Characteristics of the one-component catalysts M(CO)3X2L2 (M = Mo or W, L = PPh3 or AsPh3, X = Cl OR Br) in the ring-opening polymerization of norbornene

Journal

of Molecular

Catalysis,

28 (1985)

369

- 380

369

CHARACTERISTICS OF THE ONE-COMPONENT CATALYSTS M(C0)3X2L2 (M = MO OR W, L = PPh3 OR AsPh3, X = Cl OR Br) IN THE RING-OPENING POLYMERIZATION OF NORBORNENE LAJOS

BENCZE*

and ANNA

KRAUT-VASS

Institute of Hydrocarbon and Coal Processing, Engineering, H-8201 Veszpre’m Pf. 158 (Hungary)

Veszpre’m

University

of

Chemical

Summary M(C0)3X2L2 complexes, (where M = MO or W, X = Cl or Br, L = PPhJ or AsPh3) initiate the ring-opening polymerization of norbornene and norbornadiene. Of these one-component catalysts, tungsten derivatives are generally superior to those of molybdenum. Phosphine complexes are poor catalysts, but W(C0)3X2(AsPh3), compounds exhibit fair catalytic activity at 353 K. The catalytic activity is retained in the presence of air and moisture. The polymers produced from norbornene are moderately high-& structure and somewhat blocky with respect to cis/truns distribution. The solvent generally has little effect on the polymer yield. However there was no polymerization in nitrobenzene, a large quantity of ethyl acrylate quenched the reaction and in CC4 solutions saturated chlorinated telomers were formed exclusively. Steric crowding at the metal centre caused by coordinating molecules, such as CO, alcohol, or even olefins, generally results in increased blockiness for the polymer. A non-cage-like ‘three-ligand sequence’, i.e. the simultaneous involvment of at least three coordination sites in the coordination sphere of the propagating species, is proposed to explain the experimental results. The catalysts are surprisingly tolerant towards alcohol and water. Both additives increase the cis/truns selectivity of the catalysts and the blockiness of the polymer. However, the addition of EtOH results in high-cis polymer with the main effect in rC, while water results in high-truns polymer with the main effect in rt .

Introduction The complexes M(C0)3X2L2 (M = MO or W, X = Cl or Br, L = PPh3, AsPh3) have been used as precursors for olefin metathesis catalysts in combination with aluminium alkyls since 1971 [ 11.

*Author

to whom

0304-5102/85/$3.30

correspondence

should

be addressed.

@ Elsevier

Sequoia/Printed

in The Netherlands

370

It has recently been reported that these complexes initiate the metathesis of linear olefins, when supported on alumina or when in solution without the use of organometallic co-catalysts [2]. Besides the theoretical interest, these observations may also have some practical importance: (a) the catalysts can be conveniently handled, being insensitive to air and moisture; (b) side reactions such as alkylation, isomerization and oligomerization, usually caused by the acidic co-catalysts, can be avoided. The applicability of these catalysts has already been demonstrated by the preparation of some polymer samples for i3C NMR model studies [ 31. In this paper we report the results of our preliminary investigations on the effect of solvents, water, air, alcohol, impurities of the reactants and the composition of the catalysts on the yield and structure for ring-opened polymers of norbornene made by these catalysts.

Results Bulk polymerization

The bulk polymerization of norbornene (NBE) and norbornadiene (NBDE) may be initiated by W(C0)sC12(AsPh3)2 (I) at room temperature. The swollen polymer covering the surface of the catalyst crystals soon slows down the polymerization. On addition of alcohol, the solid polymer precipitates while the excess monomer and catalyst residues remain dissolved in the liquid phase. The yield is mainly determined by gelation, and after a reasonable reaction time (0.1 - 8 h) does not generally exceed 2 - 5%. The r3C NMR spectra of norbornene polymers in DCCls exhibit the characteristic features of a poly(cyclopentylene vinylene) [4] composed of ring-opened norbornene units (Fig. l(A)). The cis configuration of the double bonds is in slight excess in the polymer (u, = 0.57 - 0.6). The distribution of the cis and trans double-bond-containing units along the chain is somewhat blocky, i.e. the cis units are followed by cis units more frequently than anticipated for the case of random distribution. In general, the same is true for the trans double-bond-containing units. Polymers obtained from NBDE at 293 K or 353 K swelled in DCC13 but did not dissolve, possibly indicating crosslinking. Although their spectra are of poor quality, cyclopentenylene vinylene units [5] (Fig. l(B)) may easily be detected from them.

(A) Fig. 1. Ring-opened

(6) units of norbornene

(A) and norbornadiene

(B).

371

Solution polymerization Polymerization of NBE in benzene at 353 K initiated with I proceeds at an acceptable rate. The rate of polymerization (R, = 1.2 f 0.1 X 10e4 mol dmp3 s-l) is essentially independent of the monomer concentration in the range [NBE] = 0.25 - 3.0 mol dmp3. There is a slight tendency for increasing cis content (u, = 0.52 - 0.58) with increasing monomer concentration, while the increase in blockiness [5] is much more definite (r,rt = 1.1 - 3.6 + 0.1). The u, and R, values are surprisingly close to those published by Rooney et al. [6] for the polymerization of 5,5dimethylnorbornene initiated by the Fischer carbene Ph(MeO)C=W(CO)s. The rate of polymerization becomes diffusion controlled above a 2 - 5% polymer content for the reaction mixture, and gelation sets a practical limit to the conversion (Fig. 2). Yvsldl.

I

L

0

12

3

4

5

6

7

0

9

101NBEl,mol/dm3

Fig. 2. Effect of initial monomer concentration on polynorbornene (AsPh,),] = 0.7 - 9.9 X 10e3 mol drnp3 at 353 K in benzene. Reaction

yield. [W(CO)&12time, 20 - 60 min.

The molecular weight of the polymers is not very high (8, = 5000 10 000) suggesting that the rate of chain transfer plus chain termination is comparable to that of propagation (l/50 - l/100). When linear chain olefins are used as chain-transfer agents gelation can be avoided and near-quantitative conversion of norbornene attained [ 71. Benzene is an acceptable solvent for this polymerization: it dissolves the catalyst and the polymer, and is easy to remove from the product (Table 1). The yield in toluene is somewhat smaller. The solubility of the polymer in chlorobenzene is relatively small with the swollen polymer beads separating from the liquid phase at an early stage in the reaction. There was no polymerization at all in nitrobenzene. Aliphatic hydrocarbons such as octane do not dissolve the catalyst. Polymerization occurs, however, and at higher temperatures the yield is quite good.

312 TABLE Effect

1 of solvents

on yield of norbornene

polymers

a

Solvent

Yield (%)

Product

benzene toluene chlorobenzene noctane n-octane b carbon tetrachloride nitrobenzene ethyl acrylate

42 29 24 19 53

poly(cyclopentylene poly(cyclopentylene poly(cyclopentylene poly(cyclopentylene poly(cyclopentylene saturated telomers -

0 0

a [NBE] = 0.46 mol dmp3; 20 min; temp., 353 K. bReaction temp., 398 K.

[W(CO)sCl,(AsPh&]

= 0.25

x

10m3 mol dme3;reaction

vinylene) vinylene) vinylene) vinylene) vinylene)

time,

Of the various solvents tested, carbon tetrachloride appeared as one of the extremes. The conversion of the monomer in Ccl4 is very high, but the yield of solid polymer is not more than 10%. The polymer is soluble in DCCls but its double-bond content is practically zero. The majority of the products in this case are soluble in benzene/alcohol and consist mainly of 2-chloro-3-trichloromethylbicyclo [ 2.2.llheptane and its analogues. Composition of the catalysts In binary homogeneous catalyst systems the activity of tungsten compounds is generally high, while that of molybdenum compounds is just about acceptable. This is true in the cases studied here as well. The activity of molybdenum carbonyl halide complexes was at least an order of magnitude lower than that of the analogous tungsten derivatives (Table 2). The effect of the ligands L was also very significant. During the initial stages of our studies, complex I was chosen as the catalyst simply because it is not susceptible to easy decarbonylation as are phosphine derivatives [ 8, 91, i.e. WC0)3CMPPh,),

-

W(C0)2C12(PPh3)2 + CO

(1)

and for this reason a stable catalyst composition could be maintained. As can be seen from Table 2, this choice was fortunate since the activity of the triphenylarsine complex is more than an order of magnitude higher than that of the triphenylphosphine derivative. Such a great and significant difference had not been observed in heterogeneous catalytic studies [ 21. Of ligands associated with transition metals, CO seems to play a vital role. While the exchange of other ligands makes a greater or less significant alteration in the activity, the removal of carbonyl ligands destroys it entirely. Although the isoelectronic M(N0)2C12L, [lo, 111 or MC1,L2 [l] complexes are excellent precursors of two-component metathesis catalysts in combination with EtAlCl,, they are inactive as onecomponent catalysts for the polymerization of norbornene under the conditions employed in this

313 TABLE Effect

2 of catalyst

composition

on yield of ring+pened

Catalyst

Reaction

norbornene

polymers

Yield

time (min)

[catalyst]

= 0.25

(%)

0.5 0 2 4 51 19 0 0 0 0 0

20 20 20 23 20 9 120 120 120 120 120 a [NBE] = 0.26 mol dmd3; K; solvent, benzene.

a

x

10m3 mol dme3;

reaction

temperature,

353

study. However, though the presence of CO in the coordination sphere is a necessary condition for catalytic activity, it is not sufficient by itself to guarantee such behaviour: thus W(CO)6 does not initiate the ring-opening polymerization of norbornene.

Common catalyst ‘poisons’ Besides their activity, the other important characteristic of catalysts from the application point of view is their sensitivity towards the usual impurities present in the feed and permanent or accidental poisoning by air and/or moisture. Thus small quantities of water and oxygen are known to behave as activators in multicomponent catalyst systems, but in large quantities they usually quench the reaction. The reaction mixture I/NBE/benzene is not particularly sensitive to such poisons. Under the conditions studied the solution becomes saturated with water at a W/H20 molar ratio of ca. 1:15 which brings about a 20 - 25% fall in the polymer yield (Table 3). On addition of further portions of water the reproducibility of the experiments and the yield of polymer decrease, but the effect on the yield is not proportional to the amount of water added. It should be added that in these experiments the excess water was present as a separate phase and that under these conditions the influence of many uncontrolled variables, e.g. the rate of stirring, becomes significant. While the yield did not vary very much, there was a considerable change in the properties of the polymer (Table 4). Thus a, dropped to one third of the value of the control and the polymeric product was a soft, plastic material even under solvent-free conditions. It was freely soluble in benzene, toluene and chloroform, and the presence of water also caused a decrease in the cis content to cc = 0.39 although the blockiness of the polymer increased, the latter being effected entirely through rt .

374 TABLE Effect

3 of water and air on the yield of polynorbornene

Run

Water added

3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 3-11 3-12 3-13 3-14 3-15 3-16

0 0 0 2 8 16 40 50 60 80 100 110 140 160 Ob Ob

3-17 3-18 3-19

(~1)

Reaction

time (min)

55 53 48 39 39 39 34 34 25 25 25 16 0 0 57 55 50 33 29

25 20 15 20 20 20 20 20 20 20 20 20 20 20 20 15 15 25 20

;: Oe

Yield f f f + * f f + f f f

(%) 2 2 2 2 2 2 5 5 10 10 10

NBE/W/HzO 1OO:l:O 1OO:l:O 1OO:l :o 100:1:4.3 100:1:17 100:1:34 100:1:85.5 100:1:107 100:1:128 100:1:171 100:1:214 100:1:235 100:1:299 100:1:342 lOO:l:lOO:l:100:1 :lOO:l:lOO:l:-

aReactions were carried out using 250 mg NBE and 25 mg W(CO)$&(AsPh3)2 benzene at 353 K. bAir (2 ml) added. ’ At 1 .l bar air pressure. din air; benzene used as supplied. e [ NBE] distilled from Na/K alloy and stored under argon for weeks.

TABLE

mole ratio

in 10 ml

4

Effect of water and air on polynorbornene distribution are given in brackets) Run

Additive

3-2 3-11 3-17 3-18

none water air air

structurea

(calculated

rt 0.54 0.39 0.55 0.57

0.98 0.9 1.26 1.37

(1.17) (0.64) (1.22) (1.32)

1.15 1.91 1.50 1.06

values for Bernoullian

li;i,b

rcrt (0.85) (1.5) (0.8) (0.75)

1.12 1.72 1.89 1.45

(1) (1) (1) (1)

4950 1520 4860 -

a For reaction conditions, see Table 3. bVapour pressure determination in CHC13 solution.

While the effect of air on the polymer yield and on u, is small, that on the blockiness is significant. Polymerizations may be carried out in air and in unpurified benzene, but in this case the yield drops by 40% compared to the control. Storage of norbornene in a closed glass vessel under dry argon for

315

several days after distillation from Na/K alloy sometimes caused a drop of cu. 40 - 50% in the polymer yield but the reason for this is not clear. It would be anticipitated that alcohol would exhibit a smaller poisoning effect compared to water because of its lower acidity. The addition of small quantities of alcohol does not have much influence either on the yield or on the structure of the polymer (Table 5). The decrease in the yield was not greater than 50% even at a W/EtOH mole ratio of 1:400, but the cis content (u,) and the blockiness of the polymer are greatly increased. The latter was mainly effected through an increase in rc. In contrast to the effect of water and air, the effect of alcohol on the polymer properties is remarkable. TABLE 5 Effect of alcohol on polynorbornene yield and structurea noullian distribution are given in brackets)

(calculated

values for Ber-

NBE/W/EtOH mole ratio

Yield (%)

u,

rc

rt

rCrt

500:1:500:1:0.8 500:1:400

30 25 15

0.53 0.54 0.71

1.17 (1.13) 1.52 (1.17) 3.68 (2.4)

1.43 (0.89) 1.88 (0.85) 0.98 (0.4)

1.67 (1) 2.85 (1) 3.61 (1)

a [NBE] = 0.5 mol dme3; reaction time, 60 min; reaction temp., 353 K.

The yield of the polymer was not influenced to any great extent by the presence of gaseous carbon monoxide. Polymerization under 20 bar pressure of CO at 353 K with an initial concentration of NBE equal to 1 mol dm-3 in benzene initiated by I in a glass autoclave resulted in a 40% yield of polynorbornene. The value of uc at 0.50 is somewhat lower, while that of rcrt at 3.14 is higher, than that expected for this initial monomer concentration.

Discussion M(C0)3XzLz complexes initiate the metathesis of norbornene and norbornadiene. Of these onecomponent catalysts, tungsten derivatives are generally superior to those of molybdenum. The compounds W(CO)3X2(AsPh& (X = Cl or Br) demonstrated fair catalytic activity, the polymers produced from norbornene being moderately high-cis in content. The catalytic activity of the systems is retained in the presence of air and moisture. The mechanism of initiation is not discussed here. However, the relationship between W(CO)J12 (II) [ 121 and W(CO)3ClZLZ as catalysts appears to be obvious, since the link between their coordination characteristics and their chemistry has been known for a long time. The heat- and moisture-sensitive dimeric tetracarbonyl halide complexes are easily transformed in solution into a monomeric form and on addition of donor molecules (L) air-stable complexes are formed [ 81 (eqn. (2)):

376 acetone

[W(CO)4W*

W(CO)4Cl,

-

4,

W(CO)&l~L~ + co (1)

(11) (L = PPh3, AsPh,,

(2)

py, . . . etc.)

The similarity in catalytic properties between I and II is well demonstrated by their intriguing behaviour in Ccl4 solution. Thus Agapiou and McNelis [ 121 noted that the thermal activation of II in the presence of olefins gave only chlorinated products in Ccl, as a solvent. Our results are similar. When NBE was reacted with a catalytic amount of I in Ccl4 solution at 353 K, the olefin was converted to saturated products which were composed mostly of 2-chloro-3trichloromethylbicyclo[ 2.2.llheptane and its analogues (eqn. (3)): w(C0)3Clz(~Ph& 0-

*

cc14

!Q!

Cl

(n =

n

(3)

CC13

1, 2, 3

. . etc.)

This is a typical product of the peroxide-catalyzed free-radical telomerization. Whether or not this transition metal catalyzed telomerization of NBE is a genuine free-radical process has yet to be clarified. The higher activity of II in comparison to I in the metathesis of linear chain olefins might be attributed to the smaller electron density on the central atom in II and its higher susceptibility to substitution reactions. The importance of the tendency towards ligand exchange is very definitely demonstrated in the relative activity of phosphine- and amine-substituted complexes. We know from other observations [13] that equilibrium (4) exists in solution: w(co)3x2L2

-

W(CO)3XzL + L

The equilibrium constant than for the triphenylphosphine The ratedetermining step followed by coordination of the W(CO)3Cl2L + NBE ti

(4)

(KA) in case of L = AsPh, is much greater analogue (Kr): KA/KP = 20 [131. in the initiation is assumed to be reaction (4) monomer (eqn. (5)):

W(C0)3C12L(NBE)

(5)

Coordination of the olefin could also be readily envisaged in complexes of type I following their decarbonylation in reaction (1). This reaction may occur, but cannot be a ratedetermining step because (a) molybdenum complexes and triphenylphosphine derivatives are more active in reaction (1) than tungsten and triphenylarsine analogues, and (b) the presence of excess CO does not retard the rate of polymerization. However, there is another kind of CO influence on propagation; thus the blockiness of the polymer increases under CO pressure. This is not a unique effect associated with CO, since higher olefin concentration or the

presence of alcohol result in increased (T, and rCrt values as well. This phenomenon may be explained by the so-called ‘three-ligand sequence’ [ 141 (Fig. 3).

Fig. 3. Reaction

scheme

for the ‘quasi-three-ligand’

sequence.

In the absence of donor molecules (D) and at low olefin concentrations the polymerization occurs through a ‘quasi-three-ligand’ propagating species (1) in which the stereochemical relaxation of the chain-carrying carbenes, effected through the mobility of the ‘permanent’ ligands CO, Cl, etc. (but not through the rotation of the carbenes around the W=C axis) is relatively fast. For this reason the cis/truns distribution of the polymer is near to random. At increasing monomer concentration the available coordination sites become gradually more saturated (structure 4 in Fig. 4); relaxation of the propagating species is now more hindered, and the cis/truns blockiness of the polymer increases. The third coordination site may also be blocked by CO, EtOH, or even other olefins [ 71, resulting in smaller relaxation opportunities and higher cis/trans blockiness (structure 5 in Fig. 4). In Figs. 3 and 4 the symbol q depicts a real or practical ‘vacancy’, meaning a structure corresponding to 5-/4-coordinated geometry for the active species, or a coordination site occupied by a weakly bonded solvent molecule in an octahedral structure. Structures 1 -5 in this discussion are by no means exhaustive as far as the symmetry possibilities in which the intermediate metallacycle may form are involved. When the relative orientations of the carbene and the monomer(s) are considered in addition to the three permanent ligands (X, L, CO), such possibilities are numerous.

378

3 8

8

lC2

NBE-W

II/

‘I (5(5’))

-D

&

I

.,J:_,

H

@\ /H LF!L NBE-S/NBL ‘I

‘I

(4)

(3’) Fig. 4. Reaction scheme for the non-cage-like olefinic coordinating molecules).

‘three-ligand’

sequence

(D denotes

non-

What we propose here is closely related to Calderon’s ‘three-ligand sequence’ theory involving the simultaneous coordination of the carbene, monomer and the double bond(s) of the polymer to the transition metal centre [ 141. These ‘cage-like’ complexes are most probably the active species of &directing catalysts. The simultaneous involvement of at least three coordination sites in the metathesis reaction is also supported by our experimental facts. As proposed by Ivin et al. [ 151, steric crowding at the metal centre by the ligands alters the polymer structure. However, we have found that within the conditions studied steric crowding may be caused by olefins as well as by other ligands. This effect is mainly demonstrated by the rtrc values and more modestly by the cis selectivity as might be expected at the reaction temperature (353 K). The effect of alcohol on u, is surprisingly large in comparison to nonprotic ligands such as CO. The increased blockiness is effected entirely through rC, suggesting the presence of an additional driving force towards the formation of cis double-bond units. The coordination of the alcohol to the metal also creates a new function in the system. The coordinated alcohol is not liable to deprotonation in the non-polar medium [ 161 but the formation of hydrogen bonds is possible. The double bond of the polymer chain which is ultimately formed may be coordinated to the metal centre through an ---H-O---W bridge (Fig. 5). In this way the growing chain becomes a bidentate ligand which is assumed to be responsible for the increased cis selectivity of the catalyst. The influence of the acidity of the OH group and the bulkiness of the alkyl group in the alcohol on the cis-directing activity has yet to be tested. A seemingly slight modification of the catalyst, where water is employed in place of ethanol, leads to a dramatic change in the stereoselectivity. The resulting polymer mainly possesses a tram structure and has an increased r,r, value with the main effect in rt . The formation of HCl and H-W derivatives may now be expected in the system. The predominantly tram structure of the polymer may be attributed to a fast hydrogen transfer between the metal and the a-carbon atom resulting in ready rotation around

379

Fig. 5. Possible 71-H interactions norbornene polymers.

responsible

for the increasing

the W-C o-bond axis or to some secondary are in progress to test these ideas.

reaction.

rc, rcrt and (5, values for

Further

experiments

Experimental The hydrocarbons employed in this study were dried and deoxygenated by refluxing over Na/K alloy-benzophenone and were distilled immediately before use. Chlorinated solvents and other polar reactants were distilled from CaH, and stored over 4 A molecular sieves under dry argon. All manipulations were carried out in a dry argon atmosphere unless otherwise stated. The transition metal complexes were prepared as previously described [8, 17, 181. The reactions were initiated by commencing the circulation of preheated water or paraffin oil in the heating mantle of the all-glass reaction vessel. The reactions were quenched thorough the addition of a large amount of cool ethanol. The polymers were washed with ethanol and dried in uacuo. Telomers were identified by GC-MS analysis. 13C NMR spectra were obtained as described in ref. 4 but with a reduced spectral window (4000 Hz). The calculation of u,., r, and rt values was by the method of Ivin et al. [ 51, based on C* (Fig. 1) values. Correction for possible overlap of peaks was not attempted, because the errors were likely to be systematic.

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L. R. B. E.

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(1970)

528.

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11 W. B. Hughes, J. Am. Chem. Sot., 92 (1970) 532. 12 A. Agapiou and E. McNelis, J. Organometall. Chem., 99 (1975) C47. 13 Z. T&h, F. Jo& A. Kraut-Vass and L. Bencze, unpublished results. 14 N. Calderon, J. P. Lawrence and E. A. Ofstead, Adu. Organometall. Chem., I7 (1979) 449. 15 K. J. Ivin, D. T. Laverty, J. H. O’Donnell, J. J. Rooney and C. D. Stewart, Makromol. Chem., 180 (1979) 1989. 16 L. Bencze, B. Mohai and J. Kohin, Acta Chim. Acad. Sci. Hung., 113 (1983) 183. 17 L. Bencze, J. Organometall. Chem., 56 (1973) 303. 18 M. W. Anker, R. Colton and I. B. Tomkins,Aust. J. Chem., 21 (1976) 9.