Syndiospecific Propylene Polymerization with C₁ Symmetric Group 4 ansa-Metallocene Catalysts

Abstract

C1 symmetric group 4 ansa-metallocenes [Me2C(Cp)(3-CH2X-2-R-Ind)MCl2 (M ) Zr, R ) H, X ) Me (3); M ) Zr, R ) H, X ) SiMe3 (4); M ) Zr, R ) Me, X ) SiMe3 (5); M ) Hf, R ) H, X ) SiMe3 (6); M ) Hf, R ) Me, X ) SiMe3 (7); Cp ) cyclopentadienyl, Ind ) indenyl), upon activation with methylaluminoxane (MAO), catalyze the polymerization of propylene to predominantly syndiotactic polypropylene. The preparations of complexes 3-7 and the molecular structures of 3 and 4 are reported. The polypropylenes produced have [rrrr] pentad contents ranging from 43 to 75% depending on the employed transition metal, ligand substitution pattern, and polymerization conditions. The 2-Mesubstituted catalysts 5/MAO and 7/MAO exhibit higher activities and higher stereospecificities than their unsubstituted analogues 4/MAO and 6/MAO. Stereoselectivities of the zirconium catalysts 3-5/MAO are sensitive to the monomer concentration, showing decreasing syndiotacticities with decreasing propylene pressure due to increasing amount of skipped insertions. The dependence of the stereospecificity and polymerization activity on the ligand structure suggests that the conformation of the 3-(trimethylsilyl)methyl substituent can be perturbed by introduction of a 2-substituent, thus providing a tool to control the stereoerror formation. † This paper is dedicated to the memory of Professor Jan Na ¨sman.

Modification of epoxy resin with kaolin as a toughening agent

“…The 2,2′‐substitution, however, influences transfer reactions of polypropene chains 13. First insight into correlations of catalyst structure with chain‐end isomerization and termination processes in C 2 ‐, C 3 ‐homopolymerizations were recently performed by Seppälä et al14 Waymouth and coworkers reported about a correlation of the 2‐methyl substitution with the probability of chain back skip and the syndiotacticity of the resulting polypropene 15. In general three dominating mechanisms – β ‐H abstraction to metal ( A ), β ‐H transfer to monomer ( B ), and chain transfer to aluminium ( C ) – are known (Figure 5).…”

Sequence and Stereoselectivity of the C1-Symmetric Metallocene Me2Si(1-(4,7-Me2Ind))(9-Flu)ZrCl2

“…C 1 -symmetric catalysts exhibit a rich stereochemistry for R-olefin polymerization and have been observed to produce polypropylenes with a variety of microstructures: isotactic, hemi-isotactic, atactic, or syndiotactic. 1,3,[6][7][8][9][10][11][12][13][14][15][16][17][18][19] The stereoselectivity of this family of metallocenes is proposed to depend on the stereospecificity of the two heterotopic sites and the probability and sequence of olefin insertion events at the two heterotopic sites. 20 Metallocenes with heterotopic coordination sites might be expected to lead to unusual copolymer microstructures if the kinetic selectivity for the two olefins is different at the two coordination sites.…”

Mesoscopic modeling of the polymerization, morphology, and crystallization of stereoblock and stereoregular polypropylenes

“…For a long time, there has been great interest in the use of novel catalysts to obtain polypropylenes (PPs) of unusual and commercially important properties. Some studies have focused on synthetic aspects,1–17 others have examined the properties of the polymers,18–31 and still others have concentrated on relevant theory and simulations 32–49. There have also been a number of very useful reviews of some of these areas 50–59…”