Active in polymerizations

The following stage of the propagation center formation occurs through the reduction of Cr(VI) to the lower oxidation state. The compounds of Cr(II) seem to be active in polymerization in the solution of bis-triphenyl-silyl-chromate (109). For the formation of these compounds the following scheme taking into account the results (110) concerning the study of the reaction of bis-triphenylsilyl-chromate with olefins was considered (109) ... 

Recently samples of TiCl2j active in polymerization without additional activation, were prepared (156-159). The activity of TiClj in ethylene polymerization was practically the same as the activity of a conventional two-component system TiClg + AlEtjCl (see Fig. 3). The polymerization activity of TiClj depends to a large extent on the parameters tempera-... 

Measurements of polymerization rate and parallel measurements on the resultant polymer microstructure in the butadiene/DIPIP system cannot be reconciled with the supposition that only one of the above diamine solvated complexes (eg. Pi S) is active in polymerization 162). This is probably true of other diene polymerizations and other diamines. The observations suggest a more complex system than described above for styrene polymerization in presence of TMEDA, This result is clearly connected with the increased association number of uncomplexed diene living ends which permits a greater variety of complexes to be formed. 

The quantity Q therefore is the sum of two quantities, the number of polymer chains attached to metal atoms which are active in polymerization (Si CDn]) and which have the structure (IX), and species with structure (XXV) which are inactive (Xa [Z ]). In addition there are those chains with structure (IX) with monomer coordinated to the metal (52 [ E ]) these are considered to be negligibly small compared to The... 

Since the catalysts which are active in polymerization are also... 

For most of the systems reported in the literature, C/K is not known—very often, neither K nor C is known. For two-component initiator-coinitiator systems, C is usually taken to he the initiator concentration [YZ] when the coinitiator is in excess or the coinitiator concentration [I] when the initiator is in excess. C may be lower than [YZ] or [I] due to association that is, only a fraction of [YZ] or [I] may be active in polymerization. This may also he the case for one-component initiators such as triflic acid. It would be prudent to determine the actual value of C in any polymerization system—usually a difficult task and seldom achieved. Experimental difficulties have also limited our knowledge of K values, which are obtained most directly from conductivity measurements or, indirectly, from kinetic data. A comparison of polymerization in the absence and presence of a common ion salt (e.g., tetra-n-butylammonium triflate for the triflic acid initiated polymerization) is useful for ascertaining whether significant amounts of free ions are present in a reaction system. 

In addition to these supported transition-metal catalysts, some soluble transition-metal compounds exhibit considerable activity in polymerization without added aluminum alkyls.292 The most active compounds are a-organometallics of Ti and Zr with methyl, benzyl, and halogen ligands. jt-Allyl compounds of Ti, Zr, and Cr are also useful catalysts. 

Among them is the gel point conversion, if multifunctional units are present, as well as accompanying divergence of viscosity, onset of equilibrium elasticity modulus, etc. By comparing the results of modeling with experiment, one can verify to what extent the chemistry is affected by physical interactions which are practically always active in polymerizations. 

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene > mesitylene > toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

Various papers address the aspect that only a small fraction of the total amount of neodymium is active in polymerization. All figures published on the percentage of active Nd are below 10%. Monakov et al. determine an efficiency factor of 7% [93,94]. According to Pan et al. the efficiency factor is between 0.4 to 8% of the total amount of Nd present [595-598]. Sun et al. give the percentage of active neodymium in the range of 0.4 to 0.5% [599,600]. Yu... 

Ziegler-Natta-type catalysts, which are active in polymerization and oligomerization of alkenes. are also influenced by adding CO2 to the reaction mixture. The addition of CO2 changes the molecular we t and crystallinity of the products or the activity and selectivity of the catalyst, both in polyethylene [307,308] and in polypropylene production [309-312]. 

One main difference between anionic polymerization and GTP has to be found in the amount of enolates active in polymerization. In anionic polymerization, all the chains are end-capped by an enolate, which is the case for only a small part of the chains in GTP consistent with the very good control of GTP even at room temperature. In this respect, Brittain and Dicker showed that prop/ term is by far higher in GTP (250) than in classical anionic polymerization ( prop/ term = 8) . In line with slow termination compared to propagation in GTP, Bandermann and coworkers found that the amount of the nucleophilic catalyst is essential to the polymerization control. Indeed, as far as the tris(piperidino)sulfonium bifluoride-mediated GTP of MMA in THF is concerned, the polydispersity index increases with the amount of catalyst . 

Fig. 10. Effects of milling on the activity in polymerization (kg polyethylene per g of catalyst) of (MgClj + TiCl4) mixtures at different Ti contents. ( ) 2.0% Ti (O) 3.4% Ti (A) 4.2% Ti 35>. By permission of Pergamon Press Ltd.

In the case of isoprene, low molecular weight model compounds (dimer, trimer, etc.) can be prepared in benzene to produce oligomeric analogues of the polyisoprenyllithium active in polymerization [195]. The NMR spectra of such oligomers show that cis and trans forms of the lithium bearing terminal unit occur, and that one predominates at room temperature [195, 196]. It is probably the cis form, although this is difficult to establish without doubt. Except for the one unit chain, the cis—trans ratio varies reversibly with temperature. Transfer to tetrahydro-furan-rich mixtures at low temperatures shows that isomerization occurs when the solution is warmed to —40°C, probably to the trans form. This is the stable form in such solvents at all temperatures. The NMR spectra are basically the same in both hydrocarbon and ether solvents. Only the resonance due to the proton on the y-carbon is shifted upfield in polar... 

As discussed on p. 143 there are a number of species in this catalyst dependent on the temperature of reaction and molar ratios of the reactants, and only a small proportion of the titanium atoms are active in polymerization. A characteristic ESR signal appears at = 1.983 in the presence of butadiene which has been identified with the growing chains [124], and Table 17a shows its small proportion. An analogous species ( = 1.978) is obtained from the soluble Ti(acac)3/AlEt3 system but at much higher Al/Ti ratios [54]. 

Fourth, the complexes of acetylenes and diolefins react chiefly in polymerization, this tendency being most marked with the complexes of nickel and palladium. Platinum complexes are not generally active in polymerization. In the catalytic hydropolymerization of acetylene, nickel displays the greatest activity, palladium takes second place and platinum third place. The remaining noble metals are less active than platinum. Thus, again, a correlation between the two fields is observed to hold. 

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

They consider also a more general situation in which both singly and doubly coordinated complexes can exist, and both of them can be active in polymerization. The more complex rate law is... 

Vinyl polymerization using metallocomplexes commonly proceeds by a radical pathway and rarely involves an ionic mechanism. For instance, metal chelates in combination with promoters (usually halogenated hydrocarbons) are known as initiators of homo- and copolymerization of vinylacetate. Similar polymer-bound systems are also known [3]. The polymerization mechanism is not well understood, but it is believed to be not exclusively radical or cationic (as polymerization proceeds in water). The macrochelate of Cu with a polymeric ether of acetoacetic acid effectively catalyzes acrylonitrile polymerization. Meanwhile, this monomer is used as an indicator for the radical mechanism of polymerization. Mixed-ligand manganese complexes bound to carboxylated (co)polymers have been used for emulsion polymerization of a series of vinyl monomers. Macromolecular complexes of Cu(N03)2 and Fe(N03)3 with diaminocellulose in combination with CCI4 are active in polymerization of MMA, etc. 

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