one-phase supported titanium-based catalyst for polymerization of ethylene. iv. effect of alkyl...

One-Phase Supported Titanium-Based Catalyst for Polymerization of Ethylene. IV. Effect of Alkyl Group at Organoaluminium Compound on Catalyst Performance

PAVE1 SINDELAR,'** DALIBOR MATULA,' and JAROSLAV HOLECEK*

'Polymer Institute Brno, Tkalcovsk6 2, 656 49 Brno, Czech Republic, and 'Department of General and Inorganic Chemistry, University of Pardubice, N6m. Legii 565, 532 10 Pardubice, Czech Republic

SYNOPSIS

The effect of a basic layer (Si02-R3Al intermediate) in the one-phase silica supported titanium-based catalyst was investigated using the simple model catalyst systems obtained by reacting the activated silica gel consecutively with R3Al and TiC14. Mode of the interaction of SiO, with R3Al-resulting in the formation of the basic layer-was observed via analysis of the concentration of the unreacted OH groups on the silica surface employing IR spectroscopy and via analysis of the concentration of aluminium in solvent using AA spectroscopy. I t was found that nature of the alkyl group in R3Al modified the structure of the basic layer, thus influencing the catalyst performance including the concentration of both the sum of Ti2+ and Ti3+ and the ESR-active T i (111) centers. The sum of Ti2+ and Ti3+ ranged from 45 to 52 mol % and the amount of the ESR-active T i (111) species ranged from 6 to 17 mol % of the all titanium content. A significant effect of alkyl group at organoaluminium compound on the molecular weight distribution of the resulting polymer was observed. 0 1996 John Wiley & Sons, Inc. Keywords: silica titanium Ziegler-Natta catalyst ethylene polymerization IR and ESR spectroscopy

INTRODUCTION

Progress in the development of highly active Ziegler- Natta catalysts is closely connected with the introduction of support materials such as MgC121-3 or Si02,4-7 and with the discovering of the activating effect of Mg-compounds in the titanium catalysts.a11 Hence, many types of MgC12-supported titanium- based catalysts have been widely studiedl2-l6 or employed in the commercial scales. Until now, much less attention have been paid to the study of silica supported titanium-based catalysts, though silica gel concealed some advantageous properties in comparison with MgC12. It should be mentioned that OH groups covering the surface of silica gel create possibility to utilize them for anchorage of the active

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 34,2163-2171 (1996) 0 1996 John Wiley & Sons, Inc. CCC 0887-624X/96/112163-09

centers on the surface of support via chemical bonds. It was the principal idea of developing the silica- supported titanium-based catalyst in our group. The further requirement for the catalyst development was to avoid a necessity of addition of an external organometal in the polymerization reactor. The catalyst developed is prepared by a consecutive interaction of organometals and titanium compound with activated silica gel.17*'8 We found that the first step of the catalyst synthesis-impregnation of silica gel with an organoaluminium compound-predetermined catalyst performance. Type of organoaluminium compound affects the amount of reducing agents used in the next steps of catalyst synthesis" as well as the ability of the catalyst to control molecular weight of polymer by a transfer with hydrogen.20*21 These results refer to an important role of basic layer in the aforementioned catalyst. The ob- jective of this study was to gain an insight into reactions taking ,place on the silica surface during the

2163

2 164 SINDELAR, MATULA, AND HOLECEK

first two steps of the catalyst synthesis. Thus, the study was carried out using a model ternary catalyst system Si02 - R3Al - TiC1,.

EXPERIMENTAL

Chemicals

Silica gel Davison, grade 952, was calcinated in a plant activator, a t first in an air then in a nitrogen stream at 800°C. This procedure led to a reduction of the amount of OH group on the silica surface to 0.65 mmol/gsio2.

Organoaluminium compounds triethylaluminium (TEA), triisobutylaluminium (TIBA), tri-n-hex- ylaluminium (TNHA), tri-n-decylaluminium (TNDA) (all Schering) and TiC1, (Ferak, Germany) were used as received, diluted by heptane to ca. 0.5 mol/L.

Procedures

All sealed glass devices connected to a high vacuum line were employed for samples preparation and for measurement of their infrared and electron spin resonance spectra.

The SiO, - R,AI System Preparation and Analysis

Mode of the interaction between organoaluminium compound and silica gel was investigated employing two procedures. Concentration of unreacted OH groups on the silica surface was determined using infrared spectroscopy (IR), and a portion of unsupported organoaluminium compound remaining in heptane solution was evaluated from the concentration of aluminium determined using atomic absorption spectroscopy (AAS). The Si02- R3Al samples were prepared by addition of the calculated amount of organoaluminium compound to silica slurried in heptane. The amount of the selected organoaluminium compound varied in the range of 0 < CAI < 1.2 mmol per gram of SiO,. After 1 h reaction time, the excess of solvent was removed and the solid sample was dried in uacuo. Then, the dried impregnated silica was transferred into an all-sealed quartz cuvette (d = 10 mm) attached to the reactor vessel. Finally, the sample was reslurried in CCl, and used for measurement of the concentration of the unreacted OH groups. Concentration of the unreacted OH groups of the particular sample was determined from the intensity of the signal a t 4505 cm-' in accordance with the method described by Kratochvila et al.22v32 A portion of the unsupported organoaluminium compound was determined from a sample of solvent

taken off just before drying of the impregnated silica. The aluminium solution employed for the quantitative measurement of A1 was prepared by mixing of known amount of heptane solution with 30 mL of HCl solution (1 mol/L) in a graduated flask (vol- ume = 50 mL). After hydrolysis of organoaluminium compound and dissolution of white precipitation in HC1 solution, heptane was stripped off and further amount of 1 N HC1 solution was added to 50 mL.

Catalyst Prepara tion

The catalyst sample was prepared by a consecutive addition of the calculated amounts of organoaluminium and titaniumtetrachloride compound to silica gel slurried in heptane.lg The amount of organoaluminium compound did not exceed an equiva- lent amount to OH group (Al/OH = 0.95) to meet the requirement for a synthesis of the one-phase silica supported catalyst. The catalyst was dried in uucuo and transferred into small ampoules (50-350 mg) employed either for polymerization tests or titanium oxidation states analysis, and into an ESR quartz ampoule attached to the reactor vessel. The polymerization test under isobaric and isothermal conditions was used for evaluation of catalyst activity. Details of the gas-phase polymerization procedure have been given previo~sly.'~

Titanium Oxidation States Analysis

The oxidation states of titanium were determined by a combination of redox titration and elemental analysis. As the catalysts are air sensitive, a technique using special designed glass devices was developed. A three-necked bottle equipped with an in- let of highly pure nitrogen, a titration burette firmly attached to the vessel, and a Teflon valve used as a breaker was employed in the course of quantitative titrations. The titration of a stirred suspension of the catalyst in 1 N H2S04 containing KSCN as the indicator was carried out according to the method described by Jenkins.24 A ferric sulphate solution was used for direct titration of Ti3+ in this procedure. The method was checked against known TiC1,. All these analyses were performed on triplicates.

Methods

Infrared Spectroscopy

Infrared spectra of silica gel samples in the region of 4000-10,000 cm-' were measured in 1 cm quartz cell using a Perkin-Elmer 330 instrument. Peak in- tensities were evaluated from the second derivative value of the absorption curve.

ONE-PHASE SUPPORTED TITANIUM-BASED CATALYST FOR ETHYLENE POLYMERIZATION 2165

Table I. Interaction of Trialkylaluminium Compounds (0.61 mmol/g) with Silica Gel Having 0.65 mmol OH per g

Amount of Reacted OH Qualitative Parameters CAI in liquid

RBAl mmol/gsio, % mol" % molb Dimers Steric Effect ~

TEA 0.42 TIBA 0.55 TNHA 0.47 TNDA 0.43

65 85 72 67

0 0

10 25

a Related to the original content of OH groups. Related to the charged amount of R3AI.

ESR Spectroscopy

An ESR-221 spectrometer (ZGW, Berlin) operated in X-band (- 9.5 GHz) with a magnetic field mod- ulation frequency of 100 kHz was used in the study. Catalyst-after finishing its synthesis-was sealed up into the quartz ESR ampoule. A sample of di- phenylpicrylhydrazyl (DPPH) served as a precise field calibration for the calculation of the g value. Details of measurement and analysis of the catalysts were ascribed in Part IILZ5

AA Spectroscopy

Atomic absorption spectroscopic determinations were carried out with Pye Unicam (PU 9400) spectrometer provided with 5 cm air-acetylene burner. Calibration was carried out on the basis of Tritisol (Merck) standard solutions.

Gel Permeation Chromatography

GPC analyses of the PE samples were carried out in 1,2-dichlorbenzene at 135OC using a Waters GPC

0 0 0 2 0 1 0 0 n II I 0 I::

I I I I I I I O ~ / g<,oz I

Figure 1. Dependence of concentration of OH groups of silica gel on the amount of trialkyl-aluminum compound.

200 equipped with four silica gel columns (pore size 102-105 nm) and a differential refractive index de- tector. The molecular weight calibration curve was obtained on the basis of the universal calibration using several narrow molecular weight distribution polystyrene standards. The appropriate constants were taken from the article of Dawkins et a1.26

RESULTS AND DISCUSSION

Reaction of Silica Gel with Organoaluminium Compound

The interaction of silica gel with an organoaluminium compound represents the first step of synthesis of the silica-supported catalyst. Impregnating silica gel with different organoaluminium compounds (TEA, TIBA, TNHA, and TNDA), four sets of the Si02 - R3Al samples were prepared. The organoaluminium compounds were chosen with the respect to two aspects: (1) the position of the monomer- dimer equilibrium of the R3Al compound in hydrocarbon solvent^,^' and (2) the sterical effect of the alkyl group on the reactions taking place in the course of the catalyst synthesis.

The probable effects of the particular aspect of RBAl compound on catalyst properties are illustrated in the last two columns of Table I. The interaction of the R3Al with silica gel was investigated both by evaluating the concentration of the unreacted OH groups on the silica surface using IR spectroscopy, and by analyzing the supernatant for concentration of aluminium. The infrared spectra of the SiOz - R3Al samples indicated a significant effect of structure of the alkyl group on the amount of unreacted hydroxyl groups (see Fig. 1). The highest proportion of the interacted R3Al compound with hydroxyl groups presenting on the silica surface was observed for TIBA. The formation of the

Si - 0 - A1 ' species having structure I is supposed \


to be a principal reaction product of this reaction. The trialkylaluminium compounds having longer alkyl group like as hexyl- and decyl- and also TEA did not react as effectively with hydroxyl groups on the silica surface. When TEA or TIBA were employed, no aluminium was observed in the heptane supernatant until the ratio of Al/OH was equal to 1.2. As it is shown in Figure 1, the same conversion

of OH groups into the Si - 0 - A1 \ species (90%)

was attained by using both these organometals. For such a conversion it was necessary to add 0.6 mmol of TIBA or 1.2 mmol of TEA to the silica gel. We assume that the difference between the reactivity of TEA and TIBA towards functional OH groups presenting on the silica surface is based on the higher proportion of dimers in TEA heptane solution. Thus, it is supposed that the interaction of TEA with silica results in a formation of the various structures on the silica surface, for example, dimers having structure I1 or species with the structure 111. The formation of the species having structures I1 or I11 on the silica surface have already been discussed by Slotfeldt-Ellingsen et a1.28

/

Et \ /Et

/ Si ' 0 t AI-Et / Si-0, / A l \ /Et

\ Et / A ' \ E t si ' ' Et Si-0-AI

Et

I II 111

When using TNHA or TNDA in the same amount as TIBA, the reaction of these two organometals with silica gel was quite different. The amount of reacted OH groups was lower than for TIBA; moreover, a certain part of the organoaluminium compound was not anchored on the silica surface and remained in heptane see (Table I). Employing even higher amount of TNHA or TNDA than 0.61 mmol/ g led predominantly to an increase of the aluminium concentration in heptane, while only a negligible drop.of the OH group concentration on the silica surface was observed. These results say much for an inaccessibility of the part of the OH groups to the reaction with the organoaluminium compounds having longer alkyl group. For these R3Al compounds the formation of the species on the silica surface with structures I1 and I11 is negligible from the same viewpoint.

Thus, after reaction of the R3Al compound with silica gel the following species can be formed on the

silica surface: (1) - Si - 0 - A1 ' (covalently

bonded organoaluminium), formula I, (2) chemi- \

sorbed organoaluminium compound in the form of: (a) dimeric species as demonstrated by the formula 11, (b) Lewis acid-base intermediate as demonstrated by the formula 111, and 3) unreacted OH groups.

Reaction of TiCI, with SO2- R,AI

An interaction of TiC14 with silica gel impregnated by R3Al compound led to a formation of light-brown colored product. It is supposed that this color change reflected formation of Ti - A1 complexes or reduction of Ti4+ to lower valency states. The reaction product represented catalyst system polymerizing ethylene. Changes in the OH group content on the silica surface, oxidation state of titanium, and polymerization performance of the catalysts will be discussed in the next parts in more detail.

Concentration of OH Groups-IR Analysis

The addition of TiC14 to the silica impregnated by trialkylaluminium compound represents the second step of the catalyst synthesis. It was shown above that the extent of various structures formed on the silica surface strongly depended on the length of alkyl group at R3Al compound. A diversity of the particular basic layer created opportunity for TiC14 to form different species in the catalyst. The effect of the addition of TiC1, to the SiO, - R3Al systems in that the molar ratio of organoaluminium compound to OH group was kept equal to 0.95 on concetration of unreacted OH groups is demonstrated in Figure 2. The addition of 0.1 mmol of TiC14 to the system Si0,-TEA brought about a further drop in the amount of the unreacted OH groups, while for the

"."

0 W,/ TEA-TiC14 0 TEA

zl SO,/ TlOA-TiCl4 0 TIRA

0 . 4 - -

0 2 - -

0 0 I) 2 0.1 0 6 0.11

rA, Iitilriol/ g S,O%

Figure 2. Effect of an addition of TiC14 to the SiOa - R,Al systems on the concentration of OH groups on the silica surface.

ONE-PHASE SUPPORTED TITANIUM-BASED CATALYST FOR ETHYLENE POLYMERIZATION 2 167

11 ! 0 1 0 1; I1 11

, 1 1 1 1 1 1 1 1 d / ~ I \,(Id

Figure 3. of silica gel on the amount of TiCl,.

Dependence of concentration of OH groups

Si0,-TIBA system nearly no change was observed. The systems with TNHA and TNDA behaved in the same way as the TIBA system.

The lowering of the amount of unreacted OH groups after addition of TiC1, to the SO2-Et3Al system originated in all probability in a reorganization of Al-agglomerates on the silica surface by TiC1, followed by a penetration of the liberated complexes through the pores of silica. In the case of the Si02-TIBA system, where the conversion of OH groups attained nearly 90%, a formation of the Si - 0 - A1 -Ti species bonded to the silica surface was assumed as a principal reaction product. The extent of side reactions was negligible as proved by unchanged concentration of OH groups. Nearly no change in the concentration of OH group for systems with TNHA or TNDA might be explained by a dominating role of sterical effect of alkyl group in R3Al. If neither primary organoaluminium compounds could access all OH groups in the pores of silica then the A1 - Ti complex could neither do the same. It is assumed that Ti - A1 complexes formed in a such way were only weakly bonded to the silica surface. For comparison with these results, the reaction of TiC1, with the primary silica gel was also evaluated (see Fig. 3). It was found that TiC1, reacted with OH groups according to eq. (1).

I

I -Si-OH + Tic& +

I

I -Si-0-TiCl3 + HC1 (1)

Oxidation States of Titanium in the Catalyst

The concentrations of the sum of Ti2+ and Ti3+ in the catalyst were determined by redox titration, and the total titanium content was obtained by elemental analysis. The results summed up in Table I1 showed that only at about one-half of the total amount of TiC1, charged in the system was reduced to lower valency states of titanium. The quantity of the sum of Ti2+ and Ti3+ in these catalysts indicated that reducing power of organoaluminium compound was lowered as a result of a formation of the

Si - 0 - A1 ' basis. Further, the results showed

that the proportion of the reduced amount of titanium-expressed as the sum of Ti2+ and Ti3+-attained the maximum value for the systems with TIBA and TNHA. Surprisingly, the system with TEA gave the lowest amount of the sum of Ti2+ and Ti3+ within this set. The extent of reduction of titanium for the system with TNDA was found lower by 10% than the TIBA system (see Table 11). The sum of Ti2+ and Ti3+ in catalysts prepared in toluene increased by 40 mol % for the TEA catalyst, while no change was found for the TIBA catalyst. These results indicated that the dielectric constant of solvent played an important role only for catalyst in that a certain part of organoaluminium component was not firmly bonded to the silica surface. The rea- son for toluene usage will be discussed 1ater.The technique for quantitative analysis of Ti2+ is under development nowadays. Therefore, precise results of the concentration of Ti2+ will be presented to- gether with the detailed description of this method later.29

The catalyst systems consisting of silica gel impregnated with diverse types of the R3Al compounds and TiC1, exhibited ESR activity. The spectra typ- ical for solid material containing titanium species

\

Table 11. Catalyst Systems SiO, - R3Al - TiC12'

Oxidation States of Titanium in the

2 [ T ~ ~ E s R ZTi2+ + Ti3+ Ti4+ ORG 1 (mol %) (mol %) (mol %)

TEA^ 6.75 37 63 TIBA 14.80 52 48 TNDA 16.82 45 55 TEA' 10.7 52 48

a Related to the original amount of TiCl,, (mol %). bFurther signal with g-tensor parameters g, = 1.978, g,

= 1.929 and g, = 1.892 (2.38 rnol % of the original amount of TiC1, was observed. ' Catalyst synthesis carried out in toluene.


in oxidation state Ti3+ in the ligand field with low symmetry were obtained. The spectra of the particular catalytic systems differed from each other first of all in the intensity.

In the ESR spectra of all samples dominant rhombic anisotropic signals A with g-tensor parameters of g, = 1.962 + 0.003, g, = 1.945 + 0.001, g3 = 1.913 + 0.003 were observed. Moreover, the spectrum of the catalyst system SiO, - Et3A1 - TiC1, contained a subdominant rhombic signal B with g- tensor parameters of g, = 1.978 + 0.003, g, = 1.929 + 0.003 andg, = 1.892+ 0.005 representing at about 35% of the whole content of the ESR-active Ti (111) centers. For the systems containing TIBA or TNDA the amount of the ESR active species giving signal A significantly contributed (from 90 to 92%) to the overall intensity. The proportion of the subdominant signal B related to the total concentration of the ESR active centers was negligible for these systems.

Results summarized in Table I1 showed that with the increasing length of the alkyl group at organoaluminium compound increased concentration of the ESR active Ti (111) centers. The greatest increase in ESR activity was observed after replacing TEA by TIBA. However, only a moderate increase in ESR activity was observed when employing organoaluminium compound with a longer alkyl group like TNDA. In principal, the studied systems differed in the presence of the subdominant rhombic signal B that was found exclusively in the system with TEA. The amount of the ESR active species giving signal B conspicuously correlated with the amount of the part of TEA adsorbed on the silica surface or in other words with the amount of unreacted OH groups. Therefore, we tried to modify the mode of the interaction between silica gel and R3Al compound in such a way that the catalyst synthesis was carried out also in other solvents. It is known that aromatic solvents due to interaction of p-electrons shift the monomer-dimer equilibrium for the benefit of monomer. Toluene-as a first choice-was employed instead of heptane in the course of synthesis of the catalysts with TEA and TIBA. The replacement of solvent substantially increased both polymerization and ESR activity of the catalyst based on TEA only (see Tables I1 and 111). The total amount of the ESR active Ti (111) centers for catalyst prepared in heptane being equal to 6.75 mol % increased with the usage of toluene up to 10.7 mol %. Moreover, the disappearance of the rhombic signal B in the ESR spectrum of the TEA catalyst indicates that usage of toluene significantly changed the process of the active site formation. We have been already published (see Part 111)25 that the rhombic

signal B appeared in the catalyst systems where re- verse order of addition of catalyst component to silica gel was employed. This way of catalyst synthesis promoted a formation of the structures with titanium covalently bonded to the silica surface

(Si - 0 -Ti - A1 ). The same reaction was pos-

sible in a certain extent also in the TEA catalyst studied now because the organoaluminium compound did not completly cover OH groups presenting on the silica surface. The tendency to form inactive TiC13 “clusters” was also supported by a higher decrease of the relative concentration of both catalyt- ically and ESR active center with the increasing loading of TiCl, on the silica surface if the catalyst with TEA was compared with that one using TIBA (see Fig. 2 in ref. 25).

/ \

Polymerization Tests

Polymerization tests were carried out in a gas-phase process using a fluidized-bed stainless steel reactor and the observed kinetic rate-time profiles are summarized in Figure 4. All polymerization runs were carried out without the addition of cocatalyst.

Polymerization activity of the catalyst-herein- after designed as productivity-is expressed in the way that yield of the polymer obtained within 2 h is related to unite weight of the catalyst and to 1 h. Results summarized in Table I11 demonstrate the influence of the alkyl group length on productivity of the catalyst and on apparent activation energy of polymerization EAKT. The catalyst productivity has increased with the increasing length of the alkyl group at organoaluminium compound. The substi- tution of TEA for TIBA in the catalyst system brought about a substantial increase in catalyst activity as indicated values of the productivity and a substantial change in the polymerization rate profiles. We found that the kinetic curve obtained with the TEA catalyst was of the decay type, whereas those obtained with the catalysts based on TIBA, TNHA, or TNDA were of the acceleration type. The activity as well the polymerization rate of these catalysts increased with an increasing length of alkyl group. The catalyst with TIBA as well with TNHA and TNDA exhibited substantially lower apparent activation energy of polymerization compared with the system comprising TEA. Moreover, the results contained in Table I11 showed a twofold decrease in catalyst activity connected with the addition of hydrogen and a poor ability of these catalysts to control molecular weight of polymer.


Table 111. Effect of Organoaluminium Compound in Ethylene Polymerization for Supported Catalysts"

Productivity (gPE/gcat - h) MLN" Activation Energy

No. R3A1 PFLUID PFLUIDb g/10 min (kJ mol)

1 TEA 90 45 0.025 52.4 2 TIBA 115 104 0.044 34.5 3 TNHA 128 105 0.035 32.1 4 TNDA 136 108 0.035 30.7 5 TEA^ 135 87 0.023 52.1 6 TIBAd 118 105 0.045 34.2

a SiO2-R3A1-TiC4, Si02 Grace 952 (having 0.65 mmol OH group per g) was consecutively reacted with R3A1 (0.61 mmol/g) and TiCl, 0.1 mmol/g. Polymerization conditions are temperature 90°C, ethylene pressure 2.1 MPa.

Temperature 9O"C, ethylene pressure 1.7 MPa, hydrogen pressure 0.4 MPa.

Catalyst synthesis carried out in toluene instead of heptane ' Melt Index of polymer a t 21.6 N.

The difference in the values of E A K T indicate the deviation in the particular steps of the polymerization mechanism, which could be descended from the changes of both a quality of the active site and a quality of its intimate surrounding. Polymerization tests showed that the usage of toluene brought about a significant increase in productivity of the catalyst based only on TEA (see Table 111). On the other hand, the value of apparent activation energy remained the same, thus indicating that no change in the quality of the active site and its intimate surrounding took place. Taking into account also the results from ESR spectroscopy and analysis of the oxidation states of Ti, it could be concluded that toluene provided better conditions for formation a higher amount of separated active sites in comparison with heptane.

n 2 0 4 0 fin t i0 i n o i::o t ir iw [ I ~ L I I ~ ~

Figure 4. Kinetic curves obtained with catalysts having diverse types of the ORG-1 compound. Polymerization conditions: TR = 90°C, PC, = 2.1 MPa.

The polyethylenes resulted from polymerization runs with hydrogen over the studied catalysts at 90°C were analyzed with GPC. As it is shown in Figure 5 , these catalysts produced polymers with a quite broad molecular weight distribution having a dominant maximum with M,, arround lo5. The MWD curves differed in their shapes with respect to the R3A1 compound employed. An unimodal distribution observed for the TIBA catalyst reflected the presence of one kind of active site in this catalyst system. It is suggested that in this active site titanium compound is coordinately bonded via the Si-0-A1 fragment to the silica surface. The MWD curves in Figure 5 . showed that this maximum was presented also in other analyzed PE samples. This finding suggests that the active site with abovementioned arrangement was formed in all

I .o

0.11

2 0 . 6

a M

\

-5 0 . 4

0 . 2

0.0 I 2 1 1 5 6 11

log M

Figure 5. Effect of the R3A1 compound employed in the catalyst synthesis on the molecular weight distributions of polyethylenes.

2170 SINDELAR, MATULA, AND HOLECEK

studied catalysts. The appearance of an additional maximum in the PE samples obtained with the TEA or TNDA catalysts reflected formation of another type of active site in these catalysts. The location of the additional maximum on the MWD curves of the polymers depended on the organoaluminium compound employed. The catalyst with TEA tended to form a certain amount of low molecular weight (LMW) products, while the catalyst with TNDA produced a marked amount of high molecular weight (HMW) products. The presence of at least two kinds of active sites in the TEA and TNDA catalysts should be connected with the occurrence of a certain amount of unreacted R3Al compound in the SiOz - R3Al system. The position of the additional maximum on the MWD was assumed to be connected with the way of bonding of the unreacted part of R3Al compound. For the TEA catalyst, the effect of toluene employed in the course of catalyst synthesis on the MWD was evaluated. From Figure 6, it is clear that toluene caused a mild narrowing of the MWD, predominantly due to absence of the LMW products. This result supported an assump- tion that TEA agglomerates formed on the silica surface contributed to production of the LMW fractions in polymer. It is known that the ability of TiC13-based catalysts to control MW of polymer by using hydrogen is low. Hence, we assume that the presence of HMW polymer chains reflected formation of alkylated TiC13 clusters precipitated in the pores of silica in the TNDA catalyst.

CONCLUSIONS

The model ternary systems enable a detailed study of reactions taking place in the course of the first two steps of synthesis of the silica supported Ti- based catalyst. The results presented here reflected the influence of mechanism of the interaction between R3Al compound and silica gel on catalyst performance.

The reaction of organoaluminium compound with the isolated OH groups presenting on the silica surface results in formation of a potential active species

/ \ *

having the following structure - Si - 0 - A1

Through this reaction the R3Al is immobilized on the silica surface and the alkylation and reducing power of the organoaluminium compound is reduced as well as. It was shown that the length of alkyl group at R3Al influenced the reaction of organoaluminium compound with SiOz in two ways. First, the length of alkyl group significantly affected the

I I I , (i / I log hl

Figure 6. Effect of solvent type employed in the TEA catalyst synthesis on the molecular weight distribution of polyethylenes.

structures formed on the silica surface with respect to the shifts in monomer-dimer equilibrium of organoaluminium compound.27 Second, the length of the alkyl group predetermined the accessibility of OH groups on the silica surface for reaction with the organoaluminium compound. The abovementioned effects significantly influenced a quality of the catalyst's basic layer, thus affecting performance of the particular catalyst system.

The amount of R3Al compound covalently bonded to the silica surface as well as the way of bonding of the unreacted part of RBAl were the main differ- ences in these SiOz - R3Al systems. With respect to the diversities of the SiOz - R3Al systems, it is natural, that TiC14 added thereafter could form species with different stuctures in the particular system. Results presented here show that catalyst activity has increased with the increasing length of alkyl group at R3Al, with a bound change between the TEA and TIBA system. Furthermore, analyses of the catalysts showed that both the extent of reduction of titanium (expressed as a sum of Ti2+ and Ti3+) and the concentration of the titanium ESR- active centers also increased with the increasing length of alkyl group. Surprisingly, the lowest value of the sum of Ti2+ and Ti3+ and the concetration of the titanium ESR-active centers was found for the TEA system. Moreover, the TEA system was the only catalyst exhibiting a subdominant rhombic signal B. We have already demonstrated (see Part 111) 25 that ethylene polymerization activity is closely connected with the presence of the species with A rhombic signal. Species with the B-signal were found to be practical catalitically inactive. Results presented in the article have shown that process re-


sulting in formation of the B-type species can be altered to produce the A-type species if the catalyst synthesis is prepared in toluene instead of heptane. This change of solvent brought about a great increase in the sum of Ti2+ and Ti3+, the concentration of the titanium ESR-active centers as well as in catalyst activity.Replacement of solvent affected also the distribution of the polymer.

Diversities in the catalysts have manifested also in the distributions of polymers. GPC analyses of the resulting polymers reflected presence of three types of active centers in the catalysts studied. A dominant maximum with M, - lo5 was found in all polymers, demonstrating the presence of the cor- responding active center in all catalyst systems. The TIBA-based catalyst was the only catalyst giving polymer with an unimodal MWD. It was also the only system where a portion of covalently bonded organoaluminium compound to the silica surface attained 90%. A bimodal GPC curves of the other polymers demonstrated that the presence of the unreacted part of R3Al in the basic layer gave opportunity for broadening the MWD. The location of the additional peak on the GPC curve depended on the way of bonding of the unreacted part of R3A1. The TEA system where the unreacted part of R3Al formed agglomerates on the silica surface tended to produce low molecular weight polymer fractions. While the TNDA system in which the unreacted part of R3A1 remained free in solvent tended to form high molecular weight fractions.

The authors thank Dr. ZdenBk Salajka for helpful com- ments and discussions, Dr. Pave1 Hudec for performing of the GPC measurement, and Dr. Alena ObdriLlkovi for performing of analysis of titanium oxidation states.

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Received July 19, 1995 Accepted January 4, 1996

one-phase supported titanium-based catalyst for polymerization of ethylene. iv. effect of alkyl...

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