Catalyzation, metallocene

In a recent study, Yu et al. combined the palladium-diimine-catalyzed metallocene polymerization and ATRP to synthesize polyethylene-h-polystyrene and polyethylene-h-poly(butyl acrylate) [165]. A relatively new coordination olefin polymerization method - degenerative transfer coordination polymerization - was recently combined with ATRP to prepare block and graft copolymers with linear polyethylene segments [166-169]. 

M. Ohgizawa, M. Takahashi and N. Kashiwa, Metallocene catalyzed polyethylene with unique rheological property , MetCon 95 Proceedings, USA, May 1995. 

B. C. Childress, Properties of homogeneous and heterogeneous polyolefins Metallocene catalyzed versus Ziegler-Natta catalyzed resins, MetCon 94 Proceedings, USA, May 1994. 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

Metallocenes Mg(II)-catalyzed Microwave irradiation MO calculation Montmorillonite Moraceous plants Morphinadienes Mulberry tree Multifunctional catalysis Multi-step... 

Replacement of EPM copolymers and EPDM ter-polymers by saturated metallocene-catalyzed EO copolymer... 

In addition to the two-phase TPEs, two new technologies have emerged. They are the metallocene-catalyzed polyolefin plastomers (POPs, the name given to Exxon s EXACT product line) and polyolefin elastomers (POEs, DuPont Dow Elastomer s ENGAGE), and reactor-made thermoplastic polyolefin elastomers (R-TPOs). These new types of TPEs are often called metallocene elastomers-TPEs (MEs-TPEs) [87]. The new POPs and POEs are essentially very low-molecular-weight-Unear low-density PEs (VLMW-LLDPE). These new-generation TPEs exhibit mbber-like properties and can be processed on... 

Kotora M (2005) Metallocene-Catalyzed Selective Reactions. 8 57-137 Kumobayashi H, see Sumi K (2004) 6 63-96... 

Sumi K, Kumobayashi H (2004) Rhodium/Ruthenium Applications. 6 63-96 Suzuki N (2005) Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes. 8 177-215... 

Development of a reduced-order model for metallocene-catalyzed ethylene-norbornene copolymerization reaction... 

Like many homogeneously catalyzed reactions, the overall cycle (or cycles) in these polymerization reactions probably contains too many steps to be easily analyzed by any single approach. Both kinetics and model compound studies have thrown light on some of the steps. However, as indicated above, many of the model compounds isolated from the reactions of primary silanes with metallocene alkyls and hydrides are too unreactive to explain the polymerization results. 

We encounter homogeneous catalysts in both step-growth and chain-growth polymerization processes. We saw several examples of these types of reactions in Chapter 2. For example, the acid catalyzed polymerization of polyesters occurs via a homogeneous process as do some metallocene catalyzed polymerization of polyolefins. 

In practice, product developers often blend two or more resins together in order to obtain a product that has the required melt flow and solid-state characteristics. Thus, we frequently combine metallocene catalyzed linear low density polyethylene, having a most probable molecular weight distribution, with low density polyethylene, having a broad molecular weight distribution. The linear low density polyethylene provides good impact resistance, while the low density polyethylene improves melt flow characteristics. 

A series of novel, bridged Zr(IV)-boratabenzene complexes have been synthesized and structurally characterized (Scheme 28).48 The geometries of these adducts closely resemble those of classical ansa-metallocenes. In the presence of excess MAO, these boratabenzene complexes catalyze the polymerization of olefins. 

Figure 32 The dimeric organoborane chain-transfer agent, 9-BBN dimer (49), used for the in situ incorporation of B-containing group in polyolefins during the termination of a metallocene-catalyzed polymerization. (Adapted from ref. 71.)...

Figure 35 A noncoordinating carbosilane dendrimeric polyanion terminated with BR3 groups (52) used as a cocatalyst in the metallocene-catalyzed olefin polymerization. (Adapted from ref. 80.)...

Figure 36 Organoboron polymers of PS with well-defined boron-containing Lewis acids for use as a cocatalyst in metallocene-catalyzed olefin polymerizations. (Adapted from ref. 81.)... 

Figure 6 Loss tangent (<5) plotted as a function of complex modulus G for a series of syndiotactic and isotactic polypropylene. Reproduced with permission from Rojo et al. [32]. Copyright Wiley-VCH Verlag GmbH Co. KgaA. iPP, isotactic PP sPP, syndiotactic PP Met-iPP, metallocene-catalyzed PP (see Ref. [32] for full details).

More recently, it was the seminal work of Aitken, Harrod and Samuel which showed that Group IV metallocene compounds catalyzed the dehydrogenative coupling of primary silanes, RSiH3, to give polysilanes of relatively low molecular weight, [RSiH] [8], that attracted many others into the silicon/transition metal chemistry field [9],... 

By contrast, the synthesis of syndiotactic PP, s-PP, is generally catalyzed by Cs-symmetry ansa- metallocenes. For example, (16)/MAO affords PP with a pentad (rrrr) content of 86% at 25 °C.77 The stereoselectivity is highly sensitive to ligand variation. For example, substitution at the 3-position of the Cp ring with a methyl group affords heterotactic PP,78 whilst the Bu analog favors i-PP production.50,75,79... 

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