Wen-Jun Wang

Effect of long chain branching on rheological properties of metallocene polyethylene

Abstract Polyethylenes (PE) with long chain branching (LCB) densities up to 0.44 carbons/10u2008000 carbons and narrow molecular weight distributions were synthesized using a continuous stirred-tank reactor (CSTR) with the Dow Chemicals constrained geometry catalyst system, CGC-Ti/TPFPB/MMAO, in Isopar E solution at 500xa0psig and 140–190°C. Rheological properties of these metallocene polyethylenes (mPE) were evaluated and correlated to the LCB density. Measurements included steady-state viscosity, dynamic viscoelasticity, melt flow index and extrudate swell. Compared to their linear counterparts with the same molecular weights, LCB PEs gave higher viscosities at low shear rates and lower viscosities at high shear rates. These shear-thinning properties measured by a melt flow index ratio I 10 / I 2 strongly depended on the LCB density. PE with an average of fewer than one branch per six polymer molecules exhibited a I 10 / I 2 value of 25.7 compared to linear PEs I 10 / I 2 =6~8 having similar molecular weights. An increase in the LCB density also significantly reduced the ratio of loss modulus over storage modulus, and yielded longer relaxation lifetime and higher level of extrudate swell. In contrast to the rheological properties, tensile strength, melting temperature, and other physical and mechanical properties were not noticeably affected by LCB.

Long-chain branching in slurry polymerization of ethylene with zirconocene dichloride/modified methylaluminoxane

Abstract We report an experimental investigation on long chain branching (LCB) in ethylene slurry polymerization with bis(cyclopentadienyl) zirconium dichloride (Cp2ZrCl2)/modified methylaluminoxane (MMAO) using a semi-batch reactor. The effects of the reaction temperature, pressure, MMAO concentration, and catalyst feeding method on the long chain branching density (LCBD, number of branches per 10u2008000 carbons), polymer molecular weight, and shear thinning property (I 10 /I 2 ) were systematically examined. The slurry polymerization process, with its associated polymer-rich phase and the partitioning of active sites, favors the LCB formation via an in situ copolymerization of ethylene macromonomers generated by β-hydride elimination and chain transfer to monomer. Increasing the temperature from 60 to 80°C reduced the LCBD from 0.33 to 0.10, while increasing the pressure from 2 to 20xa0psig reduced the LCBD from 0.73 to 0.30. The LCB polyethylenes showed enhanced shear thinning properties, with melt flow index ratios (I 10 /I 2 ) in the range of 8.8–21.5. The feeding sequence of reactants also had a significant effect on the LCB formation. It was observed that feeding ethylene monomer before zirconocene catalyst produced polyethylenes having much higher LCBD than feeding catalyst before monomer.

Continuous solution copolymerization of ethylene and octene‐1 with constrained geometry metallocene catalyst

The solution copolymerization of ethylene (1) with octene-1 (2) in Isopar E using constrained geometry catalyst system, [C5Me4(SiMe2NtBu)]TiMe2 (CGC-Ti)/tris(pentafluorophenyl)boron (TPFPB)/modified methylaluminoxane (MMAO), has been carried out in a high-temperature, high-pressure continuous stirred-tank reactor (CSTR) at 140°C, 500 psig and with a mean residence time of 4 min. A series of copolymer samples with octene-1 content up to 0.337 mole fraction were synthesized and characterized. The estimated reactivity ratios were r1 = 7.90 and r2 = 0.099. The CGC-Ti showed a higher ability to incorporate high α-olefins than other metallocene catalysts investigated in the literature due to its open structure. The presence of octene-1 lowered the catalyst activity, particularly at octene-1 levels higher than 0.45 mole fraction. Octene-1 was also found to reduce the molecular weight of polymer and broaden the molecular weight distributions. The triad distributions were measured by 13C-NMR. A minor penultimate effect was observed. The penultimate octene-1 unit appeared to slow down monomer insertion rates. A comparison of the propagation rate of octene-1 with the incorporation rate of macromonomer in the homopolymerization of ethylene suggests that the addition of macromonomer is effectively instantaneous after it is generated with diffusion to or from the active center reaction volume playing a minor role.

Long chain branching in ethylene polymerization using constrained geometry metallocene catalyst

We report an experimental investigation on long chain branching (LCB) in ethylene polymerization with the Dow Chemicals constrained geometry catalyst system, CGC-Ti/TPFPB/MMAO, using a continuous stirred-tank reactor (CSTR) at 140°C, 3.45 × 10 3 kPa, and a mean residence time (τ) of 4 min. The effects of the catalyst (CGC-Ti) and co-catalyst (TPFPB and MMAO) concentrations on the catalyst activity, polymer molecular weight, and shear thinning were systematically examined. The boron cocatalyst had a great influence on the CGC activity. Increasing the ratio TPFPB/CGC-Ti from 0.66 to 5 gave ethylene propagation rates from 1.65 × 10 3 to 1.36 × 10 4 L mol -1 . s -1 . The addition of MMAO appeared to be essential, most likely acting as an impurity scavenger. The LCB polyethylenes showed enhanced shear thinning properties. The melt flow index ratios I 10 /I 2 were in the range of 6.96 to 23.4, with the I 2 of 0.172 to 0.681 g/10 min. The weight-average molecular weight M w was correlated to I 2 using a power equation within narrow I 10 /I 2 ranges. The exponential factors were in the range of 4.24 to 6.31. The experimental and calculated M w s were in a good agreement.

Temperature rising elution fractionation and characterization of ethylene/octene-1 copolymers synthesized with constrained geometry catalyst

We report here the fractionation and characterization of two crystalline ethylene/octene- 1 copolymer samples with octene-1 mole fractions of 0.028 and 0.089, synthesized in a solution continuous stirred-tank reactor (CSTR) using a constrained geometry catalyst (CGC) system. The samples were fractionated by preparative temperature rising elution fractionation (TREF), and the fractions were characterized by 13 C NMR, GPC and DSC. The copolymers were found to have narrow TREF curves, indicating the CGC system produces copolymers with narrow composition distributions. The fractionation was mainly determined by the short chain branching density in the copolymers. The polymer molecular weight also showed an effect on the fractionation for the copolymer with octene-1 mole fraction of 0.028. The first-order Markovian statistical model appeared to be more valid in predicting the comonomer dyad and triad distributions in the copolymer fractions. The combination of TREF and GPC was found to be an effective technique for the analysis of copolymers with low crystallinity.

Long Chain Branching in Ethylene Polymerization Using Binary Homogeneous Metallocene Catalyst System

ABSTRACTThe solution polymerization of ethylene using a binary metallocene system (BMS), [C5Me4(SiMe2NtBu)]TiMe2 (CGC-Ti) / zirconocene dichloride (Cp2ZrCl2) / tris(pentafluorophenyl)boron (TPFPB) / modified methylaluminoxane (MMAO), has been carried out in a high-temperature, high-pressure continuous stirred-tank reactor (CSTR) at 140 °C, 500 psig and with a mean residence time of 4 minutes. A series of BMS with different catalyst and cocatalyst incorporation were used. The utilization of Cp2ZrCl2 and MMAO in the BMS reduced the activity greatly, and the effect of MMAO was more severe. Formation of long chain branches decreased even though more macromonomer was produced. Polyethylenes with higher molecular weight and broader molecular weight distribution were synthesized using the binary systems than those produced using the CGC-Ti catalyst alone. The overall rate constants of ethylene propagation, long chain branching, ethylene macromonomer formation via chain transfer to monomer and β-hydride eliminati...

ESR study on styrene polymerization with CpTiCl3/MMAO: Effect of monomer addition on catalyst activity

An on-line electron spin resonance (ESR) technique was applied to investigate the syndiospecific polymerization of styrene activated by the catalyst system CpTiCl3/MMAO. The measurements included trivalent titanocene concentration and monomer conversion. The activation procedure was found to have a dramatic effect on catalyst activity. Adding the reactants in the order of (MMAO + CpTiCl3) + St gave a much higher trivalent titanocene concentration and catalyst activity than the order of (MMAO + St) + CpTiCl3. The catalyst deactivation behaviors in the temperature range of 25–70°C were followed as a function of time during polymerization. At high Al/Ti ratios (500–1000), the decay rates of trivalent titanocene in the presence of styrene were much faster than those of the pure catalyst system.

Modeling and analysis of high-impact poly(propylene) production processes, 1 Effect of chemical poisoning on particle size distribution and gel formation

This work presents a simple model for a two-stage process of high impact poly(propylene) (HIPP) production. The model predicts the bivariate distribution of particle size and polymer composition. It takes into account the effect of chemical poisoning on gel particle formation. The result shows that poisoning the solid catalyst is not an effective method for gel reduction. A better approach is to saturate the polymer particles with a co-catalyst in reactor 1 and poison the co-catalyst in reactor 2. It is also shown that the residence time distribution (RTD) of reactor 1 has a strong effect on the gel particle formation. A continuous reactor with narrow RTD is advantageous for gel reduction. The model provides some guidance for the analysis and design of the HIPP production process.