Living polymerization heterogeneous

Living polymerization does not occur with traditional Ziegler-Natta isoselective heterogeneous initiators. 

Heterogeneous Ziegler-Natta catalysts composed of titanium trichloride and alkylaluminum have been used to prepare block copolymers of ethylene with a-olefins 44-46), even though there is no known example of such a catalyst meeting the requirement for a living polymerization. The produced block copolymers have broad molecular weight distributions (Mw/Mn = 4 20) and are present in small concentrations... 

The published values of related to olefin polymerization with soluble and heterogeneous catalysts are in the range from several seconds to hours 7). It should be noted that the value of is strongly dependent upon the polymerization conditions since the rates of chain-terminating reactions Rt are functions of the temperature and concentrations of chain-terminating reagents. In a living polymerization the value of is infinite. 

Block copolymers of propylene with ethylene have been produced in commercial polymerization processes using heterogeneous Ziegler-Natta catalysts. In all processes the block copolymers are produced in small concentrations, and the major products are homopolymers. Well-defined block copolymers free of homopolymer impurities can be prepared with catalysts exhibiting a living polymerization character. In this section we deal with the synthesis of well-defined block copolymers using the living polypropylene which has been prepared with soluble vanadium-based catalysts. 

In spite of countless applications of rare earth activation in industrial heterogeneous catalysis, most soluble complexes have long been limited to more or less stoichiometric reactions. An early example is the Kagan C-C coupling mediated by samarium(II) iodide [126]. Meanwhile, true catalytic reactions have become available. Highlights are considered the organolanthanide-catalyzed hydroamina-tion of olefins [127], the living polymerization of polar and nonpolar monomers [128], and particularly the polymerization of methyl methacrylate [129]. In the first case, lanthanocene catalysts of type 27 are employed [127]. 

The other unique features of living polymerizations are that M varies directly with conversion (Fig. 22.3, cf. Fig. 22.1) and the heterogeneity index (poly-dispersity) is 1, or very close to it. 

Due to the water insolubility of these metal carbenes, aqueous polymerizations represent heterogeneous multiphase mixtures. Investigation of ROMP of the hydrophilic monomer 8 or of a hydrophobic norbomene in aqueous emulsion (catalyst precursor 7 b added as methylene chloride solution) or suspension demonstrated that the polymerization can occur in a living fashion. For example, at a monomer to catalyst ratio 8/7b = 100 with 78% yield, poly-8 of Mw/Mn 1.07 vs. polystyrene standards was obtained [68]. Using water-soluble carbene complexes of type 9 and water-soluble monomers 10, living polymerization can be carried out in aqueous solution, without the addition of surfactants or organic co-solvents [69]. 

Some coordination polymerizations also exhibit the character of a living polymerization process. For example, Natta32 described a heterogeneous catalyst yielding block polymers when the initially present monomer was replaced by another one. In that system, the lifetime of a growing polymer exceeds one-half hour, i.e. the termination caused by the hydride transfer to the catalytic center was very slow. Similar results were reported by Bier33 and by Kontos et al.34. ... 

All previously discussed examples of living cationic polymerization of vinyl ethers were based on homogeneous polymerization media. In 2007, Oashima and coworkers demonstrated the living polymerization of isobutyl vinyl ether in the presence of iron(III) oxide as heterogeneous catalyst and ethyl acetate or dioxane as base [58]. The major advantage of this heterogeneous catalytic system is the easy removal of the metal oxide catalyst. In addition, it was demonstrated that the iron(III) oxide could be reused for at least five times without a decrease in activity. 

In the literature, various reasons for formation of polymers with broad MMD on heterogeneous ZN catalysts are discussed. Convincing evidence has been obtained using the SF method that the reasrni is heterogeneity of the active centers on a surface of the catalyst [186]. In conditions of quasi-living polymerization there are no transfer reactions of the growing polymer chain and polymer is formed on the surface of catalyst in very small quantities. This polymer cannot cause diffusion restrictions, but nevertheless polymer with broad MMD (Mw/M = 3.2-4.3) is formed. The further increase in time of polymerization does not influence the width of MMD (M ,/M = 3.6). 

Heterogeneous conditions, due to poor solubility of heteropoly acid, in polymerization of isobutyl vinyl ether with H3PW12O40 in CH2CI2 were also studied. When bases like 1,4-dioxane or tetrahy-drofuran were present the molecular weight distributions were very broad. By contrast, polymerizations in the presence of dimethyl sulfide at —30°C yielded living polymerizations of the ether. Here too, the product had very narrow molecular weight distribution [139]. In summary, some typical features of living cationic polymerizations are ... 

If, however, as in living polymerization, all chains start at the same time and live throughout the polymerization, every chain will see all the changes in monomer feed composition, and consequently the composition will change along the chain and not from chain to chain. This is known as second-order chemical heterogeneity. The differences between these two kinds of heterogeneities, one between different chains, the other within different fractions of one chain, are shown in Scheme 7.3. 

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