Supported transition metal complex catalysts polymerization

A viable process for manufacturing polyolefin-clay nanocomposifes by in situ polymerization requires adequate catalytic activity, desirable polymer microstructure, and physical properties including processibility, a high level of clay exfoliation fhaf remains stable under processing conditions and, preferably, inexpensive catalysf components. The work described in the previous two sections focused on achieving in situ polymerization with clay-supported transition metal complexes, and there was less emphasis on optimization of polymer properties and/or clay dispersion. Since 2000, many more comprehensive studies have been undertaken that attempt to characterize and optimize the entire system, from the supported catalyst to the nanocomposite material. The remainder of this chapter covers work published in the past decade on clay-polyolefin nanocomposites of ethylene and propylene homopolymers, as well as their copolymers, made by in situ polymerization. The emphasis is on the catalyst compositions and catalyst-clay interactions that determine the success of one-step methods to synthesize polyolefins with enhanced physical properties. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

In conclusion, over the past 20 years, various transition metals have been successfully immobilized on polymeric resins. The catalytic activity of these transition metal complexes have been shown with little if any leaching occurring during catalysis. In several instances the supported catalyst has been re-used with little loss in activity. This proves that the reactions are truly catalyzed by anchored metal. 

In catalysis involving transition metal complexes supported on polymers it is recognized that the chemical and physical structure of the polymeric matrix can have a large effect on the performance of the catalyst. Table XI presents several different polymeric matrixes that are used for this application. 

Methods of synthesizing polymers with carbonyl groups as supports of transition metal complexes have, moreover, been analyzed [104]. The activity of these catalysts in polymerization, oligomerization and hydrogenation reactions as a function of the polymer support structure has been discussed. 

ICl patented polymerization or copolymerizati(Hi of ethylene using transition metal complex (preferably Z-N catalyst on support) at P = 30-100 MPa and T = 100 °C in pentane. Multi-reactor with different feeds and/or conditions or a tubular reactor with different reaction zones may be used. Several other LDPE manufacturers (e.g., DuPont, Mitsui)... 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

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. 

Functionalized polymers incorporating neutral, metal binding ligands such as phosphine were prepared as early as 1959 (Rabinowitz and Marcus, 1961 Issleib and Tzschach, 1959). After Merriheld introduced the concept of solid-phase synthesis, the basic idea of using polymer-immobilized transition metal complexes as catalysts burgeoned, and many more polymeric supports containing neutral donor ligands have been prepared. 

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