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Long-chain branching in PE from Cr/aluminophosphate catalysts

Paper ID Volume ID Publish Year Pages File Format Full-Text
41823 45901 2010 11 PDF Available
Title
Long-chain branching in PE from Cr/aluminophosphate catalysts
Abstract

Phillips Cr/silica catalysts have long been known to produce long-chain branching (LCB) in polyethylene (PE) through macromer incorporation. In fact, the manipulation of LCB levels through catalyst variables is a very useful way of controlling properties and molding behavior in commercial PE resins. Using alumina as the support, instead of silica, is one way of achieving very low levels of LCB.Adding small amounts of phosphate to the alumina base as a modifier causes a large rise in LCB levels. Phosphate also promotes the activity and melt index potential of the catalyst by contributing a large new low-MW population of PE chains. In this study LCB levels were measured and found to correlate nicely with the amount of phosphate added. The polymer from such a catalyst was then solvent-fractionated to determine the location of the LCB. It was found to be concentrated in this same low-MW polymer, to which the phosphate contributes, which suggests that the phosphate-associated Cr sites have a tendency to produce LCB. However, the incorporation of 1-hexene resulted in a flat short-chain branch (SCB) profile, indicating that there was no parallel behavior among the different sites between SCB and LCB incorporation.These results indicate a slightly different mechanistic pathway between the two monomers (macromer and 1-hexene). Furthermore the Cr sites must have incorporated macromers that were produced only at the same site, and not (as is usually portrayed) randomly from a general pool of vinyls contributed by all sites. To explain this finding, alternatives to the widely accepted random-incorporation mechanism are proposed, including a “tethered” vinyl end-group, and intra-, rather than the traditional inter-, molecular insertion of macromer.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (141 K)Download as PowerPoint slideResearch highlights▶ Phosphate-treated Cr/alumina catalysts produce HDPE in which the long chain branching is concentrated into the low MW. ▶ This finding contradicts the widely-accepted mechanism of LCB formation through random macromer incorporation.

Keywords
Chromium catalyst; Polymerization catalyst; Phillips catalyst; Ethylene polymerization; Polyethylene; Long-chain branching; Short-chain branching; Cross-insertion; Inter-molecular mechanism; Intra-molecular mechanism
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Publisher
Database: Elsevier - ScienceDirect
Journal: Applied Catalysis A: General - Volume 388, Issues 1–2, 20 November 2010, Pages 102–112
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us