Polymerization of olefins and functionalized monomers with zirconocene catalysts
The subject of this dissertation is the application of zirconocene catalysis to the polymerization of nonfunctional 1-olefins and functional olefins, such as methacrylates. The polymerization of 1-hexene with zirconocene/MAO catalyst systems is studied in order to elucidate the nature of the active species on the basis of kinetic experiments and, more particularly, by using molecular weight determination as a sensitive probe. It is concluded that the general classification of such catalyst systems as single-site catalysts is an oversimplification. In zirconocene/MAO systems, more than one active species is present under the applied reaction conditions, which may cause bimodal MWD of the obtained polymers. It is concluded that if the active species are in an equilibrium at any point of time, then it must be slow on the time scale of chain growth. From the experimental data some thermodynamic and kinetic parameters of the active species are determined. The observed deviation from single-site behaviour may be part of a major problem in olefin polymerization, i.e. the high excess of cocatalyst necessary in order to generate a catalytically active system. The addition of polar cosolvents to the reaction mixtures is shown to have a deleterious effect with respect to polymerization. The experimental data is interpreted in terms of the polar cosolvent and the olefin monomer competing for the vacant coordination site of the polymerization catalyst. The results indicate that copolymerization of olefins and functionalized monomers by means of zirconocene/MAO catalyzed olefin polymerization is unlikely to be successful. The application of chiral zirconocene catalysts to the polymerization of MMA is shown to be a promising tool for the rational control of polymer microstructure. Thus, with polymerization of MMA with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh4- yielding highly isotactic PMMA and polymerization of MMA with Me2C(Cp)2Zr(Me)(thf)+BPh4- yielding syndiotactic PMMA, the first example of a tight correlation between catalyst symmetry and polymer microstructure in MMA polymerization is presented. On the basis of kinetic experiments, a polymerization mechanism is proposed which is distinct from the known zirconocene catalyzed group transfer polymerization of MMA and may offer a perspective on the sequential copolymerization of 1-olefins and methacrylates by means of zirconocene catalysis. Finally, all results presented in this thesis lead to the conclusion that a (block) copolymerization of nonfunctional 1-olefins and of functional monomers may be possible, but only if the order of reactivities towards the electrophilic catalyst is respected. This means that the olefin must first be polymerized with a suitable catalyst system, following which, an active site transformation may allow for the polymerization of the functional monomer.
info:eu-repo/classification/ddc/540, Chemie, Hexen 1->, Polymerisation, Katalysator, Zirkonocenderivate, Methylaluminoxane, Methacrylsäuremethylester, Chirale Verbindungen
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