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18 Olefin Polymerizations and Copolymerizations with AlkylaluminumCocatalyst Systems III. Carbonium Ion Polymerizations with Trialkylaluminums and the Characterization of AlR -Lewis Base Systems by N M R Spectroscopy

Downloaded by CORNELL UNIV on July 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch018

3

JOSEPH P. KENNEDY and GEORGE E. MILLIMAN Corporate Research Laboratory, Esso Research and Engineering Co., Linden, N. J.

Trialkylaluminums are efficient catalysts for isobutylene polymerization in the presence of a suitable cocatalyst— e.g., tert-butyl chloride, in methyl chloride solvent at low temperatures. In the AlMe -MeCl system, investigated by NMR spectroscopy, methyl exchange is very rapid in the AlMe dimer even at low temperatures in the presence of methyl chloride. Methyl chloride enhances the rate of methyl scrambling. In addition to methyl chloride, a variety of other Lewis bases apparently also effect the exchange of methyl groups in Al Me . The significance of these findings for the theory of polymerization initiation with trialkylaluminums is discussed. 3

3

2

6

• p ecently we have described (2, 3, 4, 5, 6) and discussed from the theoretical point of view the polymerization and copolymerization of olefins, isoolefins, and aromatic hydrocarbons and dienes using trialkylaluminum and related catalyst systems. Trialkylaluminums—e.g., trimethylaluminum ( A l M e ) , triethylaluminum ( A l E t ) , and triisobutylaluminum ( A l i B u ) or dialkylaluminum halides—e.g., diethylaluminum chloride ( A l E t C l ) are weak Lewis acids and do not initiate the polymerization of cationically polymerizable hydrocarbon monomers in the absence of suitable cocatalysts. For example, isobutylene, an extremely 3

3

3

2

287

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


288

ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N PROCESSES

reactive olefin, can be stirred at reflux or at low temperatures i n methyl chloride ( M e C l ) diluent i n the presence of any of these trialkylaluminums, and no polymerization w i l l occur. However, instantaneous and vigorous polymerization occurs immediately after a small amount of a suitable cocatalyst—e.g., terf-butyl chloride—is introduced to the quiescent system. In contrast to these alkylaluminum catalyst systems, the well-known Friedel-Crafts halides—e.g., A1C1 3 , A l E t C l 2 , etc.—rapidly polymerize isobutylene to high molecular weight products i n the absence of added cocatalyst. It is usually presumed, but remains to be proved, that traces of moisture or HC1 provide the necessary cocatalyst with the latter agents. During our fundamental studies on the mechanism of carbonium ion polymerizations we became interested i n the problem of cocatalysis i n trialkylaluminum-initiated polymerizations and decided to elucidate some of the characteristic features of these systems by nuclear magnetic resonance ( N M R ) spectroscopy. This paper presents some of the intriguing phenomena we encountered with the A l M e 3 and A l i B u 3 in methyl chloride and other solvent systems, in particular an extremely rapid methyl group exchange i n the A l M e 3 dimer at very low temperatures and a very weak and a somewhat stronger complexation between A l 2 M e 6 and methyl chloride, and A l i B u 3 and methyl chloride, respectively. Experimental

A l l polymerizations were carried out i n a stainless steel enclosure (8). The purity and analysis of isobutylene, methyl chloride, and aluminum chloride have been described (9). The trialkylaluminums (Texas Alkyls, Inc.) were purified b y distillation before use; A l E t 2 C l was treated with N a C l at 80 ° C . for 2 hours to remove traces of A l E t C l 2 . AlMe 3 -*é?r*-BuCl in MeCl. T o 15 m l . isobutylene i n 60 m l . methyl chloride stirred at - 3 5 ° C , 8.3 Χ 10"5 mole A l M e 3 (0.2 m l . of 4 vol. % A l M e 3 i n M e C l solution) was added; no reaction occurred. Subsequently, a total of >~5.5 Χ 10"6 mole of terf-butyl chloride was added dropwise (a 1% teri-butyl chloride solution was used); immediate polymerization occurred; the yield was 1.469 grams or 14% ; the viscosity average molecu lar weight was 240,300. A series of experiments was carried out with this system at —35°, —55°, —78°, and —100 ° C . The general appearances of the reactions were essentially identical. AlMe 3 -ferf-BuCl in n-Pentane. To a charge of 15 ml. isobutylene in 60 m l . n-pentane stirred at - 3 5 ° C , 1.05 Χ 10"3 mole of an A l M e 3 (2 m l . of 8 vol. % A l M e 3 ) was added i n n-pentane solution; no reaction occurred. Subsequently, a tert-butyl chloride i n n-pentane solution was added dropwise until a total of 1.66 Χ 10"4 mole of tert-buty\ chloride was introduced (0.9 m l . of a 2% text-butyl chloride i n n-pentane solution); however, no polymerization occurred, and no methanol-insoluble polymer was formed. In a similar experiment A l M e 3 and teri-butyl chloride were added intermittently over 187 minutes to the above charge until a total of 6.65



18.

KENNEDY AND MILLIMAN

Alkyfoluminum-Cocatalyst

Systems

289

Χ 10" 3 mole of A l M e 3 and 7.0 Χ 10"3 mole of tert-buty\ chloride—i.e., approximately equimolar amounts—were added, but ho reaction was noticeable, and no methanol-insoluble polymer was formed. N M R Spectroscopy. A l l samples were sealed under nitrogen. L i n e width measurements were made on spectra obtained at 100-Hz sweep widths from a Varian A60 N M R spectrometer. The solvent signal was used as a reference for the adjustment of the t/-axis and curvature controls so that the maximum possible field homogeneity was obtained before the line whose width was to be measured was recorded. L i n e widths are uncorrected for inhomogeneity or uncertainty broadening. Uncertainty broadening is not expected to contribute significantly to the observed line width. Inhomogenity broadening is estimated at 0;2-0.3 H z and therefore should not contribute significantly to any of the observed line widths in Table I except those obtained from the solutions having the two highest methyl chloride concentrations. Sample temperatures were measured by inserting a sample of methanol into the probe and then calculating the temperature using the known temperature dependence of the hydroxyl signal. Chemical shifts were obtained from spectra obtained i n a Varian H A 100 N M R spectrometer operating i n the H A frequency sweep mode. The signal whose chemical shift was to be measured was recorded at a 50-Hz. sweep width, and the chemical shift was determined by reading the difference i n frequency between this signal and the lock (solvent) signal from an electronic counter. The measured chemical shifts are believed to be accurate to ± 0.1 H z . Concentrations of the sample components were calculated from the electronic integrals of the com ponents and the molar volumes of the components, assuming ideal solutions. Results

and

Discussion

Characterization of the A l M e 3 - M e C l System by N M R . It appears that the A l M e 3 in methyl chloride system has not been investigated previously by N M R spectroscopy. In related research by other workers, freezing point depression measurements of A l M e 3 in benzene solution indicated the predominant presence of dimers—i.e. (13):

The N M R spectrum of this dimer would be expected to exhibit two signals, i n the ratio of 1:2, corresponding to the two magnetically inequivalent methyl groups—viz., bridging and terminal. Indeed, whereas at room temperature the N M R spectra of A l M e 3 solutions show only one signal for the methyls (7,8), upon cooling these solutions to — 3 0 ° C . the signal broadens and is very distorted, and at — 4 0 ° C . two slightly broad-


ADDITION AND CONDENSATION POLYMERIZATION PROCESSES


290

ened signals are observed, one for each of the two types of methyls (Figure 1). [Note added i n proof: J. R. M c D i v i t t has recently observed that the 60-MHz. N M R spectrum of A l 2 M e 6 dissolved in M e C l exhibits two signals at — 8 5 ° C . for the methyls attached to aluminum at τ9.44 and τΐθ.64 i n the ratio 1:2. This observation indicates that bridge-terminal methyl exchange of the trimethylaluminum dimer is slow in methyl chlo ride at this temperature.] This behavior is typical of exchange and indi cates that the bridge and terminal methyl groups equilibrate rapidly at room temperature and that this exchange is retarded significantly at low temperatures. Noting that the activation energy for the exchange is 6-14 kcal./mole and the heat of dissociation of the dimer i n the gas phase is 20.2 kcal./mole, Muller and Pritchard (12) postulated that the exchange process was intramolecular, involving either the breaking of one bridge ( A ) or a deformation of the molecule ( B ) as shown below.

.Me

Me-Al^â£Âl-Me Me

XX

/

Me

\

Me ,

Χ

Me

Β

Me

XX \

χ

.

Me

M e

X

Me

Me

Me

X« ^

Me

Me

Me

Me

Me

X

Me

>

t CI

Me' *

CHo

CH 3 C1 +

ΑΓ

ΑΓ Me

Me

Me

Me

Me

Me4


Me

18.

Alky Muminum-C

KENNEDY AND MiLLiMAN

1

·°ι

0.9

-

0.8

-

0.7

-

293

ο catalyst Systems

1

1

1

1

1

1

1

1

1

1

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1

1

1

1

1

2.2

2.4

2.6

2.8


0 . 6 \-

0

M E T H Y L CHLORIDE CONCENTRATION,

3.0

moles per liter

Figure 2. Reciprocal of the line width of trimethylaluminum as a function of methyl chloride concentration at 60 MHz at —25°C. in cyclopentane solutions

Thus, the A l 2 M e 6 held together by electron-deficient methyl bridges is opened partially by methyl chloride, and an intermediary single-bridged hypothetical complex, A l 2 M e 6 · M e C l is formed. This complex rearranges readily, resulting in rapid methyl group scrambling. Direct evidence for the interaction between A l 2 M e 6 and M e C l has been obtained and is presented now. Interaction between Al 2 Mec and M e C l and Evidence for an Al 2 Mec · M e C l Complex. Complex formation between molecules can be detected by accurate chemical shift measurements of protons near the complexing sites. The interaction of a Lewis base such as methyl chloride with an alkylaluminum dimer might also be detected by its effect upon the linewidth of the alkyl group even if the amount of complex is very small. For slow exchange of methyl groups between the bridging and terminal positions ( two observable signals ) of trimethylaluminum dimer solutions in hydrocarbon solvents it has been shown (14) that the W (the full width at one-half peak height) is related to the average time a methyl spends in one of the two positions ( r ). This relationship is r = W 3 / 2 (corr.),where W (corr.) is the line width ( W i / 2 ) minus uncertainty and inhomogeneity broadening. For fast methyl exchange (one observ able signal) the quantity T c is defined as 1/2

eA

eA

1/2

TeATeB TeA

T B e


294

ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N PROCESSES

where ΤΘΑ and reB are the average times a methyl group spends i n a bridging and terminal position, respectively. Furthermore, reA = ^ τ Β Β since there are twice as many terminal as bridging positions. For fast exchange r e is given by 1 T e

7τ(Δν)

=

"

2

2W 1 / 2 (corr.)


where Δ ν is the difference i n chemical shifts of the bridge and terminal methyls i n the absence of exchange (14). For an intramolecular ex change process such as shown below, re is given by Me

Me

Me

Me

.Me

Me

JMe^

^ Me

Me

Me

de

^Me* Me

[dimer] rate

and therefore W1/2 is independent of [dimer]. In the presence of an added Lewis base [ B : ] which competes with the above spontaneous process i n effecting methyl exchange—viz. Me

Me*

Me

Me

^Me

^Me

* Me

.Me

ÂT

Me^

" A l ^ Me

^ M e ^

+B:

^ M e

[Dimer] kx [Dimer] + k2 [Dimer] [B:]

~

Te

and therefore *1

+

*2[B:] =

2W 1 / 2 (corr.)

i.e., the reciprocal of the line width would be proportional to the con centration of the Lewis base. According to our data shown i n Table I and Figure 2 the reciprocal of the line width of the exchanging methyl groups of the A l M e 3 dimer i n cyclopentane solvent at — 25 ° C . depends linearly on the concentration


18.


Table I.


[ C H

3

C Î \

0.09 0.10 0.28 0.28 0.30 0.50 0.77 0.80 1.13 1.17 1.27 1.37 2.39 3.13 4.23

Alky Muminum-C

ο catalyst Systems

295

NMR Data Obtained from Solutions of Trimethylaluminum and Methyl Chloride in Cyclopentane

[Al2Me6] 1.65 0.82 1.40 1.50 0.75 0.83 1.50 1.09 0.82 1.40 0.51 0.81 0.84 1.14 0.88

Chemical Shifts of CH3Cl, CH3Cl-C yclopentane, Hz. at 1 0 0 MHz., Wi/,_i, Al2MeG, Hz , - 2 5 ° C . Ambient Temp.

W1/2Al2Me6, Hz,

- 2 5 ° C .

1

20.4 27.6 14.6 12.0 14.1 9.1 5.4 5.9 5.2 4.4 4.5 4.3 2.5 2.0 1.8

0.05 0.04 0.07 0.08 0.07 0.11 0.19 0.17 0.19 0.23 0.22 0.23 0.40 0.50 0.56

134.7 134.2 134.8 134.9 134.4 134.6 135.4 135.3 135.1 135.9 135.5 135.7 137.0 137.5 138.4

of added methyl chloride. This indicates interaction between A l 2 M e 6 and M e C l and the presence of two exchange mechanisms: A l 2 M e 6 —> spontaneous exchange h

CH 3 C1 + A l 2 M e 6 —» promoted exchange F r o m arguments presented above this scheme requires that ττ(Δν)

fc + * 2 [ C H 8 C l ] = •

2

1

2 W

1

/2

The rate constants, &i and k2, are calculated from this equation and the line i n Figure 2 using the intercept and slope obtained from a leastsquares fit for a straight line; fci = 366 sec."1, and k2 = 9.9 Χ 102 liter /mole/sec. The value we obtained for fci at — 25 ° C . agrees well with that obtained by Ramey et al. (14) of 316 sec."1 using solutions of trimethylaluminum in cyclopentane. Our scheme therefore is consistent with the experimental data we obtained and with the data which Ramey obtained. W e concur with Ramey that the mechanism for the exchange of methyls most likely involves the breaking of one bridge bond:


296 Me

ADDITION

Me

AND

CONDENSATION

POLYMERIZATION

Me.

^Me

PROCESSES

Me

^ Me A l ^

M e ^

^ M e

Me *

Me

"Me

Me

fast Me


Me * Me'

Me

Al

V

s

A1

Me

Me

and suggest furthermore that the methyl chloride interacts with the trimethylaluminum to effect methyl exchange as follows: .Mev

Me CH3CI +

"ΑΓ

Al

*

Me'

s

Me, Me — - A l '

^Me

k2

^Me

Me'

,Me AT

Me* CI

^Me

^Me

fast

I CH3

-h

CH3CI

The observation that the chemical shift of added methyl chloride in Al 2 Me 6 -cyclopentane solutions is very close to the chemical shift of the same concentration of methyl chloride i n cyclopentane solutions ( Figure 3, Tables I and II) indicates that there is little if any A l 2 M e 6 * M e C l com plex formed i n solutions in cyclopentane. Therefore, the chemical shift of M e C l i n AI 2 Me 6 -cyclopentane solutions is a convenient measure of M e C l concentration, and the plot of the W~11/2 of A l M e 3 vs. the chemical shift of M e C l in cyclopentane solutions should be linear. This is observed and is illustrated i n Figure 4.



18.


Figure

3.

Alkyhluminum-Cocatalyst

Concentration dependence of the chemical ride in cyclopentane solutions

Systems

297

shift of methyl

chlo-

Table II. Chemical Shifts of Cyclopentane Solutions of Methyl Chloride [CH3Cl]

0.65 1.09 2.16 2.66

Chemical Shift of CH3Cl, CH3Cl~Cyclopentane, Hz. at 100 MHz.

134.2 134.7 136.0 136.6

W h i l e our results indicate clearly an interaction between A l 2 M e 6 and methyl chloride, we do not have direct evidence for complex formation. Evidently if a complex such as A l 2 M e 6 - M e C l exists at all, its concentration must be very low. It was theorized that chances to find evidence for the existence of a complex between a trialkylaluminum and methyl chloride would be more successful by using a monomeric trialkylaluminum, such as A l - i - B u 8 . Experiments with the Al-i-Bu ; «-MeCl system are discussed below. Characterization of the Al-i-Bu 3 -MeCl System by NMR. A l - i - B u 3 is monomeric i n hydrocarbon solutions ( I ) . Evidently the bulky isobutyl groups attached to the aluminum prevent dimerization. Since the size of the chlorine i n methyl chloride is certainly much smaller than that of an isobutyl group, we expected complex formation between A l - i - B u 3 and methyl chloride to occur. T o investigate this possibility we prepared


298

ADDITION


v

l/2

A N D CONDENSATION

POLYMERIZATION

PROCESSES

-1

134

135

136

137

138

139

140

141

142

CHEMICAL SHIFT OF METHYL CHLORIDE, HZ FROM CYCLOPENTANE AT 100 MHZ

Figure 4. Reciprocal of the line width of trimethylaluminum as a function of the chemical shift of methyl chloride. Measured at —25°C; chemical shifts measured at room temperature

Table III. Chemical Shifts of Cyclopentane Solutions of Methyl Chloride and Triisobutylaluminum

[CH3Cl]

0.10 0.30 0.88 1.10 1

[Al-i-Bu3]

Shifta of the Methylene Group of Al-i-Bu3, in Cyclopentane, Hz. at 100 MHz.

Shifta of Methyl Chloride, in Cyclopentane, Hz. at 100 MHz.

-114.7 -115.8 -119.9 -122.0

143.2 143.2 142.3 141.9

0.67 0.66 0.78 0.70

Measured from cyclopentane.

solutions of A l - i - B u 3 i n cyclopentane and added various amounts of methyl chloride. Table III and Figures 5 and 6 show our data. Figure 5 shows the chemical shift of the methylene protons of A l - i - B u 3 as a function of the ratios of concentrations of [Al-i-Bu 3 ] / [ M e C l ] i n cyclopentane solution at ambient temperature. The sharp break i n the plot at — 1.7 [ A l - i - B u 3 ] / [ M e C l ] suggests complex formation between these compounds. Similarly, the plot of the "corrected" chemical shift of methyl chloride—i.e., the chemical shift of M e C l i n Al-i-Bu 3 -cyclopentane solutions minus the chemical shift of M e C l i n the same concentration in cyclopentane solutions vs. [ A l - i - B u 3 ] / [ M e C l ] (Figure 6 ) , indicate complex formation. Obtaining more chemical shift measurements on


18.

299

Alky Muminum-C ο catalyst Systems

K E N N E D Y A N D MiLLiMAN

solutions of A l i B u 3 - M e C l i n cyclopentane w i l l lead to the determination of the equilibrium constant Kc for complex formation. AliBu 3 + MeCl ^ A l i B u 3 · MeCl These results are considered to be strong evidence for complex formation between A l i B u 3 and M e C l . 124

1

I

1

1

1

123

_

>Û- —122 0

-

_J

5 ° 1-

1-1

w< ZD Ζ

Lu

i

i! Is

Τ OF CLOPE


I l

χ

LU X

ο

°

121

-

\

120

—

\

119 118 117 116

-_

—

—

κ\

—

-

\

—

1

115 114

' '——•

0

I I 0.5 1.0

1 2.0

1 3.0

1 4.0

1 5.0

—

1 6.0

7.0

RATIO OF ALUMINUM TRIIS0BUTYL CONCENTRATION TO METHYL CHLORIDE CONCENTRATION

Figure 5. Chemical shift of the methylene protons of triisobutyMuminum as a function of the ratios of concentrations of AliBu and MeClcyclopentane solutions 3

Characterizations of Other A^Mee—Lewis Base Systems by N M R . Concurrently with our work with methyl chloride we also investigated a variety of compounds which could be regarded as Lewis bases vis-a-vis A l M e 3 — e . g . , ethyl chloride, methylene chloride, isobutylene, styrene, toluene, and butadiene. A l l these compounds are electron rich because of the presence of chlorine atoms, olefinic unsaturation, or aromatic π-electron systems. It was theorized that, similarly to methyl chloride, these compounds might also enhance the rate of methyl group scrambling in A l 2 M e 6 . N M R spectra of solutions of A l 2 M e 6 in these compounds have been obtained at low temperatures, and we have noted that the W i / 2 values for the aluminum-methyl signals i n these materials are signifi cantly less than that of A l 2 M e 6 i n cyclopentane. Values obtained for k (see second reaction scheme on p. 294) using solutions containing these 2



300

ADDITION

0

1.0

2.0

AND

CONDENSATION

3.0

4.0

POLYMERIZATION

5.0

PROCESSES

6.0

7.0

[AliBuJ/JMeCl]

Figure

6.

Dependence

of the chemical MeCl ratio

shift of MeCl

on

AliBus/

and other Lewis bases may lead to the establishment of a new "basicity" scale for Lewis bases. Such a "basicity" scale would be sensitive not only to electronic effects but also to steric and other factors. Conclusion

A Theory of Initiation and Propagation of Carbonium Ion Polymerizations with Trialkylaluminum Catalysts. Trialkylaluminums or dialkylaluminum halides i n conjunction with suitable cocatalysts i n polar solvent are active polymerization catalysts. For example, when cocatalytic amounts of tert-butyl chloride are added to a quiescent mixture of trialkylaluminums or dialkylaluminum halides i n methyl chloride solvent i n the temperature range —30° to — 1 0 0 ° C , immediate polymerization commences (2,3,4,5,6). From this and earlier published results it could be theorized that polymerization initiation involves the " i n situ' formation of A1C1 3 and/or A l E t C l 2 catalysts from A1R 3 and tert-butyl chloride or A l E t 2 C l and HC1 or tert-butyl chloride and that one of the former conventional aluminumcontaining Friedel-Crafts halides, rather than the A l M e 3 - f e r f - B u C l or A l E t 2 C l - H C l system, is the active catalytic species. However, several experimental facts speak against this possibility. First, the slopes of the


18.

Alkyhluminum-Cocatalyst


301

Systems

plots of the log molecular weight ( M W ) of polyisobutylene vs. 1/T lines obtained for A1C1 3 and A l E t 2 C l catalysts on the one hand and A l M e 3 and other trialkyl aluminums ( or dialkylaluminum halides ) on the other hand are quite different ( F i g u r e 7, Refs. 2 and 5 ) . The £p (over-all activation energy calculated from the slope of the log MW vs. 1/T plot) is 6.6 kcal./mole for the A1C1 3 and A l E t C l 2 systems whereas that for the trialkylaluminums and diethylaluminum chloride is 1.7 kcal./mole under essentially identical conditions. If A1C1 3 or A l E t C l 2 were formed from the latter systems, all the slopes of these lines would be the same—i.e., the effect of temperature on the molecular weights would be the same for all these catalyst systems. Obviously, this is not so, which indicates that the characteristics of these catalysts are i n fact dissimilar. The polymerization details ( rates, molecular weights, etc. ) which are different for the A l E t 2 C l / H C l system and the conventional A1C1 3 or A l E t C l 2 systems have been discussed previously (3).


D

τ

io l 4

1

-20 -30 -35 -50 -55

li I l«" r K

4.0

1

1

-78

ι 1

4.5 5.0 1/T X 10 3

ι 5.5

Γ

-100

e

L_J

C

I

6.0

Figure 7. Effect of temperature on the viscosity average molecular weight of polyisobutylene obtained with various aluminum-containing catalyst systems in methyl chloride diluent


302

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

Secondly, the AlMe 3 -ter£-BuCl catalyst system does not initiate the polymerization of isobutylene i n n-pentane solvent, whereas A l E t C l 2 is an efficient catalyst i n this solvent. Thirdly, the cocatalytic amounts of tert-butyl chloride required i n conjunction with A l M e 3 or A l E t 2 C l are very small ( A l M e 3 or A l E t 2 C l tert-BuCl ^ 10) so that the A1C1 or A l E t C l 2 which could be formed i n the following reaction: 3


A l M e 3 or A l E t 2 C l + tert-BuCl -> A1C13 or A l E t C l 2 + other products would certainly be insufficient to give the high conversions obtained experimentally. Fourthly, if, as it is usually assumed, A1C1 or A l E t C l 2 required the presence of cocatalysts presumably i n the form of active impurities ( e.g., H 0 or HC1) for initiation, it is hard to explain where this cocatalytic impurity would come from since the A1R or A l E t 2 C l added to the charge would have consumed these active substances before A1C1 or A l E t C l 2 could have formed. 3

2

3

3

The different polymerization results obtained with different catalysts in methyl chloride also suggest that the polymerization of isobutylene most likely does not proceed by free ion propagation alone. If free ions were involved i n propagation exclusively and the growing species were kinetically independent from the gegenion, products with identical molecular weights would be expected with all these catalysts. Again, this is not observed, which indicates that the gegenion influences the mechanism. Consequently, the concept of associated ion pairs as propagating species must be invoked. Nothing can be said about the nature of this association beyond postulating that the propagating carbonium ions are not entirely free and somehow are influenced by the corresponding gegenions. It is conceivable that associated and free ions coexist and/or are i n equilibrium with each other and that this equilibrium is affected by the nature of the solvent, temperature, concentrations, etc. It is important to emphasize that the A l M e 3 - t e r i - B u C l system is inactive i n n-pentane but is very active i n methyl chloride solvent as a polymerization catalyst for isobutylene (2). Thus, methyl chloride profoundly affects the active catalytic species in this system. Although the A l M e 3 - M e C l system is catalytically completely inactive and the addition of tert-butyl chloride is necessary to initiate the polymerization of isobutylene, our N M R studies indicate considerable interaction between A l M e 3 and M e C l . O n the basis of these facts the following theory for the initiation of isobutylene polymerization with trialkylaluminums is proposed. A t temperatures lower than — — 3 5 ° C , ferf-butyl chloride does not interact


18.

Alky Muminum-C


ο catalyst Systems

303

with AlMe 3 to the extent that methyl chloride does (7). Conceivably, the tert-butyl chloride is able to replace the less basic methyl chloride to form a new complex: Me

ÂT

Me

^Me

Al

+

I

Me^t

CH , 3 CH3—C—Cl

^ M e Me

CI

CH3


CH 3

Me

Me A l ^

M e ^ |

^

A

l

I

CI

C H 3 — C — CH

Me

M

^

+ CH 3 C1

^ M e e

3

CH3 The (CH 3 ) 3 CC1—Al 2 Me 6 bond (complex) should be stronger (more stable) than the CH 3 C1—Al 2 Me 6 bond (complex) because the f erf-butyl group is a much better electron donor than the methyl group. Conceiv ably, as the (CH 3 )C 3 C1—Al 2 Me 6 bond is forming, the ( C H 3 ) 3 C — ClAl 2 Me 6 bond is breaking, i.e., an incipient carbonium ion, (CH 3 ) 3 CQ— ClA^Mee®begins to form. In the presence of a cationically polymerizable olefin this carbonium ion ( strong Lewis acid ) attacks the monomer ( weak Lewis base ), and initiation occurs ( see reaction scheme at top of p. 304 ). This reaction might be concerted; at the same rate the ( C H 3 ) 3 C10Al 2 Me 6 bond is loosened, the ( C H ) c O — C 4 H 8 bond is formed; the driving force of the initiation reaction would be the formation of a new C—C bond and a solvated stable ion pair from an unsolvated ( C H 3 ) C—CI bond and one-half of a double bond. The methyl chloride solvent would help to open the trialkylaluminum dimer and because of its polarity (d.c. —15), would solvate and thus stabilize the forming ions. Strong indirect evidence in support of this mechanism is the fact that in the absence of isobutylene, tert-butyl chloride yileds neopentane (7). It is of interest to examine the reason for the absence of polymeriza tion in the isobutylene-Al2Me6-n-pentane system. According to our NMR results, methyl group exchange is decidedly faster in the presence of isobutylene than in pure cyclopentane, and consequently if the iso butylene monomer itself is able to interact with Al 2 Me 6 , by analogy with 3


304

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

Me.

t

Me

J ^ A l ^ Me

Me ^

CI

A

l

I

M

^ C H

^

+

3

CH2=C

^ M e

^ C H

2

.

e

I CH3—C—CH3

I


CH3

Ί θ Me

Me A l ^

M

e

Me ^ A l ^ — M e Me

^ C \

_

+

CH3

CH3

J,CH 8

GH3

methyl chloride, it opens the methyl bridge of A l 2 M e 6 . Evidently the terf-butyl chloride does not displace the isobutylene i n the C ^ s - A l s M e e complex and initiate the polymerization along the lines proposed above. The reason for this is far from obvious. It could be that the mechanism of methyl group exchange i n the A l 2 M e e - M e C l system is entirely different from that occurring i n the A l 2 M e 6 - i C 4 H 8 system, or that the polar M e C l solvent is necessary to solvate the incipient ions because unsolvated species cannot initiate. Further experiments must be performed to answer this question. Acknowledgments

W e are pleased to acknowledge the help and advice of M . T. Melchior in interpreting N M R spectra. Literature

Cited

(1) Hoffman, E. G., Trans. Faraday Soc. 58, 642 (1962). (2) Kennedy, J. P., U. S. Patent 3,349,065 (1967). (3)

Kennedy, J. P., ACS, Div. Polymer Chem., Polymer Preprints 7,

485

(1966). (4)

Kennedy, J. P., Intern. Symp. Macromol. Chem., Tokyo, Kyoto, 1966,

Abstract 2.104.


18.

KENNEDY AND MILLIMAN

Alkylaluminum-Cocatalyst Systems

305

(5) Kennedy, J. P., "Polymer Chemistry of Synthetic Elastomers," J. P. Kennedy, E. G. Tornqvist, Eds., Vol. I, Chap. 5A, Wiley, New York, 1968. (6) Kennedy, J. P., British Patent 1,094,728 (1967). (7) Kennedy, J. P., Melchior, M. T., Milliman, G. E., unpublished results. (8)

(9) (10) (11) (12) (13) (14)

Kennedy, J. P., Thomas, R. M., ADVAN. C H E M . SER. 34, 111 (1962).

Kennedy, J. P., Thomas, R. M., J. Polymer Sci. 45, 481 (1960). McCoy, G. R., Allred, A. L., J. Am. Chem. Soc. 84, 912 (1962). Mole, T., Australian J. Chem. 18, 1183 (1965). Muller, N., Pritchard, D. E., J. Am. Chem. Soc. 82, 248 (1960). Pitzer, K. S., Gutowsky, H. S., J. Am. Chem. Soc. 68, 2204 (1946). Ramey, K. C., O'Brien, J. F., Hasegawa, I., Borchert, A. E., J. Phys. Chem. 69, 3418 (1965).


(15) Williams, K. C., Brown, T. L., J. Am. Chem. Soc. 88, 5460 (1966). RECEIVED

March 11, 1968.


Olefin Polymerizations and Copolymerizations with ... - ACS Publications

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