Polyolefin-Clay Nanocomposites by In-situ Polymerization

AbolfazI Maneshi, Joao Soares, and Leonardo Simon 

Polyolefins are a major class of commodity synthetic polymers. The technology for the production of these important polymers is well estabUshed, from catalyst synthesis to polymerization reactor technology. Despite constant advancements in polyolefin production technology, applications of polyolefins are stiU mainly limited to commodity products. The recent interest in the production of polyolefin-clay nanocomposites extends the use of polyolefins to specialty and engineering plastic appHcations. Polyolefin-clay nanocomposites are lighter than conventional composites, but have thermal stability, barrier, and mechanical properties that are comparable to those of engineering plastics. 

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. 

Olefin in-situ polymerization methods for the production of polyolefin-clay nanocomposites still face many challenges before becoming industrially relevant. 

In-situ Synthesis of Polymer Nanocomposites, First Edition. Edited by Vikas Mittal. 

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Toward Polyolefin-Clay Nanocomposites by In Situ Polymerization.134... 

Maneshi A, Soares J, Simon L (2011) Polyolefin-clay nanocomposites by in-situ polymerization. In Mittal V (ed) In-situ synthesis of polymer nanocomposites. Wiley-VCH, Weinheim, pp 53-88... 

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. 

Preparation of polymer-day nanocomposites by in situ polymerization (sometimes called intercalative polymerization or polymerization compounding) circumvents the enthalpic and entropic barriers that prohibit the intercalation of nonpolar polyolefins into polar clays. Since supported olefin polymerization catalysts are desirable anyway in high-volume polyolefin manufacturing (see Section 5.1.1), the design of clay-supported catalysts to prepare nanocomposites can achieve both goals at once. In the late 1990s, researchers... 

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. 

Scott, S. L., Peoples, B. C., Yung, C., Rojas, R. S., Khaima, V., Sano, H., Suzuki, T, and Shimizu, F. 2008. Highly dispersed clay-polyolefin nanocomposites free of compatibil-izers, via the in-situ polymerization of a-olefins by clay-supported catalysts. Chemical Communications 4186 188. 

In this method, the organoclay, the catalyst precursor, and the activator (alkylalu-minum compounds or alkylaluminoxanes) are added to the reactor and the polymerization is started by introduction of the olefin monomer. This is the simplest in-situ polymerization method, and only few reports [1, 51, 70, 71] have been published investigating this method for production of polyolefin-clay nanocomposites. In comparison to the other in-situ polymerization methods, this method seems to have the lowest polymerization activities, although it is often difficult to compare catalysts activities for polymerizations done at very different conditions and following distinct procedures. 

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