Polymers metallocene catalysis

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Kaminsky W (1996) New polymers by metallocene catalysis. Macromol Chem Phys 197 3907... 

Kaminsky, W., New Polymers by Metallocene Catalysis , Macromol. Chem. Phys., 197, 3907-3945 (1996). 

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). 

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. 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

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. 

Chung, T. C., Lu, H. L., and Jankivul, W. 1997. AnovelsynthesisofPP-b-PMMA copolymers via metallocene catalysis and borane chemistry. Polymer 38 1495-1502. 

Beginning of the post-metallocene catalysis - the new ones often have imine ligand, but not Cp. They are characterized by high activity, a possibility of adjusting polymer MW and MWD as well as copolymerization of olefins with polar monomeric and macromeric species. A wide spectrum of catalysts based on fight and heavy transition metals have been published in open literature and patents. There are also several types of catalysts with intermediate stmcture - pseudo metallocene with a post-metallocene imine ligand. Unfortunately, the product shows poor thermal stability and is not ready for commercialization. 

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 the infancy of metallocene catalysis, many companies spent considerable research dollars on metallocene preparations and understanding the structure of the metallocene in relation to the polymer properties. In addition to these research dollars, large expenditures were also made to establish strong patent positions. In an effort to recoup these expenses for this budding technology, to be competitive, and to fill voids in their technology portfolios, many have established cooperative alliances or have consolidated. 

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