Thermal and rheological studies on the molecular composition and structure of metallocene‐ and Ziegler–Natta‐catalyzed ethylene–α‐olefin copolymers

Abstract

ABSTRACT:The relationship between the molecular structure and the thermal and rheological behaviors of metallocene-and Ziegler-Natta (ZN)-catalyzed ethylene copolymers and high-density polyethylenes was studied. Of special interest in this work were the differences and similarities of the metallocene-catalyzed (homogeneous) polymers with conventional coordination-catalyzed (heterogeneous) polyethylenes and low-density polyethylenes. The short-chain branching distribution was analyzed with stepwise crystallization by differential scanning calorimetry and by dynamic mechanical analysis. The metallocene copolymers exhibited much more effective comonomer incorporation in the chain than the ZN copolymers; they also exhibited narrower lamellar thickness distributions. Homogeneous, vanadium-catalyzed ZN copolymers displayed a very similar comonomer incorporation to metallocene copolymers at the same density level. The small amplitude rheological measurements revealed the expected trend of increasing viscosity with weight-average molecular weight and shear-thinning tendency with polydispersity for the heterogeneous linear low-density polyethylene and very-low-density polyethylene resins. The high activation energy values (34 -53 kJ/mol) and elevated elasticity found for some of our experimental metallocene polymers suggest the presence of long-chain branching in these polymers. This was also supported by the comparison of the relationship between low shear rate viscosity and molecular weight.

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…[74] In the rheological studies of narrowly distributed (PDI ≈ 2.0) CGC-PE homo-or copolymers, many interesting observations were made, such as higher viscosities at low shear rates, lower viscosities at high shear rates, significantly reduced ratios of loss modulus over storage modulus, longer relaxation lifetimes, higher levels of extrude at swell, [5] enhanced G′ and G″ values at low shear rates, broader relaxation spectra and thermorheologically complex behavior. [75,76] Similar findings were also reported by Löfgren group [77] and Santamarìa group [78] with other metallocene-based POs.…”

Nuclearity and cooperativity effects in binuclear catalysts and cocatalysts for olefin polymerization

“…Linear low-density polyethylene (LLDPE)-long chain polyethylene with small but significant levels of C 4 -C 6 alkyl branching-has excellent processability and high melt tension properties suitable for film manufacture (25,26). Typically, linear low-density polyethylene branching is achieved via copolymerization of ethylene with a linear ␣-olefin comonomer.…”

“…Homogeneous ''tandem catalysis'' has also received attention as an alternative approach to branched polyethylenes. Here, a single-site catalytic center produces ␣-olefin oligomers, which are subsequently incorporated, via an intermolecular process, into highmolecular-weight polyethylene by a second, different single-site catalytic center present in the same reaction medium and using the same ethylene feed (26)(27)(28). It should be evident that, at typical polymerization concentrations, these intermolecular tandem processes are inherently inefficient.…”

Thermal and mechanical analysis of metallocene‐catalyzed ethene–α‐olefin copolymers: The influence of the length and number of the crystallizing side chains

“…5 Segregation studies by stepwise annealing with differential scanning calorimetry (DSC) have been applied to different types of Z-N- [6][7][8][9][10] and metallocene-catalyzed ethene copolymers. [8][9][10][11][12] The results show that differences in the ethylene sequence distributions can be seen when the polymers are annealed in steps at successively lower temperatures. The multiple endothermic peaks observed in the melting curves after this treatment correspond to a fraction of segregated molecules that crystallized at a certain temperature.…”