Kinetics and Mechanism of Polymerization of n-Butyl Vinyl Ether

Gowoon CheonEkin D. CubukEvan R. AntoniukLavi BlumbergJoshua E. GoldbergerEvan J. Reed. The Journal of Physical Chemistry Letters 2018 9 (24), 6967- ...
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9896

J. Phys. Chem. B 2001, 105, 9896-9901

Kinetics and Mechanism of Polymerization of n-Butyl Vinyl Ether Initiated by Dichloroalane (AlHCl2): A New Cationic Initiating System Sa´ ndor Ke´ ki, Miklo´ s Nagy, Gyo1 rgy Dea´ k, and Miklo´ s Zsuga* Department of Applied Chemistry, UniVersity of Debrecen, H-4010 Debrecen, Hungary ReceiVed: NoVember 22, 2000; In Final Form: April 16, 2001

The carbocationic polymerization of n-butyl vinyl ether (BVE) using the new LiAlH4/AlCl3 initiating system is reported. The polymerizations were carried out in diethyl ether (Et2O), and in a mixture of diethyl ether and dichloromethane (1:1 v/v) at various temperatures, and the kinetics of the polymerizations was studied. The rate constants, the temperature dependence of the rate constants, and the kinetic order of the reactants were determined. The end groups of the polymers were characterized by 1H NMR and MALDI-TOF MS methods. Three different interconvertible end groups, i.e., heteroacetal, homoacetal, and aldehyde, were observed. The tacticity of the polymers formed was determined by 13C NMR. On the basis of detailed kinetic and structural investigations, a direct initiation mechanism for the polymerization is proposed.

Introduction Since the first example of living cationic polymerization of vinyl ethers was discovered with the use of hydrogen iodide and iodine as the initiating system, a variety of binary initiating systems consisting of a protonic acid and a Lewis acid has been reported.1-3 Recently, great efforts have been devoted to develop cheap and efficient initiating systems that would enable a higher control over the properties (molecular weight, stereoregularity, and end groups) of the polymers formed.3 Lewis acid (AlCl3, SnBr2, TiCl4)/protonic acid or organic halide combinations are in the focus of today’s butyl vinyl ether polymerizations.4-6 The investigations concerning vinyl ether polymerization have two main directions. One is the investigation of the effect of the solvent, co-initiator, and different additives on the rate of the reaction and the polymer structure.6-8 The other involves the examination of new initiating systems that would result in a highly stereospecific polymerization, giving an almost pure isotactic product.9 Until now, initiating systems consisting of an ionic hydride in conjunction with a Lewis acid have not been studied. In this work we used mixed aluminum hydrides AlCl3/ LiAlH4 (alanes) initiating systems. The reaction between lithium aluminum hydride and aluminum(III) chloride was investigated in details by Ahsby and Prather,10 and according to their results in this reaction alane (AlH3), chloroalane (AlH2Cl) and dichloroalane (AlHCl2) are formed depending upon the molar ratio of the reagents:

3LiAlH4 + AlCl3 f 4AlH3 + 3LiCl LiAlH4 + AlCl3 f 2AlH2Cl + LiCl LiAlH4 + 3AlCl3 f 4AlCl2H + LiCl The halogen alanes exist as monomers in ethers and do not form polimers.11,12 The Lewis acidic strength of the mixed hydrides and AlCl3 increases as shown below because of the high electronegativity of the chlorine atom:13

AlH3 < AlH2Cl < AlHCl2 < AlCl3 On the other hand, the hydrogen donating ability of these species decrease in this order.

In this report we describe a detailed kinetic and mechanistic investigation of the cationic polymerization of BVE with the use of the AlCl3/LiAlH4 initiating system. The direct initiation of the living polymerization of isobutylene and styrene with boron halogenides was investigated in detail,14-17 but to our best knowledge, direct initiation of vinyl ether polymerization has not been reported so far. Experimental Section Materials. Commercial BVE (Aldrich, purity ≈98%) was washed with 10% aqueous sodium hydroxide solution and then water, dried overnight over potassium hydroxide, allowed to reflux over calcium hydride for 2 days, and then distilled before use. AlCl3 (Aldrich, purity > 99.99%) and LiAlH4 (Aldrich) were used as received. CH2Cl2 was distilled over P2O5 and then over calcium hydride under a dry nitrogen atmosphere. Et2O was absolute grade (water content < 0.0075 ppm, Aldrich product). THF as a solvent for gel permeation chromatography (GPC) was dried overnight over potassium hydroxide and distilled over copper(I) chloride. Polymerization Procedures. The polymerizations were carried out in a drybox in special 75 cm3 test tubes under a dry nitrogen atmosphere. The initiator system was prepared by dissolving AlCl3 and LiAlH4 in a mixture of CH2Cl2 and Et2O (1:1). The concentration of the initiator stock solution was 0.1 M. The polymerizations were initiated by adding the proper volumes of the initiator described above into a mixture of the solvent (Et2O or Et2O/CH2Cl2 1:1) and BVE. After predetermined intervals, the polymerization was terminated with 2 mL of methanol. The solvents were evaporated from the quenched reaction mixture at room temperature, the crude residue was vacuum-dried, and polymer samples were analyzed. The living nature of the polymerization was checked by using the well-known “Incremental Monomer Addition” (IMA) technique.18 Measurements. Gel Permeation Chromatography. The Mn and molecular mass distribution (MMD) of the polymers were measured by GPC in THF at 35 °C with a Waters chromatograph equipped with four gel columns (7 µm Ultrastyrogel

10.1021/jp004269g CCC: $20.00 © 2001 American Chemical Society Published on Web 09/14/2001

Polymerization of n-Butyl Vinyl Ether by AlHCl2

Figure 1. 27Al NMR spectra of AlCl3 (a) and a mixture of AlCl3/ LiAlH4 (3:1 molar ratio) (b) in Et2O/dichloromethane (1/1 v/v). Experimental conditions: (a) [AlCl3] ) 0.075 mol/L; (b) [AlCl3] ) 0.075 mol/L, [LiAlH4] ) 0.025 mol/L.

columns: 500, 103, 104, 105 Å), a Waters 501 HPLC pump, and with Waters 440 UV and Waters 410 refractive index detectors. The Mn and Mw/Mn values of the polymers were calculated from their chromatograms on the basis of a polystyrene calibration. NMR. 1H and 13C NMR spectra were recorded in CDCl3 at 25 °C on a Bruker AM 360 spectrometer with tetramethylsilane as an internal standard. 27Al NMR spectra were recorded in a mixture of diethyl ether and dichloromethane (1:1 v/v) at 25 °C on a Bruker WP 200 SY spectrometer. An aqueous solution of AlCl3 was used as the standard. MALDI-TOF MS. The MALDI-TOF measurements were performed with a Bruker BIFLEX III mass spectrometer. In all cases 19 kV acceleration voltage was used with pulsed ion extraction (PIE). The positive ions were detected in the reflectron mode (20 kV). A nitrogen laser (337 nm, 1 ns pulse width) operating at 4 Hz was used to produce laser desorption, and 50-60 shots were summed. The spectra were externally calibrated with a poly(ethylene glycol) PEG standard (Mn ) 1450 g/mol, Mw/Mn ) 1.02) with linear calibration. Samples were prepared with DHB (2,5-dihydroxybenzoic acid) matrix (20 mg/mL). LiCl (5 mg/mL) was added to the matrix/analyte solution to enhance cationization. The analyte solution (5 mg/ mL) was mixed in 50:5:1 v/v ratio (matrix:analyte:LiCl). The solvent was THF. A volume of 0.5-1 µL of these solutions was deposited onto the sample plate (stainless steel) and allowed to air-dry. The differences between the measured and the calculated masses were within 0.3 Da. Calculation. Numerical integrations of the differential equations were performed with homemade software written in Turbo Pascal 7.0 using the Runge-Kutta integration method. Results and Discussion Preliminary Experiments. Experiments were carried out in order to determine which type of the alane is the most appropriate initiator for the polymerization of BVE. Alane (AlH3), monochloroalane (AlH2Cl), and dichloroalane (AlHCl2) solutions were prepared by mixing LiAlH4 and AlCl3 in different molar ratios according to the stoichiometry of the chemical

J. Phys. Chem. B, Vol. 105, No. 40, 2001 9897

Figure 2. Number-average molecular mass (Mn) vs weight of polymers (Wp) plot obtained by using the IMA technique (the data in parentheses show the Mw/Mn values). The inset shows a representative GPC trace. Experimental conditions: [I]0 ) 0.006 mol/L; ∆[BVE] ) 0.154 mol/ L; ∆Τ ) 30 min in Et2O at 0 °C. The concentration of quenching methanol ) 2 mol/L.

equations described above. The experiments were carried out in the mixture of Et2O and dichloromethane (1:1 v/v) and in pure Et2O. Significant polymer formation was observed with dichloroalane; thus AlHCl2 was selected and used in further experiments. The 27Al NMR spectra of AlCl3, and a mixture of AlCl3/LiAlH4 (molar ratio ) 3:1) are shown in Figure 1. The peaks at 101 and 94 ppm are due to the presence of the dimer of AlCl3 (Al2Cl6), and the monomeric AlCl3, respectively. The broadness of the peaks is characteristic of the fast equilibrium between Al2Cl6 and AlCl3. By adding LiAlH4 to the solution of AlCl3, a single, sharp peak appeared at 100 ppm, which can be attributed to the presence of AlHCl2 (the broad peaks of AlCl3 and Al2Cl6 disappeared). The polymerization of BVE initiated by AlHCl2 in a mixture of Et2O and dichloromethane (1:1 v/v) was relatively fast and the molecular mass distributions of the resulting polymers were broader (Mw/Mn > 1.7) than those prepared in pure Et2O (Mw/ Mn ≈ 1.3-1.5). Therefore, for further experiments Et2O was used as solvent. To check the living nature of the polymerization the “Incremental Monomer Addition “(IMA) technique18 was used. As Figure 2 shows the Mn values are apparently proportional to the weight of the polymer (Wp) formed, indicating the living nature of the polymerization. The Mw/Mn values are larger (Mw/Mn ∼ 1.5) than expected in the case of fast initiated living polymerization. On the other hand, the Mw/Mn values and the molecular mass distributions (inset of Figure 2) are characteristic of the slowly initiated living polymerizations.19 Therefore, we came to the conclusion that the BVE polymerization induced by AlHCl2 was a slowly initiated living polymerization. Effect of Proton Trap on the Polymerization. As it is known,20 initiation can also occur by the addition of a proton from traces of water and/or protic impurities on the monomer. To exclude the undesirable uncontrolled protic initiation, different proton traps such as 2,6-di-tert-butylpyridine (DTBP)21 are used. To find out the mechanism of the initiation, two parallel polymerization reactions were performed, in the presence and in the absence of DBTP under identical experimental conditions. No fundamental difference between the conversion and the molecular weights of the polymers formed was found. Figure 3 shows that according to MALDI-TOF MS examinations the fine structures of the polymer samples were also identical.

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Figure 3. Zoomed MALDI-TOF MS spectra of poly(BVE) samples: in the absence (a) and presence (b) of DTBP. Experimental conditions: [BVE]0 ) 0.62 mol/L; [I]0 ) 0.004 mol/L; [DTBP] ) 0.001 mol/L; reaction time ) 45 min in Et2O at 25 °C. The concentration of quenching methanol ) 2 mol/L.

Figure 4. 1H NMR spectrum (360 MHz) of poly(BVE) in CDCl3. Experimental conditions: [BVE]0 ) 0.62 mol/L; [AlHCl2]0 ) 0.004 mol/L; reaction time ) 5 min, in Et2O at 25 °C, Mn ) 4800 g/mol by GPC. The concentration of quenching methanol ) 2 mol/L.

From the experimental data described above it is concluded that direct initiation takes place, and the presence of protic initiation is not significant. End Group Determination. The 1H NMR spectrum and the assignments for poly(BVE) are shown in Figure 4. In the 1H NMR spectrum a signal corresponding to an aldehyde end group and the acetal protons can be assigned at 9.76 and 4.54 ppm, respectively. To get more information on the end groups of poly(BVE), MALDI-TOF MS experiments were performed. MALDI-TOF MS is a powerful technique for the determination of the molecular mass, molecular mass distribution, and the end groups because it allows ionization without significant fragmentation. The molecular mass of the peak (M) consisting of n repeating units can be expressed by eq 1, where Mr, Mend,

M ) nMr + Mend + Mx+,

(1)

and Mx+ are the mass of the repeating unit, the mass of the end groups, and the mass of the cationogen (in our case Li+) added to the sample, respectively. The zoomed MALDI-TOF MS spectrum of the poly(BVE) is shown in Figure 5. It demonstrates that poly(BVE) with three

Figure 5. Zoomed MALDI-TOF MS spectrum of poly(BVE). Experimental conditions: see caption of Figure 3.

SCHEME 1: Proposed Mechanism for the Forced Termination by Methanol

different end groups can be distinguished:methoxy (-OMe), aldehyde (-CHO). and a homoacetal type (-CH(OMe)2). On the basis of the intensity of the corresponding lithium-cationized peaks, and assuming equal ionization probability for the three types of poly(BVE)s, one can calculate that about 62%, 17%, and 21% of poly(BVE), respectively, are present with methoxy, aldehyde, and homoacetal end groups. According to the 1H NMR spectrum shown in Figure 4, 15% of poly(BVE) with an aldehyde end group (Mn ) 4800 g/mol) can be calculated, which is in very good accordance with those obtained from the MALDI-TOF MS measurements. These end groups may be developed by the elimination of chlorine from the chloroacetal end group during the forced termination process (Scheme 1). During forced termination with methanol, a mixed acetal may also be formed, which can react with the excess of methanol to form the homoacetal that is in equilibrium with the aldehyde form in the presence of traces of water. The presence of the three different end groups can be explained by the interconversion of the mixed acetal, the homoacetal and the aldehyde. Tacticity of Poly(BVE). The tacticity of poly(BVE) was investigated by 13C NMR, and the spectrum of a representative sample is shown in Figure 6.

Polymerization of n-Butyl Vinyl Ether by AlHCl2

J. Phys. Chem. B, Vol. 105, No. 40, 2001 9899 SCHEME 2: Proposed Mechanism for the Polymerization of BVE Initiated by AlHCl2

Figure 6. 13C NMR spectrum (360 MHz) of poly(BVE) in CDCl3. Experimental conditions: see caption of Figure 3.

Denoting all carbenium ions and dormant species by P+ and P, respectively, and assuming that the concentration of P+ is much lower than that of P, the following differential equations can be set up for Scheme 2,

d[P]/dt ) kc[AlHCl+AlHCl3-][M]

(2)

where [M] is the actual concentration of the monomer. Assuming fast self-ionization equilibrium in R1 (see Scheme 2), one can write

[AlHCl+AlHCl3-] ) Ki[AlHCl2]2

(3)

Substituting eq 3 into eq 2 and denoting AlHCl2 by I results in

d[P]/dt ) kcKi I2 [M]

(4)

Assuming first-order monomer dependency in R3, the rate of propagation can be given by Figure 7. ln([M]0/[M]) versus time plot as a function of the initiator concentration. Experimental conditions: [BVE]0 ) 0.62 mol/L; in diethyl ether at 25 °C.

The diad distributions of poly(BVE) were determined from the peak integral ratios of the well-resolved main chain methylene carbon signals at ≈40 ppm (where m refers to the meso and r to the raceme diads). In the case of the solvent mixture (Et2O/dichloromethane) the meso content was found to be lower (52%) than in the case of pure ether (60%). These trends demonstrate that the solvent polarity has an influence on the steric structure of the polymers formed; the more polar the solvent is, the less the meso diad content of the polymer is. Kinetic and Mechanistic Considerations. The dependence of the reaction rate on the concentrations of BVE and on the initial concentration of AlHCl2 was determined using the All Monomer In (AMI) technique at different temperatures.18 Figure 7 shows the ln([M]0/[M]) versus reaction time plot of the polymerizations at different initiator concentrations. The experimental data give a straight line starting from the origin, which suggests first-order monomer concentration dependency of the propagation. Also, it is apparent from Figure 7 that the ln([M]0/ [M])-time plots give straight lines with increasing slopes as the initiator concentration is increased. From the slopes, the apparent rate constants (kapp) of the propagation can be calculated. To describe the main kinetic features of the polymerization of BVE initiated by AlHCl2, a mechanism shown in Scheme 2 is proposed.

-d[M]/dt ) kp[P+][M] ) (kp/Kt) [P][I][M]

(5)

Since [P+] , [P], the actual initiator concentration is equal to [I]0 - [P], where [I]0 is the initial concentration of the initiator, i.e., [I] ) [I]0 - [P], thus eq 5 yields

-d[M]/dt ) (kp/Kt) [P]([I]0 - [P])[M] ) Q‚M

(6)

Interestingly, in eq 6 the product of Q ) [P]([I]0 - [P])) changes as the product of [P]([I]0 - [I]) does. The maximum value is at [P] ) [I]0/2, and if the initiator efficiency is about 50% or higher, one can get a linear ln(M]0/[M]) vs time plot and the apparent rate constant of propagation (kapp) will be nearly equal to (kp/ Kt)([I]0/2)2; i.e., at relatively high initiator efficiency eq 5 can be simplified to a first-order differential equation:

-d[M]/dt ) kapp[M]

(7)

To support this conclusion, differential eqs 4 and 5 were solved numerically and the result of this calculation is depicted in Figure 8. In this model calculation the consumption of the initiator was taken into account; however, if the consumption of the initiator was neglected (which does not apply to our case) the ln{(1 + c1/2)/(1 - c1/2)} versus time plot should give a straight line15 (where c is the conversion). The values of kp/Kt and kcKi were chosen in a way so that the results of the calculations fit to the experimental findings. As Figure 8 shows, a nearly linear dependence of ln([M]0/ [M]) with the time is obtained. The very small intercept,

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Figure 8. Calculated ln([M]0/[M]) versus time plot for the polymerization of BVE initiated by AlHCl2. For calculations kc/Ki ) 30 (mol/L )-2 min-1, kp/Kt ) 6000 (mol/L)-2 min-1, [BVE]0 ) 0.62 mol/L, and [AlHCl2] ) 0.004 mol/L values were used.

Figure 10. Experimental and calculated Mn versus conversion plots for the polymerization of BVE initiated by AlHCl2 in diethyl ether at 25 °C. For the calculations kc/Ki ) 30 (mol/L)-2 min-1, kp/Kt ) 6000 (mol/L)-2 min-1, and [AlHCl2]0 ) 0.004 mol/L values were used. The symbols and the solid lines represent the experimental and the calculated Mn values, respectively.

Figure 11. ([I]0/[P]) ln{[I]0/([I]0 - [P])} - [P] versus {[M]0 - [M]} plot at different temperatures for the polymerization of BVE initiated by AlHCl2. Experimental conditions: [BVE]0 ) 0.62 mol/L; [AlHCl2]0 ) 0.004 mol/L;in diethyl ether. Figure 9. kapp versus [AlHCl2]02 plot. Experimental conditions: [BVE]0 ) 0.62 mol/L; in diethyl ether at 25 °C.

however, can be hardly detected under the experimental conditions. The slope of the straight line is nearly equal to kapp ) kp/Kt([I]0/2)2, as mentioned above, thus, according to the mechanism drawn in Scheme 2, the apparent rate constant is directly proportional to [I]02, i.e., the kapp versus the square of the initiator concentration should give a straight line, which is in very good accordance with the experimental findings (Figure 9). Consequently, the value of kp/Kt from the slope of the ln([M]0/[M]) versus time plot can be determined using

kp/Kt ≈ 4kapp/[I]o2

(8)

The average degree of polymerization (DPn) can be expressed by eq 9 with the assumption that [P+] , [P]. The experimental and the calculated values of Mn at different initial concentrations

DPn ) ([M]0 - [M])/[P]

(9)

of the monomer are shown in Figure 10. The calculated values of Mn were obtained by solving the differential eqs 4 and 5 numerically and with eq 9. Using eqs 4-6 one can write

-d[M]/d[P] ) R[P]/([I]0 - [P])

R ) kp/(kcKiKt)

(10)

Solving the differential eq 10 gives

[M]0 - [M] ) R[I]0 ln{[I]0/([I]0 - [P])} - R[P] (11) By rearranging eq 11, one obtains eq 12, which suggests that the [I]0 ln{[I]0/([I]0 - [P])} - [P] versus {[M]0 - [M]} plot, where [P] is given by [P] ) ([M]0 - [M])/DPn, should give a

[I]0 ln{[I]0/([I]0 - [P])} - [P] ) (1/R){[M]0 - [M]} (12) straight line starting from the origin. To test eq 12 the [I]0 ln{[I]0/([I]0 - [P])} - [P] values were plotted as a function of {[M]0 - [M]} at different temperatures (Figure 11). As shown by Figure 11, the experimental data obey eq 12; thus R can be determined. Also, as the temperature decreases, the value of R increases. The calculations of R from the [I]0 ln{[I]0/([I]0 - [P])} - [P] versus {[M]0 - [M]} plots, and the values of kp/Kt (using eq 7) from the ln([M]0/[M)]-time plots at different temperatures allow the determination of the kc/Ki values (R ) kp/(kcKiKt)) and the activation energy of the initiation and propagation by means of the Arrhenius plot. The determined values for kp/Kt and kcKi at different temperatures are summarized in Table 1. The large temperature dependence of kc/Ki and kp/Kt indicates high activation energy values for the initiation and propagation. The Arrhenius plots for kc/Ki and kp/Kt are shown in Figure 12. From the slopes of the lines drawn in Figure 12, activation

Polymerization of n-Butyl Vinyl Ether by AlHCl2

J. Phys. Chem. B, Vol. 105, No. 40, 2001 9901 TOF MS measurements revealed poly(BVE) with three different, interconvertible end groups, and this recognition may be useful for carrying out additional polymer-analogue reactions. Acknowledgment. This work was financially supported by grants T019508, T025379, T025269, T030519, M28369, and F019376 given by OTKA (National Fund for Scientific Research Development, Hungary), by grant FKFP 04441/1997, and by the Bolyai Ja´nos Fellowship. We also express our thanks to CELLADAM Ltd., Hungary for providing the BIFLEX IIITM MALDI-TOF MS instrument. References and Notes

Figure 12. Arrhenius plot for the polymerization of BVE initiated by AlHCl2. Experimental conditions: [BVE]0 ) 0.62 mol/L; [AlHCl2]0 ) 0.004 mol/L; in diethyl ether.

TABLE 1: kp/Kt and kcKi Values at Different Temperatures for the Polymerization of BVE Initiated by AlHCl2 temp (°C)

kp/Kt (M-2 min-1)

kcKi (M-2 min-1)

25 0 -5 -15

5900 1025 475 300

26.3 2.38 0.57

energies of 79 and 49 kJ/mol were obtained for the initiation (kc/Ki) and the propagation (kp/Kt), respectively. The direct initiation mechanism depicted in Scheme 2 thus describes well the main kinetic features of the polymerization of BVE in the presence of AlHCl2. Conclusion A self-ionization mechanism is proposed to describe the main kinetic features of the polymerization of BVE in the presence of AlHCl2. Although the 27Al NMR spectrum of the AlHCl2 solution did not show the presence of an AlHCl+AlHCl3species, it can be concluded that this species may exist in very low concentrations that cannot be detected with NMR, but it can initiate the living polymerization of BVE. The MALDI-

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