A new approach to detailed Tref analysis of ethylene/α‐olefin copolymers prepared with multi‐center polymerization catalysts is developed. It is based on resolution of complex Tref curves into elemental components described with the Lorentz distribution function. This approach was applied to the study of a series of ethylene/1‐butene copolymers prepared with a supported Ti‐based catalyst. The analysis showed that the copolymers, which, on average, contain from 6.5 to 3.5 mol % of 1‐butene, consist of seven discrete components with different compositions, ranging from a completely amorphous material with a 1‐butene content of > 15–20 mol %, to two highly crystalline components with 1‐butene contents < 1 mol %. A comparison of these Tref results with the data on the molecular weight distribution of the copolymers (based on resolution of their GPC curves) shows that Tref and GPC data provide complimentary information on the properties of active centers in the catalysts in terms of the molecular weights of the material they produce and their ability to copolymerize α‐olefins with ethylene. Tref analysis of copolymers produced at different reaction times showed that the active centers responsible for the formation of various Tref components differ in the rates of their formation and in stability. The centers that produce copolymer molecules with a high 1‐butene content are formed rapidly but decay rapidly as well whereas the centers producing copolymer molecules with a low 1‐butene content are formed more slowly but are more stable. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4351–4362, 2005
The reaction products of dibutylmagnesium with modified silica gel surfaces have been studied as relevant olefin polymerization catalyst precursors. Reactions were carried out on thermally and/or chemically pretreated silica gel. The pretreatments effect changes in silica surface silanol type and concentration. A combination of methods including infrared spectroscopy, NMR spectroscopy, and elemental analysis were used to characterize the chemically modified surfaces. Dibutylmagnesium reacts with non-hydrogen-bonded silanols to give a bond to the surface with one surface silicon. One butyl group is retained on magnesium. Dibutylmagnesium reacts with two hydrogen-bonded silanols to give bonds to the surface with two surface silicons. Both butyl groups are lost in the reaction with non-hydrogen-bonded silanols. Dibutylmagnesium also reacts with surface siloxanes to give a singly bonded magnesium, analogous to the reaction with non-hydrogen-bonded silanols. The adjacent silicon in the siloxane is believed to react with the second butyl group to form a silicon−butyl surface species. A majority of surface-reacted magnesium on the thermally pretreated silica gels studied is a result of siloxane bond breaking.
The methylation (CH3I) of 1,3-bis(2-pyridyl)butyllithium in THF at -78°C is highly meso-selective (>98%) but the selectivity decreases with increasing cation-size or -coordination. The reaction of 1,3-bis(2-pyridyl)butyllithium with other electrophiles such as i-C3H7Br, PhCHZCl, Me3SiC1 (CD&CO and Cvinylpyridine is also stereoselective under these conditions giving meso-like products. On the other hand, addition of 2-vinylpyridine is only slightly selective (64%) and this is consistent with the 65% meso content of P2VP formed by polymerization in the presence of Li ion. The chemistry of the above reactions is rationalized by intramolecular coordination of Li or other cations by the penultimate 2-pyridyl group and this is supported by equilibria involving proton abstraction of 1,3-bis(2-pyridyl)butane and similar compounds by bases having Li, Na or K counterions. It is shown that the tendency for intramolecular chelation is highest for Li and lowest for K ion. Temperature dependence of the above equilibrium shows that intramolecular chelation of Li and Na ions is exothermic (-1.4 and -1.3 kcal respectively) whereas the AH for K ion is very small (-0.5 kcal). Entropies of chelation are slightly positive for Li (0.8 e.u.) and negative for Na and K ions (-2.60 and -0.40 e.u. respectively). The lack of stereoregular polymerization of 2-VP in the presence of Li ion is most likely due to the requirement 0 1994 Hiithig & Wepf Verlag, Zug CCC 1022-1360/95/$04.00 36 that the Li ion of the newly formed 2-pyridyl anion is coordinated with the 2-pyridyl group of the previous asymmetric center. Thus it would appear that intramolecular coordination of metal ion by penultimate 2-pyridine does not necessarily lead to isotacticpolymerization.
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