Polyethylene with Reverse Co‐monomer Incorporation: From an Industrial Serendipitous Discovery to Fundamental Understanding

Abstract

At riethylaluminium(TEAl)-modified Phillips ethylene polymerisation Cr/Ti/SiO 2 catalyst has been developed with two distinct active regions positioned respectively in the inner core and outer shell of the catalyst particle.D RIFTS, EPR, UV-Vis-NIR DRS,S TXM, SEM-EDX and GPC-IR studies revealed that the catalyst produces simultaneously two different polymers,i .e., low molecular weight linear-chain polyethylene in the Ti-abundant catalyst particle shell and high molecular weight short-chain branched polyethylene in the Tiscarce catalyst particle core.Co-monomers for the short-chain branched polymer were generated in situ within the TEAlimpregnated confined space of the Ti-scarce catalyst particle core in close proximity to the active sites that produced the high molecular weight polymer.T hese results demonstrate that the catalyst particle architecture directly affects polymer composition, offering the perspective of making high-performance polyethylene from as ingle reactor system using this modified Phillips catalyst.Several years ago at an industrial ethylene polymerisation plant, the in situ generation of co-monomer on aC rp olymerization line using the well-known Cr/Ti/SiO 2 Phillips-type catalyst was reported.[1-8] Hence,t he co-feeding of 1-hexene was significantly reduced in order to keep ap olymer with similar content of co-monomer.T he interesting finding was that 1-hexene was the major component, while butene and other oligomers were present in lower concentration. Moreover, the properties of the polyethylene produced were not affected despite the presence of butene.Onthe contrary,the polymer made even exhibited some improvements.T he hypothetical explanation for in situ co-monomer generation was ac ontamination of the recycling feeds by triethylaluminium (TEAl), as there were several lines with ac ommon recycling section. One of the polymerisation lines was running aZ iegler-Natta catalyst with TEAl as co-catalyst, [9,10] while the other polymerisation lines ran with aCr/Ti/SiO 2 Phillipstype catalyst without any co-catalyst.Preliminary tests at both pilot and bench scales confirmed the in situ generation of co-monomer with aC r/Ti/SiO 2 catalyst and TEAl. Theo bserved phenomena could be confirmed, as illustrated in Figure 1, by performing slurryphase ethylene polymerisation experiments in a4litre-sized semi-batch reactor. Thep olymerisation conditions were chosen to target the same molecular weight distribution and co-monomer content in the polymer (Table S1 and S2 in the Supporting Information). Hence,1-hexene was added in one experiment during the polymerisation when the Phillips Cr/ Ti/SiO 2 catalyst under investigation was tested without TEAl, as no co-monomer was in situ generated in that particular case.I nt he second experiment, in situ oligomerisation was induced by modification of the catalyst with TEAl. GPC-IR analysis of the polymers produced clearly shows that less comonomer was incorporated in the short polyethylene chains than in the long chains when it is generated in situ. ...

Role of Titanium in Ti/SiO2‐Supported Metallocene‐based Olefin Polymerization Catalysts. Part 1: Genesis of Active Sites and Catalytic Performance

“…The polyethylene produced were analyzed by DSC and 13 C‐NMR, to exclude the possibility that the Ti active site could synthesize a second α‐olefin to be inserted in the PE chain and introduce branching. In fact, this was the case for the Cr/Ti/SiO 2 Phillips‐type ethylene polymerization catalyst reported by Cicmil et al ., in which the Ti/SiO 2 support was obtained by a similar synthesis procedure as the one used in this work [30] . As more extensively reported in the SI, these analyses showed that the PE produced by the commercial and modified catalyst, as well ad that produced by the active Al/Ti/SiO 2 support, were all linear, non‐branched HDPE.…”

“…Hence, this study aims at elucidating the role played by such Ti modification of the SiO 2 support, which was obtained by surface modification of a commercial SiO 2 with titanium alkoxide, followed by heat treatment, resulting in a layer of TiO x on the silica surface and pore walls (further referred to as Ti/SiO 2 ). A similar titanation procedure has been used for decades with Cr‐based Phillips catalyst systems [29,30] . This Cr/Ti/SiO 2 , when activated by an aluminum‐alkyl co‐catalyst, can oligomerize ethylene into 1‐alkenes which can then co‐polymerize with ethylene, leading to a polymer product with a different chain structure having better processability properties than that obtained with the non‐titanated Cr/SiO 2 [30] .…”

“…A similar titanation procedure has been used for decades with Cr‐based Phillips catalyst systems [29,30] . This Cr/Ti/SiO 2 , when activated by an aluminum‐alkyl co‐catalyst, can oligomerize ethylene into 1‐alkenes which can then co‐polymerize with ethylene, leading to a polymer product with a different chain structure having better processability properties than that obtained with the non‐titanated Cr/SiO 2 [30] . Ti modifications of the silica support are also used in newer generations of Ziegler‐Natta catalyst materials, although in the co‐presence of MgClx [31] .…”