Abstract
A procedure to obtain very uniform polyolefin copolymers by supported metallocenes is presented. Conventional metallocene or Ziegler–Natta catalysts, both immobilized on inorganic supports, yield only copolymers with inhomogeneous comonomer incorporation and broad short-chain branching distribution. The main reasons are diffusional limitations of the monomers or the multisite character of the catalysts. By comparing inorganic and organic supports, we demonstrate that metallocenes immobilized on organic supports solve these problems. In this regard, organic and soft nanosized polystyrene particles (nPS) versus industrially used, hard, and inorganic SiO₂ were used to support [Me₂Si(Ind)₂ZrCl₂/MAO (I) and Me₂Si(Benz[e]-Ind)₂ZrCl₂/MAO (BI)] catalysts for ethylene/1-hexene copolymerization. In the inorganic case, the catalyst systems show a substantial inconsistency in the copolymers’ branching distribution, resulting in phase separation. One phase is hexene-poor with high melting temperature (Tₘ) and high molecular weight (MW). The second, hexene-rich phase, however, shows lower Tₘ and MW. By using organic supports, comonomers are uniformly inserted into the polymer chain and homogeneous microstructured copolymers are obtained. These findings are mainly attributed to diffusion processes of the monomers into the soft organic material. To prove this conclusion and to elucidate the structure of the catalyst system, various characterization techniques such as time-of-flight secondary ion mass spectrometry and scanning electron microscopy–energy-dispersive X-ray were performed.
Highlights
The copolymerization of ethylene with α-olefins has resulted in a growth of polyolefin applications over the last decades.[1,2] In particular, metallocene-catalyzed copolymerization represents a versatile synthetic route to tailor-made polyolefins.[1,3,4] In comparison to Ziegler−Natta catalysts, metallocenes have shown remarkable advantages, such as regulating the polymer microstructure by narrow molecular weight (MW) distribution and stereoselective comonomer incorporation.[5−9] homogenous metallocene catalysts cause reactor leaching/ fouling because of a lack of morphology control during polymerization.[10]
The properties of polyolefins obtained with metallocenecatalyzed polymerizations are typically modified by varying catalysts and polymerization parameters
The support is considered as an inert compound during the polymerization process, as inorganic supports have been mainly focused on
Summary
The copolymerization of ethylene with α-olefins has resulted in a growth of polyolefin applications over the last decades.[1,2] In particular, metallocene-catalyzed copolymerization represents a versatile synthetic route to tailor-made polyolefins.[1,3,4] In comparison to Ziegler−Natta catalysts, metallocenes have shown remarkable advantages, such as regulating the polymer microstructure by narrow molecular weight (MW) distribution and stereoselective comonomer incorporation.[5−9] homogenous metallocene catalysts cause reactor leaching/ fouling because of a lack of morphology control during polymerization.[10] metallocenes have been supported on micrometer-sized fragmentable porous particles. Two different silylene-bridged bis(indenyl) catalysts are chosen, which are reported as highly active to incorporate the comonomer in homogenous catalyst systems (solution polymerization).[25]
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