Insertion polymerization reactions

In addition to the wide variety of true coordination/insertion polymerization reactions with polar fimaional monomers, the phosphine sulfonate catalyst system is capable of the nonalternating copolymerization of olefins and CO. First discovered by Drent a with in situ formed catalysts, Nowack et al later prepared the first single component catalysts 104. In the course of this development, the reaction was studied in detail concerning the mechanism that leads to the additional incorporation of ethene emits (Figure 45). 

Since the above treatment would insert an additional parameter in the system (i.e. an additional piece of uncertainty), that of kwsi> and since water soluble impurities usually manifest themselves as an induction time for the polymerization reaction, the empirical treatment of (32) might be more than satisfactory. 

The molecular weight (M 10,200,000) represents the highest molecular weight known to date for a linear, synthetic copolymer. DFT calculations suggest that steric congestion, derived from the triethylsilyl group and the amine moiety, near the polymerization reaction center diminishes the rates of chain termination or transfer processes yet permits the monomer access to the active site and the monomer s insertion into the metal-carbon bond (Fig. 21). 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

The models considered in this section refer to catalytic systems for which the polymerization reaction occurs by primary insertion of the 1-alkene and neither chirality of coordination of the aromatic ligands nor chirality at the... 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Inspired by the ability of cationic ansa-zirconocene complexes to effect stereocontrolled alkene polymerization reactions, Jordan has recently reported the stereoselective insertion of simple alkenes into both the (ebi)Zr(r 2 -pyrid-2 -yl) and (ebthi) Z r (r 2 -pyr id - 2 -yl) systems [113]. As shown in Scheme 6.36, treatment of rac-(ebi)ZrMe2 114 with nBu3NH+BPh4 in the presence of 2-picoline affords the (ebi)Zr(q2-pyrid-2-yl) complex 115 (the derived B(C6F5) derivatives may also be prepared and are in fact reported to be more convenient to use). 

Depending on the nature of the active center, chain-growth reactions are subdivided into radicalic, ionic (anionic, cationic), or transition-metal mediated (coordinative, insertion) polymerizations. Accordingly, they can be induced by different initiators or catalysts. Whether a monomer polymerizes via any of these chain-growth reactions - radical, ionic, coordinative - depends on its con-... 

The Ziegler polymerizations of olefins (92, 5) and the aluminum (108), gallium, beryllium, and indium (ill) alkyl growth reactions also seem to be examples of olefin insertion reactions of metal-carbon compounds. Despite great effort concerning the mechanism of the polymerization reactions, relatively little has been learned about the actual catalytic species involved. 

The accuracy of the polymerization reaction itself, however, is insufficient to account for the high degree of fidelity in replication. Careful measurements in vitro have shown that DNA polymerases insert one incorrect nucleotide for every 104 to 105 correct ones. These... 

When the transfer reaction competes successfully with further insertion, as in the case of nickel, dimerization becomes the dominant transformation. When metal hydride elimination, in turn, is slow relative to insertion, polymeric macromolecules are formed. Ligand modification, the oxidation state of the metal, and reaction conditions affect the probability of the two reactions. Since nickel hydride, like other metal hydrides, catalyzes double-bond migration, isomeric alkenes are usually isolated. 

In the polymerization reaction the monomer is coordinated at a vacant site of the metal, followed by insertion into the metal-carbon bond (Fig. 9.5-5). Finally, the chain-propagation is terminated by transfer of a hydrogen atom in a P-position to the metal or to the coordinated monomer. 

Early mechanistic studies concerning organolanthanide-catalyzed olefin polymerization reactions showed that insertion of the unsaturated hydrocarbon into the lanthanide-carbon cr-bond is a key step. It was first demonstrated for the... 

Polymerization reactions follow an insertion mechanism, that is, alkene coordination to a vacant site on the active metal species, followed by a migratory alkyl transfer step. The addition of donor molecules which can compete with the alkene for coordination sites is therefore a means of reducing the rate of propagation and allows /3-H elimination to take place, so that a polymerization reaction might be converted to oligomerization or dimerization. On the other hand, metals which... 

In presence of dienes (Scheme 1), heterodienes, and heterotrienes (Scheme 2) rapid cycloadditions take place which prevent other reaction modes of the silylenes (insertions, polymerization, or HX-elimination-polymerization). Although a concerted [l+4]-cycloaddition is symmetry-allowed a stepwise mechanism (Scheme 3) via a three-membered intermediate is prefered or at least partly be competing to account for the formation of double-bond isomers. Ususally the formal [4+l]-cycloadducts (allylsilane-type) are the main products while the isomers with vinylsilane-units are side-products (< 30 %). Exceptions are... 

Fig. 3. Schematic representation of the energetic path followed along a polymerization reaction of the monomer M catalyzed by a catalytic centre h (such as a transition metal site or a basic surface center). The precursor species are indicated as F M, while l- M represent oligomers/polymers. The activation energy barriers for each step (A i) are represented. Also the energy barrier Afi) associated with the polymers release is represented in the perpendicular direction, as this step can potentially occur for each M insertion. In contrast to the cases displayed in Fig. 2, in this case A , > > A i (unpublished).

If the polymerization reaction occurs in the presence of steric constraints, the activation energy associated with the monomer insertion grows with the progress of the reaction, in the same way as discussed for the oligomerization reactions catalyzed by Bronsted acid sites (Section LB and Fig. 2a), because the available space is progressively reduced. Under such conditions, the formation of polymeric species is limited, and small oligomeric species can become observable. An example of this situation is ethene polymerization on Cr/silicalite, in which the transition metal center is grafted to the internal surface of a cavity (Section VI.C.l). 

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