Polymer, branched Catalysts

The branched polymers produced by the Ni(II) and Pd(II) a-diimine catalysts shown in Fig. 3 set them apart from the common early transition metal systems. The Pd catalysts, for example, are able to afford hyperbranched polymer from a feedstock of pure ethylene, a monomer which, on its own, offers no predisposition toward branch formation. Polymer branches result from metal migration along the chain due to the facile nature of late metals to perform [3-hydride elimination and reinsertion reactions. This process is similar to the early mechanism proposed by Fink briefly mentioned above [18], and is discussed in more detail below. The chain walking mechanism obviously has dramatic effects on the microstructure, or topology, of the polymer. Since P-hydride elimination is less favored in the Ni(II) catalysts compared to the Pd(II) catalysts, the former system affords polymer with a low to moderate density of short-chain branches, mostly methyl groups. 

Ionkin has reported a similar series of Ni(II) catalysts 1.50a and b bearing ortho-difuryl substituents that are noteworthy for their high thermal stability [127], The bulkier benzofuranyl-substituted catalyst 1.50b possesses the most attractive catalytic properties (Table 5, entry 9) the ability to form high molecular weight polymers (albeit in high polydispersity) and reasonable activity even at 150 °C. Even under these harsh conditions, the polymer branching density is still relatively low. 

Three processes are used commercially to make linear polyethylene-solution, slurry, and gas phase. All are called low-pressure processes (< 50 atm) to distinguish them from the free radical or high-pressure process that makes highly branched polyethylene. In the solution mode a hydrocarbon solvent at 125-170°C dissolves the polymer as it forms. The reaction usually slows as the solution becomes viscous because it becomes difficult to stir ethylene into the liquid phase. In contrast, The slurry process uses a poor solvent and low temperature (60-110°C) to prevent dissolving or even swelling of the polymer. Each catalyst particle creates a polymer particle several thousand times larger than itself. There is no viscosity limitation in the slurry method the diluent serves to transfer heat and to keep the catalyst in contact with ethylene and other reactants. Finally, the gas-phase process is much like the slurry method in that polymer particles are formed at similar temperatures. A bed of catalyst/polymer is fluidized by circulating ethylene, which also serves as a coolant. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

Since the cell wall structure of the wood is not swollen by the vinyl monomer, there is little opportunity for the monomer to reach the free radical sites generated by the gamma radiation on the cellulose to form a vinyl polymer branch. From this short discussion, it is reasonable to conjecture that there should be little if any difference in the physical properties of catalyst-heat initiated or gamma radiation initiated in situ polymerization of vinyl monomers in wood. 

The dependence of MMD and PDI on polymerization temperature is confirmed by studies reported by Oehme et al. (Nd(OCOR)3/TIBA/EASC) [169] and Pires et al. (NdV/DIBAH/ BuCl) [455]. Oehme et al. obtain a unimodal MMD at - 20 °C whereas at 80 °C two peaks are present. This is evidence for the presence of two active catalyst sites. According to Oehme et al. the active center which produces the low-molar-mass peak is more sensitive to increased temperatures than the second species. It is also possible that the latter species does not exist at low polymerization temperature. Pires et al. also explain their results on the basis of two different active catalyst sites. According to Pires et al. the less stable catalyst site which produces the low molar mass polymer gradually deteriorates in the temperature range 50-80 °C. At polymerization temperatures > 80 °C, however, an increase of PDI as well as of molar mass is observed. This effect is explained by polymer branching. 

The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and ECC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline. 

The soluble polymer-supported catalysts have also been used for asymmetrically catalyzed reactions Following a procedure for the preparation of insoluble polymeric chiral catalysts a soluble linear polystyrene-supported chiral rhodium catalyst has been prepared. This catalyst displays high enantiomeric selectivity compared to the low molecular weight catalyst. Thus, hydroformylation of styrene using this catalyst produces aldehydes in high yields. The branched chiral hy drotropaldehy de is formed in 95% selectivity. 

To show the extreme difference in behavior, the two catalysts activated at the low temperature are compared, that is, Cr/silica activated at 600 °C versus Cr/silica-titania activated at 500 °C. Notice in the left graph that the Cr/silica profile contains an "island" at low MW and very high branch frequency. This island contains almost half of all the branches in the polymer. In other words, the island represents polymer with short chains with many branches, which tends to diminish many polymer properties. In contrast, the island disappears in the right graph representing Cr/silica-titania activated at 500 °C, which indicates the lack of this harmful component. This is a graphical depiction of the effect titania has on polymer branching. 

The hydrosilanes are so effective that it is often not necessary to treat the catalyst in CO to achieve a major lowering of the polymer density. Cr(VI)/silica catalyst used with hydrosilane is sometimes quite adequate to achieve a respectable increase in polymer branching. For example in one experiment a Cr(VI)/silica catalyst, activated at 600 °C, was exposed... 

Styryl-terminated Frechet-type dendrimers have been introduced as novel polymer crosslinkers by Seebach et al. [43-45]. They are constituted of four to 16 peripheral styryl units attached to aryl end branches of dendritic TADDOL, BINOL or Salen ligands and were copolymerised with styrene by suspension polymerisation. The catalytic performance of the polymer-bound catalyst was identical to that of the homogeneous analogues however, the supported catalysts could be used in many consecutive catalytic runs with only small loss in catalytic activity. A major drawback of fixing the catalytic unit in the core of the crosslinker is the poor loading capacity of the final polymer (0.13-0.20 mmol g 0> especially when high amounts of catalysts (10-20 mol%) are needed. 

Methylmethacrylate has also been used as a reactant for hydroformy-lations with polymer-anchored rhodium catalysts. Low efficiencies were obtained at high temperatures because of competing polymerization and the branched product predominated. The polymer-supported catalysts gave lower activities and a higher proportion of branched product than the homogeneous analogue at comparable phosphine Rh ratios. 

An ideal dendrimer structure without defects features only branched units and end-groups, in addition to the core (Figure 1). The molecules are monodisperse, i.e., they all possess the same molecular weight While this perfect structure is not required for catalysis and recovery in principle, it enables the precise characterization of polymer-boimd catalyst precursors. M ALDl-TOF is a particularly useful technique. 

This is a reaction for which polymer-supported catalysts have been used extensively. The reaction sequence (shown in Scheme 14-6) involves the addition of an aldehyde group to the terminal or the internal carbon atom of an alkene. The ratio of the two aldehydes formed is dependent on the catalyst used, e.g., in the case of the two homogeneous catalysts, Rh(acac)(CO)2 and Rh(acac)(CO)PPhs, the ratios of normal to branched chain aldehydes were found to be 1.2 1 and 2.9 1, respectively (Allum et... 

Methyl vinyl ether yields a crystalline polymer only when methylene chloride is present as a solvent. However, ethyl, isopropyl, and /7-butyl vinyl ethers do not yield crystalline polymers. Branched alkyl vinyl ethers, other than isobutyl vinyl ether, and benzyl ether also yield crystalline polymers [20]. The crystallinity of the polymers (isotactic) is similar in soluble and insoluble catalyst systems [21]. 

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