Copolymerization of ethene with styrene using CGC catalysts: the effect of the cyclopentadienyl ligand substitution on the catalyst activity and copolymer structure
Introduction
A new class of highly active catalysts for polyalkene production emerged with the synthesis of (tert-butylamido)dimethyl(2,3,4,5-tetramethylcyclopentadienyl)silane which affords the η5:η1(N)-dianionic ligand [1] and, subsequently, ansa-{η5:η1(N)-1-[(tert-butylamido)dimethylsilyl]-2,3,4,5-tetramethylcyclopentadienyl}dichlorotitanium(IV) (1) [2]). These catalysts, called “constrained geometry catalysts” (CGC), arise from mixing of the ansa-amidosilylcyclopentadienyl-titanium or -zirconium dichlorides with excess methylalumoxane (MAO) [3]. Compared to titanocene or zirconocene-based single-site catalysts they provide more acidic and less sterically encumbered cationic centres [4] and, consequently, display very high activities in the polymerization of ethene, propene and in copolymerizations of ethene with terminal alkenes, cycloalkenes and styrene [5]. In attempts to tune the polymerization properties the parent CGC complex 1 was modified by replacing the cyclopentadienyl ligand by indenyl or fluorenyl ligands [6], and their rings [7] as well as the amide groups were variously substituted [8]. Recently, an effective route to CGC complexes substituted on otherwise fully methyl-substituted cyclopentadienyl ring in vicinal position to the ansa-bridge was developed affording a series of racemic complexes [TiCl2{η5:η1(N)-C5(1-SiMe2Nt-Bu-2,3,4-Me3-5-R)}], where R = H (2), Bu (3), Ph (4), 4-fluorophenyl (5), and but-2-en-2-yl) (6) [9], [10]. The substituent R has been shown to considerably modify the activity of CGC catalysts in the polymerization of ethene [11] and propene [10], and to modify molecular weights of the polymers. Polydispersities of both the polymers were close to 2, indicating a single-site character of these catalysts. NMR analyses of these polymers revealed a low branching in polyethene [11] and a low content of syndiotactic attachments in largely atactic polypropene [10].
The catalytic copolymerization of ethene and styrene which is only poorly catalyzed by titanocene-based catalysts [12] has been successfully conducted by CGC catalysts achieving a high styrene incorporation (up to 37%) [3], [13]. Particularly, the copolymerization using the parent 1/MAO catalyst was investigated in detail and the copolymers were fully characterized [14]. The 1/MAO catalyst appeared to be more active than the catalysts based on indenyl, fluorenyl or benzylamido derivatives presumably due to a higher electron density induced by fully methyl-substituted cyclopentadienyl ligand in 1 [15].
Here we report the catalytic copolymerization of ethene and styrene as depending on the substituent R in the series of catalysts 1–6/MAO under optimized conditions.
Section snippets
Ethene–styrene copolymerization
The ansa-amidocyclopentadienyltitanium dichlorides 1–6 (Scheme 1) were activated by MAO at molar ratios Al/Ti = 800, 900, and 1000 in the presence of ethene (E) and styrene (S) (molar ratio S/E = 10) in toluene solution, at temperatures 40–70 °C, and the overall concentration of the CGC catalyst (0.05–0.28 μmol/ml).
The polymerization run was started by adding the toluene solution of the respective CGC complex, and the time of polymerization was varied (30–120 min) in order to optimize the
Chemicals
Toluene was purified by treating with a solution of radical-anion formed in the sodium benzophenone/dibenzo-18-crown-6-ether system, and distilled in vacuum prior to use. Styrene (Kaučuk, a.s., Czech Republic) was dried over CaH2 and then twice distilled in vacuum just before use. Ethene, polymerization grade (kindly donated by Chemopetrol, a.s., Czech Republic) was further purified by passing through columns with a Cu deoxygenation catalyst and molecular sieve 13X before use. Methylalumoxane
Supplementary material
Crystallographic data, excluding structure factors, have been deposited at the Cambridge Crystallographic Data Centre (2: CCDC-232740, 4: CCDC-232741). Copies of the data can be obtained free of charge upon application to CCDC (e-mail: [email protected]).
Acknowledgements
This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic (project no. LN00B142). Grant Agency of the Czech Republic sponsored access to Cambridge Structure Database (grant no. 203/02/0436). KM acknowledges the financial support by the Council for Research and Development of Academy of Sciences of the Czech Republic (project no. S4040017).
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Synthesis, structure and ethylene polymerisation activity of {η<sup>5</sup>:η<sup>1</sup>(N)-1-[(tert-butylamido)diphenylsilyl)]-2,3,4,5-tetramethylcyclopentadienyl}dichlorotitanium(IV)
2020, PolyhedronCitation Excerpt :These results were reviewed in [4]. The catalytic properties of this system were modified by replacing one methyl group on the cyclopentadienyl ligand vicinal to the bridging silicon by various substituents (alkyl, alkenyl, aryl or hydrogen); their effects on polymerisation of ethene [5], propene [6] and copolymerisation of ethene and styrene were established [7]. The CGC catalysts were modified also by substituting the cyclopentadienyl ligand for an indenyl one [8].
Groups 3 and 4 single-site catalysts for styrene-ethylene and styrene-α-olefin copolymerization
2008, Coordination Chemistry ReviewsConstrained geometry complexes-Synthesis and applications
2006, Coordination Chemistry ReviewsEffects of substituents in cyclopentadienyltitanium trichlorides on electronic absorption and <sup>47,49</sup>Ti NMR spectra and styrene polymerization activated by methylalumoxane
2006, Journal of Molecular Catalysis A: ChemicalA simple modification makes a big improvement to ziegler-natta catalyst
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