Long-chain-branched isotactic polypropylenes (LCBed PP) were synthesized by copolymerizing propylene with a small amount of nonconjugated α,ω-diene (1,9-decadiene or 1,7-octadiene) using the catalyst ...system of rac-Me2Si(2-MeBenzeInd)2ZrCl2(MBI)/MMAO. In this approach, the LCB structures were introduced by the incorporation of in situ generated macromonomers with pendant 1-octenyl or 1-hexenyl groups during the polymerization. A detailed study on the effects of diene concentration on polymer properties was conducted. Polymer chain microstructures were characterized by 13C NMR, GPCV, and DSC. In the propylene/1,9-decadiene copolymerization, a series of LCBed polymer samples with the long-chain-branch density (LCBD) of up to 0.53 branch structures per 1000 carbons were produced with the diene concentrations of 0.177−3.54 mmol/L at 40 and 25 °C. A diene concentration of 35.4 mmol/L yielded cross-linked polymer gels. In the copolymerization of propylene and 1,7-octadiene, in addition to a small fraction of LCB structures produced, a cyclic seven-member ring structure was observed due to the cycloaddition of 1,7-octadiene. The cyclization significantly decreased the LCBD in the polymers. A small-amplitude oscillatory shear flow measurement was conducted to evaluate the rheological properties of the LCBed polymers. Compared to the linear samples prepared at the same polymerization conditions, the LCBed polymers exhibited enhanced low-frequency complex viscosity, improved shear-thinning, increased dynamic moduli, and reduced phase angle. The samples also showed thermorheological complexity and enhanced activation energy at low frequencies. These particular properties are related to the LCB in the polymers and become more significant with the increase of LCBD. The LCBed polypropylenes were also blended with their counterpart linear samples and demonstrated the improvement of rheological properties.
Concurrent tandem catalysis systems have shown a significant advantage in the convenient synthesis of linear low‐density polyethylene (LLDPE) from a sole ethylene monomer stock by uniquely coupling ...the tandem action between an ethylene oligomerization catalyst and an ethylene copolymerization catalyst in a single reactor. Recently, we have reported the successful synthesis of ethylene‐hexene derived LLDPE using an effective concurrent tandem catalysis system comprising (η5‐C5H4CMe2C6H5)TiCl3 (1)/MMAO and a CGC copolymerization catalyst, (η5‐C5Me4)SiMe2(tBuN)TiCl2 (2)/MMAO. In this work, we report the results from an extensive study on the important rheological properties of LLDPE grades prepared with this tandem catalysis system. Two sets of LLDPE samples having different short‐chain branching density (SCBD) were prepared with the tandem catalysis system under various catalyst concentrations and at temperatures of 25 and 45 °C. The melt rheological properties of these polymers were evaluated using small‐amplitude dynamic oscillation measurements. These polymers have been found to possess typical rheological properties found in long‐chain branched (LCB) polymers, such as enhanced zero‐shear viscosity (η0), improved shear‐thinning, elevated dynamic moduli, and thermorheological complexity, which indicate the presence of long‐chain branching in the polymers. The long‐chain branching density (LCBD) of the two respective sets of polymers were qualitatively compared and correlated to the polymerization conditions including catalyst ratio and temperature. This work represents the first study on the rheological properties of LLDPE synthesized with concurrent tandem catalysis, and it discloses another appealing feature of this unique approach—its ability to produce LCB LLDPE from a single ethylene monomer stock.
Synthesis of linear low‐density polyethylene (LLDPE) from ethylene using ethylene oligomerization catalyst and an ethylene copolymerization catalyst.
Ethylene polymerization was carried out using three nickel α-diimine catalysts ((ArN
C(An)–C(An)
NAr)NiBr
2 (
1), (ArN
C(CH
3)–C(CH
3)
NAr)NiBr
2 (
2) and (ArN
C(H)–C(H)
NAr)NiBr
2 (
3); where ...An=acenaphthene and Ar=2,6-(
i-Pr)
2C
6H
3) activated with modified methylaluminoxane (MMAO) in a slurry semi-batch reactor. We investigated the effects of ethylene pressure, reaction temperature, and α-diimine backbone structure variation on the catalyst activity and polymer properties. Changes in the α-diimine backbone structure had remarkable effect on the polymer microstructure as well as the catalyst activity. Catalyst
2 produced polymer with the highest molecular weight, while Catalyst
3 produced polymer with the lowest molecular weight. In addition, Catalyst
2 produced polymer with the lowest melting point, while Catalyst
3 produced the highest melting level exhibiting a melting behavior typical of high-density polyethylene (HDPE). With all the three catalysts, polymer molecular weight tended to decrease with increasing polymerization temperature due to the increase in chain transfer rates. In general, there was no clear and consistent trend observed for the effects of ethylene pressure on the polymer molecular weight. However, in polyethylene produced with Catalyst
2, the molecular weight was independent of ethylene pressure suggesting that chain transfer to ethylene may be a dominant mechanism for this catalyst.
Ethylene was polymerized using both homogeneous and modified methylaluminoxane (MMAO)‐treated silica supported nickel‐diimine catalysts (1,4‐bis(2,6‐diisopropylphenyl) acenaphthene diimine nickel(II) ...dibromide) in a slurry semibatch reactor. The effects of catalyst support and polymerization conditions (ethylene pressure and reaction temperature) on catalyst activity and polymer properties were systematically investigated. The supported catalyst gave far lower activity than the homogeneous catalyst. The activities of both catalyst systems increased with polymerization temperature with a maximum at 40 °C. Compared with the homogeneous catalyst, the supported catalyst system produced polyethylene with a different microstructure. Due to steric effects, the supported catalyst system exhibited lower chain walking rates than the homogeneous catalyst, producing polymers with less branching content and, thus higher melting points. Depending on polymerization conditions, two active site populations were observed during polymerization using supported catalyst; one population remained fixed on the surface of the support, and the other was extracted from the support, exhibiting the same polymerization behavior as the homogeneous catalyst.
DSC thermograms for polyethylene produced with homogeneous and supported catalysts at an ethylene pressure of 50 psig (3.45 · 105 Pa) and reaction temperature 40 °C.
Seven branched polyethylenes differing in chain topology from hyperbranched to linear structure were synthesized with chain walking Pd‐diimine catalyst, (ArNC(Me)C(Me)NAr) Pd(CH3)(NCMe)SbF6 (1), ...and Ni‐diimine catalyst, (ArNC(An)C(An)NAr) NiBr2 (2)/MMAO, respectively. An extensive rheological study, employing steady‐shear, creep‐recovery, and dynamic oscillation tests, was conducted to examine and compare the melt rheological properties of this novel series of polymers. It was found that the change of chain topology dramatically affected the polymer flow behavior, flow activation energy, and dynamic moduli (G′(ω), G″(ω)). The hyperbranched polymers exhibited typical Newtonian flow behavior and extremely low viscosity. The polymers with chain topology intermediate between hyperbranched and linear structures, however, were essentially viscoelastic materials. All the polymers obeyed the time‐temperature superposition and exhibited enhanced flow activation energy (43.8∼57.2 kJ/mol) compared to HDPE and LLDPE. In the terminal region, these polymers had different dependencies of dynamic moduli (G′(ω), G″(ω)) on angular frequency (ω) and different master curves in the log(G′) versus log(G″) plot. The hyperbranched polymer was also blended with more linear samples as a rheology modifier and was found to significantly lower the viscosity of the blends.
Structure of the Pd‐ and Ni‐catalysts used in this study.
A tandem catalytic system, composed of (η5‐C5H4CMe2C6H5)TiCl3 (1)/MMAO (modified methyl aluminoxane) and (η5‐C5Me4)SiMe2(tBuN)TiCl2 (2)/MMAO, was applied for the synthesis of ethylene–hex‐1‐ene ...copolymers with ethylene as the only monomer stock. During the reaction, 1/MMAO trimerized ethylene to hex‐1‐ene, while 2/MMAO copolymerized ethylene with the in situ produced hex‐1‐ene to poly(ethylene–hex‐1‐ene). By changing the catalyst ratio and reaction conditions, a series of copolymer grades with different hex‐1‐ene fractions at high purity were effectively produced.
The overall strategy of the tandem 1/2/MMAO catalytic system.
