Coordination polymerization single-site catalysts

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

Supported Single-Site Catalysts Catalysts supporting is considered a prerequisite for the application of most coordination catalytic systems. The most important polymerization methods (slurry and gas phase) require the use of supported or heterogenized catalysts. Particle morphology and bulk density are the main physical features of a supported catalytic system, determining its application in commercial processes [49]. 

The term single-site catalyst has a very precise meaning in coordination polymerization. From the point of view of catalyst stmcture, single-site catalysts are those where all active sites are represented by the same cdiemical species and have the same polymerization kinetic parameters. In other words, single-site catalysts can be represented with a single set of the elementary reactions described by Eqs. (1)-(14) or any other equivalent set of polymerization meciianism equations. From a polymer microstmclure point of view, single-site catalysts will produce linear... 

The graphical representation of Eq. (24) for a coordination catalyst having three distinct site types is shown in Figure 8.24. It is important to remember that the use of Eq. (24) is a direct consequence of assuming that the mechanism of polymerization for multiple-site catalysts is described with the same set of equations, Eqs. (1)-(14), used to describe single-site catalysts. In other words, Flory s distribution is the logical consequence of the mechanism adopted for coordination polymerization. 

First, from about the beginning of the early 1990s, weU-charactetized metal complexes have been shown to be highly effective as homogeneous catalysts. Such complexes, often referred to as single-site catalysts, can polymerize a wide variety of alkenes to give polymers of unique properties. Second, the molecular mechanism of polymerization can be fully explained in terms of the organometallic chemistry of metal-alkyl and metal-alkene complexes. Indeed, because of this, metal-catalyzed polymerization is often called coordination or insertion polymerization. 

Lanthanide-based catalysts, despite finding a lot of application in homogeneous catalysis, can be rather problematic due to the lability of some ligand types and the versatility of their coordination chemistry in the -1-3 oxidation state this makes the controlled synthesis of single-site Ln complexes a quite ambitious goal [92]. McLain and coworkers first demonstrated the high potential of a homoleptic yttrium complex Y(OCH2CH2NMe2)3 as ROP catalyst for the preparation of PLA from rac-lactide and that it promotes a rapid and controlled polymerization... 

Coordination Polymerization of 1,3-Dienes Single-Site (or Metallocene) Catalysts Living Radical Polymerizations Other Types of Polymerizations, Polymers Ring-Opening Polymerization... 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

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