Effect of Substituent on Titanocene/MMAO Catalyst for Ethylene/1-Hexene Copolymerization
Copolymerization of ethylene with 1-hexene was carried out using two ansa-fluorenyl titanium derivative complexes. The substituent effect on the catalytic activity, monomer reactivity ratio and polymer property was investigated. It was found that the presence of t-Bu groups on fluorenyl ring exhibited remarkable catalytic activity and produced polymer with high molecular weight. However, these catalysts produce polymer with narrow molecular weight distribution, indicating the characteristic of single-site metallocene catalyst. Based on C NMR, we can observe that monomer reactivity ratio was affected by catalyst structure. The rH values of complex 2 were lower than that of complex 1 which might be result from the higher steric hindrance leading to a reduction of 1hexene insertion step. Keywords—Constrained geometry catalyst, linear low density polyethylene, copolymerization, reactivity ratio
Octamethyloctahydrodibenzofluorenyl: Electronic Comparisons between a Sterically Expanded Ligand and Its Cyclopentadienyl Analogues
Incorporating the octamethyloctahydrodibenzofluorenyl (Oct) ligand into metallocene and constrained geometry olefin polymerization catalysts has profound catalytic consequences. The steric influences are undoubtedly important, but it is shown herein that electronics likely also play a crucial role. The electron richness of the Oct− anion was directly measured by competitive deprotonation experiments, which reveal that the pKa of OctH is 3.9 units higher than that of fluorene. The HOMO−LUMO gap decreases by about 0.6 kcal/mol for each additional tertiary alkyl group appended to the metallocene R2C(C5H4)(C13H8)ZrCl2 (R = Me or Ph) in the 2, 3, 6, and 7 positions of the fluorenyl moiety, indicating the ability of these groups to increase the HOMO energy by electron donation. The carbonyl stretching frequencies for η5-OctMn(CO)3 (2009, 1924 cm−1) demonstrated that the Oct ligand is the most electron donating in the series of CpMn(CO)3, Cp*Mn(CO)3, and (C13H9)Mn(CO)3. DFT calculations universally corroborate t...
Cytoplsmic uridine phosphorylase of rat liver. Characterization and kinetics.
Abstract Uridine phosphorylase (uridine:orthophosphate ribosyltransferase, EC 2.4.2.3) of rat liver cytoplasm has been purified 1900-fold. Initial velocity patterns of the phosphorolysis of uridine or deoxyuridine and of the synthesis of uridine indicated a sequential mechanism. Product inhibition patterns with uracil or ribose 1-phosphate as inhibitors were consistent with an "ordered Bi Bi" reaction in which inorganic phosphate is the first substrate to bind to the enzyme and ribose 1-phosphate the last product to leave the enzyme. The most highly purified enzyme fractions still retained uridine-, deoxyuridine-, and thymidine-cleaving activities in the ratio of 10:7:1 at pH 7.4. The fractions separated into a major and a minor band of protein on electrophoresis in phosphate buffer. Aging of the enzyme with loss of activity resulted in an increase in the proportion of the minor band. The pH optimum of the enzyme for uridine cleavage was 8.2, for deoxyuridine cleavage, 6.5, and for uridine synthesis, 8.5. Uridine or deoxyuridine synthesis was inhibited 50% in the presence of 1 mm uracil. The enzyme did not catalyze the direct transfer of ribose from uridine to uracil in the absence of phosphate. Uridine protected the enzyme to a greater degree than phosphate against inhibition by o-iodosobenzoate. Thus, at least one sulfhydryl group and possibly three are present at or near the active site. Uridine cleavage, uridine synthesis, and deoxyuridine cleavage were inhibited by deoxyglucosylthymine. The phosphorolysis of deoxyuridine and thymidine by thymidine phosphorylase, also purified from rat liver cytoplasm, was not inhibited by deoxyglucosylthymine. The molecular weights of both uridine and thymidine phosphorylases were estimated to be 110,000.