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Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization
Published on: November 27, 2015
Chain epimerization during propylene polymerization with metallocene catalysts: mechanistic studies using a doubly
Jeffrey C Yoder1, John E Bercaw
1Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, CA 91125, USA.
Journal of the American Chemical Society
|March 14, 2002
Summary
Chain epimerization in propylene polymerization was studied using isotopically labeled propylene. The findings suggest a mechanism involving beta-D elimination and insertion, excluding allyl/dihydrogen complexes.
Area of Science:
- Polymer Chemistry
- Organometallic Chemistry
- Spectroscopy
Background:
- Propylene polymerization using metallocene catalysts is crucial for producing polyolefins.
- Understanding chain epimerization mechanisms is key to controlling polymer stereochemistry.
- Isospecific catalysts like rac-(EBTHI)ZrCl(2) and rac-(EBI)ZrCl(2) typically yield highly stereoregular polypropylene.
Purpose of the Study:
- To elucidate the detailed mechanism of chain epimerization during propylene polymerization.
- To investigate the role of specific intermediates in stereochemical errors.
- To differentiate between proposed epimerization pathways using isotopic labeling.
Main Methods:
- Synthesis of isotopically labeled propylene (CH(2)=CD(13)CH(3)).
- Propylene polymerization using methylaluminoxane-activated rac-(EBTHI)ZrCl(2) and rac-(EBI)ZrCl(2) catalysts.
- Analysis of polymer microstructure using carbon-13 nuclear magnetic resonance ((13)C NMR) spectroscopy.
Main Results:
- Stereoerrors, attributed to chain epimerization, were observed at lower propylene concentrations and moderate temperatures.
- (13)C NMR revealed specific (13)C-labeled methylene groups and isotopomeric units ([CD(13)CH(3)], [CH(13)CH(3)]) within various pentad and heptad sequences.
- Observed errors align with a mechanism involving beta-D elimination, olefin rotation, enantiofacial interconversion, and insertion into a tertiary alkyl intermediate, followed by reverse steps.
Conclusions:
- The study provides strong evidence for a specific chain epimerization mechanism in these metallocene-catalyzed polymerizations.
- The absence of certain labeled units ([CH(2)CH(13)CH(2)D-]) in specific pentads rules out an allyl/dihydrogen complex as the mediating species.
- This detailed mechanistic understanding aids in the design of catalysts for improved control over polypropylene stereoregularity.
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