Identification of the Starting Group on the First PE Chain Produced by the Phillips Catalyst

Abstract

Upon contact with ethylene, the Phillips Cr/silica catalyst must selfalkylate before polymerization can begin. How this happens has been a mystery for over 60 years, being not only of scientific interest but also of commercial importance. Consequently, many controversial schemes have been proposed, including some that hold the initial PE chain to be different from all subsequent chains, starting with a vinyl group, instead of the usual methyl group. In this study, polymer chains produced by the Cr(II)/silica−titania catalyst at low yield were analyzed in an effort to identify the starting group on the first chain produced. The PE/Cr yield was deliberately kept low to avoid diluting that initial chain with other chains produced afterward. The polymer was then characterized by gel permeation chromatography and 13 C NMR (natural abundance). Strong signals from methyl end groups were present, but terminal vinyl groups were not detected. Likewise, analysis by proton ( 1 H) NMR also indicated a general lack of vinyl chain ends. An attempt was also made to identify the first species on the initial chain through isotopic labeling. Deutero-ethylene (C 2 D 4 ), deutero-propylene (C 3 D 6 ), or 13 Cethylene was first "preadsorbed" onto the divalent catalyst at −78 °C, where polymerization does not occur. After evacuating the excess isotopically labeled monomer, polymerization was conducted with unlabeled ethylene (C 2 H 4 ) at higher temperatures. Deuterium ( 2 H) NMR analysis of the polymers again indicated a strong methyl signal but no resonance from deuteriumcontaining vinyl chain ends. Comparison of the different NMR methods indicated a high degree of deuterium enrichment in the methyl chain ends. These results indicate that the initial chain in these experiments began with a methyl group, not a vinyl group as has sometimes been contemplated. Although helpful, this information still leaves the mechanism of site alkylation unclear because there is not enough hydrogen in ethylene to produce a methyl-initiated chain, without some other (still undefined) reaction also being involved to furnish the additional hydrogen, such as the creation of a permanent ligand.

Recent Trends in Catalytic Polymerizations

“…The development of new early-transition-metal-based catalysts remains an active topic of research in academia and industry due to the societal importance of polyolefins. ,− Despite the advances made in molecular olefin catalysts, the Phillips and Ziegler–Natta catalysts remain the preferred choice for industrial production of HDPE and iPP, respectively . Interestingly, the mechanism of activation for the Phillips catalyst remains intensely debated in the literature. − Today’s research on polyolefins focuses on expanding architecture control and polar group compatibility through polar monomer copolymerizations, postpolymerization modifications, chain transfer polymerizations, coordinative chain transfer polymerizations, and acyclic diene metathesis polymerizations.…”

Chromium complexes supported by NNO-tridentate ligands: an unprecedented activity with the requirement of a small amount of MAO

“…7 Chromium-based metal complexes, both heterogeneous and homogeneous, are among the most important polyolefin catalysts and have received increasing attention because of their wide use in both ethylene polymerization and oligomerization. [41][42][43][44][45][46][47][48][49][50][51][52] However, Cr complexes bearing phenoxy-imine ligands are rare and underdeveloped. Previously, Gibson explored this ligand family for Cr-based polymerization catalysts, but only low to modest activity (usually less than 10 5 g(PE) mol −1 (Cr) h −1 ) was obtained.…”

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

“…This at least excludes the presence of exclusively naked Cr 2 + ions and indicates that reduction by-products remain in the proximity of the Cr active sites, hereby affecting the d-d transition and CT bands. [16,[57][58][59][60][61][62][63] In order to ensure a proper fit, an additional band was required at 24000 cm À 1 , which is expected to be a CT band. This 24000 cm À 1 CT band, which is absent in the CO reduced material, can be explained by reduction by-products remaining in the coordination sphere of the Cr active site: affecting the location the d-d transition bands and the CT bands.…”