Abstract
A novel tandem catalysis system consisted of salicylaldiminato binuclear/mononuclear titanium and 2,6-bis(imino)pyridyl iron complexes was developed to catalyze ethylene in-situ copolymerization. Linear low-density polyethylene (LLDPE) with varying molecular weight and branching degree was successfully prepared with ethylene as the sole monomer feed. The polymerization conditions, including the reaction temperature, the Fi/Ti molar ratio, and the structures of bi- or mononuclear Ti complexes were found to greatly influence the catalytic performances and the properties of obtained polymers. The polymers were characterized by differential scanning calorimetry (DSC), high temperature gel permeation chromatography (GPC) and high temperature 13C NMR spectroscopy, and found to contain ethyl, butyl, as well as some longer branches. The binuclear titanium complexes demonstrated excellent catalytic activity (up to 8.95 × 106 g/molTi·h·atm) and showed a strong positive comonomer effect when combined with the bisiminopyridyl Fe complex. The branching degree can be tuned from 2.53 to 22.89/1000C by changing the reaction conditions or using different copolymerization pre-catalysts. The melting points, crystallinity and molecular weights of the products can also be modified accordingly. The binuclear complex Ti2L1 with methylthio sidearm showed higher capability for comonomer incorporation and produced polymers with higher branching degree and much higher molecular weight compared with the mononuclear analogue.
Highlights
Polyethylene contributes to about 50% of the global polyolefin consumption, and one of the fastest growing types of polyethylene is linear low density polyethylene (LLDPE), which is widely applied in film production for the packaging industry because of its high tear and impact strength
An alternative approach to the synthesis of Linear low-density polyethylene (LLDPE) is the use of tandem catalysis, where one catalyst oligomerizes ethylene to α-olefin and the other simultaneously
Polymers 2019, 11, 1114 synthesis of LLDPE is the use of tandem catalysis, where one catalyst oligomerizes ethylene to α-olefin and theexcessive other simultaneously excessive ethylene with 1B)
Summary
Polyethylene contributes to about 50% of the global polyolefin consumption, and one of the fastest growing types of polyethylene is linear low density polyethylene (LLDPE), which is widely applied in film production for the packaging industry because of its high tear and impact strength.LLDPEs are generally made by a two-step method from the copolymerization of ethylene and α-olefin comonomers such as 1-butene, 1-hexene and 1-octene catalyzed by conventional heterogeneousZiegler-Natta or metallocene catalysts, where the comonomer α-olefins were commonly generated from ethylene oligomerization (Figure 1A). An alternative approach to the synthesis of LLDPE is the use of tandem catalysis, where one catalyst oligomerizes ethylene to α-olefin and the other simultaneously. Polymers 2019, 11, 1114 synthesis of LLDPE is the use of tandem catalysis, where one catalyst oligomerizes ethylene to α-olefin and theexcessive other simultaneously excessive ethylene with 1B). The in situsingle-step produced copolymerizes ethylene withcopolymerizes the in situ produced α-olefin (Figure Such α-olefin 1B). Such single-step process used has obvious over of traditionally used process has obvious advantages over traditionally two-stageadvantages process in terms the costs of plant two-stage process in terms of the costs of plant investment, α-olefin purification, storage, and investment, α-olefin purification, storage, and transport. Such single-step process used has obvious over of traditionally used process has obvious advantages over traditionally two-stageadvantages process in terms the costs of plant two-stage process in terms of the costs of plant investment, α-olefin purification, storage, and investment, α-olefin purification, storage, and transport. transport
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