Olefin polymerization transition metal free

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

The formation of high polymers of olefins in the presence of titanium halogenides with no specially added organometallic co-catalysts was discovered long ago [see (147), and the references therein], A complete description of various alkyl-free polymerization catalysts based on the use of transition metal chlorides may be found in the review by Boor (17), where a comparison of these catalysts with traditional two-component systems is given. 

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

Mashima, K., Nakayama, Y. and Nakamura, A. Recent Trends in Polymerization of a-Olefins Catalyzed by Organometallic Complexes of Early Transition Metals. Vol. 133, pp. 1-52. Matsumoto, A. Free-Radical Crosslinking Polymerization and Copolymerization of Multivinyl Compounds. Vol. 123, pp. 41-80. 

As mentioned in Chapter 1, ethylene is always the more reactive olefin in systems used to produce copolymers involving a-olefins (LLDPE and VLDPE). An important process consideration for copolymerizations is the reactivity ratio. This ratio may be used to estimate proportions needed in reactor feeds that will achieve the target resin. However, fine tuning is often required to obtain the density or comonomer content desired. Reactivity ratios were discussed previously (Chapter 2) in the context of free radical polymerization of ethylene with polar comonomers. Reactivity ratios are also important in systems that employ transition metal catalysts for copolymerization of ethylene with a-olefins to produce LLDPE. Discussions of derivations and an extensive listing of reactivity ratios for ethylene and the commonly used a-olefins are provided by Krentsel, et al. (1). 

This contribution reviews aqueous transition metal catalyzed polymerizations of olefinic monomers. Consequently, classical free-radical polymerizations employing rnetal-containing redox-systems as initiators, for example, are not considered. 

Free radical addition of hydrosilanes to olefins was discovered in the late 1940s156. The reaction was initiated thermally, photolytically or by free radicals and was essentially limited to HSiCl3 and to olefins that did not readily polymerize. It was not until the 1950s that a suitable transition-metal catalyst was discovered for this general reaction, which is... 

Figure 2.19 illustrates Cossee s mechanism for polymerization with coordination catalysts. The active site is depicted as having a coordination vacancy that attracts the electrons in the olefin rr-bond. Coordination is followed by insertion into the polymer chain (R) and the re-establishment of the coordination vacancy for further monomer insertion. This figure also shows an important characteristic of coordination polymerization that makes it very different from free-radical polymerization the monomer is inserted between the carbon-metal bond. As a consequence, the electronic and steric environment surrounding the transition metal has a huge influence on the kinetics of polymerization. This is why... 

LDPE Copolymers. A variety of comonomers can be added to the polymerization of ethylene to make copolymers. The free-radical polymerization mechanism of LDPE production allows for the copolymerization of polar comonomers. At this time, the incorporation of polar comonomers is unique to LDPE. The transition metals used to catalyze HDPE and LLDPE production are generally poisoned by polar comonomers and therefore, only copolymers containing alpha-olefins like 1-butene, 1-hexene, and 1-octene can be made. Because the polar copolymers can be made only by the LDPE process, they command a premium in the market. The most common comonomers (and their corresponding copol5uners) are vinyl acetate (EVA), methyl acrylate (EMA), ethyl acrylate (EEA), and acrylic acid... 

Insertion of a prochiral a-olefin such as propylene creates a chiral carbon bonded to the active site. Differently from free-radical polymerization, coordination polymerization can be regio- and stereoselective, depending on the design of the ligands bonded to the transition metal. Regioselectivity determines the sequence of 1-2 or 2-1 insertions while stereoselectivity determines whether the polymer is isotactic, syndiotactic, or atactic (Figure 8.10). 

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