Ethylene/1‐hexene copolymerizations by syndioselective metallocenes: Direct comparison of Me2C(Cp)(Flu)ZrMe2 with Et(Cp)(Flu)ZrMe2 (original) (raw)
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Macromolecular Chemistry and Physics, 1995
The behaviour of catalytic systems based on zirconium compounds for the copolymerization of ethylene with 1-hexene and 1-octene is reported. The metallocenes (CH3)2SiCp2ZrCl2, Cp2ZrCl2 (Cp = η5-cyclopentadienyl), C2H4[Ind]2ZrCl2 and (Ind = η5-indenyl) were chosen for this study. The bridged catalysts, (CH3)2SiCp2ZrCl2 and C2H4[Ind]2ZrCl2, and the metallocene Cp2ZrCl2 showed similar catalytic activities for home- and copolymerization of ethylene with 1-hexene. 13C NMR analysis showed that the composition of copolymerization products depends on the catalytic system, in other words, on the ligand structure of the transition metal. Copolymers obtained using the bridged catalysts have greater incorporation of comonomer. Thermal analysis and viscosity measurements demonstrated that an increase of the comonomer concentration reduces the melting point, the crystallinity and the molecular weight of the copolymer. Results from infrared spectroscopy showed that β-elimination is one of the possible termination reactions. The monomer reactivity ratios r were determined for all catalytic systems using Fineman-Ross and 13C NMR methods. The values of r1 (M1 = ethylene) and r2 (M2 = α-olefin) showed an effect of the type of metallocene and of α-olefin on the structure of the copolymer obtained.
Copolymerizations of ethylene with 1-hexene overansa-metallocene diamide complexes
Macromolecular Research, 2004
We have performed copolymerizations of ethylene with 1-hexene using various ansa-metallocene compounds in the presence of the non-coordinative [CPh 3 ][B(C 6 F 5) 4 ] ion pair as a cocatalyst. The metallocenes chosen for this study are isospecific metallocene diamide compounds, rac-(EBI)Zr(NMe 2) 2 [1, EBI = ethylene-1,2bis(1-indenyl)], rac-(EBI)Hf(NMe 2) 2 (2), rac-(EBI)Zr(NC 4 H 8) 2 (3), and rac-(CH 3) 2 Si(1-C 5 H 2-2-CH 3-4-t C 4 H 9) 2 Zr(NMe 2) 2 (4), and syndiospecific metallocene dimethyl compounds, ethylidene(cyclopentadienyl)(9-fluorenyl) ZrMe 2 [5, Et(Flu)(Cp)ZrMe 2 ] and isopropylidene(cyclopentadienyl)(9-fluorenyl)ZrMe 2 [6, iPr(Flu)(Cp)ZrMe 2 ]. The copolymerization rate decreased in the order 4 > 1 ~ 3 > 2 > 5 > 6. The reactivity of 1-hexene decreased in the order 2 > 6 > 1 ~ 3 ~ 5 > 4. We characterized the microstructure of the resulting poly(ethylene-co-1-hexene) by 13 C NMR spectroscopy and investigated various other properties of the copolymers in detail.
Polymer Bulletin, 1995
This study employed the~3C-NMR spectroscopy to investigate the influence of the increase of the comonomer concentration on the microstruture of ethylene/1hexene and ethylene/1-octene copolymers obtained by the use of MeSiCp2ZrC12, Cp2ZrC12,, Et[Ind]2ZrC12 and [Ind]2ZrCl2 catalysts. For both comonomers butyl or hexyl branches were isolated between ethylene blocks. As the ct-olefm concentration in the copolymer increased, butyl or hexyl branches became closer, some of them, separated by only one or two ethylene units. Incorporation of ct-olefm in the copolymer was higher for the bridged catalysts, MeSiCp2ZrCI2, and Et[Ind]2ZrC12 than for the unbridged ones. The ~-olefin size did not seem to effect its reactivity towards ethylene.
Journal of Applied Polymer Science, 2002
Polyethylene copolymers prepared using the metallocene catalyst rac-Et[Ind]2ZrCl2 were fractionated by preparative Temperature Rising Elution Fractionation (p-TREF) and characterized by 13C nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and gel permeation chromatography (GPC) to study the heterogeneity caused by experimental conditions. Two ethylene–1-hexene copolymers with different 1-hexene content and an ethylene–1-octene copolymer all obtained using low (1.6 bar) ethylene pressure were compared with two ethylene–1-hexene copolymers with different 1-hexene content obtained at high ethylene pressure (7.0 bar). Samples obtained at low ethylene pressure and with low 1-hexene concentration in the reactor presented narrow distributions in composition. Samples prepared with high comonomer concentration in the reactor or with high ethylene pressure showed an heterogeneous composition. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 155–163, 2002; DOI 10.1002/app.10284
Macromolecules, 2011
This work presents novel and, to some extent, surprising information on ethene/4-methyl-1-pentene (E/Y) copolymers prepared with C 2 symmetric single center metallocene catalysts: the moderately isopecific rac-ethylenebis-(tetrahydroindenyl)zirconium dichloride (EBTHI) and the highly isospecific rac-dimethylsilylbis(2-methyl-4-phenylindenyl)zirconium dichloride (MPHI). Blocky E/Y copolymers from EBTHI, with relatively long sequences of both comonomers, underwent a thorough structural and thermal characterization, performed by combining WAXD, DSC and SSA thermal fractionation. The presence of crystallinities arising from both comonomers, as a function of the copolymer composition led to figure out the simultaneous presence of two populations of thin and defective crystals due to sequences of both comonomers, in samples with almost equimolar composition. The most isospecific metallocene, MPHI, was used exactly with the aim of finally preparing block E/Y copolymers, with long crystalline sequences of both comonomers, whose simultaneous presence could be clearly detected. The easiest 1-olefin propagation ever observed in E/Y copolymerization was obtained with MPHI. However, surprisingly, short sequences of Y were detected in the presence of short E sequences as well. Chain generation, performed for copolymers from both EBTHI and MPHI, revealed, in the latter case, a novel and unique microstructure, with very short sequences of both comonomers almost randomly distributed along the polymer chain. An amorphous nature of these copolymers was revealed by thermal analysis. This paper proposes thus an apparent paradox: on one side, it is confirmed that the 1-olefin propagation becomes easier by increasing the catalyst isoselectivity and, on the other side, short 1-olefin sequences are formed with the most isospecific metallocene. A way to come out from this impasse is proposed, taking into consideration the low steric hindrance of the most isospecific metallocene and the consequent higher reactivity for the 1-olefin, that leads to a lower concentration of 1-olefin in the polymerization bath and of 1-olefin sequences in the copolymer chain. For the first time it seems possible to tell apart the influence of isoselectivity and steric hindrance of a single center C 2 symmetric catalyst on structure and properties of ethene/1-olefin copolymers. This work reveals the existence of unexpected degrees of freedom for tuning microand macro-structures of ethene/1-olefin copolymers from C 2 symmetric metallocenes and wants to be a contribution for developing polymerizations able to control the monomer sequences, proposed as the Holy Grail in polymer science.
Macromolecules, 1996
Copolymers of ethylene and the polar monomer 6-tert-butyl-2-(1,1-dimethylhept-6-enyl)-4methylphenol were synthesized using three different homogeneous metallocene-methylalumoxane catalyst systems, i.e. rac-[1,1′-(dimethylsilylene)bis(η 5-4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (Me2Si-(IndH4)2ZrCl2)/methylalumoxane (MAO), rac-[ethylene-1,2-bis(η 5-4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (Et(IndH4)2-ZrCl2)/MAO, and dicyclopentadienylzirconium dichloride (Cp2ZrCl2)/MAO. The initial polymerization rate, compared to that of ethylene homopolymerization, increased up to almost 3 times when the sterically hindered phenolic stabilizer was added during ethylene polymerization over one of the two chiral bridged metallocene catalysts. In contrast, the addition of the phenolic monomer during ethylene polymerization over the achiral Cp2ZrCl2 catalyst did not result in an appreciable change in polymerization activity. The dissimilarity in polymerization rate behavior of chiral versus achiral metallocene catalysts may be attributed to differences in the gap aperture between the π-ligands of the catalyst and to sterical and electronic factors. The level of comonomer incorporation was also found to be different with copolymers produced over chiral versus achiral metallocene catalyst. The comonomer content was 2-3 times lower for the copolymers produced over the achiral Cp2ZrCl2 catalyst compared to the copolymers prepared over either of the two chiral catalysts under similar conditions at low temperatures. As expected, the melting points and crystallinities of copolymers decreased with increasing phenol content. According to 13 C NMR studies, the chemical shifts of the copolymer's methylene and methine backbone carbons correspond to those observed for random ethylene/1-octene copolymer with isolated hexyl branches. Thus, the produced copolymers are random copolymers, which contain isolated phenolic long chain branches. No detectable traces of phenolic homopolymer or blockcopolymer fragments were found by 13 C NMR. The thermo-oxidative stability of the copolymers prepared was high even after prolonged extraction with a mixture of refluxing (50:50) 2-propanol/cyclohexane; the oxidation induction time at 200°C ranged from 18 to 72 min for the copolymers whereas unstabilized polyethylene exhibited an oxidation induction time of only 1 min, as determined by differential scanning calorimetry (DSC). The numerical values of the ratio of weight-to-number average molecular weights of the copolymers were below 3 and thus characteristic of polymers produced by single-site catalysts. Furthermore, the copolymer molecular weights were similar to those of polyethylene prepared under similar conditions.
Macromolecules, 2005
The copolymerization of ethylene and norbornene by catalytic systems composed of i-Pr-[(Cp)(Flu)]ZrCl2 (1) and methylaluminoxane was investigated. Ethylene-norbornene (E-N) copolymers with 40.2 mol % of norbornene are highly alternating (NENE 50 mol %) and contain a significant amount of racemic ENNE (8 mol %) and no ENNN sequences. The microstructural analysis by 13 C NMR of such copolymers was completely obtained at the tetrad level by a methodology that exploits all the peak areas of the spectra and accounts for the stoichoimetric requirements of the copolymer chain. The analysis at the tetrad level allowed us to test the statistical model best describing E-N copolymerization with Cssymmetric catalyst 1 and to study the polymerization mechanism. The root-mean-square deviations between experimental and calculated tetrads demonstrate that the first-order Markov model is sufficient to describe the microstructure of E-N copolymers with 1. It is concluded that in E-N copolymerizations with this catalyst both N and E are inserted according to a Cossee's migratory insertion, and backskips of the copolymer chain to its original position occur, causing the formation of both meso and racemic NEN sequences. The probability of chain backskip is relatively high with respect to that observed in syndiotactic propylene polymerization under the same polymerization conditions. This effect seems to be due to norbornene strong coordinating ability which can influence the competition between site epimerization and chain propagation.
Journal of Applied Polymer Science, 2006
Ethylene/1-hexene copolymerization was carried out with polystyrene-supported metallocene catalyst. It was found that the kinetic of the copolymerization was strongly influenced by the steric hindrance of carrier. The influences of 1-hexene concentration in the feed on catalyst productivity and comonomer reactivity were investigated. The microstructure of resultant copolymer was analyzed by 13 C NMR. It was found that the different carriers have slight effect on the composite of copolymer.
Industrial & Engineering Chemistry Research, 2013
Two catalysts, denoted as catalyst 1 [silica/MAO/( n BuCp) 2 ZrCl 2 ] and catalyst 2 [silica/ n BuSnCl 3 /MAO/ ( n BuCp) 2 ZrCl 2 ] were synthesized and subsequently used to prepare, without separate feeding of methylaluminoxane (MAO), ethylene homopolymer 1 and homopolymer 2, respectively, and ethylene−1-hexene copolymer 1 and copolymer 2, respectively. Gel permeation chromatography (GPC), Crystaf, differential scanning calorimetry (DSC) [conventional and successive selfnucleation and annealing (SSA)], and 13 C nuclear magnetic resonance (NMR) polymer characterization results were used, as appropriate, to model the catalyst active-center distribution, ethylene sequence (equilibrium crystal) distribution, and lamellar thickness distribution (both continuous and discrete). Five different types of active centers were predicted in each catalyst, as corroborated by the SSA experiments and complemented by an extended X-ray absorption fine structure (EXAFS) report published in the literature. 13 C NMR spectroscopy also supported this active-center multiplicity. Models combined with experiments effectively illustrated how and why the active-center distribution and the variance in the design of the supported MAO anion, having different electronic and steric effects and coordination environments, influence the concerned copolymerization mechanism and polymer properties, including inter-and intrachain compositional heterogeneity and thermal behaviors. Copolymerization occurred according to the first-order Markovian terminal model, producing fairly random copolymers with minor skewedness toward blocky character. For each copolymer, the theoretical most probable ethylene sequences, n E MPDSC-GT and n E MPNMR-Flory , as well as the weight-average lamellar thicknesses, L wav DSC−GT and L wav SSA DSC , were found to be comparable. To the best of our knowledge, such a match has not previously been reported. The percentage crystallinities of the homo-and copolymers increased linearly as a function of L MPDSC-GT . This indicates that the homo-and copolymer chains folded excluding the butyl branch. The results of the present study will contribute to developing future supported metallocene catalysts that will be useful in the synthesis of new grades of ethylene−α-olefin linear low-density polyethylenes (LLDPEs).