Metallocene polymerization catalysis

It is the purpose of this section to review ways in which processes involving electrons are either explicitly accounted for in calculations on polymeric systems or in which a more or less rigorous abstraction from the electronic degrees of freedom into effective models of a coarser-grained nature is performed. The next level up from electrons is obviously atoms. Hence, this section deals mainly with the connection between quantum chemistry and atomistic (force field) simulations. Calculations which exclusively use quantum chemistry are not covered. This excludes, for example, all of the recent work on metallocene catalysis. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Table 4 summarizes the reaction conditions and the experimental results. Overall, the experimental results clearly show a new class of PE graft copolymers which can be conveniently prepared by the tranformation of metallocene catalysis to anionic graft-from polymerization. 

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

A single step of the polymerization is analogous to a diastereoselective synthesis. Thus, to achieve a certain level of chemical stereocontrol, chirality of the catalytically active species is necessary. In metallocene catalysis, chirality may be associated with the transition metal, the ligand, or the growing polymer chain (e.g., the terminal monomer unit). Therefore, two basic mechanisms of stereocontrol are possible (145,146) (i) catalytic site control (also referred to as enantiomorphic site control), which is associated with the chirality at the transition metal or the ligand and (ii) chain-end control, which is caused by the chirality of the last inserted monomer unit. These two mechanisms cause the formation of microstructures that may be described by different statistics in catalytic site control, errors are corrected by the (nature (chirality) of the catalytic site (Bernoullian statistics), but chain-end controlled propagation is not capable of correcting the subsequently inserted monomers after a monomer has been incorrectly inserted (Markovian statistics). 

Keywords Metathesis Polymerizations Metallocenes Homogeneous catalysis Heterogeneous catalysis... 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Most recently, we found that thiols cein be co-polytnerized with 1 in an erisy manner (Scheme 4), unless homopolymerization via radical or ionic initiation was not successful. Attempts to polymerize 1 by metallocene catalysis have not afforded any polymers yet. SH-Ene reaction under mild conditions does not affect the lactone structure. 

Ewen, J. A. Jones, R. L. Elder, M. J. Expanding the Scope of Metallocene Catalysis Beyond Indenyl and Fluorenyl Derivatives. In Metalorganic Catalysts for Synthesis and Polymerization Kaminsky, W., Ed. Springer Berlin, 1999, p 150. 

Most addition polymerizations involve vinyl or diene monomers. The opening of a double bond can be catalyzed in several ways. Free-radical polymerization is the most common method for styrenic monomers, whereas coordination metal catalysis (Zigler-Natta and metallocene catalysis) is important for olefin polymerizations. The specitic reaction mechanism may generate some catalyst residues, but there are no true coproducts. There are no stoichiometry requirements, and equilibrium limitations are usually unimportant so that quite long chains are formed 7iv > 500 is typical of addition polymers. 

Yang, K., Huang, Y, and Dong, J.-Y. 2007. Efficient preparation of isotactic polypropylene/ montmorillonite nanocomposites by in situ polymerization technique via a combined use of functional surfactant and metallocene catalysis. Polymer 48 6254-6261. 

S.D. Ittel, L.K. Johnson, M. Brookhart, Late-metal catalysts for ethylene homo- and copolymerization. Chem. Rev. 100, 1169-1203 (2000) K.P. Bryliakov, Post-metallocene catalysis for olefin polymerization, Russian Chem. Rev. 76(3), 253-277 (2007)... 

ADAPTATION OF METALLOCENE CATALYSIS TO EXISTING OLEFIN POLYMERIZATION PROCESSES... 

LRP is a powerful tool for the synthesis of complex polymer architectures as was shown above. However, in some cases it is desirable to combine structures that are hardly or not at all accessible via radical polymerization techniques. In such cases it may be beneficial to combine LRP with another polymerization mechanism. Many examples have been reported so far. A few examples will be listed here. Polystyrene-6-pol3risobutylene-6-polystyrene was synthesized via a combination of living cationic polymerization and ATRP (98). Polyolefin Graft Copolymers (qv) were synthesized by first polymerizing alkoxyamine-substituted olefins via metallocene catalysis, and subsequent polymerization of vinyl monomers via... 

In syndiotactic or syntactic macromolecules, the substituents have alternate positions along the chain (Figure 20.1). The macromolecule consists of 100% racemo diads. Syndiotactic polystyrene (sPS), made by metallocene catalysis polymerization, is crystalline with a melting temperature of 161°C. 

Figure 14.2 Upper. Illustration of metallocene catalyst structure, which must be specific for each stereo-specific polymer structure. Lower. Microstructures of polypropylene most can be made via metallocene catalysis polymerization.

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