Coordination polymerization late transition metal catalysts

Metallocenes or late transition metal catalysts were iiKxeas-ingly utilized over the years for the (co)polymetization of olefins including the synthesis of graft copolymers. In this last case coordination polymerization had to be combined with other polymerization processes to give access to macromonomets. However, only a few examples of coordination homopolymer-ization or copolymerization of macromonomets aimed to design comb-shaped polymers were described in the literature. In the following these examples will be described briefly. 

Broadening this comparison to include copolymers prepared by both early and late transition metal catalysts, the results discussed immediately above show that Ci-symmetric zirconocenes such as 9/MAO produce only copolymers with isolated norbornene units or alternating structures (at 30 C), mainly with isotactic (meso) configurations. C2-symmetric zirconocenes such as 2/MAO readily produce norbornene dyads that are exclusively meso-linkcd (isotactic). In accordance with their catalyst structures, Q-symmetric zirconocenes such as 8/MAO produce norbornene dyads with a rac-linkage (syndiotactic), although with a generally lower stereoselectivity. Palladium a-diimine catalysts, despite the homotopic nature of their coordination sides (that would be expected to give a mixture of meso and racemic blocks), produce norbornene dyads that are solely rac-connected. This behavior can be attributed to a chain-end control type polymerization mechanism. 

The annual production of various polymers can be measured only in billion tons of which polyolefins alone figure around 100 million tons per year. In addition to radical and ionic polymerization, a large part of this huge amount is manufactured by coordination polymerization technology. The most important Ziegler-Natta, chromium- and metallocene-based catalysts, however, contain early transition metals which are too oxophiUc to be used in aqueous media. Nevertheless, with the late transition metals there is some room for coordination polymerization in aqueous systems [1,2] and the number of studies published on this topic is steadily growing. 

Coordination polymerization of ethylene by late transition metals is a rather slow process especially when the catalyst is dissolved in water. In a study of the interaction of CH2=CH2 and [Ru(H20)6](tos)2 (tos = tosylate), both [Ru(CH2=CH2)(H20)5](tos)2 and [Ru(CH2=CH2)2(H20)4](tos)2 were isolated by evaporation of the aqueous phase which had been previously pressurized with 60 bar ethylene at room temperature for 6 and 18 hours, respectively. Longer reaction times (72 h) led to the formation of butenes with no further oligomerization. This aqueous catalytic dimerization was not selective, the product mixture contained Z-2-butene, E-2-butene and 1-butene in a 112.2122 ratio [3]. 

The use of late transition metals as olefin polymerization catalyst requires the suppression of chain transfer while at the same time a high chain growth rate should be maintained. These new catalysts have an electron-deficient, in most cases, 14-electron and cationic metal center with a vacant coordination site. The most... 

Strong interest in late transition metal olefin polymerization catalysts resulted in the development of new five-coordinate Fe and Co systems (69) that afford highly linear, crystalline, high-density polyethylene.587-589 A new class of single-component, neutral Ni catalysts based on salicylaldimine ligands (70) was reported to be active in the polymerization of ethylene 590,591... 

Coordination polymerization of dienes has progressed significantly within the last decade. Selective polymerization of 1,3-dienes is reinforced by conventional transition metal catalysts and by new organolanthanide catalysts. Nonconjugated dienes also polymerize selectively to produce polymers with cyclic units or vinyl pendant groups. Living polymerization of dienes has become common, which enabled preparation of block copolymers of dienes with alkenes and other monomers. Another new topic in this field is the polymerization of allenes and methylenecycloalkanes catalyzed by late transition metal complexes. These reactive dienes and derivatives provide polymers with novel structure as well as functionalized polymers. The precision polymerization of 1,2-, 1,3-, and l,n-dienes, achieved in recent years, will be developed to construct new polymer materials with olefin functionality. 

Not only palladium, but many more non-metallocene late (and early) transition metal catalysts for the coordination polymerization of ethene and 1-olefins were reported [11]. Among the most significant findings in this area are the disclosures of novel highly active and versatile catalysts based on (i) bidentate diimine [N,N] nickel and palladium complexes [12], (ii) tridentate 2,6-bis(imino)pyridyl [N,N,N] iron and cobalt complexes [13], and (iii) bidentate salicyl imine [N,O] nickel complexes [14]. 

The discovery of Ziegler-Natta catalysts led to many industrial and academic investigations on other kinds of metallic complexes for polymerization of different monomers. Several organometallic and coordination compounds have been synthesized and probed as catalytic systems. They have been classified based on generations or groups, transition-metal type, the chemical structure, the type of activator, and their applications in polymerization processes [2]. Currently, there are different groups of initiator systems based on early and late transition metals or lanthanide complexes, which have been studied in polymerization catalysis [3]. 

Figure 2.14 Some examples of coordination catalysts for olefin polymerization (a) Ziegler-Natta (b) Phillips (c) metallocene and (d) late-transition metal.

As an alternative to melt mixing, in-situ polymerization is an attractive technique for the preparation of polyolefin-clay nanocomposites because it can promote better clay exfoliation and dispersion in the polymer matrix [1]. During in-situ polymerization, a coordination catalyst (such as Ziegler-Natta, metallocene, or late transition metal complex) is supported onto the clay interlayer surface to make polyolefin chains directly between the clay layers, leading to their exfoliation and dispersion into the polymer phase. 

A wide variety of transition metal catalysts, such as Ti-, Mo-, W-, Ni-, and Pd-based complexes, have been investigated since the late 1950s for cyclic conjugated diene polymerization. Because this chapter focuses on the coordination polymerization of CHD and its derivatives, cationic polymerization initiated by transition metal compounds will be excluded in this section. 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

The development of organometallic initiators, both of the lithium type and of the transition-metal coordination type, occurred rapidly in the decade following the late 1950s. The lithium initiators were developed without the fanfare of coordination-type initiators. This situation developed because of the remarkable ability of the coordination catalysts to induce stereospecific polymerization of a-olefins. 

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