Chain stationary insertion

Chain stationary insertion, 16 110 Chain stiffness, of fiber polymers, 11 175 Chain-stopped alkyds, 2 152 Chain structure of PVDC, 25 699... 

Pathways (a)-(b)-(c)-(d) and (a )—(b )—(c )—(d ) correspond to the original mechanism proposed by Cossee [268,276,277] and are still valid, apart from some minor modifications [1], for heterogeneous catalysts. For metallocene-based catalysts of classes II and partially V, this mechanism gives rise to successive additions at the same site (from a configurational point of view) and is known as the chain stationary insertion mechanism ( chain skipped insertion or site isomerisation without insertion mechanism) [143, 146, 345], The (a)-(b)-(c)-(a )—(b )—(c ) pathway corresponds to the chain migratory insertion mechanism found in the case of metallocene catalysts of classes I, III, IV and partially V [143, 146]. 

Figure 3.36 Schematic representation of the chain stationary insertion mechanism of propylene polymerisation with the bridged mesogenic metallocene-based catalyst. Reproduced by permission from Ref. 402. Copyright 1995 Wiley-YCH Weinheim...

Therefore, according to Razavi et al. [127,143], chain migratory insertions and chain stationary insertions would follow each other alternately, probably involving a pen-penultimate mechanism dictated by the periodicity of the helices of the growing chain [413]. If such a coordination site switching mechanism intervenes periodically, the resulting polypropylene chain would expose a syndioisoblock microstructure. 

Chain Migratory Insertion, Chain Stationary Insertion, and Site Isomerization... 

The microstructures described above correlate to chain migratory insertion. While in the case of a C2 or C2v symmetric metallocene, due to the homotopic nature of the potentially active sites, chain stationary insertion or migratory insertion followed by site isomerization would result in the same microstructure as chain migratory insertion, in the case of Cs symmetric catalysts they result in isotactic blocks. 

Ci symmetric metallocenes are able to produce new microstructures if consecutive insertions take place on the same active site in addition to chain migratory insertion. Polypropenes containing blocks of atactic and isotactic sequences are produced, the block lengths depending on the rate of chain stationary insertion or site isomerization vs chain migratory insertion [126-133], Thus hemiisotactic polypropene represents a special case, having a chain stationary/chain migratory ratio of 1 1. 

After the chain migratory insertion mechanism, the newly formed polymer chain occupies the position that previously was the vacant coordination site, where the monomer coordinated. After a chain stationary insertion mechanism, it is predicted that the newly formed polymer chain occupies its original position. Although a distinction between these two mechanisms may not be important for ethylene polymerization, it is cmcial to understanding the polymerization of 1-alkenes, where stereochemistry is highly relevant. 

FIGURE 5.22 (See color insert following page 280.) Perspective four-cell view snapshots of various stationary-phase models, which illustrate the contrasting surface topography with bonding density and alkyl chain length. (Adapted from Lippa, K.A., et al., Anal. Chem., 11, 7852, 2005.)... 

Apolar stationary phases suffer from hydrolytic instability at pH extremes. The use of mixed phases of long (Cg, Clg) and short (C, C3) chain alkyls produces stationary phases with increased hydrolytic stability.7,8 Crowding of the long alkyl chains does not allow the alkylsilane molecules to deposit in close packing on a smooth or flat surface. Silane molecules polymerize in vertical direction, loosing contact with the silica surface. The insertion of short chain alkyls allows horizontal polymerization of the silane molecules. Thus, alkyl chains are aligned in a parallel way. The stability of the silane layer is increased consequently (figure 8.1). 

If no stationary distribution Psi(x) exists or if it is unknown, a different approach must be taken. For this purpose, we return to Eq. (1.56) and insert the forward and backward generation probabilities from Eqs. (1.61) and (1.62). Using Eq. (1.32) and applying the chain rule for products of Jacobians, we finally obtain the following expression for the acceptance probability of a forward shifting move. 

For an explanation of this effect, authors of works [28-30] proposed similar kinetic models in which the chain propagation includes two reactions of the active site with monomer molecules that differ in the value of the rate constant. The slower reaction is the insertion of the first monomer molecule (the initiation/activation of the active site), the faster reactions are the subsequent insertions of monomer molecules into activated sites (chain propagation). For a description of the stationary rate of polymerization, Eq. (2) was proposed, which takes into account the reactions of initiation (re-initiation after the chain terminations), propagation, and termination ... 

The spatial effects may be summarized, by saying that the molecules shall be extended and rectilinear as far as po.s.sible, and a ribbon shape apparently has the advantage over a cylindrical one for this purpose. Internal free rotation is undesirable stationary or nearly stationary particles are the most suitable. Zig-zagging within the chains and special branching at the ends or in the middle are at once apparent in connection with the formation of crj stalline liquid phases. Table 77 from Vorlander is cited as an example and it shows very clearly the influence of a chain inserted between two larger identical molecular components. 

The recommendation to use the same column with the same surfactant was done because, in some instances, it may be impossible to fully remove the surfactant-adsorbed layer. It was not possible to eliminate a small part of SDS adsorbed on a 1980 Hypersil ODS phase even when using a pure methanol mobile phase [8]. However, a complete SDS desorption from a 1988 Hypersil ODS phase was possible with a pure methanol mobile phase [24]. Part of the nonionic Brij 35 surfactant could not be removed from a Resolve C18 phase after a 24-hour elution of water-acetonitrile 70-30% v-v [16]. Conversely, there are many examples where the surfactant layer can be completely stripped off the stationary phase [11, 19, 21, 24]. It was suggested that partial irreversible surfactant adsorption was due to a tight insertion of the surfactant alkyl chains in the alkyl moieties of the bonded layer of densely grafted phases [35]. A column could be used with different... 

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