Two independent experiments show that in polymerization catalysts of the Phillips type the growing hydrocarbon chain is attached to the metal via a bond

Krauss, H. L.

Schmidt, Helmut K.

Universaar

Surface compounds of transition metals. - Part XIII : Nature of the polymer to chromium bond in Phillips catalysts

Scientific publications of the Saarland University

Reaction Kinetics and Catalysis Letters, VoL 3, No. 1, 83-88 (1975) SURFACE COMPOUNDS OF TRANSITION METALS, XIIL* NATURE OF THE POLYMER TO CHROMIUM BOND IN PHILLIPS CATALYSTS H.L. Kratm and H. Schmidt Imtitut fflf Anorganische Chemie, Freie Universititt Berlin 1 West Berlin 33, Fabeckstraue 34 Received March 21, 1975 Two independent experiments show that in polymerization c~mlysts of the Phillips type the growing hydrocarbon chain is attached to the metal via a a bond. ~Ba He3aBHCHMMX pff~a 9KCMepHMeHTOB yKa3bIBa~T Ha TO, ~TO npH HCHO~b3OBaHHH KaTanH3aTOpOB THna ~HnHnCa B RpoHecce HO~HMepH3aUHH~pacTymaRyI '~eBo~opo~Haff  uenoqKa npHcoe~HHReTCff K MeTa~ny CBH3b~ ~. The identification of the "active center" in the Philips polyethylene process as a coordinatlveiy unsaturated Cr2+ surface compound/2, 3/suggests that the pri- mary step in the polymerization reaction is the attachment of the C2H 4 monomer to the metal bya =bond - in analogy to the starting reaction in the Ziegler-Natta- process/4, 5, 6/. The growing polymer chain in supposed to be fixed to the metal by a Gbond, but experimental proof of this assumption is lacking. For example, a termination reaction with formation of Cr-H and a=-complexed polymer (in equl- librium with a ~ bond chain) H2 may not be excluded/7/.  9 H.cfC . *Part XII: Ref. /1 /  6* 83 KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND We used two independent experiments to solve this problem: (A) Cleavage of the O-polymer bond by HgC12, both in the "living polymer" (i. e. in the Cr-polymer complex on the surface) (A1) and in the corresponding organometallic Cr species/8, 9/in solution (A2). (B) Pulsed reaction of the "living polymer" with CO/C2H 4 cycles (BI) , and prepa- ration of the organometallic species with or without addition of CO 0]2). The first reaction (A) is known to form RHgCI and/or R2Hg if a Cr--C # bond is present/10/, while similar products from ~-bonded olefins are not obtained. Reaction (B) makes use of the well known competition of C2H 4 polymerization and CO complex formation/11/: even moderate concentrations of CO in the ethylene atmosphere ( :~ 20~o) prevent any further growth of polymer chains. Whilst the ex- change of .~-bonded olefins by CO and vice versa is well known/12/, Cr-C~bonds react via insertion, if at all/13/. Therefore, CO may act here by occupying the necessary coordination site for ~ addition of a further monomer, and/or by substi- tution of ~-bonded chains. When we compare a one-step addition of C2H 4 with a repeated cycle of C2H4/CO additions using the same gross amount of monomer, in the latter case CO should '"oite off" the shorter polymer chains formed in each single pulse of C2H 4 if ~fixation of the chain occurs. This would result in a lower molec- ular weight product with respect to the one-step polymerization. Sigma-bonded chains would remain unaffected or undergo insertion (formation of keto groups). Similarily, the cleavage ofapossible ~Joond between Cr and the polymer chain by COwould reduce the yield of the organometallic species, obtained by splitting the support-to- chromium bond of the "living polymer"/8, 9/. 84 KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND EXPERIMENTAL In the following experiments we used the "low polymer" technique described elsewhere/8/, allowing a mass spectroscopic identification of the products. The gross amount of monomer used for polymerization was limited therefore in such a way that hydrocarbon chains of less than ~ 24 C atoms were produced. All operations were carried out under strict exclusion of oxygen. Ethylene was purified from traces of oxygen according to R/eL /14/. (A1) A chromium(II) contact (1.30% Cr, oxidation umber 2.35) was reacted with a known amount of ethylene in argon, as described earlier/8/. One half of the product was extracted at 60 ~ with a 10% solution of HgCI 2 in THF. Fror~ the resulting liquid phase the solvent was removed almost completely by eva- potation. Cyicohexane was then added, the resulting solution was filtered and the solvent evaporated completely. The white residue (which slowly turns black on contact with air) was used directly for mass spectroscopy (Atlas CH 4 instru- o ment, sample temperature 60 -- 80 C, 10 eV). The appearing masses proved the presence of the ions HgCI (GnHfin+l) and Hg(CnH2n+l) with n = 4. . .  10 (maximum in intensity at n = 6). (h~2) From the second half of the product mentioned above the organochromium species was removed by extraction with CH OH/HC1 and transferred into cylco- hexane, as described earlier/8/, To 0.05 mmol of the organometallic species in cyclohexane a solution of 0.37 mmol of HgCI 2 in THF was added. After vig- orous stirring at 60 ~ for 24 hrs, the solvents were removed almost completely by evaporation. The subsequent s eps were carried out as in (A1). The mass spectra showed the presence of the same ions as in (A1). Blank exper- 85 KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND iments with HgC12 refluxed for 24 hrs in n-octene-1 did not show any masses due to the presence of organomercury compounds in the reaction mixture. (B1) As a support for the catalysts Vycor plates (Vycor 7930, Thermalquarzschmelze, Mainz, Germany)of20 mm diameter and 0.05 mm thickness were used. After impregnation with an aqueous olution of Cr% and drying at 110 ~ the Cr content was 1.74~ Mounted in a special quartz IR cuvette /14/ they were acti- vated for 5 min at 500~ reduced with CO for another 5 min at 400~ final- ly flushed with argon for 5 min at 400~ and cooled to room temperature. The catalyst plates were then reacted with small amounts of pure %H 4, while the relative quantity of reacted monomer was monitored by measuring the CH as at 9.926 cm "1 of the polyethylene formed. Equal IR absorption intensities were adjusted a)by adding the monomer in a single step, and b)by reaction with five pulses of ethylene, interrupted by flushing with argon (25 ~ 760 Tort, 5 min) and CO (9.5 ~ 760 Torr, 5 min). The plates were powdered under argon and the resulting samples transferred under inert conditions directly to the mass spectrometer (Varian MAT CH 5 D with data system MS 100, sample temperature 50 -- 200 ~ 18 eV). The mass distributions in the rage of m/e  = 200 to 700 were equal for a) and b) within a limit of + 9.0~ which is the typical error within the individual a) or b) series. No insertion reaction was observed under our conditions. (B2) The preparation of the organochromium compounds was reported earl ier/8/. The polymerization was carried out with standardized small amounts of ethylene in a "fluidized bed" reactor, followed by flushing/1/with carbon monoxide (5 rain/9.5 ~ or (2) with argon (10 rain/9.5 ~ to terminate the polymeriza- tion. Isolation and purification of the organochromium species was carried out 86 KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND as usual. The molar yields of (1)and (2)were determined photometrically in a CHzOH solution (~max = 16180 cm "1, log ~ = 2.54)and shown to be equal within a limit of 7%. RESULTS All the experiments described here lead to the same result: Cleavage of the polymer to chromium bond by HgCl 2 forms the corresponding organomercury species - which proves the existence of a chromium to carbon ~ o bond for the "living polymer" as well as for the corresponding support-free organo- metallic compounds. The "bite-off" cycles with CO leave the molecular weight distribution of the resulting polymer unchanged with respect o a one-step olymerization, excluding a a type chain to metal bond. The equal yield in organometallic species using CO or vacuum for the termination of polymerization again is contradictory to a substi- tution of a-bond chains by CO. Thfis, we have to conclude that the chains are fixed to the metal by a Cr-C #- bond. Furthermore, the obvious parallelism between the experiments with supported and support-free species hows that the Cr-C bond is essentially the same in both Cases .  Termination of the polymerization by CO (occupation of a coordination site for the monomer) or by lack of monomer (empty coordination site for monomer) are not followed by a P.elimination equilibrium. We thank the "Deutsche Forschungsgemeinsehaft" and the "Fonds der Chemischen Industrie", Germany, for the support of this work. 87 KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND REFERENCES 1. G. Hierl, H. L. Krauss: Reaction Kinetics and Catalysis Letters, 2_47 (1975) 2. H.L. Krauss, H. Staeh: Inorg. Nuel. Chem. Lett., 4....~, 393 (1968) 3. H. L. Krauss, H. Staeh: Z. Anorg. Allg. Chem., 366, 280 (1969) 4. G. Hemiei-Oliv6, S. Oliv6: Chem. -hag. Teehn., 43, 906 (1971) 5. L.L. van Reijen, P. Cos66: Disc. Faraday Soc., 41,277 (1966) 6. G. Henriei-Oliv6, S. Oily6: Angew. Chem., 8.~3,782 (1971) 7. tL A, Sehunn: Inorg. Chem., 9...~., 2566 (1970) 8. H.L. Krauss: Proc. Vth International Congress on Catalysis, p. 207, North Holland/Amer. Elsevier (197~) 9. H.L. Krauss, H. Sehmidt: Symp. on the Mechanism of Hydrocarbon Reactions, Si6fok, Hungary (1973), in press 10. H.H. Zeiss, P.A. Sneeden: Angew. Chem., 79,401 (1967) 11. J.P. Hogan: J. Pol. SeL, 8,2673 (1970) 12. M. Heberhold: Metal n Complexes, II. Elsevier Scientific, Amsterdam 1972/74 13. K. Noak, F. Calderazzo: Inorg. Chem., _7,345 (1968) 14. H. Sehmidt: Dissert. TU MUnehen, Germany, 1973 88 

Scientific documents from the Saarland University,

Reaction Kinetics and Catalysis Letters, VoL 3, No. 1, 83-88 (1975) 
SURFACE COMPOUNDS OF TRANSITION METALS, XIIL* 
NATURE OF THE POLYMER TO CHROMIUM BOND IN PHILLIPS CATALYSTS 
H.L. Kratm and H. Schmidt 
Imtitut fflf Anorganische Chemie, Freie Universititt Berlin 
1 West Berlin 33, Fabeckstraue 34 
Received March 21, 1975 
Two independent experiments show that in polymerization c~mlysts of the 
Phillips type the growing hydrocarbon chain is attached to the metal via a a bond. 
~Ba He3aBHCHMMX pff~a 9KCMepHMeHTOB yKa3bIBa~T Ha TO, 
~TO npH HCHO~b3OBaHHH KaTanH3aTOpOB THna ~HnHnCa B 
RpoHecce HO~HMepH3aUHH~pacTymaRyI '~eBo~opo~Haff  uenoqKa 
npHcoe~HHReTCff K MeTa~ny CBH3b~ ~. 
The identification of the "active center" in the Philips polyethylene process 
as a coordinatlveiy unsaturated Cr2+ surface compound/2, 3/suggests that the pri- 
mary step in the polymerization reaction is the attachment of the C2H 4 monomer 
to the metal bya =bond - in analogy to the starting reaction in the Ziegler-Natta- 
process/4, 5, 6/. The growing polymer chain in supposed to be fixed to the metal 
by a Gbond, but experimental proof of this assumption is lacking. For example, a 
termination reaction with formation of Cr-H and a=-complexed polymer (in equl- 
librium with a ~ bond chain) 
H2 
may not be excluded/7/.  
9 H.cfC . 
*Part XII: Ref. /1 /  
6* 83 
KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND 
We used two independent experiments to solve this problem: 
(A) Cleavage of the O-polymer bond by HgC12, both in the "living polymer" 
(i. e. in the Cr-polymer complex on the surface) (A1) and in the corresponding 
organometallic Cr species/8, 9/in solution (A2). 
(B) Pulsed reaction of the "living polymer" with CO/C2H 4 cycles (BI) , and prepa- 
ration of the organometallic species with or without addition of CO 0]2). 
The first reaction (A) is known to form RHgCI and/or R2Hg if a Cr--C # bond is 
present/10/, while similar products from ~-bonded olefins are not obtained. 
Reaction (B) makes use of the well known competition of C2H 4 polymerization 
and CO complex formation/11/: even moderate concentrations of CO in the ethylene 
atmosphere ( :~ 20~o) prevent any further growth of polymer chains. Whilst the ex- 
change of .~-bonded olefins by CO and vice versa is well known/12/, Cr-C~bonds 
react via insertion, if at all/13/. Therefore, CO may act here by occupying the 
necessary coordination site for ~ addition of a further monomer, and/or by substi- 
tution of ~-bonded chains. When we compare a one-step addition of C2H 4 with a 
repeated cycle of C2H4/CO additions using the same gross amount of monomer, in 
the latter case CO should '"oite off" the shorter polymer chains formed in each single 
pulse of C2H 4 if ~fixation of the chain occurs. This would result in a lower molec- 
ular weight product with respect to the one-step polymerization. Sigma-bonded chains 
would remain unaffected or undergo insertion (formation of keto groups). Similarily, 
the cleavage ofapossible ~Joond between Cr and the polymer chain by COwould 
reduce the yield of the organometallic species, obtained by splitting the support-to- 
chromium bond of the "living polymer"/8, 9/. 
84 
KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND 
EXPERIMENTAL 
In the following experiments we used the "low polymer" technique described 
elsewhere/8/, allowing a mass spectroscopic identification of the products. The 
gross amount of monomer used for polymerization was limited therefore in such a 
way that hydrocarbon chains of less than ~ 24 C atoms were produced. 
All operations were carried out under strict exclusion of oxygen. 
Ethylene was purified from traces of oxygen according to R/eL /14/. 
(A1) A chromium(II) contact (1.30% Cr, oxidation umber 2.35) was reacted with 
a known amount of ethylene in argon, as described earlier/8/. One half of the 
product was extracted at 60 ~ with a 10% solution of HgCI 2 in THF. Fror~ 
the resulting liquid phase the solvent was removed almost completely by eva- 
potation. Cyicohexane was then added, the resulting solution was filtered and 
the solvent evaporated completely. The white residue (which slowly turns black 
on contact with air) was used directly for mass spectroscopy (Atlas CH 4 instru- 
o 
ment, sample temperature 60 -- 80 C, 10 eV). The appearing masses proved 
the presence of the ions HgCI (GnHfin+l) and Hg(CnH2n+l) with n = 4. . .  10 
(maximum in intensity at n = 6). 
(h~2) From the second half of the product mentioned above the organochromium 
species was removed by extraction with CH OH/HC1 and transferred into cylco- 
hexane, as described earlier/8/, To 0.05 mmol of the organometallic species 
in cyclohexane a solution of 0.37 mmol of HgCI 2 in THF was added. After vig- 
orous stirring at 60 ~ for 24 hrs, the solvents were removed almost completely 
by evaporation. The subsequent s eps were carried out as in (A1). 
The mass spectra showed the presence of the same ions as in (A1). Blank exper- 
85 
KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND 
iments with HgC12 refluxed for 24 hrs in n-octene-1 did not show any masses 
due to the presence of organomercury compounds in the reaction mixture. 
(B1) As a support for the catalysts Vycor plates (Vycor 7930, Thermalquarzschmelze, 
Mainz, Germany)of20 mm diameter and 0.05 mm thickness were used. After 
impregnation with an aqueous olution of Cr% and drying at 110 ~ the Cr 
content was 1.74~ Mounted in a special quartz IR cuvette /14/ they were acti- 
vated for 5 min at 500~ reduced with CO for another 5 min at 400~ final- 
ly flushed with argon for 5 min at 400~ and cooled to room temperature. 
The catalyst plates were then reacted with small amounts of pure %H 4, while 
the relative quantity of reacted monomer was monitored by measuring the 
CH 
as at 9.926 cm "1 of the polyethylene formed. Equal IR absorption intensities were 
adjusted a)by adding the monomer in a single step, and b)by reaction with five 
pulses of ethylene, interrupted by flushing with argon (25 ~ 760 Tort, 5 min) 
and CO (9.5 ~ 760 Torr, 5 min). 
The plates were powdered under argon and the resulting samples transferred 
under inert conditions directly to the mass spectrometer (Varian MAT CH 5 D 
with data system MS 100, sample temperature 50 -- 200 ~ 18 eV). The mass 
distributions in the rage of m/e  = 200 to 700 were equal for a) and b) within a 
limit of + 9.0~ which is the typical error within the individual a) or b) series. 
No insertion reaction was observed under our conditions. 
(B2) The preparation of the organochromium compounds was reported earl ier/8/. 
The polymerization was carried out with standardized small amounts of ethylene 
in a "fluidized bed" reactor, followed by flushing/1/with carbon monoxide 
(5 rain/9.5 ~ or (2) with argon (10 rain/9.5 ~ to terminate the polymeriza- 
tion. Isolation and purification of the organochromium species was carried out 
86 
KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND 
as usual. The molar yields of (1)and (2)were determined photometrically in 
a CHzOH solution (~max = 16180 cm "1, log ~ = 2.54)and shown to be equal 
within a limit of 7%. 
RESULTS 
All the experiments described here lead to the same result: 
Cleavage of the polymer to chromium bond by HgCl 2 forms the corresponding 
organomercury species - which proves the existence of a chromium to carbon ~ o 
bond for the "living polymer" as well as for the corresponding support-free organo- 
metallic compounds. 
The "bite-off" cycles with CO leave the molecular weight distribution of the 
resulting polymer unchanged with respect o a one-step olymerization, excluding 
a a type chain to metal bond. The equal yield in organometallic species using CO 
or vacuum for the termination of polymerization again is contradictory to a substi- 
tution of a-bond chains by CO. 
Thfis, we have to conclude that the chains are fixed to the metal by a Cr-C #- 
bond. Furthermore, the obvious parallelism between the experiments with supported 
and support-free species hows that the Cr-C bond is essentially the same in both 
Cases .  
Termination of the polymerization by CO (occupation of a coordination site 
for the monomer) or by lack of monomer (empty coordination site for monomer) are 
not followed by a P.elimination equilibrium. 
We thank the "Deutsche Forschungsgemeinsehaft" and the "Fonds der Chemischen 
Industrie", Germany, for the support of this work. 
87 
KRAUSS, SCHMIDT: POLYMER TO CHROMIUM BOND 
REFERENCES 
1. G. Hierl, H. L. Krauss: Reaction Kinetics and Catalysis Letters, 2_47 (1975) 
2. H.L. Krauss, H. Staeh: Inorg. Nuel. Chem. Lett., 4....~, 393 (1968) 
3. H. L. Krauss, H. Staeh: Z. Anorg. Allg. Chem., 366, 280 (1969) 
4. G. Hemiei-Oliv6, S. Oliv6: Chem. -hag. Teehn., 43, 906 (1971) 
5. L.L. van Reijen, P. Cos66: Disc. Faraday Soc., 41,277 (1966) 
6. G. Henriei-Oliv6, S. Oily6: Angew. Chem., 8.~3,782 (1971) 
7. tL A, Sehunn: Inorg. Chem., 9...~., 2566 (1970) 
8. H.L. Krauss: Proc. Vth International Congress on Catalysis, p. 207, North 
Holland/Amer. Elsevier (197~) 
9. H.L. Krauss, H. Sehmidt: Symp. on the Mechanism of Hydrocarbon Reactions, 
Si6fok, Hungary (1973), in press 
10. H.H. Zeiss, P.A. Sneeden: Angew. Chem., 79,401 (1967) 
11. J.P. Hogan: J. Pol. SeL, 8,2673 (1970) 
12. M. Heberhold: Metal n Complexes, II. Elsevier Scientific, Amsterdam 1972/74 
13. K. Noak, F. Calderazzo: Inorg. Chem., _7,345 (1968) 
14. H. Sehmidt: Dissert. TU MUnehen, Germany, 1973 
88 


https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/24343/1/sm197501.pdf

Surface compounds of transition metals. - Part XIII : Nature of the polymer to chromium bond in Phillips catalysts

Abstract

Similar works

Full text

Available Versions

Universaar

Scientific publications of the Saarland University

Scientific documents from the Saarland University,