Mechanisms for Initiating Polymerization by Stable Organic Cations

20 Mechanisms for Initiating Polymerization

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by Stable Organic Cations A. LEDWITH Donnan Laboratories, University of Liverpool, Liverpool 7, England

Use of triphenylmethyl

and cycloheptatrienyl

initiators for cationic polymerization

cations as

provides a convenient

method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. the polymerization

of vinyl alkyl ethers,

and tetrahydrofuran, cussed in detail.

initiated

Mechanisms for N-vinylcarbazole,

by these reagents, are dis-

Free ions are shown to be much more

reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl

cation is less reactive

than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (k ) for free ion polymerization p

vinyl ether and N-vinylcarbazole CH Cl , 2

2

of isobutyl

have been determined in 5

and for the latter monomer the value of k is 10 p

times greater than that for the corresponding free radical polymerization.

T o u r i n g the past five years we have shown that many stable organic cations are useful initiators for polymerizing reactive olefins and cyclic ethers ( I , 2, 3, 4, 5, 27). Compared with more common initiators for cationic polymerization (31), the stable salts allow for complete characterization of the catalyst system and give rapid, highly reproducible polymerizations. In addition many of the salts used are stable indefinitely i n the crystalline state, which makes the experimental techniques easy and convenient. Stabilization of an organic cation occurs whenever the electrondeficient carbon atom is conjugated with aryl groups or with atoms containing unshared pairs of electrons such as oxygen, nitrogen, or sulfur (7). Examples of cations which are useful i n polymerization include triphenylmethyl ( I ) , cycloheptatrienyl (tropylium) ( I I ) , and various alkyl ( I I I ) , aryl ( I V ) , and benzopyrylium ( V , V I ) derivatives. 317 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

318

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

I

II

III

V

IV

VI

Ionic materials (A + B~) display ion pair dissociation equilibria, and the value of the dissociation constant (Kd) varies according to the nature of A + , B", and the dissociating ability of solvents used—e.g., A+B"

^

A7/B-

intimate or contact ion pair

solvent separated ion pair

^

A + + Bfree ions

In some cases it is possible to estimate the fraction of solvent-separated ion pairs i n equilibrium with contact ion pairs and free ions (37, 43), but conductance measurements yield values for Kd only. For most cationic (32) and anionic (37) polymerizations it is now clear that the reactivity of free ions is several orders of magnitude greater than that of corresponding ion pairs. Consequently, it is necessary to know Kd to be able to interpret correctly the reactivity i n any ionic polymerization. Table I shows values of Kd for typical stable organic cations. This article reviews the experimental evidence, accumulated i n our laboratory, relating to different mechanisms involved i n reactions of trityl and tropylium ions with reactive olefins and cyclic ethers and outlines the probable effect of ion pair equilibria on these reactions. Polymerization

of Alkyl

Vinyl

Ethers

Initiation with Triphenylmethyl Cation. Triphenylmethyl (trityl) cation derives its stability (7, 30) from resonance between the electro-

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

20.

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319

Cations

philic carbon atom and the aryl substituents:

Ph3C+

etc.

This cation can be derived from many different precursors, and we have found the hexachloroantimonate (SbCl 6 ") to be the most stable of the crystalline salts toward hydrolysis and thermal decomposition. Conse quently P h 3 C + S b C l 6 " has been used throughout this work. Trityl ion effectively initiates polymerization of olefins having strongly electron-releasing substituents, especially vinyl alkyl ethers (2, 16), Table I. Ion-Pair Dissociation Constants (Kd)b for Hexachloroanti monate Salts of Stable Organic Cations in C H 2 C l 2 (34) Temp.,

10* K d ,

Ph 3 C + SbCl 6 -

25 0 -45

1.9 3.1 5.3

Ph 3 C + SbCl 6 -

25 -45

1.4° 2.9 α

C7H7+SbCl6-

0 -45

0.3 0.7

0

0.6 1.3

°C.

Salt

M

CH3 Ph—N—CH2Ph CH3 (C2H5)4N+SbCV

s b c v

-45 0

0.7 1.2

-45

Réf. 22. Kd was evaluated according to the procedure described by Szwarc (37). Conductance was measued under high vacuum with computerized evaluation of the data. α 6


320

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES

N-vinylcarbazole, and styrenes ( 9 , 19). Tritylhexachloroantimonate ( P h 3 C + S b C l 6 " ) dissolves readily in CH 2 C1 2 , and in this solvent it is a very effective initiator for polymerizing isobutyl vinyl ether. The polymerizations are completely homogeneous, and reaction rates have been measured by an adiabatic calorimetric technique (8). Initiation is rapid and essentially complete before significant conversion occurs. Termination appears not to be a complicating factor at low temperatures, and hence the rate measurements give an estimate of the propagation rate coefficient, kp, from the rate expression: -d( m o n o m e r )

dt

= kp [ m o n o m e r ] [Ph 3 C + SbCl 6 -]

where kp — 1.5 X l O ^ M 1 sec."1 at - 2 5 ° C . and Ea = 6 kcal./mole. These kinetic measurements were made with isobutyl vinyl ether in the concentration range 10"1 to 10" 2 M and with the concentration of catalyst, P h 3 C + S b C l 6 " , in the range 10"4 to 10" 6 M. The ion pair dissociation constant (Kd) for P h 3 C + S b C l 6 i n CH 2 C1 2 at - 2 5 ° C . was measured (Table I) and found to be X 10" 4 M. It follows, therefore, that for the concentrations of salt used as initiator, dissociation into free ions w i l l occur to at least 90% of the amount of salt added. Ion pair dissociation constants for the propagating species: — CH2

C H SbCV

II

i-Bu 0 would not be expected to differ by more than one order of magnitude from the values reported i n Table I for triphenylmethyl and carbononium salts, and therefore, polymerization of isobutyl vinyl ether initiated in this manner yields propagating species which are largely free cations. The value of kp, therefore, relates to the reaction of free cation with monomer. [The experimental data reported here were obtained in a high vacuum apparatus free from greased stopcocks. The solvent CH 2 C1 2 was used within 12 hours of final purification (under vacuum) because of deterioration caused by the glass surface.] Radiation-induced polymerization of isobutyl vinyl ether i n bulk leads to an estimate (42) for kp of 1 0 + 5 M _ 1 sec."1 at 30 ° C . with Ea = 6 kcal./mole. This value, extrapolated to the temperatures used i n the present work, yields an estimate of kp within one order of magnitude of that now reported. Considering the experimental problems i n these vastly different techniques, and the fact that different solvents are involved, such agreement is remarkable and provides support for the assumptions made to evaluate kp. Poly (vinyl isobutyl ether) obtained this way is the typical, low molecular weight, (MW = 2000-8000) amorphous material normally +


20.

Initiating

LEDwiTH

Polymerization

by

321

Cations

obtained by homogeneous polymerization. Extensive monomer transfer accounts for the low molecular weights and may be confirmed readily by crude column chromatographic fractionations of polymers prepared with high catalyst concentrations. A r y l end groups are readily detected (by infrared analysis ) i n some of the fractions but not i n others. Significantly, triphenylmethane can not be detected even by G L C analysis, and hence it must be concluded that initiation involves addition of trityl cation to the reactive olefin: P h 3 C + + C H 2 = C H O R -4 P h 3 C — C H 2 — C H = O R kpi

CH2=CHOR

P h 3 C — C H — C H — C H 2 — C H = O R etc. OR It is apparent from the kinetic measurements that ki is not significantly less than kp for vinyl isobutyl ether, as expected (12) from the greater +

thermodynamic stability of, for example, C H 3 C H O C H 2 C H 3 over P h 3 C + . As represented above the initiation of vinyl ether polymerization by trityl salts involves reaction of the olefin with the aliphatic carbon atom. It is worth noting, however, that Magee, Winstein, and Heck (18) have shown previously that trityl cation reacts with the related olefin ( C H 3 ) 2 G = = C ( O C H 3 ) 2 exclusively i n the 4-position of one of the aromatic rings: CH3 \

OCH /

+> - Η + c=c J

X-

/

CH3

O /=\

zzr \

PhoC

OCH3

H

C H

Ρ

ÇH*

S

OC P

O

H

a

3

x-

^

OCH3

c

CH„

; / C— C

/

H

c —c ;+

Η / H

C

Ο / +

CH,X

\

CH3 oCH3 Undoubtedly the latter reaction is one in which steric factors dominate, although the extent to which ion pairing is a contributing cause has not


322


been characterized. Reactivity of trityl cation i n the 4-position has also been reported by Kampmeier and his collaborators (23). Initiation with Tropylium Ion. Tropylium hexachlorantimonate reacts with vinyl alkyl ethers in a manner very similar to the reactions of triphenylmethyl salts. Again, rapid initiation is followed by propaga tion without apparent termination. Termination can be demonstrated to be absent from experiments in which fresh samples of monomer are added to completed polymerizations, whereupon the measured reaction rates parallel those previously recorded (Table I I ) . Molecular weights of the polymers from isobutyl vinyl ether are very similar to those obtained with triphenylmethyl salts as initiators and again give clear evidence for excessive monomer transfer. Gas chromatographic analysis of the reaction mixtures showed that cycloheptatriene (product of hydride abstraction) was not present which indicates clearly that initiation must arise via addition of the tropylium ion to the vinyl ether—i.e., R O C H = C H 2 + C 7 H 7 + -> C 7 H 7 — C H 2 — C H

*.

II

RO+

ROCH=CH2 > C7H7CH2—CH—CH2—CH

I

Κ

II

OR R O + etc. Russian workers (26) have demonstrated that tropylium ion w i l l add to vinyl ethers as indicated below, providing support for the proposed initiation reaction. CH3N02 C7H7+Br + CH2=CHOR > C7H7—CH2—CHBr OR H20 C 7 H 7 + C 1 0 4 - + C H 2 = C H O R ->

C7H7—CH2—CH+CKV OR OH

-(ROH) C7H7CH2CHO
80% yield by chromatography


326

ADDITION A N D

CONDENSATION

POLYMERIZATION

PROCESSES

on neutral alumina. These observations clearly demonstrate that hydride ion transfer (Reaction C ) is the predominant reaction when triphenyl methyl salts interact with T H F . The fate of the cyclic carboxonium ion:

Ο is less easy to define. One possibility is instant elimination to a dihydrofuran producing H C l and SbCls which could then function as initiating molecules: +

H C l + SbCL + 2 T H F

HO(CH2)4

H C l + SbCl,,

SbCV

This idea was first suggested by Dreyfuss et al. (15) but still requires confirmation for typical polymerization conditions. In recent months, we have studied in detail ( 41 ) the kinetics of the reaction between the colored triphenylmethyl salt and T H F at temperatures which are much too low for significant polymerization. Reactions were followed spectrophotometrically and demonstrate clearly that i n bulk T H F solvent the reaction between triphenylmethyl cation and monomeric T H F is rapid even as low as — 40 ° C . A clean first-order kinetic decay of [Ph3C+SbCl6"] was observed, and evaluated rate coefficients are shown i n Table I V . The enthalpy of activation for the process was 10 kcal./mole, and G L C analysis of the mixtures even at the lowest temperatures shows clearly that triphenylmethane is formed directly in the decoloration step. Spe cifically, the reaction between triphenylmethyl hexachlorantimonate and T H F i n pure T H F yields triphenylmethane directly even at temperatures as low as — 4 0 ° C , at which point there is little or no polymerization. Although formation of triphenylmethane is rapid at low temperatures, initiation of polymerization is somewhat slower, as indicated by the slight initial curvature of typical kinetic curves at temperatures above ambient, and more dramatically by Kuntz and Melchior (25), who demonstrated significant cocatalysis by small amounts of epoxides. Cocatalysis of the polymerization of T H F using trityl hexachloroantimonate was effected


20.

Initiating

LEDWiTH

Polymerization

by

327

Cations

conveniently by using propylene oxide, and the cocatalytic effect de creases rapidly with increasing temperature. Additionally, cocatalysis by propylene oxide eliminates the dark colors which are normally observed Table I V .

Reaction of P h 3 C S b C l 6 ° w i t h Pure T H F 6 103k

Temp., °C.

-50 -40 -33 -25

(observed)"'e sec.'1

0.79 ± 0.08 2.0 ± 0.2 4.17 ± 0.08 7.8 ± 1.0

Apparent first-order rate coefficients averaged over many kinetic runs in which [Ph 3 C + SbCl 6 -] varied between 10~4 and 10" 3 M. Bulk THF = 12.3M at 25°C. c Reaction rates were followed spectrophotometrically by observing the disappearance of absorbtion caused by Ph 3 C + at 412 nm.

a

6

during the early stages of trityl initiated polymerizations. Kuntz and Melchior (25) suggest, therefore, that propylene oxide functions by reacting directly with trityl ion to give a reactive cyclic oxonium ion i n a manner similar to that suggested as initiation reaction B.

PhHC+SbCle(THF) +RlR1,C

V

CH2 ~

+/

» Ph 3 C—Ο

CRaR

\CH.,

THF

SbCl r (

O—CH2C — OCPh, R2 SbCLf Evidence for this suggestion was obtained by N M R spectroscopy of reacting mixtures which showed that triphenylmethane formation was almost eliminated and a trityl ether unit formed when the reaction systems contained propylene oxide. While this interpretation is con sistent with most of the experimental work published, Kuntz and M e l chior (25) also report that cocatalysis by propylene oxide can be ob served when the latter is added several hours after the start of polymeriza-


328

ADDITION A N D C O N D E N S A T I O N

POLYMERIZATION

PROCESSES

tion. Consequently, it is likely that the true "dormant" species i n these polymerizations has not yet been characterized. Dynamic equilibrium between trityl cation and T H F (Reaction B ) could not be demonstrated i n pure T H F as solvent (41) but has been characterized successfully (10) i n methylene chloride solutions (Table V ) , for T H F , tetrahydropyran, and 1,4-dioxane. Table V.

Association Constants (Ke) for Oxonoium Ion Equilibria of Ph 3 CSbCl 6 ~ in C H 2 C l 2 at 2 5 . 0 ° C . Κ Ether

α

M'

b

1

Tetrahydrofuran Tetrahydrofuran 1,4-Dioxane

3.7 2.0 0.5

a Ke = [Oxonium ion] J [Ph 3 C + ] e [Ether]. where [Ph 3 C + ]« represents the corrected value for free ion obtained using Kd from Table I. (Initially [Ph 3 C + SbCl e -] 2.5 X

10-4M).

6

[Ether] varied in the range 0.3 to 1.0M.

Dioxane gave the most stable system, and with this ether it was possible to demonstrate dynamic equilibrium by raising and lowering the temperature (Table V I ) . In addition low temperature quenching ( H 2 0 ) of a Ph 3 C + SbCle"-dioxane system i n C H 2 C 1 2 , such that > 90% of the trityl content was i n the form of colorless oxonium ion:

PhoC—Ο

Ο

gave a quantitative recovery of all the trityl content as triphenylcarbinol. This result shows that hydride ion transfer (forming P h 3 C H ) was not significant under the reaction conditions and confirms the proposed oxonium ion equilibrium. For all three cyclic ethers there was a concurrent slow decay of the equilibrium trityl ion concentration leading to formation of triphenyl methane Reaction C ). Thus, i n C H 2 C 1 2 , Reactions Β and C occur simul taneously whereas only C is significant i n pure T H F . Since for 10" 4 M solution of P h 3 C + S b C l 6 " , the salt is > 80% dissociated into free ions i n C H 2 C 1 2 and almost completely associated as ion pairs i n pure T H F , oxonium ion equilibria involving trityl cation are important only for the free ion. The ion pair reacts preferentially as a hydride abstracting re agent—i.e.,


20.

Initiating

LEDwiTH

Polymerization

by

329

Cations

PhaC—(/

+ SbCV

THF (K ) e

Ph3C+SbCV

Ph8C+

+ SbCl H "

free ion ion pair THF

SbCV

Ph3CH +

Decay of P h 3 C + (i.e., formation of P h 3 C H ) follows the rate expression -d[Ph 3 C+] at

:fc 2 [Ph 3 C + SbCl 6 -] [ T H F ]

for both C H 2 C 1 2 and T H F solvents. Extrapolated values of k at 25 ° C . are 21 X l O ^ M " 1 sec."1 i n pure T H F and 6 ± 3 X l O ^ M 1 sec."1 i n C H 2 C 1 2 with [ T H F ] = 0.4M. Allowing for the probable errors involved in the extrapolation and the fact that two different solvents are involved, it must be concluded that free trityl ion is much less reactive than its ion pair i n hydride abstraction from T H F . Clearly, this has important 2

Table VI. Variation in Absorbance with Temperature for the Equilibrium between Ph 3 C + SbCl 6 " and 1,4-Dioxane (0.31M) in C H 2 C l 2 Temp., °C.

Optical Density (OD)e

25 25 0 25 -35 25

2.040 b 1.890 1.660 1.895 1.195 1.780

10* [Total trityl ion] e M"

2.55 6 2.36 2.16 2.37 1.38 2.22

[Total trityl ion] e = [Ph 3 C + ] e + [Ph 3 C + SbCl 6 "] e, values are corrected for density change and quoted as at 25°C. The sealed system was cooled and reheated in the sequence shown descending thefirstcolumn. h Before adding dioxane. a


330

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

consequences for considerations of initiator efficiency and cocatalysis in polymerization of cyclic ethers. Initiation with Tropylium Ion. W h e n cycloheptatrienyl hexachlorantimonate is used as initiator for tetrahydrofuran polymerization, the reactions are somewhat cleaner, and strong colors do not develop as readily as when the corresponding trityl salts are used (17). Rates of initiation are much lower, and the reaction is hardly noticeable at room temperature. However, at 50 ° C . and above initiation is significant, and the polymerizations proceed almost to the expected theoretical conversion of monomer to polymer even when hexachlorantimonate is the anion (Table III). Therefore, the apparent low equilibrium conversion obtained with the rapidly initiating trityl salts is minimized in this case by the comparatively low rate of consumption of initiator. Once again G L C demonstrates clearly that the initiation reaction involves primarily hydride abstraction from the ether.

Cycloheptatriene ( C 7 H 8 ) is detected readily by G L C , but as before the fate of the cyclic carboxonium ion remains enigmatic. Nevertheless, polymerization of T H F proceeds smoothly and reproducibly especially at temperatures above 50 ° C . Other workers (36) have used triphenylmethyl salts to polymerize trioxane, and the general conclusion remains that hydride abstraction is a primary initiation process, although i n all cases there is no clear evidence for the fate of carboxonium ion formed.

Polymerization

of

N-Vinylcarbazole

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. W e recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylenediamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that oneelectron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical ( V I I ) .


20.

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Initiating

Polymerization

by Cations

331

2.

VII

Table VII.

Temp., °C.

0

-25

[Cat.'] 106M

Polymerization of N-Vinylcarbazole by C 7 H 7 S b C l 6 in C H 2 C l 2 [NVC], 102M

Rate(Rv), M sec. 1

10'

5

v

M " 1 sec.'1

Viscosity Av. Mol.Wt.b (χ 10- ) 5

0.987 0.987 0.750 1.00 1.00

5.39 5.40 5.40 2.70 4.04

2.60 2.42 1.97 1.15 1.87

4.88 4.53 4.87 4.27 4.63

2.59 1.68 1.92 1.38 1.85

2.00 1.75 1.50 1.00

5.40 5.39 5.40 5.39

1.42 1.90 1.09 0.703

1.31 2.01 1.35 1.30

10.2 9.00 8.83 8.57

Calculated from the rate expression RP = fcP[NVC] [C 7 H 7 + SbCl6"]. b Estimated from measurements of intrinsic viscosity in benzene at 25°C.

(38).

This i n no way detracts from the proved ability of the cyclohepta trienyl cation to oxidize reactive amines, and consequently there is always doubt as to the true nature of the reaction mechanism when tropylium ion initiates polymerization of 2V-vinylcarbazole. W e have made a de tailed kinetic study of this polymerization (34), using an adiabatic calorimetric technique. Some typical data are shown i n Table V I I . Initiation is instantaneous and complete, there is no termination, and k is evaluated readily as 4.6 X 1 0 + 5 M _ 1 sec. 1 at 0 ° C . with E = 6 kcal./mole. B y comparing data for the ion pair dissociation constant of C 7 H 7 " S b C l " c (Table I) with the catalyst concentrations employed (Table V I I ) it is apparent that free tropylium ions are the dominant initiating speices. It p

a


332

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES

is also probable that free immonium ions ( V I I I ) are the propagating entities—i.e.,

C7H7+

+

C7H7CH2

CH

CH2

CH

etc.

VIII Initiation is represented as simple addition of tropylium ion to the mono mer double bond by analogy with reactions of alkyl vinyl ethers. The value of k now reported for propagation by free cation is ap proximately five orders of magnitude greater than that for the corre sponding free radical reaction in T H F (20). Similar differences have already been noted for free cation (42) and free anion (37) propagation of styrene. p

Use of stable organic cations to initiate cationic polymerization allows characterization of free ions and ion pairs as intermediates and in some cases facilitates measurement of their respective absolute re activities. Further work is in progress to extend the range of catalysts and monomers which may react i n this way and therefore to extend our knowledge of the absolute reactivity in cationic polymerization. Acknowledgment

The studies described here were carried out i n the Donnan Labora tories, University of Liverpool with joint supervision by C . Ε . H . Bawn, C. Β. E., F. R. S. Contributions by R. M . Bell, C . Fitzsimmonds, J. Weightman, G . Cowell, D . Sherrington, J. Penfold, P. Bowyer, D . H . Iles, and P. Beresford are gratefully acknowledged. The author also thanks I. Kuntz for an advance copy of his manuscript (25).


20.

LEDWITH

Initiating Polymerization by Cations

333

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(40) Vofsi, D., Tobplsky, Α. V., J. Polymer Sci. A3, 3261 (1965). (41) Weightman, J. Α., Ph.D. Thesis, University of Liverpool, 1968. (42) Williams, F., Hayashi, K., Ueno, K., Hayashi, K., Okamura, S., Trans. Faraday Soc. 63, 1501 (1967). (43) Winstein, S., Robinson, G. C., J. Am. Chem. Soc. 80, 169 (1958). (44) Winstein, S., Appel, Β., Baker, R., Diaz, Α., Chem. Soc. (London), Spec. Publ. 19, 109 (1965). RECEIVED

April 14, 1969.


Mechanisms for Initiating Polymerization by Stable Organic Cations

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