JOURNAL OF POLYMER SCIENCE: PART A-1 VOL. 10,2609-2620 (1972)

Ethylene Polymerization with Supported Bis( Triphenylsilyl) Chromate Catalysts

WAYNE L. CARRICK, ROBERT J. TURBETT, FREDERICK J. KAROL, GEORGE L. KARAPINKA, ADRIAN S. FOX, and ROBERT N. JOHNSON, Resmrch and Development Department, Union Carbide

Corporatirm, Chemicals and Plastics, Bound Brook, New Jersey 08806

SynOpEiS

Bis(triphenylsily1) chromate is an active catalyst for ethylene polymerization with- out further treatment or additives. Catalytic activity is markedly increased when the compound is deposited on silica-alumina and is further increased if it is deposited on silica and then treated with an aluminum alkyl. Polymer molecular weight can be controlled by reaction temperature, hydrogen addition, support type, and reducing agent structure to give polymers ranging in melt index from essentially zero to >loo. In the supported catalysts the bis(triphenylsily1) chromate appears to be bound to the support and to undergo a reduction step either by reaction with ethylene or with alu- minum slkyl prior to polymerization. The active site is envisioned as chromium alkyl, bound to the support, with propagation occurring by insertion of the monomer into a Cr-C bond. Chain termination is by chain transfer to monomer.

INTRODUCTION

Bis(triphenylsiiy1) chromate is a red crystalline solid that is easily pre- pared from triphenylsilanol and chromium trioxide.'S2 The fact that it oxidatively cleaves olefins and initiates polymerization of ethylene at high pressure, without cocatalyst, has been previously reported.2 The present work extends these early results and describes the preparation of a family of highly efficient polyolefin catalysts based on bis(triphenylsily1) ~hromate.~,'

EXPERIMENTAL

Materials

All supports were obtained from Davison Chemical Company, Division of W. R. Grace, and were received as white sands.

Ethylene waa Phillips (polymerization grade), hexane was a pure grade with minimal polar impurities, nitrogen was Linde high-purity, and all were dried by passage through Linde Molecular Sieves prior to use. Aluminum alkyls were obtained from Texas Alkyls and were used without further treatment.

2609

@ 1972 by John Wiley & Sons, Inc.

2610 CARRICK ET AL.

Bis(triphenyIdy1) Chromate

This compound was prepared by the reaction of chromium trioxide and triphenylsilanol in carbon tetrachloride medium.2 The product was re- covered from the solvent by evaporation and recrystallized from hot heptane, mp 153-155°C.

Catalyst Preparation

Catalyst supports were dehydrated by fluidization in a 2-ft section of 1- in. diameter Vycor tubing mounted vertically in an electric furnace. Dry nitrogen was passed through the support bed,at a sufFicient rate to insure fluidization at the predetermined temperature (110-80O0C), usually over- night. Aliquots were removed volumetrically into a receiver, without ex- posure to air, and the precise quantity of support was determined by weight difference of the receiver.

A representative catalyst consisted of 0.4 g support (SiOz), 10 mg bis- (triphenylsilyl) chromate and, where applicable, dialkyl aluminum alkoxide (0.24 mmole). Considerable variation in ratio of reagents is permissible. The components were mixed in dry hexane with addition usually in the order (1) silyl chromate, (2) support, and (3) aluminum alkyl. The catalyst was mixed in approximately 75-100 ml of hexane and stirred for 15-30 min before being added to the reactor, although good catalytic ac- tivity also resulted when the catalyst components were mixed in the re- actor.

Polymerizations were conducted in a l-liter autoclave equipped with mechanical stirrer, temperature control, ethylene inlet, nitrogen purge, and an entry port for catalyst addition. The autoclave was charged with 500 ml of dry, degassed hexane and heated to the boiling point with nitrogen purge. Some of the diluent was allowed to boil out to complete drying and outgassing. The catalyst was then added to the reactor which was sealed, pressurized to 300 psig, and quickly brought to the desired reaction tem- perature, typically 90°C. At the end of 1 hr the reaction was cooled, de- pressurized, and opened. The particulate polymer was removed and dried without further treatment. If polymerizations are conducted above the solution temperature (>llO"C) the polymer is removed as sheets and chunks rather than as particles.

In those polymerizations employing hydrogen, the gas was added to the desired pressure prior to admitting the ethylene.

Great care must be taken to exclude air, water, or other contaminants during catalyst preparation and polymerization since the catalyst is easily poisoned.

Dialkyl Aluminum Alkoxides

The dialkyl aluminum alkoxides were prepared a t 0.5M concentration by slow addition of the appropriate alcohol to a hexane solution of the trialkyl-

ETHYLENE POLYMERIZATION 2611

aluminum. Provision for release of evolved gases and cooling in an ice bath is necessary.

Chromium VI Analysis

A sample of catalyst slurry in hexane was mixed with 200 ml degassed, distilled water and stirred for 20 min (nominally) to leach the hexavalent chromium into the aqueous phase. (If a large enough catalyst sample is used, the hexane may be drawn off before the water is added.) After the extraction, agitation was stopped, the solids were allowed to settle, and the aqueous layer was transferred to a separatory funnel. Up to this point all manipulations were conducted in a nitrogen atmosphere.

A 50-ml aliquot of the aqueous phase was taken and to this was added 5 ml4N HzS04 and 2.5 ml 1N KI. If free iodine was liberated (oxidation of K I by CrV') a yellow color developed, this was titrat.ed to near-colorless with 0.01N NazSzOa, then thyodene indicator (0.5 g) was added and any purple color titrated to colorless, again with Na2&0~. If no iodine was liberated and no color developed with the thyodene indicator, zero hexa- valent chromium was reported.

A blank titration was run on a sample consisting of anhydrous base, hexane from the reactor supply and degassed water. The titer was 0.5 ml.

Calculations

The calculations of hexavalent chromium were made as follows:

yo Crv'(based on total sample) - (ml0.01N Na2&Os) (O.Ol)(ml H20 leach/ml aliquot) (0.052) (100) -

wt. sample

% CrV1(based on total Cr) - (ml0.01N N&Oa)(O.O1) (ml HzO leach/ml aliquot) (0.052) (100) -

Cr in sample

where

g sample 3" [baae/(rpJ3iO)2Cr02 634

g Cr in sample =

RESULTS

Bis(triphenylsily1) chromate is a fairly active catalyst for ethylene poly- merization a t 20000 psi, but at low pressure (<300 psi) it gives little or no high molecular weight polyethylene. Addition of aluminum alkyl to the silyl chromate forms a feeble, low-pressure catalyst. However, when bis- (triphenylsilyl) chromate is deposited from hydrocarbon solution onto a silica-alumina of high surface area, a very active catalyst for ethylene poly-

2612

5.0 > +: 8 4.0-

5 .Y p 3.0

0

L .- u

.- L e -

CARRICK FT AL.

- 8. - 0 - t - e u 9. -0Bu 10. -0Pr I I. -0-8- Naphthyl i-2 I I 12. - 0 E t

6 -0

+%5

-

TABLE I Ethylene Polymerization with Bis (Tripheny1silyl)Chromate

[(CsHdr SiOIr Pressure, Silica- Temp, Time, Yield, MI, HLMI, HLMI/ 171, Cdz,g psig alumina, ge O C hr g dg/min dg/min MI dl/g

~

1.00 20,000b None 150 5.00 21,OOOb None 130 0.20 600 2.0 160 0.05 600 2.0 136 0.02 600 1.1 135 0.10 600 0.5 158 0.10 600 0.3 160 0.20 600 1.5 153 0.05 600 1.0 135

a Supports were dehydrated at 500'C. 8ee Baker and Camck.2

6 55 0.04 16 170 0.002 5 140 4.5 3 41 3.7 6 65 1.0 2 30 0.7 6 90 0.7 2 110 0.4 6 110 0.3

4.8 0.80

221 424 88 54 60 44 41

1 20 400 49 114 88 77 86 110 137

- 1.50 - - 2.02 2.15 2.41 - -

merization results (Table I). No cocatalyst or secondary treatment is required.3 When silica was substituted for silica-alumina, addition of an external reducing agent was necessary to obtain optimum catalytic ac- tivity. Addition of trialkyl-aluminum or dialkylaluminum alkoxides gives a highly active catalyst with commercial potential (Table 11).

A number of dialkylaluminum alkoxides were prepared from alcohols of different structure. All were good cocatalysts in the system. Interest- ingly, the melt index (MI) of the polymer (Fig. 1) waa different for different alkoxides, so that products in the commercially useful range could be pre- pared by slurry polymerizaion at 90°C. While good catalyst activity can be obtained by using either trialkylaluminum compounds or dialkyl-

01

Alkonide 1. -0Si0s 2. -0iAmyl 3. -OCHp@

4. -0CHzC1CHs)s 5. -0-i-Pr 6. -0Hexyl t -OM8

0.01 0.10 Melt Ihdrx, dg /min.

Fig. 1. Effect of alkoxide structure on melt index with silyl chromate-silica-dialkyl- aluminum alkoxide catalysts.

ETHYLENE POLYMERIZATION 2613

TABLE I1 Influence of Aluminum Alkoxide Structure

Soluble, %

(cyclo- hexane,

Cocatalyst Yield, g/hr 1P lop td,dl/g 80°C)

Melt index

EtaAl (t-Bu)aAl E t Al-OCHj EkAl-OCzH6 E t Al-OCaHT EtAl-OC4Hg EtzAl-OCsHia E t Al-O-i-CaHg EtAl-O-i-C4H9 EkAl-O-i-CjH1l E t Al-O-i-CiHla Et~Al-0-i-CsH~7 EtzAl-O-CH& (CHa)a EtAl-0-C (CHa)a

EtzAl-OCsHs

EtzAl-OCHzCsHs Etz Al-OCHzCHzCsHs

Et Al-0-2-Et Hexyl

EtZAl-0-Naphthyl (p)

Et,AI-O-@

134/3 132/2 181/1.5 195/1.5 136/1 .O 140/1.0 134/1.0 179/1.5

128/1.0 116/1.0 117/1.0

116/1.5 147/1.5

115/1.0 84/1.0 84/1 . O

139/1.0

110/1.0

128/1 .o

120/1.0

121/1.0

2.7 0.004 0.4 0.08 9.3 0.18 14.2 0.13 9.1 0.11 11.1 0.06 8.0 0.06 6.4 6.18 15.9 0.02 3.7 0.12 11.1 0.15 12.2 0.04 8.3 0.10 14.6 0.07 8.9 1.6 102 0.14 15.2 0.006 1.1 0.004 1.5

0.04 4.4

-

0.03 5.1

3.5 3.8 3.0 3.7 3.0 3.8 3.4 3.2 3.4 3.0 3.7 2.6 3.0 3.6 4.3 4.0 - - - - 4.2 4.5 3.4 5.7

3.2 1.8 9.0 3.2 11.1

-

- - - -

4.2 3.2

4.2 3.9

.,Polymerisations in 500 cc n-hexane with 0.18 mmole cocatalyst, 10 mg bis(tripheny1- sily1)chromate and 0.4 g silica dried at 200°C. Ethylene feed prwsure, 300 psig; temp, 88-91°C.

aluminum alkoxides as cocatalysts, the trialkylaluminum gives a lower base melt index than the dialkylaluminum alkoxides, and mixtures of the two may be used to give some degree of melt index control (Fig. 2), along with added catalyst stability.

The most effective supports are porous materials (silica, silica-alumina) of high surface area. In these supports most of the available surface is in- side the pores so the structure must be such that the internal surface is avail- able to the organometallic catalyst components. The data of Table I11 show that porous supports with pore diameter 1 4 7 A are poor catalysts while those with pore diameter 267 A make excellent catalysts. With these porous materials it seems reasonable that the pores must be above a certain size (267 A) in order for the catalyst components to be able to pene- trate and react effectively.

Adsorption of the silyl chromate on the support is a relatively slow process that can require several hours for completion. Both the rate and amount of adsorption can vary with the support type.

2614 CARRICK ET AL.

0 I 2 3 4 5 6 7 .01

EtOAlEtz / E t i At Fig. 2. Effect of diethylduminum ethoxide to triethylaluminum ratio on polymer melt

index.

TABLE 111 Effects of Silica Support Properties on Polymerization Activity

with Silyl Chromate Catalysts* ~

Dehy- Avg pore Support dration diameter, Surface CzH4, Temp, Yield, MI, HLMI.

typeb temp,^^ A area,ma/g psig oc g/hr dg/min dg/min

Grade 12 Grade 81

Grade 951 Grade 62

Grade 56

' I I'

I' ' 6

.I I'

Grade 952 '. I .

225 145 375 500 300 350 110 280 340 110 330

22 47 47 67

120 120 168 168 168 200 200

800 600 600 600 350 350 285 285 285 350 350

200 250 210 300 200 200 200 200 300 200 200

90 Trace - - 95 23/1 0.01 1.6

80/0.5 NF 1.3

'' 134/1 0.04 5.9 72/0.5 0.05 4.7

90 119/0.5 0.01 2.5 " 110/0.6 0.05 5.4

195/1 0.06 6 .5 I' 133/0.5 0.04 4 . 1

- - 6/1

150/1 0.18 12

~~

a Composition of catalyst was: 10 mg [(CsHi)rSiO]rCr0~. 0.4 g support, 0.24 mmole EtAOEt. Davison Chemical Company designation.

After addition of the organoaluminum compound the chromium speciea is quickly reduced in valence below the hexavalent state (Table IV), and 15 min after adding the reducing agent no hexavalent chromium was de- tected.

Gel-permeation chromatography showed the molecular weight distribu- tion of the polymer to be broader than that of most commercial polyethyl- enes with somewhat more of both the very low and high molecular weight fractions (Fig. 3).

The rate of polymerization is essentially linear with ethylene pressure (Fig. 4 and Table V), showing that the reaction is kinetically firsborder in

ETHYLENE POLYMERIZATION 2615

TABLE IV Valence Reduction in Silyl Chromate Catdystsa

~~ ~

Reduction time, Leach time, Weight CrVr chromium min hr found, mg as CrVr, %

None 3 3 3 6

15 4320 4320

0.25 18 18 18 18 18 18 0.25

20.3 0.405 0,434 0.225 0.840

None None None

77.9 1.98 2.12 1.10 4 . 1 - - -

* Catalysts formed from 0.25 g bB(triphenybiy1) chromate, 11 g silica, 4 mmole di- ethylaluminum ethoxide, and 2 mmole triethylduminum.

Supported silyl chromate + RaAlOR PolY Inor

lop LO’ 10‘ 100

Polymer Molecular Weight

Fig. 3. Comparative molecular weight distributions of commercial high-density poly- ethylene with that of the silica-supported silyl chromate catalyst.

monomer. The polymer melt index and vinyl content are fairly constant over a sixfold range of pressure.

Hydrogen acts as a mild chain transfer agent6 (Fig. 5) and over the range of 0.1-0.5 mole fraction can serve to increase the melt index by a factor of 50. Temperature also acts to regulate melt index. As shown by Figure 6, the melt index can be increased from 4 . 0 1 to -100 by increasing the reaction temperature from 90 to 160°C. In these cases the polymer forms as particles below about 100”C, as a “mush” between 100 and 120”C, and as a solution above about 120°C.

DISCUSSION

In those polymerizations conducted without metal alkyl reducing agents, there appears to be an activation step in which the bis(triphenylsily1)

2616

I I

100 200 3( Polymer Yield, grams

Fig. 4. Effect of ethylene pressure on reaction rate with the silyl cbromate9ilica-diethyl- aluminum ethoxide catalyst.

chromate reacts with ethylene to cause reduction of the chromium species and form the true active site. While the structure of the active site has not been determined, the reduction reaction is thought to be as shown h eq. (1).

When the silyl chromate is adsorbed on silica-alumina, the chemistry of the system is probably the same as the unsupported silyl chromate except for having the active site be "stabilized" by being anchored on the support. If the active species is a low-valence species, as proposed,'-1o in this and many other systems, then reaction between this species and unreduced silyl chromate could give intermediate valences, some of which

3CrII + CrV1 + 4 C F Cr" + CrvI + 2CrIVetc. (2 1

ETHYLENE POLYMERIZATION 2617

TABLE V Effect of Ethylene Pressure

Soluble, %

(cycle-

psi Yield, g MI, dg/min dg/min we% 800C) CaH4, HLMI, Vinyl, hexane

80 13 0.54 41 0.22 6 . 9 180 43 0.16 15 0.24 5 .6 280 108 0.21 18 0.19 5 . 2 380 123 0.16 14 0.21 4 . 2 480 238 0.22 21 0.19 5 .1

~~ ~

*All experiments with 0.12 g silica, 0.005 mmole silyl chromate, 0.07 mmole Etr AlOEt, 20 psig hydrogen, 1 hr at 90°C.

may not be catalytically active. Adsorption on a support would tend to immobilize and %late the separate chromium species so that such bimo- lecular reactions could not occur. This could explain the increased activity in the silyl chromate on silica-alumina catalysts used without added re- ducing agent.

The silica-supported systems are considerably more complex and require a detailed view of the structure of the support. Silica is a three-dimen- sional network of SiO, tetrahedra qJith hydroxyl groups completing the fourth valence of the silicon atoms at the surface." The surface may also contain physically adsorbed water. As the support is heated to dry it, the first step is removal of physically adsorbed water up to about 115°C. Reasonable heating between 115 and 400°C causes little change with the surface largely consisting of a layer of Si-OH groups. Heating at higher

10.0

0.1 I I I I I I 1 1 1 1 1 I * I t

.I 1.0 Hydrofpn / Ethylene P n t u n Ratio

Fig. 5. Effect of hydrogen/ethylene ratio on melt index with the silyl chromatesilica- diethylaluminum ethoxide catalyst.

2618

f / /

Ternparatwe, *C

Fig. 6. Effect of polymerization temperature on melt index of polymer from silyl chro- ma~ilica-diethylaluminluminum ethoxide catalyst.

temperatures causes loss of water by reaction of adjacent hydroxyl groups and temperatures above 800°C can cause sintering and loss of surface area. These effects can be illustrated by Scheme (3).

H\, ,H \o,'

>6OO0C isolated hydroxyls I adjacent 115 to -4000c hydroxyls

<llO"C adsorbed water

In our work it is necessary to dry the support sufficiently to remove physically bound water (>llO°C). Drying at higher temperatures may have a minor effect on rate or cause some reductiod in polymer melt index but is not necessary for good catalytic activity.

Deposition of the silyl chromate on silica is a relatively slow process which may take several hours for essential completion. The relatively

ETHYLENE POLYMERIZATION 2619

slow rate of deposition suggests that adsorption goes through an activation procem, indicative of a chemical reaction. Possible modes of deposition include physical adsorption, or transesterification of the silyl chromate with either one or two surface hydroxy groups [eqs. (4) J.z*1*-13

Adsorption + (R0)2Cr0, - Support

\ Chemical

reaction p - % ? - O R + ROH (4)

OH

where R = -Si (s>, Treatment of the supported system with the aluminum alkyl causes val-

ence reduction of the chromium species. Optimum catalytic activity re- sults a t Al/Cr ratios of 10-20 or higher. At Al/Cr >7 all the hexavalent chromium has been reduced. While these data do not rigorously establish the catalytically active valence they do prove that a reduced species is es- sential. The relatively broad molecular weight distribution shows that the system is polyfunctional so that there may well be two or more active species. l4

For initiation to occur one must transfer some ligand to the first ethylene molecule. The precise identity of this ligand is unknown; however, it seems reasonable that it is an alkyl from the aluminum alkyl or a fragment derived from reduction of the silyl chromate. When no aluminum alkyl is present, a possible initiation could involve insertion of the monomer into a Cr-0 bond. After this initial insertion the chromium species would then be a “chromium alkyl.” On the assumption that the active species is a reduced chromium compound (of undetermined valence) bound to the support, the polymerization reaction [eq. ( 5 ) ] is envisioned as insertion of the monomer into a Cr-C bond as has been previously discussed:2.16

Cr-R + CH2=CH2 - Cr-CH2-CH2-R, etc. (5)

where X denotes ligands to complete the chromium valence. Since the polymer melt index (molecular weight) is unaffected by large changes in monomer pressure, the principal termination reaction must be chain transfer to monomer [eq (6) 1.

2620 CARRICK ET AL.

Hydrogen acts as a chain-transfer agent, presumably by reaction with the active site [eq. (7)].

Propagation would continue by further addition of monomer to the Cr-H bond.

The aluminum alkyl used to reduce the chromium almost certainly re- mains bound to, or near, the catalytically active chromium center. The present data do not show whether or not the aluminum component is part of the active site, but the fact that polymerization can occur without alu- minum alkyls suggests that a second metal is not a necessary feature; earlier copolymerization work supports this view. l6 However, the changes in polymer melt index due to changes in aluminum alkyl structures may be due to these residual ligands.

References 1. F. E. Granchelli and G. B. Walker, U.S. Pat. 2,863,891 (1958). 2. L. M. Baker and W. L. Carrick, J. Org. Chem., 35,774 (1970). 3. L. M. Baker and W. L. Carrick, U.S. Pat. 3,324,101 (1967). 4. W. L. Carrick, G. L. Karapinka, and R. J. Turbett, U.S. Pat. 3,324,095 (1967). 5. J. C. Moore, J. Polym. Sci. A , 2,835 (1964). 6. R. A. V. RaE and I(. W. Do&, Crystalline Olejin Polymws,Interacience,New York,

7. W. L. Carrick, A. G. Chasm, and J. J. Smith, J. Amer. Chem. Soe., 82,5319 (1960). 8. G. Henrici-Olive, and S. Olive, Angm. Chem., 10,776 (1971). 9. L. M. Baker and W. L. Carrick, J. &g. Chem., 33,616 (1968).

1965, p. 377.

10. H. L. Kraus and H. Stach, Inorg. Nucl. Chem. LeUers, 4,393 (1968). 11. R. I(. Iler, The Colloid Chemistry of Silica and Silicates, Cornell Univ. Press,

12. J. P. Hogan, J. Polym. Sci. A-I , 8,2637 (1970). 13. I. Habeshaw and T. Hill, Proc. Intern. Cmgr. Catalysis, 3rd Cmgr., Amsterdam

14. G. W. Phillips and W. L. Carrick, J. Amer. Chem. Soe., 84,920 (1962). 15. J. Boor, Jr., in Mwomoleeuhr Reviews, A. Peterlin et al., Eds., Interscience,

16. F. J. Karol and W. L. Carrick, J. Amw. Chem. SOC., 33,2654 (1961).

Ithaca, N. Y., 1956.

2,975 (1964).

New York, 1967, pp. 115-268.

Received June 6,1972 Revised July 13,1972