Peculiarities of Ethylene Polymerization Kinetics with an Imido

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Peculiarities of ethylene polymerization kinetics with an imido-vanadium / silyl-chromate bimetallic catalyst: Effect of polymerization conditions Bao Liu, Zhou Tian, Ning Zhao, Zhen Liu, and Boping Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Industrial & Engineering Chemistry Research

Peculiarities of ethylene polymerization kinetics with an imido-vanadium /silyl-chromate bimetallic catalyst: Effect of polymerization conditions Bao Liua, Zhou Tianb*, Ning Zhaoa, Zhen Liua, Boping Liua* a

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, P.R.China

b

Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, P.R.China

*Corresponding authors: Tel: +86-021-64253627 E-mail: [email protected]; E-mail: [email protected]

Abstract The effects of various polymerization conditions such as cocatalyst concentration, pressure, temperature,

and

reaction

time

on

ethylene

homopolymerization

over

a

SiO2-supported

imido-vanadium/silyl-chromate (Cr-iV) bimetallic catalyst were systematically investigated. The monometallic silyl-chromate (S-2) and imido-vanadium (iV) catalyst were also employed for comparison. It was found that the S-2 catalyst produced polymer with relative low molecular weight, while the iV catalyst produced UHMWPE. The bimetallic catalyst was capable of producing reactor blends, with bimodal MWD and considerable amount of UHMWPE. Increasing cocatalyst concentration or decreasing polymerization temperature both enhanced the high molecular weight part of the bimodal MWD while the position of the two peaks remained unchanged. The polymerization rates all showed first-order dependences with respect to ethylene pressure for the three catalysts. Ethylene pressure variations caused no changes on the MWD of polymers made by the three catalysts, indicating that transfer to monomer is the main chain transfer mechanism. Key words: kinetics, bimetallic catalyst, ethylene homopolymerization, polymerization conditions

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1 Introduction Polyethylene is a widely used plastic in numerous application areas and it is commercially produced with Ziegler-Natta catalysts, Phillips-type catalysts, and metallocene catalysts nowadays1. In the area of Ziegler-Natta olefin polymerization, vanadium-based catalysts show a number of favorable performances. They are useful to produce polyethylene (PE) with a high molecular weight and narrow molecular weight distribution2-5. Vanadium-based catalysts are also unique in the preparation of ethylene/α-olefin copolymers with high comonomer incorporation6-9. These features increase the interests in vanadium catalysts for the ethylene polymerization as well as ethylene/α-olefin copolymerization. Blafek10 studied copolymerization of ethylene and higher 1-olefin over the vanadium catalyst anchored on the MgCl2(THF)2 carrier modified with Al(i-Bu)3. It was found that the level of comonomer incorporation to the polyethylene chain was dependent on the olefin concentration and on the hydrocarbon chain length of an olefin decreasing in the following sequence: 1-hexene > 1-octene >1-decene > 1-dodecene. Recently, Ochedzan-Siodłaket11 synthesized a silica supported vanadium catalyst by immobilization of the Cp2VCl2 precursor (V) in the pyridinium ionic liquid. The obtained polyethylene is a linear polymer, with a high molecular weight over 106 g/mol, and has a characteristic fluffy or fibrous shape. Moreover, transition metal imido complexes are also attractive as intermediates in organic syntheses as well as catalytically active species due to their proposed intermediacy in the industrially important ammoxidation of propylene12-14. Coles and Gibson15 first reported vanadium catalysts based on half-sandwich imido complexes for ethylene polymerization. Ghosh4 found that imido vanadium complex V(NAr)Cl3 supported on SiO2 in the presence of MAO as cocatalyst showed higher activity than other inorganic supports and the resulting polyethylenes had ultrahigh molecular weight. Nevertheless, the low activity of vanadium catalytic systems limits its popularity in the large-scale industrial application 7, 16. SiO2-supported organic silyl chromate (S-2) catalyst is widely used in gas-phase PE production to manufacture polyethylene products, mainly high-density polyethylene. The catalyst was first reported by Union Carbide Corporation (UCC) with a UNIPOL gas-phase process17. Usually the catalyst is formed 2 ACS Paragon Plus Environment

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by chemisorption of bis(triphenylsilyl) chromate on dehydrated silica support in hexane. Alkyl aluminum reagents are introduced into the catalyst formulations to increase catalyst productivity and control polymer molecular weight18. This catalyst now has been tailored in numerous ways for many specialized modern applications. In general, polymers having relatively high molecular weight (Mw) and broad molecular weight distribution (MWD) are suitable for articles molded by blow molding or inflation molding. In many applications, medium-to-high molecular weight polyethylenes are desirable due to their sufficient strength for applications which call for such strength (e.g., pipe applications). Thus, PE with broad or bimodal MWD is of great interest to the industry and academia because it combines the good processability of low molecular weight fraction with the excellent mechanical strength of high molecular weight fraction19. Furthermore, for enabling shear thinning in melt processing, it is an important objective in polyolefin development to control the MWD by incorporating ultra-high molecular weight polyethylene (UHMWPE)20. Small amounts of UHMWPE in PE with bimodal MWD improves the rheological and mechanical properties of polyolefins. And it also increases the melt strengthening which is required in melt extrusion and blow molding applications21, 22. Typically, a bimodal polyethylene can be produced using conventional Ziegler-Natta catalyst in a cascade process22, 23, which involves two or more reactors in series, operating at different polymerization conditions. However, this method is cumbersome, time-consuming and not economical24, 25. Another method is physically blending, but the blends usually contain high gel levels, which may lead to miscibility problems26. As a result, more and more researchers26-33 have put their efforts on the bimetallic catalyst using a mixture of two pre-catalysts supported on one carrier to synthesis bimodal polyethylene in a single reactor. More recently, Jiang34 developed a novel catalyst technology by combing hybrid catalyst, composite support, and diffusion control for the production of broad/bimodal polyethylene in a single reactor. In summary, polymerization of ethylene with S-2 catalyst has been known for many years. However, it still could not meet the diversified demand of high performance polyethylene. The use of vanadium-based catalysts allows the preparation of high molecular weight polymers, and



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ethylene/α-olefin copolymers with high α-olefin incorporation. A good idea by our group35 is to immobilize vanadium species onto S-2 catalyst in order to combine the merits of the imido vanadium catalyst and the S-2 catalyst to obtain bifunctional catalytic systems. The Cr-iV bimetallic catalyst system was found to produce PE with bimodal MWD, and show better 1-hexene incorporation ability compared with the silyl-chromate catalyst and the reported Cr-V bimetallic catalyst36. However, the polymerization kinetics at different polymerization systems were not studied. It is essential to understand the peculiarities of ethylene polymerization kinetics and the resulting polymer properties. Moreover, although the resultant bimodality of the MWD curves emerges from the corresponding Cr and V metal centers, detailed comparison between the bimetallic catalyst and the monometallic catalyst was not performed. Thus, in this work, ethylene homopolymerization is carried out with this novel catalyst in a reactor with excellent control of reaction conditions. The effect of cocatalyst concentration, polymerization temperature, ethylene pressure and reaction time on the kinetic and properties of polymer were systematically studied. For comparison, polymerizations using the corresponding silyl-chromate (S-2) and imido-vanadium (iV) catalysts were also performed. Herein, the present manuscript extends our previous work by providing detailed description of operating condition effects on the polymerization kinetics and polymer properties for this novel imido-vanadium/silyl-chromate bimetallic system. Ethylene homopolymerizations were conducted under various polymerization conditions by varying cocatalyst concentration, ethylene pressure, polymerization temperature, and reaction time. For comparison, polymerizations using the corresponding silyl-chromate (S-2) and imido-vanadium (iV) catalysts were also performed. 2 Experimental part 2.1 Materials Ammonium metavanadate was purchased from Sinopharm Chemical Reagent Co., Ltd. Bis(triphenylsilyl) chromate (BC) as chromium precursor and Davison 955 silica gel was donated by Qilu Branch Co., SINOPEC. P-tolyl isocyanate as imido agents was purchased from Alfa Aesar. Triisobutylaluminium (1.0 M in toluene) was purchased from Sigma Aldrich. The high purity dry air, 4 ACS Paragon Plus Environment

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nitrogen (N2 ≥ 99.999%) and ethylene (Polymerization Grade) were purchased from Shanghai Wetry Criterion Gas Co., Ltd. The high-purity nitrogen used in the catalyst preparation was further purified by passing through one column of 4A molecular sieves (purchased from Sinopharm Chemical Reagent Co., Ltd.) for dehydration and one column of sliver molecular sieves (purchased from Sigma-Aldrich) for deoxidation. Nitrogen and ethylene monomer for ethylene polymerization was directly purified using a commercial gas purification equipment, which was purchased from Dalian Samat Chemicals Co., Ltd., by passing through two large columns filled with different molecular sieves for dehydration and deoxidation. The solvents including n-hexane (AR grade) and toluene (AR grade), which were also purchased from Sinopharm Chemical Reagent Co., Ltd., were purged through distillation in the presence of sodium metal slices and diphenyl ketone as indicator until the indicator showed pure blue before use or for storage in a stainless-steel storage tank under purified nitrogen. 2.2 Catalyst Preparation First, about 10 g of silica was impregnated in an aqueous solution of ammonium metavanadate at 50 ff. The mixture was heated to 120 °C and dried in air for 12 h after being continuously stirred for 4 h. Then it was calcined at 600 ff and a flow rate of 600 mL·min-1 under high-purity dry air for 4 h in a fluidized-bed quartz reactor with a temperature-programmed heating controller. Thereafter, the vanadium precursor was cooled down to room temperature and transferred into a glove box under nitrogen atmosphere. Second, about 150 mL purified toluene as solvent was added to a Schlenk flask with vanadium precursor, and p-tolyl isocyanate as imido agents. The mixture was refluxed and continuously stirred for 20 h under the nitrogen atmosphere till complete reaction. Then the solvent was removed at 120 ff under dry nitrogen for 5 h and in vacuum for 30 min. Thereafter, the imido-vanadium catalyst was cooled down and transferred into a glove box. The imido-vanadium catalyst was named as iV catalyst. Third, chromium precursor BC was supported on the iV catalyst and continuously stirred for 4 h at 45 ff in a flask charged with 150 mL hexane under nitrogen atmosphere. Finally, the solution was dried



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at 80 ff under nitrogen atmosphere for 4 h and in vacuum for 30 min to remove the solvent and transferred into glove box for storage before use. The bimetallic catalyst was named as Cr-iV catalyst. For comparison, a traditional silyl chromate S-2 catalyst with 0.25 wt% chromium loading was also prepared. First, the Davison 955 silica gel was pretreated by dehydrating at 600 ff for 4 h under nitrogen atmosphere with a flow rate of 400 mL·min-1 in the fluidized-bed quartz reactor. Thereafter it was cooled down to room temperature under nitrogen atmosphere and then BC was directly supported on the pretreated silica gel in hexane. The final S-2 catalyst was obtained by removing the solvent. The bimetallic Cr-iV catalyst was prepared with 0.25 wt% chromium loading, while vanadium content was 0.24 wt% to keep Cr/V mole ratio as 1:1. Likewise, the vanadium content in the iV catalyst was also set to be 0.24 wt%. The metal loading of Cr and V in different catalysts analyzed by ICP is shown in Table S1 (Supporting Information). 2.3 Experimental Setup Polymerization reactions were carried out in a 300 mL high-pressure reactor system, which was set up in Figure 1. The reactor supplied by Parr Instruments Co. USA was designed to operate at a maximum temperature of 573 K and pressure up to 20 MPa. It was handled in the semi-batch mode at a constant total pressure and temperature throughout each run. The reactor was equipped with a double bladed impeller with a magnetic drive. All experiments were performed at 600 rpm using purified hexane as the solvent. A specially designed injection system was used to inject the catalyst components to the liquid hexane without contacting them to atmosphere, as is shown in Figure 2. A PID temperature control system was equipped by providing an external electrically heated jacket and internal cooling coils to maintain the temperature. Cold water about 30 ff lower than the reaction temperature was fed to the internal cooling coils by a circulator bath. The temperature control system was efficient, with a maximum overshoot of 0.5 to 2 ff in the first few minutes of the reaction. After that, the reactor temperature could be kept within ±0.1ff of the set point. Reactor pressure and temperature were measured by an electronic pressure gauge and thermocouple, respectively. The instantaneous consumption of ethylene monomer was recorded by an on-line mass flow meter (Brooks SLA5860). 6 ACS Paragon Plus Environment

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Figure 1. Schematic of the Reactor set up

Figure 2. Catalyst injection system 2.4 Polymerization Procedure Prior to each polymerization run, the reactor was heated to 100 ff, evacuated and thoroughly cleaned by refilling with nitrogen for at least five times. Then the reactor was first charged with 80 mL of purified hexane. A certain amount of TIBA was injected into the reactor through a bypass with a long needle which could stretch into the reactor. Another 80 mL hexane was charged into the reactor to clean the bypass. 7 ACS Paragon Plus Environment


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The catalyst was suspended into 10 mL purified hexane in the glove box before the polymerization run. Then the 20-mL vial containing the catalyst component was connected to a steel storage bottle through a long double-tipped needle. The vial was flushed with a stream of nitrogen at 0.5 bar and catalyst suspension was transferred into the steel bottle. During this process, the steel bottle remained sealed from the reactor before the polymerization run started. Once the catalyst had been added, the steel bottle was filled with a higher pressure (about 40 psi) of ethylene than that of the reactor through a by-pass on it, thus assuring the addition of catalyst to the reactor in a single pulse. Thereafter, desired pressure of ethylene monomer was charged to the reactor. When the reactor temperature was increased to the set point, the valve connected to the steel bottle was opened to start the polymerization. Polymerizations were stopped by turning off the stirrer first, closing the valve of ethylene line, and flashing off the unreacted ethylene. Then, the polymer powder was precipitated with 200 mL of ethanol and kept the polymer suspension overnight. Finally, the powder was filtered, washed with ethanol and dried overnight in a vacuum oven at 60 ff. Polymer yield calculated from the uptake rate of ethylene agreed within ± 5% with that measured by the weight of polymer produced. 2.5 Polymer Characterization The analysis of molecular weight and molecular weight distribution of produced polyethylene were performed by Polymer Laboratory SEC instrument Model 220 high-temperature GPC with two PLgel-Olexis columns. The columns and the detector were maintained at 160 ff, and HPLC-grade 1,2,4-trichlorobenzene (from Honeywell), was pumped through the columns at a flow rate of 1.0 mL·min−1 as solvent. Polystyrene (PS) samples of known molar masses were used as standards for the molar mass calibrations. The Mark - Houwink constants for the universal calibration curve were K = 4.06×10-4 and α = 0.725 for PE and K = 1.41×10-4 and α = 0.70 for PS. The thermal properties (peak melting temperature Tm and crystallinity Xc) of the polymers were analyzed by a TA DSC Q200 under N2 atmosphere. First, about 5 mg PE sample was sealed into an aluminum pan. An identical pan was used as reference. Then the temperature program was set to heat at 8 ACS Paragon Plus Environment

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a rate of 10 ff·min-1 to 160 ff (1st heating cycle) and keep at this temperature for 5 min. After being cooled down to 40 ff at 10 ff·min-1, the sample was again heated to 160 ff at the same rate (2nd heating cycle). The Tm and ∆Hf were calculated from the maximum value and integral area of the heating curve, respectively. Then the crystallinity are calculated by Xc = ∆Hf/∆Hf0, where ∆Hf is the enthalpy of fusion of the PE samples and ∆Hf0 is enthalpy of fusion of 100 % crystalline PE (∆Hf0 = 293 J·g-1). 3 Results and Discussion We studied the effects of polymerization conditions such as cocatalyst concentration, ethylene pressure, polymerization temperature and reaction time on the kinetics of ethylene homopolymerization with the Cr-iV bimetallic catalyst in the absence of hydrogen. For comparison, ethylene polymerizations with the corresponding monometallic (S-2 and iV) catalysts were also conducted to further illustrate the kinetic peculiarities of the bimetallic catalyst. It was found that the catalytic activity of iV catalyst is much lower than the other two catalysts, so the amount of iV catalyst used was 120 mg, while the dosage of S-2 and Cr-iV catalyst were both 70 mg for all the experimental runs. 3.1 Reproducibility studies A general precision of kinetic experiments is required for the kinetic study. It is known that catalysts for ethylene polymerization are extremely sensitive to purity of raw materials, cleanliness of the reactor, and the results can somewhat vary from batch to batch with the same catalyst. So special attention should be paid to obtain great reproducible kinetics curves. Figure 3 shows examples of ethylene polymerization kinetic profiles with three replicates under certain polymerization condition for the three catalyst systems. The agreement among the three replicates is outstanding. The rate pulse of the kinetic curves in the early stages of the reaction is attributed to the instantaneous catalyst injection by a high-pressure ethylene stream. The kinetic curves are a little erratic throughout the polymerization process. This is because the ethylene feed rate should compensate the variations of ethylene solubility caused by the fluctuation of temperature.



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Figure 3. Ethylene polymerization rate replicates with (a) S-2 (b) Cr-iV (c) iV catalysts, respectively. Polymerization conditions are: (a) catalyst 70 mg, 60 ff, 1 MPa, TIBA 0.25 mmol ; (b) catalyst 70 mg, 80 ff, 1 MPa ,TIBA 0.25 mmol; (c) catalyst 120 mg, 80 ff, 1 MPa ,TIBA 0.25 mmol. In all runs, 170 ml n-hexane was used as solvent and polymerization time was 1 h. 3.2 Effect of cocatalyst concentration Alky aluminums are widely used as cocatalyst in commercial reactors to change the catalyst performance and polymer properties. These compounds usually can not only act as scavengers, removing minute amounts of poisons from the reactor but also act as an activator or alkylating agents, accelerating the development of polymerization activity or changing the polymer properties37. Cocatalyst could influence catalyst activity and the alkyl aluminum concentration largely affects the polymerization kinetics through activation and deactivation of active species. In this work, TIBA is used as the cocatalyst in the subsequent investigation. Then, the concentration of TIBA in the reactor became


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an important factor which could affects the catalyst performance. A series of polymerization runs were conducted with the Cr-iV, iV and S-2 catalysts by varying cocatalyst dosages. Table 1 summarizes the polymerization results and polymer properties. The activities of the three catalysts all increase first with increasing TIBA dosages and then decrease thereafter. The prominent loss of catalytic activity caused by an overdose of cocatalyst has been reported has been reported for several catalyst systems in ethylene polymerization38-40. This is usually attributed to the over-reduction of active species. Woo et al.40 suggested that the propagation rate may be greatly lowered because of site modification of cocatalyst or the number of active centers was significantly decreased at high cocatalyst concentration. The optimal TIBA addition amount to obtain the highest activity for the Cr-iV catalyst is higher than that of the S-2 catalyst. This is because that not only chromium centers but also the extra vanadium centers need to be activated by cocatalyst. It is also noteworthy that the S-2 catalyst shows the highest activity while the iV catalyst shows the lowest even negligible activity among the three catalysts. The XPS characterization results (Table S2, Supporting Information) show that the binding energies of Cr2p become lower while the binding energy of V2p is enhanced in the Cr-iV bimetallic catalyst, compared with that of the Cr2p and V2p in the S-2 and iV catalyst, respectively. It reveals that the activity of Cr centers is suppressed while the V centers is increased due to the interaction of Cr and V. Since Cr centers are far more active than V centers, the Cr-iV bimetallic catalyst has less activity than that of S-2 catalyst. In addition, it is seen that iV catalyst mostly produces UHMWPE (having a molar mass higher than 106 g/mol), while S-2 catalyst produces PE with the lowest molecular weight (around 400,000 g/mol) among the three catalysts. The average molecular weight (around 700,000 g/mol) of the polymers made by Cr-iV catalyst falls in between the two monometallic catalysts and the MWD is much broader than that of the S-2 and iV catalyst, with an achieved polydispersity index (PDI) as high as 40. Table 1.Summary of ethylene homopolymerization results and polymer properties with different addition amount of TIBA to the reactor for Cr-iV, S-2 and iV catalyst, respectively. Polymerization conditions are: ethylene pressure 1 MPa; temperature 80 ff; n-hexane 170 ml; 1 h. Catalyst

Cocat

Activitya)

തതതതത ‫ܟۻ‬b)

PDIc)


Tm1d)

Tm2d)

Xc1e)

Xc2e)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cr-iV

iV

S-2

(mmol)

[g PE/(g cat·h)]

(×104)

0.15

31.4

64.0

0.20

90.2

0.25

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(℃)

(℃)

(%)

(%)

41.5

141

137

62.3

61.8

64.9

41.3

141

138

64.0

63.0

123.6

72.2

40.3

141

137

58.7

60.8

0.35

122.0

76.8

41.6

142

137

64.0

59.7

0.45

82.2

82.4

39.8

142

137

64.7

59.2

0.15

17.9

96.7

30.4

144

136

59.3

50.0

0.25

24.9

98.8

29.3

144

136

61.2

51.6

0.35

28.0

104.7

26.1

144

137

63.1

52.9

0.45

26.1

107.9

29.8

144

137

63.4

52.6

0.07

147.2

29.4

24.0

137

137

64.0

67.8

0.10

240.6

36.3

25.2

138

137

60.9

64.1

0.15

314.0

40.1

27.2

138

137

62.4

65.3

0.25

152.0

43.6

29.6

140

137

63.7

64.7

a) Activity of catalyst was the average number from three replicate experiments of activity. b) Mw was estimated തതതതത/Mn തതതത). d) Tm1 and Tm2 are the peak melting temperature of by HT-GPC in TCB. c) Polydispersity index (Mw nascent and recrystallized PEs, respectively. e) Xc1 and Xc2 are the crystallinity of nascent and recrystallized PEs, respectively.

Figure 4 shows the MWD curves of the polymers produced at different TIBA concentrations with S-2, Cr-iV and iV catalyst, respectively. According to the literatures, S-2 catalyst is usually known to produce broad MWD polyethylene (broader on both the high and low molecular weight ends) with a high molecular weight tail compared with the Phillips chromium-based catalyst18, 41. Productivity is particularly poor with S-2 catalyst in the absence of cocatalyst due to long induction times42. In our polymerization system, S-2 catalyst was not active if no cocatalyst is used in ethylene homopolymerization, probably attributed to the residual impurities which could deactivate the catalyst in the reactor. From Figure 4(a), it is observed that at very low TIBA concentration, MWD of the polymer is indeed broad with a high MW shoulder. Increasing cocatalyst addition enhances the high molecular weight part of the MWD curves. When TIBA addition amount is increased to 0.25 mmol thereafter, a noticeable bimodal distribution is even observed. 12 ACS Paragon Plus Environment

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Figure 4(c) shows that iV catalyst produces polymers with very high MW and relative narrow MWD. In general, vanadium based catalyst are useful for the preparation of high molecular weight polymers with narrow MWD4, 5. The GPC curves contain a long tail at the low-Mw part. With increasing cocatalyst concentrations, the average molecular weight increases and the MWD becomes narrower, while the position of the peak remains unchanged.

Figure 4. GPC analysis of polymers obtained with (a) S-2, (b) Cr-iV and (c) iV catalysts with different addition amount of TIBA to the reactor. Polymerization conditions are: ethylene pressure 1 MPa; temperature 80 ff; n-hexane 170 ml; 1 h. In Figure 4(b), it is seen that GPC curves of the polymers made by Cr-iV catalyst show bimodal distributions with a low-MW peak and an intensive high-MW peak, compared with that of the S-2 catalyst. With increasing concentrations of cocatalyst, the high-Mw peak (at about 1,250,000 g/mol) is enhanced, while the low-Mw peak (at about 55,000 g/mol) is weakened. Also, it is quite interesting to 13 ACS Paragon Plus Environment


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see that the position of the two peaks remains unchanged, corresponding to the low-Mw peak of GPC curves obtained with the S-2 catalyst in Figure 4(a) and the high-Mw peak of GPC curves obtained with the iV catalyst in Figure 4(c). Based on the above results, we may infer that both chromium and vanadium species are active centers in the bimetallic catalyst, and vanadium species is inclined to create high MW part with UHMWPE while Cr species is inclined to create low MW part. The bimetallic catalyst, hence, produces PE reactor blends with bimodal MWD containing variable amounts of UHMWPE. The content of UHMWPE in the reactor blends can be controlled to a certain degree by changing the concentration of cocatalyst. In most cases, a reduction in the molecular weight43, 44 is usually expectable since the reaction rate of chain transfer to cocatalyst increases as the concentration of cocatalyst rises. However, for all the three catalysts, the Mw of polymers increases gradually with increasing TIBA addition. Herein, it can be inferred that chain transfer to cocatalyst is not the main termination reaction that takes place in ethylene homopolymerization. On the other hand, the broad MWD of polymers made by both S-2 and iV catalysts signifies the multisite nature of the catalysts. PDI of the obtained polymers from S-2 catalyst increases gradually as cocatalyst concentration rises, indicating the formation of new sites. These sites are activated or become more active only in the presence of cocatalyst. Usually these sites tend to produce higher MW polymer37, thus increasing the molecular weight of the polymer. In the bimetallic catalyst, the two kinds of active centers (Cr and V) both exhibit a multisite nature and shows different response to alkyl aluminums. First, the molecular weight of polymers made by chromium and vanadium centers both increases when TIBA addition amount is increased according to our experimental results. Second, as is seen in Table 1, S-2 catalyst was very sensitive to alkyl aluminum, resulting in a sharp decrease in catalyst activity as TIBA dosages rise from 0.15 mmol to 0.25 mmol. Nevertheless, the activity of iV catalyst changed slightly. Therefore, the proportion of PE produced by chromium centers in the reactor blends declines. Usually vanadium centers produce UHMWPE, while chromium centers produce PEs with relative low MW. As a final consequence, the


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average molecular weight of polymers produced by the bimetallic catalyst increases and the MWD curves shift to the high molecular weight part with the increasing cocatalyst concentration.

Figure 5. MWD deconvolutions for ethylene homopolymers made by the Cr-iV bimetallic catalyst at different cocatalyst concentrations.

Figure 6. Mass fractions (a) and number average molecular weight (b) of polymer made in different site types as a function of TIBA addition amount in the reactor.



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Figure 5 compares the results for the MWD deconvolution of ethylene homopolymers made by Cr-iV catalyst with different cocatalyst dosages. Seven site types describe the MWD of these polymers remarkably well in all cases. Average molecular weight of the Flory components in these polymer samples and their contributions to the overall product are listed in Table S3 (Supporting Information). From Figure 6, it is seen that the mass fractions of polymer populations with higher Mn (sites 5 to 7) increase for higher cocatalyst concentration, while these fractions (sites 1 to 4) with lower Mn decrease. It indicates that the active sites that produce Flory components 5-7 with lower molecular weight decay faster and their Flory components contribute less to the polymers. The data on peak melting temperature (Tm) and crystallinity (Xc) in the two heating cycles measured by DSC is given in Table 1. For the iV catalyst, the nascent UHMWPE exhibits much higher melting temperature and crystallinity than melt-crystallized UHMWPE45-47. For the S-2 catalyst, Xc1 is slightly lower than Xc2 while Tm1 and Tm2 are very close. The thermal properties of the bimodal PEs balance that of the UHMWPE and low-Mw PE. Besides, Tm1 shows an upward trend with increasing cocatalyst dosages for the three catalysts. This is likely due to the increase of Mw. Usually, a polymer with higher Mw has higher physical attraction between the polymer chains and results in high Tm of the nascent samples48, 49. 3.3 Effect of polymerization temperature Figure 7 compares the kinetic curves at various polymerization temperatures for the three catalyst systems. They show complete different responses to temperature variations. For the S-2 catalyst, the rate profiles experience a long-time steady state at low temperatures (50 to 60 ff), while the catalyst deactivates more rapidly at higher temperatures (70 to 90 ff). The time for the max rate was reduced with increasing of temperature. In the case of the iV catalyst, it is interesting to note that temperature almost does not affect the shape of the kinetic curve, despite that the initial stage of the polymerization is slightly accelerated. This indicates that the activity of vanadium centers are insensitive to the variation of temperature. For the case of Cr-iV catalyst, when temperature is raised from 50 to 80 ff, the deactivation is slowed down, showing a reverse trend compared with that of the S-2 catalyst. However, 16 ACS Paragon Plus Environment

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further raising the polymerization temperature to 90 ff, the catalyst deactivated more quickly, although a higher maxima in reaction rates is observed.

Figure 7. Kinetic curves of ethylene homopolymerization with (a) S-2 (b) Cr-iV and (c)iV catalyst under various polymerization temperatures. Polymerization conditions are: temperature 80 ff; TIBA 0.25 mmol; n-hexane 170 ml; 1h. The dependence of catalyst activity on polymerization temperature is shown in Figure 8. The optimal temperature to obtain the maximum catalyst activity is different for different catalysts. For the Cr-iV catalyst the maximum activity is obtained at 80 ff while for the other two catalysts the optimal polymerization temperature is 70 ff. Plotting the effective rate constant keff = Rp/[M] (Table S4, Supporting Information) versus monomer concentration in a logarithmic-form gives activation energy of chain propagation reaction 27.19 KJ/mol for the bimetallic catalyst, higher than that of the other two monometallic S-2 and iV catalyst, 15.96 and 15.22 KJ/mol, respectively (Figure S3, Supporting 17 ACS Paragon Plus Environment


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Information). It reveals that the activity of the Cr-iV bimetallic catalyst is much more sensitive to reaction temperature than the monometallic catalyst, while the activity of S-2 and iV catalyst shows an approximated response to temperature.

Figure 8. Ethylene homopolymerization activities of three catalysts in the range of polymerization temperature from 50 to 90 ff. Polymerization conditions are: ethylene pressure 1 MPa; TIBA 0.25 mmol; n-hexane 170 ml; 1 h. Table 2 summarizes the properties of polymers made by Cr-iV, S-2 and iV catalysts at different polymerization temperatures. The Mw of the homopolymers reduces with the increase of polymerization temperature for all three catalysts, which may be attributed to the higher activation energies for lumped chain terminations than those for chain propagations50-52. For both iV and S-2 catalyst, when polymerization temperature decreases from 90 to 50 ff, PDI decreases continuously, while in the case of the bimetallic catalyst, PDI is not substantially affected by temperature in the range of 60 to 90 ff, but suddenly decreases at 50 ff. The results indicate that some of the active sites may deactivate at a low polymerization temperature. In addition, thermal properties of the polymers are also influenced by polymerization temperature. Tm1 of the PE samples increases with polymerization temperature decreasing, while Tm2 is not affected so much. With a few exceptions, Xc1 and Xc2 both show a slight upward-trend as temperature is increased. It is suggested that polymerization temperature changes the 18 ACS Paragon Plus Environment

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relative rates of molecule formation and crystallization47, 53. The metastable crystals formed at low temperatures may reorganize before melting during the thermal scans54, resulting in high Tm and low crystallinity. Table 2. Summary of polymer properties made by the three catalysts at different temperatures. Polymerization conditions are: ethylene pressure 1MPa; TIBA 0.25mmol; n-hexane 170ml; 1h. Catalyst

Cr-iV

iV

S-2

T

തതതതത ‫ܟۻ‬

℃

(× 104)

50

86.3

60

Tm1

Tm2

Xc1

Xc2

(℃)

(℃)

(%)

(%)

28.7

143

137

60.2

55.7

84.7

39.9

143

137

64.0

60.9

70

73.9

41.6

142

138

60.3

58.6

80

72.2

40.3

141

137

58.7

60.8

90

67.3

39.6

140

137

61.7

61.3

50

112.1

12.4

145

136

59.1

47.3

60

110.0

15.0

145

136

59.6

49.0

70

108.3

22.5

145

137

59.3

49.2

80

98.8

29.3

144

136

61.2

51.6

90

93.0

28.7

143

136

63.8

54.8

50

60.2

21.6

142

138

59.8

59.4

60

55.1

28.0

141

138

60.0

60.9

70

49.6

28.2

141

138

59.4

61.1

80

43.6

29.6

140

137

63.7

64.7

90

35.9

28.1

137

137

63.8

63.8

PDI

Figure 9 shows how the MWD varies with polymerization temperature for three catalyst systems. It is noted that the bimodality of GPC curves of S-2 catalyst is rather remarkable when the reaction temperature is as low as 50 ff. This could be explained as follows. First of all, the addition amount of cocatalyst (0.25 mmol) is excessive and the redundant cocatalyst will increase the high molecular weight tail in GPC curves, as we have mentioned above. Besides, low temperature usually increases molecular weight and enhance the high molecular weight part55-57. On the other hand, the decrease of temperature



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also resulted in an increase of MW for polymers made by iV catalyst, while the peak position of the MWD curves remains unchanged.

Figure 9. GPC curves of ethylene homopolymers produced at different polymerization temperature for (a) S-2 (b) Cr-iV (c) iV catalyst, respectively. Polymerization conditions are: ethylene pressure 1 MPa; TIBA 0.25 mmol; n-hexane 170 ml; 1 h. For the Cr-iV catalyst, both chromium and vanadium active centers show response to temperature variations. As we mentioned above, the high molecular weight part increases as temperature decreases for both monometallic catalysts. As a result, MWD of the polymers made the bimetallic catalyst should be much more sensitive to temperature, and in the most extreme case where the temperature is decreased to 50 ff, the low molecular weight part in GPC curves disappears and the bimodal shape is not apparent(Figure 9(b)).


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3.4 Effect of ethylene pressure Ethylene homopolymerizations with Cr-iV catalyst over a pressure range of 0.5-1.5 MPa are carried out and the kinetic profiles are shown in Figure 10. An increase in the ethylene pressure leads to a rise in the reaction rates. This is attributed to the increasing monomer concentration in the slurry polymerization system. Besides, it is also found that pressure has no effect on the shapes of the kinetic profiles. For comparison, the S-2 and iV catalyst are also employed in ethylene homopolymerization under varying ethylene pressures.

Figure 10. Kinetic curves of ethylene homopolymerization with Cr-iV catalyst under various ethylene pressures. Polymerization conditions are: temperature 80 ff; TIBA 0.25mmol; n-hexane 170ml; 1h. 3.4.1 Kinetic reaction order Determination of the kinetic reaction order is very useful for the understanding of the polymerization kinetics. However, reaction order with respect to monomer concentration (CE) or ethylene partial pressure (PE) may vary significantly with different catalyst systems and different polymerization conditions. Kissin58 reported a reaction order with respect to ethylene pressure in the range of 1.5-1.6 for gas-phase ethylene homopolymerization using a TiCl4/MgCl2-based Ziegler catalyst supported on silica. Nevertheless, when using a TiCl4/Mg(OEt)2/SiO2 Ziegler-Natta catalyst system, a reaction order of 1.8-1.9 was found under slurry conditions59. Han-Adebekun et al.60 showed a reaction 21 ACS Paragon Plus Environment


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order of 1.68 with a highly active TiCl4/MgCl2-supported catalyst. Dornik et al.61 reported a second-order dependency on the ethylene partial pressure when using metallocene catalyst. Wang et al.62 found that the pressure of monomer affects both the shapes of the rate-time profiles and the values of the polymerization rates with a Phillips chromium-based catalyst. An explanation for the observed differences in rate with pressure is greater fragmentation of the silica support with increasing pressure62. Carrick et al.18 reported the rate of polymerization is essentially linear with ethylene pressure with the S-2 catalyst. In our polymerization system, plotting the total polyethylene yield within 1 h as a function of PE in Figure 11 gives reaction order 1.0 for all three catalysts. The obtained first order with respect to ethylene pressure for the S-2 catalyst is in agreement with the findings of Carrick18.

Figure 11. Ethylene polymerization activity of the S-2, Cr-iV and iV catalyst at different ethylene pressures. Polymerization conditions are: temperature 80 ff; TIBA 0.25 mmol; n-hexane 170 ml; 1 h. 3.4.2 Effect of ethylene pressure on polymer properties The effects of ethylene pressure on Mw and MWD for the three catalyst systems are presented in Table 3. It is observed that the molecular weight and PDI appear to be relatively constant within the range of pressure variation. Figure 12 shows that the ethylene pressure almost has no effect on the MWD curves of the polymers produced by the three catalysts. Since H2 is in the absence of the polymerization system, the results indicate that chain transfer to ethylene plays a leading role in the chain transfer 22 ACS Paragon Plus Environment

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reactions while chain transfer to cocatalyst and spontaneous β-hydrogen elimination may be neglected. In the same table, it is also observed that the ethylene pressure influence the thermal properties of the polymers slightly, despite that Xc1 and Xc2 vary irregularly in a very narrow range with pressure. Table 3. Properties of ethylene homopolymers under different ethylene pressures with Cr-iV, S-2 and iV catalyst, respectively. Polymerization conditions are: temperature 80 ff; TIBA 0.25 mmol; n-hexane 170 ml; 1 h. Catalyst

Cr-iV

iV

S-2

Pressure

തതതതത ‫ܟۻ‬

(MPa)

( × 104)

0.50

71.9

0.70

Tm1

Tm2

Xc1

Xc2

(℃)

(℃)

(%)

(%)

38.3

142

137

64.8

61.9

70.0

37.9

141

137

63.2

59.7

1.00

72.2

40.3

141

137

58.7

60.8

1.20

70.7

36.7

142

137

63.5

60.1

1.50

68.6

41.7

142

137

65.0

62.6

0.70

97.4

24.2

144

137

61.3

51.6

1.00

98.8

29.3

144

136

61.2

51.6

1.20

99.0

25.8

144

137

60.9

50.9

1.50

99.6

27.0

144

137

60.8

51.2

1.70

100.0

28.4

144

137

62.5

52.9

0.30

42.0

25.0

139

136

58.8

60.4

0.50

42.2

29.3

139

137

61.8

64.5

0.70

44.9

28.1

139

137

63.1

64.4

1.00

43.6

29.6

140

137

63.7

64.7

1.20

44.4

29.2

140

138

61.0

63.0

PDI



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Figure 12. GPC curves of ethylene homopolymers obtained at different ethylene pressure for (a) S-2 (b) Cr-iV (c) iV catalyst, respectively. Polymerization conditions are: temperature 80 ff; TIBA 0.25 mmol; n-hexane 170 ml; 1 h. 3.5 Effect of reaction time The rate profiles of ethylene homopolymerization with the Cr-iV catalyst at different reaction times are shown in Figure S4 (Supporting Information). The curves superimpose perfectly, which also proves the excellent reproducibility of the experiments. Table 4 summarizes the effects of reaction time variation on catalyst activity and polymer properties. Polymer yield increases as reaction time goes by, while catalyst activity is reduced due to the deactivation of the catalyst. Average molecular weight and PDI almost do not change with reaction time. Figure 13 shows that the MWDs of the polymers remain approximately unchanged within experimental error as the duration of the polymerization increase from 10 min to 120 min. That is, Mw increases rapidly for an initial period of 10 min or even a shorter time 24 ACS Paragon Plus Environment

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(we didn’t conduct the polymerization with reaction time less than 10 min because the yield would be very low) and then reaches a constant value during the whole polymerization thereafter. These results indicates that a steady state between chain propagation and chain termination occurs after 10 min. Table 4 also shows that the reaction time has no significant effect on the crystallinity and melting point, with Tm1 and Tm2 staying around at 141 and 137 ff, respectively, while Xc1 and Xc2 both ranging from 58% to 62%.

Table 4. Summary of ethylene homopolymerization results and polymer properties at different polymerization times with the Cr-iV catalyst. Polymerization conditions are: ethylene pressure 1 MPa; TIBA 0.25 mmol; temperature 80 ff; n-hexane 170 ml. Time

Yield

Activity

തതതതത ‫ܟۻ‬ 4

PDI

Tm1

Tm2

Xc1

Xc2

(℃)

(℃)

(%)

(%)

(min)

(g)

[g PE/(g cat·h)]

(× 10 )

10

2.0

169.1

69.6

41.1

141

137

58.0

58.5

20

3.4

147.6

69.6

39.5

141

137

60.9

61.1

40

5.9

126.6

70.4

37.6

141

137

58.4

60.5

60

8.7

123.6

72.2

40.3

141

137

58.7

60.8

90

11.4

109.0

74.3

38.4

142

137

59.1

60.5

120

14.2

101.7

68.1

39.7

141

137

62.0

62.1



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Figure 13. Molecular weight distributions of polymers obtained at various polymerization times. 4 Conclusions A SiO2-supported imido-vanadium /silyl-chromate bimetallic catalyst was prepared and ethylene homopolymerization using this catalyst was performed with varying operating conditions. Together with this catalyst system, the corresponding silyl-chromate catalyst and imido-vanadium monometallic catalyst were also prepared for comparison. It was found this kind of bimetallic catalyst is capable of producing a blend of polymers, with a bimodal MWD and considerable amount of UHMWPE, due to the introduction of vanadium to chromium species. The two peaks in the bimodal MWD were identified as emerging from chromium and vanadium species separately. The chromium-associated active center produces the low-MW polymer peak while the vanadium-associated active center produces the high MW peak. The MWDs of these homopolymers shift toward higher molecular weight with higher cocatalyst dosages while the position of the two peaks remains unchanged. MWD deconvolution results reveal that mass fractions of polymer populations with higher Mn increase, while these fractions with lower Mn decrease for higher cocatalyst concentration. The peak positions of the MWDs for each population do not change. The deactivation of the Cr-iV catalyst becomes slower with increasing temperature below 80 ff, but further increasing temperature increases the deactivation rates. The Mw of the homopolymers decreases with the increase of polymerization temperature for all three catalysts. The polymerization rates all show first-order dependence with respect to ethylene pressure for both bimetallic and monometallic catalysts. Increasing ethylene pressure has no effect on MWD, suggesting that transfer to monomer is the main chain transfer mechanism. The increase in polymerization time improves the polymer yield, but reduces the catalyst activity. The Mw of the resultant polyethylenes initially increases with reaction time growing, becoming independent of the duration of polymerization after about 10 min. In addition, cocatalyst concentration and polymerization temperature greatly influence the thermal behaviors of the polymers, but in varying degrees for different catalyst systems. Ethylene pressure and reaction time seem to cause no noticeable changes of thermal properties. 26 ACS Paragon Plus Environment

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Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21406061 and 21674036) and the Fundamental Research Funds for the Central Universities.

Supporting Information The bulk composition analyzed by ICP and XPS characterization of the three catalysts, determination of the minimum number of active sites in the Cr-iV catalyst, MWD deconvolution results of the PE samples made by the three catalysts, calculation of activation energy and the rate profiles at different reaction times.



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Peculiarities of Ethylene Polymerization Kinetics with an Imido

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