The influence of the Ti 3+ species on the microstructure of ethylene/1-hexene copolymers

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1 1298 Macromol. Chem. Phys. 200, (1999) The influence of the Ti 3+ species on the microstructure of ethylene/1-hexene copolymers Kyung-Jun Chu 1, Joao B. P. Soares* 1, Alexander Penlidis 1, Son-Ki Ihm 2 1 Institute for Polymer Research, Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada jsoares@cape.uwaterloo.ca 2 Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Kusong-dong, Yusong-gu, Taejon, , Korea (Received: November 17, 1998; revised: January 19, 1999) SUMMARY: Copolymerization of ethylene and 1-hexene was carried out with catalysts having isolated Ti 3+ and multinuclear Ti 3+ species. Carbon-13 nuclear magnetic resonance spectroscopy ( 13 C NMR), crystallization analysis fractionation (CRYSTAF), and gel permeation chromatography (GPC) studies showed that the microstructure of ethylene and 1-hexene copolymers strongly depends upon the structure of the Ti 3+ species. Isolated Ti 3+ species increase the relative reactivity of ethylene in copolymerizations and produce copolymers with high molecular weight and broad short chain branching distribution (SCBD), with a large ethylene-rich fraction. Multinuclear Ti 3+ species increase the relative reactivity of 1-hexene and produce copolymers with low molecular weight and broad SCBD, with a large rubbery ethylene/1-hexene fraction. Comparative studies of the copolymer microstructure from isolated Ti 3+ and multinuclear Ti 3+ in combination with different cocatalysts, Al(CH 3 ) 3, Al(C 2 H 5 ) 3, and methylaluminoxane (MAO) were also carried out. Isolated Ti 3+ species in combination with MAO cause remarkable changes in the 1-hexene incorporation rate and SCBD in comparison with Al(CH 3 ) 3 and Al(C 2 H 5 ) 3, while multinuclear Ti 3+ species in combination with MAO do not affect as much the 1-hexene incorporation rate. This difference may be related to the mechanism of active site formation between the different Ti 3+ structures and MAO. Introduction Since the discovery of metallocene catalysts, most research efforts seem to have shifted to this type of catalysis 1, 2). However, current industrial production of polyolefins still depends mainly on Ti-based heterogeneous Ziegler-Natta catalysts 3). The reaction mechanisms of these Ziegler-Natta systems are still largely unknown and deserve a detailed study due to their large commercial importance. Most of the Ti-based heterogeneous Ziegler-Natta catalysts produce polymers with broad molecular weight distribution (MWD) and, in the case of copolymerization, broad short chain branching distribution (SCBD) 4).Itis generally accepted that, under most polymerization conditions, the effect of multiple site types of Ziegler-Natta catalysts is far more important than mass and heat transfer effects. Since polymer chains produced on each site type have different average chain lengths, comonomer compositions, and comonomer sequence lengths, the polymers made with heterogeneous Ziegler-Natta catalysts are in reality mixtures of polymer chains having very dissimilar average properties. These dissimilar average properties are reflected in the broad MWDs and SCBDs that are frequently observed in polymers made with heterogeneous Ziegler-Natta catalysts. Therefore, it is useful to understand the nature of the broad MWD and SCBD of polymers obtained with Ziegler-Natta catalysts in terms of active site types and, especially, of how to manipulate the catalyst structure to modify the microstructure of the polymers. The MWD of polymers can easily be determined by gel permeation chromatography (GPC). However, an analysis of the SCBD of copolymers is more elaborate. The most common technique for this purpose has been temperature rising elution fractionation (TREF), which was first described by Desreux and Spiegels in ), and further established by Wild et al. 6) with the development of analytical TREF, which is now a common technique in the polyolefins industry. On the other hand, a new technique to analyze the SCBD of ethylene copolymers has been recently introduced 7, 8). This technique, crystallization analysis fractionation (CRYSTAF), is based on a stepwise precipitation approach similar to TREF. By monitoring the polymer solution concentration during crystallization, the cumulative and differential SCBD can be obtained without the need to physically separate the fractions. CRYSTAF produces similar results to TREF but in a shorter time and with a simplified apparatus 7 9). Much effort has been focused on the relationship between the structure of active sites and microstructure of polymer chains because of the very broad range of product applications 10 12). In line with this research effort, in a previous paper 13), we prepared a TiCl 3 (aluminum activa- Macromol. Chem. Phys. 200, No. 6 i WILEY-VCH Verlag GmbH, D Weinheim /99/ $ /0

2 The influence of the Ti 3+ species on the microstructure of ethylene/1-hexene copolymers 1299 ted(aa))/3mgcl 2 /tetrahydrofuran (THF) catalyst (T3ME) and modified the nature of the Ti 3+ species (isolated Ti 3+ versus multinuclear Ti 3+ species) by addition of diethyl aluminum chloride (Al(C 2 H 5 ) 2 Cl) with the aim to compare the polymerization behavior of those catalysts. The objective of this work is to investigate the effect of the Ti 3+ structure on the microstructure (MWD and SCBD) of ethylene/1-hexene copolymers made with catalysts that have isolated and multinuclear Ti 3+ species. Experimental part Materials Ethylene and nitrogen were purified by removing traces of residual moisture and oxygen with columns packed with molecular sieves and an oxygen scavenger, respectively. Hexane (Aldrich) used in the polymerization and in the catalyst preparation was distilled under nitrogen atmosphere. 1- Hexene (Aldrich) was dried over molecular sieve 13X with nitrogen purging. TiCl 3 (AA, Strem Chemicals), anhydrous MgCl 2 (Aldrich), anhydrous tetrahydrofuran (THF, Aldrich), Al(C 2 H 5 ) 2 Cl (Aldrich), Al(C 2 H 5 ) 3 (Aldrich), and methylaluminoxane (MAO) (donated by Albermarle Co.) were used as received. Details on catalyst preparation have been described previously 12). The designation T3, M and E in the nomenclature for the catalysts and complexes refer to TiCl 3 (AA), MgCl 2 and electron donor (THF), respectively. D refers to Al(C 2 H 5 ) 2 Cl used to remove THF from the TiCl 3 (AA)/ 3MgCl 2 /THF (T3ME) catalyst and the number following D indicates the amount of Al(C 2 H 5 ) 2 Cl added to T3ME catalyst. T3MED2 was thus prepared by adding 0.2 mole of Al(C 2 H 5 ) 2 Cl per mole of THF contained in the T3ME catalyst. Polymerization and characterization Copolymerizations of ethylene and 1-hexene were carried out in a 250 ml glass reactor. 150 ml hexane was added to the reactor at 30 8C under nitrogen purging, followed by charging of 1-hexene and catalyst. After catalyst injection, ethylene was added to increase the total pressure to 10 psi (psi L Pa). Polymerization was started by the injection of cocatalyst and carried out for 20 min. The polymerization was terminated with an excess amount of ethanol after venting the gas monomer. The molecular weight distribution was measured by gel permeation chromatography (Waters 150CV) at 135 8C with 1,2,4-trichlorobenzene as a solvent. 13 C NMR spectra ( MHz) of copolymers in 1,2,4-trichlorobenzene were recorded at 125 8C. Copolymer microstructure was analyzed by 13 C NMR according to the method reported by Randall 14). CRYSTAF measurements were carried out as described in the literature 8). Results and discussion A previous investigation with X-ray diffraction (XRD) and electron spin resonance spectrometry (ESR) indicated that the addition of Al(C 2 H 5 ) 2 Cl to T3ME resulted in a change of the isolated Ti 3+ species in T3ME, T3MED2, and T3MED4 catalysts to a multinuclear Ti 3+ species in the T3MED8 and T3MED12 catalysts 13). In ethylene-propylene copolymerization, the presence of the multinuclear Ti 3+ species increased the relative reactivity of propylene compared to that of isolated Ti 3+ species. Tab. 1 shows polymerization results and properties of copolymers produced with different catalysts. The catalytic activity of isolated Ti 3+ (top three rows of Tab. 1) is higher than that of the multinuclear Ti 3+ species (last two rows). The difference in activity may be due to differences in the binding state of Ti 3+. The T3ME catalyst produced the copolymer with a high molecular weight and the narrowest MWD. The other catalysts produced copolymers with considerably lower molecular weights and broader MWDs than those made with T3ME. It is believed that this difference is due to the formation of more active site types due to the removal of THF from T3ME with Al(C 2 H 5 ) 2 Cl. Tab. 2 shows sequence distributions of ethylene/1-hexene (E/H) copolymers prepared with different catalysts. T3ME, T3MED2, and T3MED4 catalysts with isolated Ti 3+ produced copolymers with lower 1-hexene content (see also Tab. 1) and had no HHH triad sequences in the copolymer chains (last col- Tab. 1. Polymerization results and properties of copolymers produced with different catalyst systems a) Catalyst Activity b) M n610 3 M w610 3 M w/m n 1-Hexene content mol-% T3ME T3MED T3MED T3MED T3MED a) Polymerization at 308C, mole ratio Al/Ti = 30, M H /M E = 7 (M H and M E stand for the 1-hexene and ethylene concentration in hexane, respectively). b) Activity in g polymer/(g Ti N atm N h).

3 1300 K.-J. Chu, J. B. P. Soares, A. Penlidis, S.-K. Ihm Tab. 2. Dyad and triad sequence distributions of ethylene/1-hexene copolymers prepared with different catalysts a) Catalyst H EE EH HH EEE HEE HEH EHE HHE HHH T3ME T3MED T3MED T3MED T3MED a) Composition is in mol-%. H and E stand for 1-hexene and ethylene, respectively. Triad sequence distribution was calculated after Randall 14). All copolymer samples were prepared at M H /M E = 7 (see Tab. 1). Fig. 1. Typical analytical CRYSTAF profile of an ethylene/1-hexene copolymer studied in this work umn of Tab. 2). T3MED8 and T3MED12 catalysts with multinuclear Ti 3+ produced copolymers with HHH triad sequences and higher 1-hexene content than those made with isolated Ti 3+. From the above results, one can see that multinuclear Ti 3+ (T3MED8, T3MED12) favors 1-hexene incorporation and the formation of 1-hexene blocks in ethylene/1- hexene copolymerization. On the other hand, isolated Ti 3+ (T3ME, T3MED2, T3MED4) favors ethylene insertion in ethylene/1-hexene copolymerization. A typical analytical CRYSTAF profile for the ethylene/ 1-hexene copolymers studied in this work is shown in Fig. 1. The analysis was carried out by monitoring the polymer solution concentration during crystallization 7, 8). As the temperature goes down slowly, the most crystalline fractions, composed of molecules with no or very few branches, precipitate first (zone 3 in Fig. 1). This is followed by precipitation of fractions of increasing short chain branch content (zone 2 in Fig. 1). The last zone corresponds to the fraction that remains soluble at room temperature (zone 1 in Fig. 1). Fig. 2 shows the CRYSTAF profiles of ethylene/1-hexene copolymers made with different catalysts. The broad CRYSTAF profiles of ethylene/1-hexene copolymers reflect the active site heterogeneity of these Ziegler-Natta catalysts. In addition, there are significant differences among CRYSTAF profiles of ethylene/1-hexene copolymers made with isolated Ti 3+ and with multinuclear Ti 3+. The isolated species produced a greater fraction of ethylene-rich copolymer than the multinuclear Ti 3+ species. From the viewpoint of active site types, the CRYSTAF profiles indicate that possibly, among a multitude of active sites, two active sites dominate, i. e., ethylene favorable sites and 1-hexene favorable sites corresponding to the characteristics of the isolated Ti 3+ and multinuclear Ti 3+ species, respectively. One can verify this by the fact that the fraction of zone 2 was not affected when polymerizations were carried out with different catalysts (see Fig. 2). The effect of cocatalysts, Al(CH 3 ) 3, Al(C 2 H 5 ) 3, and MAO on the copolymer microstructure over different Ti 3+ structures was also examined. Tab. 3 and 4 show polymerization results and comonomer sequence distributions of the produced ethylene/1-hexene copolymers. The MWD and the 1-hexene content of the produced copolymers depend upon the combination of Ti 3+ species and

4 The influence of the Ti 3+ species on the microstructure of ethylene/1-hexene copolymers 1301 Fig. 2. CRYSTAF profiles of ethylene/1-hexene copolymers obtained with different catalysts Tab. 3. Polymerization results and properties of copolymers produced with different catalyst systems a) Catalyst Cocatalyst Activity b) M n610 3 M w610 3 M w/m n 1-Hexene content mol-% T3ME Al(C 2 H 5 ) Al(CH 3 ) MAO T3MED12 Al(C 2 H 5 ) Al(CH 3 ) MAO a) Cf. Tab. 1 for polymerization details. b) g polymer/(g Ti N atm N h). Tab. 4. Dyad and triad sequence distributions of ethylene/1-hexene copolymers prepared with different catalyst systems a) Catalyst Cocatalyst H EE EH HH EEE HEE HEH EHE HHE HHH T3ME Al(C 2 H 5 ) Al(CH 3 ) MAO T3MED12 Al(C 2 H 5 ) Al(CH 3 ) MAO a) Cf. Tab. 2 for details. type of cocatalyst. When MAO was used as cocatalyst, each Ti 3+ structure showed remarkable differences in catalytic activity and microstructure. MAO decreased the catalytic activity of T3ME but increased that of T3MED12. Fig. 3 and 4 show the 13 C NMR spectra of ethylene/1- hexene copolymers made with different catalyst/cocatalyst combinations. In the case of the copolymer made by T3ME with isolated Ti 3+ and MAO (Fig. 3b), the spectrum is similar to that of polyethylene. Therefore, the catalyst system T3ME/MAO did not show appreciable 1-hexene incorporation (see also last column of Tab. 3). On the other hand, T3MED12 with multinuclear Ti 3+ and MAO (Fig. 4b) showed 1-hexene incorporation, as did the other cocatalysts. Fig. 5 and 6 show CRYSTAF profiles for different catalyst/cocatalyst combinations. These show similar trends to those of the 13 C NMR spectra. The isolated Ti 3+ sites (Fig. 5) are very sensitive to MAO, while the multinuc-

5 1302 K.-J. Chu, J. B. P. Soares, A. Penlidis, S.-K. Ihm 13 Fig. 3. C NMR spectra of ethylene/1-hexene copolymers obtained with (a): T3ME and Al(C 2 H 5 ) 3, (b): T3ME and MAO lear Ti 3+ sites are not (Fig. 6). If MAO selectively activated and/or deactivated one type of Ti 3+ species only, the CRYSTAF profile of the copolymer made with multinuclear Ti 3+ and MAO would be similar to that obtained with isolated Ti 3+ and MAO. Therefore, in the same way as the removal reaction of internal electron donor by aluminum alkyl compounds affects the catalytic activity and the microstructure of the produced polymer 15, 16), it can be

6 The influence of the Ti 3+ species on the microstructure of ethylene/1-hexene copolymers Fig. 4. C NMR spectra of ethylene/1-hexene copolymers obtained with (a): T3MED12 and Al(C 2 H 5 ) 3, (b): T3MED12 and MAO speculated that the complex interaction between the environments of titanium and aluminum compounds causes the observed differences because of the high THF content (53.5 wt.-%) 13) in the T3ME catalyst. Concluding remarks The microstructure of ethylene/1-hexene copolymers strongly depends upon the structure of Ti 3+ species and the combination of catalyst and cocatalyst types. The

7 1304 K.-J. Chu, J. B. P. Soares, A. Penlidis, S.-K. Ihm Fig. 5. CRYSTAF profiles of ethylene/1-hexene copolymers obtained with T3ME and different cocatalysts Fig. 6. CRYSTAF profiles of ethylene/1-hexene copolymers obtained with T3MED12 and different cocatalysts observed broad MWDs and SCBDs are caused by the heterogeneity of active sites of MgCl 2 -supported titanium catalysts. Using CRYSTAF, it was possible to identify two dominant active site types, i. e., ethylene favorable sites that produce ethylene-rich copolymers and 1-hexene favorable sites that produce amorphous ethylene/1-hexene copolymers. These site types correspond to isolated Ti 3+ species and multinuclear Ti 3+ species, respectively. Isolated Ti 3+ species are very sensitive to MAO as a cocatalyst, leading to polymers with very little 1-hexene incorporation. Multinuclear Ti 3+, on the other hand, does not seem to be affected in its ability to incorporate 1-hexene when the cocatalyst type is varied. Acknowledgement: We are grateful to Dr. B. Monrabal from Polymer Char, Spain, for kindly analysing all copolymer samples with CRYSTAF. We are also grateful to NSERC, Canada, for financial support. 1) H. H. Brintzinger, D. Fischer, M. Mülhaupt, B. Rieger, R. Waymouth, Angew. Chem., Int. Ed. Engl. 107, 1255 (1995) 2) W. Kaminky, Catal. Today 20, 257 (1994) 3) M. Terano, Catalyst Design for Tailor-madePolyolefins, K. Soga and M. Terano, Eds., Kodansa-Elsevier Ltd., Tokyo 1994, p ) J. B. P. Soares, A. E. Hamielec, Polymer 36, 1639 (1995) 5) V. Desreux, M. L. Spiegels, Bull. Soc. Chem. Belg. 59, 476 (1950)

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