GO-CATALYSIS IN FRIEDEL-CRAFTS REACTIONS

Transcription

1 GO-CATALYSIS IN FRIEDEL-CRAFTS REACTIONS VI. POLYMERIZATION OF 2-BUTENE BY BORON FLUORIDE - METHANOL'. H. R. ALL COCK^ AND A. M. EASTHAM Division of Applied Chemistry, National Research Council, Ottawa, Canada Received November 26, 1962 ABSTRACT 2-Butene is polymerized by boron fluoride and methanol in ethylene dichloride solution at ordinary temperatures to give very low molecular weight polymers. The rate of polymerization shows exactly the same dependence on catalyst concentration as did the rate of isomerization of the cis- and trans-2-butenes, i.e. first order both in free boron fluoride and in the boron fluoride - methanol complex. The polymerization, however, shows a first-order dependence in olefin over a concentration range where the isomerization is, experimentally, virtually independent of the olefin concentration. The polymerization of olefins by strong acids is generally considered to proceed by a mechanism of the type In previous work we have attempted to obtain information about the protonation step in this process by studying the acid-catalyzed equilibration of the three isomeric olefins cis-2-, and trans-2-, and 1-butene, on the assumption that the basic difference between this isomerization and a polymerization lies simply in the relative rates with which the initial carbonium ion undergoes either deprotonation or olefin addition. The n-butenes were chosen for this work because they do not polymerize easily and hence conditions can be found where the isomerization can be observed without serious interference from polymerization. The results of our studies seem to show quite clearly that isomerization takes place according to the following reaction scheme (I), which is essentially a modification of the Polanyi-Evans mechanism for the polymerization of isobutene (2). BF3 + HX 2 HBF3X 111 Here HX, the co-catalyst, is a proton-donating material such as water, methanol, or acetic acid which reacts with the catalyst, BFR, to form the strong acid, HBF3X. This acid reacts rapidly with the olefin (reaction [2]) to form a labile complex (n-complex?) 'Issued as N.R.C. No For Part V see ref. 1. 3N.R.C. Postdoctoral Fellow, Canadian Journal of Chemistry. Volume 41 (1963) 932

2 ALLCOCK AND EASTHAM: CO-CATALYSIS 933 without disrupting the olefin double bond. Isomerization occurs when this complex reacts with a molecule of free boron fluoride but the kinetics do not show whether the rearrangement occurs during the addition of this second molecule of boron fluoride or in some subsequent process such as an ionization of the complex so formed. The rate expression for the isomerization takes the form k3[bf3] [BF3.HX] [butene] rate of isomerization = - K+ [butene], where k3 is the rate constant for the forward process in reaction [3] (the rate-controlling step) and K is the equilibrium constant for reaction [2]. However, since the values of k3 and K differ for each isomer, the equation in this form applies only to the initial rate of isomerization of a pure isomer. For the present purpose it is sufficient to point out that the rate is directly dependent upon the concentrations of the free boron fluoride and of the boron fluoride - methanol complex but will depend on butene concentration in a manner determined by the value of K in the ratio [butene]/k+butene. With acetic acid as co-catalyst at 20' C, K has the values 0.1, 0.4, and 0.5 mole 1-I respectively for the isomers cis-2-, trans-2-, and 1-butene, so the experimental order in butene varies markedly over the usual concentration range. However, in addition to this kinetic effect of changes in butene concentration, there is a medium effect which operates in the opposite direction, and the net result of the two effects is to make the observed rate virtually independent of the butene concentration over the concentration range mole 1-I with either methanol or acetic acid as co-catalyst. With the mechanism for isomerization seemingly established, it seemed worthwhile to extend the measurements to the polymerization in order to test the assumption that polymerization and isomerization are related as described above. In doing so, we have made no attempt to determine the detailed polymerization kinetics but have confined ourselves to the measurement of initial rates of monomer disappearance in ethylene dichloride solution at 0' C, using methanol - boron fluoride as the catalyst system. In order to minimize interference from the somewhat more rapid isomerization reaction we have employed as monomer a mixture of cis- and traszs-2-butene in approximately the ratio they are found in the equilibrium mixture (1:3), but 1-butene, which is also a component (3%) of the equilibrium mixture, was not added because preliminary experiments indicated that the small initial fluctuations in the concentration of this monomer should not cause significant errors in the measurements. EXPERIMENTAL, (a) Materials Ethylene dichloride, butenes, methanol, and boron fluoride were prepared and manipulated essentially as previously described (3, -1). (b) Hate Measz~rernents The rates were followed by observing the decrease in vapor pressure of butene over the reaction mixture by means of a mercury manometer closely coupled to the reaction flask. The volume of the flask and manometer was about 75 ml. For each run the methanol was distilled into the flask and was followed by 20 ml of ethylene dichloride and then by the boron fluoride. At this point the mixture was brought to 0 C, thoroughly mixed by means of a magnetic stirrer, and its vapor pressure determined as a check on its composition. It was then frozen down to remove vapor from the manometer, the manometer isolated by means of a stopcock, and the mixture again brought to 0'. The butene was rapidly vaporized into the reaction system from a small bulb attached to the reaction flask, and a few minutes allowed for equilibration of the system. The stopcock to the manometer was opened and readings taken at intervals. The readings were plotted against time, the initial rate obtained from the slope of the smoothed curve, and the results converted to concentration by means of a calibration chart prepared for the system.

3 934 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 (c) Reaction Products Reactions carried out with a low catalyst/monomer ratio stop short of completion because of catalyst consumption; the yield of polymer varies with the conditions but several hundred molecules of monomer are consumed per molecule of methanol. In order to obtain some information about catalyst destruction the vapor above the reaction mixture was circulated continuously through a gas cell in an Infracord infrared spectrophoton~eter and the strong boron fluoride band at 1500 cm-' observed during the course of the polymerization. The gases were circulated rapidly through the system by means of a high-speed, magnetically driven platinum fan mounted on Teflon bearings within the system. The sodiurn chloride windows of the absorption cell were sealed with lead-mercury amalgam and the reference cell was filled with ethylene dichloride vapor at a pressure just sufficient to balance the absorption due to solvent vapor in the reaction cell. In measuring the boron fluoride absorption it is necessary to make a small correction for a butene peak in the same region. Since butene disappears during the reaction the correction must be made continuously, so a manometer was introduced into the system and the pressure decrease observed. Correction was then made for butene absorption on the assumptiou that the pressure drop was due entirely to the decrease in butene, an assumption which may not be entirely justified but which is consistent with the change in butene absorption in other regions of the spectrum. The results thus obtained showed that the absorption due to boron fluoride remained substantially constant throughout the runs and we are satisfied that consumption of this reagent is small, perhaps well below 1% of its concentration, over the period of the rate measurements. This point is of considerable importance because the rates were followed by the fall in pressure in the system, and boron fluoride in some cases makes up a substantial part of that pressure. Since the boron fluoride concentration seems to remain constant during the runs, we conclude that it is the co-catalyst, methanol, which is consumed, but we have been unable to devise a direct test of this point. We do know, however, that under certain rather restricted conditions, methanol can be added to butene to form the ether, so it seems very likely that a reaction nf this kind is a termination process in the polymerization (1). No effort was made to examine the products of the rate runs but a few larger-scale runs were made for - this purpose, typically as follows. Methanol (0.076 g), boron fluoride (0.22 g), and trans-2-butene (43 g) were added, on the vacuum line, to ethylene dichloride (700 ml) and allowed to stand 4 days at room temperature. Boron fluoride and most of the solvent was then distilled off to leave a residue of about 100 ml, which was fractionally distilled first at atmospheric and then under reduced pressure. The fractions were found, bygas chromatography, carbon and hydrogen analysis, and cryoscopy in cyclohexane, to be hydrocarbons having molecular weights below 350. Over half of the product was made up of a mixture of butene tetramers but the range included dimers to hexamers, and in all, nearly 20 different con~pounds. The large number of products presumably results from a mixture of 1- and 2-butene uilits in the polymers. (d) Concentrations Except in the case of boron fluoride, concentrations have been expressed in nlole 1-I calculated from the amount of solvent and reactant measured into the reaction flask. However, since the boron fluoride has limited solubility, the concentration of this reagent in solution, [BF6Isoln, hc~s been used; in the present work at 0" C the dissolved boron fluoride is very close to one third of the total free boron fluoride present in the system. In calculating the concentrations it has been assumed, on the basis of earlier nark (3), that methanol is always present as the 1:l complex with boron fluoride. RESULTS The dependence of the rate of polymerization on the free boron fluoride concentration is shown in Fig. 1, where some typical results are plotted for a number of methanol concentrations. The dependence on methanol is obtained from the slopes of these lines and is shown in Fig. 2. There is a small and erratic "blank" in the absence of added methanol which may be due to adventitious moisture in the reagents or reaction system. It seems clear from the results that the polymerization depends on catalyst in exactly the same way as did the isomerization, i.e., directly on the concentrations of free boron fluoride and of boron fluoride - methanol complex. The variation of rate with monomer concentration over the concentration range mole 1-I is shown in Fig. 3 and is, experimentally, approximately of the first order. In the case of the isomerization reaction, under slightly different experimental conditions (1, 3), the order in butene approaches zero in this region so one is tempted to conclude

4 ALLCOCK AND EASTHAM: CO-CATALYSIS [GH, OH-BF, 1, mole I-lx 103 FIG. 1. Dependence of the rate of disappearance of htitene on the concentration of free boron fluoride; [butenel 0.40 mole I-', [CHBOH.BF~] as indicated on the curves. FIG. 2. Dependence of the rate of polymerization on the concentration of methanol; [butene] 0.40 mole I-', [BF3]s,l, 1.15 X mole 1-1. FIG. 3. Dependence of the rate of polymerization on butene concentration; [methanol] 4.65X10-3 mole 1-I, [BFs]ao~n 4.2X 10-3 mole 1-1.

5 936 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 that in the polymerization the overall rate of disappearance of monomer is about one order higher in monomer concentration than is the rate of initiation. While this conclusion seems reasonable and may in fact be true, it is a dangerous one because the dependence of the rate of initiation (isomerization) upon monomer concentration is complex (I) and any comparison of this kind, to be valid, should be made over a much wider concentration range than the present one. Unfortunately the range of the present method is restricted by the high rates at one end of the scale and the low vapor pressure of monomer at the other, but it seems rather doubtful that it is worth pursuing the subject further with this monomer because the complexity of the system is such that there is little hope for a solution to the polymerization equation. The chains are so short that each monomer unit will probably add at a different rate to the previous one, and furthermore the monomer itself is a mixture of three isomers each presumably with its own rate of addition. We therefore conclude only that the order in butene is somewhat higher for polymerization than for isomerization. The rate expression governing these experime~ztal results is thus and k has the value 6.5X lo2 l2 mole-2 min-l at 0' in ethylene dichloride solution. REFERENCES 1. J. M. CLAYTON and A. M. EASTHAM. J. Chem. Soc. In press. 2. A. G. EVANS and M. POLANYI. J. Chem. Soc. 242 (1947). 3. J. M. CLAYTON and A. M. EASTHAM. Can. J. Chem. 39, 138 (1961). 4. A. M. EASTHAM. J. Am. Chem. Soc. 78, 6040 (1956).