Copolymerization of Ethylene and 1-Hexene with Et(Ind)

Polymer 42 2001) 6355±6361 www.elsevier.nl/locate/polymer Copolymerization of ethylene and 1-hexene with EtInd)2ZrCl2 in hexane Mauricio L. Brittoa,1, Griselda B. Gallanda, JoaÄo Henrique Z. dos Santosa,*, Madalena C. Forteb aInstituto de QuõÂmica, Universidade Federal do Rio Grande do Sul, Av. Bento GoncËalves, 9500-Porto Alegre, 91509-900 Brazil bEscola de Engenharia, Dept. de Materiais, Universidade Federal do Rio Grande do Sul, Rua Osvaldo Cruz, 99-Porto Alegre, 90035-190 Brazil Received 2 October 2000; received in revised form 5 January 2001; accepted 6 February 2001 Abstract Copolymerizations of ethylene and 1-hexene were carried out in hexane at 608C by using ethylenebisindenyl)zirconium dichloride as catalyst and methylaluminoxane MAO) as cocatalyst in presence of triisobutylaluminum TIBA). The in¯uence of the alkylaluminum concentration and AlTOTAL/Zr molar ratio on the catalyst activity and comonomer incorporation are evaluated. Increasing the TIBA concentration, the MAO solubility augments in the polymerization milieu, as well as the catalyst activity and the comonomer incorporation. Increasing the AlTOTAL/Zr molar ratio, catalyst activity becomes higher, while comonomer incorporation remained constant at around 8 wt% for molar ratios between 0 and 5000. For AlTOTAL/Zr molar ratios higher than 5,000 comonomer incorporation increased gradually. Under the same experimental conditions dimethylsilylbisindenyl)zirconium dichloride showed higher thermal stability and led to higher comonomer incorporation. q 2001 Published by Elsevier Science Ltd. Keywords: Metallocene; MAO; Ethylene±1-hexene copolymerization 1. Introduction discovery of the metallocene/methylaluminoxane MAO) catalyst system by Sinn and Kaminsky, a great amount of Global linear low density polyethylene LLDPE) both industrial and academic research has been done. Such consumption is increasing at impressive rates as people systems has been continuously reviewed in the literature improve extrusion capabilities and search for greater [2±4]. The symmetry of the ligands in the coordination product end-use performance and value. LLDPE is sphere around the metal center determines the stereo- predicted to have the greatest increase in usage from greater speci®city of metallocene catalysts [5], affording poly- than 9% increase from 1996 through 2005 to approximately ole®ns with almost all kinds of stereoregularities. Besides, 7% from 2005 through 2010 [1]. The driving force in the metallocene catalysts produce polymers with narrow LLDPE growth can be attributed to metallocene-catalyzed molecular weight distribution Mw/Mn < 2), and uniform LLDPE resins. The demand for metallocene-catalyzed poly- comonomer distribution. ethylene is predicted to increase 45%, i.e. from 220 to A metallocene catalyst precursor can be activated with 970 kilotonnes, between 1996 and 2000 and eventually organoaluminoxanes, especially MAO, which provides grow to 5,900 kilotonnes in the year 2010. maximum activity. The structure of MAO remains not Properties of ole®n copolymers are greatly in¯uenced by completely elucidated, although it has been investigated various factors, e.g. molecular weight, molecular weight by many cryoscopic, spectroscopic and other techniques distribution, kind of comonomer, comonomer content and [6±8]. Up to now, the most acceptable proposal for the monomer sequence distribution. In turn, some of these active MAO consists of a cage-like structure. In such properties are dependent on the nature of the catalyst and cages there are monomeric AlMe3 molecules that provide cocatalyst and on the polymerization conditions. Since the the alkylation of the metallocene dichloride complex, and formal abstraction of a methyl anion from the transition metal to give a metallocene monomethyl cation that is * Corresponding author. Tel./fax: 155-51-319-1499. stabilized by a bulky MAO anion [9]. E-mail address: [email protected] J.H.Z. dos Santos). Metallocene catalysts have high solubility in aromatic 1 Present address: OPP PetroquõÂmica S.A., Centro de Pesquisas e Desen- volvimento, III PoÂlo PetroquõÂmico, Via Oeste, Lote 5, 95853-000, Triunfo, solvents like toluene due to the presence of the cyclo- Brazil. pentadienyl rings. MAO is also very soluble in aromatic 0032-3861/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S0032-386101)00117-3 6356 M.L. Britto et al. / Polymer 42 72001) 6355±6361 solvents up to 30%), but it has very low solubility in 1.2 £ 1026 M) in toluene solution was charged and, after a aliphatic solvents only 3 or 4%) [10]. However, aromatic pre-contacting time of 20 min, ethylene 6 bar) was solvents cannot be used in industrial processes due to their admitted into the reactor which was then heated up to toxicity. The main problem concerning the use of an alipha- 608C. One hour later, the non-reacted monomers were tic hydrocarbon as the polymerization milieu is the low purged and the copolymer collected under acidi®ed solubility of the catalyst components due to the low polarity methanol. The suspension was stirred for 2 h, ®ltered and of the solvent [11]. Commercial modi®ed MAO MMAO), the copolymer dried at 608C under vacuum. prepared by controlled hydrolysis of mixture of trimethyl- aluminum TMA) and triisobutylaluminum TIBA) showed 2.3. Qualitative solubility test improved solubility in aliphatic solvents, keeping good In a 2.0 l Schlenk ¯ask equipped with a mechanical polymerization ef®ciency. The introduction of TIBA in stirrer, solvent, MAO and TIBA were introduced at the the polymerization milieu as the third catalyst component same order as in the copolymerization procedure. The increases the solvent polarity, facilitating the MAO system was kept in a glass thermostatized bath, in order to solubility and the formation and stabilization of the active allow the visualization of the solution aspect. species [12]. Moreover, it is worth mentioning that this approach searches for the replacement of the expensive 2.4. Characterization of copolymers MAO cocatalyst by cheaper alkylaluminum compounds. The present paper reports on the in¯uence of the TIBA The comonomer incorporation was evaluated by infrared concentration and AlTotal/Zr molar ratio on the catalytic activ- technique by means of a Nicolet 710 FT-IR spectrometer. ity and the comonomer incorporation for the catalyst system The polymers 0.2 g) were analyzed as ®lms pressed at ethylenebisindenyl)zirconium dichloride EtInd)2ZrCl2)± 10 MPa at 1908C for 30 s. The 1-hexene content measure- MAO/TIBA using hexane as solvent. A qualitative evaluation ment was based on 1377 cm21 band height as function of the of the effect of TIBA addition on MAO solution was ®lm thickness. Calibration curve was performed with performed at different contact times and temperatures. For standard samples provided by Montell Co. presenting comparative reasons, dimethylsilylbisindenyl)zirconium 2±16 wt% 1-hexene content, which was previously deter- 13 dichloride Me2SiInd)2ZrCl2) was also investigated. The mined by CNMR. resulting copolymers were evaluated by determining their Melt ¯ow index MFI) was determined in a Tinius Olsen melt ¯ow indices MFI), intrinsic viscosityIV), melting and MP 987 extrusion plastometer at 1908C. The 2.16 and 10.0 kg crystallization temperatures. standard weights were employed. Intrinsic viscosity IV) of samples 0.04 g) was determined in decahydronaphthalin at 1358C using a SOFICA-CINEVISCO viscometer. 2. Experimental procedures Copolymer melting and crystallization temperatures, as well crystallinity were determined by differential scanning 2.1. Materials calorimeter DSC) analysis using a DSC 2910 connected to Polymerization grade ethylene 99.5% and nitrogen were a Thermal Analyst 2100 Integrator, at a heating rate of 108C/ puri®edbypassingthroughpre-activatedcolumnsofalu- min. The heating cycle was performed twice, but only the mina, molecular sieves 13 £ 4AÊ ) and cupper catalyst results of the second scan are reported, since the former is RIDOX 230). Hexane Exxon Co./Ibrasol) was dried in¯uenced by the mechanical and thermal history of the over pre-activated molecular sieves 13 £ 4AÊ ). Toluene samples. Reagen) was dried with calcium chloride while 1-hexene Molecular weight and molecular weight distributions Ethyl Co.) with calcium hydride. Both were distilled and were investigated with a Waters high-temperature GPC stored over molecular sieves 4 AÊ ) and their humidity level instrument, CV 150C, equipped with optic differential was controlled below 20 ppm by a Karl Fischer equipment. refractometer and three Styragel HT type columns HT3, HT4 and HT6) with exclusion limit 107 for polystyrene. MAO, TIBA, EtInd)2ZrCl2,andMe2SiInd)2ZrCl2,gener- ously supplied by Witco were used as received and all 1,2,4-Trichlorobenzene was used as solvent, at a ¯ow rate compounds were handled under Schlenk standard techniques. of 1 l/min. The analyses were performed at 1408C. The columns were calibrated with standard narrow molar mass 2.2. Copolymerization procedure distribution polystyrenes and then universally with linear low density polyethylenes and polypropylenes. Slurry copolymerizations of ethylene with 1-hexene were 13C NMR was employed to determine the composition carried out in a 2 l steel jacketed BuÈchi reactor 2200 rpm) and sequence distribution of the copolymers according to at 60 or 708C. The reactor was initially purged with nitrogen procedures in the literature [13]. The 13C NMR spectra were ¯ow at 708C for 1 h and then cooled to 408C. Hexane 1.0 l), recorded at 908C using

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