Metallocene catalysts: Ziegler-Natta

Keywords metallocene catalyst, Ziegler-Natta catalyst, olefin polymerization, polyolefins, homogeneous catalysts, supported catalysts, stereoregularity, molecular weight distribution (MWD), chemical composition distribution, Unipol , Novolen , stereoselectivity, single site catalyst, multiple site catalyst, gas phase process, slurry process, homopolymerization, copolymerization. 

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

Homogeneous alkylaluminium (alkylaluminoxane)-free olefin polymerisation catalysts are commonly referred to as catalysts containing group 4 metallocene cationic species. These catalysts resemble the structure and properties of the respective metallocene-based Ziegler-Natta catalysts but, by definition, are not included among the latter catalysts. The development of alkylaluminium-free metallocene-based olefin polymerisation catalysts is connected with discoveries... 

At the end of considerations of the role of alkylaluminoxanes as activators for metallocenes as Ziegler-Natta olefin polymerisation catalysts, it should be noted that the analogy between methylaluminoxane and simple acidic moieties does not appear to hold. For example, highly Lewis acidic perfluorinated boranes, such as Bf CgFs, show enhanced activity as activators when compared with [Al(Me)0]x, but the stability of the resulting zirconocene-based catalyst is drastically lower than that of the catalyst formed with [Al(Me)0]x [98]. 

In the late 1980s and in the 1990s, homogeneous metallocene-based Ziegler Natta and related aluminium-free catalysts as well as other non-metallocene-based homogeneous single-site catalysts, which are active in the syndiospecific polymerisation of styrene, were found also to promote the polymerisation of conjugated dienes [16,38 13],... 

Compare polymerizations with conventional Ziegler-Natta catalysts and metallocene-based Ziegler-Natta catalysts in respect of (a) reaction mechanism, (b) stereospecifidty of polymer product, and (c) polymer molecular weight and distribution. 

Chapter 6), grafting-from polymerization and polymer brushes (Chapter 6), living controlled radical polymerization (Chapter 6), metallocene-based Ziegler-Natta catalysts (Chapter 9), immobilized metallocene catalysts (Chapter 9), and oscillating metallocene catalysts (Chapter 9). 

The reactivity of most functional groups toward the metal catalysts requires that protection—deprotection strategies be employed. For compatibility with metallocene and Ziegler—Natta catalysts, the most commonly employed protecting groups are based on aluminum, boron, and silicon. Aluminum offers an advantage because of its ubiquitous existence in polymerization formulations. ... 

Metallocene-based Ziegler-Natta catalysts are capable of polymerizing cyclic monomers without ringopening reactions that are characteristic of heterogeneous Ziegler-Natta catalysts. Kaminsky reported the homopolymerization of cyclic monomers such as cyclobutene, cyclopentene, norbomene, and dimetha-nooctahydronaphthalene (Scheme 22) with MAO-activated zirconocene catalysts. Cyclobutene was approximately 5 times more reactive than cyclopentene, which was more reactive than norbomene. ... 

Although there is normally a preference after a 1,2-insertion for subsequent addition at the end of the longer branch, in propylene polymerizations there is no longer branch. As a result, propylene polymerizations are generally slower than polymerizations of higher a-olefins, in marked contrast to metallocene or Ziegler—Natta catalysts. 

The methods discussed above for single-site catalysts (metallocenes and late transition-metal catalysts) can be extended directly to multiple-site catalysts (Ziegler-Natta and Phillips catalysts) by assuming that each site type behaves essentially as single-site catalysts [87, 88, 99, 100, 101], Therefore, the microstructural distribution of a polymer made with a multiple-site catalyst is treated as a weighted sum of several single-type distributions [102],... 

INTRODUCTION New types of linear low-density polyethylenes (LLDPE) based on the metallocene catalyst technology have been introduced recently in the market place. Metallocene-based Ziegler-Natta catalysts utilize a new synthetic approach for the polymerization of poly(a-olefins).( 5) Metallocene precatalysts are based primarily on group IV transition metals (primarily titcuiium and zirconium straddled by a pair of cyclic alkyl molecules) and require a coactivator, which is typically methylalumoxane but certain acids containing noncoordinating anions as bases also work well. 

Sukhadia, A. M., Trade-offs in blown film LLDPE type resins from chromium, metallocene and ziegler-natta catalysts. Journal of Plastic Film Sheeting 2000,16, (1), 54-70. 

Lu, H. L., Hong, S., and Chung, T. C. 1999. Synthesis of polypropylene-co-p-methylstyrene copolymers by metallocene and Ziegler Natta catalysts. 

Further developments in metallocene- and Ziegler-Natta catalysts as well as innovative improvements in polymerization technology are guarantees for further product diversifications using the existing processes and for improvements in reactor productivities. 

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