Single-site polymerization catalysts: branched polyethylene and syndiotactic poly(alpha-olefins)
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Utilization of methylaluminoxane (MAO) activated metallocene and constrained geometry (CGC) olefin polymerization catalysts containing fluorenyl or octamethyloctahydrodibenzofluorenyl (Oct) moieties has yielded three series of syndiotactic copolymers of propylene with higher a-olefins. The melting temperatures of these polymers were analyzed, and found to correspond directly with the mole percent incorporation of comonomer, as well as with the frequency of stereoerrors in the polymers. Further analysis indicated that rmrr stereoerrors, a result of site epimerization, occur in close proximity to the incorporated comonomers. The MAO-activated fluorenyl/Oct-containing metallocene and CGC catalysts were further utilized to produce syndiotactic samples of poly(1-butene) (s-PB) and poly(1- pentene) (s-PPe). The syndiotacticity of the samples was quantified by 13C NMR and the melting temperatures determined by DSC. The samples of s-PB and s-PPe produced by Me2Si( h1-C29H36)( h1-N-tBu)ZrCl2·OEt2 (Oct-CGC) were found to melt at higher temperatures (55.9 and 43.1 °C, respectively) than any previously reported samples. The MAO-activated Oct-CGC was also used to produce polyethylene samples at a variety of polymerization temperatures and pressures. All of the samples were found to contain an unprecedented degree of branching (13-65 total branches per 1000 carbon atoms) for an early transition metal single-site catalyst. The branches were found to be almost exclusively of two or greater than five carbon atoms in length, and the levels of the longer branches could be controlled by varying the polymerization conditions. The number of ethyl branches was roughly 5 per 1000 carbon atoms for all samples. Finally, a binary catalyst system comprising the Oct-CGC and a chromium-based ethylene trimerization catalyst, ((tBuSCH2CH2)2NH)CrCl3, was developed. This MAOactivated catalyst system could be tuned to produce polyethylene samples with 17-49 total branches per 1000 carbon atoms. Between 4 and 16 of these branches were found to arise from incorporation of 1-hexene produced by the chromium oligomerization catalyst. Adjusting the ratios of oligomerization catalyst, polymerization catalyst, and activator was found to allow rational control over the branch content of the polymers. The branching levels could also be varied by altering the time between injection of the oligomerization and polymerization catalysts into the system.
constrained geometry catalyst
binary catalyst system
Schwerdtfeger, Eric Dean (2007). Single-site polymerization catalysts: branched polyethylene and syndiotactic poly(alpha-olefins). Doctoral dissertation, Texas A&M University. Available electronically from