Cyclic olefin copolymers containing both linear polyethylene and poly(ethylene-co-norbornene) segments prepared from chain shuttling copolymerization of ethylene and norbornene

Abstract

Semicrystalline polyethylene (PE) and amorphous poly(ethylene-co-norbornene) copolymers have been used in a wide variety of applications, especially in packaging. It is a big challenge to improve the transparency of high-density polyethylene while maintaining its excellent tensile strength and heat resistance. For the linear polyethylene, loss in heat resistance is usually observed when introducing randomly distributed α-olefins, while its toughness is sacrificed by inserting large amounts of cyclic comonomers. In this study, multi-block copolymers containing alternating linear polyethylene segments and poly(ethylene-co-norbornene) copolymer segments were prepared to solve this trade-off problem. Two structurally similar ansa-metallocene catalysts, ethylene bis(1-η⁵-indenyl) zirconium(IV) dichloride and methine bis(3-tert-butyl-1-η⁵-indenyl) zirconium(IV) dichloride, were adopted in this study. These two catalysts presented comparable catalytic activity but different selectivity toward ethylene/norbornene copolymerization, yielding random poly(ethylene-co-norbornene) copolymer segments and semicrystalline PE segments under the same reaction conditions, respectively. In the presence of Et₂Zn, a polymeryl exchange between the two active zirconium centers occurred and multiblock copolymers, containing both cyclic olefin copolymer segments and linear polyethylene segments, were obtained. The average norbornene incorporation and molecular weight of the copolymer, which govern the final performance parameters of the copolymers, could be easily adjusted by the Et₂Zn dosage, norbornene/ethylene ratio and/or reaction time. The composition and segment distribution of the multiblock copolymers were comprehensively studied. As proved by a light transmittance test and tensile and rheological measurements, the multiblock copolymers presented much improved transparency, tensile properties, mechanical properties and processability.