Coordination polymerization, clay

Most commercial polyolefins are produced by coordination polymerization catalysts. When compared to free radical processes used to make low-density polyethylene (LDPE), these catalysts work in comparatively gentle conditions, such as lower pressures and temperatures, while providing greater flexibility in controlling the polyolefin molecular structure. An understanding of the polymerization mechanism with coordination catalysts is essential for designing proper systems for the production of polyolefin-clay nanocomposites and wUl be covered in the next section. 

Polyolefins, which are normally defined as polymers based on alkene-1 monomers or a-olefins, are the most widely used group of thermoplastic polymers today. The use of many different coordination catalysts has been reported for the production of polyolefin/clay nanocomposites. The methods of in situ synthesis of polyolefin/clay nanocomposite by coordination catalysts mostly depends on the role of clay and can be divided into three categories (1) clay as pol5nner filler, (2) clay as catalyst or cocatalyst support, and (3) Clay acts as a cocatalyst for coordination polymerization. 

Tudor et al. [95] first used the in situ intercalative polymerization method for the preparation of PP/clay nanocomposites. They demonstrated the ability of soluble metallocene catalysts to intercalate inside silicate layers, and to promote the coordination polymerization of propylene. The silicate layers were modified by... 

A wide variety of polymer/clay nanocomposites can be synthesized by in situ coordinadmi polymerization methods, which gives the advantage of controlled molecular weight of the polymer nanocomposite. In the case of late transition metal-based coordination polymerization, the process is tolerant to the polar groups or to a little moisture in the clay or catalytic system. This moisture sensitivity can also be overcomed by treating the excess MAO or by in situ formation of MAO using TEA, TMA, or TIBA on the surface of clay, clay-MgCl2, or clay-silica hybrid material. 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

Paul, M-A., Alexandre, M., Degee, P., Calberg, C., Jerome, R., and Dubois, P. Exfoliated polylactide/clay nanocomposites by In-Situ coordination-insertion polymerization, Macromol. Rapid Commun. (2003), 24, 561-566. 

FIGURE 5.91 Schematic presentation of the process of anchoring the coordination catalytic complex, MAO/me-tallocene, at the surface in the interlayer of layered clay, followed by ethylene polymerization at the immobilized catalyst site. (After Koppl, A., Alt, H. G., and Phillips, M. D. 2001. /. Appl. Polym., 80 (3), 454. With permission.)... 

Suzuki and Suga reported the use of clays as solid acids to support and activate metallocene catalysts for olefin polymerization. They were able to use much less alkylaluminmn cocatalyst relative to solution polymerization conditions. The clays were slurried with AlMeg in toluene, then treated with a solution containing zirconocene dichloride, II, and AIMeg. The metallocenium cation was presumed formed via abstraction of chloride and/or methyl ligands by acidic sites on the surface of the clay, and the low basicity of the clay smface was proposed to stabilize the coordinatively unsaturated cation. Propylene was copolymerized with 250 psi ethylene at 70°C. For acid-treated KIO montmorillonite, an activity of 3300 X 10 kg polymer/(g Zr h) was obtained. Catalysts based on vermiculite, kaolin, and synthetic hectorite all showed lower but still appreciable activities. In this brief report, the Al/Zr ratio was not specified, and the clay dispersion was not reported. 

The calcined clay was treated with TiCl4. Attachment of Ti(IV) to the clay surface was suggested to occur at coordinatively unsaturated Mg ions (presumably formed by the removal of water during calcination) on the edges of the octahedral ribbons and to involve bridging chloride interactions, as in VII. Upon activation by AFBU3 (Al/Ti = 15), ethylene polymerization was conducted in n-hexane at atmospheric pressure and 40°C. 

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. 

In-situ Polymerization of Olefins with Coordination Catalysts Supported on Clays 59... 

The use of many different coordination catalysts has been reported in the increasing number of publications on in-situ polymerization for the production of polyolefin-clay nanocomposites. In addition to metallocenes, which are the dominant systems in this area, the use of other catalyst types has also been investigated. 

This chapter aims to give an overview on the recent advances in the synthesis of PCNs through the in-situ 1,3-butadiene homo- and copolymerization technique. To this purpose, we distinguished in-situ polymerization approaches on the basis of the polymerization method anionic or insertion/coordinative. However, before discussing on the topic of this chapter, we wish to briefly recall some peculiar aspects of clay minerals, PNs, and their methods of preparation. 

Some support materials can be rendered Lewis acidic enough to ionize dialkyl metallocenes. Marks and co-workers have reported (33) that alnmina dried at very high temperatures can react at least to some small degree with both thorium-and zirconium-based metallocene dimethyl species to yield active catalysts for polyethylene. The resulting cationic metal center is believed to remain coordinated to the surface through an Al-O-M Lewis acid/base linkage, at least prior to exposure to ethylene. Hybrid surface/cocatalyst systems based on aluminum alkyl-treated clays have been developed (34) in which the solid substrate appears to play some role in promoting polymerization activity far beyond that expected for non-methyl aluminoxane- or trialkylaluminum-activated catalysts. 

---