°C[1]. Golub et aL [2] and Chudinova et a/.[3] have shown that anhydrous perchlorates of Cu 2+, Zn 2" and Cd 2. can be prepared at ambient temperature by directinteraction of the corresponding hydrates with pure dioxane. We have found that their method can be applied to most other perchlorate hydrates as well. Since dioxane itself does not polymerize by ring opening and only copolymerizes with THF to a small extent, no interference from dioxane in the preparation of PTHF was anticipated[4]. The anhydrous perchlorate salts as complexes with dioxane were prepared by dissolving the hydrated salts in an excess of dioxane. Stirring and sometimes gently heating in cases where the salt (Zn-'*, Hg 2+, Ag+) was sparingly soluble in dioxane was used. Usually, the reaction between the metal perchlorates and dioxane was vigorous, highly exothermic, and accompanied by changes in the original salt color and crystal volume. Except for the LiCIO4 complex, which dissolved in dioxane and had to be isolated by evaporation, the complexes were filtered off from the mother liquor on a sintered glass funnel and dried overnight in a vacuum oven at room teatnperature. NH£104 was anhydrous as
R ~--~0(CH2)4 n~-~-00~Y-+
MX
(ll
where X = halogen and MY = salt of a metal M with counterion Y. All the detailed studies to date have been carried out using silver as the metal. Counterions have included most of those reported to give stable oxonium ion polymerizations, namely: S O £ F 3 , B F 4 , P F 6 , AsF6-, SbF6 and CIO4 [14]. Since silver salts are very expensive and the byproduct, AgX, darkens on exposure to light and gives black polymers, unless special care is taken[15], we were interested in surveying the possible use of other salts from other metals. Of the counterions above, only the perchlorates are commercially available with the wide variety of metal cations we wished to investigate. Thus, we chose to use metal perchlorates in our survey. Only a small amount of data concerning the use of perchlorates with organic materials can be found in the literature. Cook attributes this to the great hazards connected with their manufacture and use[l]. But the bad reputation of the perchlorates is not wholly deserved. It has been known for a long time that the perchlorates are more stable and less sensitive than the chlorates and are 23
Y. ECKSTEINand P. DREYFUSS
24
Table 1. Effect of organic halide and metal-perchlorate salt on polymerizationof tetrahydrofuran Metal Ion Ag + H~ + Pb z+ T13+ Fe 3 + Fe z + Na + A13+ Mn z+ Cr3+l Ni z+ Cuz + CO z + Cd z + Ce 3 + M~ + Ca z + Ba z + Z n z+ Li + K+ NH4 +
CH3COCI
Conversion ~ C6H 5 cocl
7Z a 70 a 70 a 60 a 60 40 60 a 40 D
a ~ Z0
a 0 0 0 0 0 0 0
CH z = C H C H z C l
Reaction time(hrs.)
73 a 68 a 0 0 0 0 0c 0
Z4 Z4 Z4 Z4 Z4 Z4 7Z 7Z
70 a 70 a 65 a 55 a 40 Z5 N 5 5b
1
0a 1
0 0 0c 0d 0
~ 5
a 0 0 0 0 0 0 not s o l u b l e
0 0 0 0 0 0
a - In a d d i t i o n to the p o l y m e r metal h a l i d e is f o r m e d . b - In a d d i t i o n
to the p o l y m e r ,
c - A white gelatinous
mass
d - A pink solution occurred
Coior polymer solution gray dark b r o w n gray gray yellow black-yellow colorless colorless light-violet colorless yellow-green yellow blue gray
iZ0
148
an a m o r p h o u s p r e c i p i t a t e
of the c o r r e s p o n d i n g
a crystalline precipitate
is f o r m e d .
formed. initially.
dissolves faster and the deep brown color appears immediately. Addition of Hg(ClO4h to THF in the absence of halide also led to polymerization. The polymerization rate was a strong function of the salt concentration, whereas the polymerization rate of THF in the presence of organic halide was much faster and much less dependent on the salt concentration. The reaction mixtures were dark (1) Ag+, Hg2+ colored. Such behavior of mercuric perchlorate suggests (2) Pb 2+, TI3~ simultaneous existence of two different mechanisms of (3) Fe 3+, A13+,Na +, Fe 2+ initiation. In the presence of organic halide the poly(4) Cr3+, Mn2+, Ni2+, Cu2+, CO:+, Cd2+ merization of THF occurs by a mechanism similar to that (5) Ce3+, Mg2+, Caz+, Ba2+, Zn2+, Li+, K +, NH4+ of Ag* salts as shown in eqn (1). In the absence of organic halide, we conclude that perchloric acid must be The observed % conversions to PTHF were 75--60, 75- one of the products of reaction of Hg(CIO4)2 with THF. 60, 60-40, 20-7 and 0 respectively. Only group (1) gave The evidence is as follows. A shift to longer wavelength polymer when the allyl halides were used. In the other and a broadening of the spectral band of the maximum in groups the order of reactivity of the halides was acetyl > the UV spectra at 200-350nm of Hg(C104)E in THF implies that polymerization of THF occurs benzoyl ~>allyl = 0. Groups (I) and (2). After addition of organic chlorides by direct interaction of the salt with THF. In the 'H to solutions of Ag+, Hg2+, Pb z+ and TI3+ perchlorates in NMR spectrum of Hg(C104) in THF a sharp far THF, white precipitates were formed. In the case of Ag~ downfield peak at about 8.3 ppm relative to Me4Si was and Pb 2+ perchlorates, AgC1 and PbC12, respectively, observed immediately after addition of Hg(C104h. After precipitated. The polymers formed became dark in color one month this peak was shifted upfield and increased in when exposed to light, probably due to reduction of the intensity. This evidence suggests the presence of a metal ions to the pure metal. However, in the case of deshielded proton and the presence of protonic acid Hgz+ and TI3+ perchlorates, the precipitates formed as a [16-18]. How the protonic acid forms is not certain. Abdulresult of reaction between the salt and the organic halide must have been Hg2CI2and TIC1 and not the expected di- Rasoul, Ledwith and Yagci have reported that the or trihalides, respectively, since we found that HgC12and polymerization of THF is readily induced in the presence T1CI3 are highly soluble in THF. On the other hand, TIC1 of AgPF6 by thermal and photochemically active sources is only slightly soluble and HgECI~is very insoluble. The of free radicals[19]. They suggested that the important solubility product of the latter in water is 1.5 x 10-'s in species must be obtained by hydrogen abstraction from comparison with that of AgC1 which is l x l 0 -m. THF and that the role of the AgPF6 is to function as a Hg(ClO4h dissolves slowly in THF, forming immediately one electron oxidant for electron donor radicals. It is a colored solution whose color varies from light brown to therefore not surprising that evidence presented here and dark red brown. Upon addition of organic halide the salt elsewhere[20] indicates that initiation of THF poly-
much safer in contact with combustible substances[l]. We encountered no difficulties in our work. The effect of organic halide and perchlorate salts on the polymerization of THF is summarized in Tablel. The perchlorate salts can be divided into five groups based on their efficiency for the preparation of polytetrahydrofuran (PTHF):
Reactions between perchlorate salts and tetrahydrofuran
25
Table 2. Effect of halide and perchlorate salt on polymerization of THF' Organic Halide
CH~ CtC L C 6 H ~ COC1 CH z C H C H Z Br CH 3 CH(CI) CH CH z
Reaction Time (hrs) i 16 1 16 1 70 1 16 1 16
Conversion Hg( C104 ) z
(~) AgCiO4
Z 7 4 6O Z3 75 4 48 6 58
0 0 5 51 iI 67 1 50 ii 65
a E a c h c h a r g e c o n s i s t e d of i0 ml THF and 0.001 m o l e s of h a l i d e a n d e i t h e r 0.001 or 0.0005 m o l e s of p e r c h l o r a t e salt, The h i g h e r h a l i d e c h a r g e w a s u s e d for the Ag + salts.
merization by Ag + or Hg 2~ without addition of a reactive species such as halide, but possibly assisted by a photon, can lead to a proton generated in situ. In Table 2 a comparison between the reactivity of mercuric and silver perchlorate salts in the presence of organic balides is presented. Based on the data in the table, Hg(CIO4)2 is as efficient a polymerizing agent as AgCIO4. Group (3). Clear, transparent solutions of PTHF were formed in the presence of Na ~, A13+, Fe 2+ and Fe 3' perchlorates. Like the salts in groups (1) and (2), Na + and AI3+ also produced precipitates during the course of the polymerization reaction but no precipitate was observed in the presence of Fe 2+ or Fe 3+ salts. The reaction of Fe(C104)3 with THF in the presence of acyl halide is very exothermic, and after 20-30 min 35% conversion to PTHF is reached. In general, PTHF formed in shorter times and in higher yields with acetyl chloride than with benzoyl chloride. These results are not surprising in view of the higher reactivity of the chloride in acetyl chloride in comparison to that in benzoyl chloride[21]. With allyl halides Na +, Fe z+ and Fe 3+ gave clear solutions while AI3+ produced a heavy white, precipitate, but no polymerization. Studies were made to establish the mechanism of THF polymerization in the presence of the salts of group (3). NaC104, A1(CIO4)3, Fe(CIO4)3 do not initiate polymerization of THF by themselves. However, addition of anhydrous Fe(CIO4h to THF leads to the appearance of a gelatinous mass which dissolves slowly with gradual addition of acetyl chloride and dissolves totally when the molar ratios Fe3+: acetyl chloride approach the value 1:3. This clear, yellow solution gave only 17% conversion to PTHF after 16 hr. Addition of one additional mole of acetyl chloride to this polymer solution gave fast polymerization. After 6 hr a conversion to 55% PTHF was reached. Another experiment in which the ratio Fe(CIO4)2: acetyl chloride was 1:4 from the very beginning, resulted in 57% conversion to PTHF after 5 hr. Such behavior suggests formation of FeCI3 during the course of the polymerization reaction. If this hypothesis is correct, polymerization of THF might occur on addition of FeCIs itself to THF. The result of such an experiment was negative. Similar, negative results were obtained with NaCI or AICI3. However, on addition of acetyl chloride to a solution of FeCl3 in THF, polymerization occurred. No polymerization was observed on addition of acetyl halide to solutions containing either NaC1 or AIC13.
The experiments leading to polymerization of THF in the presence of ferric perchlorate and acetyl chloride suggest the foliowing initiating sequence of reactions: 3RCOCI + Fe(C104)3~FeCI3 + RCO+CtO4 RCOCI + FeCL
, RCO+FeCI4 .
(3) (4)
Apparently the equilibrium in eqn (3) is far to the left, initiation is slow and the observed rate of polymerization is also slow. Addition of the fourth mole of acetyl chloride pushes the equilibrium to the right by converting the FeCI3 to stable FeCI4- and leads to rapid initiation and higher overall polymerization rates. With Na + and AI3+ percblorate salts, the initiation reaction of THF polymerization probably is similar to that given in eqn (1), the insoluble products being NaCI and some complex compound AICI3. THF, respectively. Group (4). The yields of PTHF obtained when perchlorates from this group were used were very low, %20% after 6 days. As already mentioned, the metal perchlorates in the group react very slowly with THF. The color of the resulting solutions suggest something about the mechanism of the initiation reaction. Thus, for example, the blue solution from Co(C104)2suggests the presence of CoCI~ ions and the yellow color from Cu(CIO4) the presence of CuCI2-. In almost every case a precipitate was formed. There is no reason to believe that the reactions are very different from those analogous to eqns (1)--(4). The low conversions to PTHF indicate very slow initiation reactions possibly combined with some undefined termination reactions. Group 6q. No PTHF at all was obtained when the perchlorates form this group were used. Apparently the equilibrium shown in eqn (1) is so far to the left, that is, the salt is so stable, that no interaction with the halide and THF occurs. This assumption is supported by the works of Heins"[22] and Nakahama et a/.[23]. Their studies indicate that polymerization of THF occurs in the presence of LiCIO4 only if an electric current is flowing. The lack of reactivity of the LiC104 salt in the polymerization of THF is at first unexpected. Table 3 compares its activity with that of some other lithium and sodium salts. The table shows that alkali metal salts that do not dissolve in THF are unreactive for inducing the polymerization of' THF either alone or with added halide. Both LiCIO4 and NaCIO4 are soluble, but only NaCIO4 is reactive, albeit only with a very reactive halide such as acetyl chloride. Evidently the equilibrium in eqn (1) lies to the right with NaC104 and to the left with LiC104, possibly
26
Y. ECKSTEIN and P. DREYFUSS Table 3. Comparison of reactivities of alkali metal salts for inducing polymerization of THF reactivity"
Salt
Alone
LiclO 4
with
added
allyl
halide
with.added
CH~COCl
-
L iBF4
Reference b,
22
+
-
c
7
+
c
7
LiPF 6
+
NaCIO 4
-
+
c
-
-
c
7
-
c
7
-
c
7
-
c
NaIO 4
d d
NaBF4
+
E12ctrolytically
-
b,
d NaPF 6 KCIO 4
a +
d
indicates
after
b c
d
This
that
polymerization
-
occurs;
indicates
that
no
polymerization
b,
7
7
occurs
5 days.
work.
Not
tested.
The
salt
is
insoluble
in
THF.
because NaC1 drives the reaction to the right. The results with LiBF4 are consistent with this interpretation but then the results with LiPF6 are puzzling. The relative stabilities of LiPF6 and LiC1 in THF may be just enough different from those of LiBF4 and LiCI so that the reaction in eqn (1) goes to the right with LiPF6 when reactive halide is added to THF. In the absence of halide a possible explanation can be made in terms of the relative stabilities of the LiF:PFs(LiPF6) and THF:PF5 complexes.
LiF:PF5 ON,.~ LiF."
PFs:O~
(5)
If this reaction occurs at all, it will be driven to the right as the PFs:THF complex is consumed in the imitation of the polymerization of THF[24]. A similar reaction cannot occur with LiCl04. If it occurs with LiBF4, polymerization will not ensue because BF~ alone does not initiate the polymerization of THF[14].
la Iio
lllbIIVbI Vb Vl b Vll b
i
,, Li: Na'
i i I I i I i
I , I t
I i i i i t ¢
' : t
VIII
i i
i i
i
I
i l I
i
i
I I
'
t
i
1 i
i
I
t
I
,
Cr i * Mr~°
SUMMARY AND CONCLUSIONS Figure 1 shows the location in the periodic table of the 22 metallic ions whose perchlorates were used in this study. A diagonal across the square indicates that although the element was tried, no polymerization occurred. The dotted line across Li ÷ indicates that although addition of LiCIO4 itself does not lead to THF polymerization, there are some conditions under which THF polymerization can be initiated in the presence of lithium salts. If the results in Table 1 are compared with Fig. 1, a few generalizations about the "reactivity" of the perchlorate salts relative to the location of the cation in the periodic tables can be made. (The "reactivity" under consideration here is the ability to induce THF polymerization in the presence of reactive halide.) For example, salts from the more electronegative elements in groups IB, liB, IliA and IVA are much more reactive than salts from the alkali or alkali earth elements of
I Ib
lllb lllla !IVa Va Via
VII a
l i i I
,, I t t i i
I i I l i
' I i
l I i i I
I i i
i I I
i I i
i*
AI
Fe** Cot` Ni'° CU=' Ag* Cdl°
H,g=° TI'* Pb='' Fig. 1. Location in periodic table of ions of perchlorate salts used in this study. A diagonal across the square indicates that the perchlorate salt was not suitable for initiation of THF polymerization in the presence of reactive halide. The line across Li ÷ is dotted because although LiCI04 is unable to induce polymerization of THF in the presence of halide, there are some conditions under which THF polymerization can be initiated in the presence of Li ÷ salts.
Reactions between perchlorate salts and tetrahydrofuraz groups 1A and IIA. Similarly, the more electronegative elements of the B groups, i.e. those farther to the right in the periodic table, are more reactive. Within groups, when differences in reactivity have been observed as in groups IB, IIB and IIIA, the reactivity increases down the Table. Thus in this study the order of reactivity of group I1B elements was Hgz+>Cd2+>>Zn2+-~0. The same trend toward increased reactivity down the Table would be seen in group IA, excluding hydrogen, except that KC104 does not dissolve in THF. Salts from elements in group IIA are completely unreactive. Only a few of the perchlorates could be used to initiate the polymerization of THF without added halide. Other activation is required. Ag +, for example, can induce the polymerization of THF in the presence of photons [19]. The same is probably true for Hg(CIO4):. LiCIO4 will induce the polymerization of THF only if an electric current is flowing[22-23], In these cases it is probable that the initiation sequence includes preliminary formation of protonic acid and secondary oxonium ion. One of the objectives of this investigation was to find a substitute for silver salts for the preparation of polymers according to the reaction illustrated in eqn (1). The results presented here indicate that, except for silver, the most suitable metal salts are in the sixth period. We have examined sixth period elements in groups IIB, IIIA, IVA (Hg 2', TI2+, Pb 2+) and it is possible to predict that salts of Pd, Pt and Au would also be suitable provided they are soluble in THF, Nevertheless, since Hg 2+ can lead to dark colored polymers by reactions presumably similar to those that occur with Ag + and more readily initiates the polymerization without added halide, and T13+ and PI-& are slightly less reactive, this study indicates that Ag ~ is the metal of choice for preparing PTHF according to )he scheme in eqn I1). A~ knowle&,ements--This work was supported by the U.S. Army Research Office. We thank Dan P. Lee and Qcheng-Sun Lien for helpful discussions and advice in the course of the work.
27
REFERENCES I. J. C. Schumacher, Perehlorates, Their Properties, Manufacture and Uses. Reinhold, New York, (1960). 2. A. M. Golub, O. O. Piskar'ova and L. D. Bogats'ka, Visn. Kiivs'k Unit. No. 2, Set. Astron. Fiz to Khim. 1, 78 (1961), 3. L. I, Chudinova, Russian J. Inorg. Chem. 10, 707 (1965). 4. M. P. Dreyfuss and P. Dreyfuss, J. Polym. Sci. A-l 4. 2179 f19661. 5. P. Dreyfuss, F Adaway and J. P, Kennedy, J. Appl. Poh'm. Sci. Appl. Polym. Syrup. 30, 183 (1977). 6. J. Lehmann and P. Dreyfuss, Adv. in Chem. Series 176, 587 (1979): ACS Polymer Preprints 19 (1), 693 (19781. 7. P. Dreyfuss and J. P. Kennedy, J. Polvm. Sci: 5;ymp. 56, 129 (1976). 8. K. Lee and P. Dreyfuss, ACSSymp. Series ~ , 24 (1977), 9. P. Dreyfuss and J. P. Kennedy, J. Polym. Sci.: Polvm. Syrup. 60, 47 (!977). 10. E. Franta, F. Afshar-Taromi and P. Rempp. Makromol. Chem. 177, 2191 (1976). I1. E. Franta, L. Reibel, J. Lehmann and S. Penczek..I. Polym. Sei.: Polym. ,%rap. 56. 139 (1976). 12. F. J. Burgess, A. V. Cunliffe, D. H. Richards and D. (. Sherrington, J. Polym. Sci., Polym. Lett. Edn 14, 471 (1976). 13. M. L. Hallensleben, Makromol. Chem. 178. 2125 !1977). 14. P. Dreyfuss and M. P, Dreyfuss, Adt. Polvm. ,~ci, 4, 528 (t%7). 15. D. P, Lee and P, Dreyfuss, unpublished results. 16. F. Klages, J. E, Gordon and H. A. Jung, Chem Ber. 98. 3748 (1%5). 17. M. P. Dreyfuss, J. C. Wesffahl and P. Dreyfuss. Macromol. l, 437 (l%g). 18. Y. Tanaka. J. Macromol. Sci.-Chem. All, 2198 (1977). 19. F, A. M. AbduI-Rasoul, A. Ledwith and Y. Yagci. Polvm. Bull. 1, 1 (19781. 20. M. E, Woodhouse, F. D. Lewis and T. Y. Marks, .1. Ant. Chem. Soc. 100, 996 (1978). 21. R. A. Brewsler, in Or,eanic Chemistry, p. 522. Prentice-Hal), New York (1949). 22. C. F, Heins, Polvm. Lett. 7, 625 (1969). 23. S. Nakahama, '3,. HinD and N. Yamazaki. Polym. ,I. 2. 56 (1971). 24. E. L. Muettertie~, U.S. Pa~. 2,748,559 (1, Nov. 1956)
REACTIONS BETWEEN PERCHLORATE SALTS AND TETRAHYDROFURAN Y. ECKSTEIN and P. DREYFUSS Institute of Polymer Science. The University of Akron. Akron, OH 44325, U.S.A. tReceived 11 September 1979; received forpublication 6 March 1980) Abstract--Reactions between tetrahydrofuran (THF) and 22 different perchlorate salts, most often in the presence of organic halides were studied. The use of the organic halides including acetyl, benzoyl and substituted allyl chlorides aided in arranging the perchlorates according to their ability to give good yields of tertiary oxonium ion and polymer from THF. The approximate order, based on the yield of poly(tetrahydrofuran) obtained after 24 hr was Ag +, Hg 2" > Pb 2*. TP + > Fe 3+, AP +, Na +, Fe 2+ > Cr 3+, Mnz+, Ni 2+, Cu 2+, Co 2~, Cd z+ ~>Ce st, Mg2+, Ca" ", Ba-~ , Zn 2*. Li ÷, K +, NH~ = 0. Only Ag+ and Hg:+ perchlorates gave polymer when allylhalides were used. With one other perchlorates the order of reactivity of the halide was acetyl > benzoyl ~ allyl = 0.
received. It did not dissolve or react with dioxane and was only washed and then dried as above. The amount of complexed dioxane was determined by analytical determination of the metal. Typically, 1-3 mol of dioxane was complexed with each mole of perchlorate salt. With the exception of NH4CIO4, all perchlorates used were from Ventron Corp., AIfa Division. Polymerizations were carried out at room temperature in screw cap vials as previously described[5]. Polymerization times varied from 1 to 7 days. Longer times were used when no precipitation or increase in viscosity was apparent. The polymers were characterized as previously described [6].
INTRODUCTION
Tetrahydrofuran (THF) is both an important industrial solvent and an important monomer for the preparation of polymeric materials. Metal salts with complex ions are often dissolved in T H F to serve either as supporting electrolytes or to initiate polymerization. When T H F is used as a solvent, it is desirable that the T H F should be totally inert. If the T H F is to be polymerized, fast, efficient generation of tertiary oxonium with a stable counterion is preferred. In this paper we have screened interactions between T H F and perchlorates with 22 different cations selected from almost every group of the periodic table. New insights into the reactions that occur and into the relative reactivities of the perchlorates have emerged. The reactivity of the salts is related to the position of the cation in the periodic table.
RESULTS AND DISCUSSION
The polymerization of a heterocycle such as tetrahydrofuran (THF) can be initiated by adding both a reactive halide and a suitable salt [7-13]: R-X
+ MY +
EXPERIMENTAL
Typically, the commercial perchlorates contain 3-9mol of water per mole of salt. We expected and confirmed experimentally that removal of this water is necessary for THF polymerization to occur. Drying of these salts by heating is a very tedious procedure, and is not always successful, as some of them either decompose before being totally dehydrated (e.g. Zn(CIO4)2J or as in the case of LiCIO4 the water is retained even after heating the sample for 12 hr at 300°C[1]. Golub et aL [2] and Chudinova et a/.[3] have shown that anhydrous perchlorates of Cu 2+, Zn 2" and Cd 2. can be prepared at ambient temperature by directinteraction of the corresponding hydrates with pure dioxane. We have found that their method can be applied to most other perchlorate hydrates as well. Since dioxane itself does not polymerize by ring opening and only copolymerizes with THF to a small extent, no interference from dioxane in the preparation of PTHF was anticipated[4]. The anhydrous perchlorate salts as complexes with dioxane were prepared by dissolving the hydrated salts in an excess of dioxane. Stirring and sometimes gently heating in cases where the salt (Zn-'*, Hg 2+, Ag+) was sparingly soluble in dioxane was used. Usually, the reaction between the metal perchlorates and dioxane was vigorous, highly exothermic, and accompanied by changes in the original salt color and crystal volume. Except for the LiCIO4 complex, which dissolved in dioxane and had to be isolated by evaporation, the complexes were filtered off from the mother liquor on a sintered glass funnel and dried overnight in a vacuum oven at room teatnperature. NH£104 was anhydrous as
R ~--~0(CH2)4 n~-~-00~Y-+
MX
(ll
where X = halogen and MY = salt of a metal M with counterion Y. All the detailed studies to date have been carried out using silver as the metal. Counterions have included most of those reported to give stable oxonium ion polymerizations, namely: S O £ F 3 , B F 4 , P F 6 , AsF6-, SbF6 and CIO4 [14]. Since silver salts are very expensive and the byproduct, AgX, darkens on exposure to light and gives black polymers, unless special care is taken[15], we were interested in surveying the possible use of other salts from other metals. Of the counterions above, only the perchlorates are commercially available with the wide variety of metal cations we wished to investigate. Thus, we chose to use metal perchlorates in our survey. Only a small amount of data concerning the use of perchlorates with organic materials can be found in the literature. Cook attributes this to the great hazards connected with their manufacture and use[l]. But the bad reputation of the perchlorates is not wholly deserved. It has been known for a long time that the perchlorates are more stable and less sensitive than the chlorates and are 23
Y. ECKSTEINand P. DREYFUSS
24
Table 1. Effect of organic halide and metal-perchlorate salt on polymerizationof tetrahydrofuran Metal Ion Ag + H~ + Pb z+ T13+ Fe 3 + Fe z + Na + A13+ Mn z+ Cr3+l Ni z+ Cuz + CO z + Cd z + Ce 3 + M~ + Ca z + Ba z + Z n z+ Li + K+ NH4 +
CH3COCI
Conversion ~ C6H 5 cocl
7Z a 70 a 70 a 60 a 60 40 60 a 40 D
a ~ Z0
a 0 0 0 0 0 0 0
CH z = C H C H z C l
Reaction time(hrs.)
73 a 68 a 0 0 0 0 0c 0
Z4 Z4 Z4 Z4 Z4 Z4 7Z 7Z
70 a 70 a 65 a 55 a 40 Z5 N 5 5b
1
0a 1
0 0 0c 0d 0
~ 5
a 0 0 0 0 0 0 not s o l u b l e
0 0 0 0 0 0
a - In a d d i t i o n to the p o l y m e r metal h a l i d e is f o r m e d . b - In a d d i t i o n
to the p o l y m e r ,
c - A white gelatinous
mass
d - A pink solution occurred
Coior polymer solution gray dark b r o w n gray gray yellow black-yellow colorless colorless light-violet colorless yellow-green yellow blue gray
iZ0
148
an a m o r p h o u s p r e c i p i t a t e
of the c o r r e s p o n d i n g
a crystalline precipitate
is f o r m e d .
formed. initially.
dissolves faster and the deep brown color appears immediately. Addition of Hg(ClO4h to THF in the absence of halide also led to polymerization. The polymerization rate was a strong function of the salt concentration, whereas the polymerization rate of THF in the presence of organic halide was much faster and much less dependent on the salt concentration. The reaction mixtures were dark (1) Ag+, Hg2+ colored. Such behavior of mercuric perchlorate suggests (2) Pb 2+, TI3~ simultaneous existence of two different mechanisms of (3) Fe 3+, A13+,Na +, Fe 2+ initiation. In the presence of organic halide the poly(4) Cr3+, Mn2+, Ni2+, Cu2+, CO:+, Cd2+ merization of THF occurs by a mechanism similar to that (5) Ce3+, Mg2+, Caz+, Ba2+, Zn2+, Li+, K +, NH4+ of Ag* salts as shown in eqn (1). In the absence of organic halide, we conclude that perchloric acid must be The observed % conversions to PTHF were 75--60, 75- one of the products of reaction of Hg(CIO4)2 with THF. 60, 60-40, 20-7 and 0 respectively. Only group (1) gave The evidence is as follows. A shift to longer wavelength polymer when the allyl halides were used. In the other and a broadening of the spectral band of the maximum in groups the order of reactivity of the halides was acetyl > the UV spectra at 200-350nm of Hg(C104)E in THF implies that polymerization of THF occurs benzoyl ~>allyl = 0. Groups (I) and (2). After addition of organic chlorides by direct interaction of the salt with THF. In the 'H to solutions of Ag+, Hg2+, Pb z+ and TI3+ perchlorates in NMR spectrum of Hg(C104) in THF a sharp far THF, white precipitates were formed. In the case of Ag~ downfield peak at about 8.3 ppm relative to Me4Si was and Pb 2+ perchlorates, AgC1 and PbC12, respectively, observed immediately after addition of Hg(C104h. After precipitated. The polymers formed became dark in color one month this peak was shifted upfield and increased in when exposed to light, probably due to reduction of the intensity. This evidence suggests the presence of a metal ions to the pure metal. However, in the case of deshielded proton and the presence of protonic acid Hgz+ and TI3+ perchlorates, the precipitates formed as a [16-18]. How the protonic acid forms is not certain. Abdulresult of reaction between the salt and the organic halide must have been Hg2CI2and TIC1 and not the expected di- Rasoul, Ledwith and Yagci have reported that the or trihalides, respectively, since we found that HgC12and polymerization of THF is readily induced in the presence T1CI3 are highly soluble in THF. On the other hand, TIC1 of AgPF6 by thermal and photochemically active sources is only slightly soluble and HgECI~is very insoluble. The of free radicals[19]. They suggested that the important solubility product of the latter in water is 1.5 x 10-'s in species must be obtained by hydrogen abstraction from comparison with that of AgC1 which is l x l 0 -m. THF and that the role of the AgPF6 is to function as a Hg(ClO4h dissolves slowly in THF, forming immediately one electron oxidant for electron donor radicals. It is a colored solution whose color varies from light brown to therefore not surprising that evidence presented here and dark red brown. Upon addition of organic halide the salt elsewhere[20] indicates that initiation of THF poly-
much safer in contact with combustible substances[l]. We encountered no difficulties in our work. The effect of organic halide and perchlorate salts on the polymerization of THF is summarized in Tablel. The perchlorate salts can be divided into five groups based on their efficiency for the preparation of polytetrahydrofuran (PTHF):
Reactions between perchlorate salts and tetrahydrofuran
25
Table 2. Effect of halide and perchlorate salt on polymerization of THF' Organic Halide
CH~ CtC L C 6 H ~ COC1 CH z C H C H Z Br CH 3 CH(CI) CH CH z
Reaction Time (hrs) i 16 1 16 1 70 1 16 1 16
Conversion Hg( C104 ) z
(~) AgCiO4
Z 7 4 6O Z3 75 4 48 6 58
0 0 5 51 iI 67 1 50 ii 65
a E a c h c h a r g e c o n s i s t e d of i0 ml THF and 0.001 m o l e s of h a l i d e a n d e i t h e r 0.001 or 0.0005 m o l e s of p e r c h l o r a t e salt, The h i g h e r h a l i d e c h a r g e w a s u s e d for the Ag + salts.
merization by Ag + or Hg 2~ without addition of a reactive species such as halide, but possibly assisted by a photon, can lead to a proton generated in situ. In Table 2 a comparison between the reactivity of mercuric and silver perchlorate salts in the presence of organic balides is presented. Based on the data in the table, Hg(CIO4)2 is as efficient a polymerizing agent as AgCIO4. Group (3). Clear, transparent solutions of PTHF were formed in the presence of Na ~, A13+, Fe 2+ and Fe 3' perchlorates. Like the salts in groups (1) and (2), Na + and AI3+ also produced precipitates during the course of the polymerization reaction but no precipitate was observed in the presence of Fe 2+ or Fe 3+ salts. The reaction of Fe(C104)3 with THF in the presence of acyl halide is very exothermic, and after 20-30 min 35% conversion to PTHF is reached. In general, PTHF formed in shorter times and in higher yields with acetyl chloride than with benzoyl chloride. These results are not surprising in view of the higher reactivity of the chloride in acetyl chloride in comparison to that in benzoyl chloride[21]. With allyl halides Na +, Fe z+ and Fe 3+ gave clear solutions while AI3+ produced a heavy white, precipitate, but no polymerization. Studies were made to establish the mechanism of THF polymerization in the presence of the salts of group (3). NaC104, A1(CIO4)3, Fe(CIO4)3 do not initiate polymerization of THF by themselves. However, addition of anhydrous Fe(CIO4h to THF leads to the appearance of a gelatinous mass which dissolves slowly with gradual addition of acetyl chloride and dissolves totally when the molar ratios Fe3+: acetyl chloride approach the value 1:3. This clear, yellow solution gave only 17% conversion to PTHF after 16 hr. Addition of one additional mole of acetyl chloride to this polymer solution gave fast polymerization. After 6 hr a conversion to 55% PTHF was reached. Another experiment in which the ratio Fe(CIO4)2: acetyl chloride was 1:4 from the very beginning, resulted in 57% conversion to PTHF after 5 hr. Such behavior suggests formation of FeCI3 during the course of the polymerization reaction. If this hypothesis is correct, polymerization of THF might occur on addition of FeCIs itself to THF. The result of such an experiment was negative. Similar, negative results were obtained with NaCI or AICI3. However, on addition of acetyl chloride to a solution of FeCl3 in THF, polymerization occurred. No polymerization was observed on addition of acetyl halide to solutions containing either NaC1 or AIC13.
The experiments leading to polymerization of THF in the presence of ferric perchlorate and acetyl chloride suggest the foliowing initiating sequence of reactions: 3RCOCI + Fe(C104)3~FeCI3 + RCO+CtO4 RCOCI + FeCL
, RCO+FeCI4 .
(3) (4)
Apparently the equilibrium in eqn (3) is far to the left, initiation is slow and the observed rate of polymerization is also slow. Addition of the fourth mole of acetyl chloride pushes the equilibrium to the right by converting the FeCI3 to stable FeCI4- and leads to rapid initiation and higher overall polymerization rates. With Na + and AI3+ percblorate salts, the initiation reaction of THF polymerization probably is similar to that given in eqn (1), the insoluble products being NaCI and some complex compound AICI3. THF, respectively. Group (4). The yields of PTHF obtained when perchlorates from this group were used were very low, %20% after 6 days. As already mentioned, the metal perchlorates in the group react very slowly with THF. The color of the resulting solutions suggest something about the mechanism of the initiation reaction. Thus, for example, the blue solution from Co(C104)2suggests the presence of CoCI~ ions and the yellow color from Cu(CIO4) the presence of CuCI2-. In almost every case a precipitate was formed. There is no reason to believe that the reactions are very different from those analogous to eqns (1)--(4). The low conversions to PTHF indicate very slow initiation reactions possibly combined with some undefined termination reactions. Group 6q. No PTHF at all was obtained when the perchlorates form this group were used. Apparently the equilibrium shown in eqn (1) is so far to the left, that is, the salt is so stable, that no interaction with the halide and THF occurs. This assumption is supported by the works of Heins"[22] and Nakahama et a/.[23]. Their studies indicate that polymerization of THF occurs in the presence of LiCIO4 only if an electric current is flowing. The lack of reactivity of the LiC104 salt in the polymerization of THF is at first unexpected. Table 3 compares its activity with that of some other lithium and sodium salts. The table shows that alkali metal salts that do not dissolve in THF are unreactive for inducing the polymerization of' THF either alone or with added halide. Both LiCIO4 and NaCIO4 are soluble, but only NaCIO4 is reactive, albeit only with a very reactive halide such as acetyl chloride. Evidently the equilibrium in eqn (1) lies to the right with NaC104 and to the left with LiC104, possibly
26
Y. ECKSTEIN and P. DREYFUSS Table 3. Comparison of reactivities of alkali metal salts for inducing polymerization of THF reactivity"
Salt
Alone
LiclO 4
with
added
allyl
halide
with.added
CH~COCl
-
L iBF4
Reference b,
22
+
-
c
7
+
c
7
LiPF 6
+
NaCIO 4
-
+
c
-
-
c
7
-
c
7
-
c
7
-
c
NaIO 4
d d
NaBF4
+
E12ctrolytically
-
b,
d NaPF 6 KCIO 4
a +
d
indicates
after
b c
d
This
that
polymerization
-
occurs;
indicates
that
no
polymerization
b,
7
7
occurs
5 days.
work.
Not
tested.
The
salt
is
insoluble
in
THF.
because NaC1 drives the reaction to the right. The results with LiBF4 are consistent with this interpretation but then the results with LiPF6 are puzzling. The relative stabilities of LiPF6 and LiC1 in THF may be just enough different from those of LiBF4 and LiCI so that the reaction in eqn (1) goes to the right with LiPF6 when reactive halide is added to THF. In the absence of halide a possible explanation can be made in terms of the relative stabilities of the LiF:PFs(LiPF6) and THF:PF5 complexes.
LiF:PF5 ON,.~ LiF."
PFs:O~
(5)
If this reaction occurs at all, it will be driven to the right as the PFs:THF complex is consumed in the imitation of the polymerization of THF[24]. A similar reaction cannot occur with LiCl04. If it occurs with LiBF4, polymerization will not ensue because BF~ alone does not initiate the polymerization of THF[14].
la Iio
lllbIIVbI Vb Vl b Vll b
i
,, Li: Na'
i i I I i I i
I , I t
I i i i i t ¢
' : t
VIII
i i
i i
i
I
i l I
i
i
I I
'
t
i
1 i
i
I
t
I
,
Cr i * Mr~°
SUMMARY AND CONCLUSIONS Figure 1 shows the location in the periodic table of the 22 metallic ions whose perchlorates were used in this study. A diagonal across the square indicates that although the element was tried, no polymerization occurred. The dotted line across Li ÷ indicates that although addition of LiCIO4 itself does not lead to THF polymerization, there are some conditions under which THF polymerization can be initiated in the presence of lithium salts. If the results in Table 1 are compared with Fig. 1, a few generalizations about the "reactivity" of the perchlorate salts relative to the location of the cation in the periodic tables can be made. (The "reactivity" under consideration here is the ability to induce THF polymerization in the presence of reactive halide.) For example, salts from the more electronegative elements in groups IB, liB, IliA and IVA are much more reactive than salts from the alkali or alkali earth elements of
I Ib
lllb lllla !IVa Va Via
VII a
l i i I
,, I t t i i
I i I l i
' I i
l I i i I
I i i
i I I
i I i
i*
AI
Fe** Cot` Ni'° CU=' Ag* Cdl°
H,g=° TI'* Pb='' Fig. 1. Location in periodic table of ions of perchlorate salts used in this study. A diagonal across the square indicates that the perchlorate salt was not suitable for initiation of THF polymerization in the presence of reactive halide. The line across Li ÷ is dotted because although LiCI04 is unable to induce polymerization of THF in the presence of halide, there are some conditions under which THF polymerization can be initiated in the presence of Li ÷ salts.
Reactions between perchlorate salts and tetrahydrofuraz groups 1A and IIA. Similarly, the more electronegative elements of the B groups, i.e. those farther to the right in the periodic table, are more reactive. Within groups, when differences in reactivity have been observed as in groups IB, IIB and IIIA, the reactivity increases down the Table. Thus in this study the order of reactivity of group I1B elements was Hgz+>Cd2+>>Zn2+-~0. The same trend toward increased reactivity down the Table would be seen in group IA, excluding hydrogen, except that KC104 does not dissolve in THF. Salts from elements in group IIA are completely unreactive. Only a few of the perchlorates could be used to initiate the polymerization of THF without added halide. Other activation is required. Ag +, for example, can induce the polymerization of THF in the presence of photons [19]. The same is probably true for Hg(CIO4):. LiCIO4 will induce the polymerization of THF only if an electric current is flowing[22-23], In these cases it is probable that the initiation sequence includes preliminary formation of protonic acid and secondary oxonium ion. One of the objectives of this investigation was to find a substitute for silver salts for the preparation of polymers according to the reaction illustrated in eqn (1). The results presented here indicate that, except for silver, the most suitable metal salts are in the sixth period. We have examined sixth period elements in groups IIB, IIIA, IVA (Hg 2', TI2+, Pb 2+) and it is possible to predict that salts of Pd, Pt and Au would also be suitable provided they are soluble in THF, Nevertheless, since Hg 2+ can lead to dark colored polymers by reactions presumably similar to those that occur with Ag + and more readily initiates the polymerization without added halide, and T13+ and PI-& are slightly less reactive, this study indicates that Ag ~ is the metal of choice for preparing PTHF according to )he scheme in eqn I1). A~ knowle&,ements--This work was supported by the U.S. Army Research Office. We thank Dan P. Lee and Qcheng-Sun Lien for helpful discussions and advice in the course of the work.
27
REFERENCES I. J. C. Schumacher, Perehlorates, Their Properties, Manufacture and Uses. Reinhold, New York, (1960). 2. A. M. Golub, O. O. Piskar'ova and L. D. Bogats'ka, Visn. Kiivs'k Unit. No. 2, Set. Astron. Fiz to Khim. 1, 78 (1961), 3. L. I, Chudinova, Russian J. Inorg. Chem. 10, 707 (1965). 4. M. P. Dreyfuss and P. Dreyfuss, J. Polym. Sci. A-l 4. 2179 f19661. 5. P. Dreyfuss, F Adaway and J. P, Kennedy, J. Appl. Poh'm. Sci. Appl. Polym. Syrup. 30, 183 (1977). 6. J. Lehmann and P. Dreyfuss, Adv. in Chem. Series 176, 587 (1979): ACS Polymer Preprints 19 (1), 693 (19781. 7. P. Dreyfuss and J. P. Kennedy, J. Polvm. Sci: 5;ymp. 56, 129 (1976). 8. K. Lee and P. Dreyfuss, ACSSymp. Series ~ , 24 (1977), 9. P. Dreyfuss and J. P. Kennedy, J. Polym. Sci.: Polvm. Syrup. 60, 47 (!977). 10. E. Franta, F. Afshar-Taromi and P. Rempp. Makromol. Chem. 177, 2191 (1976). I1. E. Franta, L. Reibel, J. Lehmann and S. Penczek..I. Polym. Sei.: Polym. ,%rap. 56. 139 (1976). 12. F. J. Burgess, A. V. Cunliffe, D. H. Richards and D. (. Sherrington, J. Polym. Sci., Polym. Lett. Edn 14, 471 (1976). 13. M. L. Hallensleben, Makromol. Chem. 178. 2125 !1977). 14. P. Dreyfuss and M. P, Dreyfuss, Adt. Polvm. ,~ci, 4, 528 (t%7). 15. D. P, Lee and P, Dreyfuss, unpublished results. 16. F. Klages, J. E, Gordon and H. A. Jung, Chem Ber. 98. 3748 (1%5). 17. M. P. Dreyfuss, J. C. Wesffahl and P. Dreyfuss. Macromol. l, 437 (l%g). 18. Y. Tanaka. J. Macromol. Sci.-Chem. All, 2198 (1977). 19. F, A. M. AbduI-Rasoul, A. Ledwith and Y. Yagci. Polvm. Bull. 1, 1 (19781. 20. M. E, Woodhouse, F. D. Lewis and T. Y. Marks, .1. Ant. Chem. Soc. 100, 996 (1978). 21. R. A. Brewsler, in Or,eanic Chemistry, p. 522. Prentice-Hal), New York (1949). 22. C. F, Heins, Polvm. Lett. 7, 625 (1969). 23. S. Nakahama, '3,. HinD and N. Yamazaki. Polym. ,I. 2. 56 (1971). 24. E. L. Muettertie~, U.S. Pa~. 2,748,559 (1, Nov. 1956)