Complex interplay of short- and long-chain branching on thermal and rheological properties of ethylene/α-olefin copolymers made by metallocene catalysts with oscillating ligand structure

Abstract

Unbridged oscillatory metallocenes are prone to produce heterogeneous chain microstructure, for instance, giving rise to elastomeric polypropylene. Here, we use the bis(2phenylindenyl)ZrCl 2 /MAO catalyst system, featuring oscillating ligands, to copolymerize ethylene with several α-olefins. Through investigation of chain microstructure, thermal and rheological properties, we show that the complex behavior of these materials can be justified by considering a catalyst with dual active center having different comonomer affinities. This behavior is consistent with the proposed mechanism of fast oscillating ligands, becoming locked upon proximity of cocatalyst counteranion, leading to diverse stereo-and comonomer selectivity. The unlocked, oscillating active center with lower comonomer response gives rise to mostly linear chains and is not affected by the presence of comonomer, while the locked one creates long chain branched chains, whose frequency decreases by introduction of comonomer. Copolymer samples exhibited bimodal MWD, dual crystallization mechanisms, and low frequency plateau modulus, specifically at higher comonomer levels. It was concluded that the overall behavior is determined by complex interplay of SCB and LCB, which leads to thermorheological complexity as well as phase separation.Oscillating metallocene catalyst is used in copolymerization of ethylene and several α-olefin comonomers. The dual nature of active centers leading to complex interplay of SCB and LCB is revealed by studying thermal and rheological properties. Graphical Abstract Complex interplay of short and long-chain branching on thermal and rheological properties of ethylene/α-olefin copolymers made by oscillatory metallocene catalystMostafa Ahmadi, Faeze Rezaei, Mohammad Mehdi Mortazavi, Mehdi Entezam, Florian J. StadlerOscillating metallocene catalyst is used in copolymerization of ethylene and several α-olefin comonomers. The dual nature of active centers leading to complex interplay of SCB and LCB is revealed by studying thermal and rheological properties.

Extruded-Calendered Sheets of Fully Recycled PP/Opaque PET Blends: Mechanical and Fracture Behaviour

“…Considering the Van Gurp–Palmen plots and moving from the terminal regime, a common feature that could be observed was that both blends developed an inflection point to a minimum, then increased again up to a maximum, and finally decreased as the dynamics reached the glassy regime. This behaviour has been widely reported in copolymers and binary blends [ 39 , 40 ]. Regarding the 80/20 formulation, it can be observed that a shoulder is observed in the |η*| as well as in the G′ curves before reaching the terminal region, which is shifted to lower ω.…”

“…Such behaviour is often related to composites exhibiting a physically entangled fibrillar network and percolation threshold effect of the second phase, which, in the present study, would have been created by the topological interactions of the long rPET-O microfibrils [ 40 , 43 ]. Based on the conclusions previously reported, the gradual curve step phenomenon observed in the Van Gurp–Palmen plot as the rPET-O content increases must be attributed to the increasing solid-like behaviour of the melt owing to significant changes in the relaxation mechanisms as a function of the developed morphology.…”

Resolving long-chain branch formation in tandem catalytic coordinative chain transfer polymerization of ethylene via thermal analysis

“…This is however lower than the melting temperature of linear ethylene homopolymers obtained by the traditional Ziegler-Natta catalysts [28]. That's due to the higher comonomer selectivity of metallocene catalysts, which leads to the incorporation of macromers formed by the irreversible spontaneous chain transfer or the chain transfer to monomer reaction even in the absence of any displacement catalysts [6]. By decreasing the reaction temperature, the sharp melting peaks at high temperatures, gradually disappear and new melting endotherms spanning toward lower melting temperatures appear.…”

“…Specifically, the application of GPC and NMR techniques for characterizing polyethylene copolymers requires the employment of toxic solvents at high temperatures. Moreover, as these traditional characterization methods cannot directly resolve the presence of long-chain branches, they are frequently combined with other indirect techniques that have more sensitivity to the presence of such branches [6,37]. Despite thermal properties of polyethylene are traditionally used to quantify the lamellae thickness, and consequently, the fraction of short-chain branches, since our proposed mechanism is incapable of forming such short branches, any deviation from the thermal behavior of linear chains produced in the absence of the displacement catalyst can be correlated to the presence of long-chain branches.…”

“…To this end, the development of specialized catalysts and engineering the reaction conditions toward the controlled incorporation of olefinic comonomers into the polyethylene chains has resulted in the development of new polyolefins with custom structure and superior properties [4,5]. For instance, the incorporation of α-olefins is employed for regulating the length of the crystallizable segment, whereas the cyclic olefinic comonomers like norbornene, on top of that, offer a great level of control on the glass transition temperature [6][7][8]. This, however, extremely relies on the characteristics of the employed coordination catalysts and from that view metallocene catalysts offer a flexible platform and a great potential to produce a myriad of industrially viable linear low density polyethylene (LLDPE) and cyclic olefin copolymers (COC) [9,10].…”